forces driving late pleistocene ca 77 12 ka landscape evolution in the cimarron river valley southwestern kansas abstract this study presents stratigraphic geomorphic and paleoenvironmental 13 c data that provide insight into the late pleistocene landscape evolution of the cimarron river valley in the high plains of southwestern kansas two distinct valley fills t 1 and t 2 were investigated three soils occur in the t 2 fill and five in the t 1 fill all indicating periods of landscape stability or slow sedimentation of particular interest are two cumulic soils dating to ca 48 28 and 13 12 5 ka 13 c values are consistent with regional paleoenvironmental proxy data that indicate the prevalence of warm dry conditions at these times the cimarron river is interpreted to have responded to these climatic changes and to local base level control specifically aggradation occurred during cool wet periods and slow sedimentation with cumulic soil formation occurred under warmer drier climates significant valley incision 25 m by ca 28 ka likely resulted from a lowering of local base level caused by deep seated dissolution of permian evaporite deposits

ultrashallow depth imaging of a channel stratigraphy with first arrival traveltime inversion and prestack depth migration a case history from bull creek oklahoma depth imaging in ultrashallow 50 m environments presents twofold challenge 1 coda available for depth migration is very limited and 2 conventional time processing with limited coda generally fails to estimate reliable velocity models for depth migration we studied the combining of first arrival traveltime inversion and prestack depth migration psdm for depth imaging of ultrashallow paleochannel stratigraphy associated with the bull creek drainage system oklahoma restricted by a limited number of geophones 24 we acquired data for inversion and migration through two coincident profiles the first profile for inversion has a wider survey aperture 115 m maximum shot receiver spacing and consequently sparse cmp spacing 2 5 m whereas the second profile for psdm has denser cmp spacing 1 m and consequently a narrower survey aperture 46 m maximum shot receiver spacing we also found that the velocity model from traveltime inversion of the wider aperture data set is more preferable for dep

the diversity of tectonic modes and thoughts about transitions between them plate tectonics is a particular mode of tectonic activity that characterizes the present day earth it is directly linked to not only tectonic deformation but also magmatic volcanic activity and all aspects of the rock cycle other terrestrial planets in our solar system do not operate in a plate tectonic mode but do have volcanic constructs and signs of tectonic deformation this indicates the existence of tectonic modes different from plate tectonics this article discusses the defining features of plate tectonics and reviews the range of tectonic modes that have been proposed for terrestrial planets to date a categorization of tectonic modes relates to the issue of when plate tectonics initiated on earth as it provides insights into possible pre plate tectonic behaviour the final focus of this contribution relates to transitions between tectonic modes different transition scenarios are discussed one follows classic ideas of regime transitions in which boundaries between tectonic modes are determined by the physical and chemical properties of a planet the other considers the potential that variations in temporal evolution can introduce contingencies that have a significant effect on tectonic transitions the latter scenario allows for the existence of multiple stable tectonic modes under the same physical chemical conditions the different transition potentials imply different interpretations regarding the type of variable that the tectonic mode of a planet represents under the classic regime transition view the tectonic mode of a planet is a state variable akin to temperature under the multiple stable modes view the tectonic mode of a planet is a process variable that is something that flows through the system akin to heat the different implications that follow are discussed as they relate to the questions of when did plate tectonics initiate on earth and why does earth have plate tectonics this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

a simple model of plate generation from mantle flow summary a simple model of non newtoniancreeping ow is used to evaluate classes of rheologieswhichallow viscous mantle ow to becomeplate like the model describes shallow layerlithosphericmotion driven by sources and sinks the sources represent spreading ridges while the sinks rep resent subduction zones the sources and sinks thus also prescribe the poloidal component ofthe surface ow eld the toroidal strike slip component of the ow eld is found via thesolution of the stokes equation with non newtonian rheology as a rst basic investigation ofthe model the horizontal divergence from the two dimensional rectangular velocity eld ofolson bercovici 1991 is used for the source sink eld the degree to which the induced uid ow reproducesthe rectangularplate is used to measure the success of differentrheologiesin generating plate like ows results indicate that power law rheologies even in the limit ofvery high power law index can only produce modest plate like ow for example the ratioof toroidal to poloidal kinetic energy for a source sink eld derived from a square plate is atbest 0 65 whereas a perfect square plate has a ratio of 1 0 moreover the power law rheologyappears to reach an asymptotic limit in its ability to produce plate like behavior this impliesthat plate tectonics is unlikely to arise from a power law rheology even in the limit of very high a class of rheologies that yield signi cantly more promisi ng results arise from the carreaupseudo plastic rheology with the power law index taken to be 0 one rheology in thisclass is the continuum model for stick slip earthquake behavior of whitehead gans 1974 which is essentially the carreau equation with 1 this class of rheologies referred to asthe stick slip rheologies induces a toroidal to poloidal kinetic energy ratio for a square plates ssource sink function which can be as high as0 9 the viscosity or strength distribution forthis class of rheologies also appears more plate like showing fairly uniform high viscosity re gions pseudo plates and sharply de ned low viscosity zones pseudo margins in contrast even the most nonlinear power law rheology produces spatially varying high viscosity regionsand relatively smooth low viscosity margins the greater success of the stick slip rheologiesin producing plates is attributed to a self lubricating mechanism in which the transfer of mo mentum from regions of high shear to low shear is inhibited in contrast even in the limit ofin nite power law index a power law rheology can retard but never prohibit momentum trans fer this feature is essential to the sharpening of velocity pro les into plate like pro les whichis illustrated with a simple boundary layertheory key words plate tectonics mantle convection non newtonian ow toroidal poloidal cou pling

