the outer hebrides fault a major proterozoic structure in nw britain mylonites in the outer hebrides fault zone on scalpay hams are cut by metabasite sheets which are probably of scourie dyke age 2400 2200 ma and by pegmatites of late laxfordian age c 1750 ma the mylonites contain relicts of a gneissose banding which probably formed in the archaean c 2700 ma ago these age constraints suggest that the fault formed in the inverian event c 2500 ma ago not in the caledonian silurian devonian as widely reported the fault is a long lived structure which was reactivated as an extensional fualt in the laxfordian late proterozoic torr idonian and the carboniferous early mesozoic and as a thrust in the caledonian in a regional context this interpretation has important implications for the structural evolution of the lewisian complex for the nature of the caledonian event in the laurentian craton caledonian foreland and for the age and geometry of the sedimentary basins in the hanging wall of the outer hebrides fault

upper mantle reflector structure and origin beneath the scottish caledonides the mantle reflector pattern beneath the scottish and irish caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle previous investigations interpreted the reflector as a thrust related to the caledonian orogeny silurian early devonian or as a normal fault or shear zone active during post caledonian e g devonian or permo triassic extension this study uses all available deep seismic reflection data to map mantle reflector structure along the caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology the distribution of the mantle reflector shows that it is not spatially related to either caledonian compressional or subsequent basin extensional structures in a simple way a structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform synform pair beneath the west orkney basin north of the scottish mainland and the faeroe shetland basin west of shetland respectively separated by an inferred zone of structural disruption a working hypothesis for this study is that these two surfaces may have once been part of a continuous more or less planar surface that has been offset and deformed by either or perhaps a combination of two processes 1 differential extension of crust that was at least partially coupled to the uppermost mantle beneath the west orkney and faeroe shetland basins 2 a broad approximately orthogonal i e east west trending zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust our results also suggest that the mantle reflector structure may be inherited from neoproterozoic events associated with rifting of the laurentia baltica supercontinent

deformation of the caledonide lithosphere of northwest scotland abstract an interpretation of the moist seismic reflection profile along the north coast of scotland has shown the pattern of deep crustal structure across the margin of the caledonian orogen with the hebridean cratonic foreland to the northwest it is proposed that the lower crust of the orogen is characterised by a suite of easterly dipping thrusts that divide it into flakes that were imbricated during lateral compression of the lithosphere the effects of concomitant and subsequent uplift and erosion led to the removal of the major part of the upper crust within the orogen so that the present crust and moho are largely the relics of the caledonian lower crust as time progressed the thrust front encroached further into the foreland and cut down to a basal decollement at successively deeper levels this behaviour is explained as a consequence of the response of the brittle ductile transition to changing the temperature regime rock composition and strain rate as the orogeny proceeded palinspastic reconstructions of the whole lithosphere illustrate the process following the compressional phase of the orogeny uplift led to initiation of extension and the reactivation of the thrusts as normal listric faults rotation of basement blocks gave rise to wedge shaped sedimentary basins

a regional scale climate reconstruction of the last 4000 years from lakes in the nebraska sand hills usa abstract high resolution paleohydrological reconstructions were carried out in five shallow lakes in the nebraska sand hills across an east west transect in order to 1 determine whether long term droughts of the past 4000 years were spatially and temporally coherent across the region 2 distinguish local variation in climate or hydrology from regional patterns of change and 3 compare the paleolimnological results with the existing dune inferred drought records diatom inferred lake level was reconstructed for all sites and compared with other regional records alterations between high and low lake levels were frequent during the past 4000 years which suggests that shifts between dry and wet periods were prevalent across the sand hills extended multi decadal to centennial scale droughts were more common prior to 2000 years bp while the last two millennia were hydrologically more variable and climate conditions alternated on shorter timescales despite some discrepancies among the five records the paleohydrological reconstructions refine the holocene drought history of the nebraska sand hills particularly between 2200 and 4000 cal a bp many of the observed drought events are contemporaneous with severe droughts documented at sites in the northern great plains and rocky mountains lending support for the severity and regional significance of these events in western north america

geochemical reconstruction of pacific decadal variability from the eastern north pacific during the holocene determining climate variations over the holocene requires high resolution records with well developed age models a 40 m long marine sediment core raised from effingham inlet an anoxic fjord on the west coast of vancouver island british columbia canada yields such a record forty six 14c accelerator mass spectrometry ams dates determined from terrestrial plant material form the age model downcore sampling at both 5 cm 20 year and 1 5 cm 7 year resolution indicates that high frequency oceanographic variability has prevailed at this site over the last 10 000 years spectral analysis of wt opal a proxy for diatom productivity in the basin reveals the bidecadal and pentadecadal periods of the pacific decadal oscillation pdo north pacific index npi that are related to changes in the strength of the aleutian low coherence analysis between the effingham inlet data and 18o records from jellybean lake a high elevation site in southwest yukon indicates regional coherence at periods of 45 7

holocene lake level trends in the rocky mountains u s a abstract the availability of water shapes life in the western united states and much of the water in the region originates in the rocky mountains few studies however have explicitly examined the history of water levels in the rocky mountains during the holocene here we examine the past levels of three lakes near the continental divide in montana and colorado to reconstruct holocene moisture trends using transects of sediment cores and sub surface geophysical profiles from each lake we find that mid holocene shorelines in the small lakes 4 110 ha were as much as 10 m below the modern lake surfaces our results are consistent with existing evidence from other lakes and show that a wide range of settings in the region were much drier than today before 3000 2000 years ago we also discuss evidence for millennial scale moisture variation including an abruptly initiated and terminated wet period in colorado from 4400 to 3700 cal yr bp and find only limited evidence for low lake stands during the past millennium the extent of low water levels during the mid holocene which were most severe and widespread ca 7000 4500 cal yr bp is consistent with the extent of insolation induced aridity in previously published regional climate model simulations like the simulations the lake data provide no evidence for enhanced zonal flow during the mid holocene which has been invoked to explain enhanced mid continent aridity at the time the data including widespread evidence for large changes on orbital time scales and for more limited changes during the last millennium confirm the ability of large boundary condition changes to push western water supplies beyond the range of recent natural variability

the influence of climate hydrology and permafrost on holocene peat accumulation at 3500 m on the eastern qinghai tibetan plateau peatland of the eastern qinghai tibetan plateau lies at the convergence of the east asian and indian monsoon systems in eastern asia to understand the evolution of this peatland and its potential to provide new insights into the holocene evolution of the east asian monsoon a 6 m peat core was collected from the undisturbed central part of a peat deposit near hongyuan the age depth profile was determined using 16 14c ams age dates the peat analysed for a range of environmental variables including carbon nitrogen and hydrogen concentration bulk density 13c and the associated spring water analysed for hydrogen and oxygen isotopes the age depth profile of the recovered peat sequence covers the period from 9 6 to 0 3 kyr bp and is linear indicating that the conditions governing productivity and decay varied little over the holocene using changes in carbon density organic carbon content and its 13c cold dry periods of permafrost characterised by low density and impeded surface drainage were identified the low 18o and d values of the spring water emanating around the peat deposit down to 13 8 and 102 vsmow respectively with an inverse relationship between electrical conductivity and isotopic composition indicate precipitation under colder and drier conditions relative to the present day in view of the current annual mean air temperature of 1 c this suggests conditions in the past have been conducive to permafrost inferred periods of permafrost correspond to independently recognised cold periods in other holocene records from across china at 8 6 8 2 7 8 5 6 4 2 3 1 and 1 8 1 5 kyr bp the transition to a cold dry climate appears to be more rapid than the subsequent recovery and cold dry periods at hongyuan are of longer duration than equivalent cold dry periods over central and eastern china light dark banding peat on a scale of 15 30 years from 9 6 to 5 5 kyr bp may indicate a strong influence of decadal oscillations possibly the pacific decadal oscillation and a potential link between near simultaneous climatic changes in the northwest pacific enso movement of the intertropical convergence zone and the east asian monsoon

