Ice ages and nuclear waste isolation

Engineering Geology 52 (1999) 177–192

Ice ages and nuclear waste isolation C.J. Talbot * Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, Norbyva¨gen 16, S-752 36 Uppsala, Sweden

Abstract The greatest natural threats to the integrity of the geological barriers to nuclear wastes isolated in cavities mined at depths between 400 and 800 m are likely during rapid retreats of future ice sheets. The next major glacial retreat is expected at ca 70 ka, well within the lifetime of high grade nuclear waste, but it is not yet clear how long man’s greenhouse effect may delay it. This contribution discusses the potential problems posed to European waste isolation sites during erosion by ice and over-pressurizing of meltwater and gasses in a lithosphere flexed by major ice sheets. These depend on the target rocks and the location of the site with respect to the ice-streams and margins of future ice sheets of particular size. No sites are planned under the centres of future ice sheets in Europe where end-glacial earthquakes can be expected to reactivate major faults, nor where ice can be expected to deepen and lengthen fjords along the Atlantic coast. Sites in the Alps may be vulnerable to radical changes in the patterns of glacial troughs. The stability and geohydrology of sites in coastal areas beyond future ice margins are threatened by river gorges when sea level falls ca 125 m or, in enclosed basins like the Mediterranean, ever lower. The greatest problems are likely in lowland regions exposed by the rapid retreat of thick ice fronts where large lakes on or under thick warm-based ice are dammed by more distal cold-based ice. Groundwater in subhorizontal fractures dilated by glacial unloading may reach over-pressures capable of hydraulically lifting megablocks of bedrock with fracture permeability and/or the ice damming them so that less permeable substrates are susceptible to incisions eroded to depths of ca 360 at locations controlled mainly by ice topography, kinematics and history. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Glacial incisions; Hydraulic lift; Nuclear waste; Rock permeability; Stresses

1. Introduction International law currently excludes international sites for the isolation of nuclear wastes and requires that each nation generating wastes isolates them within their national boundaries. This rules out the ocean floor where preliminary expectations of continuous uninterrupted deposition of imper* Fax: +46 471 25 91; e-mail: [email protected]

meable clays on abyssal plains were dispelled by finding ‘‘vertical offsets resembling shallow faults in acoustically complex sea floor sediments’’ ( Kuijpers and Duin, 1986). A recent study ended by speculating that ‘‘the general seabed may be riddled with vertical microseepage paths with normally defeat detection by sampling and acoustic profiling’’ (Hovland and Judd, 1988, p. 253) Low intrinsic transmissivities reported from Jurassic mudrocks in Belgium and France suggests that

0013-7952/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S0 0 1 3 -7 9 5 2 ( 9 9 ) 0 0 00 5 - 8

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their likely post-deposition permeability did not survive compaction or partial lithification. Here the focus is on high grade nuclear wastes that are almost as difficult to handle as some highly toxic chemical wastes (Milnes, 1985). Modern subsurface exploration techniques are sufficiently detailed that it will be assumed here that all sites will be designed to fit, with adequate safety barriers, within relatively impermeable rock blocks between any major fracture zones within masses of crystalline bedrock, mudrock or salt. It will also be assumed that surface facilities are expendable and that isolation will be at depths sufficiently deep to escape most surficial effects (<400 m) but not deep enough for excavation to induce major rock bursts (>800 m). Panels of emplaced canisters may not be as large or as neat as shown in promotional literature produced by the nuclear power industry, but there appears to be room to store expected national inventories even in the best known study sites. Excavation and backfilling of each isolation facility will probably disturb the mechanical stability and hydrological integrity of the geological barrier more than any natural event throughout its entire history. Even after filling and closure, large horizontal panels of mined openings will remain local nuclei of intrinsic weakness and permeability that will concentrate the regional stress fields and focus hydraulic flow to degrees that depend on their design and execution. Terrorist activities probably represent the greatest threats to the isolation of nuclear waste by conventional mining methods but are not considered here. Close impacts by astronomical bodies pose the greatest natural threat, and although not entirely predictable, are highly unlikely. It will be argued that various categories of structures generated in association with future glaciations represent problems that are both more significant and inevitable. The timing of ice ages are thought to depend mainly on astronomic cycles (Ben and Evans, 1998) and would therefore be predictable were it not for man’s greenhouse effect which may delay them to an extend not yet known. Important common elements in most recent predictions are major cold phases at between 15–35 and 50–79 ka in the future (Boulton and Payn, 1993).

Modern modelling takes account of how meteorological and glaciological processes interact with the topographies and the strengths of ice and its substrates to control the geometries and kinematics of future ice sheets (Boulton et al., 1955; Clark et al., 1996). The effects that approach mining levels relate to both the geometry and kinematics of the ice and the permeabilities of its substrates. Sites under the centres of future ice sheets are subject to only a few extremes of lithospheric depression, isostatic rebound and pore-pressures due to ice loading and unloading. Sites nearer the margins of future ice sheets are subject to repeated advances and retreats and consequent episodes of lithospheric flexure and hydrological changes. Sites beyond the margins of future ice will be subject to repeated uplift and normal-faulting stress regimes, while distal coastal sites will be subject to major changes in sea level. This discussion starts by reviewing the nature and genesis of structures associated with glaciations that approached mining levels in the Pleistocene. Structures generated under former ice centres are considered before those generated under more distal ice and then those that develop well beyond former ice margins. Other than fjords and troughs, all the deepest-reaching effects appear to date to rapid retreats of past ice sheets. Rapid retreats of future ice sheets may therefore represent the horizon to practical safety assessments for mined nuclear waste isolation facilities. This timing is explained when it is argued that fluids in bedrock can reach huge overpressures at times when subhorizontal fractures are dilated by rapid lithospheric rebound. The structures that may pose the greatest problem to nuclear waste isolation are those that are cryptic. A discussion of how rock stresses control the geometry of overpressured fluids during rapid ice retreats provides a genetic link between phenomena previously considered as separate.

