sequence age and source of silicic fallout tuffs in middle to late miocene basins of the northern basin and range province the latest cenozoic 6 ma ash beds in the western united states have been intensively studied for several decades the more widespread of these ash beds are well documented event horizons that are of great value in studies of the timing and pace of geological climatological and biological events throughout the region because explosive volcanism was not restricted to latest neogene time in this region many older ash beds are likely to prove as useful as younger beds as event horizons once they are located characterized and dated as a first step in developing a useful chronology of older cenozoic ash beds in the western united states we have sampled and analyzed silicic fallout tuffs in middle to late miocene sedimentary basins across the northern basin and range province the northern basin and range basins ideally situated in the vicinity of major coeval silicic volcanic centers contain numerous relatively unaltered silicic fallout tuffs we have correlated tuffs between all sampled sections on the basis of glass shard composition the composite stratigraphic sequence established by the correlations contains more than 200 individual tuffs including 59 widely distributed tuffs termed correlative tuffs the tuffs vary widely in composition but most are in one of two compositional groups gray metaluminous vitric tuffs gm tuffs or white metaluminous vitric tuffs wm tuffs distribution patterns compositional characteristics and correlation with ash flow tuffs show that the source for most gm tuffs was the snake river plain volcanic province along the northern edge of the northern basin and range and the source for most wm tuffs was the southwestern nevada volcanic field in the southern part of the northern basin and range the northern basin and range tuffs range in age from ca 16 6 ma the ages of individual tuffs are determined variously by direct isotopic dating by correlation to previously dated fallout and ash flow tuffs or by interpolation age estimation ages for most tuffs are known to within 0 25 m y 1 or less and for many tuffs to within 0 1 m y or less the sequence and ages of tuffs established in this study provide insights into the evolution of the northern basin and range basins and patterns of explosive volcanism in coeval volcanic centers and contribute to the development of a high resolution stratigraphy and chronology of coeval sedimentary deposits throughout the western united states

sedimentology stratigraphy and paleoclimate at the late miocene coffee ranch fossil site in the texas panhandle abstract the late miocene coffee ranch fossil assemblage contains some of the earliest evidence of c4 herbivory in north america however little is known regarding the environmental setting associated with the fauna and a general lack of detailed paleoclimate data exists for the late miocene from the north american continental interior in this study the sedimentary environments stratigraphy geochronology and paleoclimate are interpreted for a series of outcrops of the late miocene ogallala formation in the texas panhandle that includes the coffee ranch locality updated magnetostratigraphy coupled with previously published geochronology indicates that all exposed strata were deposited over a 277 kyr interval within chron c3an 2n from 6 42 6 70 ma depositional environments include fluvial channels floodplain ponds floodplain paleosols eolian paleosols riverine tufa and reworked volcanic ash trunk and tributary fluvial channels are differentiated using channel dimensions sedimentary structures and bounding surface architecture and indicate seasonally variable discharge paleosols from nine pedofacies are described and preserve a spectrum of weakly developed entisols and weakly to moderately mature inceptisols and vertisols constitutive mass balance calculations reveal that mature paleosols formed along distinct pedogenic pathways calcic vertisols accumulated pedogenic carbonate and exhibited either net volumetric dilation or collapse as a result of mineral weathering in contrast non calcic vertisols show patterns of decalcification and variable degrees of volumetric dilation and collapse the presence of the paleosols described in this study requires paleoprecipitation between of 900 1150 mm yr approximately double modern values and a westward shift of the continental udic ustic soil moisture boundary by 400 km during the late miocene this finding suggests that climates in the southern midcontinent were not sufficiently arid to select for c4 photosynthetic pathways over c3 photosynthesis in the late miocene

rheomorphic ignimbrites of the rogerson formation central snake river plain usa record of mid miocene rhyolitic explosive eruptions and associated crustal subsidence along the yellowstone hotspot track rogerson graben usa is critically placed at the intersection between the yellowstone hotspot track and the southern projection of the west snake river rift eleven rhyolitic members of the re defined 420 m thick rogerson formation record voluminous high temperature explosive eruptions emplacing extensive ashfall and rheomorphic ignimbrite sheets yet each member has subtly distinct field chemical and palaeomagnetic characteristics new regional correlations reveal that the brown s view ignimbrite covers 3300 km2 and the wooden shoe ignimbrite covers 4400 km2 and extends into nevada between 11 9 and 8 ma the average frequency of large explosive eruptions in this region was 1 per 354 ky about twice that at yellowstone the chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation this was followed by a trend towards less evolved rhyolites that may record melting and hybridisation of a mid crustal source region contemporaneous magmatism induced crustal subsidence of the central snake river basin is recorded by successive ignimbrites offlapping and thinning up the n facing limb of a regional basin margin monocline which developed between 10 59 and 8 ma the syn volcanic basin topography contrasted significantly with the present day elevated yellowstone hotspot plateau concurrent basin and range extension produced the n trending rogerson graben early uplift of the shoshone hills 10 34 ma was followed by initiation of the shoshone fault and an e sloping half graben 10 3 10 1 ma the graben asymmetry then reversed with initiation of the brown s bench fault 8 ma which remained intermittently active until the pliocene

identification of the source caldera for a pliocene ash flow tuff in northeast japan based on apatite trace element compositions and zircon u pb ages abstract in tephrochronology trace element compositions of apatite phenocrysts are useful because they change in response to various magmatic conditions as apatite crystallizes moreover because apatite tolerates burial diagenesis and welding processes without chemical changes its trace element compositions are a powerful tool for correlating pre quaternary tephra however this method is not extensively applied the hirosegawa tuff composed of a pair of pliocene ash fall and ash flow tuffs is a distinctive marker tephra that is used in geological mapping and for correlating sedimentary rocks in the eastern part of central ne japan however the source volcano of the hirosegawa tuff has not been identified because many calderas coexisted in ne japan during the pliocene analysis of the trace element compositions of apatite major element compositions of ilmenite and u pb zircon ages demonstrate that depending on the area the previously mapped hirosegawa tuff comprises different volcanic ejecta furthermore this study identified that the source volcano of the hirosegawa tuff in the type section was the shiroishi caldera located 50 km south of sendai city

comment on a synchronized dating of three greenland ice cores throughout the holocene by b m vinther et al no minoan tephra in the 1642 b c layer of the grip ice core denton j s pearce n j g 2008 comment on a synchronized dating of three greenland ice cores throughout the holocene by b m vinther et al no minoan tephra in the 1642 b c layer of the grip ice core journal of geophysical research 113 d04303 7 pp

rheology of the ronne ice shelf antarctica inferred from satellite radar interferometry data using an inverse control method 1 the antarctic ice sheet is surrounded by large floating ice shelves that spread under their own weight into the ocean ice shelf rigidity depends on ice temperature and fabrics and is influenced by ice flow and the delicate balance between bottom and surface accumulation here we use an inverse control method to infer the rigidity of the ronne ice shelf that best matches observations of ice velocity from satellite radar interferometry ice rigidity or flow law parameter b is shown to vary between 300 and 900 kpa a 1 3 ice is softer along the side margins due to frictional heating and harder along the outflow of large glaciers which advect cold continental ice melting at the bottom surface of the ice shelf increases its rigidity while freezing decreases it accurate numerical modelling of ice shelf flow must account for this spatial variability in mechanical characteristics citation larour e e rignot i joughin and d aubry 2005 rheology of the ronne ice shelf antarctica inferred from satellite radar interferometry data using an inverse control method geophys res lett 32 l05503 doi 10 1029 2004gl021693

deep sea ash layers reveal evidence for large late pleistocene and holocene explosive activity from sumatra indonesia abstract deep sea tephra layers sampled from sediment cores collected within and adjacent to the sunda trench of offshore sumatra reveal evidence for five previously undocumented and apparently large minimum volume 0 6 6 3 km3 volcanic explosivity index values of 4 5 explosive eruptions over the last 31 000 years with a presumptive source of mainland sumatra chemical analysis of glass shards and 14c age constraints are used to distinguish the five tephra layers as well as a sixth that likely correlates with the youngest toba tuff ytt the tephra layers are labeled v 1 through v 6 relative to their north to south positioning along the sunda trench the three tephra layers taken from cores west of central sumatra v 3 v 4 v 5 are well constrained by 14c age determinations whereas less reliable sedimentation rate estimates are available for the northern v 1 v 2 and southern v 6 tephra layers deposition of the northernmost tephra layer v 1 was likely accompanied by seismicity as two chemically indistinguishable tephras are separated by 12 cm of course grained turbidite layer v 2 shows a strong chemical resemblance to the ytt and age estimates do not rule out the correlation with the exception of a likely correlation with the ytt no other correlations were made between the tephras analyzed in this study with the marine or terrestrial record from the published literature the most frequent widespread and youngest marine tephra layers were found in the central region of the study area layers v 3 v 4 and v 5 were all deposited within the last 17 thousand years with minimum eruptive volumes of 0 6 to 5 2 km3 a complex depositional sequence of layer v 6 is estimated at 27 5 ka and may be associated with late pleistocene ignimbrite volcanism of southern sumatra the ages and suggested minimum volumes represented by the deep sea tephra layers are consistent with an active volcanic arc and demonstrate the need for further terrestrial studies

sierra nevada basin and range transition near reno nevada two stage development at 12 and 3 ma relative and absolute elevations of the sierra nevada and adjacent basin and range province timing of their differentiation and location amount and timing of strike slip movement between them are controversial the provincial boundary near reno developed in two stages 1 at ca 12 ma the 700 km 2 verdi boca sedimentary basin formed across what was to become the boundary probably as a result of a small magnitude but regional extensional episode that affected much of the western basin and range 2 at 3 ma the basin was complexly faulted and folded during a larger magnitude extensional episode that established the modern sierran structural and topographic boundary in this area the boundary is really a transition zone with a western edge along the donner pass california fault zone which is farther west than previously placed both episodes appear to have resulted from east west extension only which suggests that northwest motion of the sierra nevada relative to the basin and range shown by geodetic data began after 3 ma or was taken up farther east

