on the link between surface and basal structures of the jelbart ice shelf antarctica to understand the dynamics of ice shelves a knowledge of their internal and basal structure is very important as the capacity to perform local surveys is limited remote sensing provides an opportunity to obtain the relevant information we must prove however that the relevant information can be obtained from remote sensing of the surface that is the aim of this study the jelbart ice shelf antarctica exhibits a variety of surface structures appearing as stripe like features in radar imagery we performed an airborne geophysical survey across these features and compared the results to terrasar x imagery we find that the stripe like structures indicate surface troughs coinciding with the location of basal channels and crevasse like features revealed by radio echo sounding hh and vv polarizations do not show different magnitude in surface troughs the local accumulation rate is larger than at the flat surface viscoelastic modelling is used to gain an understanding of the surface undulations and their origin the surface displacement computed with a maxwell model matches the observed surface reasonably well our simulations show that the surface troughs develop over decadal to centennial timescales

carbonate ramp drowning caused by flexural subsidence the south pyrenean middle eocene foreland basin abstract a detailed sedimentological and magnetostratigraphic analysis of three selected sections of the eocene carbonate platforms of the south pyrenean jaca pamplona foreland basin has been carried out in the aragonese external sierras huesca spain the stratigraphic record spans from the middle late lutetian chron 20r sbz15 to the bartonian chron 18r sbz17 the lutetian bartonian transition was characterized by a major paleogeographic change from a turbiditic foredeep with associated peripheral carbonate ramps to deltaic sedimentation in most of the outcrops of the external sierras the peripheral carbonate platforms guara formation are overlained by prodelta and outer ramp marls arguis formation the drowning unconformity between these two formations in the outcrops located in the westernmost tip of the external sierras is documented herein for the first time in this area the guara formation is overlain by glauconitic limestones deposited in a carbonate ramp setting between these units a characteristic unconformity is observed with hard ground formation and local evidence of subaerial exposure in the shallowest domains of the study area the abundant glauconite found on top of this unconformity records a starvation period in the basin followed by the development of a new carbonate ramp system during bartonian times santo domingo member of the arguis formation magnetostratigraphic and sedimentological data show that tectonic tilting occurred in latest lutetian times prior to the drowning process this suggests the influence of the flexural subsidence in the cratonic margin of the foreland as a result of the advance of a basement thrust in the hinterland nevertheless the correlation of the uppermost lutetian drowning unconformity with a basinwide regional sea level rise which may have further been global suggests an interplay between tectonic and eustatic processes in the drowning of the late lutetian south pyrenean carbonate ramps in this work we point out a narrowing of the carbonate ramp together with a relative sea level rise as the processes responsible for the drowning of the upper guara mb carbonate ramp these processes could be responsible for the development of drowning unconformities in cratonic margins of other foreland basins as carbonate ramps have a high sediment production potential the narrowing of the carbonate factory could provide the conditions for the drowning of the carbonate ramp system in a subsequent sea level rise

late paleozoic paleomagnetism of south mongolia exploring relationships between siberia mongolia and north china abstract the tectonic evolution of a large region that includes south siberia mongolia and north china is still a matter of fierce debate this is due at least in part to the contentious proposals detailing the kinematics of the major tectonic units in that area one solution to the problem is the acquisition of better defined paleomagnetic data for the various players in the system we carried out a paleomagnetic study of early carboniferous subduction related volcanic rocks of the trans altai zone in south mongolia and successfully isolated a pre folding and presumably primary component from 22 sites a critical analysis of late paleozoic paleomagnetic data from mongolia revealed several problematic data sets while the paleolatitudes derived from more reliable studies agreed well with the expected values for siberia and were consistently different from those for the north china block we also found that late paleozoic declinations from mongolia are rotated counterclockwise through various angles with respect to siberia in addition those rotations are widespread in both inner mongolia and the north china block in this paper we put forward some testable hypotheses and propose a preliminary paleogeographic scheme of the region 1 consistent difference in late paleozoic paleolatitudes between siberia and mongolia on the one hand and the north china block on the other is largely due to strong inclination shallowing in paleomagnetic data for the latter area where most data are from sedimentary rocks 2 widespread counterclockwise rotations in mongolia inner mongolia and north china may be accounted for by a wide zone of sinistral transpression that was active in the early mesozoic 3 in the early mesozoic the mongol okhotsk ocean was much narrower than is usually assumed 4 there was no large scale oroclinal bending between siberia and north china in the permian early mesozoic

superposed deformation in the peninsula mitre recess fuegian thrust fold belt southern argentina and implications for orogenic curve kinematics abstract a detailed study of the structure of the peninsula mitre recess in the fuegian thrust fold belt of southern argentina reveals superposed deformation related to contrasting shortening directions one group of structures trend parallel to the recess and verge either towards the foreland or towards the hinterland a second group of structures accommodate contraction superposed on the previous group analysis of fault slip data reveals that the first group is related to 1 perpendicular to the surface traces of thrusts and or fold axes and are therefore compressive structures in contrast the paleostress tensors of the second group of structures show 1 orientations parallel or at a low angle with respect to local structural trends the style of the superposed structures of the second group which mostly indicate contraction subparallel to the trend of the recess is consistent with structures resulting from tangential longitudinal shortening associated with oroclinal bending this scenario implies that the peninsula mitre recess formed as a progressive curve involving rotation due to slip trajectories convergent toward the recess causing the observed tangential contraction this kinematic model supports previous interpretations of buttressing of the thrust fold belt against a basement promontory

oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

determining the timing of formation of the rawil depression in the helvetic alps by palaeomagnetic and structural methods determining the timing of formation of the rawil depression in the helvetic alps with the use of paleomagnetic and structural methods

intersections of remanence small circles new tools to improve data processing and interpretation in palaeomagnetism summary intersections of remanence small circles are increasingly used in palaeomagnetism to determine the palaeofield direction from synfolding remanences the approach small circle intersection sci method presupposes that remanence small circles from sites tilted to different directions intersect in a common point which is assumed to represent the palaeofield direction the sci method is not restricted to synfolding remanences it can be applied also to derive the palaeofield direction from pre folding remanences in addition to tilt correction and as a cross check for fold tests however the sci method has specific requirements and does not work in all cases the suitability of data sets and reliability of palaeofield estimates have to be assessed carefully and always against the background of local geology this paper examines the occurrence and distribution of sci as a tool of judgement first a data set is most suitable and a palaeofield estimate by sci most reliable when most of the theoretically possible intersections are realized and in addition clearly grouping in a narrow region second the small circles that greatly determine the result of sci are identified by the highest number of intersections they have with the others these criteria allow a systematic investigation of sci estimates upon stability and dependencies moreover scis can be an indicator of irregular unexpected tectonic settings when the intersections disperse strongly although significant bedding strike differences are given relative rotations or other unexpected displacements are likely to have occurred if so the sci estimate will be invalid and probably biased however also fisher mean calculation and most fold tests will be affected in this case the scis can help to separate the data into subsets valid for fisher statistics and sci methods the other possibility is to use tentative reconstructions to infer the true sequence of tilts and rotations and correct the data adequately

cenozoic rotation history of borneo and sundaland se asia revealed by paleomagnetism seismic tomography and kinematic reconstruction se asia comprises a heterogeneous assemblage of fragments derived from cathaysia eurasia in the north and gondwana in the south separated by suture zones representing closed former ocean basins the western part of the region comprises sundaland which was formed by late permian triassic amalgamation of continental and arc fragments now found in indochina the thai penisula peninsular malaysia and sumatra on borneo the kuching zone formed the eastern margin of sundaland since the triassic to the se of the kuching zone the gondwana derived continental fragments of sw borneo and east kalimantan accreted in the cretaceous south china derived fragments accreted to north of the kuching zone in the miocene deciphering this complex geodynamic history of se asia requires restoration of its deformation history but quantitative constraints are often sparse paleomagnetism may provide such constraints previous paleomagnetic studies demonstrated that sundaland and fragments in borneo underwent vertical axis rotations since the cretaceous we provide new paleomagnetic data from eocene miocene sedimentary rocks in the kutai basin east borneo and critically reevaluate the published database omitting sites that do not pass widely used up to date reliability criteria we use the resulting database to develop an updated kinematic restoration we test the regional or local nature of paleomagnetic rotations against fits between the restored orientation of the sunda trench and seismic tomography images of the associated slabs paleomagnetic data and mantle tomography of the sunda slab indicate that sundaland did not experience significant vertical axis rotations since the late jurassic paleomagnetic data show that borneo underwent a 35 counterclockwise rotation constrained to the late eocene and an additional 10 counterclockwise rotation since the early miocene how this rotation was accommodated relative to sundaland is enigmatic but likely involved distributed extension in the west java sea between borneo and sumatra this late eocene early oligocene rotation is contemporaneous with and may have been driven by a marked change in motion of australia relative to eurasia from eastward to northward which also has led to the initiation of subduction along the eastern sunda trench and the proto south china sea to the south and north of borneo respectively

formation of orogen perpendicular thrusts due to mechanical contrasts in the basal decollement in the central external sierras southern pyrenees spain two series of analogue models are used to explore the effect of ductile frictional contrasts of the basal decollement on the development of oblique and transverse structures during thin skinned shortening these models simulate the evolution of the central external sierras ces southern pyrenees spain which constitute the frontal emerging part of the southernmost pyrenean thrust the ces are characterised by the presence of n s to nw se anticlines perpendicular to the pyrenean structural trend and developed in the hanging wall of the thrust system they detach on unevenly distributed triassic materials evaporitic dolomitic interfingering the models simulated the effect of adjacent ductile versus frictional decollements during shortening model series a tests the thickness ratio between overburden and the ductile layer whereas model series b tests the width perpendicular to the shortening direction of frictional decollement model results confirms that deformation reaches further in areas detached on a ductile layer whereas above frictional decollement areas shortening is accommodated by additional uplift and penetrative strain this replicates the structural style of the ces higher structural relief of n s anticlines with regard to orogen parallel structures absence of a representative ductile decollement in the core plunging towards the hinterland and foreland side closure not thrusted by the south pyrenean thrust

widespread bedding parallel veins of fibrous calcite beef in a mature source rock vaca muerta fm neuquen basin argentina evidence for overpressure and horizontal compression in the neuquen basin of argentina beef bedding parallel veins of fibrous calcite is widespread within late jurassic black mudstones a typical vein consists of two grey inner zones and two white outer zones the inner zones contain inclusions of wall rock and hydrocarbons calcite fibres are perpendicular to the margins in the outer zones the angle between fibre and margin varies from about 45 at the vein tips to 90 in the centre imprints of fossils are offset proving that the fibres have grown antitaxially we infer that the veins opened in two phases during phase 1 the opening was vertical against gravity during phase 2 the veins resisted tectonic shortening so that shear stresses acted at the margins the senses of shear account for the fibre angles at outcrop igneous intrusive rocks have cut and metamorphosed the veins from burial curves maturity calculations growth strata and ages of igneous intrusions we estimate that the inner zones of the beef formed in the oil window during the aptian to albian and that the outer zones formed in the gas window during the cenomanian to campanian we infer that the beef is evidence for fluid overpressure during hydrocarbon generation