2. Summary of the problems It will be assumed here that the geometries of future ice sheets are likely to be similar to those of Pleistocene ice sheets ( Fig. 1). The extent to

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Fig. 1. Margins of Pleistocene ice sheets in Europe.

which the integrity of traditionally mined nuclear waste isolation sites are threatened by future glaciations ( Fig. 2) depend mainly on their locations relative to ice thickness, temperature and velocity. To a first order approximation the potential problems shown in Fig. 2 have simple bulls-eye patterns around ice centres. Such target patters were shared by early reconstructions of Pleistocene ice sheets which invoked basal temperatures to explain significant erosion by rapidly-moving warm-based loaded above its pressure melting point surrounded by a zone of slow-moving coldbased ice that erodes very little (Sugden, 1978). However, even on Fig. 2, the basic target pattern is complicated by mountain divides and ice-free regions starved of precipitation. Even so, there is a need for more detailed maps of the narrow tracks eroded by former streams of warm-based ice that flowed radially through thinner, slower-moving cold-based ice where substrates are almost untouched by glacial erosion (Denton and Hughes, 1981; Kleman and Borgstro¨m, 1994; Payne and Donglemans, 1997).

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Fig. 2. Crude map summarizing the potential problems posed to mined waste facilities in northern Europe by glaciationassociated structures.

3. Structures associated with glaciations and their origins 3.1. Major end-glacial faults One of the most spectacular structures indirectly associated with past glaciations are the reverse fault scarps that offset rock surfaces striated by ice under former ice centres in all the northern continents (e.g. Kujansuu, 1964). Five or six groups of major fault scars vertically offset both rockhead and till surfaces by 5–15 m locally north of 65°N in Europe (e.g. Lagerba¨ck, 1979). Individual scarps are hundreds of kilometres apart and interpolate over lengths up to 120 km; together they have a total combined length of ~500 km. Most are steep through-crustal faults (Arvidsson, 1996) but locally flare to suites of gently dipping shallow thrusts that display the palm-tree profiles of transpressive flower structures reactivating old regional faults ( Talbot, 1986). Scarps in the greenstone belts of Finland and Canada are much smaller (centimetres to millimetres) and closer (<100 m). Major end-glacial faults scarps have

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not been found in North America perhaps because the many small offsets in the well foliated bedrock there may play the same roll as the few widely spaced major displacements in the more isotropic rocks of northern Europe (Muir-Wood, 1989). However, recent modelling of glacial rebound indicates that end-glacial faults were more likely in Fennoscandian than in Laurentia because the dominant wavelength of the Fennoscandian ice sheet was closer to the critical wavelength of the elastic lithosphere (Johnston et al., 1998). Major landslides in glacial tills near the major fault scarps in Sweden area associated with seismites (graded till profiles, sand volcanoes etc.) due to ground-shake and possible tsunami (Lagerba¨ck, 1990). Some of these slides appear to have flowed around obstructions that are no longer there. Kujansuu (1964) inferred that slurries of till flowed around remnants of the last ice. Neat offsets of eskers and the precarious position of blocks poised along parts of the fault scarps point to their lack of disturbance by ice flow. The seismites, glacial shorelines, and peats in landslide scars all date to ca 8.2±0.2 ka years BP, when the last ice front was retreating past them indicating that each fault displacement was end-glacial rather than post-glacial. Attributing individual faults to single displacement implies that each scarp was displaced during an earthquake with a magnitude of ca 8 ( Talbot, 1986; Arvidsson, 1996). Only a single generation of major end-glacial faults is known and this appears to be confined to north of ca 65°N, perhaps under a former ice centre that did not melt during interstadials (Lagerba¨ck, 1990). Large examples have been postulated further south (e.g. Mo¨rner, 1990) but do not offset glacial striae on both the hanging and footwalls. Striae missing from bedrock for ten or more metres downstream of these scarps are found on the former top surfaces of large blocks carried down-ice. Such scarps are now attributed to glacial plucking and transport rather than the end-glacial release of lithospheric stresses. Major end-glacial faults and their associated landslides were originally attributed to simple postglacial rebound. However, rather than being the normal faults radial or concentric to the contours

