weak bed control of the eastern shear margin of thwaites glacier west antarctica recent acceleration and thinning of thwaites glacier west antarctica motivates investigation of the controls upon and stability of its present ice flow pattern its eastern shear margin separates thwaites glacier from slower flowing ice and the southern tributaries of pine island glacier troughs in thwaites glacier s bed topography bound nearly all of its tributaries except along this eastern shear margin which has no clear relationship with regional bed topography along most of its length here we use airborne ice penetrating radar data from the airborne geophysical survey of the amundsen sea embayment antarctica agasea to investigate the nature of the bed across this margin radar data reveal slightly higher and rougher bed topography on the slower flowing side of the margin along with lower bed reflectivity however the change in bed reflectivity across the margin is partially explained by a change in bed roughness from these observations we infer that the position of the eastern shear margin is not strongly controlled by local bed topography or other bed properties given the potential for future increases in ice flux farther downstream the eastern shear margin may be vulnerable to migration however there is no evidence that this margin is migrating presently despite ongoing changes farther downstream

carbonate diagenesis in ice the fine particles created by glacial comminution of carbonate minerals are susceptible to interaction with meltwater and the isotopic composition of fine carbonate in glacial deposits has the potential of recording this interaction here we focus on the carbonate fraction of glacial sediment and debris within basal ice at the tsanfleuron glacier switzerland size separates of basal ice debris and melt out tills show a collinear isotopic trend interpreted as a mixing line between detritus 18 o pdb 5 and 13 o pdb 0 6 and authigenic calcite which in one case had 18 o values as light as 18 7 and 13 c values as light as 5 7 in most cases the finer fractions with calcite crystals as small as 0 1 m are enriched in the isotopically light component which is interpreted to form largely by freezing in basal ice the isotopic compositions of ice and included gases could be influenced by this process melt out tills inherit the authigenic calcite thus carbonate bearing till should no longer be regarded simply as clastic sediment

direct observation of basal sliding and deformation of basal drift at sub freezing temperatures in a tunnel at the base of sub polar urumqi glacier no 1 china three new mechanisms of glacier flow at sub freezing temperatures have been observed taken individually or in combination these modes of flow can account for nearly all 60 80 of the overall glacier motion and yet they act only within the lowermost 1 2 of the effective glacier thickness these mechanisms are 1 enhanced deformation of the frozen and ice laden subglacial drift 2 motion across discrete shear planes or shear bands within the frozen drift or at the ice drift interface and 3 basal sliding at an ice rock interface at a temperature of nearly 5 c the ice laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice thus allowing very rapid shear deformation the observed rate or basal sliding at the ice rock interface agrees favorably with that predicted by the recent work of shreve 1984 if proper account is taken of the measured surface roughness and reduced ice viscosity

reconstructing past glacier dynamics and erosion from glacial geomorphic evidence snowdon north wales bedrock surfaces exposed around llyn llydaw north wales demonstrate contrasting styles of erosion beneath a late devensian ice sheet and a loch lomond stadial lls valley glacier ice sheet erosion involved lee side fracturing surface fracture wear and abrasive wear while lls erosion was primarily by abrasive wear preservation of ice sheet erosional features indicates limited rates of erosion during the lls analysis of the geometry and distribution of erosional markings suggests that the low erosional capacity of the lls glacier was due to a low basal sliding velocity this prevented the formation of lee side cavities reduced the debris flux over the bed and minimised particle bed contact loads reconstructions of the mass balance and geometry of the lls glacier indicate that most of its balance velocity could be achieved by internal deformation alone a combination of low subglacial water pressures and an unusually rough substrate explain the low sliding velocities high bed roughness is due to the absence of leeside cavities and a change in flow orientation between ice sheet and lls times which meant that the lls glacier was in contact with roughness elements which were generated in cavities beneath the ice sheet

tectono magmatic evolution of the west philippine mariana region and the origin of boninites re examination of published data on the tectomagmatic evolution of the west philippine mariana region indicate that arc magmatism and back arc extensional pulses are not synchronous but are largely asynchronous arc volcanism ceases within a few million years of the development of a back arc basin and recommences oceanward on a new arc during the final stages in the development of the back arc basin following karig s model we believe that ascent beneath the arc axial chain of a line of diapirs of morb source mantle interferes with processes of arc magma generation and arc magmatism wanes partial melting of the diapirs produces voluminous morb type tholeiites which split the arc and form the crust of a widening back arc basin we show for both the west philippine mariana region and occurrences of boninite like lavas in ophiolites that boninites appear to be erupted after arc magmatism and immediately before eruption of morb type lavas passage of a diapir of hot morb source mantle through refractory sub arc peridotite which has been enriched in lile and h2o will result in limited partial melting of the hydrous peridotite and the restricted production of boninitic magmas these are more likely to erupt modified or only slightly modified by crystal fractionation through the thin forearc crust continued ascent of the morb source mantle diapir will result in it partially melting to yield morb tholeiites which erupt shortly after and in considerably more voluminous amounts than the boninites finally we suggest that forearcs regions are likely to be incorporated into foldbelts and that boninite or low ti lava bearing ophiolites may characterize such regions

direct measurement of basal motion at a hard bedded temperate glacier glacier de tsanfleuron switzerland hubbard b 2002 direct measurement of basal motion at a hard bedded temperate glacier glacier de tsanfleuron switzerland journal of glaciology 48 160 pp 1 8 rae2008