millennial scale climate fire vegetation interactions in a mid elevation mixed coniferous forest mission range northwestern montana usa mixed coniferous forests are widespread at middle elevations in the northern rocky mountains yet relatively little is known about their long term vegetation and fire history pollen and charcoal records from twin lakes in the mission range of northwestern montana provide information on mixed coniferous forest development and fire activity over the last 15 000 years these data suggest an open parkland and minimal fire activity before 13 500 cal yr bp consistent with cold dry conditions increases in pinus pollen after 13 500 cal yr bp indicate a transition to closed forests and a slight rise in fire activity as conditions warmed and fuel biomass increased high levels of artemisia pollen between 10 000 and 6000 cal yr bp suggest an open forest during the early holocene when conditions were warmer and drier than present low severity fires likely maintained open forest shrublands but produced little charcoal during this interval present day mixed coniferous forests were established in the last 6 ka and included forest taxa characteristic of low pseudotsuga larix pinus ponderosa and high severity fire regimes picea abies increased climate variability and anthropogenic burning may have helped maintain heterogeneous mixed coniferous forests during the last several millennia when climate conditions became cooler and wetter

the influence of depth on reactivation in normal faulting abstract the study examines the influence of depth on the development of steep normal faults above a gently dipping fault which is reactivated in its deeper part and abandoned in its upper part the isotropic and anisotropic mohr coulomb anderson theory is used to determine the ability of any part of a fault to be reactivated by calculating the difference between the stress deviator necessary to induce reactivation and that required to induce fracturing when linear depth dependent properties are assumed it appears that a new normal fault can branch off a gentle pre existing fault only if the friction coefficient of the latter is much less than that used by byerlee and if the ratio cohesion of fault cohesion of intact rock is non negligible for a 30 dip fault the friction coefficient has to be less than 0 35 friction angle less than 20 neither pore fluid pressure nor constant excess fluid pressure affects the above conditions when the cohesion of the surrounding rocks increases with depth steep normal faults can branch off byerlee faults with dips as gentle as 30 a low shear strength of the lowest part of the preexisting fault also increases the chances of reactivation of the deeper part and abandonment of the upper part

from rifting to active spreading in the lau basin havre trough backarc system sw pacific locking unlocking induced by seamount chain subduction associated with pacific australia plate convergence the lau basin havre trough is an active back arc basin that has been opened since 5 5 ma by rifting and southward propagating oceanic spreading current back arc opening rates decrease from 159 mm yr 1 in the northern lau basin to 15 mm y 1 in the southern havre trough major tectonic changes occur at the transition between havre trough rifting and full oceanic spreading of the eastern lau spreading center elsc where the oblique to trench westward subducting louisville seamount chain lsc sweeps southwards along the tonga trench new swath bathymetry seismic reflection data and limited rock sampling in this area constrain a tectonic and kinematic back arc model that incorporates the effects of lsc subduction the elsc which extends southward to 24 55 s forms a deep rift valley propagating southward through older rifted arc basement present day seismicity and fresh and fractured pillow lavas at 23 42 s are consistent with rift valley neovolcanism conversely the northern havre trough has low seismicity and rifted volcanic basement ridges trending 25 45 oblique to the basin axis consistent with low levels of extensional tectonism and volcanism this latter structural fabric is interpreted as an early stage of rifting that is now locked due to compression on the arc exerted by lsc subduction while in the lau basin such effects have passed as the lsc swept along the tonga trench it is proposed that the lau havre back arc opening is controlled by tectonic constraints exerted at the limits of the system by the lsc subduction which determines the southward migration of the tonga arc pole of rotation and associated lau basin opening a discrete three stage back arc opening evolution is proposed comprising 1 an initial phase of back arc rifting along the whole length of the plate boundary beginning at 6 5 ma 2 a subsequent phase mostly present in the southern part of the back arc domain and still active in the havre trough of transpression and transtension starting at 4 ma in the north as the lsc starts to subduct and sweeps southward along the tonga trench and 3 a renewed opening phase in the northern segment of the back arc domain with rifting and spreading starting at 3 5 ma as subduction of the lsc along the northern tonga trench is progressively completed

crustal kinematics and deep structure of the northern rhine graben germany surface and subsurface geological data unpublished information gleaned from industrial seismic surveys and results from a deep reflection seismic profile across the tertiary northern rhine graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km comparison of a simple geometric kinematic cross sectional model and its depth extrapolation with the dekorp 9 n deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle the planar lower crustal reflectors probably originated mainly during late variscan permocarboniferous crustal extension and or magmatic underplating

the polarity of deep seismic reflections from the lithospheric mantle evidence for a relict subduction zone abstract birps offshore deep seismic reflection profiles to the north of scotland have revealed two bright continuous reflectors in the continental lithospheric upper mantle the dipping flannan and sub horizontal w reflectors the polarity of reflections from the flannan has been determined from normal incidence reflection data using the far field source wavelet as a starting model the effective far field wavelet was calculated by adding a receiver ghost and the effects of recording filters attenuation during transmission and sea bottom multiples to the source wavelet reflection polarity was determined in a blind test by comparing the modelled wavelet with stacks of the flannan reflection in eight out of ten stacks the reflection was picked as positive polarity two out of ten were unknown to verify this result the test was repeated for moho reflections in this case nine out of ten stacks were picked as positive the modelling also demonstrated that the flannan reflection is apparently from a simple interface complex interlayering is not required to explain the waveforms and they are not consistent with reflections from a single thin layer seismic data acquired at wide angle across the w mantle reflector show sub crustal high amplitude arrivals which can only be explained as post critical reflections only a high velocity eclogitic layer contained within normal mantle with a sharp upper boundary and a diffuse base can explain all our observations we suggest that the flannan reflector represents the top of a relict oceanic and eclogitic component of a pre caledonian subduction zone within the lithospheric mantle

