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

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

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

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

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

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

mid tertiary structural evolution of the old woman mountains area implications for crustal extension across southeastern california investigations in the old woman mountains area provide insights into mid tertiary extensional tectonism in southeastern california the old woman mountains area is a moderately extended region that lies between two highly extended terranes the central mojave extensional complex and the colorado river extensional corridor two normal faults and a high angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks these blocks correspond to the four mountain ranges of the area the old woman piute little piute and ship mountains other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys deformation occurred in part before but primarily after deposition of the peach springs tuff an 18 5 ma regional stratigraphic marker transport directions of hanging walls are westward on the west side of the old woman mountains area and east southeastward on the east side we suggest that the old woman mountains area is a less extended portion of a continuous extensional terrane that stretches from the central mojave desert across the colorado river to the transition zone of the colorado plateau lower strain in the old woman mountains area than in most of the colorado river extensional corridor and central mojave extensional complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low angle simple shear zone shifted at about 18 20 ma from a position far to the west to just east of the old woman mountains area

correlation of the peach springs tuff a large volume miocene ignimbrite sheet in california and arizona the peach springs tuff is a distinctive early miocene ignimbrite deposit that was first recognized in western arizona recent field studies and phenocryst analyses indicate that adjacent outcrops of similar tuff in the central and easten mojave desert may be correlative this proposed correlation implies that outcrops of the tuff are scattered over an area of at least 35 000 km2 from the western colorado plateau to barstow california and that the erupted volume allowing for posteruption crustal extension was at least several hundred cubic kilometres thus the peach springs tuff may be a regional stratigraphic marker useful for determining regional paleogeography and the time and extent of tertiary crustal extension

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

thrust tectonics and the role of evaporites in the ionian zone of the albanides this study focuses on the alpine fold and thrust belt in the ionian basin of albania where there are giant onshore oil fields the evaporites have acted as a detachment layer but also have been cut by thrusts that carry evaporites to the surface in their hanging walls where the detachment is located at the top of the evaporites they are dragged up in the footwall along thrusts and faults develop at the base and the top of the evaporite simple shear deformation along the thrusts causes greater than 80 thinning of the evaporite so that sheets of only 40 200 m thickness are preserved whereas the original evaporite thickness probably was well in excess of 2000 m where ramps cut through the evaporite and cause repetition of the complete evaporite layer the salt is remobilized into salt filled compressional anticlines or escapes at the surface to produce smeared out surface extrusions the most spectacular example of this is the dumrea structure where the evaporites have developed into a complex shape from an overthrusted allochthonous sheet that has been extruded at the surface the dumrea structure is up to 5 6 km thick proved by drilling and has been displaced laterally by up to 25 km and it has the effective geometry of a giant sheath fold

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

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

nature of the southwestern boundary of the central mojave tertiary province rodman mountains california tertiary volcanic and sedimentary rocks in the central mojave desert are largely confined to the barstow bristol trough an elongate tectonic depression filled with lower miocene strata the southwestern boundary of the trough is sharply defined and separates thick 2 km tertiary sequences from an area to the southwest that is virtually devoid of tertiary rocks segments of this boundary have been interpreted as a buttress unconformity as a right slip sidewall fault that formed the lateral boundary of an early miocene extensional terrane and as a dip slip fault caused by flexure of the north american plate above the subducted mendocino fracture zone the silver bell fault in the rodman mountains central mojave desert is the best exposed segment of the boundary and relations exposed along it can be used to test these hypotheses the tertiary sequence north of the silver bell fault is at least 2 3 km thick and comprises at least 1 9 km of mafic lava flows overlain by and intercalated with coarse clastic rocks derived from the south side of the fault stratigraphic relations require faulting during volcanism and indicate that the minimum throw across the fault is at least equal to the thickness of the exposed tertiary section tuff layers intercalated in the lower part of the clastic deposits are 23 24 m y old indicating that faulting and volcanism were contemporaneous with extension and plutonism in the nearby central mojave metamorphic core complex structural and stratigraphic data indicate that the silver bell fault is primarily a dip slip fault the tertiary section is folded into a west trending upright to overturned syncline located near the southern edge of exposed tertiary rocks the syncline records late cenozoic north south shortening along the northwest striking calico fault which in this area accommodated about 10 km of right slip and perhaps 1 km of reverse slip southwestward dipping strata in the areally extensive north limb of the syncline resemble tilted strata in nearby highly extended areas and have been interpreted to reflect crustal extension however bedding dip does not decrease upward in the tertiary section paleocurrent directions trend obliquely updip and northeast dipping normal faults are rare stratal tilting postdated the accumulation of the exposed section which therefore was not deposited during rotational normal faulting the new data contradict interpretation of the silver bell fault as a major right slip fault bounding the central mojave extensional terrane although they permit small magnitude extension across the fault the amount of early miocene crustal extension in this area therefore is apparently small in spite of stratal tilting and the lateral limit of large magnitude crustal extension in the central mojave block is farther northwest the data are consistent with the fault having formed by failure during plate flexure above the subducted mendocino fracture zone

