cenozoic topographic and climatic response to changing tectonic boundary conditions in western north america abstract this study presents an oxygen isotopic record from the paleocene to the pliocene based on the analysis of predominantly paleosol carbonate from intermontane basins in southwestern montana and eastern idaho 18 o values of calcite decrease by 7 to 10 between 50 and 47 ma in southwestern montana and idaho most likely as a result of an increase in elevation of 2 5 to 3 5 km this rise in elevation is roughly contemporaneous with the emplacement of the nearby challis volcanics and the formation of metamorphic core complexes in the hinterland of the sevier thrust belt moreover when compared to previous oxygen isotopic studies that show oxygen isotopic shifts of similar magnitude occurring later in the late eocene to early oligocene in northeastern nevada and late oligocene to miocene in southern nevada the results of this study add to a growing body of evidence for a spatial and temporal migration of high surface elevations from north to south in the great basin of western united states this surface uplift history supports tectonic models calling for north to south removal of the farallon slab or delamination of the mantle lithosphere

aridification of central asia and uplift of the altai and hangay mountains mongolia stable isotope evidence central asia has become increasingly arid during the cenozoic though the mechanisms behind this aridification remain unresolved much attention has focused on the influence and uplift history of the tibetan plateau however the role of ranges linked to india asia convergence but well north of the plateau including the altai sayan and hangay in creating the arid climate of central asia is poorly understood today these ranges create a prominent rain shadow effectively separating the boreal forest to the north from the deserts of central asia to explore the role of these mountains in modifying climate since the late eocene we measured carbon and oxygen stable isotopes in paleosol carbonates from three basins along a 650 km long transect at the northern edge of the gobi desert in mongolia and in the lee of the altai and hangay mountains we combine these data with modern air parcel back trajectory modeling to understand regional moisture transport pathways at each basin in all basins 13c increases with the largest increase in western mongolia the first 13c increase occurs in central and southwestern mongolia in the oligocene 13c again increases from the upper miocene to the quaternary in western and southwestern mongolia we use a 1 d soil diffusion model to demonstrate that these 13c increases are linked to declines in soil respiration driven by dramatic increases in aridity using modern day empirical relations between mean annual precipitation and soil respiration we estimate that precipitation has likely more than halved over the neogene given the importance of the hangay and altai in steering moisture in mongolia we attribute these changes to differential surface uplift of the hangay and altai surface uplift in the hangay began by the early oligocene blocking siberian moisture and aridifying the northern gobi in contrast surface uplift of the altai began in the late miocene blocking moisture from reaching western mongolia thus the northern gobi became increasingly arid east to west since the late eocene likely driven by orographic development in the hangay during the oligocene and the altai in the late miocene through pliocene

paleogeographic and sedimentary evolution of the south pyrenean foreland basin abstract during the paleogene and neogene the ne iberian plate underwent significant paleogeographic changes driven by the iberian and european plate collision and the resulting formation of the pyrenean orogen and its corresponding foreland basin shortening resulted in the advance of the orogenic wedge emplacement of allochthonous units and progressive basin partitioning sediment transfer systems reacted to the evolving paleogeographic scenario shifting from forebulge to foredeep and wedge top settings critical reorganizations included successive shifts from open to close drainage conditions which had an strong impact on accommodation and the stratigraphic architecture of the basin infill overfill and later erosion the aim of this work is to synthesize the paleogeographic and sedimentary evolution of the south pyrenean foreland with emphasis on the reconstruction of sediment routing the evaluation of sedimentation rate trends the timing of sedimentary shifts and the analysis of their causes stratigraphic data are compiled in a comprehensive magnetostratigraphy based chronostratigraphic framework besides sedimentary and structural data are put together to produce a series of palinspastically restored paleogeographic maps which reflect five key stages in the evolution of the region these stages include 1 the late cretaceous tectonic inversion of the extended iberian margin 2 the early eocene formation of the southern pyrenean foredeep 3 the middle eocene widening and overfilling 4 the late eocene shift into an internal drainage and 5 the late miocene drainage opening and erosion in the light of these results the variable role of tectonics climate and eustacy at different time scales are discussed

a numerical model of deformation and fluid flow in an evolving thrust wedge abstract to investigate deformation and fluid flow in an actively deforming tectonic wedge we model the evolution of a large two dimensional 100 km long 5 km thick mechanically and hydrologically homogeneous and isotropic pile of sedimentary strata that is deformed to become a thrust wedge we compare both dry and wet cases in order to illustrate 1 the relative importance of fluids on wedge evolution and 2 the effect of brittle deformation on fluid flow we use an elastic plastic constitutive relation including a mohr coulomb failure criterion and a non associated flow rule and coupled fluid flow with bulk rock properties that approximate typical foreland sedimentary strata simulations are made both with and without dilation the model is fully dynamic but inertial forces remain small results show that deformation within the wedge is accommodated by reverse slip movement on shear bands which migrate in both directions through the wedge as both fore and back thrusts the model has features predicted by the critical taper theory 1 overall wedge geometry 2 crudely self similar growth during evolution 3 more intense deformation toward the rear of the wedge the models show strong overall in sequence faulting behavior with major thrusts isolating relatively undeformed packages which are moved in a piggyback manner upon the active thrusts intermittent out of sequence faulting does however occur in order to maintain the wedge taper fluid flow in the deforming wedge is dominated by topography but is also strongly affected by dilational plastic deformation in all simulations there is focused fluid flow within fault zones when mechanical time stepping is shut off uncoupled flow systems evolve to steady state where inflow equals outflow by subtracting the two states we isolate the mechanical fluid response from the total coupled system response the mechanical fluid response is manifest as contours of head and pressure difference and focusing of fluid flow in areas of active deformation in non dilational simulations these zones are quite localized and are a result of the contrasting stresses in and adjacent to a brittle shear zone in simulations with dilation the zones of head and pressure difference are broad and regional in extent they are ten times greater in magnitude than in non dilation simulations and they track a rock damage front that moves through the wedge

pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols the depth to carbonate nodular bk horizon in soils d in cm is correlated with mean annual precipitation p in mm so that bk horizons are deep in subhumid regions but shallow in semiarid regions previous quantifications of this relationship are unchanged by this new compilation of 807 soils p 137 24 6 45 d 0 013 d 2 where r 2 0 52 and standard error s e 147 mm in most north american postglacial soils the bk horizon is thin and well defined whereas in monsoonal tropical soils of pakistan and kenya the bk horizon is thick and diffuse data from 675 modern soils define the relationship between the thickness of soil with nodules t in cm and mean annual range of precipitation m in mm difference between monthly means of wettest and driest months m 0 79 t 13 71 where r 2 0 58 and s e 22 mm the relationship between carbonate nodule size s in cm and soil age a in ka is quantified by 9 radiocarbon dated soils from las cruces new mexico a 3 92 s 0 34 where r 2 0 57 and s e 1 8 k y these transfer functions are applied to reconstructing paleoclimatic change from paleocene eocene paleosols of the north horn formation and flagstaff limestone of axhandle canyon utah the terminal paleocene spike of warmth recorded by fossil plants in nearby wyoming was coincident with a brief peak in both mean annual precipitation and mean annual range of precipitation in utah

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

deformation and fault activity in space and time in high resolution numerical models of doubly vergent thrust wedges a discrete element model is used to investigate the manner in which deformation and fault activity change in space and time during the development of a doubly vergent thrust wedge in the upper crust deformation is a result of shortening at a subduction slot in the base of the model a configuration which produces a dynamic backstop within the cohesionless frictional cover material a series of experiments with differing basal decollement friction are performed the distinct manners in which thrust wedges grow and the variability of fault development and activity in space and time are then examined both predicted large scale wedge geometries and individual fault fold structures are similar to those observed in sandbox models and show the complex manner in which shortening is accommodated and localized during the development of the thrust wedge when compared to a sandbox model with similar boundary conditions model results are strikingly similar in all cases deformation initiates above the subduction slot with the formation of an axial zone the wedge is then developed by displacement on a retro wedge thrust and propagation of deformation into the pro wedge region models with low coefficients of basal friction typically develop wide shallow wedges with distributed spaced deformation and rather symmetric box like structures whereas those with high coefficients of basal friction develop narrower steeper wedges consisting of a series of stacked pro wedge thrust sheets and a high displacement retro wedge thrust in general fault initiation and linkage is extremely complex in our models with several smaller faults operating until linkage occurs to form a major through going structure of particular interest is the observation that many of the faults do not develop at the basal decollement and propagate upwards through the cover but rather initiate at high levels in the cover and propagate link downwards with other deeper structures results also indicate the utility of the discrete element approach in modelling large displacement complex deformation of geological materials