a tephra lattice for greenland and a reconstruction of volcanic events spanning 25 45 ka b2k tephra layers preserved within the greenland ice cores are crucial for the independent synchronisation of these high resolution records to other palaeoclimatic archives here we present a new and detailed tephrochronological framework for the time period 25 000 45 000 a b2k that brings together results from 4 deep greenland ice cores in total 99 tephra deposits the majority of which are preserved as cryptotephra are described from the ngrip neem grip and dye 3 records the major element signatures of single glass shards within these deposits indicate that 93 are basaltic in composition all originating from iceland specifically 43 originate from grimsvotn 20 are thought to be sourced from the katla volcanic system and 17 show affinity to the kverkfjoll system robust geochemical characterisations independent ages derived from the gicc05 ice core chronology and the stratigraphic positions of these deposits relative to the dansgaard oeschger climate events represent a key framework that provides new information on the frequency and nature of volcanic events in the north atlantic region between gs 3 and gi 12 of particular importance are 19 tephra deposits that lie on the rapid climatic transitions that punctuate the last glacial period this framework of well constrained time synchronous tie lines represents an important step towards the independent synchronisation of marine terrestrial and ice core records from the north atlantic region in order to assess the phasing of rapid climatic changes during the last glacial period

mid silurian back arc spreading at the northeastern margin of gondwana the dapingzhang dacite hosted massive sulfide deposit lancangjiang zone southwestern yunnan china abstract the dapingzhang dacite hosted volcanogenic massive sulfide deposit is located in a late paleozoic collisional zone the sanjiang fold and thrust belt characterized by a collage of gondwana derived terranes at the southern margin of the south china yangtze block this area has experienced repeated paleozoic and mesozoic collisional and extensional events and is overprinted by himalayan strike slip and thrust movements the dapingzhang orebody is strongly sheared into discontinuous ore lenses and blocks and is hosted by a several hundred meter thick volcanic rock sequence of dacite with minor metabasalt spilite rhyolite and chert bulk rock nd isotope data give end 429 ma values of 2 to 5 and indicate a dominantly mantle source or origin from young continental crust tdm 800 1000 ma the dacites have distinctly low abundances of ti nb ta zr th and rees which is typical of subduction related volcanism the base metal mineralization occurs as polymetallic massive sulfide mineralization mainly pyrite sphalerite galena chalcopyrite tetrahedrite tennantite and locally barite and as pyrite chalcopyrite stockworks pervasive hydrothermal alteration is dominantly quartz sericite maximum enrichment in gold bismuth selenium and tellurium is at the interface between the stockwork and massive sulfide mineralization styles deeper parts of the stockwork zone are characterized by elevated molybdenum and rhenium whereas the distal parts to the massive sulfide mineralization have high mercury antimony zinc cadmium arsenic lead and silver the metal association and mineral assemblage are typical of volcanic rock hosted massive sulfides in back arc volcanic settings uranium pb dating on zircon by la icp ms defines an age of 429 3 ma 2 n 19 for the dacite sequence rhenium os isotope data on mo rich bulk ore samples define an isochron of 429 10 ma 2 n 9 mswd 0 21 initial 187os 188os 3 1 1 9 common os is very low and the absolute abundances of 187re and 187os define a model age of 429 4 ma n 9 95 confidence the mid silurian dacite sequence and associated massive sulfide mineralization identify a hitherto unknown early rifting and back arc seafloor spreading event at the northeastern gondwana margin related to the early evolution of the paleotethys ocean

geochemical correlation of three large volume ignimbrites from the yellowstone hotspot track idaho usa three voluminous rhyolitic ignimbrites have been identified along the southern margin of the central snake river plain as a result of wide scale correlations new volume estimates can be made for these deposits 350 km3 for the steer basin tuff and cougar point tuff xi and 1 000 km3 for cougar point tuff xiii these volumes exclude any associated regional ashfalls and correlation across to the north side of the plain which has yet to be attempted each correlation was achieved using a combination of methods including field logging whole rock and mineral chemistry magnetic polarity oxygen isotope signature and high precision 40ar 39ar geochronology the steer basin tuff cougar point tuff xi and cougar point tuff xiii have deposit characteristics typical of snake river sr type volcanism they are very dense intensely welded and rheomorphic unusually well sorted with scarce pumice and lithic lapilli these features differ significantly from those of deposits from the better known younger eruptions of yellowstone the ignimbrites also exhibit marked depletion in 18o which is known to characterise the sr type rhyolites of the central snake river plain and cumulatively represent 1 700 km3 of low 18o rhyolitic magma feldspar values 2 3 2 9 erupted within 800 000 years our work reduces the total number of ignimbrites recognised in the central snake river plain by 6 improves the link with the ashfall record of yellowstone hotspot volcanism and suggests that more large volume ignimbrites await discovery through detailed correlation work amidst the vast ignimbrite record of volcanism in this bimodal large igneous province

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america the mountain ranges of arizona and adjacent california and nevada contain large areas underlain by proterozoic anorogenic granites comprising the southwesternmost portion of a transcontinental belt of 1 4 1 5 ga old anorogenic complexes that extends across north america northeast into labrador of these a two mica monazite bearing granitic suite resides in central and southeastern arizona as part of a peraluminous subprovince that is bordered on the south southern arizona to sonora and west western arizona and adjacent portions of california and nevada by marginally metaluminous granites bearing biotite sphene hornblende and fluorite all of these 1 4 ga granites are distinctly more potassic iron enriched relative to mg and depleted in ca mg and sr in contrast to typical orogenic granitoids in general the large ion lithophile element enriched composition is a consequence of limited melting of a water deficient crustal source at depths greater than 25 37 km for the peraluminous granites this contrast is less extreme perhaps resulting from a larger degree of melting as a consequence of a greater metasedimentary component and water in its crustal source the anorogenic granitic magmas intruded into the upper crust at depths of 8 17 km or shallower at temperatures up to 790 c the most dramatic variation in the crystallization intensive parameters resides in the oxygen fugacity which spans three orders of magnitude relative to other anorogenic suites all of the magmas crystallized at elevated levels of o2 as reflected in their assignment to the anorogenic magnetite series yet a regionally significant rise in primary o2 levels unmatched elsewhere in the transcontinental belt occurs for plutons in western arizona including the holy moses and hualapai granites the most extreme case is the hualapai granite whose biotite fe fe mg ratios drop due to high o2 to a low of 0 27 down from more typical levels of 0 54 to 0 75 such extreme variations in primary levels of oxygen fugacity must be an indirect imprint of regional changes of the level of oxidation of the lower crust the high fo2 holy moses and hualapai plutons have intruded near the regional boundary between the metaluminous and peraluminous granites and appear to be imaging a major change in the level of oxidation of the lower crust this boundary is also approximately equivalent to significant changes in the nd and pb isotopic compositions of these granites and the metamorphic and magmatic character of the older orogenic terrane on a global scale the crust forming orogenies ended by 1 6 ga ago and the continents entered a long lived era dominated by localized extension and transcontinental intrusion of anorogenic potassic rapakivi granite mafic dike swarms charnockite and anorthosite the absence of orogenic deformation implies that plate consumption became intraoceanic during this time the profuse and widespread nature of the igneous activity has no phanerozoic analogue and is considered to be unique to the proterozoic a crustal overturn model ties the magmatism to heating within a largely undepleted subcontinental mantle the eventual rise of mantle plumes and the transfer of heat into the youthful undifferentiated proterozoic crust subsequent melting and rise of potassic granitic magmas from the lower crust leads to considerable crustal reorganization a process that would continue until both the mantle and crust reached a stable configuration

identification of aniakchak alaska tephra in greenland ice core challenges the 1645 bc date for minoan eruption of santorini pearce nicholas westgate j a eastwood w j preece s j 2006 identification of aniakchak alaska tephra in greenland ice core challenges the 1645 bc date for minoan eruption of santorini geochemistry geophysics and geosystems 5 pp q03005 rae2008

mid pleistocene silicic tephra beds in the auckland region new zealand their correlation and origins based on the trace element analyses of single glass shards abstract the trace element composition of individual glass shards from twelve distinct mid pleistocene silicic tephra beds from the auckland region new zealand has been determined by laser ablation la icp ms analysis this is the first application of such data to tephra correlation studies in new zealand these analyses support earlier major element based correlations between three surface sections oruarangi and okariha at port waikato and st kentigern s at east auckland and a long core patiki road west auckland the correlations provide a higher resolution record of taupo volcanic zone tvz activity than is preserved in more proximal deposits where burial and erosion obscure much of the volcanic record a combination of major and trace element analysis enables the correlation of three units exposed in the auckland region with their proximal equivalents ignimbrites from the tvz namely deposits associated with 1 the ongatiti ignimbrite 1 21 0 04 ma 2 an un named post ahuroa pre unit e ignimbrite sample ut1274 from the rocky hill section king country and 3 the unit e ignimbrite 1 ma combined major and trace element data show a marked evolution in magma chemistry with the large ongatiti and unit e ignimbrite eruptions occurring after a change in composition caused by replenishment reactivation of the magma chamber changes in magma chemistry between these deposits resulted from a combination of assimilation of metasedimentary country rocks and periods of fractional crystallisation of a plagioclase dominated assemblage trace element data and the wider dispersal of the silicic tephras in the auckland region suggests that they are all sourced from the mangakino volcanic centre mvc in the tvz this study illustrates for the first time in a new zealand context the potential for correlating tephra deposits using single glass shard trace element chemistry

age model for a continuous ca 250 ka quaternary lacustrine record from bear lake utah idaho abstract the quaternary sediments sampled by continuous 120 m long drill cores from bear lake utah idaho comprise one of the longest lacustrine sequences recovered from an extant lake the cores serve as a good case study for the construction of an age model for sequences that extend beyond the range of radiocarbon dating from a variety of potential age indicators we selected a combination of radiocarbon ages one magnetic excursion correlated to a standard sequence and a single uranium series age to develop an initial data set the reliability of the excursion and u series data require consideration of their position with respect to sediments of inferred interglacial character but not direct correlation with other paleoclimate records data omitted from the age model include amino acid age estimates which have a large amount of scatter and tephrochronology correlations which have relatively large uncertainties because the initial data set was restricted to the upper half of the bl00 1 core we inferred additional ages by direct correlation to the independently dated paleoclimate record from devils hole we developed an age model for the entire core using statistical methods that consider both the uncertainties of the original data and that of the curve fitting process with a combination of our initial data set and the climate correlations as control points this age model represents our best estimate of the chronology of deposition in bear lake because the age model contains assumptions about the correlation of bear lake to other climate records the model cannot be used to address some paleoclimate questions such as phase relationships with other areas

