channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

vector constraints on western u s deformation from space geodesy neotectonics and plate motions the rate of slip vector on the san andreas fault in central california estimated from geodetic and holocene geological data 34 3 mm yr n41 w 2 is inconsistent with the prediction of rigid plate models such as rm2 56 3 mm yr n35 w 2 this well known san andreas discrepancy is diagnostic of plate deformation distributed both east of the fault in the basin and range and west of the fault along the california continental margin we construct constraints on the integrated deformation rates across these two regions consistent with 1 the kinematical boundary conditions imposed by the rigid plate model 2 neotectonic and paleoseismic estimates of deformation rates 3 ground based geodetic measurements and 4 rates of change observed by very long baseline interferometry along seven baselines to western u s sites the space geodetic data on basin and range extension taken over a 4 year interval are compatible with geological observations averaged over the holocene the best estimate of its integrated deformation rate provided by the joint inversion of both data types is 9 7 2 1 mm yr n56 w 10 too small and in the wrong direction to account entirely for the san andreas discrepancy we therefore attribute much of it to deformation west of the sierra nevada great valley block accommodated for example on faults such as the san gregorio hosgri system of coastal california the integral of this deformation estimated by subtracting the basin and range contribution from the discrepancy vector requires significant right lateral shear parallel to the san andreas 13 5 mm yr and some compression perpendicular to it 9 3 mm yr

viscosity structure of the crust and upper mantle in western nevada from isostatic rebound patterns of the late pleistocene lake lahontan high shoreline received 13 july 2005 revised 26 october 2006 accepted 31 january 2007 published 8 june 2007 1 large lakes can both produce and record significant crustal deformation we present an analysis of the isostatic rebound pattern recorded in the shorelines of paleolake lahontan in western nevada using a layered maxwell viscoelastic model the inferred viscosity structure depends on loading history we use three variants of a well documented lake surface elevation model as input and recover corresponding estimates of viscosity and density structure a simple two layer model with an elastic plate over an inviscid half space fits the observed elevation pattern quite well with a residual variance of 32 of the data variance using multilayered finite viscosity models the residual variance is reduced to 20 of the data variance which is very near to the noise level in the higher resolution models the viscosity is below 10 18 pa s over the depth range from 80 to 160 km the minimum viscosity is very similar to the value that has been seen in the eastern great basin from similar analyses of lake bonneville shorelines but the lowviscosity zone is thinner beneath bonneville making small adjustments to a seismically derived density structure allows an improved fit to the shoreline observations additionally we find that small variations in proposed loading models can result in presumably spurious density inversions and suggest that this modeling approach provides a test for loading histories

intraplate extensional tectonics of the eastern basin range inferences on structural style from seismic reflection data regional tectonics and thermal mechanical models of brittle ductile deformation the general lack of a correlation between earthquakes and surface faulting in the interior of the western u s cordillera has motivated our efforts to evaluate the geometry structural style and mechanism of normal faulting characteristic of this region of intraplate extension to address the problem we have interpreted over 1500 km of seismic reflection data and constructed detailed cross sections of the upper crust rheological models of the continental crust were calculated to examine the possibility of shallow quasi plastic flow and its influence on faulting in the eastern basin range the western colorado plateau and the middle rocky mountains our data and interpretations have revealed the following styles of cenozoic deformation 1 steep to low angle dip normal faulting along the wasatch fault 2 low angle dip and listric normal faulting possibly associated with movement on preexisting thrusts 3 the occurrence of asymmetric mostly eastward tilted tertiary basins that are bounded by low to moderate dipping planar and listric faults and 4 at least three vertically stacked en echelon low angle reflections in the mid to upper crust that dip gently westward from 3 km beneath the wasatch plateau to over 15 km at the utah nevada border these reflections are interpreted as normal detachment faults the structural style of the pervasive low to moderate angle dipping faults cannot be easily reconciled with classic brittle failure theory but the interpreted termination of normal faults at or above the deeper low angle reflections suggests the presence of shallow zones of ductile deformation that may have accommodated slip an important observation based on interpretations of seismic reflection profiles is that normal fault zones dip more gently in the subsurface than their associated scarps in unconsolidated surficial deposits segment boundaries of the wasatch fault zone apparently coincide with the positions of east trending displacement transfer structures in thrust sheets and the position of the precambrian uinta aulacogen suggesting that preexisting crustal structure partly controls the geometry of extension to examine the influence of ductile deformation quasi plastic flow was modeled for appropriate geotherms of the basin range and colorado plateau crusts these models provide constraints on the depth to the frictional quasi plastic transition that occurs as shallow as 8 km in the eastern basin range this depth also corresponds to the approximate depth above which most accurately determined earthquake foci for small earthquakes m 5 5 appear to nucleate at 10 15 km in or near the brittle ductile transition the rheological modeling suggests that a thermally deforming continental lithosphere plays an important role in the evolution of this extending intraplate region large scale low angle reflections may represent decollements and normal faults at the top of upper crustal zones of reduced shearing strength and higher ductility