significant southern ocean warming event in the late middle eocene a prominent middle eocene warming event is identified in southern ocean deep sea cores indicating that long term cooling through the middle and late eocene was not monotonic at sites on maud rise and the kerguelen plateau a distinct negative shift in 1 8 o values 1 0 is observed ca 41 5 ma this excursion is interpreted as primarily a temperature signal with a transient warming of 4 c over 600 k y affecting both surface and middle bathyal deep waters in the indian atlantic region of the southern ocean this isotopic event is designated as the middle eocene climatic optimum and is interpreted to represent a significant climatic reversal in the midst of middle to late eocene deep sea cooling the lack of a significant negative carbon isotope excursion as observed during the paleocene eocene thermal maximum and the gradual rate of high latitude warming suggest that this event was not triggered by methane hydrate dissociation rather a transient rise in pco 2 levels is suspected possibly as a result of metamorphic decarbonation in the himalayan orogen or increased ridge arc volcanism during the late middle eocene

organic carbon burial following the middle eocene climatic optimum in the central western tethys we present trace metal geochemistry and stable isotope records for the middle eocene alano di piave section ne italy deposited during magnetochron c18n in the marginal tethys ocean we identify a sim 500 kyr long carbon isotope perturbation event we infer to be the middle eocene climatic optimum meco confirming the northern hemisphere expression and global occurrence of meco interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals le 3 toc and contemporaneous positive delta 13 c excursions these two intervals are associated with increases in the concentration of sulphur and redox sensitive trace metals and low concentrations of mn as well as coupled with the occurrence of pyrite together these changes imply low possibly dysoxic bottom water o 2 conditions promoting increased organic carbon burial we hypothesize that this rapid burial of organic carbon lowered global it p co 2 following the peak warming and returned the climate system to the general eocene cooling trend

cenozoic migration of topography in the north american cordillera continental topography is the result of complex interactions among mantle convection continental dynamics and climatic and erosional processes therefore topographic evolution of mountain belts and continental interiors reflects directly upon the coupling between mantle and surface processes it has recently been proposed that the modern topography of western north america is partly controlled by the removal of the subducting farallon plate and replacement of lithospheric mantle by hot asthenosphere creating surface uplift of the colorado plateau the southwestern united states and northern mexico while concomitant subsidence characterizes the central united states how the topography of the cenozoic north american cordillera evolved in the past is largely unknown yet currently debated tectonic models each have a predictable topographic response here we examine cenozoic surface uplift patterns of western north america based on a record of 3000 stable isotope proxy data this data set is consistent with eocene north to south surface uplift in the cordillera culminating in the assembly of an eocene oligocene highland 3 4 km in elevation the diachronous record of surface uplift and associated magmatism further supports tectonic models calling for the convective removal of mantle lithosphere or removal of the farallon slab by buckling along an east west axis the eocene oligocene development of rainout patterns similar to present day patterns along the flanks of the cordilleran orogen is therefore unlikely to be the result of late mesozoic crustal thickening and associated development of an andean style altiplano

late oligocene to early miocene humidity change recorded in terrestrial sequences in the ili basin south eastern kazakhstan central asia enhanced aridification of central asia driven by the combined effects of orogenic surface uplift paratethys retreat changes in atmospheric moisture transport and global cooling is one of the most prominent cenozoic climate change events of the northern hemisphere deciphering regional long term patterns of central asian hydrology is therefore a key element in understanding the role of northern hemisphere mid latitude drying in the global hydrological system this study characterizes long term palaeoenvironmental conditions between the late oligocene and early miocene in south eastern kazakhstan based on stable isotopes elemental geochemistry and laser ablation uranium lead geochronology from alluvial fluvial and pedogenic deposits sedimentary facies and geochemical weathering indices suggest an increased surface and ground water discharge fed by orographically enhanced precipitation in the tien shan hinterland in contrast pedogenic stable isotope data and elevated rates of magnesium fixation in clay minerals mirror enhanced rates of evaporation in the vadose zone due to protracted aridification this study posits that pronounced surface uplift of the tien shan mountains during the oligocene miocene transition promoted regionally increased orographic precipitation and the development of fluvial discharge systems this article is protected by copyright all rights reserved

mineralogical evidence for warm and dry climatic conditions in the neo tethys eastern turkey during the middle eocene abstract minerals in stratigraphic sections are valuable tools for reconstructing past environmental conditions given the state of preservation of clay minerals it is possible to determine under what conditions they formed which provides clues about continental weathering inherited minerals and geochemical conditions in the water column or pore waters neoformed or transformed of the sedimentary environment this study presents new mineralogical and chemical data from the baskil section a well preserved middle eocene neo tethys sequence from eastern turkey greater terrigenous input is marked by the increase of silicate minerals e g phyllosilicates quartz and albite in the section from 40 5 to 40 ma which diluted the marine carbonate content this period is correlative with the global middle eocene climatic optimum meco event authigenic palygorskite increased from the middle to the uppermost portion 40 to 37 3 ma of the section indicating dryer conditions in the continent and availability of si and mg in the water column and pore waters favorable for its formation additionally we suggest a stratified water column with warmer and more saline conditions at greater depths e g 200 m after 40 ma favoring palygorskite and possibly authigenic dolomite precipitation the mineralogical variations and element chemistry of rocks in the baskil section reflect how detrital sources and weathering regimes changed in this area during the middle eocene and how these changes can be related to global regional and local processes

optimal dip based on dissipation of back thrusts and hinges in fold and thrust belts 1 back thrusts and hinges are two types of transition between rigid sections of hanging walls observed in fold and thrust belts back thrusts are typical of frictional and homogeneous solids and hinges of creeping and layered materials our objective is to study the orientation of these transitions for the special case of a lower flat ramp transition in a fault bend fold with the following general two step methodology in the first step the forces acting on the transition are determined using equilibrium of each rigid section in the second step the optimal dip of the transition is obtained by minimizing the total dissipation of the structure the three sources of dissipation of comparable magnitude are at the transition on the flat and on the ramp for frictional material flows the back thrusts are velocity discontinuities with optimal dips always less than half the complementary ramp angle leading to hanging wall thickening the optimal dip agrees well with the results of physical analogue and numerical experiments for creeping and layered materials it is shown that a destabilizing deformation mechanism selected to be flexural slip is necessary for the strain to localize and the existence of hinges to be justified activation of flexural slip reduces dissipation at the transition and affects the optimal transition dip the two step methodology proposed here could be seen as a first attempt in producing mechanically balanced cross sections accounting for material rheology this approach should complement the now classical kinematic models of folding

clumped isotope palaeothermometry and la icp ms u pb dating of lava pile hydrothermal calcite veins calcite veins are a common product of hydrothermal fluid circulation clumped isotope palaeothermometry is a promising technique for fingerprinting the temperature of hydrothermal fluids but clumped isotope systematics can be reset at temperatures of ca 100 c to model whether the reconstructed temperatures represent calcite precipitation or closed system resetting the precipitation age must be known la icp ms u pb dating of calcite is a recently developed approach to direct dating of calcite and can provide precipitation ages for modelling clumped isotope systematics in calcite veins in this study clumped isotope and la icp ms u pb calcite analyses were combined in basalt hosted calcite veins from three settings in scotland samples from all three localities yielded precipitation temperatures of ca 75 115 c from clumped isotope analysis but veins from only two of the sites were dateable yielding precipitation ages of 224 8 ma and 291 33 ma 2 modelling from the dated samples enabled confident interpretation that no closed system resetting had occurred in these samples however the lack of a precipitation age from the third location meant that a range of possible thermal histories had to be modelled meaning that confidence that resetting had not occurred was lower this highlights the importance of coupling clumped isotope thermometry and la icp ms u pb calcite dating in determining the temperature of hydrothermal fluids recorded in calcite veins this paired approach is shown to be robust in constraining the timing and precipitation temperature of calcite formation and thus for tracking hydrothermal processes

capture of high altitude precipitation by a low altitude eocene lake western u s sedimentary facies of the eocene green river formation refl ect a rapid increase in water supply to lake gosiute ca 49 ma marked by a stratigraphic fi ll to spill surface deposits below this surface constitute repetitive lacustrine expansion desiccation cycles whereas those above consist of continuous profundal lacustrine mudstone grading upward into volcaniclastic deltaic sandstone above the fi ll to spill surface calcitic mudstone 18 o decreases from 26 to 20 over an interval representing 100 k y we interpret this shift to have resulted from capture of a foreland river or rivers that drained higher topography north of lake gosiute most likely in north central idaho accurate paleoelevation estimates derived from stable isotopic records in intermontane basins thus may require detailed knowledge of regional drainage systems

two massive rapid releases of carbon during the onset of the palaeocene eocene thermal maximum the release of massive amounts of carbon led to abrupt warming 55 5 million years ago an analysis of soil carbonates shows two distinct carbon injections at the event onset each releasing over 0 9 petagrams of carbon per year over hundreds to thousands of years