sanjiang tethyan metallogenesis in s w china tectonic setting metallogenic epochs and deposit types abstract tectonically the sanjiang tethyan metallogenic domain stmd is located within the eastern himalayan tibetan orogen in the sanjiang tethys southwestern china although this metallogenic domain was initiated in the early palaeozoic extensive metallogenesis occurred in the late palaeozoic late triassic and himalayan tertiary epochs corresponding tectonic settings and environments in the domain are an arc basin system related to the subduction of the palaeo tethyan oceanic slabs a post collision crustal extension setting caused by the lithospheric delamination or slab breakoff underneath the sanjiang tethys during the late triassic large scale strike slip faulting and thrusting systems due to the indo asian continent collision since the palaeocene in this metallogenic domain important gold copper base metals rare metals and tin ore belts incorporating a large number of giant deposits were developed the main types of deposits include 1 porphyry copper deposits controlled by a large scale strike slip fault system 2 vhms deposits mainly occurring in intra arc rift basins and post collision crustal extensional basins 3 shear zone type gold deposits in the ophiolitic melange zone along the thrusting shearing system 4 hydrothermal silver polymetallic deposits in the triassic intra continental rift basins and tertiary strike slip pull apart basins and 5 himalayan granite related greisen type tin and rare metallic deposits within the metallogenic epochs of the late palaeozoic to cenozoic the styles and types of the ore deposits changed from vhms types in the late palaeozoic through exhalative sedimentary type deposits in the late triassic to porphyry type copper deposits shear zone type gold deposits hydrothermal vein type silver polymetallic deposits greisen type tin and rare metal deposits in the cenozoic correspondingly ore forming metals also changed from a pb zn cu ag association through ag cu pb zn fe ag pb and ag au hg associations to ag cu pb zn cu mo au sn and li rb cs nb zr hf y ce sc associations

mass transfer along the yellowstone hotspot track ii kinematic constraints on the volume of mantle derived magma abstract this paper evaluates the neogene kinematic evolution of the eastern snake river plain esrp to estimate the volume of mafic magma transferred from mantle to crust during passage over the yellowstone hotspot four major kinematic processes affected the esrp including densification due to emplacement of gabbro into the middle crust subsidence and consequent filling of a 6 km deep basin with volcanic and plutonic rocks 20 extension via dike injection or faulting ductile attenuation and lower crustal flow from esrp to the adjacent basin and range two kinematic models involving these processes are proposed overall the two models are viable only if the initial crustal thickness were 43 47 km thick otherwise the lost mass of the esrp is not balanced by the excess mass in the basin and range in the first model densification eruption and extension by dike injection the esrp crust receives mantle derived gabbro equivalent to a 16 km thick layer but would need to lose 10 km of crust via lower crustal flow in the second model densification eruption and extension by faulting and ductile attenuation the esrp crust receives a 9 4 km equivalent thickness of mantle derived gabbro and would need to lose 6 km of crust via lower crustal flow the cumulative amount of mantle derived gabbro in the esrp crust derived by this kinematic analysis equivalent to a 9 4 16 km thick layer is nearly identical to the amount estimated independently using a mass balance petrologic model suggesting it is a robust result

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

stable isotopic evidence for a pre middle miocene rain shadow in the western basin and range implications for the paleotopography of the sierra nevada 1 a recently proposed method for approximating the paleorelief of mountain belts makes use of the predictable relationship between the isotopic depletion of precipitation and the net elevation of an orographic barrier over which an air mass rises this rain shadow effect often creates desert regions on the lee side of mountain belts in which precipitation is isotopically light changes in the isotopic composition of precipitation can be estimated from the isotopic composition of authigenic or pedogenic minerals which can then be used to infer both the development of relief during orogenesis as well as the stability of a rain shadow formed by developing mountains the 18o of smectites formed from the weathering of middle miocene to late pliocene volcanic ashes currently exposed in the rain shadow of the modern sierra nevada of california show no indication of large scale late cenozoic surface uplift of the sierra and corresponding regional rain shadow development rather smectite isotope data tentatively suggest that elevations may have decreased over this time by as much as 2000 m toward the southern end of the range and 700 m in regions farther north this suggests that the modern rain shadow cast over the western basin and range has been in existence since pre middle miocene and that the sierra nevada have been a prominent orographic barrier since before this time these interpretations are in accord with several recent studies also suggesting a possible cenozoic elevation loss of an already developed sierra nevada mountain range

geology of the ivanhoe hg au district northern nevada influence of miocene volcanism lakes and active faulting on epithermal mineralization the mercury gold deposits of the ivanhoe mining district in northern nevada formed when middle miocene rhyolitic volcanism and high angle faulting disrupted a shallow lacustrine environment sinter and replacement mercury deposits formed at and near the paleosurface and disseminated gold deposits and high grade gold silver veins formed beneath the hot spring deposits the lacustrine environment provided abundant meteoric water the rhyolites heated the water and the faults flow units and lakebeds provided fluid pathways for the hydrothermal fluids a shallow lake began to develop in the ivanhoe area about 16 5 ma the lake progressively expanded and covered the entire area with fine grained lacustrine sediments lacustrine sedimentation continued to at least 14 4 ma and periodic fluctuations in the size and extent of the lake may have been responses to both climate and nearby volcanism the eruption of rhyolite and andesite flows and domes periodically disrupted the lacustrine environment and produced interfingered flows and lake sediments the major pulse of rhyolitic volcanism took place between 15 16 0 05 and 14 92 0 05 ma high angle faulting began in the basement about 15 2 ma penetrated to and disrupted the paleosurface after 15 10 0 06 ma and largely ceased by 14 92 0 05 ma ground motion related to both faulting and volcanism created debris flows and soft sediment deformation in the lakebeds mercury gold mineralization was coeval with rhyolite volcanism and high angle faulting and it took place about 15 2 to 14 9 ma at and near the paleosurface hydrothermal fluids migrated through tuffaceous sediments above relatively impermeable volcanic and paleozoic units creating chalcedonic cinnabar bearing replacement bodies and sinters disseminated gold was deposited in sedimentary and volcanic rocks beneath the mercury deposits although the hydrologic path between the two ore types is unclear higher grade gold silver deposits formed in massive rhyolites and paleozoic quartzites at deeper levels and these mineralized zones possibly represent the feeder zones for the higher level deposits fluctuations in the ground water table locally produced hydrothermal oxidation of the near surface mercury and disseminated gold deposits the locus of mineralization shifted with time moving south and east from its inception point in the west central part of the district thus although mineralization in the district took place during a span of 300 000 years the duration of mineralization at any one place probably was much shorter the low sulfidation deposits of the ivanhoe district formed at the same time and under similar conditions as those in the nearby midas district 15 km to the northwest which includes the large high grade ken snyder gold silver epithermal vein deposit the exposures in the ivanhoe district are interpreted to represent the near surface example of the paleosurface that originally was present above the midas mineralizing system the resulting combined ivanhoe midas model provides an exploration guide for epithermal deposits in similar geologic environments in northern nevada

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

metallogeny of the northeastern gangdese pb zn ag fe mo w polymetallic belt in the lhasa terrane southern tibet abstract the northeastern gangdese pb zn ag fe mo w polymetallic belt ngpb characterized by skarn and porphyry deposits is one of the most important metallogenic belts in the himalaya tibetan continental orogenic system this belt extends for nearly four hundred kilometers along the luobadui milashan fault in the central lhasa subterrane and contains more than 10 large ore deposits with high potential for development three major types of mineralization system have been identified skarn fe systems skarn breccia pb zn ag systems and porphyry skarn mo cu w systems in this study we conducted a whole rock geochemical u pb zircon geochronological and in situ zircon hf isotopic study of ore forming rocks in the ngpb specifically the jiangga jiaduopule and rema skarn fe deposits and the yaguila pb zn ag deposit although some of these deposits porphyry mo systems formed during the post collisional stage 21 14 ma the majority these three systems developed during the main soft collision stage of the india asia continental collision 65 50 ma the skarn fe deposits are commonly associated with granodiorites monzogranites and granites and formed between 65 and 50 ma the ore forming intrusions of the pb zn ag deposits are characterized by granite quartz porphyry and granite porphyry which developed in the interval of 65 55 ma the ore forming porphyries in the sharang mo deposit formed at 53 ma the rocks from fe deposits are metaluminous and have relatively lower sio2 and higher cao mgo feo contents than the intrusions associated with mo and pb zn ag mineralization while the pb zn ag deposits are peraluminous and have high sio2 and high total alkali concentrations they all exhibit moderately fractionated ree patterns characterized by lower contents of heavy ree relative to light ree and they are enriched in large ion lithophile elements and relatively depleted in high field strength elements ore forming granites from fe deposits display 87sr 86sr i 0 7054 0 7074 and end t 4 7 to 1 3 whereas rocks from the yaguila pb zn ag deposit have 87sr 86sr i 0 7266 0 7281 and end t 13 5 to 13 3 in situ lu hf isotopic analyses of zircons from fe deposits show that ehf t values range from 7 3 to 6 6 with tdm hf c model ages of 712 to 1589 ma and yaguila pb zn ag deposit has ehf t values from 13 9 to 1 3 with tdm hf c model ages of 1216 to 2016 ma combined with existing data from the sharang mo deposit we conclude that the ore forming intrusions associated with the skarn fe and porphyry mo deposits were derived from partial melting of metasomatized lithospheric mantle and rejuvenated lower crust beneath the central lhasa subterrane respectively melting of the ancient continental material was critical for the development of the pb zn ag system therefore it is likely that the source rocks play an important role in determining the metal endowment of intrusions formed during the initial stage of the india asia continental collision

triassic volcanism in eastern heilongjiang and jilin provinces ne china chronology geochemistry and tectonic implications abstract with the aim of constraining the early mesozoic tectonic evolution of the eastern section of the central asian orogenic belt caob we undertook zircon u pb dating and geochemical analyses major and trace elements sr nd isotopes of volcanic rocks of the luoquanzhan formation and daxinggou group in eastern heilongjiang and jilin provinces china the analyzed rocks consist mainly of dacite and rhyolite with sio2 contents of 68 52 76 65 wt three samples from the luoquanzhan formation and one from the daxinggou group were analyzed using laser ablation inductively coupled plasma mass spectrometry la icp ms u pb zircon techniques three zircons with well defined oscillatory zoning yielded weighted mean 206pb 238u ages of 217 1 214 2 and 208 1 ma and one zircon with oscillatory zoning yielded a weighted mean 206pb 238u age of 201 1 ma these ages are interpreted to represent the timing of eruption of the volcanic rocks the triassic volcanic rocks are characterized by high sio2 and low mgo concentrations enrichment in large ion lithophile elements liles and light rare earth elements lrees depletion in high field strength elements hfses and heavy rare earth elements hrees 87sr 86sr i 0 7040 0 7050 luoquanzhan formation and 0 7163 0 7381 daxinggou group and end t 1 89 3 94 luoquanzhan formation and 3 42 3 68 daxinggou group these geochemical features indicate an origin involving the partial melting of juvenile lower crust nd model ages tdm2 of 651 821 ma and that compositional variation among the volcanic rocks arose from mineral fractionation and minor assimilation these volcanic rocks formed within an extensional environment following collision of the ncc and jiamusi khanka massif during the late paleozoic early triassic