of land uplift required by that view, they are reverse or oblique transpressive reactivations of old faults that displaced seismically as the margin of the last remnants of ice retreated past each example. The current working model is that a series of major earthquakes released standard plate tectonic forces stored through the preceding glaciations ( Talbot, 1986; Muir-Wood, 1989). This view is modified in a later section. No isolation sites are necessary in the region of Europe disturbed by major end-glacial faults in the past and only future ices sheets significantly larger than the Weischselian advance are likely to pose this threat south of 65°N. 3.2. Glacial troughs and fjords Erosion by ice itself is only likely to be directly significant to mined facilities where ice streams pluck, quarry and abrade channels with parabolic cross sections that reach depths up to 2 km and extend hundreds of kilometres through areas of high relief confined (in Europe) to Scandinavia, Scotland and the Alps ( Fig. 2). Fjords reach the sea and troughs do not. Both represent the effect on ice erosion rate of the positive feedback between pressure gradients induced by the topographies of the top and bottom of the ice and the temperatures and velocities of basal ice. The obliquity of the New Zealand Alps to the regional ice equilibrium line has been used to infer that it takes ca 70 ka for glaciers to erode recognizable parabolic profiles along V-shaped river valleys, ca 200 ka to develop cirques, and ca 320 ka to erode mature glacial troughs ( Kirkbride and Matthews, 1997). Once established along sinuous former river valleys or along straight major fracture zones, the dimensions of glacial troughs scale to ice catchment area and flux. Troughs deepen where ice flow converges below heads that retreat upslope. The plateaux are essentially unmodified by ice in Scandinavia, but the interfluves are narrower in the more mature glacial terrains of the Alps. No sites for nuclear waste isolation are needed in the relatively immature fjordlands of Scandinavia that may mature toward Alpine troughlands during future glaciations. However,

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sites may be needed in the already mature glacial scenery of the Alps. Here the pattern of multigenerational troughs might change dramatically as the larger glaciers erode back across former divides in the ice flow. Nevertheless, in view of the implications of later sections, sites excavated kilometres into the walls of existing troughs might have several advantages over those excavated in flat shields. 3.3. Sediment-filled fractures in bedrock The structures generated by glacial meltwater may not be as grand as those eroded by ice itself but some may approach the levels planned for nuclear waste facilities. It is not unusual for drill-hole records in old crystalline bedrock throughout the Baltic shield to document the loss of a few centimetres or decimetres of cores at various depths. Intervals through which drilling records report that the drill string fell, or where the return fluid was reduced, lost or dirty, are usually, dismissed as sub-horizontal stress-relief joints or fault gouges. Although intervals of core loss may correlate over a number of boreholes, their potential significance is rarely recognized unless extensive infills of fluvioglacial sediments are found along them in excavations within the bedrock mass. Fig. 3 illustrates an example 14 m deep in gneisses with steep fabrics beneath 3 m of till. Silty sand infills were locally graded and cross-bedded along an undulating network of mainly subhorizontal fractures that underlay an area of at least 25×300 m (Carlsson, 1979). Pockets of infill in the widest fractures (~50 cm) included pebbles up to 20 cm across and one displayed up to 50 varves affected by dewatering structures. Pollen grains among these sediments indicated reworking of glacial or interstadial sediments predating the last ice. Vertical markers in bedrock were not displaced so the surficial slab was lifted and propped but not transported horizontally. 3.4. Hydraulic lift The shallow fractures in Fig. 3 were initially attributed to fracturing by freezing ice and later

Fig. 3. Subhorizontal fractures 14 m deep open over an area of at least 75 000 m exposed in crystalline bedrock beneath 3 m 2 of till in the excavation for a nuclear complex at Forsmark were infilled by fluvioglacial sands and silts from carlsson (1979).

interstadial infilling (Stephasson and Ericksson, 1975). However, the dilation of subhorizontal fractures by groundwater overpressured beneath a retreating ice front may be more likely ( Talbot, 1990). Numerical modelling by Pusch et al. (1990) indicates that subglacial artesian water flowing 1 km behind an ice front only 200 m high is capable of lifting already loosened rock slabs 30 m thick (between 0.33 and 0.14 of the ice thickness). These workers argues that such hydraulic lift is most likely in zones of rapid ice retreat, such as that in Fig. 3 where the last ice retreated #250 m a−1. However, the Weischselian ice sheet was closer to 3500 m thick over much of Scandinavia where the ice front retreated almost 1000 km in 2000 years. This means that overpressures in water draining beneath such an ice sheet had the potential to hydraulically lift rock slabs between 0.33 and 0.14 of the ice thickness (depending on friction along their plumbing systems), that is, between 1000 and 500 m thick ( Fig. 4). Any such fractures dilated by past or future ice near waste isolation levels present a significant problem if only because they are difficult to recognize or anticipate. Core losses from subhorizontal fracture zones have been reported at intervals down to depths below 1000 m in several boreholes in Fennoscandia. The weak clay-rich materials flushed from these intervals by routine drilling

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Fig. 4. Hydraulic lift that probably dilated the sediment-filled fracture in Fig. 3 might also dilate pre-existing fractures in bedrock as deep as 600 m. A smaller version of this phenomena in sedimentary beds could account for linked hollows and megablocks (Aber, 1985).

have previously been attributed to old fault gouges ground from the surrounding bedrock. In future it will be vital to demonstrate that such infills really are old cataclastic rocks and not surficial Pleistocene sediments pumped deep into the rock mass by overpressured glacial melt water. Such tests are possible using triple-tube core barrels in drillholes deflected to collect and examine undistributed samples of infills encountered in routine exploration holes.

Several rafts of chalk in southern Sweden appear to have been carried 25 km northward before being imbricated by ice-push (Ringberg, 1988). Distances of transport are seldom constrained but examples carried 300 km from their source are reported from Poland and Alberta (Ben and Evans, 1998). The largest recognized megablock is in Saskatchewan and consists of ca 38×30 km of Cretaceous siltstones and mudrocks up to 100 m thick above a detachment containing up to 2 m of locally sheared till (Christiensen, 1971). Even this, the thickest known megablock, would pose little threat to a deep nuclear waste facility. However, whereas megablocks used to be thought of as butte-like hills frozen to the base of cold-based ice sheets, it is now recognized that some may have been pushed off a deforming substrate to warm-based ice. No megablocks of crystalline rocks have been recognized. However, although the surficial slab in Fig. 3 was not lifted and carried downstream, its vertical aspect ratio approaches that of typical megablocks, and it is possible that hydraulic overpressures could dilate fractures well below crystalline megablocks.