viscous and viscoelastic stress states at the calving front of antarctic ice shelves calving mechanisms are still poorly understood and stress states in the vicinity of ice shelf fronts are insufficiently known for the development of physically motivated calving laws that match observations a calving model requires the knowledge of maximum tensile stresses these stresses depend on different simulation approaches and material models therefore this study compares results of a two dimensional 2 d continuum approach using finite elements with results of a one dimensional 1 d beam model elaborated in reeh 1968 a purely viscous model as well as a viscoelas tic maxwell model is applied for the 2 d case the maximum tensile stress usually appears at the top surface of an ice shelf its location and magnitude are predominantly influenced by the thickness of the ice shelf and the height of the freeboard the traction free part at the ice front more precisely doub ling the thickness leads to twice the stress maximum while doubling the freeboard based on changes of the ice density results in an increase of the stress maximum by 61 poisson s ratio controls the evolu tion of the maximum stress with time the viscosity and young s modulus define the characteristic time of the maxwell model and thus the time to reach the maximum principal stress

palaeoredox conditions and sequence stratigraphy of the cretaceous storm dominated mixed siliciclastic carbonate ramp in the eastern cordillera basin colombia evidence from sedimentary geochemical proxies and facies analysis abstract the cretaceous black shales of colombia are among the most important successions in the north of south america and have attracted the attention of many geoscientists and exploration companies over the last few decades because of their high hydrocarbon potential and the presence of emerald deposits however many uncertainties still remain with regard to their tectonic setting sequence stratigraphy depositional environments palaeoxygenation conditions and organic matter preservation in order to develop a more integrated picture of these different processes and conditions we conducted a detailed sedimentological inorganic geochemical u v ni zn mn fe ti mo cu cr cd ba and sequence stratigraphic analysis of the cretaceous black shales in the magdalena tablazo sub basin eastern cordillera basin of colombia eleven lithofacies and five lithofacies associations of a storm dominated siliciclastic carbonate ramp were identified which range from basin to shallow inner ramp environments these facies were grouped into six third order stratigraphic sequences showing high order cycles of marine transgression with constrained regressive pulses during the late valanginian to early coniacian the black shales succession represents deposition under anoxic bottom water with some intervals representing dysoxic suboxic conditions the evolution of the sedimentary environments and their palaeoxygenation history reflect tectonic and eustatic sea level controls that 1 produced a variable orientation and position of the coastline throughout the cretaceous 2 conditioned the low gradient ramp geometry

the formation of an orocline by multiphase deformation a paleomagnetic investigation of the cantabria asturias arc northern spain the variscan orogeny in the cantabria asturias arc caa of northern spain represents a multiphase deformation history associated with pangea s amalgamation to determine the timing and kinematic history of deformation a paleomagnetic investigation was carried out on devonian limestones in three structural domains of the caa s hinge zone two characteristic remagnetizations in the caa are distinguished an early permian b component that postdates initial westphalian stephanian aged folding f1 and f2 and a late carboniferous c component that postdates f1 deformation both the b and c magnetizations experienced a later f3 folding phase the kinematics and geometry of post magnetization deformation are determined by bringing measured magnetic directions into agreement with reference directions these structural corrections also allow separation of deformation events in time we conclude that generally east west trending but variably plunging fold axes characterize f3 folding in the hinge zone of the caa this post magnetization deformation involved significant amounts of tilting and sub vertical axis rotation which together produced clockwise rotation in the north of the arc and counterclockwise rotation in the south of the arc to form the horseshoe shaped orocline that is observed today f3 fold axes change from near vertical to moderately inclined between and within structural domains due to the structural fabric imposed by f1 and f2 based on calculated f3 fold properties we can reconstruct pre existing f1 and f2 structural geometries of the hinge zones this analysis shows that f1 and f2 structures are the result of late carboniferous deformation as part of the same east west but temporally discrete compression regimes on the other hand f3 is controlled by permian north south

palaeomagnetic evidence for rotations and deformation in the nogueras zone central southern pyrenees spain a multicomponent palaeomagnetic remanence from the lower triassic buntsandstein redbeds of the central southern pyrenees has been interpreted as being composed of three components a low blocking temperature recent viscous magnetization l an intermediate blocking temperature eocene remagnetization i and a high blocking temperature lower triassic remanence h these palaeomagnetic results can be used in conjunction with structural evidence to identify poles of rotation during the development of the nogueras zone antiformal stack poles from individual sites reflect local deformation and folding including vertical rotation due to shearing between adjacent thrust sheets the i component was sometimes acquired syn tectonically during thrust motion and can be used to define intermediate stages of structural development results from sites located on a single thrust sheet no 7 indicate that it rotated by 45 clockwise during emplacement this is reflected in an easterly increase in displacement estimated from the construction of balanced cross sections thrust sheets appear to have been pinned close to the present day location of pont de suert during the later stages of structural development the arcuate path of thrust sheet motions and the potential movements in and out of cross section can lead to over and under estimates of minimum displacement when using line length cross section balancing techniques

a complete terrestrial oligocene magnetobiostratigraphy from the ebro basin spain abstract the integrated litho bio and magnetostratigraphic cross correlation of 11 continuous and correlatable sections in the southeastern ebro basin ne spain produced a complete oligocene magnetic polarity stratigraphy ranging from latest eocene to earliest miocene the magnetic polarity zonation of the different alluvial and shallow lacustrine sections has been correlated to the gpts using fossil mammals and the distinct sequence and pattern of polarity reversals the composite magnetic polarity sequence defines a continuous sequence from chron c15n to chron 6cn 2n from 34 8 to 23 7 ma late eocene early miocene in addition a few short normal and reverse polarity events that correlate to cryptochrons in the gpts are found i e in c11n 1r and c8n 1r the resulting regional magneto biostratigraphic framework based on fossil mammals is proposed to refine the dating of the european oligocene biochronological mammal paleogene units and correlate them with the standard oligocene marine stages and the north american land mammal ages the magnetochronology now established enables more precise dating of the late eocene and oligocene paleofloristic and paleofaunistic changes in southwestern europe which were coeval to ongoing global paleoclimatic and paleoceanographic changes on earth caused by high latitude cooling and ice sheet development in eastern antarctica

the trace metal content of recent organic carbon rich sediments implications for cretaceous black shale formation organic carbon rich sediments are enriched in several redox sensitive and stable sulfide forming trace metals tm and form an important sink in global tm cycles in today s ocean such sediments are deposited in coastal upwelling areas like those in the gulf of california off peru or off the namibian coast and euxinic basins like the black sea in an attempt to distinguish both environments by their specific tm patterns some important conclusions may be drawn 1 the tm enrichment seen in both environments broadly is rather similar 2 upwelling sediments are often enriched in cd and p and depleted in co and mn 3 sapropels have a tendency towards higher enrichments in ba and mo s re as cu ni sb and fe reflecting the strong sulfidation in an anoxic water column 4 upwelling systems cover a broad range of environmental settings where the steepness of the slope the proximity of h2s to the sediment seawater interface and the intensity of bio accumulation and regeneration play an important role for tm accumulation 5 the tm content of anoxic basins is mainly controlled by tm availability in the water column and sedimentation rate 6 enhanced bio productivity is the main switch turning an oxygenated into an anoxic environment cretaceous black shales oae 2 exhibit tm signatures of both coastal upwelling areas and sapropels the strong enrichments in cd mo tl v cu ni bi and sb demonstrate that bio accumulation and presence of h2s in the water column and associated sulfidation processes persisted during this time interval in large parts of the proto atlantic the usefulness of ba as a paleoproductivity proxy is obscured by diagenesis the extreme accumulation in ag and zn requires either an additional tm source for instance hydrothermal input or a different tm seawater composition during the cretaceous the geochemical data suggest that the major driving force for the widespread occurrence of c t black shales seems to be the increase in volcanic activity and associated co2 input throughout the cretaceous the ocean atmosphere system operated in a mode different from today during this time interval after conversion of the nutrient co2 into organic matter the system switched back into the normal operational mode

improved representation of east antarctic surface mass balance in a regional atmospheric climate model this study evaluates the impact of a recent upgrade in the physics package of the regional atmospheric climate model racmo2 on the simulated surface mass balance smb of the antarctic ice sheet the modelled smb increases in particular over the grounded ice sheet of east antarctica 44 gt a 1 with a small change in west antarctica this mainly results from an increase in precipitation which is explained by changes in the cloud microphysics including a new parameter ization for ice cloud supersaturation and changes in large scale circulation patterns which alter topographically forced precipitation the spatial changes in smb are evaluated using 3234 in situ smb observations and ice balance velocities and the temporal variability using grace satellite retrievals the in situ observations and balance velocities show a clear improvement of the spatial representation of the smb in the interior of east antarctica which has become considerably wetter no improvements are seen for west antarctica and the coastal regions a comparison of model smb temporal variability with grace satellite retrievals shows no significant change in performance

sequence stratigraphy of a tidally dominated carbonate siliciclastic ramp the tithonian early berriasian of the southern neuquen basin argentina the tithonian berriasian vaca muerta carrin cura and picun leufu formations in the southern neuquen basin were deposited on a tidally dominated mixed carbonate siliciclastic ramp basinal outer middle shallow and back ramp facies associations are recognized and a sequence stratigraphic analysis reveals that the ramp record consists of three shallowing upwards sequences ti1 ti2 and ti3 set within a lower order progradational cycle a higher order of cyclicity is superimposed on to the middle ti2 sequence the majority of the ramp facies belong to the transgressive and highstand systems tracts however at the base of ti2 a lowstand systems tract is identified characterized by a basal unconformity and an abrupt basinward shift of the shallow marine lithofacies transgressive systems tracts were characterized by slow sedimentation rates and rapid sea level rises that affected carbonate productivity highstand systems tracts show the greatest carbonate productivity and an increased progradation rate on account of a reduction in accommodation space generation palaeogeography played a major role in the development of the depositional systems partial isolation from the pacific ocean reflecting the growth of the andean magmatic arc and geographic restriction due to tectonic inversion in the central part of the basin resulted in a meso macrotidal regime that produced a tidally dominated sedimentary record in the shallow and back ramp environments coeval anoxic conditions in the central part of the neuquen basin favoured distal ramp and basinal black shale deposition during episodes of relative sea level rise