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

independent tephrochronological evidence for rapid and synchronous oceanic and atmospheric temperature rises over the greenland stadial interstadial transitions between ca 32 and 40 ka b2k abstract understanding the dynamics that drove past abrupt climate changes such as the dansgaard oeschger do events depends on combined proxy evidence from disparate archives to identify leads lags and synchronicity between different climate system components independent and robust chronologies are required cryptotephrochronology is a key geochronological tool as cryptotephra horizons can act as isochrons linking disparate and or distant records here we investigated marine sediment core md99 2284 from the norwegian sea to look for previously identified greenland ice core cryptotephra horizons and define time parallel markers between the archives we explored potential secondary transport and depositional mechanisms that could hamper the isochronous integrity of such horizons we identified six cryptotephra layers of which four correlate to previously known greenland ice core horizons none of those were identified in other marine cores and thus this study contributes greatly to the north atlantic tephra framework tripling the original amount of existing isochrons between ca 25 and 60 ka b2k the latter allow a synchronization between md99 2284 and the greenland ice cores between ca 32 40 ka b2k which is in the north atlantic the shortest time interval during the last glacial period to be constrained by four independent tephra isochrons these findings provide essential tephra based evidence for synchronous and rapid oceanic and atmospheric temperature rises during the greenland stadial interstadial transitions furthermore it enables us to estimate the average peak duration of interstadial temperature overshoots at approximately 136 years as such this well targeted high resolution investigation successfully demonstrates the use of cryptotephra for geochronological purposes in the marine realm

a genetic linkage between subduction and collision related porphyry cu deposits in continental collision zones the genesis of continental collision related porphyry cu deposits pcds remains controversial the most common hypothesis links their genesis with magmas derived from subduction modified arc lithosphere however it is unclear whether a genetic linkage exists between collision and subduction related pcds here we studied jurassic subduction related cu au and miocene collision related cu mo porphyry deposits in south tibet the jurassic pcds occur only in the western segment of the jurassic arc which has depleted mantle like isotopic compositions e g 87 sr 86 sr i 0 7041 0 7048 e nd t as high as 7 5 and e hf t as high as 18 by contrast no jurassic pcds have been found in the eastern arc segment which is isotopically less juvenile e g 87 sr 86 sr i 0 7041 0 7063 e nd t 4 5 and e hf t 12 these results imply that incorporation of crustal components during underplating of jurassic magma induced copper accumulation as sulfides at the base of the eastern jurassic arc inhibiting pcd formation at this time miocene pcds are spatially confined to the jurassic arc and the giant miocene pcds cluster in its eastern segment where no jurassic pcds occur this suggests that the arc segment barren for subduction related pcds could be fertile for collision related pcds miocene ore forming porphyries have young hf model ages and sr nd hf isotopic compositions overlapping with those of the jurassic rocks in the eastern segment whereas contemporaneous barren porphyries outside the jurassic arc have abundant zircon inheritance and crustlike sr nd hf isotopic compositions these data suggest that remelting of the lower crustal sulfide bearing cu rich jurassic cumulates triggered by cenozoic crustal thickening and or subsequent slab break off led to the formation of giant miocene pcds the spatial overlap and complementary metal endowment between subduction and collision related magmas may be used to evaluate the mineral potential for such deposits in other orogenic belts

ice flow sensitivity to geothermal heat flux of pine island glacier antarctica model projections of ice flow in a changing climate are dependent on model inputs such as surface elevation bedrock position or surface temperatures among others of all these inputs geothermal heat flux is the one for which uncertainty is greatest in the area of pine island glacier antarctica available data sets differ by up to a factor of 2 5 here we evaluate the impact of such uncertainty on ice flow using sampling analyses based on the latin hypercube method first we quantify the impact of geothermal heat flux errors on ice hardness a thermal parameter that critically controls the magnitude of ice flow second we quantify the impact of the same errors on mass balance specifically on the mass flux advecting through thirteen fluxgates distributed across pine island glacier we contrast our results with similar uncertainties generated by errors in the specification of ice thickness model outputs indicate that geothermal heat flux errors yield uncertainties on ice hardness on the order of 5 7 with maximum uncertainty reaching 15 resulting uncertainties in mass balance remain however below 1 we discuss the uncertainty distribution and its relationship to the amount of heat available at the base of the ice sheet from friction viscous and geothermal heating we also show that comparatively errors in ice thickness contribute more to model uncertainty than errors in geothermal heat flux especially for fast flowing ice streams 2012 american geophysical union all rights reserved

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

late miocene climate and life on land in oregon within a context of neogene global change abstract clarendonian 12 ma fossil soils plants molluscs fish and mammals of eastern oregon allow reconstruction of late miocene paleogeography paleoclimate and paleoecology on land between the global thermal maximum of the middle miocene 16 ma and global cooling and drying of the late miocene 7 ma six different pedotypes of paleosols recognized near unity and juntura allow reinterpretation of local mammalian paleoecology fossil beavers dominated gleyed entisols of riparian forest abundant camels and common hipparionine horses dominated alfisols of wooded grassland and grassy woodland diatomites overlying mammal bearing beds have bullhead catfish ictalurus ameiurus vespertinus as well as fossil leaves dominated by live oak quercus pollardiana fossil plants and soils of unity and juntura are most like those of grassy live oak woodland and savanna on the western slopes of the sierra nevada in northern california today fossil plants and soils indicate mean annual temperature of 12 9 7 7 17 7 c and mean annual precipitation of 879 604 1098 mm miocene paleoclimatic changes in eastern oregon show no relationship to changes in oxygen isotopic composition of marine foraminifera usually taken as an index of global paleoclimatic change mismatch between land and sea paleoclimatic records is most likely an artefact of global ice volume perturbation of oxygen isotopic values instead miocene paleoclimatic change in eastern oregon parallels changes in carbon isotopic composition of marine foraminifera presumably through fluctuations in greenhouse gases

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

grassland expansion as an instrument of hydrologic change in neogene western north america abstract the evapotranspiration et flux accounts for approximately two thirds of terrestrial precipitation worldwide and in grassland regions et is equivalent in magnitude to precipitation regional contributions to the terrestrial hydrologic budget however have been far from constant in the past as distribution of vegetation changed dramatically the rise of grass dominated ecosystems is one of the most profound paleoecological changes in the cenozoic why then would grassland expansion not feature prominently in the record of neogene hydrologic change despite numerous stable isotope paleoenvironmental studies in neogene north america the contributions of land cover change have been largely ignored we present a compilation of 16 oxygen isotope studies of pedogenic carbonate and smectite from western north america including 4 new records nearly all records from california the basin and range the rocky mountains and the great plains show increases in o 18 on the order of 2 6 in order to assess the role of et in the hydrologic cycle we developed an isotopic water vapor transport model wherein we manipulated et parameters along a specified air mass trajectory grasslands lead to o 18 of precipitation o p 18 values that are up to 5 greater than broadleaf and needleleaf vegetation at inland study sites these results demonstrate that changes in vegetation played a critical role in establishing the modern hydrologic regime in western north america we suggest that this isotopic increase is due to three primary reasons 1 increased evaporation and transpiration fluxes in grassland regions affect water balance 2 shallower rooting depths of grasses lead to the transpiration of soil water enriched in o 18 due to evaporation and 3 grasslands transpire o 18 rich waters during a shorter more punctuated growing season we argue that the observed isotope signals are indicative of a feedback mechanism wherein grasslands not only respond to regional and global climatic trends but also act as a driver of hydrologic change by enhancing seasonality and aridity grasslands transmit hydrologic disturbances downstream engineering climatic conditions favorable for their expansion

spatial and temporal variation of cenozoic surface elevation in the great basin and sierra nevada the surface uplift of mountain belts caused by tectonism plays an important role in determining the long term climate evolution of the earth however the general lack of information on the paleotopography of mountain belts limits our ability to identify the links and feedbacks between topography tectonics and climate change on geologic time scales here we present a 1 8 o and d record of authigenic minerals for the northern great basin that captures the timing and magnitude of regional surface uplift and subsidence events in the western united states during the cenozoic authigenic calcite smectite and chert 1 8 o values suggest the northern great basin region experienced 2km of surface uplift between the middle eocene and early oligocene followed by 1 to 2km of surface subsidence in the southern great basin and or sierra nevada since the middle miocene these data when combined with previously published work show that the surface uplift history varied in both space and time surface uplift migrated from north to south with high elevations in southern british columbia and northeastern washington in the middle eocene and development of surface uplift in north and central nevada in the oligocene this pattern of north to south surface uplift is similar to the timing of magmatism in the western cordillera a result that supports tectonic models linking magamtism with removal of mantle lithosphere and or a subducting slab