3.6. Block fields and caves 3.5. Megablock rafting Glacial floes, rafts (schollen) or megablocks, are terms applied to large subhorizontal slabs of bedrock that have not only been lifted but transported and stranded above thin glacial or fluvioglacial deposits (Ben and Evans, 1998, pp. 470–472). The allochthonous nature of megablocks totally or partially buried by till is almost as cryptic as fractures beneath hydraulically lifted blocks. Unless obviously exotic, most megablocks are mapped as bedrock unless underlying glacial or fluvial sediments are recognized by drilling or excavation. Many megablocks of erratic impermeable sediments (>1 km2 and ≤30 m thick) have been documented in the Canadian prairies (Sauer, 1978), Poland (Ruszczynska-Szenajch, 1976) England, Denmark and Sweden (Aber, 1985). Megablocks have typical length to thickness ratios ca 100 and are thought to have been carried by flowing ice.

Fields of bedrock blocks striated by ice on one surface are jumbled into piles above and around hills riddled by caves considerably further south of the end-glacial faults in northern Europe (Sjo¨berg, 1987). Steep fractures connecting at least three storeys of pre-existing subhorizontal fractures ca 3–4 m apart appear to have dilated in low hills hundreds of metres across. Empty caves are propped open by fracture misfits and a few exotic boulders too large to reach their present locations without fractures having been wider when they entered. Blocks above and around the caves are so delicately poised that they cannot have been disturbed by flowing ice and must have been dilated and left empty at end-glacial times. There is spatial correlation between the caves and some of the block fields with areas of greatest Gaussian curvature during post post-glacial uplift ( Ekman, 1988). However, this correlation is incomplete and other processes were probably

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involved. Attributing the cave systems to the explosive release of groundwater or gasses overpressured beneath the ice in hills with side boundaries (Bjo¨rk, 1989) would make them on-shore equivalents of some of the pockmarks recognized offshore (see below). Explosive release of gasses accumulated beneath seals of gas hydrates loaded by ice would account for empty fractures being temporarily widened for the entry of boulders too large to enter since.

3.7. Seabed pockmarks and major craters Pockmarks, craters <150 m across and up to 2–3 m deep, occur in great concentrations (>40 km2) in soft muddy seabeds at all depths throughout the world (Hovland and Judd, 1988). Most appear to be craters cleared by seepages of groundwater (hot or cold, fresh or saline) and/or gases (thermogenic, biogenic or hydrothermal ). Disturbance of their substrates seldom exceed ca 30 m but methane has been inferred to have risen from hydro-carbon sources above salt diapirs 800 m below parts of the North Sea (Hovland and Judd, 1988). Minimal erosion or deposition in the North Sea over the last 8000 years means that the present population of pockmarks there accumulated. As active and inactive pockmarks cannot be distinguished, it is not known whether there were surges of gas escape during ice retreat. However, this seems likely for a cluster of craters several hundreds of metres across in Triassic bedrocks in the central Barents Sea has been attributed to the accumulation of hydrothermal gases beneath a thick layer of gas-hydrates during overloading by grounded Late Weischselian ice (Solheim and Elerhoi, 1993). As rapidly as the ice-loaded lifted, the hydrates decomposed from the base upward and blew-out through the hydrate seal to form major craters that might also be considered endglacial structures (Anndreassen, 1995). Pockmarks and gas blow-outs cause severe problems to the offshore oil industry but their main relevance to nuclear waste isolation appears to be their implication that gas accumulations beneath seals of ice, permafrost, or gas hydrates can gener-

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ate craters offshore and, perhaps, cave systems and block fields onshore during ice retreat. Abandoned offshore oil fields suggested as potential waste sites are likely to be particularly vulnerable to gas blow-outs. 3.8. Melt water canyons Channels eroded by meltwaters from Pleistocene ice are generally much larger than those expected of current sediment-transport systems. They occur locally along the paths of former ice streams anywhere between former ice centres to former ice margins. Lacking shoreline and lacustrine deposits, their regional patterns integrate better with late glacial ice-directed drainage patterns than with bedrock lithologies or structures. Some parallel the inferred edge of the ice but most parallel the inferred ice slope. Their dimensions and potential significance to nuclear waste isolation generally increase downstream. Meltwater canyons above the highest post-glacial shoreline are attributed to sub-aerial stream erosion in crevasses or along the margins of coldbased ice, or the superposition of supra-glacial streams onto pre-glacial sceneries (Olvmo, 1989). Subglacial meltwater canyons eroded in lowlands beneath warm-based ice lower than late glacial shorelines have similar dimension but are distinguished by floors rising as much as 75 m downstream (Sissons, 1961). Few meltwater canyons are more than a few kilometres long, or wider or deeper than 100 m. However, many extend into lowlands or offshore and end in major glacial incisions that are much more significant ( Wingfield, 1990). 3.9. Major glacial incisions and tunnel valleys Major glacial incisions appear to be offshore equivalents of the tunnel valleys buried beneath the northern lowlands of Europe, Asia and the Americas (Ben and Evans, 1998). They have been defined by Wingfield (1990) as enclosed depressions eroded >100 m into continental shelves or adjacent lowlands with widths >2 km and floors below −140 m (the likely extreme lowstand of