sedimentology and sequence stratigraphy of a tithonian valanginian carbonate ramp vaca muerta formation a misunderstood exceptional source rock in the southern mendoza area of the neuquen basin argentina abstract the vaca muerta formation early tithonian early valanginian is a rhythmic succession of marls and limestones cropping out in the neuquen basin west central argentina this lithostratigraphic unit was traditionally interpreted as basinal to slope deposits detailed facies analysis allows to differentiate seven facies associations representing basinal to middle ramp facies of a homoclinal ramp system prograding westward from the eastern margin and slope facies attributed to a distally steepened ramp system that progrades eastward from the andean volcanic arc in the west two sequence hierarchies are recognized five third order depositional sequences and fifteen fourth order high frequency sequences fluctuations in organic matter content within the vaca muerta formation suggest relationship with depositional sequences finding the highest values associated with transgressive system tracts this work represents an important advance in the understanding of the sedimentary and stratigraphic evolution of this exceptional unconventional reservoir our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter facies and sea level changes

bedding parallel fibrous veins beef and cone in cone worldwide occurrence and possible significance in terms of fluid overpressure hydrocarbon generation and mineralization abstract bedding parallel fibrous veins are common worldwide in sedimentary basins especially within strata of low permeability the term beef refers to bedding parallel veins of fibrous minerals where the fibres are mutually parallel and have formed quasi vertically more complex on a smaller scale are cone in cone structures yet these are also common within bedding parallel veins for both beef and cone in cone we have compiled a worldwide catalogue 157 localities typically the veins consist mainly of white gangue minerals for example calcite gypsum or quartz but may also contain accessory minerals of economic interest for example bitumen sulphides emerald pitchblende or gold fluid inclusions may contain oil or gas calcite beef 110 localities is common in organic rich shale of marine carbonate origin especially of 1 cambrian ordovician 2 devonian carboniferous 3 early jurassic or 4 cretaceous to palaeogene ages gypsum beef 30 localities is common in evaporitic or lacustrine strata of continental origin especially of triassic or neogene ages quartz beef 17 localities is common within meta turbidite sequences especially of ordovician or proterozoic ages because these modal ages seem to reflect climatic controls we infer that the fibre forming mineral species have not travelled far vertically the same conclusion holds for accessory minerals typical temperatures of formation are 1 up to 60 c for gypsum beef 2 70 c to 120 c for calcite beef and 3 200 c to 350 c for quartz beef hydrocarbon bearing calcite beef may be a good indicator of a petroleum system in which oil or gas migrate together with aqueous solutions we argue that beef and cone in cone layers result from tensile fracturing and vertical dilation coeval with fibre growth possible causes are either 1 force of crystallization or 2 seepage forces due to fluid overpressure for layers that form at depths of several km fluid overpressure is the more likely cause

conical folds an artefact of using simple geometric shapes to describe a complex geologic structure abstract reliance on diagrams and tangent plots to describe fold geometry has created an unwarranted status for conical folds at the expense of periclines analysis of virtual periclines using synthetic stereograms tangent diagrams and geologic curvature analysis demonstrates small portions of periclines are cylindrical the more significant non cylindrical portion of periclines allows for a mathematically permissible but unrealistic geometrical representation as portions of cones we demonstrate cones are an extremely poor geometric representation of both the virtual periclines and examples of natural non cylindrical folds conical folds if they exist terminate at a point the amplitude to width ratio and the plunge of the crestal line must remain constant towards the fold terminus with a gaussian curvature of zero in contrast the amplitude to width ratio plunge along the crestal line and gaussian curvature of periclines vary towards the fold terminus these differences have important implications for the rheological modeling of folds and while realistic dynamic models for periclines exist models for conical fold formation remain conceptual we suggest that in order to advance our understanding of how folds form it may be pointless to continue the misconception of conical folds as an accurate geometric representation of how folds end

new model for evolution of fold and thrust belt curvature based on integrated structural and paleomagnetic results from the pennsylvania salient in a number of curved fold and thrust belts worldwide such as the cantabrian arc and the wyoming idaho salient paleomagnetic data indicate vertical axis rotations inconsistent with structural findings this apparent conflict is especially pronounced in the pennsylvania salient where the paleomagnetically defined vertical axis rotations are opposite of the rotation sense indicated by structural studies we resolve this apparent disparity by developing an integrated and kinematically admissible model that has implications for other curved mountain belts the curvature of the pennsylvania salient may be explained by deformation partitioning in an initially laterally tapered basin between the cambrian ordovician carbonates and the overlying siliciclastic rocks earliest deformation occurred in the lowest strata and progressed toward the foreland and up section with time lateral differences in initial layer parallel shortening in the cambrian ordovician carbonates caused differential translation and rotations about a vertical axis in the cover sequence because of initial basin geometry the orogenic wedge developed a lateral taper with further shortening in the wedge and involvement of the cover rocks in the fold and thrust deformation gravitational driving forces became more important and the paleostress trajectories diverged in opposite directions on the two arms of the salient in response to the lateral taper

proximal distal facies relationships and sedimentary processes in a storm dominated carbonate ramp kimmeridgian northwest of the iberian ranges spain abstract facies analysis of the upper kimmeridgian rocks in the outcrops located near ricla zaragoza province northeast spain and the integration of the resultant data in a broader context the northern part of the iberian basin has produced two general models showing the facies distribution and the processes that controlled the sedimentation in the kimmeridgian carbonate ramp using these two models the transition from shallow to relatively deep environments of the carbonate ramp is examined in detail model 1 corresponds to the development of a mixed carbonate siliciclastic ramp during a slow rise and stillstand of sea level sequence 1 hst whereas model 2 represents the growth of a pure carbonate ramp during a rapid rise of sea level sequence 2 tst carbonate production was higher in the shallow ramp domains coral reefs and oolitic shoals in model 1 and reefs in model 2 than in deeper domains where there is no indication of significant pelagic or benthic production the activity of unidirectional return flows induced by winter storms and hurricanes played an important role in the redistribution of the sediment across the ramp generating different coarse grained deposits in the inner and mid ramp settings dunes lower scale bedforms and tempestites occur in model 1 and storm lobes bars and tempestites in model 2 moreover a significant bulk of the carbonate mud produced in shallow areas would eventually be resedimented in the outer ramp as suspended load in the density currents stillstand of sea level in model 1 involved a rapid progradation of the inner and proximal mid ramp carbonate and siliciclastic facies the rapid relative sea level rise of model 2 is determined by the dominance of the carbonate facies and by the presence of aggradational geometries in the transitional area between shallow and deep ramp domains the presence of relatively thick sections in the outer ramp settings instead of condensed sections as observed in model 1 during times of sea level rise model 2 can mainly be explained by the increase of the shallow production in the reef dominated areas

vertical axis rotations in fold and thrust belts comparison of ams and paleomagnetic data in the western external sierras southern pyrenees abstract geometry and kinematics of fold and thrust belts can be complex settings when oblique structures and vertical axis rotations var take place during thrust emplacement many techniques can be used to unravel such complex tectonic histories from classic ones like strain analysis and changes of paleocurrents to more modern approaches such as paleomagnetism and calcite twin analysis in this paper anisotropy of magnetic susceptibility is compared to vertical axis rotations deduced from paleomagnetic data along three cross sections in the external sierras and the jaca basin southwestern pyrenees these data enable us to state that 1 ams represents pre tilting deformation fabrics and magnetic lineation behaves like a passive marker during later deformation and 2 a primary record of the paleomagnetic field can be found in the area therefore the difference with the paleomagnetic reference is the local var the comparison of the two datasets allows us to draw the following conclusions a the changes in both paleomagnetic var and magnetic lineation orientation between neighbouring sites are similar and b there is a significant linear regression between var and trend changes in magnetic lineation deviations in magnetic lineation with respect to the expected local magnetic lineation permit vertical axis rotations to be quantified using the ams dataset the combined analysis of paleomagnetic and ams data enables 1 a quicker comparison of the kinematic evolution in areas with similar ams patterns 2 the identification of anomalous orientations of magnetic lineation linked to vars and 3 the use of ams analysis as a homogeneity test for vars determined from other techniques ams is proposed as a suitable preliminary technique to select further sites for paleomagnetic analysis since paleomagnetic laboratory procedures are much more time consuming than ams measurements ams does not usually modify sample remanence and hence samples can be used for both types of analysis

the vaca muerta quintuco system tithonian valanginian in the neuquen basin argentina a view from the outcrops in the chos malal fold and thrust belt the vaca muerta quintuco system uppermost lower tithonian lower valanginian is a thick shallowing upward sedimentary cycle consisting of dark bituminous shales marlstones limestones and sandstones cropping out in the neuquen basin west central argentina this paper analyzes three outcrop sections in chos malal area northem neuquen province detailed facies analysis allows us to differentiate six facies associations representing basinal to proximal outer ramp facies of a homoclinal carbonate ramp system vaca muerta formation and basinal to shoreface facies of a mixed carbonate siliciclastic shelf system quintuco formation prograding westward from the eastern margin of the basin two sequence hierarchies were recognized 5 composite depositional sequences third order and 15 high frequency sequences fourth order fluctuations in organic matter content within the vaca muerta formation suggest a relationship with depositional sequences finding the highest values associated with transgressive systems tract whereas the transition to the quintuco formation shows a strong decrease in total organic carbon the x ray diffraction studies show an increase of clay minerals and quartz in the transgressive systems tract of the vaca muerta formation and an increase in the content of calcite in highstand systems tracts this pattern is reversed in the quintuco formation our sequence stratigraphic approach contributes to the understanding of the relationship between organic matter clay minerals facies stacking pattern and relative sea level changes in this exceptional shale oil and shale gas unconventional reservoir this study may be helpful for a better postulate of petrophysical and geomechanical models for unconventional exploration

conical folds and apparent rotations in paleomagnetism a case study in the southern pyrenees abstract the so called apparent rotation was defined as the angular deviation between a local paleomagnetic direction after the standard bedding correction and their corresponding paleomagnetic reference j geophys res 85 1980 3659 in this paper we make a theoretical exploration on this concept and we conclude that depending on the number sequence orientation and magnitude of the deformation axes that have affected to the rock volume the apparent rotation may be the addition of a vertical axis rotation plus a spurious rotation the later is an error whose origin is the inappropriate application of the bedding correction during the restoration which does not fit the reverse sequence of deformations then apparent rot s l spurious rot vertical axis rot conical folds are complex geometries that cannot be restored by using the bedding correction however appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts in this paper we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection the modelling has to be based on a good characterization of the geometry fold axis orientation and understanding of the kinematics of the fold general modelling has also allowed us to predict the apparent rotation in conical synclines its magnitude depends on the semiapical angle and on the degree of development of the fold the sense of the rotation clockwise or counter clockwise will depend on the sense of rotation the fold axis the western external sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall ebro foreland basin of the south pyrenean sole thrust in addition a vertical axis clockwise rotation up to 47 32 in average has been detected in the hagingwall an apparent rotation up to 28 20 in average is observed in the footwall of the structure when a simple bedding correction is used this deviation does not fit with the expected ebro basin direction reference and is caused by the effect of the riglos conical syncline developed by the flexure of the foot wall ramp of the south pyrenean sole thrust the forward modelling carried out considering the geometry and kinematics non significant rotations in the autochthonous foot wall of this structure predicts very well the paleomagnetic observations in the field geographic coordinates with angular departures of only 5 in average the only application of the bedding correction would introduce errors spurious rotations up to 21 12 in average related to the conical geometry that would not allow the differentiation of these distinct structural units