mid to late holocene climate change in north central mongolia as recorded in the sediments of lake telmen abstract a record of climatic change has been reconstructed for the past 7110 calibrated calendar cal years for lake telmen mongolia based upon sedimentologic and geomorphic evidence from the lake watershed system lake telmen is a saline presently 4 g l 1 closed basin lake and is therefore sensitive to changes in effective moisture balance between 7110 and 6260 cal yr ago conditions in the lake telmen region were hyperarid and a small saline approximately 20 g l 1 lake was present increased effective moisture balance but still arid conditions prevailed between 6260 and 4390 cal yr ago since 4390 cal yr ago generally more humid conditions prevailed in the lake telmen region as the lake deepened a hypoxic hypolimnion became established below a well developed summer thermocline resulting in varved sediment accumulation since 4390 cal yr ago between about 2710 and 1260 cal yr ago a greater than present day effective moisture balance is recorded by a series of four lake highstand terraces sedimentologic profiles from lacustrine cores suggest an increase in effective moisture during the past approximately 680 cal yr

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

paleoshoreline geomorphology of boon tsagaan nuur tsagaan nuur and orog nuur the valley of lakes mongolia abstract we conducted a preliminary study of paleoshoreline features associated with boon tsagaan nuur tsagaan nuur and orog nuur lakes located in the gobi altai transition zone of the valley of lakes dolina ozor which stretches from central to western mongolia the paleoshoreline features were first identified on radarsat satellite sar imagery we investigated the features during the 1998 field season of the joint mongolian russian american archaeological expedition to the gobi altai region we identified paleoshorelines of multiple elevations in the field which are considered to be relict beach ridges and wave cut terraces other paleolake landforms include spits and gilbert type deltas these landforms are complex large and well established implying that the paleolakes were stable for extended periods the reconstructed paleolakes cover extensive areas of the valley floor implying that hydrological and climatic conditions were very different in the past paleolake expansions may have occurred under a variety of circumstances one hypothesis is that the high lake stands occurred during the wetter period corresponding to the oxygen isotope stage 3 prior to the last glacial maximum lgm during the warmest early holocene and the late holocene or during all these periods if low evaporation rates due to lower temperatures glacier meltwater and possibly increased precipitation are important factors then the expansions may have occurred during the terminal late glacial period after the last glacial maximum the greatly expanded lakes in the gobi altai could have significantly affected the quaternary human demography and migration in the region

glacio isostatic crustal movements caused by historical volume change of the vatnajokull ice cap iceland measurements of the lake level of lake langisjor at the sw edge of the vatnajokull ice cap indicate a tilt of 0 26 0 06 rad year away from the ice cap in the years 1959 1991 the tilt is too large to be explained as an elastic earth response to ice retreat this century or to be caused by change in the gravitational pull of the ice cap but it can be explained by sub lithospheric viscous adjustment regional subsidence in historical times in se iceland can similarly be attributed to viscous adjustment resulting from the increased load of vatnajokull during the little ice age the inferred sub lithospheric viscosity is 1 1018 5 1019 pa s

state of stress and modern deformation of the northern basin and range province constraints on the current stress regime of the actively extending northern basin and range province are provided by deformation data focal mechanisms and fault slip studies hydraulic fracturing in situ stress measurements borehole elongation breakouts analyses and alignment of young volcanic vents the integrated data indicate significant variations both in principal stress orientations and magnitudes an approximately e w least principal stress direction appears to characterize both the eastern and western margins of the basin and range province whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented nw to n60 w the contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins active deformation along the southeastern and western province margins is characterized by a combination of strike slip and normal faulting focal mechanisms along northeastern province margin wasatch front and in central nevada indicate a combination of normal and oblique normal faulting temporal regional and depth dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles however some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated evidence for significant temporal variation or multiple cycles of variation in relative stress magnitude comes from the sierran front basin and range boundary region where recent earthquakes are predominantly strike slip whereas the profound relative vertical relief across the sierra frontal fault zone in the last 9 10 m y implies a normal faulting stress regime using the best data on stress orientation relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements analysis of well constrained slip vectors in the owens valley california area indicate that large temporal variations in the magnitude of the approximately n s oriented maximum horizontal stress are required to explain dominantly dip slip and strike slip offsets on subparallel faults similar faulting relations are observed throughout much of the boundary zone between the basin and range sierra nevada including the walker lane belt along the eastern province margin in the wasatch front area in utah available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of 4 5 km earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth furthermore superimposed sets of young fault striae along a segment of the wasatch fault also indicate temporal variations of relative stress magnitudes sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion thermally induced flow and the influence of the san andreas plate boundary although difficult to date accurately the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle 1000 5000 years and multiple earthquake cycles 10 000 years

