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1.
The influence of glacier hydrology on the time-dependent morphology and flow behaviour of the late Weichselian Scandinavian ice sheet is explored using a simple one-dimensional ice sheet model. The model is driven by orbitally induced radiation variations, ice-albedo feedback and eustatic sea-level change. The influence of hydrology is most marked during deglaciation and on the southern side of the ice sheet, where a marginal zone of rapid sliding, thin ice and low surface slopes develops. Such a zone is absent when hydrology is omitted from the model, and its formation results in earlier and more rapid deglaciation than occurs in the no-hydrology model. The final advance to the glacial maximum position results from an increase in the rate of basal sliding as climate warms after 23000 yr BP. Channelised subglacial drainage develops only episodically, and is associated with relatively low meltwater discharges and high hydraulic gradients. The predominance of iceberg calving as an ablation mechanism on the northern side of the ice sheet restricts the occurrence of surface melting. Lack of meltwater penetration to the glacier bed in this area means that ice flow is predominantly by internal deformation and the ice sheet adopts a classical parabolic surface profile.  相似文献   

2.
Results from geophysical explorations of three deep valleys, selected from different tectonic regimes in the Eastern Alps (Ötz-, Oichten-, and Drau Valley), are presented and discussed. Ongoing tectonic deformation may use tectonic structures related to these valleys. However, seismic activity is low there. During the Würm ice age, the thickness of the ice cover ranged between 300 and 1,500 m above present ground elevation. The geophysical investigations comprised reflection seismology, gravity- and resistivity surveys. The maximum depth down to the erosional base of the valleys varies from ~340 to 700 m. Distinct layer packages of the valley-infill at depths greater than 250 m were termed “old valley-fill”. Geophysical parameters and a comparison with the reflection seismic image of an intermediate layer at the recent Pasterze glacier suggest that the top of the “old valley-fill” represents the glacier bed during the decay of the Würm glaciation. Deep erosion is not related to high basal shear stress. The confluence of tributary glaciers is apparently not a significant factor for deep erosion in our examples of deep alpine valleys. We conclude that deep erosion may be related to high water pressure at the glacier bed, supported by specific processes of tectonic weakening.  相似文献   

3.
祁连山老虎沟12号冰川冰下形态特征分析   总被引：4，自引：1，他引：3  
武震  刘时银  张世强 《地球科学进展》2009,24(10):1149-1158
利用探地雷达对老虎沟12号冰川进行了多个剖面观测,分析了雷达图像反映的冰川内部特征.结果表明通过分析雷达图像中的纹理和位置,雷达波形的振幅和极性变化等信息能够有效的辅助识别冰川内部介质的变化界面位置和估算其规模.在老虎沟12号冰川的部分剖面内部存在冰内空洞、冰内融水空洞、冰下裂隙、冰下河等地貌形态,在冰岩界面和冰川边缘存在规模不等的碎屑层.这说明即使在典型的大陆型冰川,冰川内部结构也可能较为复杂,在冰岩界面处可能存在较强的冰川作用.  相似文献   

4.
The Cairngorm Mountain area of Scotland is a classic example of a landscape of selective linear glacial erosion, with sharp contrasts in the intensity of glacial erosion between the deeply incised troughs and valleys and the undulating high plateau. This article examines the Quaternary development of Glen Avon, a 200 m deep glacial trough set within the high plateau of the mountains. Evidence concerning the aggregate basal thermal regimes of the topographically controlled ice streams that formerly developed in this area is reconstructed from the geomorphological record, including bedforms indicative of wet-based, sliding ice and of dry-based ice frozen to its bed. This mapping indicates that basal sliding was not confined exclusively to the troughs but extended towards valley heads and on to parts of the plateau adjacent to troughs. The extent of basal sliding appears to have been greatest beneath pre-Late Devensian ice sheets. Basal ice temperatures are modelled under steady-state conditions for the last ice sheet at c. 18 ka BP. Basal thermal regimes are predicted using a reconstruction of the preglacial relief and for the current topography of the area. Convergent flow of ice through the preglacial valley system appears to have been sufficient to induce basal melting and therefore to initiate valley deepening. This effect is enhanced when the model is run across the present topography. Comparison of results of the geomorphological mapping and the modelling reveals significant differences between the actual and predicted extent of basal sliding outside the main ice stream. The overall conclusion is that many ice streams in mountainous terrain are inherited from the locations of preglacial valleys, which serve to accelerate ice flow and promote frictional heating beneath ice sheets.  相似文献   