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

role of a low viscosity zone in stabilizing plate tectonics implications for comparative terrestrial planetology 1 earth s near surface layer its lithosphere is broken into quasi rigid plates that form the upper thermal boundary layer for mantle convection since the discovery of plate tectonics it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone lvz over which the plates glide easily the lvz or asthenosphere concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the earth s gravity field here we show in both two dimensional 2 d cartesian and 3 d spherical earth models that combining a pronounced lvz and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection with 30 toroidal surface motion for the 3 d case recycling of water into the upper mantle at subduction zones is a plausible cause of earth s lvz whereas venus is dry and lacks both an lvz and plate tectonics

holocene environments of the central great plains multi proxy evidence from alluvial sequences southeastern nebraska abstract pollen plant macrofossils phytoliths carbon isotopes and alluvial history from sediments exposed along the south fork of the big nemaha river southeastern nebraska usa provide an integrated reconstruction of changes in holocene vegetation climate and fluvial activity from 9000 to 8500 uncalibrated 14 c yr bp climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly upland deciduous forest declined and prairie attained its holocene dominance from 8500 to 5800 yr bp upland forest elements disappeared and even riparian trees were sparse under dry climatic conditions alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr bp from 5800 to 3100 yr bp riparian forests returned to prominence and droughts were intermittent alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley aggradation on alluvial fans slowed and finally ceased near the end of this period during a short dry interval from 3100 to 2700 yr bp riparian trees except elm disappeared and prairie and weedy species became more abundant this interval is represented by the organic roberts creek member and the alluvial setting was a slightly incised meandering channel belt habitats became similar to presettlement conditions during the last 2700 yr bp weedy taxa dominate modern samples reflecting widespread disturbance alluvial fans and terrace surfaces were stable during the last 2500 years but episodes of floodplain aggradation were punctuated by incision of the main channel

cool wet conditions late in the younger dryas in semi arid new mexico abstract a thick alluvial sequence in central new mexico contains the scholle wet meadow deposit that traces upstream to a paleospring the wet meadow sediments contain an abundant fauna of twenty one species of freshwater and terrestrial mollusks and ten species of ostracodes the mollusks and ostracodes are indicative of a local high alluvial water table with spring supported perennial flow but without standing water pollen analysis documents shrub grassland vegetation with sedges willow and alder in a riparian community stable carbon isotopes from the wet meadow sediments have 13c values ranging from 22 8 to 23 3 indicating that 80 of the organic carbon in the sediment is derived from c3 species the wet meadow deposit is ams dated 10 400 to 9700 14c yr bp corresponding to 12 300 to 11 100 cal yr bp and overlapping in time with the younger dryas event yd the wet meadow became active about 500 yr after the beginning of the yd and persisted 400 yr after the yd ended the scholle wet meadow is the only record of perennial flow and high water table conditions in the abo arroyo drainage basin during the past 13 ka

late quaternary pollen record from cheyenne bottoms kansas abstract a sediment and pollen record from cheyenne bottoms a large 166 km2 enclosed basin in central kansas provides evidence for local and regional vegetation and climate change during the late quaternary ca 30 000 yr although radiocarbon dating of the carbonate rich lacustrine sediments remains problematic a basic chronological framework for the section is established two major litho and biostratigraphic units a farmdalian zone ca 30 000 to 24 000 yr b p and a holocene zone ca 11 000 yr b p to present are separated by a major unconformity spanning the woodfordian ca 24 000 to 11 000 yr b p pollen and sedimentary data indicate a period of basin wide drying preceding this unconformity the sustained absence of sediment accumulation within this playa like basin suggests that early woodfordian conditions were increasingly arid with strengthened surface winds before this persistent shallow water marshland dominated the local basin bottom vegetation regional upland vegetation was an open grassland sage steppe throughout the farmdalian with limited populations of spruce juniper aspen birch and boxelder in riparian settings and escarpments throughout the holocene water levels within the basin fluctuated changes in wetland vegetation resulting from water level fluctuation have increased during the last 3 000 yr indicating that periodic episodes of wetland loss and rebound are not unique to postsettlement conditions but are an ongoing phenomenon at cheyenne bottoms

holocene aridification vegetation change sedimentation regime and limits of carbon isotope data as indicated by the alluvial pollen sites of arroyo grande and el molino in north central mexico abstract a multi proxy record of palynological sedimentological archaeological and carbon isotopic data of alluvial sites in the upper nazas basin arroyo grande durango 1855 m amsl and the parras basin el molino coahuila 1245 m amsl is introduced describing environmental changes across a short grassland desert ecotone from the eastern margins of the sierra madre occidental smo these sites reconstruct holocene vegetation history in semi arid north central mexico and integrate fluvial geomorphology and palynology data in particular pollen data links a rise of xeric flora with higher energy deposition and increased erosion in upland basins of the eastern smo according to archaeological finds at examined sites human occupation is linked to mesic or afforested conditions however limitations of carbon isotope data for environmental historical reconstruction are indicated by somewhat stable values at arroyo grande durango especially while pollen sites show a replacement of short grasses and park woodlands by a more xeric flora at higher elevations with a development of chihuahuan desert flora at low elevations el molino coahuila vegetation reconstructions are supported by pollen analog data collected from the cuatro cienegas region in coahuila mexico

thermal convection in a cylindrical annulus with a non newtonian outer surface this study presents the results of numerical simulations of a model for lithospheremantle coupling in a terrestrial type planet to first order a geologically active terrestrial type planet may consist of a metallic core silicate mantle and lithosphere with the lithosphere being rheologically different from the mantle therefore we have developed a numerical model consisting of a thin non newtonian fluid hoop that is dynamically coupled to a thick newtonian fluid cylindrical annulus thus the rheological dichotomy between mantle and lithosphere is built into the model time dependent calculations show the existence of at least two regimes of behaviors in one regime the behavior of the hoop switches between periods characterized by low or high speeds in response to changes in convective vigor and planform this regime may apply to the planet venus where the available evidence indicates that prior to 500 myr ago the planet was resurfaced on a time scale of 100 myr since that time large scale tectonic activity on venus has been sharply curtailed in the other regime which is more like plate tectonics on earth the hoop speeds rise and fall on short time scales

