rapid eocene extension in the robinson district white pine county nevada constraints from 40ar 39ar dating precise 40ar 39ar ages on pretectonic syntectonic and posttectonic rhyolites from the robinson mining district which is within a highly extended domain in east central nevada indicate that extreme extension 400 occurred in fewer than 900 k y during the eocene the duration of extension is tightly delimited by an eocene lacustrine unit that was horizontal prior to extension and by younger rhyolitic units the rhyolites represent at least five eruptive episodes including the preextensional and synextensional rhyolite of white hill emplaced between 37 6 and 37 4 ma and the postextensional rhyolite of garnet hill emplaced at 36 7 ma although some of it is perhaps as old as 37 1 ma thus the duration of extreme extension at robinson was between 0 9 and 0 3 m y comparable to or shorter than other highly extended areas in western north america the extensional domain in east central nevada is a composite feature produced by localized episodic rapid extension that began in early eocene time and continued to at least middle miocene time

modelling heat flow in a cold high altitude glacier interpretation of measurements from colle gnifetti swiss alps modelling the heat flow in small cold high altitude glaciers is important for the interpretation of paleoclimatic data from ice cores coupled glacier flow and heat flow models are presented that incorporate the densification heat advection and possible phase transitions at the permafrost boundaries within the bedrock marked bends observed in the temperature profiles from two recent boreholes on colle gnifetti swiss alps are interpreted with the help of a transient heat flow model driven with a temperature history the conclusion is that substantial warming of the mean firn temperature at shallow depths has taken place over the last few decades this has not been observed before in cold firn regions of the alps modelled heat fluxes in the monte rosa massif are strongly influenced by the mountain topography this leads to a spatial variability of the temperature gradient near the glacier base which has been observed in boreholes to the bedrock in order to match the measured temperature profiles in the glacier the vertical heat flux at great depth must be set to an extremely low value it is shown with the help of the transient heat flow model that this is a paleoclimatic effect possibly enhanced by a degrading permafrost base

very high elevation mont blanc glaciated areas not affected by the 20th century climate change 1 this paper analyses the impact of climate change over the last 100 years on high elevation glaciated areas of the mont blanc range comprising ice fields covering the top of the mont blanc 4808 m and dome du gouter 4300 m areas surface ablation is negligible for these high elevation areas and the surface mass balance is mainly controlled by snow accumulation at dome du gouter ice fluxes have been calculated through two transversal sections by two independent methods in order to assess long term surface accumulation a comparison between these results and recent accumulation observations together with the strong relationship between valley precipitation and snow accumulation suggests that surface accumulation rates did not change significantly over the entire 20th century moreover the small ice thickness changes less than 3 m on the average observed at mont blanc and dome du gouter between 1905 and 2005 clearly reveal that these high elevation glaciated areas have not been significantly affected by climate change over the last 100 years

atmospheric temperature changes over the 20th century at very high elevations in the european alps from englacial temperatures 1 given the paucity of observations a great deal of uncertainty remains concerning temperature changes at very high altitudes over the last century englacial temperature measurements performed in boreholes provide a very good indicator of atmospheric temperatures for very high elevations although they are not directly related to air temperatures temperature profiles from seven deep boreholes drilled at three different sites between 4240 and 4300 m above sea level in the mont blanc area french alps have been analyzed using a heat flow model and a bayesian inverse modeling approach atmospheric temperature changes over the last century were estimated by simultaneous inversion of these temperature profiles a mean warming rate of 0 14 c decade between 1900 and 2004 was found this is similar to the observed regional low altitude trend in the northwestern alps suggesting that air temperature trends are not altitude dependent

seismic wave attenuation associated with crustal faults in the new madrid seismic zone the attenuation of upper crustal seismic waves that are refracted with a velocity of about 6 kilometers per second varies greatly among profiles in the area of the new madrid seismic zone in the central mississippi valley the waves that have the strongest attenuation pass through the seismic trend along the axis of the reelfoot rift in the area of the blytheville arch defocusing of the waves in a low velocity zone and or seismic scattering and absorption could cause the attenuation these effects are most likely associated with the highly deformed rocks along the arch consequently strong seismic wave attenuation may be a useful criterion for identifying seismogenic fault zones