asiras airborne radar resolves internal annual layers in the dry snow zone of greenland 1 we present data from the european space agency s airborne sar interferometric radar altimeter system asiras flown during the cryovex 2004 field calibration validation campaign and new high resolution depth profiles of snow density measured in the field by neutron scattering we combine these data to calculate the depth of internal reflecting horizons in the asiras data the high resolution density data allow us to identify annual layers in the snow density profile and correlate their peaks with the reflecting horizons we use the thickness of the annual layers combined with the density profile to determine the spatial and temporal pattern of snow accumulation along the radar track for a period of 6 years from 1995 2002 our mean annual accumulation rate is 0 47 0 09 ma 1 water equivalent in agreement with the value determined from a core taken in this location in 1992 similarly our inter annual variability shows the same trends as recent model estimates over the entire ice sheet because asiras was designed to mimic as closely as possible the characteristics of the sar interferometric radar altimeter siral the principal payload of cryosat the detection of internal layering with asiras illuminates the possibility of detecting internal layers with a space borne radar and thus the possible application of this technique to the dry snow zones of antarctica greenland and smaller ice bodies

apparent tectonic rotations declination anomaly equations and declination anomaly charts the conventional method for tilt correction of paleomagnetic data assumes that rock bodies rotate about horizontal axes during deformations apparent tectonic rotations which may produce anomalous declination readings of paleomagnetic directions and other linear elements can result from the use of such a procedure if the rock bodies deform about inclined axes the error declination anomaly resulting from two apparent tectonic rotations 1 single rotations about an oblique axis and 2 continuous rotations about a precessing fold axis can be determined algebraically for given orientations of rotation axis and angles of rotation the declination anomaly does not depend on the orientation of the paleomagnetic vector or the trend of the rotation axis rather it is a function of the plunge of the rotation axis and the angle of rotation a declination anomaly chart dac is constructed for each of the two processes to determine graphically whether anomalous paleomagnetic data obtained from a study area can result from the process in the dacs the plunge of the bedding pole vector and the declination anomaly resulting from the apparent tectonic rotations are plotted against the plunge of the rotation axis and the angle of rotation the declination anomaly for an individual site is taken as the deviation of the measured remanent magnetic direction at the site from the reference direction for coeval rocks each data point is plotted at the intersection of the declination anomaly curve and the beddingpole curve in the dacs if the dispersion in remanent magnetic directions over a study area results from one of the apparent tectonic rotations the data points will lie along a straight line in the corresponding dac potential applications of the dacs include testing the nature of anomalous paleomagnetic directions determining the deformation style of folds and restoring structural orientations of tilted blocks

conical folding in the core of an orocline a geometric analysis from the cantabrian arc variscan belt of nw iberia abstract the cantabrian arc of the variscan belt has been recently defined as a true orocline constraining kinematics and deformation timing this curved sector of the orogenic belt is characterized by two different fold sets 1 one runs parallel to the outcrops of the main thrusts and describes a horseshoe shape concave towards the east and 2 another is radial to the arc a detailed geometric study of the fold interference patterns in the cantabrian arc revealed the conical nature of the folds belonging to the radial set these conical folds developed with different geometrical characteristics semi apical angles and axis attitudes depending on the initial orientation and geometry of the folded surfaces they are interpreted to result from a vertical axis rotation during oroclinal buckling of the variscan belt in nw iberia this study of conical folds in the cantabrian arc highlights that conical folds in curved orogenic arcs are a powerful tool for establishing the sequence of tectonic events because interference patterns due to vertical axis secondary differential rotations provide unique geometrical characteristics observed in the cantabrian arc that can be extrapolated to other oroclines

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

albian syndepositional block rotation and its geological consequences basque cantabrian basin western pyrenees stratigraphic structural palaeocurrent and palaeomagnetic analyses of upper albian deep water deposits in and around the deba block northern iberia are presented results indicate an anticlockwise vertical axis rotation of this block by 35 during a maximum time span of c 1 ma late albian intra c auritus ammonite subzone this albian syndepositional block rotation is interpreted to be the consequence of the coeval activity of conjugate major sinistral strike slip faults and minor antithetic dextral strike slip faults which border the deba block on the base of conservative estimations a minimum block rotation rate of 35 ma 1 and a sinistral strike slip rate of 1 2 km ma 1 are calculated as a consequence of the interaction of the rotated deba block with adjacent non rotated blocks its corners experienced coeval transpressive nw and se corners and transtensional deformations sw and possibly ne corners at the transtensional sw corner two domal high reflective seismic structures have been recorded and interpreted as high level magmatic laccoliths these magmatic intrusions triggered the development of a mineralizing hydrothermal system which vented to the late albian seafloor warm to hot hydrocarbon rich fluids vented hydrocarbon was generated from albian organic rich sediments by contact alteration with hydrothermal fluids

cretaceous black shales of venezuelan andes preliminary results on stratigraphy and paleoenvironmental interpretations abstract this paper deals mainly with the late cretaceous la luna formation on the venezuelan andean platform the studied area the so called transversale de barquisimeto is located at the contact zone between the andes of merida and allochthonous slabs thrusted from the caribbean domain and is crosscut by a major active transform fault the bocono fault which is part of the contact zone between the caribbean and south american plates the la luna formation is made of dark organic rich laminated limestones and shales and though always appearing with black shale facies is different in the areas located in the north or in the south of the bocono fault we provide here some new information about the biostratigraphic frame of the la luna formation and of the overlying colon formation in the studied area the depositional environment of the la luna formation was certainly rather shallow near storm wave base and stratified during deposition of la luna formation dysaerobic to anoxic conditions prevailed which may be linked with a regional anoxic event perhaps related to seasonal upwelling along the northern coast of south america during late cretaceous time

basin development and tectonic history of the llanos basin eastern cordillera and middle magdalena valley colombia the middle magdalena valley eastern cordillera and llanos basin constituted a major regional sedimentary basin from the triassic to the middle miocene basin development began during the triassic to the earliest cretaceous with a synrift megasequence related to the separation of north and south america in the proto caribbean the synrift megasequence began with deposition in a continental environment that became paralic and shallow marine in the early cretaceous basin development continued into the cretaceous in a back arc setting east of the andean subduction zone the back arc megasequence was dominated by shallow marine sedimentation and produced an excellent regional source rock during the turonian coniacian marine deposition was abruptly terminated during the ear y maastrichtian due to the final accretion of the western cordillera accretion of the western cordillera created the early pre andean foreland basin megasequence of late maastrichtian to early eocene age this depositional episode consists of coal rich alluvial plain coastal plain and estuarine deposits throughout the middle magdalena valley eastern cordillera and eastern llanos basin the megasequence was terminated by middle eocene deformation in the magdalena valley which ended sediment deposition throughout colombia loading effects of this deformation reestablished the basin in which the late pre andean foreland basin megasequence was deposited until the early miocene this megasequence also consists of alluvial plain coastal plain and estuarine deposits including the primary reservoir in the llanos foothills the upper eocene mirador fo mation the megasequence also includes a series of four major grossly coarsening upward cycles in the llanos basin these cycles correspond to changes in sea level sediment supply and foreland basin loading the mudstone in the lowermost of these cycles is the regional seal in the llanos basin and foothills the middle miocene onset of andean deformation in the eastern cordillera isolated the middle magdalena valley from the llanos basin the deformation was dominated by inversion of the basin controlling faults the resultant loading of the lithosphere created the accommodation space for the andean foreland basin megasequence a major transgression into the llanos basin coeval with this deformation caused deposition of marine mudstones in the lower part of the megasequence however the majority of the andean foreland basin megasequence consists of the guayabo formation a classic molasse sequence deposited in a high energy coarse grained bed load dominated fluvial system that was supplied by the developing mountains of the eastern cordillera

carbonate cementation in a sequence stratigraphic framework upper cretaceous sandstones book cliffs utah colorado abstract three macroscopic diagenetic features can be recognized in the sandstones of the upper cretaceous desert member of the blackhawk formation and castlegate sandstone of the mesaverde group exposed in the book cliffs utah each of which have distinctive form geometry and stratigraphic distribution diagenetic alterations are 1 leached zones whitecaps up to 10 m thick beneath coal beds 2 large up to 8 m concretionary carbonate cemented bodies in amalgamated shoreface and thin fluvial sandstones and 3 thin up to 2 m laterally extensive carbonate cemented horizons beneath major marine flooding surfaces each feature has distinct petrographic and geochemical signatures and formed through discrete diagenetic processes large isolated carbonate cemented bodies are composed of ferroan dolomite most of which precipitated during early diagenesis field and petrographic data coupled with stable isotope data early cements 13c 2 5 to 3 4o oo vpdb 18o 7 8 to 12 0o oo vpdb 87sr 86sr 0 7078 later cements 13c 3 1 to 5 7o oo vpdb 18o 12 0 to 15 1o oo vpdb 87sr 86sr 0 7093 suggest precipitation from meteoric fluids input into sediments during times of relative sea level fall the source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events relative sea level rise cement precipitation in these horizons continued through sediment burial as a result of organic matter oxidation reactions in overlying organic rich mudstones the results of this study show a link between sedimentation related to changes in relative sea level and diagenesis leading to the potential for the development of process based predictive models of early diagenesis in depositional successions

remagnetizations as a tool to analyze the tectonic history of inverted sedimentary basins a case study from the basque cantabrian basin north spain 1 in this work we use syntectonic remagnetization data to determine the synrift geometry of inverted basins this methodology analyzes remagnetization directions in order to obtain the tilting of beds at the moment of the acquisition of the overprint filtering the subsequent compressional deformation we collected samples from jurassic and lower cretaceous limestones and red beds cabuerniga and pas groups in the cabuerniga basin one of the most important mesozoic basins developed near the iberia europe mesozoic plate boundary in northern spain that later inverted during the uplift of the pyrenees these deposits represent the synrift sequence linked to a widespread extensional stage in the iberian basins during the mesozoic all sampled rocks show a normal polarity remagnetization fold and conglomerate tests indicate that remagnetization postdates the main extensional stage of basin formation late jurassic barremian and was acquired before the subsequent compressional deformation the analysis of these overprinted directions allows the basin geometry of the studied area to be restored at the moment of acquisition of the remagnetization this approach represents a powerful tool to study basinal evolution and uplift of sedimentary basins to be tested in other inverted basins with syn extensional related secondary overprints