5.
On the meltwater genesis of drumlins   总被引：1，自引：0，他引：1  
Large subglacial cavities in basal ice. the presumed precursors of depositional drumlins, can be created by corrasion from suspended sediment during a large water sheet outburst flood. The cavities are primarily the result of vortex action. For a given flood discharge the corrasion rate of the ice roof increases with water sheet velocity, V, to a power greater than V16/3, Because of ice displacment during a flood, V can vary spatially along a flow line by up to an order of magnitude. The highest velocities normally occur downstream from major bed depressions or near the ice margin. Fields of large drumlins frequently occur at such sites. The process of formation of large subglacial cavities is predicted to be more velocity-sensitive than the erosion of bedrock. No accurate estimate of water sheet flood volumes can be made at this time but they could be at least an order of magnitude less than the 80 000 km3, which was previously estimated.  相似文献   

6.
Transverse-to-iceflow ribbed moraine occurs in abundance in the coastal zone of northern Sweden, particularly in areas below the highest shoreline (200–230 m a.s.l.), but occasionally also slightly above. Based on detailed sedimentological and structural investigations of machine-dug sections across five ribbed moraine ridges, it is concluded that these vertically and distally prograding moraine ridges were formed as a result of subglacial folding/thrust stacking and lee-side cavity deposition. The proximal part of the moraines (Proximal Element) was formed by subglacial folding and thrust stacking of sequences of pre-existing sediments, whereas the distal part (Distal Element) was formed by glaciofluvial and gravity-flow deposition in lee-side cavities. The initial thrusting and folding is suggested to be a result of differences in bed rheology at the ice-marginal zone during the early or late melt season, and that generated a compressive zone transverse to ice flow as a result of a more mobile bed up-glacier compared to a less mobile bed down-glacier. It is considered that the lee-side cavities were formed as a result of ice-bed separation on the distal slope of the thrust/fold-created obstruction. The lee-side cavities formed an integral part of a subglacial linked-cavity drainage network regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal iceflow velocity. The proximal and distal elements of the ribbed moraine ridges are erosively cut and/or draped with a consistently more homogeneous deforming bed till (Draping Element) marking the final phase of ribbed moraine formation considered to be contemporaneous with De Geer moraine formation further down-flow at the receding ice-sheet margin.  相似文献   

7.
The 1982–1983 surge of Variegated Glacier involved the development, growth and downglacier propagation of a velocity peak associated with rapid basal sliding facilitated by high subglacial water pressures. Passage of the velocity peak through the glacier was preceded by an episode of longitudinal shortening and followed by an episode of elongation. The deformation history of the glacier ice was dependent upon location relative to the surge nucleus and the final position reached by the propagating velocity peak. Ice above the surge nucleus experienced continuous and cumulative elongation; ice below the final position of the velocity peak experienced continuous and cumulative shortening; ice between these two points experienced shortening followed by elongation and low cumulative strain. The large-scale pattern of ice structure development reflects these deformation histories. Surging is equivalent to thrust sheet emplacement by a combination of gravity gliding over a weakened basal layer and ‘push from behind’, with the gravity-driven motion of the surging part of the glacier providing the push which allows the surge front to propagate. The relationships established between the deformation history of surging glaciers and the development of ice structures may facilitate the interpretation of structures in thrust sheets.  相似文献   

8.
Aario  R. 《GeoJournal》1977,1(6):65-72
Associations of flutings, drumlins, hummocks and transverse ridges were created in the inner marginal zone of the ice sheet, where, nearer the ice margin, the ice was already thinner and the former higher flow velocity had slowed down due to the decreased volume of transported ice. However, as the ice flow mechanics tend to favour certain higher flow velocities, which can, to some extent, even be self-supporting, the speed of the ice did not slow down homogenously throughout the ice mass. Certain parts of the ice continued to move at the higher speed but an increasing number of units appeared, where the velocity had dropped down to a considerably lower level. In the resulting flow pattern, with fast and slow units of ice flowing side by side, a drag effect contributed to a spiral secondary flow between the flow units of differential velocity.Both depositional and erosional processes were involved and, depending on the balance between them, the resulting bed configuration could be formed by either one or by a combined effect of both of these processes. However, faster flow regime more often favoured erosion and in areas of lower regime deposition more often prevailed. Consequently troughs were formed in the areas of higher velocity, while ridges and other positive landforms were created in the areas of lower velocity. The spiral flow frequently transferred drift from the trough area to the area of lower flow regime, and therefore the flute-ridges and drumlins were often formed depositionally by the combined effect of the slow parallel flow and the spiral flow component. Rogen ridges indicate an undulating flow. This could also exist simultaneously with the other kinds of flow and many landforms owe their characteristics to their combined effect.  相似文献   

