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

clean subglacial access prospects for future deep hot water drilling accessing and sampling subglacial environments deep beneath the antarctic ice sheet presents several challenges to existing drilling technologies with over half of the ice sheet believed to be resting on a wet bed drilling down to this environment must conform to international agreements on environmental stewardship and protection making clean hot water drilling the most viable option such a drill and its water recovery system must be capable of accessing significantly greater ice depths than previous hot water drills and remain fully operational after connecting with the basal hydrological system the subglacial lake ellsworth sle project developed a comprehensive plan for deep greater than 3000 m subglacial lake research involving the design and development of a clean deep ice hot water drill however during fieldwork in december 2012 drilling was halted after a succession of equipment issues culminated in a failure to link with a subsurface cavity and abandonment of the access holes the lessons learned from this experience are presented here combining knowledge gained from these lessons with experience from other hot water drilling programmes and recent field testing we describe the most viable technical options and operational procedures for future clean entry into sle and other deep subglacial access targets

a hot water ice coring drill a new method of ice core drilling uses an annulus of hot water jets to melt out a cylindrical ice core this lightweight device used in combination with a fast hot water drill can quickly obtain ice cores from any depth

fluvial bedrock incision in the active mountain belt of taiwan from in situ produced cosmogenic nuclides the concentration of cosmogenic nuclides in rocks exposed at the earth s surface is proportional to the total duration of their exposure this is the basis for bedrock surface exposure dating and has been used to constrain valley lowering rates in the taroko gorge eastern central range taiwan taroko gorge contains a uniquely complete geomorphic record of fluvial valley lowering continuous fluvially sculpted surfaces are present in the lower 200 m of this marble gorge assuming no post fluvial erosion of the gorge wall the concentration of in situ produced cosmogenic 36 cl measured in gorge wall marbles reveals exposure ages from 0 2 ka in the active channel to 6 5 ka at 165 m above the present river these ages imply an average fluvial incision rate of 26 3m m a 1 throughout the middle and late holocene taking into account lateral gorge wall retreat after initial thalweg lowering would give rise to calculated older exposure ages without considering gorge wall retreat our estimates therefore represent maximum incision rates estimated maximum holocene incision rates are higher than the long term exhumation rates derived from fission track dating the longterm gorge development governed by tectonic uplift is superimposed by short term variations in incision rates caused by climatic or regional tectonic changes copyright 2005 john wiley sons ltd

paleogeographic reconstruction of the eocene idaho river north american cordillera eocene lake gosiute in southwestern wyo ming was progressively fi lled in by volcani clastic sediment between 49 6 and 47 0 ma the source of this material has long been thought to have been the absaroka volcanic province immediately north of the greater green river basin lead isotope compositions of sandstone from this interval however are consistent with derivation from the challis volcanic fi eld the 40 ar 39 ar ages of single de trital k feldspar crystals from greater green river basin sandstones are nearly identical to 40 ar 39 ar eruptive ages for volcanic rocks of the challis volcanic fi eld 49 8 45 5 ma but we also identify mesozoic and proterozoic crystals that are consistent with cooling ages for rocks that were likely exposed in the idaho segment of the north american cordillera during the eocene most of these rocks are traversed by or are up gradient of a major eocene paleovalley in central idaho the sud den appearance of challis derived sediment in the greater green river basin indicates that a major river here named the idaho river connected the interior of the north american cordillera to the greater green river basin this connection requires 500 km to reach from central idaho to the greater green river ba sin the idaho river probably carried detritus that was stripped from distant uplifted moun tains above the active challis volcanic fi eld as far south as the piceance creek basin suggest ing a total length of at least 1000 km middle eocene metamorphic core complexes in the northern cordillera likely produced a major highland in the headwaters of the idaho river which generated river systems that drained both eastward into the wyoming foreland and westward into the oregon coast ranges

influence of late cretaceous magmatism on the sevier orogenic wedge western montana late cretaceous large volume pluton emplacement and accompanying volcanism within the evolving western montana thrust wedge may have played important roles in determining the geometric and kinematic development of the thrust wedge thereby influencing patterns of sediment dispersal and subsidence in the adjacent foreland basin intrusion of the upper cretaceous boulder batholith and coeval eruption of the elkhorn mountains volcanics were focused mainly at the trailing margin of the lombard eldorado allochthon the resultant thick 16 17 km igneous culmination evolved over a relatively short interval 80 70 ma thickening the orogenic wedge to the point of supercritical taper and facilitating continued motion along the lombard eldorado thrust system thrust imbrication at the wedge toe and forelandward translation of the helena salient delivery of eroded volcaniclastic detritus from the thickened thrust wedge and accelerated basin subsidence due to thrust loading resulted in accumulation of a thick 4 5 km sequence of campanian maastrichtian volcaniclastic strata in the foreland basin livingston group the structural and sedimentological effects of this structural magmatic culmination are similar to those of basement cored culminations elsewhere in the sevier orogen and to the neogene central andean orogenic wedge

stratigraphic architecture magnetostratigraphy and incised valley systems of the pliocene pleistocene collisional marine foreland basin of taiwan lithofacies analysis magnetostratigraphy and seismic profiles of pliocene pleistocene foreland basin deposits of taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin we have recognized several scales of stratigraphic packages and unconformities in deposits of the taiwan foreland basin small scale 20 to 150 m thick stratigraphic sequences contain upward shallowing marine lithofacies successions that are bracketed by thin coquina sandstones we interpret the small scale stratigraphic packages as parasequences in the traditional sequence stratigraphy model the thin coquina sandstones representing marine flooding intervals the average duration of individual small scale packages was in the range of 37 5 k y on the basis of our magnetostratigraphy these sequences are interpreted as the product of eustatic sea level change possibly related to the orbital time series of obliquity intermediate scale stratigraphic sequences are 150 to 1000 m thick and are bounded by unconformities that are well exposed in outcrop and can be clearly identified in seismic sections the unconformity surfaces have several hundred meters of relief and represent periods of major fluvial valley incision in the foreland basin one of the unconformities is locally an angular one that we interpret as representing a growth structure that formed during structural uplift of the proximal margin of the foreland basin at ca 1 25 ma across this angular unconformity there were marked increases in rates of sediment accumulation and tectonic subsidence in the foreland basin other major unconformities that bound intermediate scale stratigraphic sequences are high relief disconformities these unconformities may be the product of eustatic changes because there has been little change in rates of sediment accumulation and tectonic subsidence across these unconformities the duration of individual intermediate scale packages ranges from 100 000 to 700 000 yr on the basis of magnetostratigraphy and biostratigraphy we interpret the intermediate scale sequences as sequences in the traditional sequence stratigraphy model our analysis of the pliocene pleistocene deposits of the taiwan foreland basin has several implications for understanding the stratigraphic evolution of this collisional marine foreland basin 1 deposition in the taiwan foreland basin appears to have been punctuated by at least five episodes of erosion and major fluvial valley incision large volumes of sediment were eroded from the proximal margin of the foreland basin and transported to more distal parts of the foreland basin or to depocenters outside the foreland basin system during all stages of basin development 2 the presence of high relief unconformities and growth structures in the pliocene pleistocene foreland basin deposits suggests a well developed wedge top depozone in the foreland basin system 3 the pliocene pleistocene strata of the foreland basin of taiwan record 2 3 m y of deposition on the basis of our magnetostratigraphy sediment accumulation rate was on the order of 950 m m y during the earlier stages of basin development during the later stages of basin development sediment accumulation rate increased to 1900 m m y sediment accumulation rates in the collisional marine foreland basin of taiwan are much higher than previously published rates from more extensively studied retroarc foreland basins and collisional nonmarine foreland basins