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

a mass conservation approach for mapping glacier ice thickness the traditional method for interpolating ice thickness data from airborne radar sounding surveys onto regular grids is to employ geostatistical techniques such as kriging while this approach provides continuous maps of ice thickness it generates products that are not consistent with ice flow dynamics and are impractical for high resolution ice flow simulations here we present a novel approach that combines sparse ice thickness data collected by airborne radar sounding profilers with high resolution swath mapping of ice velocity derived from satellite synthetic aperture interferometry to obtain a high resolution map of ice thickness that conserves mass and minimizes the departure from observations we apply this approach to the case of nioghalvfjerdsfjorden 79north glacier a major outlet glacier in northeast greenland that has been relatively stable in recent decades the results show that our mass conserving method removes the anomalies in mass flux divergence yields interpolated data that are within about 5 of the original data and produces thickness maps that are directly usable in high spatial resolution high order ice flow models we discuss the application of this method to the broad and detailed radar surveys of ice sheet and glacier thickness copyright 2011 by the american geophysical union

timing and development of the heise volcanic field snake river plain idaho western usa the snake river plain srp developed over the last 16 ma as a bimodal volcanic province in response to the southwest movement of the north american plate over a fixed melting anomaly volcanism along the srp is dominatedby eruptions of explosive high silica rhyolites and represents some of the largest eruptions known basaltic eruptions represent the final stages of volcanism forming a thin cap above voluminous rhyolitic deposits volcanism progressed generally from west to east along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera forming eruptions within a particular field separated by ca 0 5 1 ma similar to and in continuation with the present day yellowstone plateau volcanic field passage of the north american plate over the melting anomaly at a particular point in time and space was accompanied by uplift regional tectonism massive explosive eruptions and caldera subsidence and followed by basaltic volcanism and general subsidence the heise volcanic field in the eastern srp idaho represents an adjacent and slightly older field immediately to the southwest of the yellowstone plateau volcanic field five large volume 0 5 km 3 rhyolitic ignimbrites constitute a time stratigraphic framework of late miocene to early pliocene volcanism for the study region field relations and high precision 4 0 ar 3 9 ar age determinations establish that four of these regional ignimbrites were erupted from the heise volcanic field and form the framework of the heise group these are the blacktail creek tuff 6 62 0 03 ma walcott tuff 6 27 0 04 ma conant creek tuff 5 51 0 13 ma and kilgore tuff 4 45 0 05 ma all errors reported at 2a the fifth widespread ignimbrite in the region is the arbon valley tuff member of the star light formation 10 21 0 03 ma which erupted from a caldera source outside of the heise volcanic field these results establish the conant creek tuff as a distinct and widespread ignimbrite in the heise volcanic field eliminating former confusion resulting from previous discordant k ar and fission track dates new 4 0 ar 3 9 ar determinations when combined with geochemical lithologic geophysical and field data define the volcanic and tectonic history of the heise volcanic field and surrounding areas volcanic units erupted from the heise volcanic field also provide temporal control for tectonic events associated with late cenozoic extension in the snake range and with uplift of the teton range wyoming in the snake range movement of large 0 10 km 3 slide blocks of mississippian limestone exposed 50 km to the east of the heise field occurred between 6 3 and 5 5 ma and may have been catastrophically triggered by the caldera eruption of the 5 51 0 13 ma conant creek tuff this slide block movement of 300 vertical meters indicates that the snake range had significant relief by at least 5 5 ma in jackson hole the distribution of outflow facies of the 4 45 0 05 ma kilgore tuff related to eruption from the kilgore caldera in the heise volcanic field on the eastern srp indicates that the northern teton range was not a significant topographic feature at this time

the yellowstone hotspot in space and time nd and hf isotopes in silicic magmas abstract over the course of its 16 m y history the yellowstone hotspot has produced silicic magmas exhibiting systematic and often sympathetic variations in isotopic and chemical composition temperature and frequency of eruption nd and hf isotopic ratios vary systematically from initial eruptions at 16 ma contemporaneous with basaltic volcanism in eastern oregon and washington to the present day yellowstone volcanic plateau nd and hf isotopic ratios co vary and span the range of most terrestrial samples reflecting mixing of mantle and crustal sources earliest erupted silicic magmas were hot in excess of 1050 c relatively less evolved and have isotopic ratios within the range of contemporaneous columbia river flood basalts the transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the precambrian craton at 15 ma is marked by shifts in e nd from 4 to 11 and in e hf from 10 to 10 the duration of the transit yields a crustal magma source diameter of 70 km in the interval from 14 to 9 ma e nd systematically increases from 11 to 7 recording a minimum increase in the mantle component from 5 to 30 the mantle component could be twice as great depending upon the isotopic composition of crust and mantle reservoirs in this same interval peak temperatures of 1000 c occurred at 9 ma the last 8 m y are characterized by less frequent eruption of lower temperature 830 900 c and more compositionally evolved magmas

cryptotephrochronology of the eemian and the last interglacial glacial transition in the north east atlantic the eemian interglacial and the onset of the subsequent glacial period serve as the most recent analogue for the natural operation of the climate system during the current interglacial pronounced climatic oscillations occurred during this period but their nature and pattern are poorly understood due to dating limitations and unknown phase relationships between different regions and archives tephrochronology offers considerable potential for precise correlation of disparate palaeoclimatic archives preserving evidence of these rapid climatic transitions through the tracing of common isochronous tephra horizons we outline the identification of three previously unknown cryptotephra horizons within a marine core from the rockall trough north east atlantic this sequence preserves a high resolution record of this interval and shard size geochemical heterogeneity and the co variance of shard concentrations with ice rafted debris data are utilized to demonstrate that primary airfall was the most likely transport and depositional pathway the main geochemical populations of these horizons have similar transitional alkali major and trace element compositions suggesting that they were derived from a common icelandic source potentially the oraefajokull volcanic system these tephra horizons represent additions to the north atlantic event stratigraphy for this period and tentative correlations to icelandic terrestrial deposits are proposed for two horizons

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the sanjiang region sw china abstract the sanjiang region in se tibet plateau and nw yunnan is known to have formed by amalgamation of gondwana derived continental blocks and arc terranes as a result of oceanic subduction followed by continental collision from paleozoic to mesozoic in this paper we provide a synthesis of tectonic evolution magmatism and metallogeny in the region based on data from literatures early paleozoic ophiolites 473 439 ma in the changning menglian belt indicate the existence of a proto tethys ocean in this region two episodes of subduction related magmatism in the early paleozoic one occurred in the baoshan and tengchong blocks at 502 455 ma and the other occurred in the simao block at 421 401 ma are regarded as evidence for two different events of subduction of the proto tethys ocean at different locations the proto tethys was succeeded in early devonian by the paleo tethys which comprised the main ocean and three branches ailaoshan jinshajiang and garze litang the changning menglian main ocean existed from middle devonian to middle triassic the remnants of the oceanic crust are preserved in a few places in the longmu tso shuanghu suture as well as in the changning menglian ophiolite belt the eastward subduction of the main oceanic plate from early permian to early triassic formed a prominent arc terrane stretching 1500 km from yunnan to eastern tibet from the waning stage of subduction to post subduction numerous s type granite plutons with ages varying between 230 and 219 ma such as the lincang batholith in yunnan were emplaced at or close to the suture this event produced several hydrothermal w sn deposits in the region the tectonic evolution and associated magmatism of the jinshajiang and ailaoshan branch oceans are generally comparable to those of the main ocean however the branch oceans were subducted westward instead the garze litang branch ocean also underwent westward subduction from middle devonian to late triassic arc related high sr y porphyry intrusions and associated porphyry skarn cu mo au deposits are common in the jinshajiang ailaoshan region especially in the yidun arc which formed prior to jurassic the vms deposits in the sanjiang region formed in diverse tectonic settings including middle silurian back arc basins carboniferous oceanic islands paleozoic subduction zones and triassic post subduction rifting environments the mesozoic and early cenozoic evolution of the baoshan and tengchong blocks was largely influenced by eastward oceanic subduction of the meso and neo tethys from late permian to middle cretaceous and from late cretaceous to 50 ma respectively abundant early cretaceous granitoids and associated skarn type pb zn and sn fe deposits in the baoshan and tengchong blocks were produced in the background of the shan boundary oceanic slab subduction to the west and the break off of the nujiang bitu oceanic slab to the north the subduction of the neo tethys oceanic plate beneath the tengchong block from late cretaceous to paleogene formed abundant s type granitoids and many skarn type and greisen type sn w deposits granitoids formed at 105 to 81 ma and contemporaneous hydrothermal w mo ag and au deposits which temporally coincided with the subduction of the neo tethys are common in the yidun arc terrane

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

the nature and timing of crustal thickening in southern tibet geochemical and zircon hf isotopic constraints from postcollisional adakites abstract rising as the roof of the world the tibetan plateau is now underlain with the thickest continental crust on earth how and when was this crust formed which would have exerted pivotal controls to the formation of the plateau has long been an issue of hot debates this paper reports zircon u pb ages and hf isotope data for postcollisional 30 9 ma adakites in the southern lhasa terrane southern tibet a comparative analysis of whole rock rare earth element geochemistry and zircon hf isotopes between the adakites and associated gangdese igneous rocks suggests that the tibetan crust underwent a major phase of tectonic thickening between ca 45 and 30 ma in the region the lower part of the thickened crust consisted prevailingly of mafic lithologies which we argue to have resulted from intense basaltic underplating and subsequent remelting that took place during the late cretaceous and eocene time related to the neotethyan subduction processes including breakoff of the subducted slab at ca 50 ma in the early stage of the india asia collision these processes were responsible for not only the juvenile crust formation but also for the creation of a thermally softened lithosphere in the southern lhasa terrane the indentation of india consequently caused distributed lithospheric thickening with formation of an orogenic root beneath southern tibet root foundering during the oligocene gave rise to the adakitic magmatism regional topographic uplift and onset of northward underthrusting of the indian plate that has since played a key role in forming the entire tibetan plateau

environmental change in the great plains an isotopic record from fossil horses abstract carbon and oxygen isotope ratios of fossil horse tooth enamel from nebraska and texas show evidence for late neogene environmental changes in the great plains the earliest unambiguous c4 dietary signal among texas equids coincides with the age of the classic late hemphillian age coffee ranch fauna which we suggest is 6 6 ma based on volcanic ash correlations c4 vegetation was present in the diets of a small fraction of late hemphillian equids in nebraska and was thereafter ordinary in the diets of both nebraska and texas equids there is no unequivocal evidence for abundant c4 vegetation in the diets of pre late hemphillian equids and we suggest that the ensuing dietary change reflects c4 biomass expansion in the latest miocene carbon isotope ratios of post hemphillian horses in nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co occur with horse remains indicating that the two proxies carbon isotopes and presence absence of tort