the holocene giant lake chad revealed by digital elevation models abstract the chad basin is a closed basin in the central part of north africa a southern sub basin which is supplied by the humid tropics includes the present day lake chad the northern sub basin is presently dry as it only receives drainage from sahara rivers in these areas the existence of a giant lake known as the lake mega chad has been debated for a long time its level would be controlled by the mayo kebbi threshold in the southern part of the lake which joined the lake mega chad and the niger river via the benoue trough digital elevation models the topo6 and globe data sets are used to characterise a well defined shelf like morphology locally up to 50 km wide followed at a constant elevation over hundreds of kilometers this strictly horizontal geomorphic feature is interpreted as a wave cut lacustrine shoreline terrace it is associated with a sandridge that represents a barrier island system built by wave generated currents this study demonstrates that a holocene lake mega chad was present from 11 n to 18 n across the southern and northern sub basins the influence of such a water vapour source must be considered in palaeoclimatological reconstructions

lake size variations in the lahontan and bonneville basins between 13 000 and 9000 14c yr b p abstract recessions of lakes lahontan and bonneville that commenced 13 500 14 c yr b p were interrupted at 11 500 14 c yr b p in the lahontan basin and 12 200 14 c yr b p in the bonneville basin by relatively large perturbations in lake level that persisted for 2000 years minor glacial readvances in the sierra nevada and white mountains of california nevada occurred during the latter half of this interval between 11 000 and 9700 14 c yr b p the hydrologic response of lakes lahontan and bonneville and the mountain glacial advances were concurrent with the allerod younger dryas climatic intervals recorded in vegetational and glacial records of western and central europe

carbonate deposition pyramid lake subbasin nevada 4 comparison of the stable isotope values of carbonate deposits tufas and the lahontan lake level record abstract in this paper the fundamental importance of changes in hydrologic balance and hydrologic state on the 18o and 13c values of water and dissolved inorganic carbon dic in lakes of the lahontan basin is illustrated abrupt changes in 18o and 13c values of carbonate deposits tufas from the pyramid lake subbasin nevada coincide with abrupt changes in lake level and hydrologic state minima in lake level at 26 000 15 500 and 12 000 yr b p are associated with relatively heavy 18o and 13c values maxima in the lake level record at 14 000 and 10 500 yr b p are associated with relatively light 18o and 13c values we believe that the correlation between maxima and minima in the lake level and 18o records reflect the fundamental effect of lake level dynamics on the 18o value of lake water evaporation increases the 18o value of lake water whereas streamflow discharge and on lake precipitation decrease the 18o value variation in the 18o value of lake water therefore indicates change in the hydrologic balance increases in 18o accompany decreases in lake volume and decreases in 18o accompany increases in lake volume covariance of 13c and 18o indicates that change in 13c values of dic also accompany change in lake volume we offer the hypothesis first put forward by j a mckenzie that change in the productivity photosynthesis respiration balance is responsible for much of the observed variation in 13c most great basin lakes including lake lahontan experienced changes in hydrologic state during the late wisconsin when a lake becomes hydrologically open the residence time of water decreases the greater the rate of spill the greater the volume of evaporated 18o enriched water removed from the spilling lake and the more negative the 18o value of water remaining in the spilling lake the concentration of dic as well as the concentrations of photosynthesis limiting nutrients e g phosphorus nitrogen silica molybdenum decrease as spill increases increasing rates of spill therefore lead to overall decreases in photosynthetic rates relative to respiration rates and as a consequence the 13c values of dic become more negative

paleoclimatic evolution of the uvs nuur basin and adjacent areas western mongolia abstract the investigations presented in this paper focus on the shifts in pleistocene glaciations and the geomorphic changes in landforms as well as lake level changes and aeolian deposits of the last glacial interglacial cycle including the holocene geomorphic evidence and high lake levels show that the climate was more humid before the last glacial maximum lgm however at least one arid phase also occurred during the second half of the lgm the climate was dry and cold turning to wet and cold during the late glacial of the last ice age fluctuations in humidity and temperature occurred during the holocene since about 2000 yr bp the impact of human activity has increased