a new strategy for discrete element numerical models 2 sandbox applications 1 here we present a series of numerical experiments using a new formulation of the discrete element method dem that improves performance in modeling faults and shear zones in the new method named the stress based discrete element method sdem which is introduced in the companion paper by egholm stress tensors are stored at each circular particle further sdem includes rotational resistivity of particles and elastoplastic constitutive rules for governing particle deformation when combining these new features the sdem is capable of reproducing the friction properties of rocks and soils without the need for the ad hoc calibration routines normally associated with dem in contrast to the conventional dem the friction properties of a sdem particle system are in agreement with the mohr coulomb constitutive model with friction angles specified on a particle level benchmark sandbox models show that unlike most commonly used numerical methods sdem faults and shear zones develop at angles in agreement with general observations from structural geology and analogue modeling studies citation egholm d l m sandiford o r clausen and s b nielsen 2007 a new strategy for discrete element numerical models 2 sandbox applications j geophys res 112 b05204 doi 10 1029 2006jb004558

mechanics of blind thrusting comparison of numerical and physical modeling abstract physical models and numerical simulations offer alternative approaches to developing a mechanical understanding of deformation histories this contribution 1 compares results from two independent studies one employing the finite element method the other using dynamically scaled physical models of blind thrusting and 2 discusses the implications that these results have for the role that boundary conditions play in thrust system mechanics the finite element models were designed to assess roof sequence response to duplex emplacement whereas the physical modeling focused on triangle zone evolution despite this difference both studies address the same fundamental question how does a roof sequence respond to the propagation and displacement of blind thrust faults the emplacement of thrust horses leads to a deformation partitioning in the overlying roof sequence deformation is partitioned between translation and internal distortion our results show that one of the controls on deformation

the cenozoic climatic and topographic evolution of the western north american cordillera herein we present oxygen isotope records from cretaceous to recent terrestrial sediments in the western north american cordillera the purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the cenozoic to do this we constructed 18o maps of surface waters for time intervals that trace the development of topography of western north america these maps are based on 4861 oxygen isotope analyses from both published 4478 and new 383 data we determined the 18o values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors these data suggest that in the late cretaceous to early eocene the sevier hinterland formed a plateau of unknown height around 50 ma a topographic wave developed in british columbia and eastern washington that swept southward reaching northeastern nevada at 40 to 38 ma and southern nevada 23 ma this southward encroachment of an eocene plateau sweep caused reorganization of drainage patterns such that the intraforeland basins of wyoming and utah drainages extended deep within the sevier hinterland as the wave swept southward the landscape within the sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of 4 km and relief of 1 5 km this eocene highland was bordered on the west by a high sierra nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands the spatial and temporal evolution of this highland correlates with the timing of volcanism and extension these observations support tectonic models that call for north to south removal of the farallon slab or piecemeal removal of mantle lithosphere the isotopic data show that prior to growth of this highland the north american monsoon nam penetrated much farther north in the paleocene eocene than today the combined effects of global cooling increasing latitudinal temperature gradients and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the nam front by the oligocene the hydrologic regime that we observe today was in place it has been modified since then as a result of basin and range extension and collapse of the highlands in the mid miocene this collapse allowed the nam to penetrate farther north into the great basin of nevada and utah

u pb dates of paleosols constraints on late paleozoic cycle durations and boundary ages u pb ages of paleosol samples from cyclothemic sections in the sacramento mountains of new mexico and the permian basin of west texas bracket the pennsylvanian permian and carboniferous permian boundaries linear interpolation and extrapolation to boundaries are accomplished by using cyclothems as periodic subdivisions the carboniferous permian boundary is 301 2 ma 2 the pennsylvanian permian boundary is 302 2 4 2 and the missourian virgilian boundary is 307 3 ma 2 these ages are within the uncertainties of recent estimates for the boundaries but are significantly more precise the average cycle duration for this upper pennsylvanian and lower permian interval is 143 64 ka 2 within the uncertainty of the 100 k y cycle that dominated pleistocene sea level history this new estimate which is much shorter than those published previously suggests that the pennsylvanian and early permian cyclothems may have been driven in a manner similar to that which caused glacio eustatic cycles of the pleistocene

prediction of thrusting sequences in accretionary wedges 1 the main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp the development with the construction of the relief and the arrest because of the onset of another thrusting event a simple kinematics is proposed for the geometry of the developing thrust fold based on rigid regions separated by velocity discontinuities along which work is dissipated according to the coulomb criterion the evolution of the thrust fold satisfies mechanical equilibrium and is optimized at every time of the three stages to provide the least upper bound in tectonic force according to the maximum strength theorem the development of the thrust or its arrest because of the initiation of another thrust is decided by selecting the event which leads to the least upper bound in tectonic force the approach is first validated by proving that the critical slope angle c for the classical triangular wedge is properly captured it is shown that a perturbation in the form of an extra relief in this perfectly triangular geometry leads to the onset of thrusting with the ramp or the back thrust outcropping either at the back or to the front of the perturbation respectively for a range of slope dip close to the critical angle c the study of normal thrust sequences from the rear to the front in the wedge toe reveals that weakening of the ramp accounted for by changing its friction angle from an initial to a smaller final value is necessary for each thrust to have a finite life span this life span is longer with a larger relief buildup for more pronounced weakening decreasing the decollement friction angle results in an increase in the number of thrusts in the sequence each thrust creating milder relief the normal sequence is ended with the first out of sequence thrust which occurs earlier for smaller weakening over the ramp the proposed methodology is partly used to construct an inverse method proposed to assess the likeliness for the transfer of activity from the active to the incipient thrust in a section of nankai s accretionary wedge the inverse method provides the initial friction angle over the incipient ramp and the final friction angle over the fully active ramp from the geometry of the corresponding thrusts and the topography it is shown that the friction angle over the incipient ramp is most likely to be larger then the one over the active ramp justifying a key hypothesis needed to predict discrete sequences of thrusting

a calcite reference material for la icp ms u pb geochronology u pb dating of calcite is an emerging but rapidly growing field of application in geochronology with great potential to inform problems in landscape basin and mountain belt evolution through age determination of diagenetic cements vein mineralisation and geological formations difficult to date otherwise in this brief we present isotope dilution u pb isotope measurements on a sample of calcite wc 1 that has been and will continue to be used as a reference material for in situ u pb laser ablation inductively coupled plasma mass spectrometry la icp ms dating and which is suitable to be distributed to the geochronological community we present in situ measurements using la icp ms to demonstrate the suitability of wc 1 for use as a u pb dating reference material in spite of it not being isotopically homogeneous the wc 1 calcite sample is 254 4 6 4 ma old and comprised of 85 to 98 radiogenic lead it presents a suitable reference material that can facilitate dating of calcite ranging in age from precambrian to late neogene age

greenhouse crises of the past 300 million years proxies of past co 2 and climate over the past 300 m y now reveal multiple global climate change events in unprecedented detail evidence for past co 2 spikes comes from expanded and refined data on stomatal index of fossil ginkgo and related leaves new evidence for synchronous climatic change comes from paleosols in montana utah and neighboring states each co 2 spike was coeval with unusually clayey red and decalcified paleosols which can be traced throughout the colorado plateau spikes in atmospheric co 2 also were coeval with increases in paleosol alkali depletion as an indication of high temperature and spikes in paleosol base depletion and depth to calcic horizons as indications of high precipitation in the colorado plateau times of warmer climate were also more humid perhaps due to greater moisture potential of warmer air seasonality of climate did not increase during warm wet spikes the mesozoic greenhouse was not persistently hot with cool spells but warm with hot flashes these data furnish power laws predicting the sensitivity and magnitude of change in mean annual temperature mat and mean annual precipitation map due to rising co 2 in a mid latitude mid continental region the magnitude of the coming anthropogenic greenhouse pales in comparison with past greenhouse spikes at times of global mass extinctions

high temperature environments of human evolution in east africa based on bond ordering in paleosol carbonates many important hominid bearing fossil localities in east africa are in regions that are extremely hot and dry although humans are well adapted to such conditions it has been inferred that east african environments were cooler or more wooded during the pliocene and pleistocene when this region was a central stage of human evolution here we show that the turkana basin kenya today one of the hottest places on earth has been continually hot during the past 4 million years the distribution of 13c 18o bonds in paleosol carbonates indicates that soil temperatures during periods of carbonate formation were typically above 30 c and often in excess of 35 c similar soil temperatures are observed today in the turkana basin and reflect high air temperatures combined with solar heating of the soil surface these results are specific to periods of soil carbonate formation and we suggest that such periods composed a large fraction of integrated time in the turkana basin if correct this interpretation has implications for human thermophysiology and implies a long standing human association with marginal environments