stonehenge rhyolitic bluestone sources and the application of zircon chemistry as a new tool for provenancing rhyolitic lithics abstract the source of the bluestones at stonehenge has long been debated and while there is general consensus that the so called spotted dolerites are derived from a relatively small number of outcrops exposed in the highest parts of mynydd preseli in southwest wales the source of the rhyolitic component has attracted relatively little detailed attention this is largely because unlike the uniqueness of the spots in the coarser grained doleritic rocks the rhyolites are fine grained in character and lack any obvious distinctiveness especially in hand specimen this makes their provenancing difficult a recent study however suggested that there was a close lithological similarity between the informally termed rhyolite with fabric bluestone component and rhyolitic rocks from the ordovician fishguard volcanic group exposed in the pont saeson area of north pembrokeshire this study aims to see if the chemistry of zircons which are present in both sets of samples could be used to support the petrographical association analyses for certain high field strength elements including the rare earth elements obtained by la icp ms showed that indeed the analyses were nearly identical when compared using a range of statistical approaches including similarity coefficients statistical distance and principal component analysis while showing clear differences to sample sets which had no reason to be correlated with the pont saeson samples there are two important conclusions arising from this study firstly the identification of the pont saeson source of the rhyolite with fabric bluestone from outcrops in low ground to the north of the mynydd preseli will without doubt lead to fresh debates about the mechanisms of transport of this component of the bluestones to the stonehenge site secondly the chemistry of zircons may well prove to have a wider application in the provenancing of fine grained rhyolitic rocks which have an archaeological context

excision and the original low dip of the miocene pliocene bannock detachment system se idaho northern cousin of the sevier desert detachment geologic mapping and structural analysis in the bannock range se idaho indicate that the bulk of cenozoic extension in southeast idaho was accommodated by slip on the low angle bannock detachment system the bannock detachment system consists of low angle normal faults with a likely n s extent of 130 km it was active from ca 10 3 ma to ca 3 4 ma and accommodated 15 km of top to the wsw extension the master low angle detachment fault cuts steeper older normal faults and extensional folds in its hanging wall crosscutting relationships and prior work show that the hanging wall to the detachment system began as a cohesive block that later broke up along normal faults that either sole into or are cut by the master detachment fault after breakup began the master detachment fault was folded isostatically by a nnw trending extensional anticline the oxford ridge anticline the folding produced a new listric low angle normal fault east of the oxford ridge anticline to replace the back tilted portion of the detachment fault this new low angle normal fault excised part of the hanging wall cut hanging wall structures and served as a secondary breakaway for the detachment west of the oxford ridge anticline the master detachment fault initiated and slipped at a low angle is the youngest low angle normal fault of the system and has not been rotated to a low dip angle through time the exhumed bannock detachment system although somewhat smaller shorter lived and more disrupted by younger basin and range normal faults is similar to the buried sevier desert detachment system beneath central utah both top to the west systems formed near the transition between the sevier thrust belt and its hinterland collapsed broad thrust related culminations eroded to cambrian rocks are paired with partly coeval top to east metamorphic core complexes 100 160 km to the west lie east of major thrust ramps formed and slipped at low angles in the most extended part of the system and exhibit a translation and breakup phase of deformation both are late cenozoic in age and have voluminous lacustrine synrift deposits that record saline and alkaline conditions during early extension similar processes likely produced both detachment systems

new boundary conditions for the west antarctic ice sheet subglacial topography of the thwaites and smith glacier catchments airborne radar sounding over the thwaites glacier tg catchment and its surroundings provides the first comprehensive view of subglacial topography in this dynamic part of the west antarctic ice sheet wais and reveals that tg is underlain by a single broad basin fed by a dendritic pattern of valleys while smith glacier lies within an extremely deep narrow trench subglacial topography in the tg catchment slopes inland from a broad low relief coastal sill to the thickest ice of the wais and makes deep connections to both pine island glacier and the ross sea embayment enabling dynamic interactions across the wais during deglaciation simple isostatic rebound modeling shows that most of this landscape would be submarine after deglaciation aside from an island chain near the present day ross amundsen ice divide the lack of topographic confinement along tg s eastern margin implies that it may continue to widen in response to grounding line retreat

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

trace element and isotopic evidence for two types of crustal melting beneath a high cascade volcanic center mt jefferson oregon mt jefferson is an andesite dacite composite volcano in the cascade range the locus of andesite and dacite dominated volcanism for at least 1 million years a large trace element data set for mt jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model fc or afc for the generation of mt jefferson andesites several incompatible element zr nb y concentrations decrease in the range from basalt to andesite and then increase in the range from andesite to rhyodacite others ba rb la th remain constant or show a slight increase in the basalt to andesite range with modest increases from andesite to rhyodacite systematic variations in highly incompatible element ratios such as ba la and rb th suggest magma mixing dominates the trace element signatures rhyodacites are isotopically uniform 87sr 86sr 0 70325 0 70343 206pb 204pb 18 75 18 85 18o 6 3 0 3 whereas andesite and dacite are more variable 87sr 86sr 0 70291 0 70353 206pb 204pb 18 59 18 86 18o 6 0 0 6 typical basaltic andesite has 87sr 86sr 0 70326 0 70358 206pb 204pb 18 78 18 85 and 18o 5 9 0 4 sr rich 1 000 ppm basaltic andesite is more variable 87sr 86sr 0 70300 0 70360 206pb 204pb 18 70 18 89 18o 5 9 0 4 the data define mixing arrays with one end member at 87sr 86sr 0 7029 206pb 204pb 18 59 another at rhyodacite and a third at 87sr 86sr 0 7036 206pb 204pb 18 89 the first end member is defined by sr rich 800 1 200 ppm andesite with high al2o3 and low k2o ba and rb th the third one by k2o and very sr rich 2 000 ppm shoshonite isotopic data for basalts in northern oregon preclude any fractionation relationship between basalt and either rhyodacite or sr rich andesite e g the minimum 206pb 204pb ratio in basalt is 18 83 considered in light of geophysical models for the cascades these data suggest two types of crustal melting beneath the arc rhyodacite may be generated at 25 30 km depth by partial melting of arc basalt like amphibolite at 850 900 c sr rich andesite may be formed by partial melting of depleted morb like mafic granulite at 35 45 km depth at 1 000 1 100 c experimental and ree evidence supports these interpretations as does the restriction of sr rich andesite in the cascades to the area south of the 100 mw m2 heat flow contour between mt jefferson and mt hood thick crust and high heat flow are necessary to produce such andesite

petrogenesis of eocene mineralized porphyry in bijiashan eastern margin of tibet plateau constraints from geochronology geochemistry and hf isotopes abstract the ailaoshan jinshajiang porphyry belt located in the sanjiang area of eastern tibetan plateau is one of the major polymetallic metallogenic belts in china however the nature and genesis of magmatic intrusions associated with the porphyry copper mineralization in this belt remain contentious here we focus on the bijiashan deposit one of the large scale porphyry copper deposits in this area to gain insights on the host rock petrogenetic mechanism tectonic setting based on geochemistry zircon u pb geochronology and lu hf isotopic compositions zircon u pb dating of the ore hosting porphyry yielded an age of 35 6 0 2 ma the major elements of the porphyry show wide range in sio2 58 33 70 high k2o 2 83 8 85 and na2o k2o mostly 8 and moderate range in a cnk 0 56 1 17 our data suggest that the porphyry belongs to high kcalc alkaline alkaline and metaluminous peraluminous series the trace elements are characterized by enrichment in k rb ba la and nd but depletion in ta nb p and ti and enrichment in lree but depletion in hree with lree hree of 9 78 20 10 the main intrusion associated with mineralization corresponds to alkali richa type granitoid with high tfeo 0 49 3 4 low na2o k2o 0 23 1 62 and high 1000 ga al 2 17 2 64 except 1 57 and right declinedree patterns however some of the geochemical features also are consitent with the typical characteristics of c adakite rocks including high contents of al2o3 13 49 16 12 and sr 400 ppm lower mgo mostly

extensive soft sediment deformation and peperite formation at the base of a rhyolite lava owyhee mountains sw idaho usa in the northern owyhee mountains sw idaho a 200 m thick flow of the miocene jump creek rhyolite was erupted on to a sequence of tuffs lapilli tuffs breccias and lacustrine siltstones of the sucker creek formation the rhyolite lava flowed over steep palaeotopography resulting in the forceful emplacement of lava into poorly consolidated sediments the lava invaded this sequence liquefying and mobilising the sediment propagating sediment subvertically in large metre scale fluidal diapirs and sediment injectites the heat and the overlying pressure of the thick jump creek rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the sucker creek formation units and the formation of metre scale loading structures simple and pendulous load casts detached pseudonodules density contrasts between the semi molten rhyolite and liquefied sediment produced highly fluidal rayleigh taylor structures local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment the result is a 30 40 m zone beneath the rhyolite lava of extremely deformed stratigraphy brittle failure and folding is recorded in more consolidated sediments indicating a differential response to loading due to the consolidation state of the sediments the lava sediment interaction is interpreted as being a function of 1 the poorly consolidated nature of the sediments 2 the thickness and heat retention of the rhyolite lava 3 the density contrast between the lava and the sediment and 4 the forceful emplacement of the lava this study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture

stable isotope chemistry of fossil bone as a new paleoclimate indicator abstract during fossilization bone is thought to recrystallize and alter chemically on timescales of kyr to a few tens of kyr i e similar to the timescale for formation of soils therefore c and o isotope compositions of bone apatite should correlate with trends in soil water composition and aridity and serve as paleoclimate indicators this hypothesis was tested by analyzing c and o isotope compositions of the co 3 component of fossil bone apatite from mid oligocene through late pleistocene units in oregon and western idaho including the john day 19 4 30 0 ma mascall 15 2 15 8 ma and rattlesnake 7 2 7 8 ma formations whose paleosol sequences have been studied in detail and the juntura 10 11 ma hagerman 3 2 ma and fossil lake 18 o values provide a baseline of meteoric water compositions stable isotope compositions of bone co 3 do change in response to broad climatic trends but show poor correlation with compositions of corresponding paleosol co 3 at specific horizons instead compositional deviations between bone and paleosol co 3 correlate with compositional deviations with the next higher paleosol this suggests that the timescale for fossilization exceeds one paleosol cycle based on stratigraphic evidence and simple alteration models fossilization timescales are estimated at 20 50 kyr indicating that bone co 3 will prove most useful for sequences spanning 100 kyr c isotopes show negative and strong positive deviations during wet and dry climates respectively and short term trends correspond well with changes in aridity within the mascall and rattlesnake formations as inferred from paleosols a proposed correction to 18 o values based on 13 c anomalies implies a small 1 5 increase in meteoric water 18 o during the late oligocene global warming event consistent with a minimum temperature increase of 4 c a strong inferred decrease in 18 o of 4 5 after 7 ma closely parallels compositional changes in tooth enamel and reflects a doubling in the height of the cascade range