climate warming revealed by englacial temperatures at col du dome 4250 m mont blanc area 1 temperatures were measured in two deep boreholes drilled at the same location in the ice at col du dome 4250 m in 1994 and 2005 providing clear evidence of atmospheric warming the 1994 temperature profile was already far from steady state conditions results from a heat transfer model reveal that the englacial temperature increase cannot be explained solely by atmospheric temperature rise the latent heat produced by the refreezing of surface meltwater below the surface also contributes to the englacial temperature increase although surface melting is normally very low at this altitude this contribution became significant after 1980 for temperatures at the top of the borehole simulations for different climatic scenarios show that glaciated areas located between 3500 and 4250 m could become temperate in the future this warming could have a major impact on the stability of hanging glaciers frozen to their beds if the melting point is reached

contour mapping of relic structures in the precambrian basement of the reelfoot rift north american midcontinent a new contour map of the basement of the reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features the major features are two asymmetric intrarift basins bounded by three structural highs and the rift margins the basins are oriented normal to the northeast trend of the rift two of the highs appear to be ridges of undetermined width that extend across the rift the third high is an isolated dome or platform located between the basins the minor features are three linear structures of low relief oriented subparallel to the trend of the rift two of these located within the rift basins may divide the rift basins into paired subbasins these mapped features may be the remnants of initial extensional rifting half graben faulting and basement subsidence the rift basins are interpreted as having formed as opposing half graben and the structural highs are interpreted as having formed as associated accommodation zones some of these features appear to be reactivated seismogenic structures within the modern midcontinent compressional stress regime a detailed knowledge of the geometries of the reelfoot rift s basement features therefore is essential when evaluating their seismic risk potential

neotectonics of the upper mississippi embayment abstract although the upper mississippi embayment is an area of low relief the region has been subjected to tectonic influence throughout its history and continues to be so today tectonic activity can be recognized through seismicity patterns and geological indicators of activity either those as a direct result of earthquakes or longer term geomorphic structural and sedimentological signatures the rate of seismic activity in the upper mississippi embayment is generally lower than at the margins of tectonic plates the embayment however is the most seismically active region east of the rocky mountains with activity concentrated in the new madrid seismic zone this zone produced the very large new madrid earthquakes of 1811 and 1812 geological and geophysical evidence of neotectonic activity in the upper mississippi embayment includes faulting in the benton hills and thebes gap in missouri paleoliquefaction in the western lowlands of missouri subsurface faulting beneath and tilting of crowley s ridge in northeastern arkansas and southeastern missouri subsurface faulting along the crittenden county fault zone near memphis tennessee faulting along the east flank of the tiptonville dome and numerous indicators of historic and prehistoric large earthquakes in the new madrid seismic zone paleoearthquake studies in the new madrid seismic zone have used trenching seismic reflection shallow coring pedology geomorphology archaeology and dendrochronology to identify and date faulting deposits of liquefied sand and areas of uplift and subsidence the cause of today s relatively high rate of tectonic activity in the mississippi embayment remains elusive it is also not clear whether this activity rate is a short term phenomenon or has been constant over millions of years ongoing geodetic and geological studies should provide more insight as to the precise manner in which crustal strain is accumulating and perhaps allow improved regional neotectonic models