13c 18o isotope signatures and clumped isotope thermometry in foraminifera and coccoliths accurate constraints on past ocean temperatures and compositions are critical for documenting climate change and resolving its causes most proxies for temperature are not thermodynamically based appear to be subject to biological processes require regional calibrations and or are influenced by fluid composition as a result their interpretation becomes uncertain when they are applied in settings not necessarily resembling those in which they were empirically calibrated independent proxies for past temperature could provide an important means of testing and or expanding on existing reconstructions here we report measurements of abundances of stable isotopologues of calcitic and aragonitic benthic and planktic foraminifera and coccoliths relate those abundances to independently estimated growth temperatures and discuss the possible scope of equilibrium and kinetic isotope effects the proportions of 13c 18o bonds in these samples exhibits a temperature dependence that is generally similar to that previously been reported for inorganic calcite and other biologically precipitated carbonate containing minerals apatite from fish reptile and mammal teeth calcitic brachiopods and molluscs aragonitic coral and mollusks most species that exhibit non equilibrium 18o 16o 18o and 13c 12c 13c ratios are characterized by 13c 18o bond abundances that are similar to inorganic calcite and are generally indistinguishable from apparent equilibrium with possible exceptions among benthic foraminiferal samples from the arctic ocean where temperatures are near freezing observed isotope ratios in biogenic carbonates can be explained if carbonate minerals generally preserve a state of ordering that reflects the extent of isotopic equilibration of the dissolved inorganic carbon species

new biostratigraphic magnetostratigraphic and isotopic insights into the middle eocene climatic optimum in low latitudes the middle eocene climatic optimum meco was a warming event that interrupted the long term eocene cooling trend while this event is well documented at high southern and mid latitudes it is poorly known from low latitudes and its timing and duration are not well constrained because of problems of hiati microfossil preservation and weak magnetic polarity in key sedimentary sections here we report the results of a study designed to improve the bio magneto and chemostratigraphy of the meco interval using high resolution records from two low latitude sections in the atlantic ocean ocean drilling program odp sites 1051 and 1260 we present the first detailed benthic foraminiferal stable isotope records of the meco from the low latitudes as well as the biostratigraphic counts of orbulinoides beckmanni and new magnetostratigraphic results our data demonstrate a 750 kyr long duration for the meco characterized by increasing 13c and decreasing 18o with minimum 18o values lasting 40 kyr at 40 1 ma coincident with a short lived negative 13c excursion thereafter 18o and 13c values recover rapidly the shift to minimum 18o values at 40 1 ma is coincident with a marked increase in the abundance of the planktonic foraminifera o beckmanni consistent with its inferred warm water preference o beckmanni is an important eocene biostratigraphic marker defining planktonic foraminiferal zone e12 with its lowest and highest occurrences lo and hos our new records reveal that the lo of o beckmanni is distinctly diachronous appearing 500 kyr earlier in the equatorial atlantic than in the subtropics 40 5 versus 41 0 ma we also show that at both sites the ho of o beckmanni at 39 5 ma is younger than the published calibrations increasing the duration of zone e12 by at least 400 kyr in accordance with the tropical origins of o beckmanni this range expansion to higher latitudes may have occurred in response to sea surface warming during the meco and subsequently disappeared with cooling of surface waters

the clumped isotope geothermometer in soil and paleosol carbonate we studied both modern soils and buried paleosols in order to understand the relationship of temperature t c 47 estimated from clumped isotope compositions 47 of soil carbonates to actual surface and burial temperatures carbonates from modern soils with differing rainfall seasonality were sampled from arizona nevada tibet pakistan and india t c 47 obtained from these soils shows that soil carbonate forms in the warmest months of the year in the late morning to afternoon and probably in response to intense soil dewatering t c 47 obtained from modern soil carbonate ranges from 10 8 to 39 5 c on average t c 47 exceeds mean annual temperature by 10 15 c due to summertime bias in soil carbonate formation and to summertime ground heating by incident solar radiation secondary controls on t c 47 are soil depth and shading site mean annual air temperature maat across a broad range 0 30 c of site temperatures is highly correlated with t c 47 from soils following the equation maat c 1 20 t c 47 0 21 72 r 2 0 92 where t c 47 0 is the effective air temperature at the site estimated from t c 47 the effective air temperature represents the air temperature required to account for the t c 47 at each site after consideration of variations in t c 47 with soil depth and ground heating the highly correlated relationship in this equation should now permit mean annual temperature in the past to be reconstructed from t c 47 in paleosol carbonate assuming one is studying paleosols that formed in environments generally similar in seasonality and ground cover to our calibration sites t c 47 0 decreases systematically with elevation gain in the himalaya following the equation elevation m 229 t c 47 0 9300 r 2 0 95 assuming that temperature varied similarly with elevation in the past this equation can be used to reconstruct paleoelevation from clumped isotope analysis of ancient soil carbonates we also measured t c 47 from long sequences of deeply buried 5 km paleosol carbonate in the himalayan foreland in order to evaluate potential diagenetic resetting of clumped isotope composition we found that paleosol carbonate faithfully records plausible soil t c 47 down to 2 5 4 km burial depth or 90 125 c deeper than this and above this temperature t c 47 in paleosol carbonate is reset to temperatures 40 c we observe 40 c as the upper limit for t c 47 in modern soils from soil depths 25 cm and therefore that t c 47 40 c obtained from ancient soil carbonate indicates substantially warmer climate regimes compared to the present or non primary temperatures produced by resetting during diagenesis if representative this limits the use of t c 47 to reconstruct ancient surface temperature to modestly buried 3 4 km paleosol carbonates despite diagenetic resetting of 47 values 18 o and 13 c values of the same deeply buried paleosol carbonate appear unaltered we conclude that solid state reordering or recrystallization of clumping of carbon and oxygen isotopes can occur in the absence of open system exchange of paleosol carbonate with significant quantities of water or other phases

a revised calibration of the clumped isotope thermometer abstract a growing number of materials and environmental settings are studied using the carbonate clumped isotope 47 thermometer the method has been applied in both biogenic and non biogenic carbonate systems in marine and terrestrial settings over a wide range of geological timescales the current 47 temperature calibration gives good temperature estimates for most biogenic materials however systematic biases are commonly observed at low temperatures in this study we report additional calibration data that covers a wider temperature range at more evenly distributed temperatures and are measured at higher analytical precision than the original calibration combining these data with the original calibration yields a 47 t relationship that is similar to the original calibration though slightly less steep 47 0 0526 0 0025 10 6 t 2 0 0520 0 0284 this revised calibration is in better agreement with biogenic carbonates including those grown at low temperatures the difference between the original and revised calibrations is significant for carbonates forming below 16 c or above 49 c 47 values of 0 68 and 0 56 additionally we include a comprehensive analysis of the sources of error associated with 47 measurements and estimated temperatures and recommend measurement strategies for obtaining the desired precision as an illustration we apply the revised calibration and uncertainty analysis to 3 previously published studies at low temperatures the revised calibration results in significant differences from the original calibration and hence affects the interpretation of the environmental signal recorded in light of our 47 errors analysis in cases where the temperature signals are small we find that replicate analyses are critical to identify a temperature signal

calibration of the clumped isotope geothermometer in soil carbonate in wyoming and nebraska usa implications for paleoelevation and paleoclimate reconstruction abstract recent work has shown that soil carbonates typically form as the soil dries after seasonal rainfall and therefore record seasonal aspects of climate rather than mean annual conditions because soil carbonate formation is closely related to the timing of local rainfall and drying it is necessary to understand the formation seasonality and temperature and soil water o 18 values recorded in modern soil carbonate before accurate estimates of stable isotope based paleoelevation and paleoclimate can be made here we study carbonate clumped isotope 47 and oxygen isotope o 18 compositions of modern soil carbonates and seasonal variations of soil moisture and temperature of nearby climate stations in wyoming and western nebraska usa to understand the seasonality of soil carbonate formation in semi arid to arid temperate montane settings we find that soil carbonate clumped isotope temperatures t 47 are 3 5 c higher than mean summer air temperature and are similar to or higher than average summer soil temperature at depths 40 cm soil moisture dramatically decreases in early summer following the cessation of spring rains and snowmelt and shows only brief increases after major mid late summer rain events soil water o 18 values calculated using carbonate o 18 and t 47 values are similar to the o 18 values of local mean summer precipitation these lines of evidence suggest that soil carbonates in our study area formed during times of soil dewatering in early summer and after major summer storm events in mid late summer and in or near equilibrium with mean summer precipitation o 18 values t 47 values of modern soil carbonates are inversely correlated with elevation with a lapse rate 4 0 c km similar to the modern air and soil temperature lapse rates calculated soil water o 18 values also are inversely correlated with elevation with a lapse rate of 3 7 km sample elevations can be reconstructed using both t 47 values and calculated soil water o 18 values between 900 m and 2600 m in our studied area with an accuracy of 0 5 km or less however o 18 values of precipitation and soil water reflect the complex interaction of climatic environmental and topographic conditions prior to carbonate formation whereas t 47 values may be influenced by site specific conditions we suggest that both soil carbonate t 47 values and calculated soil water o 18 values should be used to corroborate paleoelevation reconstruction and the seasonal nature of soil carbonate formation should be considered when soil carbonate t 47 values and o 18 values are applied for paleoclimate reconstruction