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

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

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

phase equilibrium geobarometers for silicic rocks based on rhyolite melts part 3 application to the peach spring tuff arizona california nevada usa establishing the depths of magma accumulation is critical to understanding how magmas evolve and erupt but developing methods to constrain these pressures is challenging we apply the new rhyolite melts phase equilibria geobarometer based on the equilibrium between melt quartz and two feldspars to matrix glass compositions from peach spring tuff arizona california nevada usa high silica rhyolite we compare the results to those from amphibole geothermobarometry projection of glass compositions onto the haplogranitic ternary and glass sio2 geobarometry quartz 2 feldspar rhyolite melts pressures span a relatively small range 185 230 mpa consistent with nearly homogeneous crystal compositions and are similar to estimates based on projection onto the haplogranitic ternary 250 50 mpa and on glass sio2 255 275 mpa amphibole geothermobarometry gives much wider pressure ranges temperature independent 65 300 mpa temperature dependent 75 295 mpa amphibole only 80 950 mpa average anderson and smith am mineral 80 549 559 1995 blundy and holland contrib miner petrol 104 208 224 1990 or holland and blundy contrib miner petrol 116 433 447 1994 thermometer a b pressures are most similar to phase equilibria results 220 210 190 mpa respectively crystallization temperatures determined previously with rhyolite melts 742 c zr in sphene 769 20 c and zircon saturation 770 780 c geothermometry are similar but temperatures from amphibole geothermometry 450 955 c are notably different the average anderson and smith holland and blundy 1994 thermometer b 710 c temperature is most consistent with previous estimates the rhyolite melts geobarometer effectively culls glass compositions affected by alteration or analytical issues peach spring glass compositions that yield pressure estimates reveal a tight range of plausible na2o and k2o contents suggesting that low na2o and high k2o contents of many peach spring samples are due to alteration use of altered whole pumice compositions in rhyolite melts simulations is likely the cause of the incorrect crystallization sequence reported previously for peach spring compositions using the rhyolite melts geobarometer we estimate a more realistic composition for peach spring tuff high silica rhyolite and the calculated composition finds close matches with some analyzed rocks and results in the expected sequence of crystallization

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

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

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

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

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

miocene synextension sedimentation patterns whipple mountains southeastern california implications for the geometry of the whipple detachment system a sedimentary basin formed in the southern and eastern whipple mountains during early to middle miocene detachment faulting stratigraphic relations and facies patterns indicate that the shape and size of the sedimentary basin was controlled both by fault activity and by sedimentary processes high angle normal faults formed the southwestern and perhaps also the northeastern boundary of the half graben whereas a transfer fault probably formed the southeastern boundary the transfer fault influenced the earliest synextension sedimentation in the basin the location of this fault and similar faults in the mohave mountains and aubrey hills of arizona is inferred to have been controlled by the position of primary irregularities or corrugations of the detachment fault shifts in facies patterns particularly a shift in the position of the basin depocenter reflect changes in basin geometry comparison with adjacent areas is necessary to distinguish between tectonic and nontectonic causes a shift in depocentral position parallel to regional transport direction in the whipple mountains basin is interpreted as resulting from a change in the location of the high angle normal fault forming the southwestern margin of the basin whereas a shift perpendicular to transport direction probably resulted from overtopping of topographic barriers within a fault block the most active phase of faulting occurred prior to deposition of stratigraphic units younger than the 18 5 ma peach springs tuff relative uplift of the core of the whipple mountains probably as a result of isostatic rebound following tectonic denudation influenced sedimentation patterns after deposition of the peach springs tuff