early tertiary anaconda metamorphic core complex southwestern montana a sinuous zone of gently southeast dipping low angle tertiary normal faults is exposed for 100 km along the eastern margins of the anaconda and flint creek ranges in southwest montana faults in the zone variously place mesoproterozoic through paleozoic sedimentary rocks on younger tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks lower plate rocks are lineated and mylonitic at the main fault and below the mylonitic front are cut by mylonitic mesoscopic to microscopic shear zones the upper plate consists of an imbricate stack of younger on older sedimentary rocks that are locally mylonitic at the main lowermost detachment fault but are characteristically strongly brecciated or broken kinematic indicators in the lineated mylonite indicate tectonic transport to the east southeast syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging wall rocks in low lying areas between fault blocks

stratigraphic record of the yellowstone hotspot track neogene sixmile creek formation grabens southwest montana abstract the sixmile creek formation fills deep grabens in southwest montana and preserves a stratigraphic record of the evolution of the yellowstone hotspot track from 17 ma to 2 ma the ruby beaverhead big hole deer lodge medicine lodge grasshopper three forks canyon ferry jefferson melrose wise river and paradise grabens were active during outbreak of the hotspot they appear to be parts of a radial system of extensional structures that may have formed on a broad dome that was centered on the hotspot outbreak area in southwest idaho and southeast oregon early in the evolution of the grabens massive debris flows surged down paleogene paleovalleys from uplifted and tilted horst blocks and accumulated in the grabens the grabens captured runoff from the hotspot dome with thick deposits of river gravel that appear to have been derived in part from east central idaho as the hotspot track propagated along the eastern snake river plain silicic ash fell into the graben drainage basins and was reworked into thick fluvial beds along graben axes the grabens were cross cut by northwest trending late neogene grabens that propagated on the flanks of silicic volcanic centers along the hotspot track the late neogene grabens diverted the middle miocene drainage patterns in southwest montana

widespread basement erosion during the late paleocene early eocene in the laramide rocky mountains inferred from 87sr 86sr ratios of freshwater bivalve fossils reconstruction of the timing and pattern of exhumation of basement involved uplifts during the laramide is essential to the understanding of the mechanisms of crustal shortening and thickening in the region we use reconstructions of the 87 sr 86 sr ratios of late cretaceous early cenozoic river water from fossil shells in six basins of the rocky mountains contextualized by modern river water sr chemistry studies to trace the erosion of precambrian basement cores in the laramide ranges the 87 sr 86 sr ratios and sr concentration of modern river water in the rocky mountains are controlled by river bedrock lithology weathering of precambrian silicate rocks in the cores of laramide ranges produces high 87 sr 86 sr ratios in highland rivers weathering of paleozoic and mesozoic carbonates along the basin margins reduces the 87 sr 86 sr ratios and increases sr concentration of rivers as they flow basinward lowland rivers that have headwaters in precambrian basement mostly have 87 sr 86 sr ratios 0 711 whereas rivers confined to or with very long reaches in basins have 87 sr 86 sr ratios between 0 709 and 0 711 river water 18 o values do not change in response to changes in catchment elevation our results from fossil shells show that carbonate weathering dominated the river water sr chemistry in the alberta foreland williston and crazy mountains basins and that proterozoic low grade metamorphic carbonates in the belt purcell supergroup were not exposed in the canadian rocky mountains during late cretaceous early paleocene time silicate weathering influenced the sr chemistry of surface waters in the powder river basin during the late paleocene and the washakie basin during the early eocene suggesting precambrian silicate basement rock was extensively exhumed and eroded the observation that precambrian basement was eroded earlier around the powder river basin than around the washakie basin is consistent with a previous study that suggests laramide ranges in northeast wyoming reached high elevation earlier than the ranges adjacent to the sevier thrust belt widespread basement erosion in late paleocene early eocene time was controlled by tectonic exhumation and may have been intensified by the wet and warm global climate

enabling clean access into subglacial lake whillans development and use of the wissard hot water drill system clean hot water drill systems chwdss are used with clean access protocols for the exploration of subglacial lakes and other subglacial aquatic environments e g ice shelf cavities in antarctica a chwds developed for the whillans ice stream subglacial access research drilling wissard project by the science management office at the university of nebraska lincoln unl smo usa was specifically designed for use in west antarctica where the us antarctic program s south pole traverse could assist with logistical support the initial goal was to provide clean access holes through ice up to 1000 m thick following environmental stewardship guidelines however the existing design allows this chwds to be used for ice thicknesses up to 2000 m following modifications to accommodate longer hose lengths in january 2013 the wissard chwds successfully provided for the first time a clean access borehole through 800 m of ice into subglacial lake whillans beneath the west antarctic ice sheet for the deployment of scientific instruments and sampling tools the development and initial use of the wissard chwds required the project team to address a number of constraints while providing contingencies to meet the defined project scope schedule and budget

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

land based evidence for tertiary climatic variations northern rockies paleoclimatic interpretations of sedimentation patterns paleosol mineralogy and vertebrate faunal assemblages of extensive tertiary continental clastic sediments of montana and idaho indicate four major changes in precipitation in this area during tertiary time wet to dry in late eocene dry to wet in late early miocene wet to dry in early middle miocene and dry to wet in late miocene pliocene time this climate pattern is synchronous with similar climatic changes inferred from deep ocean sedimentation rates for the indian pacific and atlantic oceans and to oxygen isotope paleo temperature data for the north pacific ocean and new zealand other paleoclimatic evidence supports the pattern the evidence suggests widespread synchronous tertiary climate changes that may provide useful time stratigraphic markers

sedimentation in a tectonically partitioned nonmarine foreland basin the lower cretaceous kootenai formation southwestern montana the lower cretaceous kootenai formation in southwestern montana was deposited in the nonmarine cordilleran foreland basin in the united states during a period of intensified uplift in the westward adjacent but increasingly impingent sevier fold thrust belt concurrently the foreland basin was partitioned by uplift of intra foreland structural elements and incipient plutonism kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture the kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin the following criteria for recognition of tectonic partitioning of a nonmarine foreland basin are demonstrated by the kootenai 1 complex internalized drainage patterns which locally diverge from the over all regional paleoslope direction and a typical longitudinal pattern 2 intraformational and interformational unconformities both proximal to the fold thrust belt and within the foreland basin 3 evidence for cannibalization and redistribution of foreland basin sediments 4 abrupt deflections of paleodrainage axes in response to initiation of intraforeland uplifts and 5 dramatic base level alterations and consequent changes of fluvial geomorphology as a result of tectonic partitioning and segmentation of the foreland basin kootenai sedimentation can be divided into six phases 1 a tectonic pulse in the sevier fold thrust belt produced a thin but widespread sheet of gravel alluvium referred to as the basal conglomerate member which was deposited by a shallow perhaps ephemeral gravel bed braided stream system drainage was predominantly eastward across the axis of the foreland basin 2 subsequently initial activation of intra foreland structures over the present tobacco root madison gravelly and beartooth archean blocks and the boulder batholith caused reworking of the basal conglomerate in all but the westernmost foreland basin although the topographic manifestations of these uplifts were subtle the foreland basin was effectively partitioned into at least four distinct drainage basins the resulting deposit commonly called the pryor member but here referred to as the first sandstone member was deposited by sand and gravel dominant braided stream systems in the west and east respectively paleodrainage directions in the western foreland basin were southward but over all drainage was northward 3 a period of mud dominant sedimentation followed deposition of the first sandstone probably resulting from a combination of decreased tectonism influx of siliceous volcanic ash and change in source lithology 4 renewed tectonic activity in the fold thrust belt generated a second episode of coarse grained alluvial sedimentation producing the kootenai second sandstone member basin partitioning was accentuated but drainage directions remained essentially unchanged initial activation of the blacktail snowcrest uplift dammed the southern portion of the second sandstone fluvial system base level was elevated and a major anastomosed stream system developed upstream from the structural dam downstream from the axis of the uplift a transport efficient probably entrenched meandering system developed 5 the remainder of kootenai siliciclastic deposition was dominated by muddy fluvial and fluviolacustrine systems the foreland basin configuration remained essentially unchanged as drainage became ponded in response to further elevation of base level as basin partitioning continued an extensive carbonate lacustrine interval punctuated this period of sedimentation 6 gradual southward transgression of the cretaceous sea left a transitional fluviolacustrine to marine imprint on the sedimentary record of the uppermost kootenai and overlying blackleaf formation rejuvenation of orogenesis in the fold thrust belt augmented by cratonic and major intraforeland volcanic and sedimentary lithic sources renewed the supply of coarse grained detritus to the foreland basin during deposition of the blackleaf