the younger dryas impact hypothesis a cosmic catastrophe in this paper we review the evidence for the younger dryas impact hypothesis ydih which proposes that at 12 9k cal a bp north america south america europe and the middle east were subjected to some sort of extraterrestrial event this purported event is proposed as a catastrophic process responsible for terminal pleistocene environmental changes onset of yd cooling continent scale wildfires extinction of late pleistocene mammals and demise of the clovis culture in north america the earliest well documented continent scale settlement of the region the basic physics in the ydih is not in accord with the physics of impacts nor the basic laws of physics no yd boundary ydb crater craters or other direct indicators of an impact are known age control is weak to non existent at 26 of the 29 localities claimed to have evidence for the ydih attempts to reproduce the results of physical and geochemical analyses used to support the ydih have failed or show that many indicators are not unique to an impact nor to 12 9k cal a bp the depositional environments of purported indicators at most sites tend to concentrate particulate matter and probably created many ydb zones geomorphic stratigraphic and fire records show no evidence of any sort of catastrophic changes in the environment at or immediately following the ydb late pleistocene extinctions varied in time and across space archeological data provide no indication of population decline demographic collapse or major adaptive shifts at or just after 12 9 ka the data and the hypotheses generated by ydih proponents are contradictory inconsistent and incoherent copyright 2014 john wiley sons ltd

isotopic and rock magnetic expression of environmental change at the pleistocene holocene transition in the central great plains abstract in a region too dry for the effective use of the traditional palynological approach loess deposits of the central great plains offer a source of proxy information for the reconstruction of past climates the brady soil a ubiquitous feature developed within the uppermost late pleistocene peoria loess recorded the character of vegetation change associated with climate shifts during the pleistocene holocene transition stable carbon isotope ratio analysis and two room temperature magnetic parameters were used to examine the nature of the change during this period 13 c values derived from soil organic matter indicate that in the late pleistocene minor populations of drought tolerant c4 plants were interspersed among the dominant cool season grasses c3 in the west while c3 grasses were accompanied by trees in the east by the early holocene c4 grasses were the dominant vegetation in the west and contributed substantially to the 13 c signal in the east magnitude of the 13 c shift suggests an increase in july mean daily temperature of 8 10 c according to existing models magnetic susceptibility and frequency dependence signal the cessation of loess deposition and development of the brady soil and indicate a spatial pattern in the intensity of weathering across the region

deglacial hydroclimate of midcontinental north america abstract during the last deglaciation temperatures over midcontinental north america warmed dramatically through the bolling allerod underwent a cool period associated with the younger dryas and then reverted to warmer near modern temperatures during the early holocene however paleo proxy records of the hydroclimate of this period have presented divergent evidence we reconstruct summer relative humidity rh across the last deglacial period using a mechanistic model of cellulose and leaf water 18o and d combined with a pollen based temperature proxy to interpret stable isotopes of sub fossil wood midcontinental rh was similar to modern conditions during the last glacial maximum progressively increased during the bolling allerod peaked during the younger dryas and declined sharply during the early holocene this rh record suggests deglacial summers were cooler and characterized by greater advection of moisture laden air masses from the gulf of mexico and subsequent entrainment over the mid continent by a high pressure system over the laurentide ice sheet these patterns help explain the formation of dark colored cumulic horizons in many great plains paleosol sequences and the development of no analog vegetation types common to the midwest during the last deglacial period likewise reduced early holocene rh and precipitation correspond with a diminished glacial high pressure system during the latter stages of ice sheet collapse

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

generation of plate tectonics with two phase grain damage and pinning source sink model and toroidal flow abstract the grain damage and pinning mechanism of bercovici and ricard 2012 for lithospheric shear localization is employed in two dimensional flow calculations to test its ability to generate toroidal strike slip motion and influence plate evolution this mechanism posits that damage to the interface between phases in a polycrystalline material like peridotite composed primarily of olivine and pyroxene increases the number of small zener pinning surfaces which then constrain mineral grains to ever smaller sizes regardless of creep mechanism this effect allows a self softening feedback in which damage and grain reduction can co exist with a grain size dependent diffusion creep rheology moreover grain growth and weak zone healing are greatly impeded by zener pinning thereby leading to long lived relic weak zones the fluid dynamical calculations employ source sink driven flow as a proxy for convective poloidal flow upwelling downwelling and divergent convergent motion and the coupling of this flow with non linear rheological mechanisms excites toroidal or strike slip motion the numerical experiments show that pure dislocation creep rheology and grain damage without zener pinning as occurs in a single phase assemblages permit only weak localization and toroidal flow however the full grain damage with pinning readily allows focussed localization and intense plate like toroidal motion and strike slip deformation rapid plate motion changes are also tested with abrupt rotations of the source sink field after a plate like configuration is developed the post rotation flow and material property fields retain memory of the original configuration for extensive periods leading to suboptimally aligned plate boundaries e g strike slip margins non parallel to plate motion oblique subduction and highly localized weak and long lived acute plate boundary junctions such as at what is observed at the aleutian kurile intersection the grain damage and pinning theory therefore readily satisfies key plate tectonic metrics of localized toroidal motion and plate boundary inheritance and thus provides a predictive theory for the generation of plate tectonics on earth and other planets

understanding holocene variations in the vegetation of sao joao river basin southeastern coast of brazil using phytolith and carbon isotopic analyses abstract to understand variations in the vegetation of the southeastern coast of brazil during the holocene and to identify roughly when they took place we make use of phytolith morphology and carbon stable isotope analyses coupled with 14c dating of soil profiles the soil profile studied is located in the sao joao river basin close to cabo frio rio de janeiro state we evaluated the soil characteristics which showed great variations in granulometry with larger texture in upper horizons that enabled the movement of particles to lower layers the accumulation soil horizons revealed several palaeoclimatic aspects higher phytolith stocks a higher density of trees and a lower water stress at the bottom of the profile and a predominance of c3 grasses throughout the holocene carbon stable isotopic ratios of the soil profile confirm the predominance of c3 plants except for e bt horizon which was enriched in 13c the soil organic matter mean residence time ranges from post bomb topsoil to 10 245 years cal bp at the base of the profile the results of this study are compatible with other evidence of a more humid environment in the first half of the holocene