3 d magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins summary 20 magnetotelluric mt soundings were collected on the isle of skye scotland to provide a high resolution three dimensional 3 d electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity seismic geological and mt data the full 3 d inversion of the mt data jointly interpreted with gravity data reveals upper crustal structure the main features of the model are interpreted in conjunction with previous geological mapping and borehole data our model extends to 13 km depth several kilometres below the top of the lewisian basement the top of the lewisian basement is at approximately 7 8 km depth and the topography of its surface was controlled by precambrian rifting during which a 4 5 km thick sequence of torridonian sediments was deposited the mesozoic sediments above which can reach up to 2 2 km thick have small scale depocentres and are covered by up to 600 m of tertiary lava flows the interpretation of the resistivity model shows that 3 d mt inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units where conventional seismic reflection methods may fail

patterns of terrestrial and limnologic development in the northern greater yellowstone ecosystem usa during the late glacial early holocene transition abstract a high resolution record of pollen charcoal diatom and lithologic data from dailey lake in southwestern montana describes postglacial terrestrial and limnologic development from ice retreat ca 16 000 cal yr bp through the early holocene following deglaciation the landscape surrounding dailey lake was sparsely vegetated and erosional input into the lake was high as summer insolation increased and ice recessional processes subsided picea parkland developed and diatoms established in the lake at 13 300 cal yr bp closed subalpine forests of picea abies and pinus established at 12 300 cal yr bp followed by the development of open pinus and pseudotsuga forests at 10 200 cal yr bp increased planktic diatom abundance indicates a step like warming at 13 100 cal yr bp and alternations between planktic and tychoplankic taxa suggest changes in lake thermal structure between 12 400 and 11 400 cal yr bp an increasingly open forest in combination with increased benthic diatoms indicates warm dry summers during the early holocene after 11 400 cal yr bp in contrast to nearby records in northern yellowstone that register prolonged summer wet conditions until ca 8000 cal yr bp because of its low elevation dailey lake was apparently sensitive to the direct effects of increased summer insolation on temperature and effective moisture registering dry summers in contrast higher elevations in northern yellowstone responded to the indirect effects of an amplified seasonal insolation cycle on atmospheric circulation including elevated winter snowpack and or increased summer convective storms as a result of enhanced monsoonal circulation

age and structure of the southern rockall trough new evidence abstract the rockall trough separates the rockall plateau microcontinent from the shelf and slope west of the british isles the structure and age of the trough has been the source of considerable discussion although widely considered to be of oceanic origin postulated ages for the spreading range from permian to cretaceous new seismic profiles linked to the ipod sites in the bay of biscay and to oceanic anomalies of known age are used to present a new assessment of the age and structure of the southern rockall trough it is concluded that about 120 km of ocean crust is present in the trough and that spreading took place in the albian maastrichtian interval

a diatom record of late holocene climate variation in the northern range of yellowstone national park usa the changes in diatom species composition in a sediment core from crevice lake yellowstone national park spanning the past 2550 yr were used to reconstruct long term limnological and ecological conditions that may be related to late holocene climate variability planktic forms dominate the fossil diatom assemblages throughout this record but changes in species dominance indicate varying nutrient levels over time particularly phosphorus the changes in the nutrient concentrations in the lake were probably driven by changes in temperature and wind strength that affected the duration of water column mixing and thus the extent of nutrient recycling from deep waters prior to 2100 cal before present bp stephanodiscus minutulus and synedra tenera dominated suggesting long cool springs with extensive regeneration of phosphorus from the hypolimnion that resulted from isothermal mixing from 2100 to 800 cal bp these species were replaced by cyclotella michiganiana and cyclotella bodanica these species are characteristic of lower nutrient concentrations and are interpreted here to reflect warm summers with long periods of thermal stratification from 800 to 50 cal bp s minutulus dominated the diatom assemblage suggesting a return to lengthy mixing during spring the most dramatic late holocene changes in the fossil diatom assemblages occurred during the transition from the medieval period to the little ice age approximately 800 cal bp

three dimensional modeling of lacustrine diatom habitat areas improving paleolimnological interpretation of planktic benthic ratios three dimensional modeling of lake morphometry enables the calculation of lake volume planar surface area and basin surface area which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level we have applied three dimensional modeling to foy lake montana and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake the model allows us to produce semiquantitative depth reconstructions and greatly improves lake level reconstruction of this morphometrically complex basin the conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record comparison of the model results with both long and short sedimentary records from foy lake montana suggests that planktic benthic ratios may be a better predictor of mean lake depth than maximum lake depth the model developed here is simple enough to be easily implemented has few requirements can be adapted to suit any lake basin and can be augmented to examine a variety of different lake processes that affect habitat availability

evidence for extensional shear zones in the mantle offshore britain and their implications for the extension of the continental lithosphere deep seismic profiles recorded in the seas around the british isles have revealed bands of seismic reflections in the uppermost mantle on the basis of their appearance geophysical characteristics and association with major crustal faults these features are best interpreted as mantle shear zones furthermore in the north sea the mantle reflectors appear to follow an extensional trend and when interpreted as extensional shear zones help explain the regional stratigraphy of the overlying basin the crosscutting relationships between mantle reflections to the west of scotland and a major strike slip fault the great glen fault ggf suggest that the mantle structure postdates the ggf and hence is probably related to subsequent extension furthermore kinematic analysis of lithospheric deformation to the north of scotland is consistent with the structure observed on the seismic data if the flannan mantle reflection is an extensional shear zone reconstruction of the lithosphere prior to permian extension restores the flannan to the edge of the rockall trough a rift probably initiated in the carboniferous thus the flannan fault may have developed during carboniferous rifting it is concluded that most of the mantle reflections are some form of extensional shear zone in the mantle and the role of such structures during lithospheric extension is considered

holocene climate recorded by magnetic properties of lake sediments in the northern rocky mountains usa we present two hypotheses regarding the evolution of holocene climate in the northern rocky mountains that stem from a previously unpublished environmental magnetic record from jones lake montana

the structure of the rockall trough from seismic refraction and wide angle reflection measurements there are few detailed descriptions of the deep crustal structure of failed cratonic rifts and passive margins despite their import ance in constraining the variety of structural models invoked to account for their common structural symmetry or gross half graben form1 3 such deep crustal structure profiles also have considerable value in defining the continent ocean boundary con straining plate reconstructions and determining the overall setting of sedimentary basin development the rockall trough in the north atlantic ocean is a sinuous elongate bathymetric depression 200 km wide and more than 2 000 m deep lying between rockall plateau and the british isles and the structure nature and age of its underlying crust are disputed in the absence of deep crustal structure profiles the presence of oceanic crust has been assumed because the trough has to be accommodated in reconstructions of the north atlantic although stretched continental crust is equally plausible the nature of the crust is important for the precise reconstruction of the late jurassic early cretaceous plate history and setting of sedimentary basins of the north atlantic4 8 the nature of the crust beneath the trough as well as the structure thickness and composition of the sedimentary cover is also relevant to oil exploration here we report the first observations of deep crustal structure across the rockall trough and its margins from seismic refraction and wide angle reflection profiles our results show that the northern rockall trough is underlain by highly stretched continental crust and not by oceanic crust as previously postulated

the variscan fold thrust belt of southeast ireland and its control on early mesozoic extension and deposition a method to predict the sherwood sandstone

northern scotland as an atlantic north sea divide president9s anniversary address 1984 from the end of the caledonian orogeny until the present northern scotland has acted as a structural high separating the north sea and atlantic basins or their forerunners the geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre existing basement structure from devonian to early palaeocene times the massif was generally low lying and its behaviour with respect to vertical movements was essentially neutral the land surface carved in the crystalline basement during the devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins tectonic events of major importance in the basins such as the formation of the north sea and hebridean rift systems produced little change of conditions in the intervening massif it is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching the uneventful history of the massif was interrupted by two phases of mafic igneous activity in late carboniferous and palaeocene times which record responses to regional plate motions the formation of the palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the north sea the style of these events and the anomalous character of the continental margin off nw britain may have been determined by the refractory nature of the precambrian basement which inhibited stretching prior to sea floor spreading