physical conditions producing slab stagnation constraints of the clapeyron slope mantle viscosity trench retreat and dip angles abstract recent seismic tomography has revealed various morphologies in the subducted lithosphere in particular significant flattening and stagnation of slabs around the 660 km boundary are seen in some areas beneath the northwestern pacific subduction zones we examined the cause of slab stagnation in terms of the clapeyron slope of the phase transformation from ringwoodite to perovskite magnesiowustite trench retreat velocity dip angles and high viscosity of the lower mantle based on two dimensional 2 d numerical simulations of thermal convection in particular we examined the conditions necessary for slab stagnation assuming a very small absolute value of the clapeyron slope which were proposed based on recent high pressure high temperature high p t experiments our calculations show that slabs tend to stagnate above the 660 km boundary with an increasing absolute value of the clapeyron slope viscosity jump at the boundary and trench retreat velocity and a decreasing initial dip angle stagnant slabs could be obtained numerically for a realistic range of parameters obtained from high p t experiments and other geophysical observations combining buoyancy high lower mantle viscosity and trench retreat we found that a low dip angle of a descending slab at the bottom of the upper mantle plays an important role in slab stagnation two main regimes underlie slab stagnation buoyancy dominated and viscosity dominated regimes in the viscosity dominated regime it is possible for slabs to stagnate above the 660 km boundary even when the value of the clapeyron slope is 0 mpa k

icecube enhanced hot water drill functional description the icecube neutrino observatory was constructed at the south pole during the 2004 05 to 2010 11 austral summer seasons icecube transforms 1 km3 of antarctic ice into an astrophysical particle detector composed of 86 cables strings of optical sensors buried deep beneath the surface each string required drilling a borehole 60 cm in diameter to a depth of 2500 m the 5 mw enhanced hot water drill was designed and built specifically for this task capable of producing the required boreholes at a rate of one hole per 48 hours hot water drilling on this scale presented unique challenges and was rich in lessons learned yielding a collection of notable developments and takeaways e g fuel saving measures thermal modeling firn drilling and closed loop computer control descriptions of system functionality and of lessons learned from icecube drilling are presented

structural and depositional history jefferson and madison basins southwestern montana abstract recent seismic and gravity data from the cenozoic jefferson and madison basins provide new information concerning their structural and depositional histories both basins are north south elongated structural basins formed as a result of horizontal extension after laramide horizontal thrusting each basin is bounded on the east side by a sinuous faulted steep mountain front and large west sloping alluvial fans extend almost completely across both basins gravity data show that each basin in the subsurface is asymmetric with a large steep west dipping fault on the east flank and one or more east dipping fault s of smaller magnitude on the west flank the deep axis of each basin runs parallel to the east mountain front and lies east of the surface geographic central axis jefferson basin has two deep closed structural lows one east of silver star and one east of twin bridges which are separated by a structural arch sediment depth on the arch exceeds 3 000 m 10 000 ft madison basin is shallow on its north end approximately 2 100 m 7 100 ft where it is terminated by the prominent northwest southeast spanish peaks structural trend and progressively becomes deeper 4 500 m 5 000 ft or more south of ennis montana seismic data confirm or support the gravity data seismic also shows the folded and thrusted rocks of the east mountain footwall block dipping steeply westward to where they gradually disappear beneath the thick tertiary sediments tertiary strata lying directly against the large west dipping basin fault show dip reversal caused by drag folding during basin subsidence downthrown rollover type anticlines are thus present on the east side of the basins numerous small faults many antithetic cut the deeper strata and diminish in throw upward strata seen in the seismic sections can be subdivided into a lower set which forms the bulk of the basin fill possibly equivalent to the renova formation late eocene to early miocene a thinner middle set unconformably overlying the lower set equivalent to the sixmile creek formation miocene and pliocene and an upper set composed of west dipping quaternary alluvial fan deposits each set thickens toward the east basin bounding fault in the lower renova set lacustrine intervals are indicated by their consistent lateral seismic character whereas fluvial intervals appear to terminate abruptly end of article last page 1352

evidence from paleosols for ecosystem changes across the cretaceous tertiary boundary in eastern montana ancient soils paleosols of the latest cretaceous hell creek formation are mildly calcareous have clayey subsurface bt horizons and exhibit abundant large root traces as is typical of forested soils in subhumid climates the fact that some of the paleosols are capped by thin impure coals is evidence for seasonally dry swamps the paleosol evidence thus supports published reconstructions based on fossil leaves pollen and vertebrates that this area was subtropical seasonally dry subhumid and forested mainly by angiosperms paleosols within the earliest tertiary paleocene tullock formation have thicker coaly surface o and a horizons and are more drab colored and less calcareous than paleosols of the hell creek formation these features are indications of waterlogging and of a humid climate large root traces and clayey subsurface bt horizons are evidence of swamp woodland and forest inferred base level and paleoclimate are compatible with evidence from fossil leaves and pollen that indicates more abundant deciduous early successional angiosperms and swamp conifers compared to those of late cretaceous time most of the paleosols have drab munsell hues and can be expected to preserve a reliable fossil record of pollen and other plant remains the carbonate content of the paleosols declines toward the top of the hell creek formation and the uppermost 3 m of the formation is noncalcareous because of this the decline in diversity and abundance of bone over this interval is interpreted as a taphonomic artifact evidence from paleosols supports paleobotani cal evidence for catastrophic change in ecosystems at the cretaceous tertiary boundary

a microbial ecosystem beneath the west antarctic ice sheet liquid water has been known to occur beneath the antarctic ice sheet for more than 40 years but only recently have these subglacial aqueous environments been recognized as microbial ecosystems that may influence biogeochemical transformations on a global scale here we present the first geomicrobiological description of water and surficial sediments obtained from direct sampling of a subglacial antarctic lake subglacial lake whillans slw lies beneath approximately 800 m of ice on the lower portion of the whillans ice stream wis in west antarctica and is part of an extensive and evolving subglacial drainage network the water column of slw contained metabolically active microorganisms and was derived primarily from glacial ice melt with solute sources from lithogenic weathering and a minor seawater component heterotrophic and autotrophic production data together with small subunit ribosomal rna gene sequencing and biogeochemical data indicate that slw is a chemosynthetically driven ecosystem inhabited by a diverse assemblage of bacteria and archaea our results confirm that aquatic environments beneath the antarctic ice sheet support viable microbial ecosystems corroborating previous reports suggesting that they contain globally relevant pools of carbon and microbes that can mobilize elements from the lithosphere and influence southern ocean geochemical and biological systems