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

late quaternary vegetation and climate changes in central texas based on the isotopic composition of organic carbon stable carbon isotope analysis of organic carbon in alluvial deposits and soils of three streams in central texas reveals significant shifts in the ratio of c3 to c4 plant biomass production during the past 15 000 yr these temporal changes in vegetation appear to be in response to changes in climate during the late pleistocene c4 plants comprised only about 45 to 50 of the vegetative biomass in this area suggesting that conditions were cooler and wetter than at any time during the past 15 000 yr the time between 11 000 and 8000 yr b p is interpreted as transitional between late pleistocene conditions and warmer and drier holocene conditions based on a slight increase in the abundance of c4 plant biomass during the middle holocene between approximately 6000 and 5000 yr b p mixed c3 c4 plant communities were replaced almost completely by c4 dominated communities indicating prairie expansion and warmer and drier climatic conditions by 4000 yr b p the abundance of c4 plant biomass decreased to levels similar to the early holocene transitional period suggesting a return to cooler and wetter climatic conditions no significant shift in the ratio of c3 to c4 productivity has occurred during the last 4000 yr except for a slight increase in the abundance of c4 plant biomass around 2000 yr b p the results of this investigation correlate well with other regional late quaternary climatic interpretations for central and north texas the southern plains region and with other portions of the great plains

quantifying the distribution of nanodiamonds in pre younger dryas to recent age deposits along bull creek oklahoma panhandle usa high levels of nanodiamonds nds have been used to support the transformative hypothesis that an extraterrestrial et event comet explosion triggered younger dryas changes in temperature flora and fauna assemblages and human adaptations firestone rb et al 2007 proc natl acad sci usa 104 41 16016 16021 we evaluate this hypothesis by establishing the distribution of nds within the bull creek drainage of the beaver river basin in the oklahoma panhandle the earlier report of an abundance spike of nds in the bull creek i younger dryas boundary soil is confirmed although no pure cubic diamonds were identified the lack of hexagonal nds suggests bull creek i is not near any impact site potential hexagonal nds at bull creek were found to be more consistent with graphene graphane an additional nd spike is found in deposits of late holocene through the modern age indicating nds are not unique to the younger dryas boundary nd distributions do not correlate with depositional environment pedogenesis climate perturbations periods of surface stability or cultural activity

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

role of faults nonlinear rheology and viscosity structure in generating plates from instantaneous mantle flow models concentrated strain within plate margins and a significant toroidal component in global plate motion are among the most fundamental features of plate tectonics a significant proportion of strain in plate margins is accommodated through motion on major tectonic faults the decoupling influence of faulted plate margins primarily results from history dependent lithospheric deformation rather than from instantaneous stress weakening rheologies for instantaneous mantle flow models we argue that faults should be treated as preexisting mechanical structures with models incorporating preexisting faults a power law rheology with an exponent of 3 and slab pull and ridge push forces we demonstrate that nonlinear interaction between weak faults and this power law rheology produces plate like motion our models show that in order to produce plate like motion the frictional stress on faults needs to be small and the asthenosphere viscosity should be much weaker than that of lithosphere while both plateness and the ratio of toroidal to poloidal velocities are reduced with increasing fault coupling the viscosity contrast between the lithosphere and asthenosphere only influences plateness this shows that both diagnostics plateness and the ratio of toroidal to poloidal velocities are necessary to characterize plate motion the models demonstrate that weak transform faults can guide plate motion this guiding property of transform faults and the decoupling of thrust faults result in oblique subduction where the strike of subducted slabs is oblique to transform faults subducted slabs beneath a dipping fault produce oceanic trench and fore bulge topography and principal stresses consistent with subduction zone observations

differential rotation between lithosphere and mantle a consequence of lateral mantle viscosity variations the description of plate motions in the so called hotspot reference frame introduces a global rotation of the lithosphere with respect to the mantle this rotation called toroidal field of degree 1 is roughly westward it reaches an amplitude of about 2 cm yr and has been consistently found in the different generation of plate tectonic models various authors have tried to relate this observation to the deceleration of the earth s rotation to polar wander or to tidal drag however these different physical mechanisms cannot explain the requested amplitude in this paper we compare the values of this rotation vector using different relative plate motion models expressed in the hotspot reference frame in a model earth with lateral viscosity variations a differential rotation is predicted the observed net lithospheric rotation is consistent with the dynamics of a model earth where the asthenospheric viscosity below the oceans is at least one order of magnitude lower than underneath the continents this relative westward drift of the lithosphere may account for the significant structural differences between east or west dipping subduction zones

paleo environmental reconstruction and bio stratigraphy oklahoma panhandle usa worldwide the period between 12 000 and 9000 radiocarbon years before present rcybp witnessed the shift from glacial to post glacial times and marks the transition from the pleistocene to the holocene on the southern plains of north america this period is marked by rapidly changing climatic conditions shifting flora and faunal associations and cultural adaptations a recently discovered locality along bull creek oklahoma panhandle provides the opportunity to study the environmental context of changing megafauna presence during this transition the paleo environment is reconstructed from sediment particle size distribution stable carbon isotope pollen and phytolith analyses from the bull creek site and compared with the megafaunal biostratigraphy in this area