a 3 d thermal regime model suitable for cold accumulation zones of polythermal mountain glaciers analysis of the thermal and mechanical response of high altitude glaciers to climate change is crucial to assess future glacier hazards associated with thermal regime changes this paper presents a new fully thermo mechanically coupled transient thermal regime model including enthalpy transport firn densification full stokes porous flow free surface evolution strain heating surface meltwater percolation and refreezing the model is forced by daily air temperature data and can therefore be used to perform prognostic simulations for different future climate scenarios the set of equations is solved using the finite element ice sheet ice flow model elmer ice this model is applied to the col du dome glacier mont blanc area 4250 m a s l france where a comprehensive data set is available the results show that the model is capable of reproducing observed density and velocity fields as well as borehole temperature evolution the strong spatial variability of englacial temperature change observed at col du dome is well reproduced this spatial variability is mainly a result of the variability of the slope aspect of the glacier surface and snow accumulation results support the use of this model to study the influence of climate change on cold accumulation zones in particular to estimate where and under what conditions glaciers will become temperate in the future

rift structure of the northern mississippi embayment from the analysis of gravity and magnetic data gravity and magnetic data have been simultaneously inverted to derive a crustal model of the northern mississippi embayment which is compatible with geologic and seismic refraction data the results indicate that this region is a site of a late precambrian early paleozoic rift system an anomalously dense layer is present at the base of the crust and thickens beneath a broad northeast trending graben that formed during the initial phases of rifting the thickest part of anomalous crust occurs beneath the region of greatest seismic activity which lies within the geographic limits of the graben it is suggested from this correlation that strain accumulates within the anomalous crust layer which acts as an inhomogeneity in a relatively homogeneous lithosphere based on the results of this study and previous geophysical studies the complex tectonic evolution of the upper mississippi embayment since late precambrian time is rift formation possibly along a pre existing shear zone graben development in cambrian time reactivations in permian and cretaceous time and subsequent formation of the present mississippi embayment and fault zones with large influences from rift structures

atmospheric warming at a high elevation tropical site revealed by englacial temperatures at illimani bolivia 6340 m above sea level 16 s 67 w in june 1999 a deep 138 7 m ice core was extracted from the summit glacier of illimani bolivia 6340 m above sea level 16 39 s 67 47 w and an englacial temperature profile was measured in the borehole using on site and regional meteorological data as well as ice core stratigraphy past surface temperatures were reconstructed with a heat flow model the englacial temperature measurements exhibit a profile that is far from a steady state reflecting an increasing atmospheric temperature over several years and nonstationary climatic conditions englacial temperature interpretation using air temperature data borehole temperature inversion and melting rate quantification based on ice core density shows two warming phases from 1900 to 1960 0 5 0 3 k starting approximately in 1920 1930 and from 1985 to 1999 0 6 0 2 k corresponding to a mean atmospheric temperature rise of 1 1 0 2 k over the 20th century according to various climate change scenarios the future evolution of englacial temperatures was simulated to estimate when and under what conditions this high elevation site on the illimani summit glacier could become temperate in the future results show that this glacier might remain cold for more than 90 years in the case of a 2 k rise over the 21st century but could become temperate in the first 20 m depth between 2050 and 2060 if warming reaches 5 k

cold firn and ice of high altitude glaciers in the alps measurements and distribution modelling the thermal regime of high altitude accumulation areas in the swiss alps was systematically investigated on the jungfraufirn bernese alps on the breithornplateau valais alps and on grenzgletscher valais alps in 1991 1992 and 1994 temperatures were measured in a deep hole 120 m deep and in several shallow holes 14 30 m deep whereas the wide neve of the jungfraufirn at 3400 3600 m a s l and the 3800 m high breithorn plateau seems to be predominantly temperate cold firn and ice temperatures were measured throughout on grenzgletscher 3900 4450 m a s l mean firn temperatures on grenzgletscher vary strongly and range between 3 and 14 c a comparison between the measured temperature profiles and a one dimensional heat conduction calculation shows that the release of latent heat by penetrating and refreezing meltwater decisively influences the thermal pattern of the firn pack a multiple linear regression model based on measured firn temperatures from the european alps and the parameters altitude and aspect yields aspect dependent lower boundaries for the occurrence of cold firn ranging between 3400 northerly aspects and 4150 m a s l southerly aspects a total of 120 glaciers with cold firn areas are found when applying the model to glacier inventory data from the european alps