quantitative characterization and elevation estimation of lake lahontan shoreline terraces from high resolution digital elevation models the precision with which the elevation of a feature such as a terrace can be measured depends on the characterization of the noise contaminating the measurement a method for identification and extraction of terrace feature elevations is presented and the topographic noise due to erosion as well as measurement error is quantified high resolution digital elevation models dem are acquired at six wave cut volcanic bedrock terrace sites from around the highstand of paleo lake lahontan in the western great basin local dems which are tied to regional geodetic control were acquired using conventional total station rapid postprocessed and real time kinematic global positioning system methods the topographic signal is processed with derivative filters for geomorphic feature recognition and averaging for noise reduction results indicate that noise levels for identifiable features such as riser crest knickpoint and slope inflection point are statistically equivalent and on the order of 0 5 m standard deviation averaging within topographic bins spanning 50 m along terrace strike yields feature elevation estimates with standard errors on the order of 0 12 m the mean bench window elevation between the riser crest and knickpoint has the lowest standard error and is systematically related to water level propagation of surveying geoid estimation and terrace feature elevation estimation errors indicates that displacements on the order of 0 5 m may be resolved using these methods elevation estimate interpretation involves terrace development degradation and neotectonics but this new methodology has significant advantages in studies of neotectonic or geomorphic processes using local terrace elevation measurements

a reevaluation of the isostatic rebound of lake bonneville recent data pertaining to the uplift of the provo and bonneville shorelines of the late pleistocene lake bonneville are evaluated in a reanalysis using a rigorous model of deformation of a plane earth comprising an elastic layer overlying a viscoelastic channel the new observations constrain the wavelength and amplitude of the uplift of two shorelines formed at two different epochs and they partly resolve the ambiguities in the plausible mechanical parameters found in previous solutions the results indicate that the effective thickness of the elastic lithosphere is 28 to 30 km equal to the seismically determined crustal thickness in the region the estimate of the upper mantle viscosity in the region varies from 1 5 3 4 lo19 pa s to 2 1 5 8 l018 pa s depending on whether the upper mantle is assumed to be of uniform viscosity or the flow is confined to a narrow channel in the asthenosphere the predicted rate of present uplift at the center of the desiccated lake is 0 06 mm yr and 0 03 mm yr for the shallow channel and uniform mantle models respectively these values are beyond the first order leveling accuracies and the question of the existence of a narrow channel in the region cannot be resolved with the available observations

level of lake lahontan during deposition of the trego hot springs tephra about 23 400 years ago abstract the trego hot springs tephra bed is a silicic tephra about 23 400 yr old found at several localities in pluvial lake sediments in northern nevada southern oregon and northeastern california it has been characterized petrographically by the major and minor element chemistry of its glass and by its stratigraphic position with respect to other tephra layers at a newly described locality on squaw creek northwest of gerlach nevada at the north end of the smoke creek desert trego hot springs tephra has been found in sediments of the sehoo and indian lakes formations the depositional environments of these sediments show that when the tephra fell pluvial lake lahontan stood between 1256 and 1260 m and that immediately thereafter the lake rose to at least 1275 m these data corroborate earlier findings by benson quaternary research 9 300 318 from radiometric dating of calcareous tufa however the lake lahontan area has been affected by isostatic subsidence and rebound in response to changing water loads so that caution is required in the use of lakeshore elevations in correlation

particle size and shape in pebble beach sedimentation an in depth investigation was made of beach gravel fabrics at two beach sites located on the glamorgan heritage coast wales u k granulometric data from 31 080 pebbles were obtained and analysis confirmed the existence of a basic shape differentiation structure when the b axis was used at gileston beach a low energy location and nash beach a high energy location the use of the b axis in size and shape analysis seems to have led some workers to incorrect rejection of size as a factor in determining the transportational and depositional potential of a particle an inferred energy model can be constructed where the influence of particle size on sorting is predominant when energy conditions are high and particle shape factors predominate when energy conditions are low the c axis standard deviation value was the most effective discriminator of variation in shore parallel sedimentation structures at both beaches for shore normal structural discrimination the c axis mean and standard deviation values were the most effective