upper mantle shear structure beneath the northwest atlantic ocean we have modeled the sh motion from earthquakes in the northwest atlantic ocean to derive an oceanic upper mantle shear velocity model the signals were recorded on long period wwssn and canadian network stations on the east coast of north america the travel times and waveforms of seismograms in the distance range 11 16 were used to constrain the lid structure these data indicate a fast 4 75 km s lid of about 100 km thickness in the older western atlantic from 16 to 28 the s waves pass through triplications due to velocity jumps near 400 and 660 km the branches from both discontinuities are visible in the s wave data these arrivals were modeled using synthetic seismograms to obtain accurate travel times the two triplications are also apparent in ss data from 30 to 50 the triplication curves derived from the s waves agree with the ss data given the lid structure the waveforms and travel times from the more distant data put tight constraints on the shear velocities at greater depths we find shear velocities from 200 to 400 km to be slow in relation to a shear model of the canadian shield derived by a similar technique indicating continent ocean heterogeneity to about 400 km

predominance of long wavelength heterogeneity in the mantle analysis of travel time anomalies of long period shear waves recorded by the global digital seismographic network indicates that mantle heterogeneities which probably reflect patterns of mantle convection occur most prominently on length scales greater than about 6 000 km when projected onto a two dimensional map at the earth s surface models of mantle convection will need to be able to produce a similar power spectrum

geochronology and physical context of oldowan site formation at kanjera south kenya oldowan sites in primary geologic context are rare in the archaeological record here we describe the depositional environment of oldowan occurrences at kanjera south kenya based on field descriptions and granulometric analysis excavations there have recovered a large oldowan artefact sample as well as the oldest substantial sample of archaeological fauna the deposits at kanjera south consist of 30 m of fluvial colluvial and lacustrine sediments magneto and bio stratigraphy indicate the kanjera south member of the kanjera formation was deposited between 2 3 and 1 92 million years ago ma with 2 0 ma being a likely age for the archaeological occurrences oldowan artefacts and associated fauna were deposited in the colluvial and alluvial silts and sands of beds ks 1 to ks 3 in the margins of a lake basin field descriptions and granulometric analysis of the sediment fine fraction indicates sediments from within the main archaeological horizon were emplaced as a combination of tractional and hyperconcentrated flows with limited evidence of debris flow deposition this style of deposition is unlikely to significantly erode or disturb the underlying surface and therefore promotes preservation of surface archaeological accumulations hominins were repeatedly attracted to the site locale and rapid sedimentation minimal bone weathering and an absence of bone or artefact rounding further indicate that fossils and artefacts were quickly buried

chronological evidence fails to support claim of an isochronous widespread layer of cosmic impact indicators dated to 12 800 years ago according to the younger dryas impact hypothesis ydih 12 800 calendar years before present north america experienced an extraterrestrial impact that triggered the younger dryas and devastated human populations and biotic communities on this continent and elsewhere this supposed event is reportedly marked by multiple impact indicators but critics have challenged this evidence and considerable controversy now surrounds the ydih proponents of the ydih state that a key test of the hypothesis is whether those indicators are isochronous and securely dated to the younger dryas onset they are not we have examined the age basis of the supposed younger dryas boundary layer at the 29 sites and regions in north and south america europe and the middle east in which proponents report its occurrence several of the sites lack any age control others have radiometric ages that are chronologically irrelevant nearly a dozen have ages inferred by statistically and chronologically flawed age depth interpolations and in several the ages directly on the supposed impact layer are older or younger than 12 800 calendar years ago only 3 of the 29 sites fall within the temporal window of the yd onset as defined by ydih proponents the ydih fails the critical chronological test of an isochronous event at the yd onset which coupled with the many published concerns about the extraterrestrial origin of the purported impact markers renders the ydih unsupported there is no reason or compelling evidence to accept the claim that a cosmic impact occurred 12 800 y ago and caused the younger dryas

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

late quaternary environmental reconstructions of playa lunette system evolution on the central high plains of kansas united states playas are small ephemeral wetlands found throughout semiarid and arid regions around the world lunettes are dunes that form downwind of some large playas two playa lunette systems on the high plains were investigated to reconstruct paleoenvironments during playa lunette system evolution both playa lunette systems are composed of sediment spanning 40 k y with earliest deposits accumulating during at least marine isotope stage mis 3 and possibly starting as early as mis 5 the gilman canyon formation pedocomplex dating between 42 and 28 ka dominates this period and is ubiquitous throughout the playa lunette systems climate during mis 3 was similar to modern with warm temperatures low effective moisture and playa floors exposed long enough to allow pedogenesis during mis 2 climate was relatively cool with higher effective precipitation and playas were inundated for longer periods the regionally expressed brady soil of the pleistocene holocene transition caps these deposits and dates between 11 8 and 9 4 ka during the pleistocene holocene transition climate warmed yet moisture availability remained relatively high several distinct shifts in 13 c identify rapid climate changes associated with the bolling allerod younger dryas chronosequence warming continued into the holocene though moisture availability was highly variable holocene soils are common and date between 8 3 and 3 6 ka playa lunette system stratigraphy represents a continuum of the upland loess sequence though deposits are altered by playa hydrology geomorphic processes alternated between dissolution and fluvial eolian driven processes as climate changed and playas developed due to the aggradational environment and sensitivity of these systems high resolution records of environmental change throughout their evolution are preserved