crustal accretion in the pan african nd and sr isotope evidence from the arabian shield abstract nd and sr isotope determinations on late precambrian to early palaeozoic igneous and sedimentary rocks from the arabian shield are used to investigate the proportion of reworked older crust and the rate at which new crust was generated during the pan african event eight rb sr whole rock isochrons on igneous suites yield ages in the range 770 590 ma and initial 87 sr 86 sr ratios of 0 7038 0 7023 these data confirm that magmatism in this area was largely restricted to the period 850 550 ma and the initial ratios are sufficiently low to preclude significant contributions from a long lived upper crustal source the initial 143 nd 144 nd ratios of a variety of lithologies including several samples of possible basement are all higher than the contemporaneous values for chur e nd 1 6 to 6 9 suggesting that many were derived directly from the upper mantle and that any inferred crustal source regions for the remainder could not have separated from likely lree depleted mantle reservoirs before 1200 ma the arabian shield therefore provides an example of rapid crustal growth during the late proterozoic and contrasts with the damara intracratonic belt of namibia where nd and sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period

organized flow from the south pole to the filchner ronne ice shelf an assessment of balance velocities in interior east antarctica using radio echo sounding data 1 ice flow through central antarctica has the potential to transmit accumulation changes from deep interior east antarctica rapidly to the shelf but it is poorly constrained owing to a dearth of ice velocity observations we use parameters derived from airborne radio echo sounding res data to examine the onset areal extent and englacial conditions of an organized flow network tributaries feeding an ice stream draining from the south pole to the filchner ronne ice shelf we classified res flight tracks covering the region according to whether englacial stratigraphy was disrupted i e internal layers diverged significantly from the surface and bed echoes or undisrupted i e internal layers closely parallel surface and basal topography and we calculated subglacial roughness along basal reflectors where satellite measured surface ice flow speeds are available covering 39 of the study region regions of fast and tributary flow correspond with res flight tracks that exhibit more disrupted internal layers and smoother subglacial topography than their counterparts in regions of slow flow this suggests that disrupted internal layering and smooth subglacial topography identified from res profiles can be treated as indicators of past or present enhanced flow tributaries where neither satellite nor ground based ice flow measurements are available we therefore use these res derived parameters to assess the balance flux modeled steady state flow regime between the south pole and filchner ronne ice shelf the res analysis confirms that an organized flow network drains a wide region around the south pole into the filchner ronne ice shelf however the spatial extent of this network as delineated by the res data diverges from that predicted by currently available balance flux models

the basal roughness of pine island glacier west antarctica we assess basal roughness beneath pine island glacier pig west antarctica based on a recent airborne radio echo sounding dataset we identify a clear relationship between faster ice flow and decreased basal roughness in significant parts of pig the central portion and two of its tributaries are particularly smooth but the majority of the tributaries feeding the main trunk are rougher we interpret the presence of a smooth bed as being a consequence of the deposition of marine sediments following disappearance of the west antarctic ice sheet in the pliocene or pleistocene and conversely a lack of marine sedimentation where the bed is rough importantly we also identify a patchy distribution of marine sediments and thus a bed over which the controls on flow vary while there is a notable correspondence between ice velocity and bed roughness we do not assume a direct causal relationship but find that an indirect one is likely where low basal roughness results in low basal resistance to flow a lower driving stress is required to produce the flux required to achieve mass balance this in turn means that the surface in that area will be lower than surrounding areas with a rougher bed and this will tend to draw flow into the area with low bed roughness since our studies shows that bed roughness beneath the tributaries of the trunk varies substantially there is a strong likelihood that these tributaries will differ in the rate at which they transmit current velocity changes on the main trunk into the interior of the glacier basin

bedrock surface roughness and the distribution of subglacially precipitated carbonate deposits implications for formation at glacier de tsanfleuron switzerland two carbonate deposits are identified on the exposed bedrock surface in the forefield of glacier de tsanfleuron switzerland macrocrystalline sparite and microcrystalline micrite comparison of the distributions of these forms with lee side slope facets identified by high pass filtering of a flow parallel bedrock profile at a range of frequencies reveals two significant results first while the distribution of sparite is consistent with formation in the lee side of subglacial bedrock hummocks that of micrite is not this contrasts with previous investigations in which both sparite and micrite have been considered to form by mineral concentration and precipitation during the refreezing of regelation related basal meltwaters in the lee side of bedrock hummocks alternative mechanisms of micrite formation involving carbonate deposition and or precipitation within subglacial bedrock hollows are proposed second the distribution of sparite is most strongly correlated with the distribution of lee side slope facets identified by filtering at a frequency equivalent to a hummock wavelength of c 0 1 m this correspondence indicates empirically that pressure related melting and refreezing regelation operates most effectively around bedrock hummocks that are shorter than c 0 1 m 1998 john wiley sons ltd

the paleozoic petroleum geology of central arabia exploratory drilling in central saudi arabia has established significant paleozoic petroleum potential in this region in recent discoveries the permian interval of the unayzah formation has been identified as the primary oil and gas reservoir the unayzah formation consists of alluvial and fluvial to shallow marine siliciclastic sandstones and siltstones that were deposited on a major pre unayzah unconformity saudi arabian paleozoic oil is a high gravity 43 53 degrees api crude with low sulfur content usually less than 0 07 that is classified as arabian super light oil gas with condensate has also been discovered in the unayzah formation the identified paleozoic traps are moderate relief fault generated structures with generally 30 100 m of closure unayzah structural traps developed primarily during the triassic to early jurassic however an earlier phase of significant pre unayzah structural development is also indicated between the devonian and late carboniferous and is related to a hercynian structural event post triassic structure in central arabia is mainly regional and results principally from late cretaceous to tertiary development of the central arabian arch the established source rock is the basal qusaiba shale a widespread organic rich shale that was deposited following regional deglaciation in the early silurian migration occurred regionally updip through unayzah formation reservoirs from the underlying qusaiba shale subcrop and or vertically along fault bounded zones on the flanks of structures from the underlying qusaiba shale interbedded upper permian shales and evaporites provide a top seal for the unayzah and form the basal sequence of the overlying transgressive khuff carbonate pre qusaiba reservoir targets include the cambrian ordovician saq sandstone the ordovician qasim formation and upper ordovician to lower silurian periglacial clastics of the zarqa and sarah formations high gravity low sulfur oil has also been discovered in this pre qusaiba clastic section the reservoirs of the pre qusaiba structures are fault bounded and sources laterally by the down faulted qusaiba shale

interfacial water in polar glaciers and glacier sliding at 17 c we have observed sliding at a cold 17 oc ice rock interface beneath meserve glacier antarctica and the segregation of ice into clean lenses amidst the dirty basal layers of this glacier we interpret these as manifestation s of thin water films at ice rock interfaces we use shreve s theory for sub freezing sliding to esti mate the nominal film thickness to be at least tens of nanometers such water films should exist around rocks in most polar ices and likely have high solute concen trations due to solute rejection during regelation and due to exchange with veins and grain boundaries where impurities reside