natural examples of olivine lattice preferred orientation patterns with a flow normal a axis maximum tectonic plate motion is thought to cause solid state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals the mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction this has been explained by the existence of an olivine preferred orientation with an a axis maximum parallel to the induced mantle flow direction in subduction zones however the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion which has usually been ascribed to localized complex mantle flow patterns recent experimental work suggests an alternative explanation under conditions of high water activity a b type olivine preferred orientation may form with the a axis maximum perpendicular to the flow direction natural examples of such b type preferred orientation are however almost entirely unknown here we document widespread b type olivine preferred orientation patterns from a subduction type metamorphic belt in southwest japan and show that these patterns developed in the presence of water our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought

shrimp u pb and 40ar 39ar age constraints for relating plutonism and mineralization in the boulder batholith region montana the composite boulder batholith montana hosts a variety of mineral deposit types including important silver rich polymetallic quartz vein districts in the northern part of the batholith and the giant butte porphyry copper molybdenum pre main stage system and crosscutting copper rich main stage vein system in the southern part of the batholith previous dating studies have identified ambiguous relationships among igneous and mineralizing events mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at butte have all been considered to be late stage differentiates of the boulder batholith however previous dating studies indicated that the boulder batholith plutons cooled from about 78 to 72 ma whereas copper rich main stage veins at butte were dated at about 61 ma recent efforts to date the porphyry copper molybdenum pre main stage deposits at butte resulted in conflicting estimates of both 64 and 76 ma for the mineralizing events silver rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously to resolve this controversy we used the u s geological survey stanford shrimp rg ion microprobe to date single age domains within zircons from plutonic rock samples and 40 ar 39 ar geochronology to date white mica biotite and k feldspar from mineral deposits u pb zircon ages are rader creek granodiorite 80 4 1 2 ma unionville granodiorite 78 2 0 8 ma pulpit rock granite 76 5 0 8 ma butte granite 74 5 0 9 ma altered steward type quartz porphyry dike i 15 roadcut 66 5 1 0 ma altered steward type quartz porphyry dike continental pit 65 7 0 9 ma and quartz monzodiorite of boulder baldy big belt mountains 66 2 0 9 ma zircons from rader creek granodiorite and quartz porphyry dike samples contain archean inheritance the 40 ar 39 ar ages are muscovite silver rich polymetallic quartz vein basin district 74 4 0 3 ma muscovite silver rich polymetallic quartz vein boulder district 74 4 1 2 ma muscovite early dark micaceous vein continental pit 63 6 0 2 ma biotite early dark micaceous vein continental pit 63 6 0 2 ma potassium feldspar early dark micaceous vein continental pit 63 to 59 ma and biotite biotite breccia dike continental pit 63 6 0 2 ma outlying silver rich polymetallic quartz veins of the basin and boulder mining districts probably are directly related to the 74 5 ma butte granite whereas steward type east west quartz porphyry dikes and butte pre main stage deposits are parts of a 66 to 64 ma magmatic mineralization system unrelated to emplacement of the boulder batholith the age of the crosscutting main stage veins may be about 61 ma as originally reported but can only be bracketed as younger than the 64 ma pre main stage mineralization and older than the about 50 ma eocene lowland creek intrusions the 66 ma age for the quartz monzodiorite of boulder baldy and consideration of previous dating studies in the region indicate that small ca 66 ma plutonic systems may be common in the boulder batholith region and especially to the east the approximately 64 ma porphyry copper systems at butte and gold mineralization at miller mountain are indicative of regionally important mineralizing systems of this age in the boulder batholith region resolution of the age and probable magmatic source of the butte pre main stage porphyry copper molybdenum system and of the silver rich polymetallic quartz vein systems in the northern part of the boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events

the effect of rheological parameters on plate behaviour in a self consistent model of mantle convection abstract in a three dimensional numerical model of mantle convection we have explored the self consistent formation and evolution of plates this was done by investigating the influence of temperature stress and pressure dependence of the viscosity on plate like behaviour further the role of a depth dependent thermal expansivity and of internal heating was examined first order self consistent plate tectonics can be obtained for temperature and stress dependent viscosity we observe virtually undeformed moving plates and the phenomenon of trench migration however plate motion only appears during short intervals which are interrupted by long phases characterised by an immobile lid an improvement is achieved by adding pressure dependence of the viscosity and of the thermal expansivity to the model this leads to extended plates with continuous plate motion and the appearance of a low viscosity zone internal heating accelerates subduction and gives an increase in plate velocity the plates are otherwise left intact we find that the surface velocity is largely determined by subduction rather than by upwellings plate like behaviour is restricted to a narrow parameter window the window is bordered by an episodic or a stagnant lid type of flow on the one side and by a domain in which convection is akin to constant viscosity flow on the other side with these different convective regimes the behaviour of various terrestrial planets can be described self consistently occasionally even a change from the plate regime to the stagnant lid regime occurs such a type of transition has possibly changed the style of tectonics on some terrestrial planets such as mars

two new methods of paleocurrent determination from trough cross stratification abstract trough cross stratification is the most abundant accurate and efficient palcocurrent direction indicator in fluvial deposits however exposures in which trough axes can be measured are rare two new methods are presented for the determination of paleocurrent directions from the more common exposure types namely the use of three dimensional exposures of oblique cuts through individual trough limbs and two dimensional exposures of sets of trough cross strata three dimensional exposures of trough limbs can be measured and treated statistically on a stereographic plot to determine the average trough axis orientation in two dimensional exposures characteristic asymmetry of the basal scour surface and truncation of foreset laminae can aid in estimation within about 25 of pal ocurrent direction

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

influence of temperature and depth dependent viscosity structures on postseismic deformation predictions for the large 1946 nankai subduction zone earthquake abstract to investigate the influence of spatial change of viscosity on postseismic deformation associated with the interplate 1946 nankai earthquake m 8 0 at the nankai trough southwest japan we newly constructed a realistic viscoelastic structure model taking into account temperature and depth dependent viscosity of materials for this purpose we first compiled leveling and triangulation data during postseismic periods and clarified characteristics of the amount and spatial patterns of postseismic vertical displacement and principal strain fields then we calculated the spatial distributions of viscosity from temperature and flow fields which were obtained from 2d subduction models by incorporating the obtained viscosity structure into 3d viscoelastic finite element models we constructed a temperature and depth dependent viscosity structure model model p2 based on model p2 we constructed a viscoelastic structure model taking into account poisson s ratio for the oceanic plate and low velocity regions and the existence of low viscosity materials beneath the shikoku and chugoku districts model p3 which were revealed from seismic tomography we also constructed a conventional layered viscoelastic structure model model l1 and plate subduction model model p1 with constant viscosity for each region and evaluated the effects of different viscoelastic structures on postseismic surface deformations using the coseismic slip distribution obtained by inversion analyses of geodetic data we also compared the calculated surface deformations with the observed postseismic crustal deformations in and around shikoku the results show that postseismic surface deformation fields for the newly constructed model p2 are rather different from those for models l1 and p1 landward horizontal displacements for model p2 are smaller than those for models l1 and p1 seaward horizontal displacements are negligible and vertical displacement is characterized by small subsidence over shikoku the postseismic horizontal principal strain field for model p2 is characterized by contractions in the n s to nw se directions at amounts smaller than those for models l1 and p1 postseismic surface deformations for model p3 are almost the same as those for model p2 the observed postseismic vertical displacement and horizontal principal strain fields could not be explained by the viscoelastic response for the realistic viscoelastic structure models p2 and p3 this indicates that the effects of elastic and viscoelastic responses due to interplate coupling on the plate interface after slip at the extension of the coseismic slipped region and poroelasticity should be taken into account to precisely estimate postseismic surface deformation this also suggests that in order to evaluate postseismic crustal deformations derived from a large interplate subduction zone earthquake it is essential to use realistic temperature and depth dependent viscoelastic structure models