application of the redundant lifting scheme for ground roll attenuation in near surface characterization using full waveform inversion on p wave seismic data abstract seismic modelling of the shallow subsurface within the first few metres is often challenging when the data are dominated by ground roll and devoid of reflection we showed that even when transmission is the only available phase for analysis fine scale and interpretable p wave velocity vp and attenuation qp 1 models can still be prepared using full waveform inversion with data being preconditioned for ground roll to prove this idea we suppressed the ground roll in two different ways before full waveform inversion modelling first through a bottom mute second through a novel wavelet transform based method known as the redundant lifting scheme the applicability of full waveform inversion is tested through imaging two buried targets these include a pair of utility water pipes with known diameters of 0 8 m and burial depths of 1 5 m respectively the second target is the poorly documented backfill which was the former location of the pipe s the data for full waveform inversion are acquired along a 2d profile using a static array of 24 40 hz vertical component geophones and a buried point source the results show that a the redundant lifting scheme better suppresses the ground roll which in turn provides better images of the targets in full waveform inversion and b the vp and qp 1 models from full waveform inversion of redundant lifting scheme data could detect the two targets adequately

non newtonian stagnant lid convection and magmatic resur facing on venus abstract mantle convection on venus is likely to occur in the regime known as stagnant lid convection we investigate this regime for internally heated convection with temperature and pressure dependent power law viscosity dislocation creep scaling relationships obtained for large aspect ratio convection are different from steady state square box calculations but agree well with scaling theory and boundary layer stability analysis results for arrhenius viscosity and pressure dependent viscosity show that the efficiency of heat transport is sensitive to the viscosity function at the bottom of the lid new scaling relationships are applied to parameterized convection calculations of the thermal history of venus assuming that plate tectonics could not occur during evolution the onset of convection beneath the lid is delayed even for initial potential temperatures near the solidus during the conductive regime melting is suppressed due to the development of a thick cold lid at the surface after convection begins the lid becomes thinner and the planet undergoes a period of widespread melting and volcanism the timing of the beginning and ending of melting depends on various factors such as the initial conditions and mantle rheology the episode of melting predicted by the models can be reconciled with the cessation of global resurfacing on venus 300 800 myr ago the models yield present day lithospheric thicknesses around 200 km which is similar to previously suggested estimates

toroidal poloidal partitioning of plate motions since 120 ma changes in plate motions and plate configurations during the cenozoic and mesozoic have been investigated extensively but most geodynamical models have concentrated on present day plate motions we have investigated the recent evolution of plate tectonics by examining the history of toroidal poloidal partitioning of plate motions taking into account estimated errors our results suggest a significant increase in the ratio of toroidal to poloidal motions postdating the hawaiian emperor h e bend at 43 ma corresponding to an overall decrease in global plate motions these changes may reflect greater mantle plume activity in the mesozoic but a causal mechanism is not obvious in general observed cenozoic and mesozoic plate motions do not appear to be random which implies that they are correlated we also find perhaps three significant changes in net rotation of the lithosphere with respect to hotspots since 120 ma

geomorphic and sedimentary responses of the bull creek valley southern high plains usa to pleistocene and holocene environmental change abstract fluvial geomorphology and stratigraphy often reflect past environmental and climate conditions this study examines the response of bull creek a small ephemeral creek in the oklahoma panhandle to environmental conditions through the late pleistocene and holocene fluvial terraces were mapped and their stratigraphy and sedimentology documented throughout the course of the main valley based on their elevations terraces were broadly grouped into a late pleistocene fill terrace t3 and two holocene fill cut terrace sets t2 and t1 terrace systems are marked by similar stratigraphies recording the general environmental conditions of the time sedimentary sequences preserved in terrace fills record the transition from a perennial fluvial system during the late glacial period and the younger dryas to a semiarid environment dominated by loess accumulation and punctuated by flood events during the middle to late holocene the highest rates of aeolian accumulation within the valley occurred during the early to middle holocene our data provide significant new information regarding the late pleistocene and holocene environmental history for this region located between the well studied southern and central high plains of north america

the potential role of non newtonian rheology in the resurfacing of venus several analyses of the impact cratering record on venus have concluded that 80 of venus was resurfaced sometime in the last 300 m y 700 m y the cratering record further indicates that the resurfacing took place over a 30 m y 100 m y interval since this period there has been little large scale tectonic activity on venus this study proposes that such behavior may result from the interaction of phase change induced avalanches with a non newtonian lithosphere the mechanism proposed here suggests that because of non newtonian rheology the venusian lithosphere becomes highly mobile during brief periods when avalanches occur afterwards due to the reduction in stress levels the lithosphere becomes rigid and large scale tectonics ceases

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

two dimensional convection with a self lubricating simple damage rheology summary we present 2 d simulations of convection in the earth s mantle with temperature and damagedependent viscosity in a basally heated system the equation governing the temporal evolution of damage includes a source term for damage a healing term and an advection term a systematic study of the influence of the different damage terms on the convection pattern shows that 1 at least four different convective regimes can be distinguished depending on the size of the damage source term and 2 self lubricating behaviour is possible only if the advection term in the damage equation is small enough compared with the other terms we also demonstrate that good plate like behaviour in terms of a focused low viscosity bands lvb b homogeneous surface velocities within the plates and abrupt velocity jumps across the plate boundaries and c asymmetric subduction can be obtained with this kind of rheology although an increase in subduction asymmetry tends to make convection highly time dependent

variation in radiocarbon ages of soil organic matter fractions from late quaternary buried soils radiocarbon dating of three organic matter fractions total humic acid and residue isolated from late quaternary buried soils of the central great plains reveals that there often are considerable differences among but no consistent order to the ages of fractions for late holocene soils the residue fraction or the total fraction generally produces the oldest age for late pleistocene soils however no fraction was consistently the oldest the absence of a consistent sequence of fraction ages is attributed to postburial contamination of soils when bulk samples from the same soil were split and sent to two laboratories different radiocarbon ages were usually obtained the variability in radiocarbon ages of soil organic matter confirms that caution should be taken when using radiocarbon ages obtained from different laboratories to make regional stratigraphic correlations