9.
Subglacial deformation is crucial to reconstructing glacier dynamics. Sediments associated with the Late Ordovician ice sheet in the Djado Basin, Niger, exhibit detailed structures of the subglacial shear zone. Three main types of subglacial shear zones (SSZ) are discriminated. The lowermost SSZ, developed on sandstones, displays Riedel macrostructures and cataclastic microstructures. These resulted from brittle deformation associated with strong glacier/bed coupling and low pore-water pressure. Where they developed on a clay-rich bed, the overlying SSZ display S–C to S–C′ fabrics, sheath folds, and dewatering structures. These features indicate high ductile shear strain and water overpressure. On fine-grained sand beds, the SSZ exhibit homogenized sand units with sand stringers, interpreted as fluidized sliding beds. The succession of subglacial deformation processes depends on fluid-pressure behavior in relation to subglacial sediment permeability. Fluid overpressure allows subglacial sediment shear strength and ice/bed coupling to be lowered, leading to ice streaming.  相似文献   

10.
From mapping and consideration of prominent drift ridges at Stockdale Head, western Lake District, northern England it is inferred that the ridges are the products of dissection of a glacigenic or soliflual drift sheet rather than landforms constructed at the margins of a Loch Lomond Stade (LLS) valley‐head glacier. This proposal has implications for the recognition of LLS glacier limits and, possibly, understanding the dearth of moraine ridges associated with Dimlington ice in Lake District valleys. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

11.
This is a study of Late Devensian drumlins formed in southern Anglesey and Arvon, northwest Wales. This area was affected by ice sheet coalescence when the Welsh ice sheet met with the lrish Sea ice sheet, and drumlins were formed once the two had coalesced. It is suggested that the drumlins were the result of net subglacial soft-bed erosion, and that they represent more resistant cores within the subglacial deforming layer. The drumlins have either gravel or till cores, and where the core was deformable, large-scale compressive glaciotectonic structures were seen (e.g. Dinas Dinlle) with local subglacial compression of –59%. Where the cores were more resistant (e.g. Lleiniog) these were not deformed but remained as more competent masses within the deforming layer. It is suggested that the less competent material flowed around the cores, some remaining as a thin carapace, but most of the material being removed down glacier, leaving the drumlins as erosional remnants. In northwest Wales there is a multi-till sequence that traditionally has been interpreted as having been deposited as the result of separate ice-sheet advances and retreats. However, in this study, it is suggested that the different tills were deposited as the result of ice-sheet coalescence, and that sites such as Dinas Dinlle do not show evidence of a major readvance in the retreat of the Devensian ice, but are indicative of continuously changing conditions within the subglacial deforming bed.  相似文献   

12.
The glacial landscape beneath the Maudheimvidda ice sheet in East Antarctica was most probably formed during a more temperate phase of Antarctic glaciation than the present. Overdeepened glacial cirques and U-shaped valleys are found in the Heimefrontfjella and Vestfjella mountain ranges. These glacial landforms, located beneath the ice sheet, have been mapped with radio-echo sounders. The present ice sheet covering these landforms is cold and frozen to its bed, and has a negligible erosive effect on the substrate. Ice sheet thickening during the Quaternary glacial periods is not believed to have caused any significant increase in erosion at the investigated sites. Instead, the glacial morphology was most likely formed by smaller, temperate glaciers when the Antarctic climate was warmer than at present. Datings of foraminifera and ash layers from the Transantarc-tic Mountains indicate that the present cold ice sheet was formed 2.5 Ma years ago. Other studies imply that a cold Antarctic ice sheet has lasted even longer. The glacial landforms in Maudheimvidda may thus be of a pre-Quaternary age.  相似文献   