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

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

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

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

mineralogy and temporal relations of coexisting authigenic minerals in altered silicic tuffs and their utility as potential low temperature dateable minerals abstract coexisting fine grained 0 1 20 m authigenic silicate minerals separated from altered tuffs in miocene and plio pleistocene lacustrine deposits were characterized petrographically and using x ray powder diffraction the authigenic minerals are dominated by clinoptilolite erionite phillipsite k feldspar silica calcite smectite and randomly interstratified illite smectite minor accessories of opal ct cristobalite and barite are present with the major alteration minerals authigenic minerals from altered tuffs were dated using the k ar method to evaluate the utility of these minerals for determining the time of alteration in low temperature diagenetic environments the eruption ages of some of these zeolite rich tuffs were determined using the 40 ar 39 ar method on single sanidine and plagioclase minerals the k ar isotopic ages of the fine grained k feldspar show minimal variation compared with results from the clinoptilolite separates the isotopic ages from the authigenic k feldspar 15 13 8 ma and some of the zeolites 16 6 7 ma are similar to the eruption ages of the tuffs and indicate early alteration despite their open framework structure zeolites apparently can retain part or all of their radiogenic argon under favorable conditions e g saturated environment how much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the k ar method although zeolite isotopic ages should be interpreted with caution they may be useful to constrain temporal relations of low temperature diagenetic processes when used in conjunction with other dateable minerals

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

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

periodic features of seismic activity in albania by statistica means the temporal variation of the seismic activity in albania is discussed in this study we have investigated the seismic energy release for this century also using the periodogram technique to study the time series of the seismic energy release

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

evidence for periodic seismicity in the inner aegean seismic zone the most complete and reliable data of strong m s 6 5 shallow h 70 km earthquakes which occurred in the inner aegean seismic zone have been utilized to describe its seismicity time variation during 1800 1986 by two independent statistical models the first is a sequentially stationary model of seismicity rates which shows that intervals of low seismicity rate lasting for some 37 years alternate with high rate intervals of 8 12 years duration the second model is a statistical model according which seismic energy released within 5 year time windows approximates a harmonic curve within a period of about 50 years this model is in agreement with the notion that the time series of strong earthquake occurrences in the inner aegean seismic zone consists of a random shocks withm s 6 5 6 8 and a nonrandom component m s 6 9 maxima and minima of the harmonic curve coincide with the high and low rate intervals respectively a model of regional stationary accumulation of thermal stresses along certain seismic belts and their cyclic relaxation may explain this periodicity

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

migration of tertiary volcanism in the southwestern united states and subduction of the mendocino fracture zone abstract a compilation of 417 isotopic dates on mid and late cenozoic igneous rocks from the southwestern united states shows that volcanism migrated northward with time the principal locus of volcanism at any given time was an east west band that corresponded closely with the calculated position of the subducted mendocino fracture zone mfz under the north american plate this correspondence supports the theory that volcanism was triggered by subduction of the mfz which was a major 1 km north facing topographic step in the farallon plate both volcanism and the mfz moved northward at about 3 1 cm year andesites and rhyolites show close correspondence to mfz passage but many basalts were erupted significantly later cooling dates on basement rocks in southern arizona cluster at the time the mfz passed under that area earlier models proposing rapid late tertiary steepening of the subducted farallon plate were based on a westward sweep of volcanism our compilation shows no evidence for such a westward sweep

background seismicity rates and precursory seismic quiescence imperial valley california the problem of recognizing precursory seismicity rate changes consists of two steps first a background or normal rate must be determined and second anomalous rates must be recognized in this paper we address the first of these steps determining a background seismicity rate two problems must be dealt with if a meaningful background rate is to be determined first dependent events must be recognized and removed from consideration and second detection or reporting changes must be accounted for a number of techniques are available for recognizing dependent events we used the one developed by mcnally 1976 in this work the technique we used for recgonizing detection and reporting changes was developed by habermann 1983 we demonstrate these techniques on seismicity from the imperial valley region of california after a reasonable background rate is determined we examine seismicity rates in this region prior to the imperial valley earthquake of 15 october 1979 the source area of the earthquake shows a possible precursory quiescence which started during july 1979 the time of this rate decrease is close to the time of a change in the seismic instrumentation in the imperial valley which makes it difficult to determine whether the decrease is real or related to detection several other seismicity rate changes observed in the imperial valley appear to be related to changes in the operation of the network there these include quiescent periods which occur only if magnitude cutoffs are applied to the data and periods of low reporting of events with m 2 5 these observations suggest that magnitudes were not reported for some events during several time periods all of these observations indicate that determination of detection histories is crucial for meaningful seismicity studies