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

resolving the internal and basal geometry of ice masses using imaging phase sensitive radar the phase sensitive radio echo sounder pres is a powerful new instrument that can measure the depth of internal layers and the glacier bed to millimetre accuracy we use a stationary 16 antenna pres array on store glacier in west greenland to measure the three dimensional orientation of dipping internal reflectors extending the capabilities of pres beyond conventional depth sounding this novel technique portrays the effectiveness of pres in deriving the orientation of dipping internal layers that may complement profiles obtained through other geophysical surveying methods deriving ice vertical strain rates from changes in layer depth as measured by a sequence of pres observations assumes that the internal reflections come from vertically beneath the antenna by revealing the orientation of internal reflectors and the potential deviation from nadir of their associated reflections the use of an antenna array can correct this assumption while the array configuration was able to resolve the geometry of englacial layers the same configuration could not be used to accurately image the glacier bed here we use simulations of the performance of different array geometries to identify configurations that can be tailored to study different types of basal geometry for future deployments

geochemistry petrology and rbsr dating of trondhjemite and granophyre associated with jabal tays ophiolite idsas area saudi arabia abstract jabal tays trondhjemite and granophyre are leucocratic acidic rocks that intrude the melange zone of an ophiolitic complex they consist of two small stocks 200 m in diameter and some dikes and vein like bodies petrographically they are medium fine grained hypidiomorphic granular rocks consisting mainly of plagioclase an10 60 and quartz which commonly shows myrmekitic texture the mafic minerals are primarily biotite and muscovite with occasional sericite chlorite and epidote pseudomorphs after hornblende the bulk chemical composition and trace elements are similar to trondhjemites from other ophiolite complexes they also show calc alkaline affinity the k rb rb sr and initial 87sr 86sr 0 7030 0001 ratios as well as the depletion of the lil elements suggest an origin by partial melting of basic parent material in an island arc tectonic setting over a subduction zone the granophyres probably evolved from the trondhjemites either by crystal fractionation or crustal contamination the whole rock rb sr isochron age of 620 40 ma places an upper limit on the geologic and tectonic evolution of the jabal tays ophiolitic complex

basal roughness of the institute and moller ice streams west antarctica process determination and landscape interpretation we present a detailed analysis of bed roughness beneath institute and moller ice streams west antarctica using radio echo sounding data res acquired in the austral summer of 2010 11 we assess roughness using a two parameter approach and also assess the directionality of roughness relative to present day ice flow our work highlights the wealth of additional information that resides in analyses of bed roughness employing these multiple approaches we show that spatially variable roughness patterns are partly a consequence of the ability of flowing ice not only to smooth the bed but also to redistribute and remove sediments and to do this along flow accordingly we identify some fast flow tributaries underlain by topography that has been streamlined and other tributaries that are underlain by sediments we also identify locations that are currently protected from erosion but where more ancient erosion may once have occurred we conclude that detailed roughness analysis is a useful tool for landscape interpretation and we suggest that the roughness of an ice sheet s bed should be viewed not only as the consequence of ancient marine sedimentation but also as a product of more contemporary erosion and redistribution of sediments as well as bedrock smoothing that is ongoing because of continuing dynamic activity in this way basal roughness has the potential to evolve continually with ice sheet form and flow and should not be viewed simply as a snapshot of either present day or palaeo basal conditions

metamorphism and 40ar 39ar dating of the halaban ophiolite and associated units evidence for two stage orogenesis in the eastern arabian shield the arabian shield has long been recognized as a region where plate tectonic processes have been in action during most of the late proterozoic resulting in the amalgamation of the shield s five constituent terranes along four major suture zones studies thus far carried out on these collisional belts have concentrated on the origin and dating of intermediate to acidic igneous rocks and very little emphasis has been placed on the understanding of metamorphic processes operative during plate convergence the al amar suture separates the afif microplate from the ar rayn block in the easternmost section of the shield among several ophiolite occurrences in this suture the halaban belt is by far the largest and preserves within and around it a record of a number of thermal structural events related to the opening and eventual closure of a supra subduction zone back arc basin that existed in the period 695 680 ma the bimodal nature of 40 ar 39 ar dates from the halaban ophiolite and associated units is indicative of a two stage orogenesis climaxing at 680 and 600 ma instead of the previously proposed model of a single orogenic episode between 670 and 630 ma the first stage 680 ma is believed to be related to basin closure and ophiolite emplacement as the ar rayn island arc collided with the afif microcontinent the second episode 600 ma was the outcome of a major collisional event between the arabian craton and a large continental mass east of the ar rayn block