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Weischselian sea level in the North Sea). Known examples have lengths up to 30 km, widths up to 5 km and flat bottoms with side slopes averaging ca 1 in 10. The deepest known glacial incision reaches 360±40 m below the ambient surface ( Wingfield, 1990). Most incised depressions narrow downstream and are deepest 1/3 of the way from their discharge end. Broad swathes of three generations of major incisions formed near the limits of lowland or tidewater ice-sheets on the continental shelf of Europe during the late stages of each of the last three glacial stages. Incisions of each generation differed slightly in age and many are superimposed in series, parallel or crosscutting patterns so that composite incisions develop at local intersections. Such complications imply complex histories in which outburst floods alternated with intervals of reshaping by ice. Wingfield (1990) estimated that British waters alone cover ca 400 major incisions of the first ( later Elstarian) generation, ca 200 of the intermediate ( late Saalian) generation, and at least 400 of the last ( late Weischselian) generation, ca 1000 altogether.

3.10. Overdeepening by overpressured meltwater Subglacial meltwater channels range from the relatively minor canyons eroded beneath proximal ice to giant incisions and tunnel valleys eroded beneath re-entrants between lobes in the ice margins. All are distinguished by local stretches that have been overdeepened. Subglacial conduits responsible for overdeepened channel sections are kept open by frictional melting and meltwater pressures that match ice pressures. Channels generally follow pressuregradients imposed by ice topography and can overdeepen upslope sections. Subglacial channels can be overdeepened in any substrates where ice flow lowers their roofs as fast as erosion by the piping of sediment-laden water deepens their floors. Over-deepening subglacial channels are thus filled mainly by flowing ice so that tunnel valleys or incisions end up much larger than the initial channel. It has been suggested that substrates of plastic sediment could also be flushed

out as fast as they are squeezed upward into subglacial channels. However, incisions have similarities in geometries, size and infills in substrates that range from unlithified Pleistocene sediments through Tertiary–Mesozoic sedimentary rocks and basalts to Precambrian crystalline rocks. These similarities point to a common origin ( Wingfield, 1989).

3.11. Jo¨kulhlaups Although tunnel valleys and major glacial incisions could be considered as merely extreme cases of overdeepening by overpressured sub-glacial meltwater, they are so voluminous that an extra factor is usually invoked: Jo¨kulhlaups (e.g. Ehlers, 1981; Wingfield, 1990). Jo¨kulhlaup (‘glacier flood’) is an Icelandic term for the periodic or irregular catastrophic outbursts of glacially-stored water that are known in mainland Europe as debacles. Jo¨kulhlaups in Iceland emerge from ice caps melted from below by geothermal heat or volcanic eruptions. Grimsvo¨tn in Iceland releases jo¨kulhlaups of ca 4.5 km3 at intervals of ca 6 years. The released water travels 50 km under the margins of Vatnajo¨kul ice cap before emerging as floods that can last ca 30 days. Measured discharge rates usually peak at 5×104 m3 s−1 up to 20 days after they start and one of largest peaked at 1.5×106 m3 s−1. However, modern Jo¨kulhlaups in Iceland are minor compared to those released by Pleistocene ice sheets. Glacial Lake at Missoula in Montana is estimated to have had a maximum volume of ca 2500 km3 and to have catastrophically released the volume of Lake Ontario (ca 2184 km3) several times (Ben and Evans, 1998, p. 343). Together these floods scoured 60 m of loess over 4×104 km2 and plucked and scoured channels in basalts underlying the Channelled Scablands of Washington state; each flood may have lasted only a few days. Many of the resulting tunnel valleys are 100–200 m deep and the Columbia river gorge reaches ca 300 m. Most of this erosion is attributed to a few floods that have been estimated to have discharged at up 2.7 or 21×106 m3 s−1, equivalent

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to the mean flow of all the world’s rivers into the oceans (Ben and Evans, 1998, p. 343). 3.12. Mechanisms of jo¨kulhlaups Jo¨kulhlaups have been attributed to a sudden increase in subglacial drainage rate when distributed Darcian flow through porous substrate is overtaken by piping along conduit systems or tunnel valleys ( Fowler and Ng, 1996; Piotrowski, 1997). The largest jo¨kulhlaups are thought to occur when huge water bodies accumulate behind dams of more distal cold-based ice. Such lakes discharge when their depths exceed 90% of the thickness of the ice dam (Nye, 1976). However, Bjo¨rnsson (1992) showed that Icelandic Jo¨kulhlaups often flood before lakes and cupola can simply float the ice damming them. He calculated that beams of ice floating upstream can cantilevering the ice dam off its frozen substrate. Once flow beneath the ice dam becomes continuous, the initial discharge-tunnel widens due to thermal and mechanical energy with positive feedback between frictional melting and the outrush. The initial breach then enlarges rapidly by headward erosion until the lake is either totally discharged or the low and wide discharge tunnel closes (Bjo¨rnsson, 1992). However, they develop, major incisions are likely to downcut #350 m below the surface of the present continental shelves or coastal lowlands during the next glacial retreat. Hydraulic lift along subhorizontal fractures or bedding in the ice substrate would account for the floors of incisions being generally flat and it is possible that other fractures may dilate beneath not only the flanks of the deepest incisions but even their floors. 3.13. Can tunnel valleys in salt close? It has long been known that rocksalt can flow into deep boreholes and mine workings remarkably rapidly. Rocksalt is so ductile that the steep walls of any tunnel valleys incised into the tops of salt structures by Pleistocene ice might have closed due to gravity spreading and the fall of salt. Recent studies on a river cliff now being cut in an active salt extrusion in Iran suggest that gorges 400 m