13.
Rarely-preserved features indicative of clast lodgement are exceptionally well preserved near Peoria, Illinois, on the contact surface between Illinoian till and underlying glacifluvial sand due to synsedimentary cementation of the substrate contact. Features preserved on the cemented contact surface record a history of particle transport, lodgement by ploughing into a deformable substrate, and subsequent overriding by abrasive debris-rich ice. Linear grooves and frontal sediment prows suggest that clasts embedded in the glacier sole ploughed through the soft, deformable bed. Increasing form resistance by the enlargement of sediment prows that developed on the lee side of clasts and deeper penetration of the clasts into the substrate eventually exceeded the force exerted on the clasts by ice flow, and the clasts lodged. Subsequently, clasts were abraded on their up-ice flanks and plucked on their down-ice flanks, resulting in stoss-lee morphology. These features offer direct information on the nature of the interface between aglacier and a soft, deformable substrate such as characterized large areas of former ice sheets.  相似文献   

14.
《Earth》2007,83(3-4):143-179
Calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world. Calving rates can increase dramatically in response to increases in velocity and/or retreat of the glacier margin, with important implications for sea level change. Despite their importance, calving and related dynamic processes are poorly represented in the current generation of ice sheet models. This is largely because understanding the ‘calving problem’ involves several other long-standing problems in glaciology, combined with the difficulties and dangers of field data collection. In this paper, we systematically review different aspects of the calving problem, and outline a new framework for representing calving processes in ice sheet models. We define a hierarchy of calving processes, to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events. The first-order control on calving is the strain rate arising from spatial variations in velocity (particularly sliding speed), which determines the location and depth of surface crevasses. Superimposed on this first-order process are second-order processes that can further erode the ice margin. These include: fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front; undercutting of the glacier terminus by melting at or below the waterline; and bending at the junction between grounded and buoyant parts of an ice tongue. Calving of projecting, submerged ‘ice feet’ can be regarded as a third-order process, because it is paced by first- or second-order calving above the waterline.First-order calving can be represented in glacier models using a calving criterion based on crevasse depth, which is a function of longitudinal strain rate. Modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution. Realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law. Models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini, but an important limitation is that they cannot be used to model floating glacier termini or ice shelves. Alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models. Such models can explain a remarkable range of observed phenomena within a simple, unifying framework, including: downglacier increases in velocity and strain rates where basal and/or lateral drag diminishes; flow acceleration in response to thinning through time; the tendency for glaciers to stabilise at ‘pinning points’ in relatively shallow water or fjord narrowings; the constraints on ice shelf stability; and the contrasts in calving rates between tidewater and freshwater calving glaciers. Many unresolved issues remain, however, including the role played by the removal of backstress in the acceleration of retreating calving glaciers, and the controls on melting at and below the waterline.  相似文献   

15.
天山托木尔峰科其喀尔巴西冰川表面运动速度特征分析   总被引：6，自引：2，他引：4  
天山托木尔峰科其喀尔巴西冰川是典型的树枝状山谷冰川,利用3组(6期)ASTER遥感影像通过COSI-corr软件反演了该冰川表面运动速度.与花杆测量数据进行对比,反演冰川表面运动速度平均绝对误差为3.1 m·a-1,相对误差为11.9%,二者在空间上的分布基本一致,表明其反演精度符合要求.在此基础上,分析冰川表面运动速...  相似文献   

16.
Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3-5m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times.  相似文献   

17.
This study investigates the marginal subglacial bedrock bedforms of Jakobshavns Isbrae, West Greenland, in order to examine the processes governing bedform evolution in ice stream and ice sheet areas, and to reconstruct the interplay between ice stream and ice sheet dynamics. Differences in bedform morphology (roche moutonnee or whaleback) are used to explore contrasts in basal conditions between fast and slow ice flow. Bedform density is higher in ice stream areas and whalebacks are common. We interpret that this is related to higher ice velocities and thicker ice which suppress bed separation. However, modification of whalebacks by plucking occurs during deglaciation due to ice thinning, flow deceleration, crevassing and fluctuations in basal water pressure. The bedform evidence points to widespread basal sliding during past advances of Jakobshavns Isbrae. This was encouraged by increased basal temperatures and melting at depth, as well as the steep marginal gradients of Jakobshavns Isfjord which allowed rapid downslope evacuation of meltwater leading to strong ice/bedrock coupling and scouring. In contrast to soft-bedded ice stream bedforms, the occurrence of fixed basal perturbations and higher bed roughness in rigid bed settings prevents the basal ice subsole from maintaining a stable form which, coupled with secondary plucking, counteracts the development of bedforms with high elongation ratios. Cross-cutting striae and double-plucked, rectilinear bedforms suggest that Jakobshavns Isbrae became partially unconfined during growth phases, causing localised diffluent flow and changes in ice sheet dynamics around Disko Bugt. It is likely that Disko Bugt harboured a convergent ice flow system during repeated glacial cycles, resulting in the formation of a large coalesced ice stream which reached the continental shelf edge.  相似文献   