paleolimnological records of climatic change in north america lacustrine fossil records provide long time series of data on limnological and climatic conditions these data are useful for establishing natural patterns of climate variability and for generating testable hypotheses about atmospheric circulation and climate ecosystem linkages shoreline features can indicate past lake level fluctuations that may reflect changes in moisture balance but often these records are discontinuous and are evidence of only extreme conditions the organisms geochemistry and sedimentology of lake sediments may provide a more continuous sequence of direct and indirect lake climate interactions in the past the most clearly interpretable paleolirnnological records of climatic change are those that use several lines of evidence to corroborate a climatic hypothesis and are from sites near an ecotone or in regions of extreme climate in all cases hydrologic setting mediates a lake s response to climate and must be considered in interpreting sedimentary sequences discussion of the impact of recent and future climatic changes necessitates a baseline against which the magnitude of observed or predicted changes can be measured commonly we look to recent history as a reference point for comparison however the landscape of recent decades or in some cases even centuries has been altered greatly by human activities thus to determine whether recent or future climatic change is unique or unusual we need long natural preanthropogenic time series against which observed or predicted climatic states and variability can be compared paleolimnological archives are a tool for looking at environmental patterns prior to anthropogenic impact and show a range of climatic states far greater than those documented by written records former strand lines of pluvial lakes bonneville and lahontan in the western u s for example testify to moisture extremes unsurpassed in recent centuries or even millennia in addition one can us the paleolimnological record as an independent climatic proxy against which patterns and rates of terrestrial response to climatic change can be compared here i present a broad overview of paleoclimatic reconstruction in north america from lacustrine records with a focus on the late quaternary the review is limited to climatic inference from paleolirnnological records a term i use in a restrictive sense to refer to records of past lake states physical biological and geochemical as opposed to records of past stream hydrology e g ely et al 1993 knox 1993 or terrestrial conditions i summarize paleolimnological tools available for reconstruction of past climate the capabilities and limitations of these tools

holocene seasonal variability inferred from multiple proxy records from crevice lake yellowstone national park usa article i nfo a 9400 yr old record from crevice lake a semi closed alkaline lake in northern yellowstone national park was analyzed for pollen charcoal geochemistry mineralogy diatoms and stable isotopes to develop a nu anced understanding of holocene environmental history in a region of northern rocky mountains that re ceives both summer and winter precipitation the limited surface area conical bathymetry and deep water 31 m of crevice lake create oxygen deficient conditions in the hypolimnion and preserve annually laminated sediment varves for much of the record pollen data indicate that the watershed supported a closed pinus dominated forest and low fire frequency prior to 8200 cal yr bp followed by open parkland until 2600 cal yr bp and open mixed conifer forest thereafter fire activity shifted from infrequent stand replacing fires initially to frequent surface fires in the middle holocene and stand replacing events in recent centuries low values of 18 o suggest high winter precipitation in the early holocene followed by steadily drier conditions after 8500 cal yr bp carbonate rich sediments before 5000 cal yr bp imply warmer summer conditions than after 5000 cal yr bp high values of molybdenum mo uranium u and sulfur s indicate anoxic bottom waters before 8000 cal yr bp between 4400 and 3900 cal yr bp and after 2400 cal yr bp the diatom record indicates extensive water column mixing in spring and early summer through much of the holocene but a period between 2200 and 800 cal yr bp had strong summer stratification phosphate limita tion and oxygen deficient bottom waters together the proxy data suggest wet winters protracted springs and warm effectively wet summers in the early holocene and less snowpack cool springs warm dry sum mers in the middle holocene in the late holocene the region and lake experienced extreme changes in win ter spring and summer conditions with particularly short springs and dry summers and winters during the roman warm period 2000 cal yr bp and medieval climate anomaly 1200 800 cal yr bp long springs and mild summers occurred during the little ice age and these conditions persist to the present although the proxy data indicate effectively wet summer conditions in the early holocene and drier conditions in the middle and late holocene none point specifically to changes in summer precipitation as the cause in stead summer conditions were governed by multi seasonal controls on effective moisture that operated over multiple time scales

a 2200 yr record of hydrologic variability from foy lake montana usa inferred from diatom and geochemical data abstract a 2200 yr long high resolution 5 yr record of drought variability in northwest montana is inferred from diatoms and 18o values of bio induced carbonate preserved in a varved lacustrine core from foy lake a previously developed model of the diatom response to lake level fluctuations is used to constrain estimates of paleolake levels derived from the diatom data high frequency decadal fluctuations in the de trended 18o record mirror variations in wet dry cycles inferred from banff tree rings demonstrating the sensitivity of the oxygen isotope values to changes in regional moisture balance low frequency multi centennial isotopic changes may be associated with shifts in the seasonal distribution of precipitation from 200 b c to a d 800 both diatom and isotope records indicate that climate was dry and lake level low with poor diatom preservation and high organic carbon nitrogen ratios subsequently lake level rose slightly although the climate was drier and more stable than modern conditions at a d 1200 lake level increased to approximately 6 m below present elevation after which the lake fluctuated between this elevation and full stage with particularly cool and or wetter conditions after 1700 the hydrologic balance of the lake shifted abruptly at 1894 because of the establishment of a lumber mill at the lake s outlet spectral analysis of the 18o data indicates that severe droughts occurred with multi decadal 50 to 70 yr frequency

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

increased northeast pacific climatic variability during the warm middle holocene 1 we use a pair of decadally resolved planktonic foraminiferal 18o records from santa barbara basin sbb to show that the warmest interval of the current interglacial the middle holocene was characterized by increased decadal to centennial scale climatic variability modern relationships between the pacific decadal oscillation pdo the el nino southern oscillation enso and upper water conditions in sbb also suggest a preference of the northeast ne pacific climate during the warm mid holocene toward the warm phase of the pdo and more intense enso warm events el ninos in the context of future climate this study suggests increasing decadal to centennial scale climatic variability in a globally warmed world

modes of climate variability synthesis and review of proxy based reconstructions through the holocene abstract modes of climate variability affect global and regional climates on different spatio temporal scales and they have important impacts on human activities and ecosystems as these modes are a useful tool for simplifying the understanding of the climate system it is crucial that we gain improved knowledge of their long term past evolution and interactions over time to contextualise their present and future behaviour we review the literature focused on proxy based reconstructions of modes of climate variability during the holocene i e the last 11 7 thousand years with a special emphasis on i proxy based reconstruction methods ii available proxy based reconstructions of the main modes of variability i e el nino southern oscillation pacific decadal variability atlantic multidecadal variability the north atlantic oscillation the southern annular mode and the indian ocean dipole iii major interactions between these modes and iv external forcing mechanisms related to the evolution of these modes this review shows that modes of variability can be reconstructed using proxy based records from a wide range of natural archives but these reconstructions are scarce beyond the last millennium partly due to the lack of robust chronologies with reduced dating uncertainties technical issues related to proxy calibration and difficulty elucidating their stationary impact or not on regional climates over time while for each mode the available reconstructions tend to agree at mutidecadal timescales they show notable disagreement on shorter timescales beyond the instrumental period the reviewed evidence suggests that the intrinsic variability of modes can be modulated by external forcing such as orbital solar volcanic and anthropogenic forcing the review also highlights some modes experience higher variability over the instrumental period which is partly ascribed to anthropogenic forcing these features stress the paramount importance of further studying their past variations using long climate proxy records for the progress of climate science