highest pluvial lake shorelines and pleistocene climate of the western great basin abstract shoreline altitudes of several pluvial lakes in the western great basin of north america record successively smaller lakes from the early to the late pleistocene this decrease in lake size indicates a long term drying trend in the regional climate that is not seen in global marine oxygen isotope records at 70 m above its late pleistocene shoreline lake lahontan in the early middle pleistocene submerged some basins previously thought to have been isolated other basins known to contain records of older pluvial lakes that exceeded late pleistocene levels include columbus fish lake lake columbus rennie kobeh diamond lakes jonathan and diamond newark long lake hubbs and clover very high stands of some of these lakes probably triggered overflows of previously internally drained basins adding to the size of lake lahontan simple calculations based on differences in lake area suggest that the highest levels of these pluvial lakes required a regional increase in effective moisture by a factor of 1 2 to 3 relative to late pleistocene pluvial amounts assuming that effective moisture is directly proportional to the hydrologic index or lake area tributary basin area these previously unknown lake levels reflect significant changes in climate tectonics and or drainage basin configurations and could have facilitated migration of aquatic species in the great basin

dating of paleolakes in the central altiplano of bolivia abstract the deposits of the extensive paleolakes of the bolivian altiplano correspond to an alternation of lacustrine episodes and dry periods a phenomenon commonly explained by climatic changes the lack of precise dating of the deposits has prevented detailed reconstruction of the evolution of paleolakes that would allow a determination of the relative influence of interconnections between neighboring basins and local climatic conditions among the five paleolakes of the northern altiplano titicaca basin and the three of the central altiplano poopo coipasa and uyuni basins only the two most recent paleolakes have been dated a 121 m long sediment core recovered from the salar de uyuni contained 11 lacustrine layers l1 l11 separated by 12 salt crusts radiocarbon dating shows that the younger layers l1 and l2 are related to the tauca lacustrine phase 12 000 16 000 cal yr bp and layers l3 and l4 to the minchin lacustrine phase 30 000 73 000 cal yr bp layer l5 located at 46 m below the surface contains a volcanic ash with well preserved biotites is here dated by the 40 ar 39 ar method the measured age of 191 000 5000 y is the first date ever obtained from a pre minchin paleolake lake escara this result combined with 14 c and u th data obtained on lakes tauca and minchin from various authors suggests that the duration of the lacustrine events increased regularly during the pleistocene in contrast paleolake levels in the northern altiplano decreased from the oldest known lake mataro 3950 m to the present titicaca level 3806 m a progressive erosion of the threshold between the northern and central altiplano may have allowed important volumes of water to overflow from the northern basin into the central altiplano the existence of large paleolakes in the central altiplano was thus not only dependent on local climatic conditions but also on additional inputs from the north where climatic conditions could have been quite different from those prevailing in the south

lake bonneville constraints on lithospheric thickness and upper mantle viscosity from isostatic warping of bonneville provo and gilbert stage shorelines the deformed shorelines of the late pleistocene lake bonneville provide an excellent opportunity to analyze the response of the earth to surface loads of relatively short wavelengths 400 km and short duration 30 kyr recently revised estimates of the lake loading history and an expanded compilation of shoreline elevations for the bonneville 1550 m provo 1440 m and gilbert 1300 m shorelines have prompted us to reexamine the deflection the earth model we use is identical with that employed by nakiboglu and lambeck and consists of an elastic plate overlying a maxwell viscoelastic channel and a rigid base our calculation differs from that of nakiboglu and lambeck in two respects we used a significantly more detailed load model and we compared our results with observations from three distinct shorelines the lake elevation history spans 32 kyr in 18 piecewise linear segments the water load is computed at each time step from a digital terrain model 4 4 km cell size and a specified lake elevation we find that the use of three separate shorelines significantly reduces the acceptable region of parameter space the effective elastic lithospheric thickness is 23 2 km the mantle viscosity is 1 2 0 2 1020 pa s and the depth to a significant viscosity increase is no less than 300 km though it might be much more examination of the residual deflections differences between observed and computed shoreline elevations reveals a regional tilt down to the ne of 6 10 5 which we attribute to collapse of the peripheral bulge produced by the laurentide ice sheet