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deep in salt can close by ductile flow that reaches rates of 1 m month−1 on a time-scale of years between major falls ( Talbot, 1999). Overlying sediments and insoluble cap rocks were removed along a tunnel valley across the crest of Gorleben salt dome in Germany where an exploratory mine is being excavated 840 m below surface to decide in 2005 whether radioactive waste will be isolated there by the year 2030. Potash salts have been replaced down to ca 4000 m below surface by a steep lens of gravel beneath the most overdeepened stretch of this valley. Such replacement could be attributed to the salt itself having been eroded to this depth by a major Late Elserian ice incision (ca 300 ka BP) which closed rapidly as the ice front withdrew and salt was no longer removed from the enclosed basin. The volumes of salt necessary to close the cross sectional area of any glacial incision eroded in the top of Gorleben dome is likely to have been significant. Its closure might have been sufficiently rapid to be beyond the resolution of backstripping by Zirngast (1996) who had to average post-Palaeocene salt-velocities over the last 23 million years. 3.14. River gorges The current land-exit points of most major rivers world-wide are underlain by two generations of buried gorges dating from the two deepest Pleistocene lowstands. These date from rivers working down toward base levels lowered 125–150 m below current sea level. Around enclosed basins like the Caspian and Baltic Seas and the American Great Lakes, base level lowering could be extremely rapid with overspill in the latter two cases being controlled by the extent of adjacent ice sheets. Thus the Mediterranean desiccated during the Messinnian salinity crisis and deep river gorges far beyond the ice front retreated hundreds of kilometres inland exposing potential sites in mudrock nearby to slope failure by gravity and drawn down. The sill of the Mediterranean is now too deep to be affected by direct glacial and hydrostatic effects but could be raised by tectonic forces so that the Mediterranean could desiccate again during future lowstands.

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4. Stress fields and bedrock permeability 4.1. Current situation The axis of maximum stress in most of Europe is a NW–SE horizontal compression that parallels the compressive stress trajectories between the midAtlantic ridge and the Alps and Pyrenees (Muller et al., 1997). In areas of low relief not subject to Pleistocene ice loading and unloading, the minimum principal stress is generally horizontal and NE–SW so that the upper 15 km of crust is prone to strike slip-strains [crust deeper than 15 km may be in a normal stress regime (Plenefisch and Bonjer, 1997)]. Stress fields are more complicated where the lithosphere is still undergoing postglacial rebound at an arctan rate (Pa˚sse, 1996) and still has 20 ka to run (Ekman, 1996). Here, the axes of principal stress tend to swap one for another so that stress regimes change with both depth and location. Principal stresses swap where their values change smoothly while they maintain essentially constant orientations so that boundaries between stress regimes are marked by changes in the relative values of principal axes rather than their rotation. Three stress regime are prone to thrust, strike-slip or normal strains where the vertical stress is the smallest, intermediate or largest principal compressions, respectively. Stresses measured in deep vertical boreholes in Sweden usually define regimes that are prone to thrusting within a few tens or hundreds of metres of such mechanical free surfaces as rock head and major fracture zones, and strike-slip-prone regimes for a few hundred metres in-between ( Talbot, 1990). The surficial thrust-prone regime appears not to be ubiquitous. The maximum horizontal stress trends near NW–SE in ca 80% of determinations and swaps to near NE–SW elsewhere. Simple extrapolation between in situ stress determinations down individual boreholes suggest that narrow zones prone to normal faulting may exist along major fracture zones (e.g. Fig. 5). Neither fracture zones nor stress regimes can be extrapolated straight across many other major fracture zones. Understanding how transmissivities in old crystalline rocks vary between 10−6 and 10−12 m s−1

for completing the nuclear energy cycle requires further development of the new subject of cratonic structural geology (Munier and Talbot, 1993). Here the focus is not on the obvious ocean ridges, trenches or orogenic mountain chains that develop along plate margins where oceans open and close, but the inconspicuous fracture patterns that reactivate or grow contemporaneously in the distant plates (Munier and Talbot, 1993). At every candidate waste isolation site, it should be possible to assign particular suites of contemporaneous new and/or reactivated veins, faults or joints with appropriate kinematics and infills to every major tectonic event at the surrounding plate boundaries and every rise and fall of the craton due to loading and unloading by nappes, sediments, volcanics, ice or water. This story should also match the history of deposition, quiescence, erosion and deformation in surrounding sedimentary basins. ˚ spo¨ Hard Precambrian crystalline rocks in the A Rock Laboratory in Sweden appear to have accumulated sets of fractures and fracture zones with a sufficiently wide range in orientations and spatial densities that, by Caledonian times, they had already fracture-saturated on all scales. Saturation means (Renshaw, 1997) that, rather than develop new fractures or reactive old veins strengthened by infills of quartz and/or epidote, post-Caledonian events jostled rock blocks by reactivating fractures with such weak infills as chlorite, zeolites, iron oxides, carbonates or gouges (Munier and Talbot, 1993). Careful sub-surface studies in northern Sweden failed to identify a single definite new fracture attributable to Pleistocene glaciations beneath surficial levels (Ba¨ckblom and Stanfors, 1989). As Pleistocene rock breakage appears to have exploited pre-exiting fractures, it seems likely that the fracture patterns due to be dilated at isolation levels by lithospheric stresses induced by future glacial rebounds not only exist but are already known. About 10% of the fractures mapped ˚ spo¨ HRL in Sweden now underground at the A bear water (Munier, 1993). Most of these hydraulic conductors are in the relatively easily recognized fracture zones within which the hydraulic conductivity is anisotropic, being highest along the most