multidecadal drought and holocene climate instability in the rocky mountains time series analysis of a diatom inferred drought record suggests that holocene hydroclimate of the northern rocky mountains has been characterized by oscillation between two mean climate states the dominant climate state was initiated at the onset of the holocene ca 11 ka under this climate state drought was strongly cyclic recurring at frequencies that are similar to twentieth century multidecadal phase changes of the pacific decadal oscillation this pattern remained consistent throughout much of the midholocene continuing until ca 4 5 ka after this time the mean climate state changed and drought recurrence became unstable periods of cyclic drought alternated with periods of less predictable drought the timing of this shift in climate was coincident with widespread severe drought in the mid continent of north america overall the strongest periodicity in severe drought occurred during the mid holocene when temperatures in the northern rocky mountains were warmer than today

support for tropically driven pacific decadal variability based on paleoproxy evidence two independent proxy reconstructions of sea surface temperature reflect a common pattern of pacific decadal sea surface temperature variability over the past two centuries since the pattern extends to both the north ern and southern hemispheres this result supports the idea that pacific decadal variability is a basin wide phenomenon originating in thetropics the causes of pacific decadal climate variability here termed pdv are the subject of much debate most work describing pdv has focused on the north pacific where the leading eof of low pass filtered sea surface temperature

crustal structure of the basin and ridge system west of new caledonia southwest pacific from wide angle and reflection seismic data 1 during the zoneco 11 marine geophysical survey september 2004 two deep reflection seismic profiles recorded by ocean bottom seismometers were acquired in the offshore domain west of new caledonia the northern profile crosses the new caledonia basin the fairway ridge the fairway basin and the lord howe rise the southern profile crosses the norfolk rise south of new caledonia the new caledonia basin the fairway ridge and basin and ends at the foot of lord howe rise on the northern profile the lord howe rise has a crustal thickness of 23 km and exhibits seismic velocities and velocity gradients characteristic of continental crust the crust thins to 12 15 km in the neighboring fairway basin which is interpreted to be of thinned continental origin based on the seismic velocities the crustal thickness of the fairway rise is 22 km and it is also interpreted to be of continental origin the new caledonian basin is underlain by crust of 10 km thickness which shows unusally high velocities between 7 0 and 7 4 uncharacteristic for either thinned continental or oceanic crust on the southern profile the norfolk rise is also found to be of continental nature here the new caledonia basin shows velocities crustal thickness and basement roughness characteristic of typical oceanic crust the crust in the fairway basin shows higher velocities than on the northern profile which could be caused by volcanic intrusions into the crust during extension a deep reflector in the upper mantle was imaged underneath the new caledonian basin on the northern profile

paleolimnological response to millennial and centennial scale climate variability during mis 3 and 2 as suggested by the diatom record in les echets france a 27 m long sediment sequence retrieved from the central part of the les echets basin in france has been analysed in sub centennial resolution for biogenic silica and fossil diatom remains the sequence corresponds to the later part of marine isotope stage mis 3 and to most of mis 2 distinct changes in diatom productivity diversity and taxonomic composition between 36 2 and 31 7 kyr bp appear to relate to dansgaard oeschger do climate variability intervals characterized by low diversity productivity and small sized benthic diatom taxa are most likely a response to colder conditions in relation to do stadials in contrast higher diversity productivity and a high abundance of planktonic taxa indicate a response to warmer temperatures during do interstadials the time interval between 30 3 and 15 7 kyr bp is characterized by continuous low diatom productivity and a benthic dominated community with intermediate species richness suggesting a transition to more stable conditions three time intervals with extremely low concentrations of diatom valves 46 1 36 2 31 7 30 3 and 26 3 23 6 kyr bp overlap with ages reported for heinrich h events 4 3 and 2 we speculate that the lake at les echets suffered from severe ecological stress as a response to h events this is the first detailed study exemplifying the response of a lake based on diatoms to climate variability during late part of mis 3 and most of mis 2 in europe

high resolution sea surface reconstructions off cape hatteras over the last 10 ka 1 this study presents high resolution foraminiferal based sea surface temperature sea surface salinity and upper water column stratification reconstructions off cape hatteras a region sensitive to atmospheric and thermohaline circulation changes associated with the gulf stream we focus on the last 10 000 years 10 ka to study the surface hydrology changes under our current climate conditions and discuss the centennial to millennial time scale variability we observed opposite evolutions between the conditions off cape hatteras and those south of iceland known today for the north atlantic oscillation pattern we interpret the temperature and salinity changes in both regions as co variation of activities of the subtropical and subpolar gyres around 8 3 ka and 5 2 3 5 ka positive salinity anomalies are reconstructed off cape hatteras we demonstrate for the 5 2 3 5 ka period that the salinity increase was caused by the cessation of the low salinity surface flow coming from the north a northward displacement of the gulf stream blocking the southbound low salinity flow concomitant to a reduced meridional overturning circulation is the most likely scenario finally wavelet transform analysis revealed a 1000 year period pacing the 18o signal over the early holocene this 1000 year frequency band is significantly coherent with the 1000 year frequency band of total solar irradiance tsi between 9 5 ka and 7 ka and both signals are in phase over the rest of the studied period

holocene vegetation fire and climate history of the sawtooth range central idaho usa the paucity of low and middle elevation paleoecologic records in the northern rocky mountains limits our ability to assess current environmental change in light of past conditions a 10 500 yr long vegetation fire and climate history from lower decker lake in the sawtooth range provides information from a new region initial forests dominated by pine and douglas fir were replaced by open douglas fir forest at 8420 cal yr bp marking the onset of warmer conditions than present presence of closed douglas fir forest between 6000 and 2650 cal yr bp suggests heightened summer drought in the middle holocene closed lodgepole pine forest developed at 2650 cal yr bp and fires became more frequent after 1450 cal yr bp this shift from douglas fir to lodgepole pine forest was probably facilitated by a combination of cooler summers cold winters and more severe fires than before five drought episodes including those at 8200 cal yr bp and during the medieval climate anomaly were registered by brief intervals of lodgepole pine decline an increase in fire activity and mistletoe infestation the importance of a holocene perspective when assessing the historical range of variability is illustrated by the striking difference between the modern forest and that which existed 3000 yr ago

displacement of the moho by the outer isles thrust shown by seismic modelling deep seismic data fig 1 from the caledonides off the north west coast of scotland collected 1981 82 by the british institutions reflection profiling syndicate birps show evidence that caledonian thrusts have been reactivated as mesozoic normal faults1 2 the best example is the outer isles oi thrust which is mapped on land as a compressional fault and is revealed by these data to be overlain offshore by a thick wedge shaped mesozoic rift basin1 known as the north lewis nl basin here we have used synthetic seismic sections to identify distortions of the oi thrust reflection due to the nl basin models which compensate for the effects of this distortion show that the oi thrust has a steeper angle of dip than has been predicted previously assuming no distortion causing the oi thrust to project into a 2 3 km offset of the moho and hence to penetrate the entire crust our data suggesting that shear strains affecting the lower crust may be localized in character have adverse implications for rheological models which predict to the contrary that the lower crust is a region where strain is distributed