measuring glacier surface roughness using plot scale close range digital photogrammetry glacier roughness at sub metre scales is an important control on the ice surface energy balance and has implications for scattering energy measured by remote sensing instruments ice surface roughness is dynamic as a consequence of spatial and temporal variation in ablation to date studies relying on singular and or spatially discrete two dimensional profiles to describe ice surface roughness have failed to resolve common patterns or causes of variation in glacier surface morphology here we demonstrate the potential of close range digital photogrammetry as a rapid and cost effective method to retrieve three dimensional data detailing plot scale supraglacial topography the photogrammetric approach here employed a calibrated consumer grade 5 mpix digital camera repeatedly imaging a plot scale 25 m2 ice surface area on midtre lovenbreen svalbard from stereo pair images digital surface models dsms with sub centimetre horizontal resolution and 3 mm vertical precision were achieved at plot scales 4 m2 extraction of roughness metrics including estimates of aerodynamic roughness length z 0 was readily achievable and temporal variations in the glacier surface topography were captured close range photogrammetry with appropriate camera calibration and image acquisition geometry is shown to be a robust method to record sub centimetre variations in ablating ice topography while the dsm plot area may be limited through use of stereo pair images and issues of obliquity emerging photogrammetric packages are likely to overcome such limitations

u pb zircon geochronology and geological evolution of the halaban al amar region of the eastern arabian shield kingdom of saudi arabia u pb zircon model ages for eleven major units from the halaban al amar region of the eastern arabian shield indicate three stages of evolution 1 plate convergence 2 plate collision and 3 post orogenic intracratonic activity convergence occurred between the western afif and eastern ar rayn plates that were separated by oceanic crust remnants of oceanic crust now comprise the ophiolitic complexes of the urd group the oldest plutonic unit in the study is from one of these complexes and gave an age of 694 8 ma detrital zircons from the sedimentary abt formation of the urd group which is intercalated with the ophiolitic rocks were derived from source rocks with a mean age of 710 ma the abt formation may be an accretionary wedge on the western margin of the ar rayn plate plate convergence was terminated by collision of the afif and ar rayn plates during the al amar orogeny which began about 670 ma during collision the urd group rocks were deformed and in part obducted on to one or both plates synorogenic leucogranitoid rocks were intruded from 670 to 640 ma from about 640 to 630 ma widespread unfoliated dioritic plutons were emplaced in the ar rayn block and represent the end of orogenesis related to collision there is no definitive evidence for a significantly older basement beneath the study region

the tectono metallogenic evolution of the arabian shield abstract the arabian shield comprises a group of diverse terranes accreted during the late proterozoic their boundaries are suture zones characterised by tectonised ultramafic bodies aligned within distinct orogenic belts the asir hijaz and midyan terranes consist of a series of late proterozoic immature island arcs and together form the western half of the shield in the east however the afif and ar rayn terranes have a continental affinity with the afif terrane being much older at least early proterozoic and possibly archaean metallogeny within the arabian shield is also diverse and includes cu ni and cr mineralisation associated with proterozoic oceanic crust or ophiolite arc type volcanogenic base and precious metal deposits associated with formerly active plate margins shear zone hosted mesothermal gold mineralisation related to accretionary and strike slip tectonics as well as sn w ta mo ree mineralisation associated with post cratonisation alkali granite magmatism of late proterozic to early phanerozoic age it is possible to trace a systematic evolution in metallogeny both within each terrane and throughout the craton as a whole as a function of crustal development and tectonics although the age range of rocks exposed in the shield is from early proterozoic to tertiary all known styles of primary metallogeny are associated with its cratonisation in the late proterozoic

timing of tectonic events in the menderes massif western turkey implications for tectonic evolution and evidence for pan african basement in turkey the menderes massif forms the western of the two large metamorphic culminations within the turkish alpide orogen it has three major lithologic units with a gneissic core at the base and a schist and a marble envelope overlying it successively although relations between them have been largely obscured by the polymetamorphic and structurally complex history of the massif we present a review of the available stratigraphic evidence and combine it with new isotopic ages from the central and southern parts of the massif to constrain the timing of major tectonic events that shaped the massif since the late proterozoic pt3 in the southern part of the massif sensu stricto three episodes of deformation and metamorphism are distinguished whereas in its northern part there are four the first episode occurred at 500 10 ma with intense deformation and metamorphism at high grade amphibolite facies with local anatexis in the central part of the massif the 470 9 ma old now highly deformed tonalitic and granitic intrusions mark the end of this episode on a cambrian reconstruction of continents around the eastern mediterranean the menderes massif forms the southern end of the pan african orogenic collage of northeastern africa and arabia the area of earliest palaeozoic deformation in the menderes may connect with the northwest african orogenic collage along the strike via the bozburun and saricicek diabases arkoses and schists in the center of the karacahisar dome interpreted herein as fragments of a pan african suture the earliest palaeozoic deformations to affect the rocks were probably related to the last pan african collisions and associated postcollisional convergence the southern part of the massif was undeformed from early ordovician to the early later eocene whereas the northern part was deformed metamorphosed and intruded possibly during the late triassic related to the closure of the karakaya marginal basin of palaeo tethys the next major event affecting the whole of the massif was the intense deformation and widespread metamorphism that reached high amphibolite grade in the structurally lower parts and only greenschist grade in the outermost envelope this metamorphism here called the main menderes metamorphism mmm is biostratigraphically constrained between early eocene and early oligocene time rb sr isotopic data show a spread of ages between 60 ma and 25 ma with the greatest number of determinations around 35 5 ma this number is in excellent agreement with the stratigraphic evidence and shows that mmm took place during latest eocene time or at the transition from eocene to oligocene time along the northern border of the massif deformation and metamorphism had already taken place during the late cretaceous in a high pressure low temperature hp lt metamorphic belt northernmost part of menderes massif sensu lato which was then covered by palaeocene molasse the evolution of the hp lt belt was probably related to the obduction of the bozkir ophiolites from the izmir ankara branch of the neo tethys ocean and preceded terminal collision in the evolution of the massif the mmm was a product of the latest palaeocene collision across neo tethys and the consequent internal imbrication of the menderes taurus block that resulted in the burial of the menderes massif area beneath the lycian nappe complex a low grade metamorphic event dated at 10 to 5 ma is interpreted to be related to the extensional deformation in the aegean and western turkey which has been dated stratigraphically to have begun during the tortonian 11 5 ma