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Fig. 5. (a) Current in situ stresses calculated from overcoring and hydraulic fracturing (horizontal ) or density–gravity–depth (vertical ) ˚ spo¨ HRL in Sweden. Levels where the vertical principal stress is second largest (intermediate shading) are prone in a borehole at A to strike-slip strains. Darkest levels are where the vertical principal stress is lowest and prone to dilation of horizontal fractures if pore pressure are high. A possible regime prone to ‘normal faulting’ is shown (pale) where the vertical principal stress exceeds extrapolated horizontal stresses so that steep pre-existing fractures tend to dilate. The same shading convention is used in subsequent ˚ spo¨ HRL, Sweden, relate to the figures. (b) The values and geometries of current rock permabilities/transmissivities measured in A geometry of old fractures dilated by in situ stresses (shown schematically). These are subvertical in strike-slip (wrench) regimes and subhorizontal in thrust regimes. The strike-slip (wrench) regimes general in the rock mass is laced by thrust regimes along fracture zones which may have core regimes of normal stress.

common intersections between the closely spaced shear fractures oblique across each zone. However, significant proportions of wet fractures occur within the intervening blocks in which fracture permeabilities are lower but still significant. The comparatively simple intrablock fractures patterns known probably first developed as paleo-ocean lapetus opened and closed hundreds of kilometres away between ca 1 and 0.4 Ga. Stress levels insufficient to generate either earthquakes or new fractures still control bedrock permeability; sets of pre-existing fractures are dilated by the locally current stress regime controlled by continued glacial rebound. The most conductive sets generally subparallel the current maximum and intermediate stress axes which swap between regimes with orthogonal distance from fracture

zones or the surface (Fig. 5). In the strike-slip regimes between depths of 200 and 500 m in most ˚ spo¨ HRL ( Fig. 5), the most dilated fracof the A tures are subvertical, NW–SE trending, single fractures hundreds of square metres in area and tens of metres apart. These seem to occur as single parallel fractures in diffuse fracture ‘swarms’ with only a few connections along steep steps and splays (Munier, 1993). The relative trasmissivites of subvertical and ˚ spo¨ HRL swap subhorizontal fracture sets in the A with the principal stress axes so that, in stress regimes prone to thrusting, water transmissivity is greater along gently dipping fractures than along steep fractures (Fig. 5). Fracture sets with different hydraulic transmissivities intersect along the same horizontal NW–SE axis in both stress regimes.

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However, the most significant are subvertical in strike-slip regimes and subhorizontal in thrust regimes ( Fig. 5). Intrablock fractures held open by current forces are relatively minor with transmissivities between 10−6 and 10−9 m2 s−1. Their orientations and relative significance appear to relate to the current local stress regime and, although their locations are not predictable, they should be visible to underground radar. It is not yet clear whether individual cavities tailored to receive waster canisters can avoid intrablock fractures or whether such fractures need to be grouted. 4.2. Changes due to ice loading and freezing Like ice sheets past and present, future ice sheets will flex the lithosphere and lead to fluctuations in patterns of stress and permeability as well as sea level. Large Pleistocene ice sheets exceeded 3 km in thickness over radii approaching 1000 km in Fennoscandia (as in current Greenland ) and 2000 km in Laurentia (as in current Antarctica); ice sheets in Iceland, Scotland and the mountain chains of Europe were smaller and thinner. The thickest ice had the potential for depressing rockhead something like 960 m but, because the resulting flow in the asthenosphere is so slow, equilibrium was probably never reached, and the maximum depression was probably closer to 600 m (Boulton and Payn, 1993). Changes in the atmosphere, cryosphere, hydrosphere and lithosphere are considerably faster than those possible deep in the underlying astheonsphere. As a result, vertical ice loading and unloading is almost instantaneous compared to slower but greater changes in lateral loading due to slower depression or rebound of the elastic lithosphere [Fig. 6(A)]. Vertical rock stresses therefore change much more rapidly than lateral stresses [Fig. 6(B)]. However, lateral compression generally increases with depth at about twice the rate of vertical compression ( Fig. 5). This means that although horizontal stresses change more slowly in time, they soon reach higher values. Stress regimes at rockhead change when and where relative values of the principal stresses swap one for another [Fig. 6(B)]. Fig. 6(C ) is designed to give a qualita-

Fig. 6. A simplified loading history for a site in central Sweden during the last ( Weischselian) major glaciation. (A) Ice thickness appears above the time axis and consequent depression of rock head below. Ice load depressed the lithosphere toward a maximum depth 0.17 times the ice thickness that it never reached because of viscous retardation by the mantle. (B) Principal stresses (sV, sH and sh) at rock head with time. Stress regimes change where relative values of principal stresses swap one for another. Lithospheric flexure is assumed to increase lateral compressive stresses at a gradient with depth twice that for vertical stresses with a relative time lag near 10–20 ka. (C ) Changes in stress regimes with depth (vertical ) and time (horizontal ) assuming that the pre-Weischselian stress field was similar to today. Notice that the whole section was in the thrust regime so that subhorizontal fractures and zones were dilated during ice retreat when meltwater was being released at potentially high pressures.