thrust dip and thrust refraction in fault bend folds analogue models and theoretical predictions analogue models made of two layers of loose sand separated by a thin layer of micro glass beads were shortened by a rigid emerging ramp dipping at either 30 or 45 and possessing a high intermediate or null surface friction shortening resulted in formation of closely spaced back thrusts in the sand layers the dips of the back thrusts vary within a range of 30 depending on the ramp friction and 7 depending on its dip an increase in ramp friction or to a lesser extent in ramp dip decreases the thrust dips in the model the second important observation is that when friction is greater along the ramp than along the layer of glass beads then the glass beads layer acts as a separate upper ramp above which the back thrusts steepen the theory proposed to explain these observations predicts the thrust dips through a two step procedure first global equilibrium of forces in the two layers is required to yield the mean forces at stake along the ramps and thrusts second the total dissipation is minimized with respect to the dips of the back thrusts the relevant frictional properties of our analogue materials have been measured in stress conditions as close as possible to the experimental ones below 1 kpa and used with the theory to yield optimal back thrust dips that are all within 3 of the measured dips this is a surprisingly good fit when considering that we did not take into account geometric changes strain softening and dilatancy or compaction due to slip on the thrusts we conclude that this general two step theoretical procedure is validated in the context of analogue frictional materials we also propose a possible mechanism for thrust refraction and top to the foreland sense of shear observed in the hanging walls of lower flat to ramp transition in sedimentary piles that is based on the triggering of secondary upper ramps finally this mechanical approach can also be seen as complementary to the kinematic models of fold thrust structures which by definition cannot grasp the strong effects of friction on the kinematics

combined high precision 48 and 47 analysis of carbonates abstract high precision analysis of the excess abundance relative to the stochastic distribution of mass 48 isotopologues in co2 evolved from acid digestion of carbonates 48 has not been possible until recently due to the relatively low natural abundance of 18o here we show that the 253 plus gas source mass spectrometer equipped with faraday cups and 1013 resistors can perform combined 47 and 48 analyses on carbonates with external reproducibilities 1sd of 0 010 and 0 030 respectively 10 mg aliquots of five carbonate reference materials eth 1 eth 2 eth 3 eth 4 and carrara are digested with phosphoric acid at 90 c using a common acid bath the evolved co2 is purified using an automated gas preparation system including cryotraps and a gc and analyzed for its 47 and 48 compositions using the dual inlet system of a 253 plus gas source mass spectrometer raw 47 and 48 values are finally normalized to the carbon dioxide equilibrium scale cdes in 47 cdes 90 c vs 48 cdes 90 c space calcite reference materials carrara eth 3 and eth 4 agree with the equilibrium curve for calcite after adding semi empirically determined 90 c acid fractionation factors of 0 196 for 47 and 0 136 for 48 to theoretical 63 and 64 data agreement between measured and theoretically expected 48 cdes 90 c highlights the accuracy of our high precision clumped isotope analytical setup combined analysis of the abundances of mass 47 and mass 48 isotopologues in co2 evolved from acid digestion of natural carbonates has excellent potential for the determination of accurate and highly precise paleotemperatures as well as for the identification of rate limiting kinetic processes involved in biomineralization a formation temperature of 15 2 c is obtained on the 95 confidence level for the upper cretaceous chalk sample eth 3

hot or not impact of seasonally variable soil carbonate formation on paleotemperature and o isotope records from clumped isotope thermometry quantifying the timing and temperature of soil carbonate formation is important for interpreting isotopic records of earth surface temperature and soil water composition from paleosols pedogenic carbonates typically have been assumed to form at mean annual temperature but recent work suggests warm season bias in carbonate formation could impact the interpretation of d 18 o values and carbonate

aridification in continental asia after the middle eocene climatic optimum meco global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transitional interval during the eocene epoch characterized by incipient polar ice sheet formation as well as short lived warming events of which the middle eocene climatic optimum meco is most noticeable understanding this critical period requires high resolution records that are being gathered in marine basins but are still lacking in the terrestrial realm here we provide a precisely dated terrestrial record crossing the meco time interval from the xining basin nw china we document a rapid aridification step and the onset of obliquity dominated climate cyclicity indicated by lithofacies and pollen records dated at 40 0 ma at the base of magnetochron c18n 2n this shift is concomitant within error with the meco peak warming in ocean drilling program site 1258 for which we reassessed the magnetostratigraphic age at 40 0 ma also at base of magnetochron c18n 2n the rapidity of the shift observed in the xining basin and the region wide aridification and monsoonal intensification reported around 40 ma suggests asian paleoenvironments were responding to global climate changes associated with the meco however the xining records show only the permanent shift but not the transient peak warming observed in marine meco records we thus relate this permanent aridification to occur during the post meco cooling we propose the mechanisms linking global climate to asian paleoenvironments may be eustatic fluctuations driving the stepwise retreat of the proto paratethys epicontinental sea or simply global cooling reducing moisture supply to the continental interior in any case eocene global climate cooling from greenhouse to icehouse conditions seem to have played a primary role in shaping asian paleoenvironments

erosional control of the kinematics and geometry of fold and thrust belts imaged in a physical and numerical sandbox 1 we investigate the effect of systematically varying erosion intensity k on the geometry and kinematics of fold and thrust belts modeled using equivalent conditions in physical and numerical experiments similar material properties and boundary conditions were used to compare numerical experiments to those modeled in the sandbox using an erosion rule that removes mass from the scaled sand wedges according to rates expected from bedrock fluvial incision the geometry of both modeling approaches is quantitatively compared with that expected from analytical theory the fold and thrust belt growth rate is inversely proportional to k and is well predicted by theory except for the high erosion case in which both the numerical and the experimental sandbox grows supercritically a direct relationship exists between the erosion intensity and the number of fore shear bands and shear strain magnitude the number of fore shear bands decreases and their strain increases with erosion intensity the results indicate that realistic and mechanistic erosion rules can be applied to physical experiments to model mass removal and this approach opens up the possibility of calibrating strain history to erosion intensities for predictions in natural settings quantitative comparisons between the physical and numerical sandbox experiments indicate that deformation style and geometry of the thrust and fold belts are similar when identical rheologies and boundary conditions are used this benchmarking suggests that numerical experiments realistically model conditions observed in the simple cohesionless rheologies commonly employed in sandbox experiments this suggests that numerical simulation may reliably model experimental deformation in geological situations that are difficult to represent in scaled experiments because of the different scales and rheologies of natural orogenic wedges

cenozoic paleoclimate on land in north america abstract paleotemperature and paleoprecipitation over the past 40 m yr can be inferred from the degree of chemical weathering and depth of carbonate nodules in paleosols of oregon montana and nebraska paleosol records show that late eocene 35 ma middle miocene 16 ma late miocene 7 ma and early pliocene 4 ma warm climatic episodes were also times of a wet climate in oregon montana and nebraska oregon and nebraska were humid during warm wet times but montana was no wetter than subhumid within the rain shadow of intermontane basins global warm wet paleoclimatic spikes steepened rather than flattened geographic gradients of rocky mountain rain shadows long lived mountain barriers created dusty dry basins with sedimentation rates high enough to preserve milankovitch scale 100 41 kyr global paleoclimatic variation in some sequences of paleosols greenhouse warm wet climates indicated by paleosols were also peaks of diversity for north american plants and animals and coincided with advance

tectonic and sedimentary evidence for synchronous and out of sequence thrusting larache acilah area western moroccan rif syn tectonic turbidites were analysed using measured sections structural relationships and bio and litho correlations of sedimentary units to determine the tectonic evolution of the alpine thrust belt of the northwestern moroccan rif the external zones of the northwestern moroccan rif contain two tertiary turbidite sequences which can be sub divided into type i ii and iii turbidite packages sensu mutti 1984 one turbidite sequence the acilah sandstone oligocene earliest miocene is found at the leading edge of the tangier thrust sheet while a second the sidi mrait flysch is seen in the underlying prerif thrust sheet the lower sidi mrait flysch was probably deposited in a foredeep basin at the end of the eocene early oligocene the advancing thrust front incorporated the foredeep basin deposits into the thrust belt subsequent turbidite deposition occurred in piggyback basins palaeocurrent directions indicate sediment transport from sse to nnw along troughs elongated parallel to the tectonic strike creation of piggyback basins turbidite onlap relationship unconformities and structural relationships indicate that complex synchronous and out of sequence thrusting accompanied the overall foreland progressing deformation sequence