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

regional gravity studies in southeastern california western arizona and southern nevada bouguer gravity anomaly low pass and band pass maps of southeastern california western arizona and southern nevada are analyzed to determine regional crustal structure based on 100 250 km band pass wavelength filtering of the bouguer anomaly map a low amplitude gravity maximum is associated with the highly extended terrane along the colorado river region of california arizona and nevada two crustal models constructed using bouguer gravity anomaly data and constrained by geological mapping aeromagnetic data and seismic refraction data are consistent with the dome shaped midcrustal lens of probable dioritic composition centered under the whipple mountains proposed by mccarthy et al 1989 although other models are not precluded this midcrustal lens extends from the western edge of the colorado river extensional corridor to east of the buckskin and hualapai mountains in arizona dioritic material is exposed in various mountain ranges in the colorado river extensional corridor and may be part of the source of the gravity maximum that is associated with each range

ignimbrite correlation using whole rock geochemistry an example from the sulcis sw sardinia italy ignimbrites are useful chronological markers in the geological record at local and regional scales they also provide information on the dynamics of the eruption that produced them making their study of great importance in terms of volcanic hazard assessment however their study is usually hampered by their lateral variation and discontinuity when stratigraphic and lithologic criteria are not sufficient for correlation purposes the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension volume and facies variations whole rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics these may introduce vertical and horizontal geochemical heterogeneity in the final deposit in addition the occurrence of zoned ignimbrites due to magma supply of changing composition is common in this work we show that if appropriately used whole rock geochemistry can be a valid and highly useful tool for ignimbrite correlation we provide an example from the study of an ignimbrite sequence containing 18 units sensu lato in the sulcis region sw sardinia italy a protocol has been developed for unit recognition based on successive simple binary diagrams where the whole rock composition of a problem sample can be plotted immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering the diagrams provided here are designed for the sulcis but the methodology followed to develop them may be applied to other study areas

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

hafnium oxygen neodymium strontium and lead isotopic constraints on magmatic evolution of the supereruptive southern black mountains volcanic center arizona u s a a combined lass zircon whole rock study the 700 km 3 peach spring tuff pst erupted at 18 8 ma from the silver creek caldera in the southern black mountains volcanic center sbmvc of western arizona is the only supereruption scale ignimbrite in the northern colorado river extensional corridor the sbmvc contains pre and post caldera volcanic rocks and caldera related intrusions 19 17 ma that provide a detailed petrologic record of ignimbrite antecedence and aftermath whole rock sr nd pb hf isotopic data combined with complementary zircon o and hf isotopic data from a suite of pre through post pst samples provide robust constraints on 1 how the sbmvc evolved with respect to magmatic sources and processes throughout its 2 ma history and 2 the petrogenetic relationships between the pst and slightly younger intracaldera plutons both pre and post pst units have isotopic ranges nd 8 3 to 11 6 hf 8 2 to 14 0 87 sr 86 sr i 0 709 0 712 206 pb 204 pb 18 19 18 49 207 pb 204 pb 15 60 15 62 208 pb 204 pb 38 95 39 29 that fall within the spectrum of miocene colorado river extensional corridor rocks and are consistent with mixing of substantial fractions of proterozoic mojave crust and juvenile material derived from regional enriched mantle compared to the pst which has relatively uniform isotopic ratios nd 11 4 to 11 7 hf 13 8 to 14 3 87 sr 86 sr i 0 709 0 712 206 pb 204 pb 18 20 18 29 207 pb 204 pb 15 60 15 62 208 pb 204 pb 39 02 39 33 individual pre and post pst units are isotopically more variable and generally more primitive consistent with whole rock isotopes zircon hf 8 to 14 and oxygen 18 o 4 5 to 7 2 for most pre and post pst units also have wider ranges and more mantle like values than those of the pst 12 to 15 6 1 to 7 1 moreover zircon isotopic compositions decrease in post pst samples a few zircons from post pst intrusions have 18 o values lower than mantle values whole rock and zircon elemental and isotopic analyses indicate that 1 most pre and post pst units are less evolved and less homogenized than the pst itself 2 intrusions in the silver creek caldera are petrogenetically distinct from the pst and therefore represent discrete magmatic pulses not unerupted pst mush 3 enriched mantle input increased in the sbmvc following the paroxysmal pst eruption 4 post pst history of the sbmvc was characterized by periodic influx of magmas with varying juvenile fractions into pre existing mushy or solidified intrusions resulting in variable and incomplete hybridization and 5 melting and assimilation of hydrothermally altered crust played a relatively minor role in the generation and evolution of magmas in the sbmvc