stratigraphy of the younger dryas chronozone and paleoenvironmental implications central and southern great plains abstract the great plains of the united states was the setting for some of the earliest research in north america into patterns and changes in the character of late pleistocene environments and their effects on contemporary human populations many localities in the region have well stratified records of terminal pleistocene and early holocene human paleoindian activity and past environments these have proven important in debates over the character of the younger dryas chronozone ydc 11 000 10 000 14c bp 12 900 11 700 cal bp in the continental interior this paper reviews the lithostratigraphic record of the ydc on the central and southern great plains and summarizes paleobiological records largely isotopic the goal is to determine if there is any uniformity in the timing character direction and or magnitude of changes in depositional environments or broader geomorphic systems before during or after the ydc in order to address the question of the character of environments through this time the stratigraphic records of the late pleistocene to early holocene transition and in particular the stratigraphic records of the ydc vary through time and space the data clearly show that a host of geomorphic processes produced the terminal pleistocene and early holocene stratigraphic records of the great plains moreover the ydc is not necessarily manifest as a distinct lithostratigraphic or biostratigraphic entity in these different types of deposits and soils the various geomorphic systems of the great plains did not behave synchronously in response to any common climate driver these stratigraphic records reflect local environmental conditions and probably a complex response to the reorganization of mid latitude climates in the terminal pleistocene and early holocene

convection driven tectonics on venus an analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on venus theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a newtonian viscous medium with an exponential depth dependence of viscosity a green s function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth it is shown that during the transient phase of the solution the field quantities stress velocity displacement in both media depend upon the rheology of both media whereas in steady state the solutions become uncoupled due to this uncoupling stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate i show for such a plate that 1 shear stress imparted at the base of the lower boundary can induce large in plane stretching mode forces and 2 these forces can be estimated directly from free air gravity anomalies without detailed knowledge of the density distribution in the underlying flow free air gravity anomalies for venus imply in plane forces of about 2 3 1012 n m in plane forces of this magnitude are capable of extensive disruption of the venusian lithosphere to demonstrate this an elastic plastic analysis is carried out for the deformation of a model venus lithosphere the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow the results predict that dynamic uplift of venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere geoid anomalies and geoid to topography ratios for venusian topographic features are large compared to values for thermal swells in earth s ocean basins this suggests that dynamic support of topography must be an important process on venus further a low viscosity zone is probably not present beneath the venusian lithosphere a comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on earth is unimportant

mantle convection under simulated plates effects of heating modes and ridge and trench migration and implications for the core mantle boundary bathymetry the geoid and benioff zones summary numerical convection models are presented in which plates are simulated by imposing piecewise constant horizontal velocities on the upper boundary a 4 1 box of constant viscosity fluid and two dimensional 2 d flow is assumed four heating modes are compared the four combinations of internal or bottom heating and prescribed bottom temperature or heat flux the case with internal heating and an isothermal base is relevant to lower mantle or whole mantle convection and it yields a lower thermal boundary layer which is laterally variable and can be locally reversed corresponding to heat flowing back into the core locally when scaled to the whole mantle the surface deflections and gravity and geoid perturbations calculated from the models are comparable to those observed at the earth s surface for models with migrating ridges and trenches the flow structure lags well behind the changing surface plate configurations this may help to explain the poor correlation between the main geoid features and plate boundaries trench migration substantially affects the dip of the cool descending fluid because of induced horizontal shear in the vicinity of the trench such shear is small for whole mantle convection but is large for upper mantle convection and would probably result in the tonga benioff zone dipping to the se opposite to the observed dip for the case of upper mantle convection

a hybrid radial basis function pseudospectral method for thermal convection in a 3 d spherical shell a novel hybrid spectral method that combines radial basis function rbf and chebyshev pseudospectral ps methods in a 2 1 approach is presented for numerically simulating thermal convection in a 3 d spherical shell this is the first study to apply rbfs to a full 3d physical model in spherical geometry in addition to being spectrally accurate rbfs are not defined in terms of any surface based coordinate system such as spherical coordinates as a result when used in the lateral directions as in this study they completely circumvent the pole issue with the further advantage that nodes can be scattered over the surface of a sphere in the radial direction chebyshev polynomials are used which are also spectrally accurate and provide the necessary clustering near the boundaries to resolve boundary layers applications of this new hybrid methodology are given to the problem of convection in the earth s mantle which is modeled by a boussinesq fluid at infinite prandtl number to see whether this numerical technique warrants further investigation the study limits itself to an isoviscous mantle benchmark comparisons are presented with other currently used mantle convection codes for rayleigh number 7 103 and 105 the algorithmic simplicity of the code mostly due to rbfs allows it to be written in less than 400 lines of matlab and run on a single workstation we find that our method is very competitive with those currently used in the literature

geoarchaeology and geochronology of the miami clovis site southern high plains of texas abstract the miami site excavated in 1937 is in a small playa basin on the high plains surface the site is one of the earliest documented co occurrences of clovis points and mammoth reinvestigation of the site and related collections was undertaken to better understand the stratigraphy geochronology and archaeology the basin 23 m diameter 1 6 m deep filled with 1 dark gray silty clay and 2 near the top of the section a lens of well sorted silt or loess the basin started to fill ca 13 700 yr b p the loess dates to ca 11 400 yr b p and the bone bed probably dates to ca 11 400 10 500 yr b p the loess may be the local manifestation of a clovis drought the partial remains of five mammoths three adults and two juveniles were recovered in 1937 no other animal remains are known the bone is heavily weathered and there are no clear indications of human modification artifacts found at the site include three clovis points and a scraper found among the bones and two flakes and a scraper found on the surface near the playa the origins of the bone and stone assemblage are uncertain but four scenarios are offered a successful mammoth kill an unsuccessful kill with wounded animals dying at the watering hole opportunistic scavenging following natural deaths or a palimpsest of multiple deaths following both natural and human causes