inverse method applied to a sand wedge estimation of friction parameters and uncertainty analysis we examine the fundamental mechanism of frontal accretion in a sand wedge from the occurrence of a forward thrust ramp evolving into a fault bend fold to the jump of deformation to a new frontal ramp ahead we use inverse problem theory to extract quantitative information on friction parameters from the systematic comparison of experimental observations and theoretical predictions the observables are locations dips and lifetime of thrust ramps hinge and associated compression the experimental values observed data are cast into statistical models describing the error bars the theory of limit analysis provides calculated data requiring five parameters material density friction coefficient of the decollement plane friction coefficient of the bulk material and the variation of friction with slip on the ramp as well as the distance for this variation the misfit between observed data and calculated data is determined for all physically admissible values of the parameters values yielding a small misfit are interpreted as highly probable the mean misfit per observable is within their error bars and therefore application of the theory reproduces the observables bulk and decollement friction coefficient values with high probability are compared to independent measurements the inversion also reveals systematic discrepancies the frictional weakening on the ramps is overestimated while the force is underestimated the calculated thrust sheet is longer than observed and the calculated jump to a second ramp occurs earlier than observed these conclusions allow us to identify necessary improvements for the experimental set up and of the theoretical assumptions

enhanced siliceous plankton productivity in response to middle eocene warming at southern ocean odp sites 748 and 749 the middle eocene climatic optimum meco is a major transient warming event that occurred at 40 ma and reversed a long term cooling trend through the early and middle eocene we report the results of a high resolution quantitative study of siliceous microfossils at ocean drilling program sites 748 and 749 southern kerguelen plateau southern ocean 58 s across a 1 4 myr interval spanning the meco event at both sites a significant increase in biosiliceous sedimentation is associated with the meco event rich siliceous planktonic microfossil assemblages in this interval are unusual in that they are dominated by ebridians with radiolarians as a secondary major component silicoflagellates and diatoms comprise only a minor fraction of the assemblage in contrast to siliceous microfossil assemblages that characterize modern southern ocean sediments based on our new siliceous microfossil records we interpret two 300 kyr periods of elevated nutrient availability in southern ocean surface waters which span the peak warming interval of the meco and the post meco cooling interval a diverse assemblage of large silicoflagellates belonging to the dictyocha grandis plexus is linked to the rapid rise in sea surface temperatures immediately prior to peak warmth and a pronounced turnover is observed in both ebridian and silicoflagellate assemblages at the onset of peak warming the interval of peak warmth is also characterized by high abundance of cosmopolitan ebridians e g ammodochium spp and silicoflagellates e g naviculopsis spp and increased abundance of tropical and subtropical diatom genera e g asterolampra and azpeitia these observations confirm the relative pattern of temperature change interpreted from geochemical proxy data at multiple southern ocean sites furthermore rapid assemblage changes in both autotrophic and heterotrophic siliceous microfossil groups indicate a reorganization of southern ocean plankton communities in response to greenhouse warming during the meco event

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

chronologic constraints on hominin dispersal outside africa since 2 48 ma from the zarqa valley jordan abstract recent discoveries constrain the presence of hominins in north africa since ca 2 4 ma and in china since ca 2 1 ma providing a new temporal framework for the earliest migration out of africa no paleolithic sites of such age exist in the levant the natural corridor between africa and asia the dawqara formation in the zarqa valley jordan has been known since the early 1980s because of the presence of artifacts at different stratigraphic levels within its fluvial sediments consisting of choppers cores and flakes although most of the artifacts display signs of transport they bear unambiguous evidence of manufacture and document hominin presence in the zarqa valley during the deposition of dawqara formation based on integrated chronology provided by paleomagnetic 40ar 39ar and u pb dating methods our study shows that the dawqara formation was deposited between 2 52 0 01 ma and the matuyama olduvai geomagnetic reversal 1 95 ma by linear interpolation the artifact bearing stratigraphic levels within the dawqara formation have ages of ca 2 48 ma 2 24 ma 2 16 ma 2 06 ma and 1 95 ma respectively possibly documenting continuous hominin presence in the zarqa valley these new ages for the dawqara assemblage constrain the earliest hominin dispersal out of africa to the beginning of the pleistocene and pre date by ca 300 kyr the hominin occupation of chinese loess plateau

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

rapid middle eocene temperature change in western north america abstract eocene hyperthermals are among the most enigmatic phenomena of cenozoic climate dynamics these hyperthermals represent temperature extremes superimposed on an already warm eocene climate and dramatically affected the marine and terrestrial biosphere yet our knowledge of temperature and rainfall in continental interiors is still rather limited we present stable isotope 18o and clumped isotope temperature 47 records from a middle eocene 41 to 40 ma high elevation mammal fossil locality in the north american continental interior montana usa 47 paleotemperatures of soil carbonates delineate a rapid 9 11 c temperature excursion in the paleosol record 47 temperatures progressively increase from 23 c 3 c to peak temperatures of 32 c 3 c and subsequently drop by 11 c this hyperthermal event in the middle eocene is accompanied by low 18o values and reduced pedogenic carbonate concentrations in paleosols based on laser ablation u pb geochronology of paleosol carbonates in combination with magnetostratigraphy biostratigraphy stable isotope and 47 evidence we suggest that this pronounced warming event reflects the middle eocene climatic optimum meco in western north america the terrestrial expression of northern hemisphere meco in western north america appears to be characterized by warmer and wetter sub humid conditions compared to the post meco phase large and rapid shifts in 18o values of precipitation and pedogenic caco3 contents parallel temperature changes indicating the profound impact of the meco on atmospheric circulation and rainfall patterns in the western north american continental interior during this transient warming event

synoptic reconstruction of a major ancient lake system eocene green river formation western united states numerous 40ar 39ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the greater green river piceance creek and uinta basins of wyoming colorado and utah constrain 8 m y of the eocene epoch multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance 40ar loss and 39ar recoil when considered in conjunction with existing radioisotopic ages and lithostratigraphy biostratigraphy and magnetostratigraphy these new age determinations facilitate temporal correlation of linked eocene lake basins in the laramide rocky mountain region at a significantly increased level of precision to compare our results to the geomagnetic polarity time scale and the regional volcanic record the ages of eocene magnetic anomalies c24 through c20 were recalibrated using seven 40ar 39ar ages overall the ages obtained for this study are consistent with the isochroneity of north american land mammal ages throughout the study area and provide precise radioisotopic constraints on several important biostratigraphic boundaries applying these new ages average sediment accumulation rates in the greater green river basin wyoming were approximately three times faster at the center of the basin versus its ramp like northern margin during deposition of the underfilled wilkins peak member in contrast sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled tipton and laney members sediment accumulation patterns thus reflect basin center focused accumulation rates when the basin was underfilled and supply limited accumulation when the basin was balanced filled to overfilled sediment accumulation in the uinta basin at indian canyon utah was relatively constant at 150 mm k y during deposition of over 5 m y of both evaporative and fluctuating profundal facies which likely reflects the basin margin position of the measured section the most rapid sediment accumulation for the entire system 1 m k y occurred between 49 0 and 47 5 ma when volcaniclastic materials from the absaroka and or challis volcanic fields entered the green river formation lakes from the north our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the green river formation basins coupled with previously published paleocurrent observations our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes the overall character of eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding laramide landscape freshwater overfilled lakes were initially dominant 53 5 52 0 ma possibly related to high erosion rates of remnant cretaceous strata on adjacent uplifts expansion of balanced fill lakes first occurred in all green river formation basins at 52 0 51 3 ma and again between 49 6 and 48 5 ma evaporative underfilled lakes occurred in various basins between 51 3 and 45 1 ma coincident with the end of the early eocene climatic optima and subsequent onset of global cooling defined from marine record however evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate evaporative terminal sinks were initially located in the greater green river and piceance creek basins 51 3 48 9 ma then gradually migrated southward to the uinta basin 47 1 45 2 ma this history is likely related to progressive southward construction of the absaroka volcanic province which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient the greater green river and piceance creek basins were eventually filled from north to south with absarokaderived detritus at sedimentation rates 1 2 orders of magnitude greater than the underlying lake deposits