mid miocene rhyolite volcanism in northeastern nevada the jarbidge rhyolite and its relationship to the cenozoic evolution of the northern great basin usa we present new physical geochemical geochronologic and oxygen isotope constraints on the mid miocene jarbidge rhyolite in northeastern nevada usa providing new constraints on the tectonomagmatic evolution of the cenozoic northern great basin widespread extension due to rapid collapse of the nevadaplano began at ca 17 16 ma across the northern great basin coeval with this event was compositionally bimodal basalt rhyolite volcanism that is often attributed to the inception of the yellowstone hotspot the most widespread mid miocene volcanic units in northeastern nevada are lavas and domes of the jarbidge rhyolite the thickest and most areally extensive exposures of these lavas include and are found just west of the jarbidge mountains nevada this study focuses on jarbidge rhyolite directly south of the central snake river plain adjacent to the thickest exposures in the vicinity of jarbidge nevada textures on a range of scales indicate that the jarbidge rhyolite consists primarily of phenocryst rich lavas laser 40 ar 39 ar ages for sanidine are consistent with effusive eruption of metaluminous to slightly peraluminous ferroan calc alkalic rhyolite from 16 1 to 15 0 ma prior k ar ages suggest that some activity occurred over a slightly longer duration major and trace element data coupled with new stable and prior radiogenic isotope measurements suggest that jarbidge rhyolite magmas formed primarily via melting of quartzofeldspathic crust the jarbidge rhyolite lavas are geochemically dissimilar from younger snake river plain rhyolites e g lower mgo lower nb higher rb nb and are more similar to coeval rhyolites erupted to the west on or adjacent to the oregon plateau the distribution of the jarbidge rhyolite lavas in northeastern nevada reflects an intimate association with temporally and spatially coincident extension rather than the yellowstone hotspot

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

old crow tephra across eastern beringia a single cataclysmic eruption at the close of marine isotope stage 6 abstract old crow tephra is the largest and most widespread quaternary eruption presently known in eastern beringia its major and trace element geochemistry fe ti oxides and stratigraphic and paleoecological context indicate that it is the result of a single cataclysmic eruption the proximal region may well have experienced tephra fallout from small eruptions just prior to or after the old crow event but there is no evidence to indicate that the distal area was affected we recalculate the glass fission track age at 124 10 ka which coupled with stratigraphic and paleoenvironmental reconstructions indicates that deposition occurred prior to development of the last interglacial boreal forest which suggests a latest marine isotope stage mis 6 age the bulk tephra volume erupted is estimated by three different approaches that are in broad agreement at 200 km 3 but this result must be considered as tentative given the poor controls on definition of isopachs over such a large area the source caldera although presently unrecognized is located in the eastern aleutian arc possibly at or near the emmons lake volcanic center

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

physical volcanology of the pavagadh rhyolites northern deccan traps stratigraphic structural and textural record of explosive and effusive eruptions abstract mount pavagadh 840 m in the northern deccan traps india is mainly composed of mafic lava flows picrites ankaramites mugearites basalts and capped by rhyolites rhyolites also found at lower elevations on and around the mountain are thought to represent blocks of the eroded upper rhyolites or local eruptions in situ although the pavagadh sequence has been studied for 150 years the stratigraphy structures textures and physical volcanology of the pavagadh rhyolites have been poorly documented and are discussed here at length the upper rhyolite sequence contains tens of non welded crystal bearing glass and pumice rich ash beds of 47 m total thickness these represent a primary fallout ash succession derived from distant plinian eruptions possibly corresponding with massive ignimbrite deposits recently recognised in saurashtra several hundred kilometers to the southwest the fallout ash succession is overlain by a 50 m thick lava flow showing well developed flow banding and folding basal and upper breccias and a local basal vitrophyre the lava flow is overlain by an ignimbrite with a basal vitrophyre both these units showing excellent vitroclastic textures the upper rhyolites thus represent alternating explosive effusive explosive eruptions the lower level rhyolites in the area showing intense rheomorphic deformation in outcrop but lacking vitroclastic textures are probably also lavas however they are not blocks of the eroded upper rhyolite lava flow but from separate eruptions the pavagadh rhyolites were probably far more laterally extensive than their present outcrops with a minimum original volume of 9 km3 their source areas remain unknown geochemical and geochronological work on the pavagadh rhyolites is needed in the stratigraphic and volcanological context provided here to understand their petrogenesis and potential relationship with the cretaceous palaeogene k pg boundary mass extinction

identification of the icelandic landnam tephra ad 871 2 in scottish fjordic sediment certain marginal marine environments such as the scottish fjord systems contain high resolution records of palaeoclimatic change in which decadal to centennial climatic events can be resolved this paper explores the possibilities of using tephrochronology to stratigraphically constrain the timing of such events in the loch sunart record md04 2831 on the nw coast of scotland uk one tephra horizon containing both silicic and basaltic shards is identified within late holocene sediment with geochemical analyses of the basaltic shards suggesting an origin in the veidivotn bar arbunga volcanic system radiocarbon age estimates and stratigraphic information suggests that the ad 871 landnam tephra is the most likely candidate the shards identified within this horizon appear hydrated and indicate the operation of post depositional weathering processes possibly influenced by the saline conditions of the fjord environment

kinematic evolution of the olinghouse fault and the role of a major sinistral fault in the walker lane dextral shear zone nevada usa abstract the olinghouse fault is a highly oblique sinistral fault in the northern walker lane dextral shear zone in this study we use geologic mapping and structural analysis of 56 fault surfaces and several folds to constrain the evolution and kinematic role of the olinghouse fault within the walker lane we document a two stage development of the olinghouse fault with an early phase of extension between 12 and 10 ma followed by left lateral shear with a component of extension beginning at 8 to 5 ma the sinistral olinghouse fault has played a relatively complex kinematic role in the evolution of the walker lane including acting as a boundary for clockwise vertical axis block rotation a strain transfer zone between loci of extension and a structural domain boundary redistributing strain between the carson and pyramid lake domains the fault has accumulated 2 5 3 km of left lateral offset at an average rate of 0 3 0 6 mm yr the complexity of this sinistral normal fault zone within a broad zone of dextral shear was likely influenced by the propagation of walker lane dextral shear into a zone of pre existing crustal structure

diverse mid miocene silicic volcanism associated with the yellowstone newberry thermal anomaly the santa rosa calico volcanic field sc of northern nevada is a complex multi vent mid miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism santa rosa calico volcanism initiated at 16 7 ma concurrent with regional steens columbia river flood basalt activity and is characterized by a complete compositional spectrum of basalt through high silica rhyolite to better understand the relationships between upwelling mafic magmatism coeval extension and magmatic system development on the oregon plateau we have conducted the first compre hensive study of santa rosa calico silicic volcanism detailed stratigraphic based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins at least five texturally distinct silicic units are found in the western santa rosa calico volcanic field including abun dant lava flows near vent deposits and shallow intrusive bodies similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present the western and eastern santa rosa calico units are characterized by abundant macro and microscopic disequilibrium textures reflecting a complex petrogenetic history additionally unlike other mid mio cene oregon plateau volcanic fields e g mcdermitt the santa rosa calico volcanic field is characterized by a paucity of caldera forming volcanism only the cold springs tuff which crops out across the central portion of the volcanic field was caldera derived major and trace element geochemical variations are present within and between eastern and western santa rosa calico silicic units and these chemical differences coupled with the observed disequilibrium textures illustrate the action of open system petrogenetic processes and melt derivation from heterogeneous source materials the processes and styles of santa rosa calico silicic magmatism are linked to three primary factors local focusing of and thermal and material contributions from the regional flood basalt event lithospheric extension within the northern portion of the northern nevada rift and interaction of mid miocene silicic magmas with pre santa rosa calico lithosphere similar processes and styles of mid miocene silicic volcanism likely occurred across the oregon plateau in regions characterized by both focused lithospheric exten sion and localized mafic magmatism

stable isotope records of hydrologic change and paleotemperature from smectite in cenozoic western north america abstract the oxygen and hydrogen isotopic composition of soil water 18ow and dw hereafter reflect the history of water through processes such as source evaporation precipitation and vapor recycling temperature humidity evaporation and post condensation processes can affect 18ow and dw as such isotope proxy records are often limited in their ability to constrain paleoclimate paleoecology or paleoelevation without independently corroborating data smectite preserves both the hydrogen and oxygen isotope signature of parent water and therefore provides critical insight into meteoric water line relationships and paleotemperature here we use in situ pedogenic smectite 18o and d records to characterize the evolution of the hydrologic cycle in cenozoic western north america we incorporate 192 samples 119 of which are previously unpublished from 11 cenozoic basins representing a range of environments in the basin and range rocky mountains and great plains our results indicate that the processes controlling smectite isotopic compositions vary both regionally and temporally in some localities such as oligocene to pleistocene western nebraska change in temperature is the primary control on smectite isotopic composition in other basins such as in miocene trapper creek id isotope values lie along the meteoric water line suggesting change in meteoric water composition is responsible for the variation in most basins especially those in the neogene basin and range smectite line slope suggests either evaporation of previously meteoric water or a combination of change in paleotemperature and meteoric water composition smectite geothermometry suggests mineral formation temperatures of 30 40 c in the middle miocene in the rocky mountains great plains and basin and range and a decrease of 10 15 c since the middle miocene climatic optimum consistent with clumped isotope and paleofloral temperature estimates