petrography and environmental controls on the formation of phanerozoic marine carbonate hardgrounds early marine seafloor lithification of carbonate sediments leads to the formation of hardgrounds and is known from rocks as old as the proterozoic hardground surfaces however are more commonly recorded in the phanerozoic sedimentological archive while ecological studies of hardground biota abound the environmental and physico chemical parameters leading to the development of seafloor lithification remain in many cases poorly understood and documented this paper reviews published evidence on the petrography mineralogy and geochemistry of phanerozoic carbonate hardgrounds within a process oriented context of their environmental controls hardgrounds typically develop in warm and shallow agitated tropical waters that are saturated with respect to caco3 but are also reported from hemi pelagic to bathyal realms and from cool water temperate settings a range of early marine cement types are documented from present day hardgrounds whereas preserved cement fabrics and related mineralogies are less diverse in early phanerozoic hardgrounds carbonate hardgrounds are widespread during calcite sea periods e g ordovician and cretaceous as opposed to some preservation bias ancient aragonite seas e g permian and triassic obviously the relation between seawater chemistry and hardground abundance and attributes is not an easy one here the concept of aragonite versus calcite seas serves as a first order template for the sake of a structured discussion whereas modern aragonite mode oceans document a significant diversity in spatial and bathymetric seawater properties this spatio temporal complexity is perhaps reflected in the scarce record of calcite sea hardgrounds from the devonian or paleogene rock record contrasted by the abundance in aragonite holocene and recent seas holocene hardgrounds allow for the direct assessment of rates of and reasons for early subaquatic lithification and in most cases escaped subsequent non marine diagenetic overprint conversely studies of ancient hardgrounds are often biased by early biological and mechanical erosion the degree of diagenetic overprint and tectonic to orogenic processes the formation distribution and physical properties of hardgrounds depend on the global climatic context greenhouse icehouse modes on sea level changes on ocean stratification on the spatial extent of epicontinental seas and carbonate depositional environments and on the spatially different mineralogies and rates of carbonate production the fabrics and mineralogies of early hardgrounds cement seem to substantially hinge on variations in atmospheric co2 thus the application of lessons from modern to ancient hardground case examples is not straightforward a holistic model explaining the full variability of controls on submarine hardground formation and their cement petrography throughout the phanerozoic is as yet lacking a more critical view on secular changes in hardground formation patterns is clearly needed future work should rely on the combination of sedimentological stratigraphic palaeoecological petrographic and geochemical approaches specific emphasis should be given to time intervals triassic carboniferous cambrian etc where the bulk of reported case studies is at best scarce

geochemistry palaeoenvironments and timing of aptian organic rich beds of the paja formation curiti eastern cordillera colombia at the curiti quarry eastern cordillera of colombia a 23 35 m section includes a 6 3 m interval of organic rich marlstones calcareous clay shales and clay shales devoid of benthic fossils and bioturbation at the base of the paja formation it overlies carbonate ramp deposits of the rosablanca formation with the topmost layer containing reworked and phosphatized assemblages of middle barremian to lowest aptian ammonites of the genera pulchellia gerhardtia toxancycloceras karsteniceras and prodeshayesites high resolution analyses of the section including total inorganic carbon tic 0 09 50 wt total organic carbon toc up to 8 4 and stable carbon isotope measurements 13 c org between 22 05 and 20 47 allow the determination of the stratigraphic relationship between the organic rich level within the paja formation and oceanic anoxic event 1a oae 1a the range of values and the overall pattern of the 13 c org curve are comparable with the lower aptian interval c7 thus the organic rich shale interval of the paja formation at the curiti section showing characteristics of oxygen depleted conditions was deposited after oae 1a which is known to occur between isotopic levels c3 and c6

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

basin infill architecture and evolution from magnetostratigraphic cross basin correlations in the southeastern pyrenean foreland basin magnetostratigraphy numerical dating and facies mapping have been combined to provide the first cross basin correlation of the eocene southeastern pyrenean foreland basin in northeastern spain this has enabled 1 depositional systems on the northern and southern margins of the basin to be dated and correlated across the basin and 2 changes in the rates of sediment accumulation in time and space to be evaluated by sampling correlative intervals at several locations in a transect across the basin magnetozones were detected that otherwise may have been misinterpreted due to recent magnetic overprinting or missed by wide sampling intervals in previous magnetostratigraphic surveys results of this study indicate that marginal marine strata of the southeastern part of the foreland near the vic area of spain are 5 m y older than previously thought the base of the interval 46 ma being middle lutetian in age by proposing an age of at least 35 68 ma for the overlying cardona evaporite unit the age span of the marine interval in the studied part of the foreland is 10 m y contrary to earlier interpretations no major southward migration of the southeastern pyrenean foreland basin depocenter is recorded by the basin stratigraphy during the initial stages of middle eocene marine sedimentation in the southern margin of the basin however marked basin asymmetry is observed later in its evolution as the top of the marine deposits records northward increase in the thickness of chron c17n 1n 37 47 36 61 ma this asymmetry is also proved by the presence of a larger number of magnetozones within marine strata at the top of the marine interval in the northern sector these results imply that previous age calibrations and the estimated rates of tectonic processes and sedimentation within the foreland need to be reassessed the implications for basin infill architecture evolution through time and space subsidence analysis and depocenter evolution through time are addressed

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

vertical axis rotation associated with folding and thrusting an example from the eastern subbetic zone of southern spain the differential rotation about a vertical axis across a southeast vergent recumbent fold and thrust fault within a single nappe from the eastern subbetic zone of southern spain has been determined using paleomagnetism the upper limb has rotated clockwise by 68 22 relative to the lower limb mapping of the structure indicates that the syncline is an isoclinal fold in the northeast and an open fold in the southwest the differential vertical axis rotation across this structure appears to have been accommodated by a pro gressive increase in along strike horizontal shortening reaching 8 5 km at 3 7 km from the pivot point this differential shortening and the rotation are probably the result of increased drag on the underlying thrust close to the pivot point compared to the farthest extent of the structure the lateral boundary of the rotating sheet is a sinistral strike slip fault which is consistent with the rotational geometry pinning is probably enhanced by lateral heterogeneities within the stratigraphy this result ties rotational deformation to early middle miocene folding and thrusting rather than to late miocene holocene strike slip deformation rotation by pinning of thrust sheets may prove to be an important aspect of deformation in many fold and thrust belts

palaeomagnetic analysis of plunging fold structures errors and a simple fold test abstract the conventional corrections for bedding dip in palaeomagnetic studies involve either untilting about strike or about some inclined axis the choice is usually governed by the perceived fold hinge orientation while it has been recognised that untilting bedding about strike can be erroneous if the beds lie within plunging fold structures there are several types of fold which have plunging hinges but whose limbs have rotated about horizontal axes examples are interference structures and forced folds restoration about inclined axes may be incorrect in these cases the angular errors imposed upon palaeomagnetic lineation data via the wrong choice of rotation axis during unfolding are calculated here and presented for lineations in any orientation which could be associated with an upright symmetrical fold this extends to palaeomagnetic data previous analyses which were relevant to bedding parallel lineations this numerical analysis highlights the influence of various parameters which describe fold geometry and relative lineation orientation upon the angular error imparted to lineation data by the wrong unfolding method the effect of each parameter is described and the interaction of the parameters in producing the final error is discussed structural and palaeomagnetic data are cited from two field examples of fold structures which illustrate the alternative kinematic histories both are from thin skinned thrust belts but the data show that one is a true plunging fold formed by rotation about its inclined hinge whereas the other is an interference structure produced by rotation of the limbs about non parallel horizontal axes since the angle between the palaeomagnetic lineations and the inclined fold hinge is equal on both limbs in the former type of structure but varies from limb to limb in the latter a simple test can be defined which uses palaeomagnetic lineation data to identify rotation axes and hence fold type this test can use pre or syn folding lineations and could be useful in areas of non coaxial folding

subglacial melt channels and fracture in the floating part of pine island glacier antarctica 1 a dense grid of ice penetrating radar sections acquired over pine island glacier west antarctica has revealed a network of sinuous subglacial channels typically 500 m to 3 km wide and up to 200 m high in the ice shelf base these subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf above the apex of most channels the radar shows isolated reflections from within the ice shelf comparison of the radar data with acoustic data obtained using an autonomous submersible confirms that these echoes arise from open basal crevasses 50 100 m wide aligned with the subglacial channels and penetrating up to 1 3 of the ice thickness analogous sets of surface crevasses appear on the ridges between the basal channels we suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and if the channels were formed sufficiently rapidly would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets we conclude that ice shelf basal melting plays a role in determining patterns of surface and basal crevassing increased delivery of warm ocean water into the sub ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf perhaps as a prelude to eventual collapse

paleomagnetic study of thrust sheet motion along the rocky mountain front in montana near wolf creek montana the fold and thrust belt intersects the central montana trough at the northern edge of the helena salient remanent magnetizations from 18 of 31 paleomagnetic sites in the late cretaceous two medicine formation 79 ma document counterclockwise rotations of thrust bound rocks along the margin of the salient results from 12 reliable sites in the upper of two thrust sheets gives a mean direction of dec 328 4 inc 67 9 95 5 63 results from 6 reliable sites in the underlying thrust sheet yield dec 321 9 inc 69 3 95 11 87 the difference in these directions is not significant at the 95 confidence level the thrust sheets show no relative rotation combining all 18 sites gives an average direction of dec 326 3 inc 68 4 95 4 94 this average direction is 25 2 14 0 counterclockwise from a late cretaceous reference direction dec 351 5 inc 69 3 established from the adjacent adel mountain volcanic rocks sim 76 ma comparison to an older 88 ma pole established from the niobrara formation indicates 11 7 11 4 of counterclockwise rotation the paleomagnetic results with other geologic evidence show that both thrust sheets interacted with an oblique lateral foreland ramp

quantifying vertical axis rotation in curved orogens correlating multiple data sets with a refined weighted least squares strike test 1 map scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems paleomagnetic analysis combined with detailed structural studies is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales this paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test this refined method provides measures of best fit slope confidence interval and goodness of fit between map scale structural trend paleomagnetic rotations and deformation fabric orientations structural trend is estimated by averaging fold axial trace formation contact and bed strike data over kilometer scale areas paleomagnetic and deformation fabric site mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics weighting factors are estimated from combined measurement uncertainty structural noise related to small scale block rotation and stress strain refraction and variations in restoration paths the number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites evaluation of appropriate weighting factors and statistical analysis a case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the wyoming salient of the sevier thrust belt paleomagnetic data yield a strike test slope of 0 76 0 11 indicating that the wyoming salient is a progressive arc with 3 4 secondary curvature related to vertical axis rotation synchronous with large scale thrusting and 1 4 initial curvature finite strain anisotropy of magnetic susceptibility and mesoscopic structural orientations which are related to early layer parallel shortening all yield strike test slopes of 0 9 0 1 comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations by integrating systematic paleomagnetic and structural data using statistical analysis curvature models for the wyoming salient are closely constrained