a paleopedological approach to the interpretation of terrestrial sedimentary rocks the mid tertiary fossil soils of badlands national park south dakota sedimentology is concerned with processes environments and products of sedimentation whereas paleopedology is concerned with soil formation on these materials between times of sedimentation both approaches can contribute much to an understanding of ancient terrestrial environments fossil soils can be recognized from irregular vertical tubular structures fossil root traces and burrows massive bioturbated layers with gradational contacts soil horizons complex cracking and veining peds and cutans and distinctive petrographic textures sepic plasmic fabrics in badlands national park south dakota numerous other features such as patterns of fossil bone accumulation horizons of calcareous nodules and layers and local variations in mineralogy and trace and major chemical elements were also evidence of fossil soils in the pinnacles area of badlands national park there are at least 87 successive fossil soils in 143 m of stratigraphic section ancient soil formation is a kind of early diagenesis which can be obscured by additional changes after burial of the soil late diagenesis in badlands national park those few late diagenetic features identified did not seriously alter traces of ancient soil formation fossil soils can be mapped and named in the field using the soil mapping procedures of the u s department of agriculture at least ten such nongenetic units fossil soil series can be recognized among fossil soils of the pinnacles area of badlands national park each represents a particular ancient environment in most cases these fossil soil types could be identified within modern soil classifications such as those of the u s department of agriculture and the australian c s i r o in addition fossil soils can be evidence of past soil forming factors such as climate organisms topographic relief parent material and time of formation climatic interpretations from fossil soils may be useful confirmation of other data although some immature or waterlogged soils reveal nothing of climate the kind of former vegetation such as woodland or grassland can be interpreted from fossil soils even where direct fossil evidence of its floristic composition is lacking fossil soils may be a guide to the degree nature and location of preservational biases of animal and plant fossils fossil soils can also be used to assess the degree of adaptation of associated fossil mammals because they are evidence of vegetation independent of other fossils from features of fossil soils related to waterlogging or drainage it may be possible to assess paleotopographic changes as well as some aspects of their tectonic control little altered parent material is usually preserved under fossil soils in some cases as in the region of the south dakota badlands during the oligocene parent material appears to have been complexly interrelated with vegetation climate and fluvial regimen times of formation of fossil soils can be estimated by comparison with studies of modern soil formation from such estimates which for the sequence in the pinnacles area of badlands national park appear to be crude but not unrealistic the completeness and rates of sediment accumulation of different parts of a sequence can be compared providing insights into changing factors in the long term behavior of fluvial systems

drainage basin responses to climate change recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion the sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution the golem model simulates basin evolution under the action of weathering processes hillslope transport and fluvial bedrock erosion and sediment transport results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change an increase in runoff intensity or a decrease in vegetation cover will lead to a rapid expansion of the channel network with the resulting increase in sediment supply initially generating aggradation along the main network followed by downcutting as the sediment supply tapers off by contrast a decrease in runoff intensity or an increase in the erosion threshold will lead to a retraction of the active channel network and a much more gradual geomorphic response cyclic changes in runoff intensity are shown to produce aggradational degradational cycles that resemble those observed in the field cyclic variations in runoff also lead to highly punctuated denudation rates with denudation concentrated during periods of increasing runoff intensity and or decreasing vegetation cover the sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes a finding which underscores the need for caution in interpreting modern sediment yield data

paleogene postcompressional intermontane basin evolution along the frontal cordilleran fold and thrust belt of southwestern montana the paleogene renova formation is the earliest record of postcompressional sedimentation within and adjacent to the helena salient of the cordilleran fold and thrust belt in southwestern montana paleocurrent and compositional data from basin margin facies document radiating paleodispersal away from high relief 2 km highlands coincident with modern mountainous areas source rocks within the paleohighlands included the same archean metamorphic proterozoic paleozoic and mesozoic sedimentary and mesozoic plutonic and vo l canic rocks as exposed in modern uplifts paleocurrent and compositional data from trunk fluvial conglomerates and sandstones document the existence of an interbasinal drainage system that connected the three forks western gallatin and townsend basins with headwaters farther to the west and southwest near the present day montana idaho border overall the distribution of paleogene mountainous areas and basins closely resembled modern geography and the paleogene drainage network was strikingly similar to the modern missouri river headwater system the renova formation records the early stages of decay of the cordilleran orogenic belt including the evolution of a complex intermontane basin network in southwestern montana high energy late cretaceous to early eocene fluvial systems carved deep large scale paleovalleys into the orogenic wedge along zones of structural and stratigraphic weakness at least a 5 km thickness of overburden was removed during this time incision was temporally correlative with early cenozoic regional uplift and subtropical climatic conditions subsequent deposition of the renova formation was temporally correlative with the cessation of uplift the initiation of crustal extension and climatic cooling however extension is not interpreted to have played a major role in earliest basin development

processes facies and architecture of fluvial distributary system deposits abstract there is evidence from the stratigraphic record of examples of fluvial deposits that were the products of deposition from river systems which had decreasing discharge down flow and transitions from proximal channelised to distal unconfined flow these deposits form fan shaped bodies several tens of kilometres in radius and their stratigraphic architecture is aggradational with no evidence of deep incision driven by base level fall the fluvial systems that generated these deposits formed under conditions for which there is no complete analogue today an endorheic basin with a relatively arid climate adjacent to an uplifted area with higher precipitation a conceptual model for fluvial systems of this type has therefore been built on the basis of outcrop examples and a consideration of the controls on sedimentation proximal areas are characterised by amalgamated coarse pebbly and sandy channel deposits with little preservation of overbank facies channel dimensions are generally smaller in the medial areas but sizes are variable deposits are of braided meandering and simple channels which show varying degrees of lateral migration the channel fills may be mud or sand with overbank flow processes playing an important role in filling channels abandoned on the floodplain after avulsion the proportion of overbank deposits increases distally with sheets of sand deposited as lateral and terminal splays by unconfined flow interconnection of sandstone bodies is poor in the distal areas because channel fill bodies are sparse small and are not deeply incised the radial pattern of the sediment body forms by the repeated avulsion of channels active channels build up lobes on the alluvial plain and rivers switch position to follow courses on lower lying areas the term fluvial distributary system is here used to describe a river system which has a downstream decrease in discharge and has a distal zone which is characterised either by terminal splays on to a dry alluvial plain or a lake delta during periods of lake highstand

ocean stratification and low melt rates at the ross ice shelf grounding zone ocean driven melting of ice shelves is a primary mechanism for ice loss from antarctica however due to the difficulty in accessing the sub ice shelf ocean cavity the relationship between ice shelf melting and ocean conditions is poorly understood particularly near the grounding zone where the ice transitions from grounded to floating we present the first borehole oceanographic observations from the grounding zone of the ross ice shelf antarctica s largest ice shelf by area contrary to predictions that tidal currents near grounding zones mix the water column we found that ross ice shelf waters were vertically stratified current velocities at middepth in the ocean cavity did not change significantly over measurement periods at two different parts of the tidal cycle the observed stratification resulted in low melt rates near this portion of the grounding zone inferred from phase sensitive radar observations these melt rates were generally 10 cm year which is lower than average for the ross ice shelf 20 cm year melt rates may be higher at portions of the grounding zone that experience higher subglacial discharge or stronger tidal mixing stratification in the cavity at the borehole site was prone to diffusive convection as a result of ice shelf melting since diffusive convection influences vertical heat and salt fluxes differently than shear driven turbulence this process may affect ice shelf melting and merits further consideration in ocean models of sub ice shelf circulation