heterogeneities of the instrumental seismicity catalog 1904 1980 for strong shallow earthquakes we analyze the world wide consistency of teleseismic reporting completeness of the seismicity record and homogeneity of magnitude determination for strong shallow earthquakes magnitude m 6 depth h 70 km in the period 1904 to 1980 for earthquakes with magnitude m 7 we used the catalog given by abe 1981 which lists the surface wave magnitude ms of every reported large event that occurred between 1904 and 1980 according to the original formulation of gutenberg 1945 under the postulate that the rate of earthquake occurrence for the entire world as a whole is consistent on a time scale of decades and that since the installation of the world wide standardized seismograph network in the early 1960s the earthquake catalog for strong m 6 shocks is complete and the seismicity rates are typical of all periods in the century we find that for instrumental related reasons the ms of large events prior to 1908 have been systemically overestimated by at least 0 5 magnitude unit and the ms of events in the period 1908 to 1948 have been consistently overestimated by 0 2 unit relative to the ms assigned to shocks occurring after 1948 when these corrections are taken into account the catalog of events with ms 7 0 is shown to be largely complete i e nearly all the large shallow shocks which occurred in the earth are listed for most of the world since early in the century for earthquakes as the magnitude 6 level we used the international seismological center isc tape and regional catalogues of earthquakes this file compiles the earthquake data given by gutenberg and richter 1954 for the first half of the century and data from various international agencies after 1945 we find a highly incomplete seismicity catalog at this magnitude level for the first half of the century mainly due to numerous increases and decreased in the earthquake detection and reporting capabilities a more complete but extremely inhomogeneous catalog for the period 1950 to 1963 due to the use of different formulations and criteria to calculate and report the parameter magnitude by the various seismological agencies and a largely complete and homogeneous in ms record since 1964

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

basal layered deposits of the peach springs tuff northwestern arizona usa basal layered deposits of the large volume peach springs tuff occur beneath the main pyroclastic flow deposit over a minimum lateral distance of 70 km in northwestern arizona usa the basal deposits are interpreted to record initial blasting and pyroclastic surge events at the beginning of the eruption the pyroclastic surges traveled a minimum of 100 km from the as yet unknown source changes in bedding structures with increasing flow distance are related to the decreasing sediment load of the surges some bed forms in the most proximal part of the study area kingman arizona can be interpreted as being shock induced reflecting a blast origin for the surges component analyses support a hydrovolcanic origin for some of the blasting and subsequent pyroclastic surges the eruption apparently began with magmatic blasts which were replaced by hydrovolcanic blasts hydrovolcanic activity may be partially related to failure of the conduit walls that temporarily plugged the vent a single large volume pyroclastic flow immediately followed the blast phase and no evidence has been observed for a plinian eruption column the stratigraphic sequence indicates that powerful hydrovolcanic blasting rapidly widened the vent thus bypassing a plinian fallout phase and causing rapid evolution to a collapsing eruption column similar processes may occur in other large volume ignimbrite eruptions which commonly lack significant plinian fallout deposits

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

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

a catalogue of seismicity in greece and adjacent areas summary a new earthquake catalogue for greece has been formed to cover the instrumental period 1901 78 in particular 605 earthquakes for the period 1917 63 inclusive are relocated using first arrival data from the international seismological summary these relocations incorporate macro seismically and other well controlled master events into an ensuing joint epicentre determination technique the largest annual average shift is 165 km for 78 earthquakes during the decade after 1917 decreasing to 17 km for the later years 1957 63 magnitudes are redetermined mainly using readings from the swedish network and uppsala since as early as 1908 catalogue completeness exists for magnitudes around 5 5 for at least the last 60 years but magnitudes of about 4 7 are completely reported only during the most recent 15 years the new epicentral positions lead to a better delineation of the seismic zones than has previously been achieved the majority of shallow earthquakes are contained in a belt parallel to the hellenic arc which extends north into albania in the south east the epicentral locations give a more diffuse extension of this zone into the west coast of turkey depths of intermediate earthquakes along the hellenic arc tend to relocate to shallower depths in the south west part of crete no earthquake focus deeper than 100 km is found although in the south eastern section of the arc a tendency to increased depths is observed a second zone starts at leukas island in the west and extends through central greece to near volos on the east coast where it divides into two branches which are less well defined but which eventually join the seismicity of western turkey a third zone follows the saronikos and corinth gulfs