architecture and sequence stratigraphy of a late neogene incised valley at the shelf margin southern celtic sea abstract valleys on the outer celtic sea shelf were cut and filled during the late pliocene early pleistocene the kaiser valley is one of several valleys forming an anastomosed network the main valley directed 20 azimuth true n is 50 m deep and 10 km wide it is connected to the parallel dompaire and parsons valleys by several 120 140 directed incisions of lesser width and depth analyzed by mean of very high resolution seismic data the kaiser valley is interpreted as containing a compound fill consisting of eight erosionally based depositional sequences a typical sequence comprises two facies 1 fluvial channels at the base which represent lowstand to early transgressive deposits and 2 onlapping transgressive bay fill deposits that are locally interbedded at the top with isolated small channels attributed to flood tidal deltas the erosional bases of the fluvial facies correspond to sequence boundaries these are interpreted to result from relative sea level falls successive fluvial and bay fill facies are separated by flat erosional surfaces of high acoustic amplitude which extend laterally across the entire composite valley locally beveling sequence boundaries and creating terraces on the valley walls these flat facies contacts are interpreted as bay ravinement surfaces produced by waves in an estuarine setting the larger scale stacking pattern of the depositional sequences defines a progradational retrogradational trend in which the lowest sequence is mainly constituted by fluvial channel deposits whereas upper sequences display mostly bay fill facies the sequences are related to fifth order glacioeustatic fluctuations whereas their progradational retrogradational trend reflects fourth order eustatic variations and or rapid tectonic tilting of the area as indicated by the presence of two incision orientations the preservation of the system took place during a third order sea level rise and was favored by subsidence of the margin leading to its present occurrence down to 240 m below present sea level

geomorphological investigations and luminescence dating in the southern part of the khangay and the valley of the gobi lakes central mongolia geomorphological investigations in the catchment area of the baydragiyn river along the southern slope of the khangay and in the valley of the gobi lakes in western mongolia provide evidence for late quaternary glaciations and lake level changes thermoluminescence tl and infrared optically stimulated luminescence irsl dating of aeolian and colluvial sediments from the central khangay place the sediments that overlie fluvial and glaciofluvial terraces in the holocene an age of 21 ka is determined for a sand deposit overlying the terrace that is related to the last glacial ice margin lacustrine sediments from higher beach lines in the valley of the gobi lakes provide evidence for a slightly more humid period around 1 5 ka and a larger extent of the lakes in the early holocene at about 8 5 ka as also reported from other parts of central asia remnants of lacustrine sediments buried by alluvial gravel and indicating a huge palaeolake in the basin of the orog nuur however date to the early stage of the last glaciation around 70 ka copyright 2001 john wiley sons ltd

mobility of continental mantle evidence from postseismic geodetic observations following the 1992 landers earthquake the crust around the rupture zone of the 1992 landers earthquake has continued to deform in the years following the earthquake at rates 3 times greater than pre earthquake rates we use a combination of global positioning system gps and synthetic aperture radar insar data collected during a 3 year epoch following the earthquake in order to investigate postseismic mechanisms responsible for the high transient velocities in order to maximize the potential signal from viscoelastic relaxation we evaluate and model postseismic relaxation following the first few months of documented accelerated deformation the combination of gps and insar data allows us to establish viscoelastic relaxation of the lower crust and upper mantle as the dominant postseismic process and to discriminate among possible viscoelastic models the data particularly require the presence of a highly ductile uppermost mantle beneath the central mojave domain with temperature between the wet and dry basalt solidus this is consistent with independent seismic and geochemical inferences of a regionally warm uppermost mantle further consideration of seismic velocity variations in conjunction with faulting patterns within the mojave desert suggests that the primary faulting characteristics of the mojave desert namely the pervasive late cenozoic deformation within the eastern california shear zone versus the near absence of faults in the western mojave domain are controlled by the rheology of the uppermost mantle

lake level variation in the lahontan basin for the past 50 000 years selected radiocarbon data on surficial materials from the lahontan basin nevada and california provide a chronology of lake level variation for the past 50 000 yr a moderate sized lake connected three western lahontan subbasins the smoke creek black rock desert subbasin the pyramid lake subbasin and the winnemucca dry lake subbasin from about 45 000 to 16 500 yr b p between 50 000 and 45 000 yr b p walker lake rose to its sill level in adrian valley and spilled to the carson desert subbasin by 20 000 yr b p lake level in the western lahontan subbasins had risen to about 1265 m above sea level where it remained for 3500 yr by 16 000 yr b p lake level in the western lahontan subbasins had fallen to 1240 m this recession appears synchronous with a desiccation of walker lake however whether the walker lake desiccation resulted from climate change or from diversion of the walker river is not known from about 15 000 to 13 500 yr b p lake level rapidly rose so that lake lahontan was a single body of water by 14 000 yr b p the lake appears to have reached a maximum highstand altitude of 1330 m by 13 500 yr b p a condition that persisted until about 12 500 yr b p at which time lake level fell 100 m no data exist that indicate the level of lakes in the various subbasins between 12 000 and 10 000 yr b p during the holocene the lahontan basin was the site of shallow lakes with many subbasins being the site of one or more periods of desiccation the shape of the lake level curve for the three western subbasins indicates that past changes in the hydrologic balance and hence climate of the lahontan basin were large in magnitude and took place in a rapid step like manner the rapid changes in lake level are hypothesized to have resulted from changes in the mean position of the jet stream as it was forced north or south by the changing size and shape of the continental ice sheet