decadal sea surface temperature variability in the subtropical south pacific from 1726 to 1997 a d we present a 271 year record of sr ca variability in a coral from rarotonga in the south pacific gyre calibration with monthly sea surface temperature sst from satellite and ship measurements made in a grid measuring 1 degrees by 1 degrees over the period from 1981 to 1997 indicates that this sr ca record is an excellent proxy for sst comparison with sst from ship measurements made since 1950 in a grid measuring 5 degrees by 5 degrees also shows that the sr ca data accurately record decadal changes in sst the entire sr ca record back to 1726 shows a distinct pattern of decadal variability with repeated decadal and interdecadal sst regime shifts greater than 0 75 degrees c comparison with decadal climate variability in the north pacific as represented by the pacific decadal oscillation index 1900 1997 indicates that several of the largest decadal scale sst variations at rarotonga are coherent with sst regime shifts in the north pacific this hemispheric symmetry suggests that tropical forcing may be an important factor in at least some of the decadal variability observed in the pacific ocean

mesozoic rocks from the hebrides shelf and implications for hydrocarbon prospectivity in the northern rockall trough abstract shallow boreholes drilled in the west lewis and west flannan basins outer hebrides shelf have proved rocks of probable latest bajocian bathonian to barremian age where previously a mainly permo triassic infill has been assumed the full geological succession of the basins remains unproved although the permo triassic history of the basins may be similar the rocks encountered suggest that the basins may have dissimilar geological histories during the jurassic and early cretaceous lithologies are mainly fine grained poorly sorted sandstones or organic rich mudstones some correlation can be made with the isle of skye and the west shetland area the sandstones are poor hydrocarbon reservoirs but the middle jurassic and ryazanian mudstones although immature where drilled have excellent oil source rock potential and may have generated large amounts of hydrocarbons if they are present at depth in the adjacent northern rockall trough

hinterland directed transtensional faulting at an orogen structural front the example of the cap chat melange quebec appalachians the cap chat melange is a regional scale dismembered unit that occurs close to the quebec appalachian front the melange records early d 1 taconian and late d 3 acadian regional deformations the greater degree of disruption of the melange compared with surrounding units is due to the superimposition of d 2 structures that are rare outside the melange d 2 structures include both layer extensional features such as block disruption along shear zones and faults and layer contractional features such as folds that record the transtensional collapse of the taconian orogenic wedge along a rheologically weak deformation zone the cap chat melange is interpreted as the westernmost remnant of a zone of mechanical decoupling in the upper crust which allows normal or transtensional faulting in the inner parts of the orogen this paper presents evidence that beyond the general complexity of melange units geological observations may still contribute to unraveling their tectonic history and that transtensional reactivation of original thrust faults is a viable mechanism in the frontal parts of orogens

new accelerator mass spectrometry radiocarbon ages for the mazama tephra layer from kootenay national park british columbia canada charcoal fragments recovered from the mazama air fall tephra layer in cores from dog and cobb lakes kootenay national park british columbia yielded accelerator mass spectrometry ages of 6720 70 and 6760 70 14c years bp respectively these two new ages together with other previously published radiocarbon ages on charcoal and twig fragments from mazama air fall deposits indicate that the climactic eruption of mount mazama occurred 6730 40 14c years bp

deep seismic reflection structure of the caledonian orogenic front west of shetland although the moine thrust has been well studied on land in northwestern scotland the subject of its deep crustal structure and regional continuation offshore remains controversial here we use post stack migration and filtering of deep seismic reflection profiles crossing the projection of the moine thrust and coincident caledonian orogenic front around the western margin of the shetland platform in order to characterize the crustal structure of the thrust in an area not substantially affected by basin formation and extension and where onshore geological constraints can be more readily applied this study provides for the first time seismic reflection constraints on middle and lower crustal structure down to the level of the moho discontinuity underlying the thrust the seismic data reveal a thick wedge of east dipping reflections in the lower crust capped by a highly coherent reflection which can be correlated to the postulated moine thrust as previously mapped on shetland the continuation of this wedge from the middle crust down to the moho against which individual reflections are abruptly truncated suggests a thick skinned interpretation for the caledonian orogen beneath the western flank of the shetland platform the available seismic data from around shetland imply that the moine thrust caledonian front forms a boundary between an upper plate which is relatively featureless structurally and a lower plate composed of interpreted northwest verging thrusts possibly a crustal duplex structurally below the level of the moine nappe our observations are thus in contrast with previous interpretations made from seismic data in the west orkney basin north of the northwest highlands that describe a highly reflective moinian upper plate

late quaternary eolian sand depositional record for southwestern kansas landscape sensitivity to droughts abstract eolian deposits and landforms are ubiquitous in western kansas particularly south of the arkansas river with the presence of a stabilized dune field stratigraphic studies and associated optical dating reveal a complex depositional history for this dune field spanning the late quaternary the oldest eolian deposits from ca 16 to 12 ka completely or partially bury high terraces with ages of ca 16 and 33 ka it is unknown whether these eolian deposits reflect regional aridity and or a change in sediment availability with the transition of the arkansas river from a single channel to a braided system there is pervasive evidence for episodic eolian erosion and deposition ca 9 8 to 6 3 ka generally coincident with loess deposition on upland surfaces olson c g nettleton w d porter d a brasher b r 1997 middle holocene aeolian activity on the high plains of west central kansas holocene 7 3 255 261 and thus it is inferred to reflect regional aridity sites within the dune field show a sequence of eolian sands and weak buried soils reflecting either dune migration or sand sheet accretion at ca 1490 430 380 320 180 and 70 yr ago which correspond well with continental scale droughts in the tree ring record eolian sand in the arkansas river dune field may be derived principally with variability in fluvial activity and in climate during the late pleistocene to the middle holocene with substantial reactivation of eolian systems during decadal scale drought variability in the past 2000 yr

three dimensional mapping of seismic reflections from the crust and upper mantle northwest of scotland abstract a survey area 190 km by 110 km to the northwest of scotland on the uk continental shelf has been the site of intense exploration using the deep seismic reflection technique a closely spaced grid of deep reflection data totalling 1600 line km has been acquired by the birps group between 1981 and 1987 the majority of the data were recorded to 15 s two way travel time 50 km depth profiles with recording times up to 60 s 230 km depth were also acquired the survey includes a two ship synthetic aperture profile the slave line which synthesises a 0 16 km offset cdp profile the main features revealed and consistently imaged across the survey area by this combined dataset are an upper crust containing half graben style basins bounded to the west by planar faults a reflective lower crust and dipping and sub horizontal high amplitude reflections within the mantle three dimensional mapping of the dipping reflectors shows a complex package of reflections which updomes into the lower crust in the central area of the survey disrupting the moho and continuing into the uppermost mantle where it thins to a couplet of reflections in the north and south of the survey area this structure is observed only in the mantle to a depth of 80 km this dataset is unique in deep seismic reflection profiling in terms of the quality of the image returned the great depth to which reflections are imaged and the intensity of surveying of the lower crust and upper mantle the continuity and strength of the reflectors seen on these records show that even at the great depths at which these structures exist the seismic reflection technique is able to image them as high amplitude coherent and highly continuous reflections the reflections are interpreted as shear zones or faults within the lower crust and upper mantle this indicates that in this part of the world at least both the lower crust and upper mantle are able to sustain discrete zones of deformation