modeling hydraulic fracture of glaciers using continuum damage mechanics the presence of water filled crevasses is known to increase the penetration depth of crevasses and this has been hypothesized to play an important role controlling iceberg calving rate here we develop a continuum damage based poro mechanics formulation that enables the simulation of water filled basal and surface crevasse propagation the formulation incorporates a scalar isotropic damage variable into a maxwell type viscoelastic constitutive model for glacial ice and the effect of the water pressure on fracture propagation using the concept of effective solid stress we illustrate the model by simulating quasi static hydrofracture in idealized rectangular slabs of ice in contact with the ocean our results indicate that water filled basal crevasses only propagate when the water pressure is sufficiently large and that the interaction between simultaneously propagating water filled surface and basal crevasses can have a mutually positive influence leading to deeper crevasse propagation which can critically affect glacial stability therefore this study supports the hypothesis that hydraulic fracture is a plausible mechanism for the accelerated breakdown of glaciers

rotation velocity of a thrust a paleomagnetic study in the external sierras southern pyrenees abstract an accurate calibration of chronologic and geologic data biostratigraphic and magnetostratigraphic with gpts has been performed for the middle and upper eocene syntectonic deposits in the surrounding areas of the pico del aguila anticline south pyrenean external sierras this calibration shows a variable sedimentation rate that changes from 4 8 cm ka at the bottom to 34 cm ka at the top with a maximum of 58 6 cm ka in the upper part of the studied stratigraphic section and gives a fine and continuous chronologic frame to understand the rotational kinematics of the infrajacent thrust system at the same time a detailed paleomagnetic study has been carried out 14 sites and a total of 157 thermally demagnetised samples to know the amount of rotation in the area taking advantage the tape recording effect of the syntectonic materials these samples display a chrm between 250 and 400 c both polarities that agree very well with the sequence of magnetostratigraphic zones and consistency with the magnetic reference due to the decreasing value of the magnetic declination from 46 to 3 of the primary components checked by fold and reversal test the calibration has been useful to assign an absolute age to every rotation value from 40 32 to 37 05 ma a time versus rotation curve does not show significant differences of rotation between the flanks of the pico del aguila anticline less than 8 thus the clockwise rotations detected in the study area result from the kinematics of the imbricate basal thrust system which is also responsible for the development of detachment folds e g pico del aguila anticline in a time against rotation graph the simplest fit for the overall rotation values is given by a linear segment rotation velocity of 10 ma however the best fit is given by a segment of a parabolic curve this pattern implies that there were changes of the rotation velocities between 21 ma and 5 3 ma that in this segment of the external sierras were caused by an acceleration of the rotational movement between 9 ma2 and 5 5 ma2 the thrust sheet shows maximum values of at least 40 of clockwise rotation

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

systematic sequence scale controls on carbonate cementation in a siliciclastic sedimentary basin examples from upper cretaceous shallow marine deposits of utah and coloradao usa whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies these studies have been generally restricted to observations upon individual sequences and or limited sub surface data in this paper long term 5 million years large scale 200 km stratigraphic controls on carbonate cementation patterns are documented from the upper cretaceous panther tongue member blackhawk formation and castlegate sandstone exposed in the book cliffs in utah and colorado usa together these comprise eight progradational wedges of sandstones which interfinger with the mancos shale deposited within the western interior seaway foreland basin petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early relative to burial diagenesis within the host sandstones stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements are always present down dip of sequence boundaries and or leached whitecaps beneath coals in addition the lateral distribution of cement bodies increases consistently up succession from less than 5 km in extent in the older sequences to 30 km in extent in the youngest sequences these changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence boundary development in younger sequences the implications of these data are that whilst localized spatial patterns of diagenesis and in particular carbonate cementation are predictable and controlled by the nature and presence of individual stratal surfaces systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger scale stratigraphic development and basin evolution this evolution may be driven by eustatic shifts or through tectonic or climatic driven base level shifts these observations allow an improved insight into the basin scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub surface hydrocarbon reservoirs

identification of systems tracts in low angle carbonate ramps examples from the upper jurassic of the iberian chain spain abstract the upper jurassic of the iberian chain in northeast spain was selected for the study and identification of sequence boundaries and systems tracts in low relief carbonate ramp systems the study area provides a special opportunity for sequence stratigraphic analysis because the outcropping upper jurassic carbonate ramps are relatively undeformed well exposed from proximal to distal areas and have an established ammonite zonation two sequences are recognized sequence a uppermost callovian to uppermost oxfordian in age up to 60 m thick in the proximal areas and between 15 and 5 m thick in the middle and distal areas and sequence b kimmeridgian to lowermost tithonian from 80 to 160 m in thickness across the basin the evolution of the first sequence is related to regional relative sea level changes i e tectono eustatic changes superimposed on the sedimentary basin on the other hand the larger accommodation created during the second sequence is regarded as controlled mainly by steady subsidence sequence boundaries indicative of subaerial exposure show as unconformities with stratigraphic hiatuses in the proximal areas of the basin the lowstand systems tract in sequence a is represented by a decimetre thick reworked level consisting of several ammonite biozones in sequence b this systems tract consists of a thick wedge shaped marly unit in both sequences the transgressive surface overlaying these lowstand deposits is generally planar and marks a sharp boundary between the restricted and open marine fossils however in sequence b in the middle part of the basin the transgressive surface can be absent instead a transitional change to open marine facies exists the transgressive systems tracts show sedimentary condensation at least in the distal areas of the basin with mostly ammonites and benthic fossil wackestones the transgressive unit is generally overlain by a hard ground surface that corresponds to a maximum flooding surface finally in the upper part of both sequences highstand systems tracts are represented by progradational siliciclastic and shallow carbonate complexes in the proximal areas of the basin in sequence a sedimentation is condensed in the distal areas with glauconite marl levels present the descriptions of sedimentary features of the systems tracts of these two sequences developed in a low angle carbonate ramp are suggested to be used as tools for the recognition of systems tracts patterns within sequences in comparable depositional settings

structure of the alice anticline papua new guinea serial balanced cross sections and their restoration abstract the structure of the western part of the papuan fold belt is characterised by both thin skinned thrusting and basement involved structures basement involvement is attributed to inversion of earlier formed extensional faults the alice anticline is a frontal structure of the papuan fold belt and formed due to inversion of a tertiary extensional fault system complex forelimb geometry of the alice anticline changes markedly along strike and suggests varying amounts of shortening in this part of the structure balanced cross sections quantify differential shortening along the anticline whereas palinspastic restoration shows that the differential shortening is compatible between cross sections three dimensional restoration of the alice anticline makes use of a series of balanced cross sections and is based on a line length method paradoxically the restoration reveals non plane strain in the balanced cross sections upon which it relies however the restoration also reveals and quantifies a component of rotation about vertical axes which would not be detected by application of conventional methods of structural analysis rotations about vertical axes are attributed to pinning of progressive foreland propagating deformation the distribution of rotations about vertical axes suggest that zones of pinning are coincident with relay zones in the early extensional fault geometry two relay zones associated with the original geometry acted as obstructions to deformation and have effectively pinned contractional structures during their formation causing the rotations about vertical axes such rotations are not manifested in changing fold axis orientations and have only been derived from a consideration of material balance during deformation

magnetic reorientation induced by pressure solution a potential mechanism for orogenic scale remagnetizations abstract new paleomagnetic analyses of triassic cretaceous and eocene strata in the south central pyrenees show evidence for a widespread remagnetization located along the southern border of the axial zone the internal sierras and the northern part of the jaca pamplona basin this remagnetization always reversed in polarity was acquired after an extensive period of late eocene early oligocene folding and tilting in the area and affects limestones sandstones marls and red beds in addition a characteristic prefolding component was identified in 30 of upper cretaceous and triassic red beds these results together with a revaluation of previously published paleomagnetic data from the central pyrenees indicate that the spatial distribution of the postfolding remagnetization coincides with that of a domain of pressure solution cleavage a relationship between the intensity of the remagnetization and the characteristic prefolding component with respect to the density of cleavage surfaces leads us to propose a mechanism for the remagnetization related to the development of pressure solution cleavage that is framed within the tectonic evolution of the central pyrenees partial dissolution of rock under tectonic compression leads to the liberation and subsequent accumulation of insoluble minerals in cleavage planes magnetic grains are part of the relatively insoluble residue and they reorient in the presence of the ambient field after they are freed during dissolution of the rock matrix chemical reequilibrium dissolution and or neoformation of magnetic carriers during this process cannot be excluded the remagnetization mechanism we propose can help to explain widespread remagnetizations in low to moderately deformed rocks without the need of large scale migration of orogenic fluids

late cretaceous upwelling and anoxic sedimentation in northwestern south america abstract black shales and bituminous limestones were widely deposited in northwestern south america during the maximum transgression of late cretaceous seas these cenomanian to santonian campanian anoxic sediments are distributed in two main subprovinces thick chert and phosphorite accumulated in the trough subprovince landward of a possibly partially emergent cordillera central colombia only minor intercalations of chert were deposited in the platform subprovince lying northward in the maracaibo basin northwestern venezuela faunal evidence depauperate benthic fauna planktic foraminiferal assemblages high dominance clupeoid fish assemblages and sedimentological evidence suggest anoxic condition prevailed throughout the region probably the result of dynamic upwelling in the trough subprovince but the reflection of a global anoxic event within the platform subprovince

trace element behavior and redox facies in core shales of upper pennsylvanian kansas type cyclothems the black shale submember of core shales of pennsylvanian kansas type cyclothems is highly enriched in redox sensitive trace elements tes relative to the overlying gray shale submember as well as to many other modern and ancient organic rich deposits controls on trace element behavior in the black shale can be inferred from trace element enrichment factors efs relative to gray shale and the relationship of tes to total organic carbon toc most tes conformed to one of two patterns 1 mo u v zn and pb exhibited moderate efs and strong covariation with toc in samples having 10 wt toc and 2 cu ni cr and co exhibited low efs and moderate to strong covariation with toc in all samples these patterns are inferred to represent different responses to benthic redox conditions the first pattern is characteristic of tes of strong euxinic affinity i e those taken up in solid solution by fe sulfide or involved in other reactions catalyzed by free h2s and resident mainly in authigenic phases whereas the second pattern is characteristic of tes of weak euxinic affinity i e those not strongly influenced by the presence of free h2s and resident mainly in the organic carbon or detrital fractions of the sediment these inferences allowed development of a multiproxy technique for assessing redox facies in black shale samples euxinic conditions were considered to have existed if at least two of four redox indicator trace elements mo u v zn showed euxinic levels of enrichment and nonsulfidic anoxic conditions were inferred otherwise the validity of the procedure is indicated by 1 agreement among all four redox indicators for a large majority of samples 69 of 185 2 among samples yielding a mixed redox signal a systematic sequence of te enrichment v zn mo u and 3 for tes of weak euxinic affinity reduced variance among samples representing each redox facies sequential enrichment may be a response to differential redox thresholds for accumulation e g v or postdepositional remobilization of trace elements e g u and may provide a basis for finer assessment of redox conditions in low oxygen paleoenvironments than permitted by simple classification schemes the multiproxy procedure for redox facies analysis developed in this study is likely to be more reliable than widely used single proxy indicators based on trace elements e g mo authigenic u or v v ni or c s fe systematics e g s toc or dop d 2004 elsevier b v all rights reserved