an emerging thermochronometer for carbonate bearing rocks 47 u pb assessing the thermal evolution of sedimentary basins is critical for understanding the origin of natural resources including ores geothermal fluids or hydrocarbons and for deciphering larger scale tectonic and geodynamic evolutions modern reconstructions of past subsurface temperatures mostly rely on thermochronometers that are not applicable to carbonate rocks e g fission track and u th he analyses here by coupling carbonate clumped isotope 47 thermometry and laser ablation u pb geochronology on a complete paragenetic sequence we demonstrate the applicability of an emerging thermochronometer for carbonate bearing rocks paired 47 and u pb data were obtained for calcite and dolomite phases precipitated in a middle jurassic carbonate hydrocarbon reservoir of the paris basin depocenter france the absolute thermochronological data allow the precise reconstruction of the thermal history of these rocks from shallow burial temperatures 40 c occurring in the late jurassic toward a progressive burial and heating stage up to 87 c during the cretaceous followed by a cooling stage down to 69 c during the tertiary uplift of the basin the inferred time temperature path based on 47 u pb data is mostly consistent with the thermal scenario independently deduced from organic maturity indicators from the underlying lower jurassic shales the 47 u pb thermochronological data also highlight a thermal anomaly during aptian albian time that requires revisiting the accepted timing for hydrocarbon migration in the middle jurassic reservoir carbonates

analogue benchmarks of shortening and extension experiments we report a direct comparison of scaled analogue experiments to test thereproducibility of model results among ten different experimental modelling laboratories we present results for two experimen

coupled greenhouse warming and deep sea acidification in the middle eocene 1 the middle eocene climatic optimum meco is an enigmatic warming event that represents an abrupt reversal in long term cooling through the eocene in order to further assess the timing and nature of this event we have assembled stable isotope and calcium carbonate concentration records from multiple deep sea drilling project and ocean drilling program sites for the time interval between 43 and 38 ma revised stratigraphy at several sites and compilation of 18o records place peak warming during the meco event at 40 0 ma chron c18n 2n the identification of the 18o excursion at sites in different geographic regions indicates that the climatic effects of this event were globally extensive the total duration of the meco event is estimated at 500 ka with peak warming lasting 100 ka assuming minimal glaciation in the late middle eocene 4 6 c total warming of both surface and deep waters is estimated during the meco at the study sites the interval of peak warming at 40 0 ma also coincided with a worldwide decline in carbonate accumulation at sites below 3000 m depth reflecting a temporary shoaling of the calcite compensation depth the synchroneity of deep water acidification and globally extensive warming makes a persuasive argument that the meco event was linked to a transient increase in atmospheric pco2 the results of this study confirm previous reports of significant climatic instability during the middle eocene furthermore the direct link between warming and changes in the carbonate chemistry of the deep ocean provides strong evidence that changes in greenhouse gas concentrations exerted a primary control on short term climate variability during this critical period of eocene climate evolution

early cenozoic evolution of topography climate and stable isotopes in precipitation in the north american cordillera paleoelevation reconstructions of the north american cordillera in ferred from the oxygen delta 18o and hydrogen delta d isotope ratios of terrestrial paleoclimate proxy materials soils ashes lake sediments suggest rapid north to south migration of topography in the early cenozoic pre 49 ma to 28 ma the validation of this reconstruction relies on an accurate understanding of the delta 18op and the associated regional climate change in response to the uplift of the western north america here we study this response using a global climate model gcm with explicit delta 18op diagnostics echam5 wiso focusing on the isotopic effects of different types of precipitation vapor mixing recycling and moisture source and compare the response against estimates made using a rayleigh distillation models of moist adiabatic condensation rdm four experiments are performed with eocene topography inferred from terrestrial stable isotope paleoaltimetry records to investigate how southward propagation of topography affects regional climate temperature precipitation and circulation pattern and dela 18op over north america our experiments predict delta 18op patterns that are broadly consistent with maps of temporally binned proxy delta 18o and generally support an early cenozoic north to south propagation of high topogra phy in the north american cordillera they do not support the commonly made assumption that isotopic fractionation occurs primarily through rainout following rayleigh distillation nor the application of modern empirical delta 18op lapse rates to past environments in our gcm simulations precipitation processes and climate changes that are not captured by rdms substantially affect delta 18op these processes include shifts in local precipitation type between convective and large scale rain and between rain and snow intensification of low level vapor recycling particularly on leeward slopes develop ment of air mass mixing and changes in wind direction and moisture source each of these processes can have significant 2 influences on delta 18op that are comparable in magnitude to surface uplift of hundreds or even thousands of meters in many regions these processes fortuitously compensate each other explaining the apparent agreement between echam5 wiso and proxy delta 18o and more broadly between rdm estimates and observed delta 18o elevation relationships in some regions compensation is incomplete and as a result echam5 wiso delta 18op does not agree with estimates from the rdm in these regions including the interior of the northern cordillera and the eastern flank of the southern cordillera moderate adjustments of paleoelevations may be in order

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

constraints on friction coefficients by an inverse analysis of sand box thrust dips abstract we measured the dips of thrusts forming in a uniform sand layer under horizontal shortening by a rigid emerging ramp in conditions of full erosion various ramp dips and four types of ramp material were used throughout the 12 experiments in three experiments the ramp was shortcut by a less steep thrust forming a spontaneous ramp in the sand the forward modelling proposed to predict these observations involves i computing the total dissipation in the structure by using global force equilibrium and a parameterised kinematic framework including the possibility of shortcut development and ii minimising the total dissipation of mechanical work with respect to the kinematic parameters i e the thrust dips for which two analytic solutions are found the inverse problem involves computing probability densities over the parametric space of friction coefficients including the internal friction angle of the sand and friction angles of the four materials used on the rigid ramps practically this involves computing a misfit using the l2 norm between the observed and predicted thrust dips for every possible value of the friction angles the resulting probability density provides all the quantitative information that can be retrieved from the experiments in the light of our forward modelling ramp friction angles exhibit sharp probability changes at values that trigger the occurrence of shortcuts thus yielding non trivial bounds on the ramp friction values that are directly related to the observations of ramp and back thrust geometry also the probability distribution of the internal sand friction and those of the ramp frictions is in good agreement with independent measurements this approach is designed to be applicable to field studies of brittle deformation where geometries are observed and rheological parameters are sought

sedimentation and paleogeography of an eocene to oligocene rift zone idaho and montana examination of sedimentary basins in east central idaho and analysis of previous work in western montana indicate that late middle eocene to oligocene extension north of the eastern snake river plain was confined to a north to north northwest trending rift zone 100 25 km wide in east central idaho six remnants of half grabens preserve up to 2 6 km of eocene to oligocene boulder to pebble conglomerate sandstone and intercalated tuff the paleogene basin bounding normal faults strike north to north northwest dip west and accommodated up to 10 km of dip slip clasts at the base of each basin fill sequence were derived almost exclusively from underlying middle eocene volcanic rocks whereas higher in the section the detritus was derived increasingly from pre tertiary formations exposed in the footwall of the basin bounding normal faults the uppermost units of the basin fill deposits typically lack volcanic clasts pebble count data indicate that the basin fill deposits were produced by unroofing of proterozoic to tertiary rocks in the uplifted footwall east of each basin slip on the basin bounding normal faults created substantial relief and spawned gravity emplaced slide blocks and megabreccias that interfinger with thick alluvial fan deposits in the basins in contrast previous studies of the upper eocene to lower miocene renova formation in southwestern montana suggest that relatively low relief and tectonic quiescence characterized the region east of the proposed rift zone i speculate that a river flowing from the rift zone into a basin east of the rift shoulder may have carried fine grained sediment into the renova basin throughout east central idaho initial slip on the north to north northwest striking normal faults was closely associated with rhyolitic volcanism during the waning phases of challis magmatism ca 47 45 ma 40ar 30ar ages however show that sedimentation and extension outlasted volcanism by more than 15 m y and were largely amagmatic gravitational collapse of previously thickened crust probably drove this episode of extension within and west of the sevier fold and thrust belt i interpret the rift zone which can be traced into western montana and southern british columbia as a northward continuation of a west directed belt of late paleogene extension previously documented in the great basin

a middle eocene carbon cycle conundrum a 500 000 year long period of warmth in the middle eocene was marked by high atmospheric carbon dioxide concentrations and prolonged dissolution of carbonate in the deep oceans numerical simulations attempting to capture these features identify gaps in our understanding of the causes of this and similar perturbations