arc parallel flow within the mantle wedge evidence from the accreted talkeetna arc south central alaska 1 residual mantle exposures in the accreted talkeetna arc alaska provide the first rock analog for the arc parallel flow that is inferred from seismic anisotropy at several modern arcs the peridotites exposed at the base of the jurassic talkeetna arc have a moho parallel foliation and indicate dislocation creep of olivine at temperatures of 1000 to 1100 c slip occurred chiefly on the 001 100 slip system which has only rarely been observed to be the dominant slip system in olivine stretching lineations and olivine 100 slip directions are subparallel to the long axis of the talkeetna arc for over 200 km indicating that mantle flow was parallel to the arc axis the alignment of the olivine 100 axes yields a calculated s wave anisotropy with the fast polarization direction parallel to the arc thus 1 the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog 2 arc parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones fluid filled pockets or glide on the 010 001 h2o induced slip system 3 seismic anisotropy beneath modern arcs may be due to slip on 001 100 with a horizontal foliation rather than slip on 010 100 with a vertical foliation and 4 the observed dominance of the 001 100 slip system may be due to high h2o concentrations suggesting that strain in the oceanic upper mantle may be accommodated dominantly by 001 100 olivine slip

ancient hillslope deposits missing links in the study of climate controls on sedimentation rare outcrops of miocene pliocene buried hillslopes and colluvium present in proximal basin fill exposures in the arid southwestern united states in settings free of tectonic influences reveal hillslope sediment production and delivery to piedmonts that vary as a function of climate change a petrologic weathering index for sand clay mineral data and stratigraphic relations indicates that episodes of greater production and delivery of colluvium from hillslopes coincide with more intense chemical weathering and thus greater effective moisture strata recording limited colluviation less intense weathering and piedmonts dominated by eolian processes very likely correspond to drier conditions the mode of climate driven hillslope and piedmont landscape responses evident in these older stratigraphic records corroborates the conceptual model developed by quaternary researchers for arid environments even though climate change was operating at higher amplitudes and different frequencies in the quaternary this evidence of long term continuity in the mode of landscape response to climate change provides a new level of support for using the geologic record to understand the effects of future climate change

climate driven bedrock incision in an active mountain belt measurements of fluvial bedrock incision were made with submillimeter precision in the east central range of taiwan where long term exhumation rates and precipitation driven river discharge are independently known they indicate that valley lowering is driven by relatively frequent flows of moderate intensity abrasion by suspended sediment is an important fluvial wear process and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

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

sequence stratigraphy of cenozoic continental rocks southwestern montana because the presently used bozeman group lithostratigraphy is difficult to apply to cenozoic basin fill rocks in the jefferson beaverhead melrose and divide valleys southwestern montana we advocate the use of sequence stratigraphy surface sequence boundaries are in many cases marked by calcic paleosol zones these zones can be projected into the subsurface and tied to seismic reflection data density velocity contrasts between calcic paleosol zones and overlying nonpedogenic rocks generate bright reflectors that define sequence boundaries sequence boundary delineation is further enhanced by reflection termination patterns similar to those found in marine strata sequence age calibration is based upon vertebrate fossil and radiometric age data five sequences separated by basin wide unconformities are recognized most sequence bounding unconformities are marked by paleosols an occurrence not previously reported for cenozoic strata of southwestern montana the sequences and their age ranges are 1 bridgerian uintan approximately 50 to 44 ma calc alkaline volcanic flows and interstratified sedimentary rocks 2 duchesnian to whitneyan approximately 42 to 30 ma terrestrial sedimentary and volcanic rocks 3 arikareean approximately 27 to 21 ma terrestrial sedimentary and volcanic rocks 4 barstovian to blancan approximately 16 to 4 ma terrestrial sedimentary and volcanic rocks and 5 early quaternary approximately 1 8 ma to holocene terrestrial sedimentary and volcanic rocks

paleomagnetic and isotopic dating of thrust belt deformation along the eastern edge of the helena salient northern crazy mountains basin montana thrust belt deformation along the eastern edge of the disturbed belt in the helena salient of montana has not been well dated owing to a lack of syn or postorogenic strata in situ paleomagnetic data from alkaline intrusions exposed in the easternmost folds of the salient in the northern crazy mountains basin previously described as pre syn or posttectonic with respect to deformation are well grouped dec 343 inc 61 95 5 0 k 46 n 16 sites a negative fold test is significant at minimally 95 confidence intrusion and magnetization acquisition thus postdate fold and thrust deformation k ar age determinations of selected intrusions range from 52 to 48 ma paleomagnetic and isotopic age data combined with stratigraphic information indicate that folding in the northern crazy mountains basin is middle to late paleocene in age this age is in agreement with suggested dates for deformation in the northern montana disturbed belt but is recognizably older than the youngest episodes of frontal deformation in the utah idaho wyoming salient data from this study and existing structural and stratigraphic information demonstrate that deformation in the overthrust belt and foreland provinces of southwest montana overlapped temporally and spatially ranged from late cretaceous to earliest eocene in age and progressed from west to east through time

fault controlled pluton emplacement in the sevier fold and thrust belt of southwest montana usa abstract problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening not extension occurs on a regional scale new structural data from the pioneer and boulder batholiths of southwest montana usa suggest emplacement at the top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km frontal thrust facilitated pluton emplacement was accommodated by 1 a magma feeder zone created along the ramp interface 2 providing releasing steps at ramp tops that serve as initial points of emplacement and subsequent pluton growth and 3 localizing antithetic back thrusts that assist in pluton ascent a model of magma emplacement is proposed that involves these elements this model for syntectonic ramp top emplacement of plutons helps explain how space is made for plutons within fold and thrust belts

viscous anisotropy of two phase composites and applications to rocks and structures abstract anisotropy of viscosity in rocks has a significant effect on the development of geological structures as demonstrated in several theoretical and analogue model studies this paper develops the results of a self consistent mechanics approach to modelling the bulk viscous properties of two phase composites to reveal the strength of the viscous anisotropy anisotropy factor is defined as the ratio of normal to shear viscosity n s the value of is determined for different types of composites of two viscous phases with varying viscosity ratio m 5 10 100 ranging from bilaminates to those with elliptical shape fabrics given by axial ratio r bilaminates with equal layer thickness are the most highly anisotropic systems where simply depends on m these results are applied to rocks modelled as two phase composites of competent and incompetent lithologies with the aim of quantifying the viscous anisotropy that is associated with natural rock deformation and the formation of geological structures it is deduced that m 5 or 10 is an appropriate maximum viscosity contrast for many metasedimentary rocks e g psammite pelite composites but that m 100 might be effectively reached in limestone marl pairs or in rocks containing surfaces of slip or zones of recrystallisation or grain boundary sliding these m values give rise to maximum values of 3 5 5 and 25 5 respectively an anisotropy factor of 25 is effectively a composite of a rigid and a viscous phase and may only be achieved in solid melt mixtures anisotropic rocks whether layered or containing a strong shape fabric will develop folds more easily than isotropic rocks characteristic structures are similar folds chevron folds kink bands and shear bands geometric features of folds especially fold angularity may provide methods of quantifying the viscous anisotropy of rocks and composites

drilling to the beds of the greenland and antarctic ice sheets a review we give a review of drilling to the beds of the greenland and antarctic ice sheets with a history of just a century we use a chronological rather than a geographical ordering to emphasize the way drilling has developed the review is divided into two parts core drilling and hole only drilling for access to the beds or the deep interior of the ice sheets this is an overview not a comprehensive report on each project while we have endeavoured to include all projects that fit our selection criteria we have not provided all the technical details