sedimentary basins formed and carried piggyback on active thrust sheets small sedimentary basins that formed on the apennine southern margin of the po basin complex northern italy have been studied using borehole and seismic reflection information and their evolution can be dated using the largely marine biostratigraphy of the miocene to quaternary sediments these studies show that the basins formed and were filled while being carried on moving thrust sheets and they are therefore named piggyback basins each of these basins corresponds to the active phase of a different thrust front ramp on the pyrenean northern margin of the ebro basin complex northern spain similar piggyback basins formed between eocene and mid miocene time they have since been uplifted and eroded and may therefore be studied in outcrop in northern spain much of the basin fill sediment was deposited in nonmarine environments

rotational basement kinematics deduced from remagnetized cover rocks internal sierras southwestern pyrenees 1 paleomagnetic data from the internal sierras unravel the kinematic relationship between basement guarga gavarnie bielsa and cover thrusts larra monte perdido the new data come from an area of 100 km along strike and 5 15 km across strike of the southwestern pyrenees the area covers a variety of structural positions that range from cover to basement thrusts from 84 sites located in upper cretaceous two meaningful magnetic components are found 1 the higher temperature c component unblocks at 575 c presents two polarities is prefolding has a mean declination and inclination d i of 010 44 95 8 k 15 and is considered primary and 2 the intermediate b component unblocks between 250 c and 450 c has a mean d i of 198 43 95 4 k 18 and has always reverse polarity and postfolding the nine sites located in the eocene flysch south of the upper cretaceous show the same postfolding component b the acquisition of this intermediate component alludes to a widespread tectonic related remagnetization event affecting the internal sierras this event must be younger than the folding of the flysch deposits middle upper eocene the tilting induced by the basement thrust gavarnie bielsa in the axial zone does not alter the inclination of the b so that the sediments acquire the b component later than priabonian emplacement age of the gavarnie thrust the b component is rotated clockwise with respect to the tertiary reference this means that the rotation takes place after the remagnetization event the position of the c component before the rotation of the b component is negligibly rotated d i of 002 44 95 7 k 94 with respect to the cretaceous reference therefore the b component records the last and only rotational event in the region the rotation is related to the latest basement deformation guarga thrust during upper oligocene times and it reflects the regional shortening gradient in the southwestern pyrenees the emplacement of the larra monte perdido cover thrust system that took place prior to basement thrusts mentioned above did not undergo any significant rotation

stacking in paleomagnetism application to marine sediments with weak nrm plio pleistocene marine marls and clays from southern italy often have weak natural remanent magnetizations nrm and show poor thermal demagnetization results some random scatter appears after treatment at higher temperatures due to the low remaining intensities and promoted by the acquisition of viscous magnetizations since these problems hamper a uniform analysis of the demagnetization results a method is proposed to improve their representation and interpretation a simple stacking routine reduces the amount of scatter considerably and makes it possible to separate different nrm components in a more consistent and reliable manner in addition the construction of nrm decay curves is more straightforward from stacked diagrams and thus a more accurate information is obtained about the remanence carrying minerals the application to two sites from southern italy serves as an illustration for the method

flexural unfolding of horizons using paleomagnetic vectors this paper introduces a new restoration method pmag3drest designed for complex folded structures non cylindrical non coaxial it combines paleomagnetic vectors and bedding markers setting up a reference system that allows deformed and undeformed surfaces to be related to one another we assume flexural conditions during the deformation consequently the stratigraphic horizons are considered to be globally developable surfaces with total area preservation except in specific deformation areas using paleomagnetism in the proposed restoration process pmag3drest helps to locate these areas with greater accuracy it is similar to other approaches based on triangulations but it forces the available paleomagnetic data to converge with the paleomagnetic reference vector during the restoration process our experiments use computer and analog models in which the deformed and undeformed surfaces are perfectly known this enables us to apply the restoration method to the deformed surface and compare the parameters of the restored surface with those of the initial undeformed surface to quantify the quality of the method paleomagnetic data anchor the surface leading to more accurate results

restoring paleomagnetic data in complex superposed folding settings the boltana anticline southern pyrenees abstract complex kinematic scenarios in fold and thrust belts often produce superposed and non coaxial folding interpretation of primary linear indicators must be based on a careful restoration to the undeformed stage following the reverse order of the deformation events therefore sequential restoration to the ancient coordinate system is of key importance to obtain reliable kinematic interpretations using paleomagnetic data in this paper a new paleomagnetic study in the western flank of the boltana anticline southern pyrenees illustrates a case study of a complex tectonic setting having superposed non coaxial folds the first stage of nw se folding linked to the oblique boltana anticline took place during lutetian times the second stage was linked to the vertical axis rotation and placed the boltana anticline in its present day n s configuration our data support a long lasting lutetian to priabonian period with main rotational activity during the bartonian priabonian other authors support a var coeval with anticlinal growth the third stage resulted in southwards tilting related to the emplacement of the n120e striking guarga basement thrust oligocene early miocene based on this deformational history a sequential restoration was applied and compared with the classic bedding correction at the site scale single bedding correction gives errors ranging between 31 and 31 in the estimation of vertical axis rotations at the locality scale in sites grouped in three folds from w to e arbella planillo and san felizes the bedding corrected data display rotation values in accordance with those found in the ainsa basin by other authors sequential restoration based on the afore mentioned evolution in three steps improves both some locality means and the internal consistency of the data therefore reasonably constrained sequential restoration becomes essential to reconstruct the actual history of superposed folding areas

geochemical paleoredox indicators in devonian mississippian black shales central appalachian basin usa the degree of anoxia that existed during accumulation of devonian mississippian black shales in central kentucky has been debated widely in this study geochemical data were used to elucidate paleodepositional environments for these units using analyses from ten cores collected from the outcrop belt in central kentucky sulfur carbon and selected trace element relationships were compared the sunbury shale tournasian lower mississippian and the cleveland and huron members of the ohio new albany shale fammenian upper devonian show different relationships between carbon sulfur and trace elements c s fe relationships for the sunbury suggest anoxic possibly even euxinic conditions prevailed during sediment accumulation analysis of c s fe data suggests that the cleveland may have accumulated under anoxic conditions with intermittent dysoxia whereas the huron member may have accumulated under a relatively wide range of conditions from anoxic to dysoxic to possibly oxic these three units exhibit different degrees of trace element enrichment with the approximate order of enrichment relative to an average shale being mo pb zn v ni cu cr co the sunbury shows the highest levels of enrichment followed by the cleveland with the huron showing only slight enrichment for most of these trace elements differing degrees of enrichment may reflect differences in depositional environment during accumulation high mo concentrations in the sunbury generally 200 550 ppm may infer euxinic conditions prevailed during sediment accumulation the cleveland and the huron both show considerably lower mo contents averaging around 100 ppm geochemical ratios including ni co v cr and v v ni also indicate variable paleoredox conditions for these shales based on previously established thresholds v cr and ni co ratios infer at least anoxic conditions during accumulation of the sunbury anoxic to dysoxic conditions during cleveland accumulation and dysoxic to oxic conditions for the huron v v ni ratios tend to indicate consistently lower oxygen regimes than do other paleoredox indicators and this discrepancy is greatest for the huron member it is suggested that thresholds established for paleoredox indicators in previous studies should not be applied strictly but that relative differences in these indicators collectively can infer variations in the degree of anoxia in addition relationships between corg and redox elements may help elucidate the role of anoxia in om accumulation it may be concluded from this study that the devonian mississippian black shales of central kentucky accumulated under variable bottom water conditions at least anoxic conditions prevailed during accumulation of much of the sunbury shale and the upper part of the cleveland member and possibly euxinic conditions for the sunbury bottom water conditions may have been intermittently anoxic and dysoxic during deposition of the lower cleveland during accumulation of the huron member it is likely that conditions ranged from anoxic to dysoxic to marginally oxic possibly being close to normal marine conditions at times during accumulation of the lowermost huron sediments

escape models of the alpine carpathian pannonian region in the light of palaeomagnetic observations recent tectonic models of the alpine carpatho pannonian region alcapa assume a large eastward shift of the transdanubian range domain in the cenozoic since palaeomagnetism is one of the most powerful tools in solving geodynamic processes the authors present an approach to the escape problem by using all available and relevant palaeomagnetic data this data set demonstrates consistency with models put forward by geologists for jurassic and older ages from the mid jurassic on the northern calcareous alps nca did not share the rotations of the transdanubian range domain and of the southern alps after individual movements from neocomian to miocene the transdanubian range domain must have drifted northward in the mid miocene up to the southern margin of the northern calcareous alps before starting the escape in the geologists definition

paleogene propagation of the southern pyrenean thrust wedge revealed by finite strain analysis in frontal thrust sheets implications for mountain building abstract to better evaluate the sequence of shortening and estimate shortening rates associated with the propagation of the southern pyrenean front we propose an original approach based on the reconstruction of finite displacement and finite vertical axis rotations this study is based on the quantitative analysis of published datasets including balanced cross sections paleomagnetic and thermochronologic data we distinguish three main episodes of shortening between 41 and 37 ma 37 and 30 ma and between 30 and 16 ma they are associated with a decrease of accretionary rates from 3 km ma to 0 39 0 27 km ma on the pro wedge side of the pyrenean orogenic wedge based on the comparison between frontal accretionary flux and outcoming fluxes in the axial zone of the pyrenees we show that the period ranging from 37 to 30 ma recorded a remarkable increase in erosion that is not compensated by accretion in the wedge in the context of the overall decrease in the iberia eurasia plate convergence we suggest that the main cause of the acceleration of erosion on top of the growing pyrenean orogen is related to climate changes that occurred at the eocene oligocene boundary