thermochronological constraints on the timing and magnitude of miocene and pliocene extension in the central wassuk range western nevada apatite fission track and u th he thermochronological data provide new constraints on the timing of faulting and exhumation of the wassuk range western nevada where east dipping normal faults have accommodated large magnitude ene wsw oriented extension extensional deformation has resulted in the exhumation of structurally coherent fault blocks that expose sections of preextensional mostly granitic upper crust in the grey hills and central wassuk range these fault blocks display westward tilts of 60 and expose preextensional paleodepths of up to 8 5 km based on the structural reconstruction of tilted preextensional tertiary andesite flows that unconformably overlie mesozoic basement rocks apatite fission track and u th he thermochronological data from the fault blocks constrain the onset of rapid footwall exhumation at 15 ma fission track modeling results indicate rapid fault block exhumation occurred between 15 and 12 ma which is in agreement with miocene volcanic rocks that bracket the tilting history in addition fission track and u th he data suggest reduced rates of cooling following major extension as well as renewed cooling related to active high angle faulting along the present day range front starting at 4 ma thermochronological data from structurally restored fault blocks indicate a preextensional miocene geothermal gradient of 27 5 c km the thermochronological constraints on the timing of extensional faulting and the eruptive history in the wassuk range imply a model for extension where crustal heating and volcanism precede the onset of rapid large magnitude extension and where synextensional magmatism is suppressed during the highest rates of extension

a tibetan model for the early tertiary western united states a comparative study of the geology of the early tertiary western united states and the modern tibetan and turkish iranian plateaux indicates that these three regions situated in the hinterland of major orogenic belts share a common taphrogenic evolutionary path both the tibetan and turkish iranian plateaux have high mean elevations above sea level 5 km and 2 0 2 5 km respectively have experienced fast uplift and widespread unroofing during the last 10 8 ma and are undergoing extension nearly perpendicular to the plate convergence direction successive accretionary and collisional events accompanied by significant shortening and underplating during the formation of these plateaux resulted in the generation of overthickened continental crust 50 km that is currently in the process of extending a two stage magmatism characterized by widespread calc alkaline volcanism followed and or overlapped by bimodal basaltic to rhyolitic volcanism occurs synchronously with crustal extension and thinning the plateaux in general have an internal closed drainage system and are drained by antecedent rivers near their edges the early tertiary western us cordillera displayed similar geological and geomorphic features to those observed in the tibetan and turkish iranian plateaux in the aftermath of a prolonged mesozoic orogeny and underwent an orogenic collapse starting around mid tertiary time a n trending crustal welt with a maximum thickness of 60 km and an elevation around 3 km formed a highland with alpine flora in the hinterland of the cordilleran orogenic belt and separated internal drainage basins and saline lakes on the east from major flood plains of antecedent river systems on the west this highland subsequently became the locus of crustal extension metamorphic core complex formation and wide spread calcalkaline volcanism 9ignimbrite flare up9 this early stage of orogenic collapse was followed by the basin and range extension characterized by block faulting and bimodal volcanism starting around 18 16 ma the basin and range extension and associated subsidence resulted in development of the great basin with a mean elevation of 1 5 km above sea level and a crustal thickness of 30 km similarities in the post orogenic evolutionary paths of these three regions indicate that continental lithosphere reacts to overthickening in a consistent way through large scale crustal remobilization and extensional collapse regardless of the mode and nature of its orogenic build up