18.
This study investigates the variations in erosional processes beneath Briksdalsbreen; a Norwegian valley glacier, with a thin coarse grained deforming bed. The subglacial zone was investigated in situ, close to the central glacial flow line (Site A) and close to the valley wall (Site B), via boreholes. The till was collected using two different sediment samplers and the bed was examined using a borehole video camera. In order to evaluate how representative borehole sampling was of the subglacial environment, the results from in situ subglacial samples were compared with random samples taken from an exposed subglacial surface in the glacier foreland, as well as a sample taken from the site after glacier retreat. The results indicated that the texture of the exposed subglacial surface was similar to in situ samples from subglacial Site B (close to the valley wall), and that the samples collected with the ‘small’ sediment sampler had the closest fit. SEM analysis revealed eroded quartz grains at both sites as a result of rotation (abrasion) and fracture. The samples from close to the valley wall were very poorly sorted, with evidence for rounding and preservation of pre-weathered surfaces, which suggest that a relatively simple pattern of erosion from crushing to rotation dominated. However, the till from the centre of the glacier was better sorted, more angular, and with few pre-weathered surfaces. We suggest that this results from a more complex glacial erosional history associated with greater strain and longer residence time within the deforming layer. When compared with other deformation tills, it was confirmed that there is a relationship between grain size and erosional style. Clay-rich tills show little comminution, fine sand-rich tills generate a significant silt component, whilst the coarse sand-rich tills at Briksdalsbreen showed complex deformational styles but no significant silt component.  相似文献   

19.
ABSTRACT The local climatic regime and the mass balance state are important determinants of the dynamics of terrestrial and marine glacier fronts, which in turn determine the sediments and landforms produced at the glacier front. Many modern glaciers undergoing overall retreat in areas of'maritime'climate produce winter push moraines during a late winter readvance, followed by a summer retreat, whilst in more'continental'regions no significant winter readvance occurs and annual push-moraines are absent. The frontal dynamics which lead to these changes are analysed and the form, structure, sequence and field relations of both terrestrial and marine push-moraines are described from Iceland, Spitsbergen and Baffin Island.
Long-term changes in mass balance leading to major glacier advances or readvances also generate large push-moraines. In terrestrial environments push-moraine formation is accompanied by uplift, rejuvenation and down-cutting of outwash systems whose sediments become closely associated with glaciotectonic structures, which permit pre-, syn- and post-tectonic sequences to be identified.
The development of ice marginal fan/moraine complexes is modelled as a function of the relative magnitude of two parameters: the velocity of ice movement and the calving rate. A high ice velocity just exceeded by the calving rate gives closely spaced push-moraines and confluent ice marginal fans. A high velocity far exceeded by the calving rate produces closely spaced moraines but separate ice marginal fans. A low ice velocity in combination with a high calving rate results in well separated and feebly developed push-moraines, while a low ice velocity and a low calving rate produces feeble push-moraines and coalescent fans.  相似文献   

20.
The occurrence of diatoms (both marine and freshwater) in sediments beneath the West Antarctic Ice Sheet (WAIS) is suggestive of past ice-sheet collapse. However, it is not the only model explaining such occurrences. We propose another mechanism for introducing diatoms beneath ice sheets by considering the fate of a diatom placed (by eolian processes) on top of an ice sheet. Mathematical modeling indicates that the route the diatom will take through the ice sheet is dictated by the basal melting rate. If no basal melting takes place, flowlines will crop out at the ice-sheet margin. However, if basal melting is as low as 0.01 m/yr the trajectories of all Howlines except for those nearest the margin will intersect the bed, with those diatoms deposited near the dome reaching the bed about halfway down the Howband. Larger values of basal melting lead to the diatoms reaching the bed even faster and closer to the point of origin. In light of these results, the presence of diatoms in sediments beneath the WAIS does not lead to a unique solution; it is not necessary to invoke past ice-sheet collapse to account for their presence.  相似文献   

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