vertical movement and gravity change near the la grande 2 reservoir quebec leveling and precise gravity observations were made along a 50 km profile before and after the filling of the la grande 2 reservoir 53 7 n 77 w in 1979 anomalous relative vertical uplift of 4 cm and relative changes in gravity of 14 gal 140 nm s2 were observed along the profile the observed changes are anomalous with respect to theoretical results for loading of a layered elastic earth homogeneous isotropic porous elastic models with reasonable hydraulic properties are also unable to explain the observations in an effort to account for the discrepancies the accuracies of the leveling and the gravity data were investigated corrections were made to the leveling data for refraction errors rod calibrations magnetic effects and deflection of the vertical by the water mass although the corrections for systematic errors were significant the discrepancy with respect to homogeneous isotropic loading models remains the anomalous increase in gravity is thought to be the result of lateral transport of water in a high permeability formerly undersaturated zone hydraulically coupled to the reservoir the anomalous uplift is thought to be the result of elastic expansion of cracks and fractures of low aspect ratio in the high permeability zone or the result of an unexpectedly high rate of regional tilting in the lagrande 2 area

tropical climate changes at millennial and orbital timescales on the bolivian altiplano tropical south america is one of the three main centres of the global zonal overturning circulation of the equatorial atmosphere generally termed the walker circulation1 although this area plays a key role in global climate cycles little is known about south american climate history here we describe sediment cores and down hole logging results of deep drilling in the salar de uyuni on the bolivian altiplano located in the tropical andes we demonstrate that during the past 50 000 years the altiplano underwent important changes in effective moisture at both orbital 20 000 year and millennial timescales long duration wet periods such as the last glacial maximum marked in the drill core by continuous deposition of lacustrine sediments appear to have occurred in phase with summer insolation maxima produced by the earth s precessional cycle short duration millennial events correlate well with north atlantic cold events including heinrich events 1 and 2 as well as the younger dryas episode at both millennial and orbital timescales cold sea surface temperatures in the high latitude north atlantic were coeval with wet conditions in tropical south america suggesting a common forcing

quaternary paleoenvironmental change on the tibetan plateau and adjacent areas western china and western mongolia abstract this paper summarises the nature of climatic change during the last glacial interglacial cycle on the tibetan plateau and adjacent areas the results are derived from inland glacier fluctuations lake level changes and dust loess records of central asia including tibet these are based on our field investigations from 1988 to 1998 and literature surveys the changing extents of different climatic controlled geomorphic landscapes and vegetation zones help to provide estimates of the magnitude of climatic change features such as ice wedge casts loess and palaeosols ela reconstruction s and palaeobotanical data are used to help reconstruct palaeoprecipitation and palaeotemperatures weathering characteristics the overlying strata and some luminescence dates indicate that there are two main glacial ice advances during the last glacial cycle these correspond to marine oxygen isotope stages ois 2 and 4 in some areas as the eastern or northern margin of the tibetan plateau remnants of older glaciations are preserved higher lake levels in the deserts of central asia and on the tibetan plateau are dated to 40 to 25 ka ois 3 and to late glacial early to mid holocene periods our work supports the view that in many areas of central asia cold phases during the last glacial correspond with the maximum extent of glaciers and the periglacial activity the last glaciation produced large alluvial fans alternating with periods of high lake stands on the tibetan plateau however at the northern margin of the plateau in the qaidam basin and in some particular desert areas in western china high lake levels occurred also during the pleistocene and are related with alluvial fans

surface vertical displacements and level plane changes in the front reservoir area caused by filling the three gorges reservoir on the basis of the results of geological surveys and seismic prospecting in the three gorges area two earth models are constructed which are appropriate to the crustal structure in the front three gorges reservoir area utilizing load induced deformation theory we model the surface vertical displacements and level plane changes in the area caused by increased water loads of the reservoir the two quantities are needed to determine height changes on the earth s surface the numerical results demonstrate that the heights will be significantly changed as the reservoir fills up for example when the reservoir water level reaches an elevation of 175 m the crust subsides by 1 0 48 3 mm the level plane uplifts by 1 1 7 5 mm and the height falls by 2 1 45 0 mm accordingly our results provide the reliable corrected values for discriminating between the earthquake related crustal motion and the height changes that occur due to reservoir impoundment moreover if the height data previously observed in the area need to be used for engineering surveys after the reservoir is filled they must be corrected