geologic structures related to new madrid earthquakes near memphis tennessee based on gravity and magnetic interpretations new inversions of gravity and magnetic data in the region north of memphis tennessee and south of latitude 36 define boundaries of regional structures and igneous complexes in the upper crust microseismicity patterns near interpreted boundaries suggest that igneous complexes influence the locations of microseismicity a weak seismicity cluster occurs near one intrusion covington pluton at the intersection of the southwest margin of the missouri batholith and the southeast margin of the reelfoot rift a narrow seismicity trend along the reelfoot rift axis becomes diffuse near a second intrusion osceola intrusive complex and changes direction to an area along the northwest flank of the intrusion the axial seismicity trend also contains a tight cluster of earthquakes located just outside the osceola intrusive complex the mechanical explanation of the two seismicity patterns is uncertain but the first cluster may be caused by stress concentration due to the high elastic stiffness and strength of the covington intrusion the spatially changing seismicity pattern near the osceola complex may be caused by the preceding factors plus interaction with faulting along the rift axis the axial seismicity strand itself is one of several connected and interacting active strands that may produce stress concentrations at strand ends and junctions the microseismicity clusters at the peripheries of the two intrusions lead us to conclude that these stress concentrations or stressed volumes may be locations of future moderate to large earthquakes near memphis

thermal conductivity of snow firn and porous ice from 3 d image based computations estimating thermal conductivity of snow firn and porous ice is key for modeling the thermal regime of alpine and polar glaciers whereas thermal conductivity of snow was widely investigated studies on firn and porous ice are very scarce this study presents the effective thermal conductivity tensor computed from 64 3 d images of microstructures of snow antarctic firn and porous ice at 3 20 and 60 c we show that in contrast with snow conductivity of firn and porous ice correlates linearly with density is approximately isotropic and is largely impacted by temperature we report that performances of commonly used estimates of thermal conductivity vary largely with density in particular formulas designed for snow lead to significant underestimations when applied to denser ice structures we present a new formulation to accurately estimate the thermal conductivity throughout the whole density range from fresh snow to bubbly ice and for any temperature conditions encountered in glaciers

volume and frequency of ice avalanches from taconnaz hanging glacier french alps very large volumes of ice break off regularly from taconnaz hanging glacier french alps during winter when the snow mantle is unstable these collapses can trigger very large avalanches that represent a serious threat to inhabited areas below photogrammetric measurements have been performed over 1 year to assess the volume and frequency of the largest collapses major collapses occur when the glacier reaches a critical geometry after a major ice collapse the glacier is in a minimal position and subsequently recharges over 6 months to reach the maximum position again this critical geometry is a necessary but not sufficient condition for further large collapses large collapses do not systematically occur in the maximum position as ice is often removed by disintegration into small ice blocks for two major collapses the volume of ice breaking off has been assessed at 275000m 3 photogrammetric measurements were used to determine an ice flux of 820000m 3 a 1 through the studied ice stream in agreement with an assessment based on ice flow modeling this ice flux estimation was used to determine the average ice volumes breaking off over surveyed periods

petrologic model of the northern mississippi embayment based on satellite magnetic and ground based geophysical data abstract a petrologic model of the northern mississippi embayment derived from gravity seismic and rift data is evaluated by converting the model to a magnetization model which is compared with satellite magnetic anomaly models a magnetization contrast of approximately 0 54 a m determined from the petrologic model of the embayment compares favorably to values of 0 62 a m and 0 45 a m from a magsat united states apparent magnetization contrast map and a published pogo magnetization contrast model respectively the petrologic model suggests that the magnetic anomaly low associated with the mississippi embayment may be largely due to the intrusion under non oxidizing conditions of low curie temperature gabbroic material at the base of the crust of the embayment near surface mafic plutons bordering the mississippi valley graben appear from aeromagnetic data to have higher magnetizations than the deeper gabbroic material however it is impossible to ascertain if this is due to compositional differences or similar material at shallower lower temperature depths these results indicate that variations in the curie temperatures of intrusions accompanying rifting may account for a large part of the wide range of magnetic anomalies associated with presently inactive rifts with normal heat flow