inherited structural control from repeated cretaceous to recent extension in the north mernoo fault zone western chatham rise new zealand abstract the active north mernoo fault zone nmfz is the youngest of three discrete phases of basement involved extensional faulting to deform the northwestern corner of the continental chatham rise since the cretaceous the first two phases inferred to be of late cretaceous and eocene age respectively were both part of widespread extensional systems that developed over most of the chatham rise region the third phase the late miocene to recent nmfz developed on the northwestern corner of the rise as a consequence of the interaction between the chatham rise plateau and the southern end of the hikurangi subduction zone a study of seismic reflection data from the northwestern chatham rise has revealed 1 1 the detailed structure that relates to each phase of normal faulting 2 2 the patterns of fault reactivation during successive episodes of extension 3 3 the significant structural control that older deformational fabrics have had on the development of the nmfz during the late cretaceous phase of extension two major fault bounded half grabens together with other smaller basins developed beneath the upper continental slope these e w trending s dipping faults are inferred to have accommodated about 2 7 km of horizontal extension across a 30 km wide region the early cenozoic extensional system reactivated some cretaceous half graben structures but mainly formed new faults with similar geometry in crust north of the cretaceous structures the early cenozoic faults are inferred to have accommodated about 1 4 km of horizontal extension across a 60 km wide region the late cenozoic nmfz also reactivated some of the cretaceous normal faults and almost all of the major early cenozoic normal faults in this region in addition many new extensional structures developed and produced an e w to wnw ese trending southward dipping domino style array of faults up to 100 km in width although the density of faults in the nmfz is greater than that of the two previous extensional systems to include this region the total horizontal extension since late early pliocene times is estimated to be no more than 3 km the common reactivation of old normal faults during the late cenozoic development of the nmfz indicates a significant level of structural inheritance from the previous episodes of extension to effect the region furthermore the parallelism between the newly developed faults and the older two extensional systems suggests that the normal faults could be reactivating structures in the torlesse terrane basement which in turn relate to an early mesozoic phase 100 ma of accretionary tectonics the kinematics of active extension within the nmfz is therefore likely to be influenced by the pre existing structural discontinuities that resulted from a history of poly phase deformation

geological inheritance and crystal dynamics of the northwest scottish continental shelf abstract deep seismic reflection profiles have been combined with conventional seismic surveys across the northwest scottish continental shelf to interpret basin development in the region post caledonian basins are built upon partially reactivated older structures which have strongly influenced their internal structure and stratigraphy we show that the minch basin along with several others developed in the hangingwall of the partially reactivated outer isles fault a planar fault dipping at about 30 ese that reaches nearly to the moho the internal structure of these hangingwall basins is compared with appropriate scaled analogue models to show that they have developed by passive hangingwall collapse above a footwall block that moved towards the wnw this geometry is incorporated into a lithospheric dynamic model with northwest extension towards the rockall trough from a fixed lithosphere beneath the scottish highlands this model requires non linear lithosphere extension which is accommodated by movements on faults and block rotation in the upper crust extension and thinning of the lower crust along a network of anastomosing ductile shears extensional movement of the flannan fault in the upper mantle and horizontal displacement along the moho beneath the outer hebrides this dynamic model has wider application for other extensional regions

the influence of pre existing thrust faults on normal fault geometry in nature and in experiments relations between normal faults and pre existing thrust faults are classically described in terms of three basic situations normal faults can cross cut thrust faults they can branch out from thrust faults at depth on a de collement level or they can entirely reactivate thrust planes the mechanical aspects of these types of interaction were studied by analogue modelling in which sand simulates the brittle rocks and silicone putty an interlayered de collement the models underwent compression producing thrust faults with variable dips followed by extension three possible ways of interaction are described here a no interaction occurs in the case of low dip thrust faults 32 1 and normal faults are developed independently displaying a listric geometry b branching at depth on the de collement level occurs when dip of the thrust faults reaches 32 1 in this case the dip of the normal faults whose geometry becomes planar decreases with increasing thrust dip we suggest that this change in dip of normal faults depends upon the rotation of stress tensor axes along the pre existing fault zone where a drop in the friction coefficient is likely to occur c reactivation occurs in brittle material when dip of the pre existing fault exceeds 41 1

crustal architecture across phanerozoic australia along the eromanga brisbane geoscience transect evolution and analogues abstract the crustal architecture across phanerozoic australia has been established from seismic data along an 1100 km long east west transect the eromanga brisbane geoscience transect this has enabled a better understanding of deep structures and processes that have controlled the development of major sedimentary basin systems it has shown that crustal dynamics throughout geological history have played an important role in the development of these sedimentary basins and that structures developed during the early palaeozoic have influenced and continue to influence basin systems the transect crosses three major basement provinces of the tasman orogenic system in eastern australia the thomson northern lachlan and new england orogens the basement geology in the transect region has until now been only poorly understood because it is largely obscured by the mesozoic cover rocks of the eromanga surat and clarence moreton basins the transect interpretation has firmly identified crustal scale ramp structures multiple intra crustal detachment surfaces strike slip fault architecture lower crustal magmatism underplating mono remobilisation and intra crustal terranes in the geological reconstructions of southern queensland the boundaries between the orogens the foyleview and burunga mooki geosutures have been identified as lithospheric scale structures that have influenced the evolution of the tasman orogenic system as a whole

deep structure of the foreland to the caledonian orogen nw scotland results of the birps winch profile the winch marine deep seismic reflection profile crosses the hebridean shelf the proterozoic foreland to the caledonian orogen west of scotland the data quality is very good the upper crust is largely devoid of coherent seismic reflections although this may in part be due to acquisition techniques being inappropriate for this problem in contrast the middle and lower crust 10 25 km depth exhibits good reflections the mid crust contains reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust offshore in a normal sense in mesozoic times caused the formation of the minch and north lewis basins the mono is defined by a strong band of reflections at about 27 km depth which correlates well with results from the hmse explosion seismic survey moho depth is apparently rather uniform in the area of the foreland crossed by winch winch and hmse results together suggest a mean crustal velocity for the hebridean shelf of 6 4 0 1 km s 1 the eastward dipping flannan thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth well into the upper mantle neither the flannan thrust nor the outer isles thrust appear to pass straight through the reflective lower crust suggesting that the lower crust is a region of high strain the outer hebrides is a positive block probably formed as an isostatic response to mesozoic normal faulting which reactivated the outer isles thrust

linkage between mantle and crustal structures and its bearing on inherited structures in northwestern scotland deep seismic reflection profiles in scotland reveal mantle structures beneath a crust with a polyphase tectonic history that resulted in several generations of structures continuum mechanics suggests that coeval mantle and crustal structures must be kinematically linked inherited structures imply relative ages for the reflectors ages that can be placed into the context of the geological history of the near surface rocks of northern scotland thus some mantle reflectors are assigned triassic ages related to the opening of the west orkney and related marginal basins of the atlantic ocean other mantle reflectors are cut by late caledonian structures associated with the great glen fault zone and therefore older than c 400 ma many of these structures also track the late precambrian margin of laurentia and may be related to either the opening 900 600 ma or closing 500 600 ma of the iapetus ocean some reflective structures may also be attributed to 1800 1700 ma laxfordian deformation that was part of a global scale orogenic belt