the source magmatic contamination and alteration of the idaho batholith zircons from cretaceous and tertiary granitic rocks of the idaho batholith preserve the best record of magmatic oxygen isotope ratios the coupling of oxygen isotope data from refractory minerals such as zircon and garnet with that from less refractory minerals such as quartz reveals the complex evolution and alteration history of the idaho batholith values of 18o zircon throughout the batholith are relatively homogeneous at 6 9 0 9 1 sd n 21 rocks despite variations in age and chemistry samples from the bitterroot lobe have an average 18o zircon of 7 1 0 3 n 7 rocks and samples from the atlanta lobe have an average 18o zircon of 7 0 1 0 n 9 rocks four epizonal tertiary plutons have an average 18o zircon of 7 2 0 2 n 4 rocks these data indicate a similar magmatic source for all granitic rocks of the idaho batholith except the tertiary epizonal casto pluton which has a 18o zircon of 4 0 0 1 n 1 rock the zircon data show that the range of normal magmatic 18o is considerably more restricted than was previously inferred despite some samples containing two micas and or garnet or having a mildly peraluminous character quartz is more variable than zircon in 18o average 18o quartz for the entire batholith is 10 4 1 1 1 sd n 65 rocks with a range of 7 7 to 12 4 excluding the casto pluton the spread of quartz values is 2 7 larger and typically higher than that calculated relative to zircon quartz data from the bitterroot lobe and epizonal tertiary plutons suggest the alteration of primary 18o during metamorphic events quartz data from the atlanta lobe suggest closed system diffusional exchange during cooling igneous garnet 18o values are higher than and out of equilibrium with those of coexisting zircon by 0 1 0 9 assuming an average 18o for unaltered wall rock metasediments the incorporation of 10 metasediment into a magma after crystallization of zircon raises the 18o magma by 0 9 the largest measured garnet zircon these quartz and garnet 18o from the atlanta and bitterroot lobes suggest magmatic contamination with a high 18o rock after the crystallization of zircon and before that of garnet and quartz

faunal stability during the early oligocene climatic crash abstract traditional neo darwinism views species as highly flexible entities which adapt to climatic change by gradually evolving new morphologies of the 177 species of mammals from the upper eocene lower oligocene 37 30 ma white river group in the high plains most species are static for 2 4 million years on average and some persist much longer only three examples of gradualism can be documented in the entire fauna and these are mostly size changes contrary to expectations most mammalian species show no change during the earliest oligocene climatic crash 33 2 ma in spite of the fact that the vegetation changed from dense forests to open forested grassland mean annual temperatures dropped 13 c and conditions got much drier and more seasonal only a few mammalian lineages speciated a few more went extinct and the vast majority 62 out of 70 persisted through this climatic event with no observable response whatsoever this evidence shows that organisms are much less responsive to the environment than short term neontological studies suggest

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

long distance longitudinal transport of gravel across the cordilleran thrust belt of montana and idaho two newly identified middle eocene paleovalleys 100 km long preserved on top of the southwest montana reentrant of the cordilleran fold and thrust belt indicate long lived longitudinal flow across the thrust belt and resolve a long standing debate about the source of the voluminous quartzite debris in the upper cretaceous to lower tertiary divide harebell and pinyon conglomerates of montana idaho and wyoming geologic mapping stratigraphic provenance and geochronologic studies revealed that eocene volcanic and sedimentary rocks in the paleovalleys are as thick as 2 km onlap preexisting bedrock and interfinger with well rounded conglomerate derived from formations exposed only to the west the middle eocene paleovalleys are the youngest expression of a major paleoriver system that transported sediment toward the foreland during the sevier orogeny an eocene subcrop map shows that the headwaters of the eocene paleovalleys coincided with structural culminations in the thrust belt that supplied sediment to the divide conglomerate of the upper cretaceous to lower tertiary beaverhead group ultimately the lemhi pass and hawley creek paleovalleys provided several thousand cubic kilometers of quartzite debris to the pinyon and harebell conglomerates of northwest wyoming 200 350 km away and formed the northwest half of a giant longitudinal drainage system sevier contraction not the rising idaho batholith first uplifted vast culminations beneath the headwaters of this river system

wissard at subglacial lake whillans west antarctica scientific operations and initial observations a clean hot water drill was used to gain access to subglacial lake whillans slw in late january 2013 as part of the whillans ice stream subglacial access research drilling wissard project over 3 days we deployed an array of scientific tools through the slw borehole a downhole camera a conductivity temperature depth ctd probe a niskin water sampler an in situ filtration unit three different sediment corers a geothermal probe and a geophysical sensor string our observations confirm the existence of a subglacial water reservoir whose presence was previously inferred from satellite altimetry and surface geophysics subglacial water is about two orders of magnitude less saline than sea water 0 37 0 41 psu vs 35 psu and two orders of magnitude more saline than pure drill meltwater 0 002 psu it reaches a minimum temperature of 0 558c consistent with depression of the freezing point by 7 019 mpa of water pressure subglacial water was turbid and remained turbid following filtration through 0 45 mm filters the recovered sediment cores which sampled down to 0 8 m below the lake bottom contained a macroscopically structureless diamicton with shear strength between 2 and 6 kpa our main operational recommendation for future subglacial access through water filled boreholes is to supply enough heat to the top of the borehole to keep it from freezing

high initial sr isotopic ratios and evidence for magma mixing in the pioneer batholith of southwest montana the northeast part of the composite pioneer batholith of southwest montana consists of late cretaceous calc alkaline plutons that vary in composition as a function of age changing from minor hornblende biotite quartz diorite at about 76 m y to a small pluton of hornblende biotite tonalite at about 74 m y to abundant biotite hornblende granodiorite and biotite granite at about 72 m y to some biotite granodiorite and 2 mica granite at 67 to 65 m y initial 87 sr 86 sr ratios sirs for the plutons range from 0 7112 to 0 7160 magmas having a sir near 0 7112 appear in most episodes of emplacement whereas those having higher values are restricted to individual episodes two granodiorite plutons of 72 m y age show a mutual gradational contact across which the sir varies over a distance of 7 km from 0 711 to 0 714 the variation is ascribed to mixing between contemporaneous but isotopically distinct granodiorite magmas the high sirs of the batholith are interpreted as reflecting partial melting

eocene to recent variations in erosion across the central andean fold thrust belt northern bolivia implications for plateau evolution abstract quantifying the erosional and kinematic evolution of orogenic plateaus has been limited by insufficient age constraints on their deformation and erosion histories palinspastic restorations suggest the central andean fold thrust belt and plateau evolved concurrently in bolivia we present an analysis that synthesizes 19 new and 32 previous apatite and zircon fission track and u th he mineral cooling ages along a 200 km traverse across the plateau margin and entire thrust belt in northern bolivia the new apatite fission track data are interpreted using a grain age deconvolution algorithm with inverse thermal modeling of track lengths grain ages and mineral composition proxy data results suggest 1 eo oligocene 40 25 ma initial rapid erosion of the plateau margin 2 accelerated distributed erosion across the entire thrust belt since the early to mid miocene 15 ma and 3 the magnitude of erosion decreases eastward from 10 to 4 km we compare these results with two end member models of the central andes that contrast in duration and magnitude of deformation the rapid eo oligocene 40 25 ma erosion is only consistent with the end member that emphasizes the long duration and large magnitude deformation controlled by the sequential stacking of basement thrust sheets however the distributed miocene 15 ma to recent erosion is consistent with both end members because the recorded cooling could have resulted from active deformation protracted erosion or both if the long duration model is correct the time between the two phases of accelerated cooling brackets the cessation of the first basement thrust sheet and implies the early development of the andean plateau analogous to its modern width but unknown elevation by the early miocene 20 ma

prediction of anisotropy from flow models a comparison of three methods observations of anisotropy in earth are used regularly as constraints for models of deformation using various assumptions about the relationship between deformation and the resulting anisotropic fabric we compare three methods for calculating fabric from velocity fields tracking of finite strain ellipses a kinematic crystallographic code and the evolution of directors we find that the use of the finite strain ellipse provides only limited prediction capabilities as it cannot reproduce experimental observations that involve recrystallization the crystallographic code we tested a variant of the popular code d rex provides a more complete fabric prediction but at a much higher computational cost the directors method provides an intermediate solution while it does not include some of the more complex crystallographic processes that d rex does the results of this method closely resemble those of d rex at a lower computational cost the directors are also capable of tracking anisotropy at much larger strains than d rex we conclude that when computation speed is important for example in self consistent geodynamic models with anisotropic rheology the directors method provides an appropriate approximation