late pleistocene and late holocene lake highstands in the pyramid lake subbasin of lake lahontan nevada usa shoreline geomorphology shoreline stratigraphy and radiocarbon dates of organic material incorporated in constructional beach ridges record large lakes during the late pleistocene and late holocene in the pyramid lake subbasin of lake lahontan nevada usa during the late holocene a transgression began at or after 3595 35 14 c yr b p and continued perhaps in pulses through 2635 40 14 c yr b p resulting in a lake as high as 1199 m during the latest pleistocene and overlapping with the earliest part of the younger dryas interval a lake stood at approximately 1212 m at 10 820 35 14 c yr b p and a geomorphically and stratigraphically distinct suite of constructional shorelines associated with this lake can be traced to 1230 m these two lake highstands correspond to periods of elevated regional wetness in the western basin and range that are not clearly represented in existing northern sierra nevada climate proxy records

sources of middle proterozoic to early cambrian siliciclastic sedimentary rocks in the great basin a nd isotope study the nd isotopic compositions of middle proterozoic to early cambrian terrigenous sedimentary rocks in the great basin united states were determined to identify the source areas of the siliciclastic detritus and to constrain models for the early tectonic evolution of the western margin of the continental united states in the southwestern great basin and vicinity nd isotope data from the intracratonic middle to late proterozoic pahrump group reveal that the coarse grained detritus from the crystal springs and the younger kingston peak formations including apparent glacial dropstones was derived from the local proterozoic basement province 1 as defined by nd isotope data fine grained detritus derived from other proterozoic crustal terranes provinces 2 or 3 was identified in the southernmost exposed portions of the pahrump group and corresponds to more distally derived material originally eroded from proterozoic basement farther to the south and east coarse and fine grained sedimentary material composing the overlying early cambrian miogeoclinal strata were also derived primarily from local province 1 basement rocks but the occurrence of detritus derived from province 2 or 3 crust is widespread geographically the deposition of sediment derived from province 2 or 3 across the developing continental shelf presumably reflects thermal subsidence and progressive marine encroachment of the ancient continental margin within the middle proterozoic to early cambrian section only the arkosic middle member of the wood canyon formation and the correlative deeper marine andrews member of the campito formation have measured nd values greater than 15 6 to 4 these units were apparently derived from a local 1 b y old granitic source exposed during the early cambrian period to the northeast of present day southern nevada the tectonic settings of the formation and subsequent uplift of this granitic terrane are unknown but erosion of the granite generated excellent nd isotopic marker beds that should prove useful in correlating early cambrian miogeoclinal sedimentary rocks throughout southwestern north america in the north central great basin late proterozoic miogeoclinal sedimentary rocks of the trout creek and mccoy creek groups show a marked shift from province 2 or 3 to province 1 sources through time this shift is consistent with a change in topography related to the emergence of the tooele uinta arch as an area of positive relief the nd data provide the first evidence that this structure was present in this region as early as latest proterozoic time as in the southern great basin however the isotope data indicate that sediments deposited in the late proterozoic era in the northern regions were dominantly derived from sources in the local precambrian basement

changes of shell microstructural characteristics of cerastoderma edule bivalvia a novel proxy for water temperature abstract shells of bivalves potentially provide an excellent archive for high resolution paleoclimate studies however quantification of environmental variables specifically water temperature remains a very challenging task here we explore the possibility to infer water temperature from changes of microstructural characteristics of shells of the common cockle cerastoderma edule the size and elongation of individual microstructural units i e prisms in the outer shell layer of seven three to five year old specimens collected alive from the intertidal zone of the north sea near texel the netherlands and schillig germany were measured by means of automatic image processing growth patterns circatidal ciralunidian and fortnightly increments and lines shell oxygen isotope values and mark and recovery experiments were used to place the shell record in a precise temporal context irrespective of the locality and ontogenetic age size and elongation of the prisms increased non linearly with water temperature small 0 12 0 05 m 2 and round prisms elongation 2 42 0 31 were formed at temperatures of ca 10 c late april whereas larger 0 33 0 11 m 2 and more elongated prisms 3 26 0 28 occurred during hot summer ca 22 c no clear cut or consistent correlation existed between microstructural characteristics and growth rate as well as a variety of other environmental variables such as salinity chlorophyll a and turbidity based on these findings a model was constructed from three shells at texel that enables reconstruction of water temperature with a precision of 1 7 1 0 c from prism size and elongation sst 9 02 17 25 p s 1 10 p e this model was successfully tested at four shells from schillig the new temperature proxy can be of particular interest for paleoclimate studies in nearshore settings when non recrystallized c edule shells are available future studies are required to verify our findings and check if other species with the same and different microstructures show similar relationships with water temperature