cambrian to holocene structural and burial history of nashville dome about 14 000 ft 4 270 m of strata covered basement over the present crest of the nashville dome by the end of the paleozoic calculated by estimating the geothermal gradient and using temperatures of veins in stones river group and knox dolomite at least 7 500 ft 2 290 m of post devonian strata have been removed by subsequent erosion estimates of other erosional episodes include 350 ft 107 m of upper knox during the middle ordovician and 500 ft 152 m of devonian ordovician during the late devonian mesozoic to holocene uplift was at least 6 350 ft 1 940 m 1 500 ft or 460 m 25 of which occurred in the latest 100 m y and 450 ft or 140 m 7 during the latest 2 m y a rate ranging from about 15 ft m y 4 6 m m y for the longer term to over 225 ft y 70 m m y in the pleistocene to holocene earliest structure of the area was a series of elongate basins probably rifts synchronous with reelfoot rift to the west uplifts trending n10 e moved about 40 mi 65 km westward during the middle ordovician these may relate to similar trending and moving appalachian orogenic events a change to uplifts trending n50 e parallel to strikes of appalachian thrusts occurred in the late ordovician and continued to the devonian this may reflect a similar late ordovician change in the orientation of appalachian tectonism in the interval from post mississippian to late cretaceous the dome curved westward to join the pascola arch in response to ouachita activity

model of the southeastern margin of the mississippi valley graben near memphis tennessee from interpretation of truck magnetometer data the geometry and magnetic properties of the southeastern margin of the mississippi valley graben ate studied by analyzing detailed magnetic anomaly profiles that are based on measurements made along roads near memphis tennessee the closely spaced measurements were obtained with a truck mounted magnetometer system developed by the u s geological survey modeling results suggest that the graben margins represent both structural boundaries and conduits for ascending magma about 2 km of vertical offset associated with normal faulting occurs within an interpreted 5 5 km wide zone in which magnetic basement has an average dip of 20 nw into the graben this zone may contain several large normal faults formed during a stage of rifting in late precambrian or early paleozoic time subsequent erosion of the scarps probably aided in forming the gentle northwestward slope of the buried precambrian surface across the fault zone the high apparent susceptibility about 3 10 3 emu of magnetic basement associated with this fault zone and with the upblock suggests either that ascending magma intruded the upblock or that the two blocks differed lithologically prior to the formation of the graben

avalanching glacier instabilities review on processes and early warning perspectives avalanching glacier instabilities are gravity driven rupture phenomena that might cause major disasters especially when they are at the origin of a chain of processes reliably forecasting such events combined with a timely evacuation of endangered inhabited areas often constitute the most efficient action recently considerable efforts in monitoring analyzing and modeling such phenomena have led to significant advances in destabilization process understanding improving early warning perspectives the purpose of this paper is to review the recent progress in this domain three different types of instabilities can be identified depending on the thermal properties of the ice bed interface if cold 1 the maturation of the rupture is associated with a typical time evolution of surface velocities and passive seismic activity a prediction of the final break off is possible using these precursory signs for the two other types water plays a key role in the development of the instability if the ice bed interface is partly temperate 2 the presence of meltwater may reduce the basal resistance which promotes the instability no clear and easily detectable precursory signs are known in this case and the only way to infer any potential instability is to monitor the temporal evolution of the thermal regime the last type of instability 3 concerns steep temperate glacier tongues switching for several days weeks during the melting season into a so called active phase followed in rare cases by a major break off event although the prediction of such events is still far from being achievable critical conditions promoting the final instability can be identified

tectonic geomorphology of the mississippi valley between osceola arkansas and friars point mississippi abstract a geomorphic study of the mississippi river and its alluvial valley between osceola arkansas and friars point mississippi has identified anomalous surface features that can be linked to known geological structures for example relatively recent deformation along big creek fault zone white river fault zone bolivar mansfield tectonic zone blytheville arch crittenden county fault zone and reelfoot rift margins is suggested by river and topographic anomalies faults and plutons appear to affect drainage networks and the morphology of crowleys ridge suggests significant fault control the many anomalies probably reflect a fractured suballuvial surface although movement along these fractures will most likely occur in seismically active areas the probability of movement elsewhere