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

sedimentary response to orogenic exhumation in the northern rocky mountain basin and range province flint creek basin west central montana middle eocene through upper miocene sedimentary and volcanic rocks of the flint creek basin in western montana accumulated during a period of significant paleoclimatic change and extension across the northern rocky mountain basin and range province gravity modelling borehole data and geologic mapping from the flint creek basin indicate that subsidence was focused along an extensionally reactivated sevier thrust fault which accommodated up to 800 m of basin fill while relaying stress between the dextral transtensional lewis and clark lineament to the north and the anaconda core complex to the south northwesterly paleocurrent indicators foliated metamorphic lithics 64 ma 40ar 39ar muscovite grains and 76 ma u pb zircons in a ca 27 ma arkosic sandstone are consistent with oligocene exhumation and erosion of the anaconda core complex the core complex and volcanic and magmatic rocks in its hangingwall created an important drainage divide during the paleogene shedding detritus to the nnw and ese

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

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

anisotropic viscosity changes subduction zone thermal structure abundant observations of seismic anisotropy in subduction zones attest that the material in the mantle wedge has a strong fabric and therefore should be mechanically anisotropic in this paper we examine the effect of anisotropic viscosity on the thermal structure of subduction zone mantle wedges and quantify its importance relative to other thermal and rheological factors using two dimensional finite element kinematic models we find that anisotropic viscosity results in two substantial changes a hotter slab wedge interface and time variability of the melt production rate and excess temperatures although not as significant as the effect of temperature dependent viscosity anisotropy leads to an increase of up to 35 c in the temperature along the slab wedge interface a hotter slab wedge interface can change the depth extent of the seismogenic zone limit the depth to which hydrous minerals can carry water and influence flux melting time variability of the thermal field is a novel result of adding anisotropic viscosity to our models this time variability results from heterogeneity in material alignment and could explain temporal changes in subduction zone magmatism without invoking a change in the wedge geometry slab age or composition

tectonics of the yellowstone hotspot wake in southwestern montana deposits off a neogene paleovalley provide a geologic datum in southwestern montana that brackets the age of faulting associated with the yellowstone hotspot ages of paleovalley deposits on top of the modern blacktail ruby tendoy and centennial ranges and in adjacent valleys range from 16 to 2 ma at least 2 km of offset has occurred since emplacement of the timber hill basalt 6 0 0 1 ma much may have occurred after deposition of the 2 0 ma huckleberry ridge tuff which is now found in isolated outcrops as far north as the ruby range gravel clasts within the paleovalley have a northern provenance whereas volcanic and volcaniclastic rocks were derived from the snake river plain to the south we propose that uplift associated with passage of the hotspot caused the drainage reversal

paleosols in clastic sedimentary rocks their geologic applications interest in pre quaternary paleosols has increased over the past decade in large part because they have proved to be beneficial in solving diverse geological problems the majority of paleosols are described from continental deposits most commonly from alluvial strata criteria for recognizing these paleosols have been extensively described however classifying them has proved more complicated pre quaternary paleosols are generally classified according to one or more modern soil classification systems although one new classification has been proposed exclusively for paleosols to avoid problems using the modern soil classifications in addition to taxonomic classification paleosols can be categorized according to the interplay among deposition erosion and the rate of pedogenesis when they formed paleosols can be solitary if they formed during a period of landscape stability following the development of an unconformity such paleosols are commonly thick and extremely well developed more commonly paleosols are vertically stacked or multistory because they formed in sedimentary systems undergoing net aggradation if erosion was insignificant and sedimentation was rapid and unsteady compound paleosols generally formed if the rate of pedogenesis exceeded the rate of deposition composite paleosols developed thick cumulative paleosols indicate that erosion was insignificant and that sedimentation was relatively steady both autogenic and allogenic processes can influence depositional and erosion patterns and thus affect the kinds of soils that form consequently paleosols can help to interpret the history of sediment deposition and the autogenic and allogenic processes that influenced a sedimentary basin paleosols are also helpful in stratigraphic studies including sequence stratigraphic analyses they are used for stratigraphic correlations at the local and basinal scale and some workers have calculated sediment accumulation rates based on the degree of paleosol development in addition to their stratigraphic applications paleosols can be used to interpret landscapes of the past by analyzing paleosol landscape associations at different spatial scales ranging from local to basin wide in scope at the local scale lateral changes in paleosol properties are largely the result of variations in grain size and topography at the scale of the sedimentary basin paleosols in different locations differ because of basinal variations in topography grain size climate and subsidence rate paleosols are used to reconstruct ancient climates even to estimate ancient mean annual precipitation map and mean annual temperature mat ancient climatic conditions can be interpreted from modern soil analogs or by identifying particular pedogenic properties that modern studies show to have climatic significance stable carbon and oxygen isotopes are also used to interpret ancient climate and some effort has been made to estimate mat from isotopic composition on the basis of

oxygen isotope evidence for high altitude snow in the laramide rocky mountains of north america during the late cretaceous and paleogene the 18 o values of river waters in late cretaceous and paleogene basins of alberta montana wyoming and colorado are calculated from the isotopic composition of unaltered aragonite mollusk fossils the isotopic composition of these river waters has significant implications for the uplift and climate history of the mountains surrounding the basins our data show that the 18 o values of river water varied dramatically ranging from 23 to 5 relative to vienna standard mean ocean water with an uncertainty of 2 close stratigraphic association of river deposits with very different isotopic values indicates two different water sources rainfall in the warm low elevation basins and snowfall in surrounding mountains the 18 o difference between basinal and highland precipitation suggests that local relief was similar to today9s on the order of 2 5 3 km seasonal variation in shell 18 o values also indicates that there are times when large snow fields in the maastrichtian and paleocene mountains of montana and wyoming survived summer melting and were multiannual features

biogeochemistry and microbial diversity in the marine cavity beneath the mcmurdo ice shelf antarctica ice shelves surround 75 of antarctica s coastline and are highly sensitive to climate change several have recently collapsed and others are predicted to in the near future marine waters beneath ice shelves harbor active ecosystems while adjacent seas can be important areas of bottom water formation despite their oceanographic significance logistical constraints have resulted in few opportunities to directly sample sub ice shelf cavities here we present the first data on microbial diversity and biogeochemistry beneath the mcmurdo ice shelf mis near ross island antarctica physicochemical profiles obtained via a 56 m deep borehole through the mis revealed three vertically layered water masses antarctic surface water aasw ice shelf water isw and modified high salinity shelf water mhssw metabolically active moderately diverse shannon diversity from 2 06 to 5 74 microbial communities were detected in the aasw and mhssw heterotrophic bacterial production and dissolved organic matter concentrations were higher 12 37 and 24 respectively in mhssw relative to aasw chemoautotrophic production was 5 3 nmol c l 1 d 1 and 6 0 nmol c l 1 d 1 in the aasw and mhssw respectively phytoplankton cells were more abundant and larger in the mhssw sample relative to the aasw which indicates sinking of phytoplankton produced in surface waters and together with southerly flowing currents 0 09 0 16 m s 1 horizontal advection of phytoplankton from mcmurdo sound advected phytoplankton carbon together with in situ chemoautotrophic production provide important sources of organic matter and other reduced compounds to support ecosystem processes in the dark waters in the ice shelf cavity