statistical analysis of an experimental compressional sand wedge the quasi static deformation of dry sand is widely used as an analogue to the brittle deformation of the upper crust the quantitative comparison of analogue to natural tectonics or to mechanical predictions requires identifying sources of biases and estimating the intrinsic variability of the experimental results we develop experimental and statistical methods that fulfill these requirements we consider an initially perfect wedge resting on a flat layer made of a uniform dry sand in a rectangular glass box the box is shortened lengthwise by translating one of its end walls towards the other the lateral walls can remain fixed or be translated with the moving end wall upon shortening the wedge is thrusted above the flat layer forming classical fore and backthrusts as essentially plane strain structures lifetimes locations and dips of all thrusts constitute seven quantifiable output parameters called observables in addition to the shortening forces monitored at both end walls during shortening up to seventy measurements of each observable were performed in seven final state cross sections of ten experiments a three step statistical analysis allows us to prove that first the observables vary independently justifying their modeling with independent distributions second the ergodic hypothesis holds meaning that along strike variations can be used to infer the intrinsic experimental variability measurements can thus be repeated on successive cross sections in each experiment third our data set is free from bias due to friction on the lateral walls or due to the finite length of the box we then construct statistical models of each observable using either gauss or laplace distributions for example forethrusts dip at 38 3 2 and backthrusts at 41 3 3 we finally show how to apply these statistical models to experiments using a different initial geometry the statistical methods presented here are applicable to experiments with different setups materials and observables although the ergodic hypothesis is relevant only to plane strain experiments

paleoenvironmental changes during the middle eocene climatic optimum meco and its aftermath the benthic foraminiferal record from the alano section ne italy the middle eocene climatic optimum meco was one of the most severe short term global climate perturbations of the cenozoic that occurred at ca 40 ma and was characterized by a gradual 4 6 c temperature increase of intermediate and deep waters we investigated the response to the meco of the deep sea ecosystem in the central western tethys through a quantitative study of bathyal benthic foraminiferal assemblages in the expanded and continuous alano section northeastern italy for which data on stratigraphy lithology isotope and trace element geochemistry and calcareous microplankton were available during the gradual warming of meco lasting between 350 and 650 kyr marine export productivity increased causing a significant but transient restructuring of benthic foraminiferal faunas which changed gradually from assemblages typical for oligo mesotrophic sea floor conditions to assemblages indicative of more eutrophic conditions just after the peak meco conditions which lasted less than 100 kyr a prolonged phase of environmental instability 500 kyr occurred marked by even more highly increased export productivity leading to bottom water oxygen depletion as reflected in deposition of organic rich sediments and multiple peaks of bi triserial opportunistic benthic foraminiferal taxa including buliminids bolivinids and uvigerinids the high productivity may have been caused by a strong influx of nutrient bearing fresh water into the basin due to the increased vigour of the hydrological cycle during the warm period and this increased fresh water influx might have been a factor in enhancing water column stratification thus exacerbating the hypoxic conditions which persisted about 400 500 kyr after deposition of the organic rich layers the environmental perturbation ended and benthic foraminiferal assemblages recovered while conditions became very similar to what they were before the meco the environmental disturbance during and directly after the meco thus strongly but transiently affected benthic foraminiferal assemblages in the central western tethys

u pb dating of calcite cement and diagenetic history in microporous carbonate reservoirs case of the urgonian limestone france microporous carbonates can constitute excellent hydrocarbon reservoirs if their micro pore and or nanopore structure is sufficiently developed and continuous in such deposits assessing the exact timing of reservoir property stabilization is critical to better understand the postdepositional processes favorable to the creation or preservation of porosity however placing reliable and accurate chronological constraints on the formation of microporosity in these reservoirs is a major challenge in this study we performed absolute u pb dating of calcite cements occurring in the urgonian microporous limestone northern tethys margin of southeastern france u pb ages ranging between 96 7 4 9 ma and 90 5 1 6 ma were obtained on the major calcitic phase responsible for the cementation and therefore the stabilization of microporosity indicating that this diagenetic process occurred synchronously at the regional scale following an extended subaerial exposure our results show that 1 the mineralogical stabilization process responsible for the formation of an excellent pervasive microporous network took place relatively early and 2 the so acquired reservoir quality was preserved for more than 90 m y these observations emphasize the importance of long exposure periods and associated meteoric influx for the formation and preservation of good microporous reservoirs

rapid change in high elevation precipitation patterns of western north america during the middle eocene climatic optimum meco we present eocene terrestrial oxygen 18o carbon 13c and strontium 87sr 86sr isotope lake and paleosol records from two high elevation sites on the western north american plateau these records represent terrestrial stable isotope evidence of the middle eocene climatic optimum meco an enigmatic and rapid global warming event that interrupted protracted eocene global cooling at around 40 0 ma revised stratigraphy based on 15 40ar 39ar ages from air fall and water lain ash in the elko basin nevada places evaporative lake conditions followed by rapid 20 decrease in 13c values already during the pre meco cooling phase requires that in contrast to pre meco climate conditions meco temperature seasonality in western north america was sufficiently strong to re establish regular lake overturn the rate and magnitude of the rapid negative lacustrine 18o shift during meco in the elko lake basin is inconsistent with a simple scenario of regional surface uplift affecting decrease of 18o in precipitation and associated lake freshening we consider the overall negative shift in 18o of non evaporatively 18o enriched lake waters 9 3 1 8 between the 42 to 43 ma elko formation and the ca 38 ma base of the indian well formation to be composed of a ca 3 to 4 permil decrease in 18o of lake carbonate as a consequence of post meco cooling and an additional ca 5 to 6 permil decrease in 18o of riverine lake input as a response to 43 to 38 ma surface uplift when combined with isotope enabled global circulation model results the elko lake data further suggest that once critical elevations were attained a change in upstream moisture transport including strengthening of monsoonal summer rainfall on the south eastern flanks of the cordillera and a larger fraction of air parcel trajectories to the eastern lee of the cordilleran highlands that had passed over the western continental interior were responsible for the rapid middle eocene decrease in 18o of precipitation in the central cordilleran hinterland

seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils from central new mexico usa in order to better calibrate pedogenic carbonate as a proxy for past environments we compared the stable isotopic composition of soil co 2 soil water and pedogenic carbonate in young soils from central new mexico usa seasonal changes in the 13 c value of soil co 2 the 18 o value of soil water and the soil temperature were monitored to establish the timing of isotopic equilibrium with the carbonate calcite solubility was calculated from measured temperatures and co 2 concentrations in the soil this approach allowed us to determine the conditions associated with pedogenic carbonate formation carbon isotope equilibrium oxygen isotope equilibrium and minimum calcite solubility all occurred simultaneously during warm dry conditions in may 2008 when soil co 2 concentrations were low it is therefore concluded that pedogenic carbonate forms during warm dry periods and does not record mean growing season conditions as typically assumed the seasonal bias in pedogenic carbonate formation may explain the occurrence of pedogenic carbonate in monsoon climates and its absence in regions where annual precipitation is more uniformly distributed the implications of the seasonal bias for stable isotope based paleoenvironmental reconstructions are that paleoelevations may have been previously over or underestimated paleoatmospheric co 2 concentrations likely have been signifi cantly overestimated and pedogenic carbonate provides a c 4 biased record of paleovegetation especially in dry soils

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

sadiola hill a world class carbonate hosted gold deposit in mali west africa the 8 moz sadiola hill gold deposit is located in the kedougou kenieba inlier a window of deformed ca 2200 to 2050 ma rocks that crop out in eastern senegal and western mali the geology of the inlier differs from other paleoproterozoic granite greenstone belts and sedimentary basins by the abundance of carbonate rocks the sadiola hill gold deposit occurs within 3 km of the senegal mali shear zone and country rocks in the region have undergone polycyclic deformation the lithostratigraphy of the open pit consists of impure limestones overlain by younger sequences of detrital sedimentary rocks which include arenite wacke and siltstone the sedimentary rocks have been subjected to regional greenschist facies metamorphism and have been intruded by multiple generations of synkinematic calc alkaline stocks and dikes the bulk of the ore is hosted within a n s to nnw trending 10 to 50 m wide brittle ductile dilational shear zone defined as the sadiola shear zone gold lodes also occur along an array of steep nne trending shear zones geometric and kinematic analyses indicate that the ore hosting structures were undergoing sinistral displacement at the time of mineralization locally defined as the d3s nnw sse shortening event hydrothermal alteration is polyphase and includes an early high temperature calc silicate phase i e porphyroblastic growth of actinolite tremolite followed by a potassic phase i e biotite calcite quartz k feldspar tourmaline actinolite that was synchronous with ore mineral deposition paragenetic studies reveal a multistage ore development that includes an as rich sulfide stage followed by an au sb stage the ore is associated with a metal enrichment suite of au as sb cu fe w mo ag bi zn pb te the sadiola hill deposit shares a similar relative timing and structural setting to that of other world class orogenic gold systems in the west african craton e g ashanti loulo with gold deposition occurring during a period of transcurrent tectonics after the cessation of region wide compressional deformation the high temperature paragenesis at sadiola hill is atypical of gold mineralization in the craton and indicates that the late eburnean tectono magmatic activity between ca 2080 and 2060 ma played an important role in the dynamics of hydrothermal fluid circulation along the senegal mali shear zone