complex network of channels beneath an antarctic ice shelf ice shelves play an important role in stabilizing the interior grounded ice of the large ice sheets the thinning of major ice shelves observed in recent years possibly in connection to warmer ocean waters coming into contact with the ice shelf base has focused attention on the ice ocean interface here we reveal a complex network of sub ice shelf channels under the fimbul ice shelf antarctica mapped using ground penetrating radar over a 100 km2 grid the channels are 300 500 m wide and 50 m high among the narrowest of any reported observing narrow channels beneath an ice shelf that is mainly surrounded by cold ocean waters with temperatures close to the surface freezing point shows that channelized basal melting is not restricted to rapidly melting ice shelves indicating that spatial melt patterns around antarctica are likely to vary on scales that are not yet incorporated in ice ocean models

palaeomagnetism of the early devonian lower old red sandstones of south wales implications to variscan overprinting and differential regional rotations component structures of magnetic remanence in the old red sandstones of south wales are related to tectonic history by detailed study of three contrasting areas 1 remote from 2 bordering and 3 within the variscan orogenic belt a uniform southerly shallow directed component dominates remanence in each sector in area 1 where it is considered most representative of the regional field this magnetisation has a mean di 191 1 8 0 151 samples in situ 95 2 equivalent to a pole position at 341 1 e 41 2 s fold tests show that it was acquired at an intermediate stage of orogenesis and during the carboniferous permian reversed superchron it is interpreted as a pervasive magnetic overprint imparted by northward fluid migration consequent upon loading of the orogen to the south remote from the orogenic front a higher blocking temperature dual polarity remanence of probable lower devonian age has sporadically survived di 246 38 27 samples tilt adjusted 95 7 2 corresponding to a pole position at 297 e 3 n this remanence is entirely replaced close to the frontal zone of the orogen within the orogen low blocking temperature components are more prominent and chemical alteration obscures the highest blocking temperature portion of the remanence spectrum this region was deformed into upright folds before the magnetising event following remanence acquisition continuing deformation produced asymmetry in fold structure producing a plunge and steepening of the southern limbs structures in the south were already more deformed than those in the north at the time of magnetisation implying that the causative fluid front moved northwards with time the palaeomagnetic vectors are rotated clockwise to the west in south wales and in part record the effects of regional dextral transpression the largest rotation however is observed within the same thrust block and apparent differential rotation here is probably a consequence of strain

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

paleomagnetism and magnetic fabric of the eastern cordillera of colombia evidence for oblique convergence and nonrotational reactivation of a mesozoic intracontinental rift we report the paleomagnetic and magnetic fabric results of 58 sites from cretaceous miocene marine and continental strata from the eastern cordillera ec and the cucuta zone at the junction between the santander massif and the merida andes of colombia the ec is an intracontinental doubly vergent range inverting a triassic to early cretaceous rift zone twenty three sites reveal nonsystematic tectonic rotations including unrotated areas of the ec range with respect to stable south america our data show that the ec inverted a nne oriented rift zone and that the orientation of the mesozoic rift and the mountain chain roughly correspond interestingly magnetic lineations from anisotropy of magnetic susceptibility analysis do not trend parallel to the chain but rather are oblique to the main orogenic trend by also considering gps evidence of a 1 cm yr ene displacement of central western colombia accommodated by the ec we suggest that the miocene recent deformation event of this belt arises from ene oblique convergence reactivating a nne oriented rift zone oblique shortening was likely partitioned into pure dip slip shear characterizing thick skinned frontal thrust sheets well known along both chain fronts and by range parallel right lateral strike slip faults which have not been identified yet but likely exist in the axial part of the ec finally the 35 9 clockwise rotation observed in four post miocene magnetically overprinted sites from the cucuta zone reflects late cenozoic and ongoing right lateral strike slip displacement occurring along faults parallel to the bocono fault system possibly connected with the right lateral faults inferred to exist along the axial part of the ec

layer parallelisation an unrecognised mechanism for palaeomagnetic rotations in fold belts abstract we report a new driving mechanism for vertical axis rotation in fold belts during directed folding there always are layers which are tilted oblique to the regional shortening direction these layers are geometrically incompatible with fold closure and must become accommodated towards parallelism with the other layers in the course of further shortening accommodation is achieved through vertical axis rotations of the layers towards the shortening direction a revision of palaeomagnetic data sets from fold belts shows that 1 incompatible layers occur in almost all data sets reflecting a certain degree of non cylindrical folding and 2 a parallelisation of the layers by true vertical axis rotations occurred when folding became intense layer parallelisation is a potential source of disturbance for palaeomagnetic and tectonic interpretation it can be the explanation for some of the frequent outliers in palaeomagnetic data sets and a basic model for the rotation pattern of palaeomagnetic directions

geometry and kinematics of vertical axis rotations in fold and thrust belts abstract vertical axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts the kinematics of rotating thrust sheets may be best described as a combination of a far field rotation or translation in a planar geometry and a near field or local rotation so that the instantaneous rotation axis lies outside the body of the sheet this simplifies the thrust sheet kinematics significantly as it means that all the points on the sheet have a component of forward motion and the bounding fault is a thrust over all its area the consequences of a simple thrust sheet rotation for the interpretation of fault lineation data are investigated the lineations recorded in the footwall and the hangingwall are likely to be different the hangingwall lineations being skewed towards the direction of rotation the principal mechanism for local thrust sheet rotation is differential shortening either on folds or thrusts or by a combination of these structures the forward propagation of thrusting towards the foreland can accumulate large 120 rotations on the uppermost sheets these rotational thrust sheets cannot extend indefinitely along strike instead they terminate against lateral ramps or transfer faults sidewall duplexes may form as a consequence of rotation over a lateral ramp examples of rotational geometries from the subbetic zone of southern spain are described on a larger scale obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike slip motion reliable palaeomagnetic data are not always available for fold and thrust belts so independent evidence for rotational deformation is of great importance fold and thrust geometries particularly along strike variability in fold geometries highly variable axial traces of folds evidence for near synchronous refolding and the deviation of regional trends may all be a consequence of thrust sheet rotation detailed mapping and particularly the identification of the position of footwall and hangingwall cut offs can also provide strong evidence for rotation

relationship between inferred redox potential of the depositional environment and geochemistry of the upper pennsylvanian missourian stark shale member of the dennis limestone wabaunsee county kansas u s a analyses of 21 samples collected from a core of the 52 8 cm thick stark shale member of the dennis limestone in wabaunsee county kansas demonstrate four cycles with two orders of magnitude variations in contents of cd mo p v and zn and order of magnitude variations in contents of organic carbon cr ni se and u the observed variability in amounts and or ratios of many metals and amounts and compositions of the organic matter appear related to the cause and degree of water column stratification and the resulting absence presence of dissolved o2 or h2s high cd mo u v zn and s contents a high degree of pyritization dop 0 75 0 88 and high high v v ni 0 84 0 89 indicate the presence of h2s in a strongly stratified water column intermediate contents of metals and s intermediate dop 0 67 0 75 and intermediate v v ni 054 0 82 indicate a less strongly stratified anoxic water column whereas low metal contents and low v v ni 0 46 0 60 indicate a weakly stratified dysoxic water column high p contents at the top of the organic matter rich intervals within the stark shale member indicate that phosphate precipitation was enhanced near the boundary between anoxic and dysoxic water compositions relatively abundant terrestrial organic matter in intervals deposited from the more strongly stratified h2s bearing water column indicates a combined halocline thermocline with the fresher near surface water the transport mode for the terrestrial organic matter the predominance of algal organic matter in intervals deposited from a less strongly stratified water column indicates the absence of the halocline and the presence of the more generally established thermocline relatively low amounts of degraded hydrogen poor organic matter characterize intervals deposited in a weakly stratified dysoxic water column the inferred variability in chemistry of the depositional environments may be related to climate variations and or minor changes in sea level during the general phase of deeper water deposition responsible for this widespread shale member

reconstructing the kinematic evolution of curved mountain belts a paleomagnetic study of triassic red beds from the wyoming salient sevier thrust belt u s a determining the kinematic history and mechanics of curved fold and thrust belts is fundamental to understanding the tectonic evolution of mountain systems to better understand the development of a classic curved fold and thrust belt we completed an integrated paleomagnetic and strain study of the wyoming salient paleomagnetic data are reported here from 154 sites collected from red beds of the triassic ankareh formation in the salient and nine sites collected from the relatively stable foreland red beds display three components with distinctly different magnetic behaviors 1 a near primary triassic magnetization carried by hematite that is stable up to 680 c tr component 91 sites 2 a cretaceous chemical remagnetization carried partly by magnetite k component 32 sites and 3 a recent viscous magnetization that is mostly removed by 350 c site mean vectors for the tr and k components show a high degree of scatter from expected triassic and cretaceous reference directions suggesting significant tilt and rotation subsequent to magnetization acquisition restoration of tilt and folding for individual site means results in well clustered shallow and moderate inclinations for the tr and k components respectively and in variable declinations related to systematic vertical axis rotations statistical analysis of declinations for both components indicates that 75 of present day salient curvature resulted from secondary rotation and 25 of primary curvature was likely related to sedimentary basin architecture analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets 80 compared to the frontal thrust sheets 65 suggesting that rotations were concentrated near the leading edge of the propagating fold and thrust wedge with only minor additional rotation of internal sheets transfer zones oblique ramps and more deformed overturned fold limbs display locally more complex patterns which can be understood through careful structural analysis when combined with internal strain data and regional structural relations paleomagnetic data support a kinematic model of a progressive arc with curved thrust slip paths and differential shortening that rotated early layer parallel shortening fabrics and produced minor strike parallel extension this kinematic history likely reflects a combination of processes including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient presence of a weak basal detachment and fault zone weakening that favored lower taper and buttressing by laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient

the mid cretaceous north atlantic nutrient trap black shales and oaes organic rich sediments are the salient marine sedimentation product in the mid cretaceous of the ocean basins formed in the mesozoic oceanic anoxic events oaes are discrete and particularly organic rich intervals within these mid cretaceous organic rich sequences and are defined by pronounced carbon isotope excursions marine productivity during oaes appears to have been enhanced by the increased availability of biolimiting nutrients in seawater due to hydrothermal alteration of submarine basalts in the pacific and proto indian oceans the exact mechanisms behind the deposition of organic rich sediments in the mid cretaceous are still a matter of discussion but a hypothesis which is often put forward is that their deposition was a consequence of the coupling of a particular paleogeography with changes in ocean circulation and nutrient supply in this study we used a global coupled climate model to investigate oceanic processes that affect the interbasinal exchange of nutrients as well as their spatial distribution and bioavailability we conclude that the mid cretaceous north atlantic was a nutrient trap as a consequence of an estuarine circulation with respect to the pacific organic rich sediments in the north atlantic were deposited below regions of intense upwelling we suggest that enhanced productivity during oaes was a consequence of upwelling of pacific derived nutrient rich seawater associated with submarine igneous events