finite element modeling of subglacial cavities and related friction law 1 sliding velocity and basal drag are strongly influenced by changes in subglacial water pressure or subglacial water storage associated with opening and closing of water cavities in the lee of bedrock obstacles to better understand this influence finite element simulations of ice flowing past bedrock obstacles with cavity formation are carried out for different synthetic periodic bedrock shapes in the numerical model the cavity roof is treated as an unknown free surface and is part of the solution as an improvement over earlier studies the cases of nonlinear ice rheology and infinite bedrock slopes are treated our results show that the relationship between basal drag and sliding velocity the friction law can be easily extended from linear to nonlinear ice rheology and is bounded even for bedrocks with locally infinite slopes combining our results with earlier works by others a phenomenological friction law is proposed that includes three independent parameters that depend only on the bedrock geometry this formulation yields an upper bound of the basal drag for finite sliding velocity and a decrease in the basal drag at low effective pressure or high velocity this law should dramatically alter results of models of temperate glaciers and should also have important repercussions on models of glacier surges and fast glacier flows

multiscale analysis of topographic surface roughness in the midland valley scotland surface roughness is an important geomorphological variable which has been used in the earth and planetary sciences to infer material properties current past processes and the time elapsed since formation no single definition exists however within the context of geomorphometry we use surface roughness as an expression of the variability of a topographic surface at a given scale where the scale of analysis is determined by the size of the landforms or geomorphic features of interest six techniques for the calculation of surface roughness were selected for an assessment of the parameter s behavior at different spatial scales and data set resolutions area ratio operated independently of scale providing consistent results across spatial resolutions vector dispersion produced results with increasing roughness and homogenization of terrain at coarser resolutions and larger window sizes standard deviation of residual topography highlighted local features and did not detect regional relief standard deviation of elevation correctly identified breaks of slope and was good at detecting regional relief standard deviation of slope sdslope also correctly identified smooth sloping areas and breaks of slope providing the best results for geomorphological analysis standard deviation of profile curvature identified the breaks of slope although not as strongly as sdslope and it is sensitive to noise and spurious data in general sdslope offered good performance at a variety of scales while the simplicity of calculation is perhaps its single greatest benefit

linking bedrock discontinuities to glacial quarrying quarrying and abrasion are the two principal processes responsible for glacial erosion of bedrock the morphologies of glacier hard beds depend on the relative effectiveness of these two processes as abrasion tends to smooth bedrock surfaces and quarrying tends to roughen them here we analyze concentrations of bedrock discontinuities in the tsanfleuron forefield switzerland to help determine the geologic conditions that favor glacial quarrying over abrasion aerial discontinuity concentrations are measured from scaled drone based photos where fractures and bedding planes in the bedrock are manually mapped a tukey honest significant difference test indicates that aerial concentration of bed normal bedrock discontinuities is not significantly different between quarried and non quarried areas of the forefield thus an alternative explanation is needed to account for the spatial variability of quarried areas to investigate the role that bed parallel discontinuities might play in quarrying we use a finite element model to simulate bed normal fracture propagation within a stepped bed with different step heights results indicate that higher steps larger spacing of bed parallel discontinuities propagate bed normal fractures more readily than smaller steps thus the spacing of bed parallel discontinuities could exert strong control on quarrying by determining the rate that blocks can be loosened from the host rock

the upper proterozoic ophiolite melange zones of the easternmost arabian shield the arabian nubian shield was produced by the episodic cratonization and accretion of intra oceanic arc systems over the period 1100 500 ma bp the easternmost part of the arabian shield is dominated by the n s al amar fault zone long believed to represent an easterly inclined upper proterozoic suture between either two island arc systems or oceanic and continental lithosphere in the area studied there are two n s ophiolitic melange zones which separate a central belt of metamorphosed trench sediments abt schists from flanking granitoid zones that represent the plutonic cores of arc systems the melange blocks are of layered gabbro chromitite homogeneous gabbro with dykes plagiogranite sheeted dykes and pillow lavas set in a sheared serpentinite matrix the blocks are large c 1 km and gabbroic in the western zone and smaller 1 100 m and of fine grained basic rocks in the eastern zone all are compositionally similar and petrographically typical of rock types found in ophiolite complexes elsewhere however the zr ti and nb content of all these rocks is too low even for the back arc marginal basin setting commonly invoked for ophiolite genesis to explain the unusually low high field strength element abundances it is suggested that either these rocks were generated in or near a transform fault zone and then deposited in an oceanic trench when they began to be subducted or that their boninitic character identifies them as the products of fore arc igneous activity and by analogy with the tectono magmatic evolution of the w phillipine mariana region in the w pacific they mark the site of arc separation prior to back arc sea floor spreading the latter explanation is preferred

spatial variability in the water content and rheology of temperate glaciers glacier de tsanfleuron switzerland the physical properties of eight ice coresrecovered from along a flowline at glacier detsanfleuron switzerland have led to the identification of three distinctive internal zones we use variations in the bulk ionic chemistry of these zonesto approximate their relative liquid water concentrations and ice viscosities results suggest that relative bulk water concentration and ice softness vary by over an order of magnitude between the zones implications of this variability for predictions of the glacier s response to climate change are evaluated by incorporating these relative softnesses into a multi layered two dimensional model of ice flow model output is compared with that from an identical model constrainedwith a spatially uniform ice viscosity under advance and retreatmodel ling scenarios the former scenario isusedto tuneviscosity by growinga glacier to itspres ent long section geometry resulting in best fit ice hardness values of 1 2a 1 bar 3 for the englacial ice in the multi layered model and 7 0a 1 bar 3 for all of the ice in the single layered model both result in close approximations to the current long profile yielding rmsdeviationsbetween measuredandmodelledicethicknessesthatare 55m in contrast a single layered model constrained with a hardness of 1 2a 1 bar 3 overestimates the current measured long section area by 31 having a rms ice thickness error of 15 0m under the retreat modelling scenario which gauges the response of the glacier to an imposed 75m rise in equilibrium line altitude ela the multi layered model predicts a long section area reduction of 78 this contrasts with a reduction of 64 for the single layered model hardness 7 0a 1 bar 3 and 85 forthe single layered model hardness 1 2a 1 bar 3

numerical simulation of three dimensional velocity fields in pressurized and non pressurized nye channels channels incised into bedrock or nye channels often form an important component of subglacial drainage at temperate glaciers and their structure exerts control over patterns and rates of a channel erosion b water flow velocity and c water pressure the latter in turn exerts a strong control over basal traction and thus ice dynamics in order to investigate these controls it is necessary to quantify detailed flow processes in subglacial nye channels however it is effectively impossible to acquire such measurements from fully pressurized subglacial channels to solve this problem we here apply a three dimensional finite volume solution of the reynolds averaged navier stokes rans equations with a one equation mixing length turbulence closure to simulate flow in a 3 m long section of an active nye channel located in the immediate foreground of glacier de tsanfleuron switzerland numerical model output permits high resolution visualization of water flow through the channel reach and enables evaluation of the experimental manipulation of the pressure field adopted across the overlying ice lid this yields an increased theoretical understanding of the hydraulic behaviour of nye channels and in the future of their effect on glacier drainage geomorphology and ice dynamics

tectonic and deposition model of late precambrian cambrian arabian and adjoining plates during the late precambrian the terranes of the arabian and adjoining plates were fused along the northeastern flank of the african plate in gondwanaland this phase which ended approximately 640 to 620 ma was followed by continental failure 620 to 580 ma and intracontinental extension 600 to approximately 550 ma during the infracambrian extensional phase a triple junction may have evolved near the sinai peninsula and may have consisted of the 1 jordan valley and dead sea rift branch 2 sinai and north egypt rift branch and 3 the najd wrench rift branch the najd hawasina and zagros fault systems may have been transverse faults that accompanied rifting in the arabian gulf and zagros mountains southern oman pakistan and kerman in central iran while the area was extending and subsiding the tethys ocean flooded the eastern side of the arabian plate and iran and deposited calcareous clastics carbonates and evaporites including the hormuz and ara halites this transgression extended into the western part of the arabian plate via the najd rift system the termination of the extensional phase during the late early cambrian was accompanied by a major regression and terrestrial conditions on the arabian peninsula however by the early ordovician as sea level peaked to a highstand the arabian plate was blanketed with marginal marine sediments

a mechanism of basal ice layer formation involving major ice fabric changes an opportunity to analyse the mechanism for the formation of a basal ice layer was provided by glacier de tsanfleuron in the swiss alps this glacier has some lateral subglacial cavities in which a basal ice layer forms on the bedrock floor and is subsequently incorporated at the base of the glacier petrographic and crystallographic analyses of the different types of ice have provided a means of investigating the successive stages of the process these analyses show major structural changes which provide field confirmation of the deformation mechanisms studied by several authors in laboratory experiments

neoproterozoic to early cambrian franciscan type melanges in the tepla barrandian unit bohemian massif evidence of modern style accretionary processes along the cadomian active margin of gondwana abstract melanges have been reported from a variety of modern tectonic settings and are a hallmark of the franciscan type subduction accretion complexes in the precambrian however well documented examples of melanges become extremely scarce this paper describes in detail neoproterozoic to early cambrian melanges recently recognized in the cadomian basement of the tepla barrandian unit bohemian massif the melanges are of sedimentary and tectonic origin and include voluminous successions of graywackes interpreted to represent mass transport deposits presumably along the upper slope of an accretionary wedge the clast compositions indicate multiple episodes of mixing of terrigenous arc derived and deep water pelagic material moreover abundant bodies of meta basalts interpreted as dismembered seamounts or pillow volcanoes were emplaced tectonically into various structural levels of this sedimentary melange by two different mechanisms at an upper level the irregular to elongated bodies of basalts were offscraped only weakly strained and incorporated in the sedimentary matrix that exhibits steep cleavage indicating horizontal pure shear shortening and oblate strain in contrast at a lower level disc shaped and closely spaced basalt bodies are ductilely sheared and reworked into a flat lying prolate to plane strain fabric suggesting horizontal ductile flow presumably along the base of the accretionary wedge during flat slab oceanic subduction our new u pb ages also reveal that the melange formation continued until the early cambrian

characterization of subglacial landscapes by a two parameter roughness index previous studies using fourier transformation ft methods to analyze subglacial roughness have shown promise for distinguishing between different types of subglacial landscape from raw subglacial elevation data we derive a two parameter ft roughness index whereis based on the ft of elevation as previously considered in isolation andis based on both the ft of elevation and the ft of bed slope profile in this way we take account of both vertical and horizontal irregularities in subglacial surfaces we demonstrate the statistical veracity of using to consider roughness in terms of obstacle amplitudes and spacing and consider the use of in studies of ice dynamics and subglacial geomorphological interpretation we show that can be linked to basal sliding rates on the metre scale and can be used to differentiate further than single parameter roughness indices between different classes of subglacial landscape in particular between erosional and depositional settings

magnetically inferred basement structure in central saudi arabia abstract a compilation of magnetic data acquired during the past three decades for a region in central saudi arabia where precambrian basement is partly exposed on the arabian shield and partly concealed by overlying phanerozoic strata shows a central sector of conspicuous n s trending anomalies a heterogeneous western sector of short wavelength high intensity anomalies and an eastern sector of low to moderate intensity broad wavelength anomalies anomalies in the western and central sectors correlate with neoproterozoic metavolcanic metasedimentary and intrusive rocks of the arabian shield and are interpreted as delineating extensions of shield type rocks down dip beneath phanerozoic cover these rocks constitute terranes making up part of a neoproterozoic orogenic belt that underlies northeast africa and western arabia and it is proposed that their magnetically indicated easternmost extent marks the concealed eastern edge of the orogenic belt in central arabia the flat magnetic signature of the eastern sector not entirely accounted for as an effect of deep burial may reflect the presence of a crustal block different in character to the terranes of the orogenic belt and speculatively may outline a continental block that according to some tectonic models of the region collided with the neoproterozoic terranes and thereby caused their deformation and tectonic accretion

late proterozoic evolution of the northern part of the hamisana zone northeast sudan constraints on pan african accretionary tectonics new structural data from the red sea hills northeast sudan add constraints to accretionary tectonic processes that acted in the arabian nubian shield during the late proterozoic geometric constraints provided by field data are used together with landsat thematic mapper imagery to interpret the regional distribution and structural relations of spectrally and lithologically distinctive rock assemblages in contrast to interpretations reported elsewhere these studies indicate that the hamisana zone a prominent structural feature in the northern red sea hills post dated terrane accretion it is characterized by east west crustal shortening and by steep folds and thrust faults calc alkaline magmatism was contemporaneous with and outlasted main phase deformation thus deformation occurred in an intra arc setting thrust faults were reactivated after east west contraction compromising their usefulness in delineating 9sutures9 or as piercing points across older faults pan african accretionary processes may have been analogous to phanerozoic ophiolite and island arc accretion in the western north american cordillera where penetrative deformation occurred in response to periodic intra plate shortening events rather than an ultimate collision of unrelated crustal fragments such an analogy implies a mechanism for crustal thickening by structural as well as igneous processes during evolution of an accretionary belt in a long lived convergent margin setting consistent with the long standing observation that pan african magmatism became more evolved with time this view emphasizes that an interplay of deformation and magmatism resulted in protracted accretion and crustal thickening in contrast to models where crustal shortening results directly from collision of a volcanic arc microcontinent or other buoyant features

late proterozoic transpression on the nabitah fault system implications for the assembly of the arabian shield abstract the longest proposed suture zone in saudi arabia the nabitah suture can be traced as a string of ophiolite complexes for 1200 km along the north south axis of the arabian shield results of a field study in the north central shield between 23 and 26 n indicate that the nabitah suture is indeed a major crustal discontinuity across which hundreds of kilometers of displacement may have occurred on north south trending subvertical faults of the nabitah fault system although not a unique solution many structures within and near these faults can be reconciled with transpression i e convergent strike slip and syntectonic emplacement of calc alkaline plutonic rocks transcurrent motion on the nabitah fault system appears to have began prior to 710 ma was active circa 680 ma and terminated prior to significant left lateral strike slip on the najd fault system which began sometime after 650 ma northwest directed subduction in the eastern shield could have produced the observed association of calc alkaline magmatism and left lateral transpressive strike slip and is consistent with interpretation of the abt schist and sedimentary rocks of the murdama group as relics of the associated accretionary wedge and fore arc basin

sea level higher than present 3500 years ago on the northern main hawaiian islands new data from an emerged coastal bench and associated fossil beach on kapapa island oahu hawaii preserve a detailed history of middle to late holocene sea level these include 29 new calibrated radiocarbon ages and elevations indicating mean sea level reached a maximum position of 2 00 0 35 m ca 3500 yr b p these results correlate with additional evidence from hawaii and other pacific islands and provide constraints on oahu s long term uplift rate 0 03 0 07 mm yr previously based solely on pleistocene age shorelines our sea level reconstruction is consistent with geophysical model predictions of earth s geoid response to the last deglaciation and with observations of increased antarctic ice volume during the late holocene

quantifying bed roughness beneath contemporary and palaeo ice streams bed roughness is an important control on ice stream location and dynamics the majority of previous bed roughness studies have been based on data derived from radio echo sounding res transects across antarctica and greenland however the wide spacing of res transects means that the links between roughness and flow are poorly constrained here we use digital terrain model dtm bathymetry data from a well preserved palaeo ice stream to investigate basal controls on the behaviour of contemporary ice streams artificial transects were set up across the minch palaeo ice stream nw scotland to mimic res flight lines over institute and moller ice streams antarctica we then explored how different data resolution transect orientation and spacing and different methods impact upon roughness measurements our results show that fast palaeo ice flow can occur over a rough hard bed not just a smooth soft bed which much previous work has suggested smooth areas of the bed occur over both bedrock and sediment covered regions similar trends in bed roughness values were found using fast fourier transform analysis and standard deviation methods smoothing of bed roughness results can hide important details we propose that the typical spacing of res transects is too wide to capture different landform assemblages and that transect orientation influences bed roughness measurements in both contemporary and palaeo ice stream setting

subglacial carbonate deposition an isotopic study of a present day case abstract different types of calcite deposits may be formed in the subglacial environment patchy coatings on polished and striated bedrock pavements occur on outcrops recently exposed by retreating temperature glaciers coated pebbles and thin limestone crusts in fissures are present in recent basal till exposures freezing of meltwater is the main process susceptible to explain most of these carbonate deposits partial freezing is accompanied by a rejection of ions from the growing ice and a concentration increase in the residual water eventually until saturation is reached glacier sliding over a protuberance by pressure melting on the stoss side and refreezing on the lee side is likely to be responsible for subglacial carbonate deposition a fixed bedrock protuberance will give rise to fluted and furrowed coatings on its lee side a mobile protuberance like a pebble at the ice bedrock interface because it is susceptible to rotate while in traction will have a coating on most of its surface a present day case in the swiss alps is studied by a comparison of the isotopic composition of calcite deposits ice and meltwater from the glacier this study indicates that melting of the basal ice layer not of glacier ice is at the origin of the initial water giving rise to the precipitate since the basal ice layer is formed in the marginal zone of the glacier subglacial carbonate deposition is a marginal feature in this case

drainage through subglacial water sheets 1 subglacial drainage plays an important role in controlling coupling between glacial ice and underlying bed here we study the flow of water in thin macroporous sheets between ice and bed previous work shows that small perturbations in depth of a nearly parallel sided water film grow unstably because these areas have enhanced viscous dissipation that leads to enhanced melting of an ice roof we argue that in the presence of bed protrusions bridging a water sheet downward motion of the ice roof can stabilize this sheet stability results when the rate of roof closure increases faster with water depth than the rate of viscous dissipation we therefore modify existing theory to include protrusions that partially support the overlying glacier differences in the pressure on protrusions relative to water pressure drive roof closure the mechanisms of both regelation and creep normal to the bed close the overlying ice roof and decrease the ice bed gap in order to account for multiple protrusion sizes along the bed for instance resulting from an assortment of various sized sediment grains we incorporate a method of partitioning overburden pressure among different protrusion size classes and the water sheet partitioning is dependent on the amount of ice protrusion contact and therefore water depth this method allows prediction of roof closure rates we then investigate stable steady sheet configurations for reasonable parameter choices and find that these steady states can occur for modest water depths at very low effective pressures as is appropriate for ice streams moreover we find that multiple steady sheet thicknesses exist raising the possibility of switches between low and high hydraulic conductivity regimes for the subglacial water system

spectral roughness of glaciated bedrock geomorphic surfaces implications for glacier sliding a microroughness meter mrm was used to measure the high frequency roughness of a number of geomorphic surfaces in the forefield of glacier de tsanfleuron switzerland resulting spectral power densities are added to low frequency spectra measured by electro optical distance meter edm to generate composite roughness spectra that include almost 5 orders of magnitude of roughness in the frequency domain these are used to define two roughness indices a general index of bed roughness is defined as the integral of the raw spectral power densities and a sliding related index of bed roughness is defined as the integral of the spectral power densities weighted to account for the optimum dependence of glacier sliding speed on hummock wavelength results indicate that mrm measured geomorphic components vary in roughness by 3 orders of magnitude principally depending on the surface microenvironment measured and profile orientation relative to the direction of former ice flow both mrm and edm measured roughnesses are lower parallel to the direction of former ice flow than perpendicular to it composite roughness spectra consequently indicate that the glacier bed is smoothed in the direction of former ice flow at all horizontal scales from 1 mm to 40 m typically resulting in an order of magnitude decrease in sliding related roughness relative to that measured perpendicular to ice flow comparison of data from two survey sites located adjacent to and 1 2 km from the current glacier margin indicates that postglacial subaerial weathering homogenizes bedrock roughness in particular reducing high frequency flow orthogonal roughness accounting for the effect of 28 ice bedrock separation over one of the profiles reduces net sliding dependent roughness by between 27 and 43 depending on the transition wave number used

magma in forearcs implication for ophiolite generation abstract forearc areas non volcanic arcs of contemporary island arcs at convergent plate boundaries contain magmatic rocks geological evidence seismic profiles heat flow data density considerations and petrological and geochemical arguments suggest that a forearc tholeiitic association fat containing high mg calc alkaline andesites is present in non volcanic arcs at some stage of island arc development the fractionated as well as primitive magma is unable to penetrate low density sediments and underplates thick piles of unconsolidated accreting rocks the underplating causes upwelling the occurrence of magma in forearcs provides an alternative interpretation for the tectonic setting of some ophiolitic masses rather than ocean ridge formation and later obduction it offers an autochthonous island arc bound and geologically substantiated interpretation for the ophiolite suite

quaternary evolution of glaciated gneiss terrains pre glacial weathering vs glacial erosion vast areas previously covered by pleistocene ice sheets consist of rugged bedrock dominated terrain of innumerable knolls and lake filled rock basins the cnoc and lochan landscape or landscape of areal scour these landscapes typically form on gneissose or granitic lithologies and are interpreted 1 either to be the result of strong and widespread glacial erosion over numerous glacial cycles or 2 formed by stripping of a saprolitic weathering mantle from an older deeply weathered landscape we analyse bedrock structure erosional landforms and weathering remnants and within the cnoc and lochan gneiss terrain of a rough peneplain in nw scotland and compare this with a geomorphologically similar gneiss terrain in a non glacial arid setting namaqualand south africa we find that the topography of the gneiss landscapes in nw scotland and namaqualand closely follows the old bedrock saprolite contact weathering front the roughness of the weathering front is caused by deep fracture zones providing a highly irregular surface area for weathering to proceed the weathering front represents a significant change in bedrock physical properties glacial erosion and aeolian erosion in namaqualand is an efficient way of stripping saprolite but is far less effective in eroding hard unweathered bedrock significant glacial erosion of hard gneiss probably only occurs beneath palaeo ice streams we conclude that the rough topography of glaciated cnoc and lochan gneiss terrains is formed by a multistage process 1 long term pre glacial chemical weathering forming deep saprolite with an irregular weathering front 2 stripping of weak saprolite by glacial erosion during the first glaciation s resulting in a rough land surface broadly conforming to the pre existing weathering front etch surface 3 further modification of exposed hard bedrock by glacial erosion in most areas glacial erosion is limited but can be significant beneath palaeo ice stream the roughness of glaciated gneiss terrains is crucial for modelling of the glacial dynamics of present day ice sheets this roughness is shown here to depend on the intensity of pre glacial weathering as well as glacial erosion during successive glaciations