grassland vegetation in the southern great plains during the last glacial maximum abstract new pollen records from white lake in the southern high plains and from friesenhahn cave on the southeastern edwards plateau of texas indicate that the glacial age vegetation of the southern great plains was a grassland the high plains was a treeless artemisia grassland and the edwards plateau at the south edge of the great plains was a grassland with pinyon pines and deciduous trees in canyons and riparian habitats the glacial age grasslands differ from modern shortgrass and tallgrass prairies and may have no modern analog the dominance of prairie vegetation during the last glacial maximum is compatible with late pleistocene mammalian faunas and late glacial grassland pollen records from the region earlier interpretations of a pine spruce forest on the high plains were based on pollen assemblages that are here shown to have been altered by postdepositional deterioration resulting in differential preservation of conifer pollen grains accordingly the tahoka pluvial and other climatic episodes defined by high plains pollen records are abandoned

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

models of convection driven tectonic plates a comparison of methods and results summary recent numerical studies of convection in the earth s mantle have included various features of plate tectonics a number of different methods for modelling plate like behaviour have been used the differences in the methods of modelling plates may assume or predict significantly different plate deformation we describe three methods of modelling plates through material properties force balance and a thin power law sheet approximation we compare the results obtained using each method on a series of simple calculations from these results we are able to develop scaling relations between the different parametrizations while each method produces different degrees of deformation within the surface plate the surface heat flux and average plate velocity agree to within a few per cent the main results are not dependent upon the plate modelling method and therefore are representative of the physical system we set out to model

plate tectonics on super earths equally or more likely than on earth abstract the discovery of extra solar super earths has prompted interest in their possible mantle dynamics and evolution and in whether their lithospheres are most likely to be undergoing active plate tectonics like on earth or be stagnant lids like on mars and venus the origin of plate tectonics is poorly understood for earth likely involving a complex interplay of rheological compositional melting and thermal effects which makes it challenging to make reliable predictions for other planets nevertheless as a starting point it is common to parameterize the complex processes involved as a simple yield stress that is either constant or has a byerlee s law dependence on pressure because the simplifying assumptions made in developing analytical scalings may not be valid over all parameter ranges numerical simulations are needed one numerical study on super earths finds that plate tectonics is less likely on a larger planet o neill and lenardic 2007 in apparent contradiction of an analytical scaling study valencia et al 2007 to try and understand this we here present new calculations of yielding induced plate tectonics as a function of planet size focusing on the idealized end members of internal heating or basal heating as well as different strength profiles and compared to analytical scalings in the present study we model super earths as simple scaled up versions of earth i e assuming constant physical properties keeping the ratio of core mantle radii constant and applying the same temperature difference between top bottom boundaries and the same internal heating rate effects that originate outside of the planet such as tidal forces meteor impacts and intense surface heating from a nearby star are not considered we find that for internally heated convection plate tectonics is equally likely for terrestrial planets of any size whereas for basally heated convection plate tectonics becomes more likely with increasing planet size this is indicated both by analytical scalings and the presented numerical results which agree with each other when scalings are adjusted to account for increasing mean density with increasing planet size plate tectonics becomes more likely with increasing planet size for all scenarios the influence of the pressure variation of viscosity thermal expansivity and conductivity may however act in the opposite sense and needs to be determined in future studies at least in the simplest case factors other than planet size such as the presence of surface water are likely most important for determining the presence or absence of plate tectonics

self consistent generation of tectonic plates in time dependent three dimensional mantle convection simulations 2 strain weakening and asthenosphere 1 in a previous paper tackley 2000b it was shown that a combination of temperature dependent viscosity and viscoplastic yielding is sufficient to give rudimentary plate tectonic like behavior in three dimensional models of mantle convection here the calculations are extended to include two complexities that have been suggested as being important in localizing deformation at plate boundaries strain weakening and the presence of a low viscosity asthenosphere introducing an asthenosphere by reducing the viscosity by a factor of 10 where material reaches a solidus 1 dramatically improves plate quality even if the asthenosphere is restricted to regions around spreading centers 2 gives good smoothly evolving platelike behavior over a wide range of yield stress values spanning an order of magnitude and 3 gives bimodal stable solutions over a range of yield stresses either immobile lid or plate behavior may be obtained depending on initial condition by contrast introducing an asthenosphere by using depth dependent viscosity has a much smaller effect on system behavior increasing strain weakening 1 improves localization at spreading centers but leads to an increasingly complex network of spreading centers fragmenting the plates 2 weakens convergent zones and can make downwellings highly episodic and 3 does not lead to pure transform boundaries in these calculations time dependent damage evolution and instantaneous strain rate weakening give very similar results all cases that display platelike behavior have very long wavelength mantle heterogeneity consistent with earth s

geochemistry of platinum group elements of the zoned ultramafic alto condoto complex northwest colombia the alto condoto complex dated at 20 ma is a zoned ultramafic intrusion with a magmatic rock sequence ranging from dunite to hornblendite the different petrographic units are cumulates formed by fractional crystallization processes parts of the central dunite are anomalously enriched in ptfe alloys and to a minor degree in iros alloys all other lithologic units of the complex display normal fractionation trends of platinum group elements pge with a general increase of total pge from wehrlite to horn blende clinopyroxenite and hornblendite this normal trend is best described by increasing pd and decreasing ir abundances the controlling factor for pge mineralization in zoned ultramafic complexes is probably an effective ptfe alloy precipitation during the crystallization of olivine this may be initiated by rapid tectonic movements causing pressure decrease and overheating of the magma with pge alloy formation as a result of the f sub o 2 shift during olivine crystallization