oldest lamproites from peninsular india track the onset of paleoproterozoic plume induced rifting and the birth of large igneous province abstract potassic and ultrapotassic magmatism from deep lithospheric sources in intra cratonic settings can be the signal of subsequent voluminous mafic magmatism and the formation of large igneous provinces lips triggered by mantle plumes here we report for the first time precise zircon u pb age data from a suite of lamproites in the bastar craton of central india that mark the onset of paleoproterozoic rifting and culminating in the formation of extensive mafic dyke swarms as the bar codes of one of the major lip events during the precambrian evolution of the indian shield the lamproites from the nuapada field occur as dismembered dykes and are composed of phenocrysts and microphenocrysts of altered olivine together with microphenocrysts of phlogopite and magnetite within a groundmass of chlorite and calcite with accessory rutile apatite and zircon the rocks compositionally correspond to olivine phlogopite lamproite and phlogopite lamproite geochemical features of the lamproites correlate with their counterparts in peninsular india and other similar suites elsewhere in the world related to rift settings and also indicate oib like magma source the associated syenite shows subduction related features possibly generated in a post collisional setting magmatic zircon grains with high th u ratios in the syenite from the nuapada lamproite form a coherent group with an upper intercept age of 2473 8 ma representing the timing of emplacement of the magma zircon grains in three lamproite samples yield four distinct age groups at ca 2 4 ga 2 2 ga 2 0 ga and 0 8 ga the 2 4 ga group corresponds to xenocrysts entrained from the syenite whereas the 2 2 ga group is considered to represent the timing of emplacement of the lamproites the ca 2 0 ga zircon grains correlate with the major thermal imprint associated with mafic magmatism and dyke emplacement in southern bastar and the adjacent dharwar cratons a few young zircon grains in the syenite and lamproites show a range of early to middle neoproterozoic ages from 879 to 651 ma corresponding to younger thermal event s as also represented by granitic veins cutting across these rocks and extensive silicification zircon lu hf isotope data suggest magma derivation from a refertilized paleo mesoarchean sub continental lithospheric mantle or oib type sources the differences in hf isotope composition among the zircon grains from different age groups indicate that the mantle sources of the lamproite are heterogeneous at the regional scale a combination of the features from geochemical and zircon hf isotope data is consistent with asthenosphere lithosphere interaction during the lamproite magma evolution the timing of lamproite emplacement in central india correlates with the global 2 2 ga record of lips we link the origin of the related mantle plume to the recycling of subducted slabs associated with the prolonged subduction accretion history prior to the neoarchean cratonization as well as the thermal blanket effect of the earth s oldest supercontinent pulsating plumes and continued rifting generated voluminous dyke swarms across the bastar and dharwar cratons forming part of a major global rifting and lip event

downward remagnetization of a 74 m thick zone in lake sediments from palaeo lake idaho nw united states locating the gauss matuyama geomagnetic boundary within a dual polarity zone

photosynthetic pathway of grass fossils from the upper miocene dove spring formation mojave desert california abstract the spread of grasslands in the miocene and of c4 grasses in the late miocene pliocene represents a major development in terrestrial plant evolution that affected the climate system and faunal evolution the macrofossil record of grasses is sparse likely due to the limited preservation potential of grasses diagnosis of the c3 or c4 photosynthetic pathway depends on preservation of both cellular structures and organic carbon for isotope analysis here we analyze the anatomical and isotopic composition of newly collected grass fossils from the dove spring formation red rock canyon state park california usa located in the el paso basin on the western side of the basin and range province a site previously identified as one of the earliest known c4 grass fossil bearing localities we analyzed the anatomical and geochemical characteristics of these new grass fossils dated to 12 01 12 15 ma the fossils analyzed in this study include grass shoots and in cross section display anatomy indicative of the c3 photosynthetic pathway we isolated organic carbon from the stem fossils and determined the carbon isotopic composition to be 24 8 0 5 together the anatomical and geochemical analyses confirm that these plants used the c3 photosynthetic pathway our findings are consistent with dietary evidence based on tooth enamel from grazing mammals of available c3 resources in the same sections these newly reported miocene age c3 grass fossils contribute to a sparse macrofossil record of grass evolution overall paleoecological reconstructions at this site indicate more humid conditions during the miocene compared to the modern mojave desert with c3 grasses and diverse grazing mammals

integrating the intimate records using tephrochronology rising to the challenge little has challenged our understanding of climate change more so than the abruptness with which large scale shifts in temperature occurred during the late quaternary the causal mechanisms driving

systematic mineralogical diversity in a type granitic intrusions control of magmatic source and geological processes the origin and causes of mineralogical diversity of a type granites are debated the series of a type granite plutons with distinct mineralogical differences emplaced along an upper paleozoic crustal scale shear zone in the cobequid highlands nova scotia provide an opportunity to examine the origin of different a type plutons in a similar tectonic setting based on the ferromagnesian minerals present the plutons are classified into sodic granites with sodic amphibole calcic granites with calcic amphibole and biotite granites sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone whereas plutons in the western shear zone with lesser gabbro are solely biotite granites trace elements and radiogenic isotopes show that the three granite types have different sources intensive parameters including temperature pressure and water in melt contents were estimated from mineralogical and geochemical data modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20 40 partial melting of intracrustal feldspathic rocks whereas the sodic granites are extreme fractionates 90 of coeval mafic magma we propose that supply of upper paleozoic mafic magma probably related to regional extension and decompression melting beneath the magdalen basin created a deep crustal hot zone in the eastern cobequid highlands and extreme fractionation of underplated mafic sills produced the sodic granites heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust creating batches of biotite and calcic granitic melts in different depths fractionated and crustally derived melts segregated along crustal scale faults constructing the complex plutons in the east melting of the crust was further facilitated by the release of water from the crustal rocks upon heating in the eastern shear zone water was released predominantly by magmatic rocks and in lesser amounts compared to the west where neoproterozoic sedimentary and volcaniclastic rocks are more abundant the volatile rich granitic melts in the western part of the shear zone were crystallized rapidly stabilizing only biotite this study demonstrates that the mineralogical variations in a type granites arise from rather similar magma compositions but they are important petrogenetic indicators of varying sources specific magmatic processes and emplacement conditions

chronology of polyphase extension in the windermere hills northeast nevada fission track and 40 ar 39 ar dating and chemical correlation of volcanic strata exposed in the windermere hills and northern pequop mountains northeast nevada indicate a protracted polyphase history of tertiary late eocene late miocene extension along the northern margin of a major cordilleran metamorphic core complex early extension is recorded by a west tilted half graben filled with early oligocene 34 79 0 18 39 18 0 12 ma sedimentary rocks in the eastern windermere hills above the lowangle black mountain detachment fault the early oligocene half graben conformably overlies a widespread suite of late eocene 39 18 0 12 40 38 0 06 ma calc alkaline volcanic rocks reflecting a temporal link between early extension at a high structural level and the end of the ignimbrite flare up these strata are cut by east west striking normal faults which are exposed along and parallel to the northern margin of the metamorphic complex available age data e g between 14 93 0 08 and 34 79 0 18 ma permit the interpretation that the east west striking faults formed at the same time as or after large magnitude unroofing of highgrade rocks we interpret the east west striking faults to accommodate differential uplift of greenschist grade metamorphic rocks in the upper crust above a lateral ramp in a west northwest directed mylonitic shear zone subsequent extension in the windermere hills is defined by deep rapidly filled half grabens of middle miocene 7 42 2 0 to 14 93 0 08 ma age that unconformably overlie older faults and synextensional deposits these are the youngest half grabens in the region and are inferred to be initiated by extensional stresses imparted to the base of the lithosphere by a laterally spreading mantle plume e g the yellowstone hotspot located in southeastern oregon at this time

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

celadonite in continental flood basalts of the columbia river basalt group celadonite is a common alteration product of basalts in marine environments it has been argued that marine fluids are necessary for celadonite formation possibly by providing a source of k and other dissolved cations laterally extensive deposits of celadonite occur in basalts of the grande ronde basalt of the columbia river basalt group the celadonite is found in scoriaceous flow tops of layered basalt flows where it fills vesicles and replaces the surrounding groundmass evolved interstitial glasses are present in the basalts and dissolution of these glasses may provide sufficient k for celadonite formation whereas dissolution of groundmass augite provides a source of mg and fe these observations show that alteration by seawater or any other external source of dissolved ions is not necessarily required for celadonite formation

correlating tephras and cryptotephras using glass compositional analyses and numerical and statistical methods review and evaluation abstract we define tephras and cryptotephras and their components mainly ash sized particles of glass crystals in distal deposits and summarize the basis of tephrochronology as a chronostratigraphic correlational and dating tool for palaeoenvironmental geological and archaeological research we then document and appraise recent advances in analytical methods used to determine the major minor and trace elements of individual glass shards from tephra or cryptotephra deposits to aid their correlation and application protocols developed recently for the electron probe microanalysis of major elements in individual glass shards help to improve data quality and standardize reporting procedures a narrow electron beam diameter 3 5 m can now be used to analyze smaller glass shards than previously attainable reliable analyses of microshards defined here as glass shards in part 2 of the review we describe critically assess and recommend ways in which tephras or cryptotephras can be correlated in conjunction with other information using numerical or statistical analyses of compositional data statistical methods provide a less subjective means of dealing with analytical data pertaining to tephra components usually glass or crystals phenocrysts than heuristic alternatives they enable a better understanding of relationships among the data from multiple viewpoints to be developed and help quantify the degree of uncertainty in establishing correlations in common with other scientific hypothesis testing it is easier to infer using such analysis that two or more tephras are different rather than the same adding stratigraphic chronological spatial or palaeoenvironmental data i e multiple criteria is usually necessary and allows for more robust correlations to be made a two stage approach is useful the first focussed on differences in the mean composition of samples or their range which can be visualised graphically via scatterplot matrices or bivariate plots coupled with the use of statistical tools such as distance measures similarity coefficients hierarchical cluster analysis informed by distance measures or similarity or cophenetic coefficients and principal components analysis pca some statistical methods cluster analysis discriminant analysis are referred to as machine learning in the computing literature the second stage examines sample variance and the degree of compositional similarity so that sample equivalence or otherwise can be established on a statistical basis this stage may involve discriminant function analysis dfa support vector machines svms canonical variates analysis cva and anova or manova or its two sample special case the hotelling two sample t2 test randomization tests can be used where distributional assumptions such as multivariate normality underlying parametric tests are doubtful compositional data may be transformed and scaled before being subjected to multivariate statistical procedures including calculation of distance matrices hierarchical cluster analysis and pca such transformations may make the assumption of multivariate normality more appropriate a sequential procedure using mahalanobis distance and the hotelling two sample t2 test is illustrated using glass major element data from trachytic to phonolitic kenyan tephras all these methods require a broad range of high quality compositional data which can be used to compare unknowns with reference training sets that are sufficiently complete to account for all possible correlatives including tephras with heterogeneous glasses that contain multiple compositional groups currently incomplete databases are tending to limit correlation efficacy the development of an open online global database to facilitate progress towards integrated high quality tephrostratigraphic frameworks for different regions is encouraged

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved