were deforming subglacial beds beneath past ice sheets really widespread abstract the concept of widespread large strain deformation of subglacial unconsolidated unfrozen sediments during pleistocene glaciations has inconsistencies in the geological record numerous properties of tills and related sediments are difficult to reconcile with pervasive strains as predicted by the deforming bed theory while accepting glacier bed deformation as a geological process occurring under certain circumstances we propose that it was much less widespread than believed by some instead we believe that basal sliding and englacial transport are major ice movement and debris transport mechanisms

response of sediment to ice sheet loading in northwestern germany effective stresses and glacier bed stability laboratory tests on sediment over ridden by the last ice sheet in northwestern germany reveal very low ice induced pre consolidation and high palaeo pore water pressures sediment consolidation at the base of the glacier was largely controlled by hydraulic properties of the substratum generally low permeabilities of the bed caused sustained high pore water pressure in over ridden sediments close to the flotation point this implies a serious possibility of hydraulic lifting of the ice sheet it is believed that the reduced basal coupling limited the transformation of glacier shear stress on to the bed sediments which is indicated by a lack of sedimentological evidence for widespread pervasive bed deformation ice motion was probably focused at the glacier sole by some combination of sliding and ploughing however isolated spots with deformation occur so that the subglacial system in the study area can be characterized as a stable deforming mosaic

dust and ice deposition in the martian geologic record abstract the polar layered deposits of mars demonstrate that thick accumulations of dust and ice deposits can develop on the planet if environmental conditions are favorable these deposits appear to be hundreds of millions of years old and other deposits of similar size but of greater age in nonpolar regions may have formed by similar processes possible relict dust deposits include from oldest to youngest noachian intercrater materials including arabia mantle deposits noachian to early hesperian south polar pitted deposits early hesperian hellas and argyre basin deposits late hesperian electris deposits and the amazonian medusae fossae formation these deposits typically are hundreds of meters to a couple kilometers thick and cover upward of a million or more square kilometers the apparent persistence of dust sedimentation at the south pole back to the early hesperian or earlier and the early growth of tharsis during the late noachian and perhaps earlier indicates that extensive polar wandering is unlikely following the middle noachian a scenario for the overall history of dust and perhaps ice deposition on mars includes widespread voluminous accumulations perhaps planetwide during the noachian as impacts volcanism and surface processes generated large amounts of dust the arabia deposits may have formed as ice availability and dust accumulation waned during the early hesperian thick dust sedimentation became restricted to the south pole and the deep hellas and argyre basins the north polar sedimentary record prior to the amazonian is largely obscured deposits at electris and medusae fossae may have resulted from local sources of fine grained material perhaps volcanic eruptions

chasma australe mars structural framework for a catastrophic outflow origin abstract chasma australe 500 km long and up to 80 km wide is the most remarkable of the martian south pole erosional reentrants carved in the polar layered deposits we have interpreted chasma australe erosional and depositional features as evidence for a flood origin which we have reconstructed using a modified manning equation the main characteristics of the flow are a water velocity between 30 and 50 m s 1 and discharge values between 7 10 8 and 3 10 9 m 3 s 1 very near to mgs data based estimations for martian outflow channels d e smith et al 1998 science 279 1686 1692 we thus postulate that chasma australe originated in a catastrophic flood the tectonic study of an area roughly 20 million km 2 in size around mars south pole included the measurement and projection in rose diagrams of more than 300 lineaments of which 85 were wrinkle ridges and the rest straight scarps the whole set of lineaments canbe explained by a stress field with a 1 n10 e in strike the wrinkle ridges being reverse faults and the other lineaments direct and strike slip faults the straight layout of parts of chasma australe almost 200 km long suggests that the chasma was carved following a fracture network the effectiveness of the erosional process the canyon is up to 1000 m deep leads us to suspect that this carving was preceded by a sapping period endogenetic and exogenetic processes would thus have contributed to the origin of this landform

distinct erosional progressions in the medusae fossae formation mars indicate contrasting environmental conditions abstract the form of erosional remnants of the medusae fossae formation mff on mars provide evidence of their development progression and implicate two spatially distinct environments in the equatorial regions of mars ubiquitous yardangs are clearly the product of strong unidirectional winds acting over time on variably indurated deposits yardang orientation is used as a proxy to map regional and local wind direction at meso scale resolution in other more limited areas not subjected to strong unidirectional winds randomly oriented kilometer scale mesas and buttes are found to be remnants of progressive cliff recession through mass wasting as support is lost from within the mff lithology at its margins the differing processes that dominate the formation of the distinctive landforms have implications for meso scale variations in climate that remain unresolved by current modeling efforts additionally the variability of erosional forms within the deposit emphasizes the overall complexity of this extensive formation

medusae fossae formation new perspectives from mars global surveyor 1 the nature and origin of the medusae fossae formation mff on mars has been debated since the return of the first viking images the mff s young age distinctive surface texture and lack of obvious source have prompted multiple hypotheses for its origin this study uses data from the mars global surveyor mgs mission to examine the mff at all available scales we discuss and quantify observations from mars orbiter laser altimeter mola topography and mars orbiter camera moc images to better constrain the origin of the mff topographic grid estimates yield a present extent of 2 1 106 km2 and a volume of 1 4 106 km3 however remnant yardang deposits observed far from the thicker lobes of mff material suggest that it may have once covered up to 5 106 km2 we do not find compelling evidence for extensive fluvial reworking of the mff however in several regions buried channels are apparent in the mff because the formation is draped over underlying topography layering is apparent at all scales from submeter to hundreds of meters with variable resistance to weathering continuity of layers appears to be local to regional but not likely formation wide yardangs form both parallel and bidirectional patterns with resistant layers and jointing probably influencing their orientations a comparative study of mff regional topography and surface expression indicates that the mff is quantitatively dissimilar to martian polar layered deposits the material is most likely a friable and irregularly consolidated air fall deposit of probable volcanic origin

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

strike slip faulting of ridged plains near valles marineris mars the surface of mars shows abundant evidence of extensional and contractional deformation in the form of normal faults grabens and wrinkle ridges 1 3 in contrast strike slip faults have been considered to be extremely rare or absent on mars4 5 this view is based on the lack of significant lateral offset of pre existing markers such as craters across suspected faults thus mars is believed to have evolved without plate tectonics or other large scale lateral motions of the lithosphere6 however careful study of viking orbiter images is revealing that strike slip faults may be present on mars7 here i identify and document several well preserved examples of martian strike slip faults and examine their relationships to wrinkle ridges the strike slip faulting predates or overlaps periods of wrinkle ridge growth south east of valles marineris and some wrinkle ridges may have nucleated and grown as a result of strike slip displacements along the echelon fault arrays lateral displacements of several kilometres inferred along these arrays may be related to tectonism in tharsis

ignimbrites of amazonis planitia region of mars a series of postulated ignimbrite units is mapped in the amazonis memnonia and aeolis quadrangles of mars the units cover about 2 2 106 km2 within a broad but discontinuous and irregular belt trending east west along the highland lowland boundary the ignimbrites overlie parts of the western and southern aureole materials of olympus mons but are embayed in places by the lava plains of the lowlands stratigraphic relations between the basalt flows from the tharsis montes region and the ignimbrites are not clearly defined crater counts suggest that the younger ignimbrites postdate the lava flows crater counts per square kilometer for the ignimbrites range from 7 29 1 95 10 4 to 6 36 2 01 10 5 for craters larger than 1 km in diameter the ignimbrite materials form thick 100 m extensive relatively flat sheets that are smooth to grooved or gently undulating grooved surfaces appear to be yardangs and in most places are not alined with prevailing wind directions the seven mapped ignimbrite units are characterized by morphologic expression stratigraphic position and crater counts similarities to ignimbrites in the pancake range of central nevada include 1 rounded patches of smooth high albedo nonwelded material superposed on jointed low albedo welded material 2 local complementary joint sets in welded materials and 3 thick flow sheets of great areal extent that follow but subdue underlying topography four major eruptive centers occur in areas where units are thickest and where a dominant nnw sse structural trend is expressed locally by unit margins elongate collapse features and normal faulting a minimum volume of 3 85 106 km3 for the deposits has been calculated from thickness estimates based on shadow measurements and crater rim height relations

origin chronology and climatological significance of annual moraine ridges at myrdalsjokull iceland minor transverse moraine ridges have formed annually at the retreating north margin of myrdalsjokull 550 600 m a s l in south iceland during the period 1928 1981 they are composed of slabs of clast paved lodgement till and their formation at the glacier edge results from subglacial freezing on of sediment slabs during winter and the annual cycle of ice front activity this type of annual moraine ridge is likely to be characteristic of retreating temperate glaciers where the glacier toe is thin and winters are cold the speed of glacier retreat is finely balanced with changes in mean summer temperature suggesting that the short term ice front fluctuations are climatically induced and not a result of surge activity

new martian paradigms several interrelated paradigms concerning the nature and the evolution of mars are discussed these include an impact origin of the northern lowland mars as a water rich planet former oceans in the northern lowlands a warm wet climate on early mars and the possibility of ancient life on mars locations on mars suggested for the search of possible ancient life include the heavily cratered terrain dissected by valley networks possible lacustrine deposits in valles marineris the oxidizing layer below the surface sites along the fringes of the hypothetical northern circumpolar ocean and in evaporite deposits

subglacial water escape structures and till structures slettjokull iceland beyond slettjokull the north part of the ice cap m rdalsjokull south iceland an undulating till plain is dissected by meltwater streams throughout a transect from the ice margin to the 1900 ice marginal moraine a distance of about 1 2 km finely laminated wedge shaped water escape structures wes are present originating in the till and disappearing down ice into the underlying sands and fine gravels a second set of much smaller and deformed black wes occurring within the till has been produced by upward injection into the till as a response to stiffening of the till after dewatering by the downward developing wes at present the toe of the glacier is frozen to its bed and the frozen bed material is more than 2 m thick during the relatively cold period around the turn of the century and until the late 1920s a similar submarginal body of permafrost must have existed the sequence reveals that subglacial meltwater attributed to drainage following the 1918 katla eruption was blocked by a frozen glacier toe causing the formation of water escape structures subsequently glacier flow deformed the substratum and produced the threefold division of the till bed the study has important implications for theories envisaging subglacial floods because it demonstrates that it is very difficult for temperate glaciers with a temporary frozen bed in the toe region to retain subglacial meltwater copyright 1999 john wiley sons ltd

is the gordii dorsum escarpment on mars an exhumed transcurrent fault investigators of deformation features on the surface of mars have generally concluded that fault zones with major horizontal displacements do not occur or have not been preserved1 5 here we present evidence from high resolution viking imagery for an array of structures along the gordii dorsum escarpment which suggest that it is a left lateral strike slip fault zone of lithospheric proportion the escarpment is one of an array of subparallel ridges6 7 preserved in an equatorial region to the west of tharsis montes the region is of transitional character between the youthful amazonian age cover of the amazonia planitia to the north and older noachian age intensely cratered surfaces of the highlands to the south relations between these ridges and the gordii dorsum suggest that the ridges are exhumed ancient fault zones these features imply that mars like the earth today may have had in its early stages a more dynamic tectonic style of planetary evolution

geologic context of the mars radar stealth region in southwestern tharsis stealth is a region on mars that has no distinguishable radar return stealth was discovered in 1988 by muhleman et al 1991 in 3 5 cm earth based radar images and it was found to extend 2000 km along the martian equator west of arsia and pavonis montes stealth was proposed to be a deposit of unconsolidated granular material with a minimum thickness of several meters with the most likely explanation being that stealth is a volcanic tephra deposit muhleman et al 1991 1995 stealth overlies some of the youngest geologic units on mars including lava flows and a giant curvilinear ridged lobate unit on the west flank of arsia mons stealth also overlies part of a region of smooth and wind scoured materials called the medusae fossae formation examination of high resolution viking orbiter images suggests that stealth is the youngest of several discontinuous wind worked mantles of fine grained 1 cm sediment older deposits of fine sediment appear to be interbedded with lava flows and show evidence of aeolian deposition dunes and deposits in the lee of topographic obstacles and erosion yardangs and pedestal craters the geologic context supports a volcanic ash origin for stealth if stealth is composed of tephra then its extent and the nature of landforms in the region indicate that it resulted from multiple eruptions that occurred relatively late in martian history possible source vents include linear depressions flanked by constructive volcanic landforms that are located on the lower western slopes of arsia and pavonis montes

strike slip faults on mars observations and implications for global tectonics and geodynamics 1 the tectonic evolution of mars has been driven primarily by the interaction of tharsis induced loading stresses with a uniform contractional stress field leading to global assemblages of graben and wrinkle ridges until recently strike slip faults have appeared to be largely absent from the tectonic record we here present evidence for a new set of noachian to early hesperian strike slip faults southwest of tharsis the best preserved fault in the newly mapped set has a length of 200 km and measured lateral offset of 5 9 km we use the locations of both previously mapped and newly identified strike slip faults to investigate the tectonic and geodynamic evolution of mars we model the tectonic evolution of the planet in response to the evolving balance between the loading and contractional stresses finding widespread regions in which strike slip faulting was favored the observed tectonic history is consistent with a scenario in which loading induced stresses peaked early in mars history followed by the growth of contractional stresses leading to a shift in the preferred locus of strike slip faulting from regions southwest of tharsis in the noachian to early hesperian to northwest of tharsis in the early amazonian the contractional strain history inferred from the ages and locations of the strike slip faults is consistent with the strains calculated from models of the thermal evolution of mars in which the gradual secular cooling of the planet over time was augmented by plume induced contraction during tharsis formation in the noachian

what causes till thickness to change over distance answers from mýrdalsjokull iceland abstract many explanations have been provided for variations of the spatial distribution and thickness of till sheets this paper gives new insight into the architecture of a stratigraphically distinct till sheet as a function of the type of substratum and preadvance topography at a scale of 10 km this emphasises the sensitivity of the subglacial system to changes in the basal drainage conditions the study area is the forefield of slettjokull at the northern margin of the m rdalsjokull ice cap central south iceland here detailed lithostratigraphical and sedimentological investigations including mapping of the thickness for two till units sediment logging clast fabric and geotechnical measurements provide a basis for interpretations the results show that the thickness of a stratigraphically distinct till sheet is directly correlated to the type of substratum where the substratum consists of sorted sediments the till is thin conversely the till is thick where the substratum consists of till overlying sorted sediments a sedimentological model is presented in which till thickness is tied to the variable hydraulic conditions experienced in a deforming subglacial bed

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

high porosity of basal till at burroughs glacier southeastern alaska debris rich basal ice at burroughs glacier southeastern alaska has 60 vol to 70 vol debris recently deposited basal till exceeds 60 vol sediment with 30 to almost 40 porosity where basal ice is very rich in debris basal till is deposited through melt out with only slight compaction of the debris porosity this high in till is commonly associated with subglacially deforming and dilated sediment however the recently deposited basal melt out till at burroughs glacier has not been deformed after deposition but has porosity values similar to tills elsewhere interpreted to be subglacially deforming and dilated in an unfrozen state high porosity can occur in basal melt out till deposited directly by basal melt out

fault type predictions from stress distributions on planetary surfaces importance of fault initiation depth faults on planetary surfaces initiate at depth where stresses due to the weight of overburden are important consequently the prediction of fault type from calculated stresses at depth can yield different faults than those predicted using the surface criteria commonly employed in geophysical models for elastic plate flexure models of mascon loading on the moon stresses calculated at the surface predict that the most likely faulting is strike slip for which there is no direct evidence at a distance from the basin center where grabens are found modifying these plate flexure calculations to account for the weak surface layer of ejecta megaregolith 1 3 km thick that overlies the relatively strong lunar lithosphere results in model stresses in the outer layers of the moon tens of bars that are an order of magnitude lower than in the lithosphere hundreds of bars because faults bounding lunar grabens initiate at the mechanical discontinuity between the megaregolith and lithosphere and because the reverse faults likely responsible for wrinkle ridges probably initiate at the basalt basin floor contact 0 5 2 km deep model stresses must be calculated at these depths superposing nonisotropic stresses due to the weight of the overburden at the depths of these mechanical discontinuities with plate flexure model stresses correctly predicts maximum stress differences in an interior zone of expected wrinkle ridges surrounded by a zone of concentric normal faults or grabens and effectively eliminates the strike slip zone in agreement with observations the absence of strike slip faults from almost all planetary and satellite surfaces may be explainable if lithospheric deformation model stresses are calculated at important shallow crustal mechanical discontinuities where the maximum compressive stress is vertical thereby favoring the formation of normal faults if true mechanical discontinuities in the shallow crusts of planets and satellites play an extremely important and hitherto unrecognized role in the formation of structures observed on their surfaces

observations models and mechanisms of failure of surface rocks surrounding planetary surface loads geophysical models of flexural stresses in an elastic lithosphere due to an axisymmetric surface load typically predict a transition with increased distance from the center of the load of radial thrust faults to strike slip faults to concentric normal faults these model predictions are in conflict with the absence of annular zones of strike slip faults around prominent loads such as lunar maria martian volcanoes and the martian tharsis rise we suggest that this paradox arises from difficulties in relating failure criteria for brittle rocks to the stress models indications that model stresses are inappropriate for use in fault type prediction include 1 tensile principal stresses larger than realistic values of rock tensile strength and or 2 stress differences significantly larger than those allowed by rock strength criteria predictions of surface faulting that are consistent with observations can be obtained instead by using tensile and shear failure criteria along with calculated stress differences and trajectories with model stress states not greatly in excess of the maximum allowed by rock fracture criteria

ribbed moraine formation ribbed rogen moraines are conspicuous landforms found in interior parts of formerly glaciated areas two major theories for ribbed moraine formation have been suggested in recent years i the shear and stack theory which explains ribbed moraine formation by shearing and stacking of till slabs or englacially entrained material during compressive flow followed by basal melt out of transverse moraine ridges and ii the fracturing theory according to which ribbed moraines form by fracturing of frozen pre existing till sheets at the transition from cold to warm based conditions under deglaciating ice sheets in this paper we present new data on the distribution of ribbed moraines and their close association with areas of frozen bed conditions under ice sheets in addition we show examples of ribbed moraine ridges that fit together like a jig saw puzzle these observations indicate that fracturing and extension of a pre existing till sheet may be a predominant process in ribbed moraine formation in summary we conclude that all described characteristics of ribbed moraines are compatible with the fracturing theory while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detailed morphology of ribbed moraines one implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen bed to thawed bed conditions under former ice sheets

origin of the medusae fossae formation mars insights from a synoptic approach 1 the geologic origin of the medusae fossae formation mff has remained a mystery despite three decades of research to better constrain its formation an in depth analysis of observations made in the literature was combined with a new survey of over 700 mars orbiter camera narrow angle images of the mff to identify morphologic characteristics and material properties that define this formation as a whole while previous work has identified clear agreement on some characteristics our analysis identifies yardangs collapse features and layering as pervasive features of the mff whereas collapse features and layering may implicate several different physical and chemical processes yardangs provide vital information on material properties that inform about mechanical properties of the mff lithology aspect ratios of megayardangs range from 3 1 to 50 1 and slope analyses reveal heights of up to 200 m with cliffs that are almost vertical other yardangs show lower aspect ratios and topographic profiles these characteristics coupled to the presence of serrated margins suggest that mff lithology must be of weakly to heavily indurated material that lends itself to jointing the characteristics and properties of the mff are inconsistent with those of terrestrial pyroclastic fall deposits or loess but are in common with large terrestrial ignimbrites a hypothesis that explains all key observations with a single mechanism yardang fields developed in regionally extensive ignimbrite sheets in the central andes display morphologic characteristics that correlate with degree of induration of the host lithology and suggest an origin by pyroclastic flow for the mff

hirise images of yardangs and sinuous ridges in the lower member of the medusae fossae formation mars abstract hirise images of the lower member of the medusae fossae formation mff were used to identify characteristics of two specific landforms that are well expressed in this particular geologic unit yardangs and sinuous ridges yardangs are wind eroded ridges that are usually confined to arid environments where the bedrock materials can be easily eroded by windblown sand yardangs are common in the lower member of mff where many individual yardangs show evidence of a caprock unit overlying a more friable unit most consistent with an ignimbrite origin for these mff deposits heights of the yardangs in the lower member materials are generally less than a few tens of meters in contrast to yardangs in the thicker middle member mff materials to the east of the study area the yardangs may form in materials comprised of discrete depositional units and there is good evidence that at least a dozen such depositional events contributed to the emplacement of the lower member of mff the lower member yardang heights indicate aeolian erosion has removed at least 19 000 km 3 of lower member mff materials sinuous ridges are elongate positive relief landforms that have been attributed to a variety of possible fluvial flow processes on mars sinuous ridges are very common within exposures of the lower member of mff multiple ridge types are present but all forms seen at hirise scale are most consistent with some form of aqueous channel flow rather than other possible origins the results from this initial examination of hirise images indicate the potential utility of comparing yardangs and sinuous ridges in the lower member to other members of mff although it remains to be determined if sinuous ridges are abundant in the younger mff members

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

evidences for a noachian hesperian orogeny in mars a number of tectonic structures have been located at the thaumasia plateau daedalia planum and aonia terra mars they include isolated folds with axial traces up to 200 km long trains of tightly folded structures tens of km long and thrusts their size and geometry are similar to those on earth and the direction of compression seems to have varied amply with time suggesting a complex tectonic evolution crater counts on the deformed terrain point to noachian to early hesperian ages on the basis of the geometry and geological relations of these structures we propose that they form part of an old martian orogen the thaumasia aonia orogen which embraced not only the thaumasia plateau but areas of daedalia planum aonia terra and nereidum montes as well a regional coherent layering is previous to the deformation and could represent the trace of even older stresses on the martian lithosphere

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

mode of subglacial transport deduced from till properties mýrdalsjokull iceland abstract a multi methological study of recent deposited till yields new insights into the mode of subglacial transport at slettjokull the northern margin of the m rdalsjokull ice cap the ice flow direction transport distance transport path and glacier regime are known for a well defined point source this knowledge permits an investigation of the lateral development of till properties and the reconstruction of a sedimentological model comprising some of the parameters affecting the modification of sediment in subglacial transport one of the most remarkable results from this investigation is the very short transport distance that is required for the different till properties to reach a mature state within the first 250 m of transport distance significant changes occur in the grain size distribution clast morphology geochemical and mineral magnetic properties however the variation beyond 250 m suggests that till properties are likely to be a function of parameters other than transport distance it is inferred on the basis of a renewed depositional model that the physical processes and conditions in a deformable bed beneath slettjokull are highly responsive to any change in type of substratum furthermore it is proposed that the ability of comminution processes to act on the sediment in transport is controlled by the prevailing subglacial drainage conditions and its influence on the rheological behaviour of a deformable bed as a consequence it may be demonstrated that the sedimentary properties of the deforming bed till might change significantly over very short distances without direct correspondence to transport distance

oblique rifting at tempe fossae mars this work shows the results of a structural study of the faults observed at the tempe rift northeastern tharsis region mars a new detailed map of faults and fault systems was used to geometrically characterize the fracture architecture of the tempe rift and to measure fault length displacement and spacing data to analyze the spatial distribution of fault centroids and to investigate the fractal nature of fault trace maps a comparison with analog models and the use of conventional techniques of fault population analysis show that the tempe rift was most probably generated under sinistral oblique rifting processes which highlights the importance of the presence of inherited fractures in the tectonic evolution of the noachian crust the angle between the extension direction and the rift axis varies along the tempe rift ranging from 50o 60o at its central southern part to 66o 88o to the southwest fault scaling relationships are similar to those found at midocean ridges on earth with exponential fault length frequency distributions localized inhomogeneous deformation generated weakly interacting faults spanning the entire thickness of the mechanical layer this thickness decreased from southwest to northeast along the rift along with distance from the central part of the tharsis dome

tharsis dome mars new evidence for noachian hesperian thick skin and amazonian thin skin tectonics a photogeological reconnaissance of viking mosaics and images of the tharsis dome has been carried out fifteen new areas of transcurrent faulting have been located which together with other structures previously detected support a model in which the thaumasia plateau the southeastern part of the tharsis dome is proposed to be an independent lithospheric block that experienced buckling and thrust faulting in late noachian or early hesperian times as a result of an e w directed compression evidence is presented that this stress field rather than the tharsis uplift was decisive in the inception of valles marineris which we consider a transtensive dextral accident the buckling spacing permits us moreover to tentatively reconstruct a martian hesperian lithosphere similar in elastic thickness to the mean present terrestrial oceanic lithosphere thus supporting the possibility of a restricted lithospheric mobility in that period tharsis lithosphere was again subjected to shear stresses in amazonian times a period in which important accidents such as strike slip faults wrinkle ridges and straight and sigmoidal graben were formed under a thin skin tectonic regime while the lithosphere as a mechanical unit had become too thick and strong to buckle the possible causes of those stresses and especially their relationships to a putative period of plate tectonics are discussed

evidence for an ancient impact basin in daedalia planum mars geologic mapping has provided observations that some features on mars are morphologically similar to lunar imbrium sculpture materials these grooved features occur in clustered sets of a dozen or more are spaced 1 to 10 km apart and are unlike other linear martian features such as yardangs faults ancient valley networks and mass wasting features we propose that they represent impact materials from an ancient basin the orientations of these features were plotted as lines on a schmidt net and the density of the resulting intersections suggests a likely source area at approximately 26 0 125 0 or roughly the center of daedalia planum numerous features geologic materials and earth based radar data indicate that a basin with a diameter of approximately 4500 km with an early noachian age is centered at this location four interior rings with diameters of 1100 1500 2200 and 3200 km and an exterior ring with a diameter of approximately 6400 km were found the rings are spaced with a ratio of 2 0 1 which is similar to the basin ring spacing on the other terrestrial planets both concentric and infrequently radial basin related faults were recognized including claritas fossae gordii dorsum and the thaumasia graben faults or zones of weakness related to the basin rings may have influenced the position of the mangala valles source area and arsia mons based on the location and morphology of early hesperian to early amazonian volcanics composing daedalia planum the daedalia basin may also have extensive deep seated interior faults capable of extruding mare materials to the surface

reconstructing the transport history of glacigenic sediments a new approach based on the co variance of clast form indices abstract two indices are used to describe the aggregate shape and roundness characteristics of glacially transported clasts the c40 index the percentage of clasts with c a ratios 0 4 and the ra index the percentage of very angular and angular clasts in a sample analysis of the between and within sample co variance of these indices provides a powerful tool for discriminating actively and passively transported clasts in glacigenic deposits use of these indices is illustrated in an analysis of the form characteristics of clasts from lateral and frontal moraines at storbreen jotunheimen these fall between those of actively transported clasts in basal till low c40 and ra values and those of unmodified frost weathered clasts high c40 and ra values both indices decline down moraine within sample analyses of clast shape demonstrates that these aggregate clast form gradients reflect changes in the relative proportions of actively and passively transported debris on the moraines rather than progressive modification of actively transported clasts these analyses also indicate that actively transported clasts form the dominant component of the moraines particularly near the former glacier terminus further applications of the approach developed here are suggested

inverted fluvial features in the aeolis zephyria plana western medusae fossae formation mars evidence for post formation modification 1 the aeolis and zephyria plana contain the western most portion of the medusae fossae formation mff an enigmatic and extensive light toned deposit located in the martian equatorial region and dated from the hesperian to amazonian epochs this area hosts a large population of sinuous ridges srs interpreted as inverted fluvial features formed by precipitation indurated by chemical cementation buried by subsequent deposition and finally exhumed this interpretation of srs as uniformly fluvial represents a modification to an earlier hypothesis for one particular sr of possible glaciofluvial i e esker formation these srs provide a tool to investigate the degree and character of post fluvial modification processes in this region we combined digital terrain models made from context camera ctx and high resolution imaging science experiment hirise stereo image pairs with individual data points from the mars orbiter laser altimeter mola to estimate relief cross sectional profiles longitudinal profiles and slope directions of selected srs longitudinal profiles of several srs display undulations with amplitudes of up to order 100 m while some of the lower amplitude undulations may be due to differential erosion undulations having amplitudes in excess of sr relief require alternative explanations our combined morphologic and topographic analysis suggests that multiple post flow processes including compaction of the deposits and tectonic displacements have modified the original sr profiles specification of the type s and magnitudes of these modification processes will contribute to understanding both the potential of post flow modification of fluvial profiles elsewhere on mars as well as the nature and properties of the mff

review of subglacial hydro mechanical coupling trapridge glacier yukon territory canada abstract the interaction of basal processes with the subglacial drainage system is a critical issue in understanding glacier dynamics since the recognition that many glaciers and ice masses overlie soft sediments rather than hard bedrock much research has been undertaken to investigate how mechanical and hydrological conditions of a deformable substrate control the coupling at the ice bed interface and thus affect fast ice flow and glacier surging in research undertaken on trapridge glacier a small surge type glacier in the st elias mountains yukon territory canada we have combined extensive field investigations using novel measurement techniques and theoretical modelling to study hydro mechanical coupling processes measurements of subglacial water pressure indicate that the basal water system can be dramatically inhomogeneous both spatially and temporally since ice bed coupling is strongly influenced by subglacial water pressure the stresses at the bed are also markedly heterogeneous and are expected to form a patchwork distribution which mimics the pressure distribution of the basal water system this heterogeneity in the stress field at the ice bed interface introduces a pronounced variability to the basal motion mechanics as such basal sliding and subglacial sediment deformation are not steady and continuous processes instead the variability of the subglacial water system leads to a spatial and temporal interplay of increased ice bed coupling at low water pressures at one site or time with ice bed decoupling during rising water pressures at other sites or times thus on the one hand there is downglacier shear deformation of the bed and accumulation of elastic strain in ice and sediment while on the other hand there is enhanced slip sliding of the glacier and upglacier shear motion of the bed due to an elastic relaxation of the sediment