miocene extension and fault related folding in the highland range southern nevada a three dimensional perspective abstract the highland range of southern nevada contains a major anticline and syncline that constitute the westernmost segments of the black mountains accommodation zone in the highly extended colorado river extensional corridor the folds are defined by thick tilted sections of miocene volcanic and sedimentary strata that accumulated immediately prior to and during regional extension the folds are generally symmetrical with interlimb angles that exceed 90 subhorizontal hingelines and steeply inclined axial surfaces east and west dipping normal faults dominate the west and east tilted limbs of the folds respectively the limbs of the folds are parts of major half grabens tilt fanning within these half grabens and 15 new 40ar 39ar dates bracket major extension between about 16 5 and 11 ma tilting of the opposing fold limbs occurred simultaneously and was contemporaneous with extension the anticline and syncline are therefore interpreted as fault related extensional folds produced by the partial along strike overlap of oppositely dipping normal fault systems and attendant tilt block domains the anticline developed between overlapping listric normal faults that dip toward one another including the east dipping mccullough range and west dipping keyhole canyon faults each limb of the anticline is a rollover fold developed in the hanging wall of the inwardly dipping listric normal faults the syncline formed between overlapping outwardly dipping listric faults as adjacent fault blocks were tilted toward one another the dominant folding style was fault bend folding with drag folding and displacement gradient folding playing subsidiary roles the anticline and syncline significantly affected depositional patterns with synextensional units including two major ash flow tuffs thinning toward the anticlinal hinge and thickening toward the synclinal hinge the black mountains accommodation zone is largely composed of intersecting northwest trending anticlines and northeast trending synclines both occurring at a variety of scales depending on the amount of overlap between the opposing normal fault systems this three dimensional geometry contrasts with the typical style of folding in contractional settings and may characterize extensional accommodation zones that include the partial along strike overlap of multiple oppositely dipping normal faults the folds in the highland range serve as well exposed analogues of similar features in hydrocarbon rich areas of submerged continental margins

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

an introduction to the application of x ray microtomography to the three dimensional study of igneous rocks imaging rocks in three dimensions through x ray microtomography enables routine visualization of structures in samples which can be spatially resolved down to the sub micron scale although x ray tomography has been applied in biomedical research and clinical settings for decades it has only recently been applied to studies of rocks and few geoscientists realize its value and potential this contribution provides an introduction to the principles and techniques of x ray microtomography to the study of igneous rock textures as well as reviewing the current state of the art we hope that this short review will encourage more geoscientists to apply x ray microtomography in their research and that this will lead to new insights into the processes that occur in magmatic as well as other geological systems

correlation of ignimbrites in the central anatolian volcanic province using zircon and plagioclase ages and zircon compositions abstract episodes of high eruptive fluxes 10 3 km 3 year in continental environments are associated with magmatism related to subduction post orogenic collapse intra plate hot spots or rifting during such episodes voluminous ignimbrite deposits are produced which cover landscapes over 10 4 10 5 km 2 in such sequences brief eruptive recurrence and chemical similarity limit the applicability of geochronological and geochemical correlation methods here we present complementary geochronological data 40 ar 39 ar plagioclase eruption and 206 pb 238 u zircon crystallization ages for ignimbrites from the miocene holocene central anatolian volcanic province cavp in addition we successfully employed zircon geochemistry trace elements oxygen isotopes as an alteration resistant indicator to correlate rhyodacitic to rhyolitic ignimbrites whose eruption age differences are too brief to be resolved by 40 ar 39 ar geochronology by applying this method we dismiss previous correlations between stratigraphic members i e sofular and gordeles sofular and sarimadentepe but demonstrate close relationships for other cavp ignimbrites i e kavak units 1 to 4 cemilkoy ignimbrite and overlying fallout deposits our chronostratigraphy reveals two previously unrecognized eruptive pulses at 9 8 ma and 7 5 ma which are characterized by increasing magmatic temperatures 75 100 c within each cycle despite a long term 10 ma eruptive productivity that is about one order of magnitude smaller than in other magmatically active continental plateaus the cavp achieved high eruptive fluxes during brief 1 2 ma intervals