oxygen isotope records of holocene climate variability in the pacific northwest abstract oxygen isotope 18o measurements of authigenic carbonate from cleland lake southeastern british columbia paradise lake central british columbia and lime lake eastern washington provide a 9000 year holocene record of precipitation evaporation balance variations in the pacific northwest both cleland lake and paradise lake are small surficially closed basin systems with no active inflows or outflows lime lake is surficially open with a seasonally active overflow water isotope values from cleland and paradise plot along the local evaporation line indicating that precipitation evaporation balance is a strong influence on lake hydrology in contrast lime lake water isotope values plot on the local meteoric water line signifying minimal influence by evaporation to infer past hydrologic balance variations at a high temporal resolution we sampled the cleland paradise and lime lake sediment cores at 1 60 mm intervals 3 33 years per sample on average and measured the isotopic composition of fine grained

basin structure and tertiary magmatism on skye nw scotland the emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre existing basin structures these controls are investigated using examples from the tertiary igneous complexes of skye part of the sea of hebrides basin of nw scotland the basin consists of an array of half graben related to se dipping normal faults these pre volcanic mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields the unconformity represents a period of cretaceous uplift and denudation of the basin and its flanks entirely pre dating the tertiary volcanism of nw scotland this unconformity seals stratigraphically the major basin faults preserving field relationships that permit the tracing of these faults in the country rocks to the tertiary intrusions the major camasunary fault is separated from the raasay fault via a series of minor graben linked by a series of steep nw se trending faults that transfered mesozoic displacements between the principal fault strands a broad range of igneous material of various compositions was intruded into part of the mesozoic sea of hebrides basins and their flanks during palaeocene times different emplacement styles and different structural controls are found the major gabbroic centres do not appear to be controlled by upper crustal structures having been emplaced into the footwalls of major faults however minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions thereby facilitating emplacement in contrast the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile reactivating segments of the basin fault network doming occurred from an array of sills the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions the sill levels and their transgressive forms are strongly related to inferred mesozoic basin structures the major fold structures of tertiary age in southern skye are interpreted as accommodating granitic emplacement rather than crustal shortening the nw se mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes these interpretations are illustrated using field relationships mapped in the vicinity of the coire uaigneich granophyre it is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions the deeper levels of the mesozoic faults show no evidence of substantial reactivation

lake sediment records of holocene hydroclimate and impacts of the mount mazama eruption north central washington usa abstract multi proxy sediment records from castor lake and scanlon lake north central washington provide a late quaternary perspective on lake catchment hydrologic and ecosystem responses to climate change and the mazama volcanic ashfall event analyses of authigenic carbonate mineral oxygen and carbon isotope values organic carbon and nitrogen content and sedimentological facies were conducted on sediment from castor lake in order to reconstruct lake catchment hydrologic balance the source and abundance of organic matter and variations in lake level sedimentary facies characterization was conducted on the scanlon lake sediment in order to provide supporting evidence for the lake level shifts inferred from the castor lake data marked changes in castor lake proxy values occur from 12 860 to 11 440 calendar years before present yr bp suggesting lake catchment responses to the younger dryas cold reversal including higher lake levels substantial variability in productivity and increased catchment erosion high 18o values at 9630 yr bp indicate that lake levels were lower than at present and that the early holocene was dry sedimentological analyses and 18o data demonstrate that subsequent to the mazama climactic eruption 7600 yr bp a transition to the lowest lake levels of the holocene occurred over several centuries with maximum 18o values at 7290 90 uncertainty range 7020 7500 yr bp and low lake levels persisting until 6190 5960 6410 yr bp lithological changes in the scanlon lake record support these inferences the lowest 18o values of the holocene occur in sediment from 5000 yr bp indicating high lake levels at this time after which a secular trend toward higher 18o values and lower lake levels occurred the prolonged lowstand in the centuries following the mazama eruption suggests that catchment hydrologic characteristics were strongly impacted by the ashfall through effects on soil water retention capacity that similar responses may have occurred in comparable settings within the mazama ashfall zone and that such volcanic events have the potential confound the climate signal in lake sediment records were a similar event to happen in the future catchment hydrology stream river discharge and lake catchment ecosystem dynamics could be affected for centuries

boundary conditions in sandbox models of crustal extension an analysis using distinct elements abstract the geometries of faults that form in the brittle hanging wall above large low angle detachments has been an active topic of research in petroleum geology and geophysics for the last decade in order to understand the progressive development of the geologic structures and to identify possible petroleum traps experimentalists have used sandbox models to try to gain insight into the kinematics and mechanics of the deformation of overburden above detachments although the structures observed in some of the models resemble those observed in nature boundary conditions and constitutive properties of these models have rarely been measured precisely so the experiments are often difficult to interpret we use an entirely numerical approach based on the distinct element method dem in which the overburden is represented by hundreds of individual elements that interact mechanically through contact forces the faults that form in an assemblage of elements resemble those observed in nature the boundary conditions are prescribed as are the material reactions of individual elements so the numerical experiments are more easily interpreted we suggest that dem models complement sandbox models and can be used to interpret the progressive development of geologic structures

decadal scale climate drivers for glacial dynamics in glacier national park montana usa 1 little ice age 14th 19th centuries a d glacial maxima and 20th century retreat have been well documented in glacier national park montana usa however the influence of regional and pacific basin driven climate variability on these events is poorly understood we use tree ring reconstructions of north pacific surface temperature anomalies and summer drought as proxies for winter glacial accumulation and summer ablation respectively over the past three centuries these records show that the 1850 s glacial maximum was likely produced by 70 yrs of cool wet summers coupled with high snowpack post 1850 glacial retreat coincides with an extended period 50 yr of summer drought and low snowpack culminating in the exceptional events of 1917 to 1941 when retreat rates for some glaciers exceeded 100 m yr this research highlights potential local and ocean based drivers of glacial dynamics and difficulties in separating the effects of global climate change from regional expressions of decadal scale climate variability

some observations on basin evolution and dynamics sedimentary basin evolution is intimately bound in with continental lithosphere dynamics deep seismic reflection profiling has revealed images of the crust and upper mantle that suggest linkages between the shape and structure of sedimentary basins as they evolve with the rheological responses to tectonic stress of upper and lower crust and upper mantle components of the lithosphere this is well illustrated by the european geotraverse lithosphere profile from north cape norway to tunisia the alpine mountain belt has a lithospheric root as well as a crustal root by virtue of its asymmetric disposition this may be the cause of compressive and tensile forces in close association in the upper crust which have created mountain uplift and basin subsidence extension and thrusting all in juxtaposition evidence for an elastic upper layer of crust above a plastic substratum can be used as a basis for modelling basin geometry structure and stratigraphy built upon the flexural nature of the upper crust such models go a long way towards simulating the evolution of basins such as the north sea viking graben in more detail analogue structural models can reveal the geometry of basin fill in relationship to crustal movement and the relationships between structure and sedimentation patterns however sedimentary basin modelling needs to begin with the quantitative simulation of depositional environments in relation to such factors as tectonics sea level variations sediment supply and erosion in order to match sedimentary architecture only then can the range of deposits be placed in space and time within a basin environment as building blocks within a dynamically evolving structural framework to create predictive models for comparison with geological reality