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tive feel for changes in vertical profiles of stress regimes with depth from the beginning of Weischselian ice loading to 2 ka in the future. The only obvious locations where patterns of pre-, synand post glacial stress regimes are likely to differ from those in Fig. 6 is where principal stresses are not orthogonal to the flat surface of a shield but deflected around high relief. Ice loading faster than the lithospheric can be depressed tends to drive existing stress regimes toward or further into regimes of normal faulting [Fig. 6(C )]. Long periods with only slow fluctuations in loads tend to emphasize strike-slip stress regimes. Ice-loads falling faster than the lithosphere can rebound expands thrust regimes at the expense of other regimes [Fig. 6(C )]. Whether rapid rebound drives thrust regimes into all or only parts of the upper crust, gently dipping bedrock fractures and fracture zones tend to dilate at a time when huge volumes of meltwater or gasses are being liberated. High pressures in water (or gas) sealed deep in bedrock by ice (or gas hydrates) probably account for why the most significant structures associated with meltwaters or gasses appear to have occurred during the last rapid ice retreat.

4.3. Regional scale The patterns and magnitudes of stress in a flexed lithosphere depend on the dominant wavelength of the load (i.e. twice its diameter) relative to the elastic thickness of the lithosphere; the most critical wavelength is near ca 930 km (Johnston et al., 1998). Loads with wavelengths less than four times the lithospheric thickness are borne largely by the elastic plate and vertical displacements are small. The greatest stress changes are induced by loads with dominant wavelengths close to 12 times the elastic thickness of the plate when horizontal stresses can increase to six times more than vertical stresses (Johnston et al., 1998). Ice sheets with wavelengths much greater than the elastic thickness of the lithosphere stabilize faults under thickening ice and destabilize them during melting. Vertical loads and displacements do not extend beyond the flexure hinge close to the ice margin but horizontal

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stresses do and, were it not for the plate tectonic forces, would induce a regime strongly prone to normal faulting in the peripheral bulge (Johnston et al., 1998). Models neglecting the effects of horizontal tectonic stresses, topographies and pore pressures by Johnston et al. (1998) suggest that lateral stresses increase under an isolated sheet 333 km in radius and 1000 m thick, reach 23 MPa, ca 2.5 times the 9 MPa increase in vertical stress. This would have been the case in Britain during Weischselian times were it not for the fact that Britain was in the peripheral bulge of Fennoscandian ice. Models of a Fennoscandian ice sheet 1000 m thick indicate that increases in horizontal stresses were lower than in Britain during the last glacial maximum and that increases in horizontal stresses were only about twice the increase in vertical stress; substituting ice thickness nearer a more realistic 3000 m is expected to raise stress changes toward those calculated for an isolated Britain (Johnston et al., 1998). However, modelled horizontal stresses remained high and soon far exceeded vertical stresses falling as fast as the Fennoscandia ice retreated. The end-glacial faults under the Fennoscandia ice centre can be attributed to fractures that started to destabilize at ca 12 ka BP and were most unstable at ca 9 ka BP. It would be possible to monitor earthquakes with magnitudes down to magnitude 4 in Antarctica and Greenland but they do not occur; this suggests that large continental ice sheets somehow suppress seismic activity (Johnston, 1987). It is conceivable that sub-glacial water dilates faults and that plate tectonic strains still occur by stable aseismic slip. However, it is more likely that standard plate tectonic stress trajectories are swamped by ice-induced stresses around and beneath iceloaded lithospheric depressions and accumulate as potential energy in surrounding regions. Approximately concentric-outward zones of thrust, strike slip and normal stress regimes would insulate depressions from the general strike-slip tectonic stress field and wax and wane in response to ice expansion and contraction (Johnston et al., 1998). Their sudden convergence on former centres of long-lived ice sheets with critical wavelength

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could lead to catastrophic release of pent up forces and the end-glacial fault scarps in Lapland. 4.4. Synthesis The greatest natural threats to the integrity of the geological barriers to mined isolation levels are posed by the high pressures induced by future icesheets in deep ice, meltwater and gasses and the high differential stresses induced in bedrock by rapid rebound. Rapidly flowing ice streams erode fjords and troughs throughout glacial periods. The restriction of major end-glacial faults to Lapland indicate that glacial rebound stresses reached sufficient differentials to reactive major faults only when Late Weischselian ice sheets close to the critical wavelength of the lithosphere retreated rapidly into its long-lived centre in Fennoscandia. Problems arise elsewhere because differential stresses insufficient to seismically offset the crust still dilate the rock mass by opening old fractures to high-pressure fluids. Subglacial water pressures are likely to be the highest where huge bodies of water on, in or under warm-base ice can reach deep fracture networks beneath dams of coldbased distal ice. Accumulations of deep hydrocarbon gasses may be capable of blowing craters in sedimentary rocks or cave systems in crystalline bedrocks, when the load of grounded ice responsible for their gas hydrate seals is suddenly lifted. Ice-substrates of different character are susceptible in different ways to the huge fluid overpressures that can develop when large ice sheets retreat rapidly. All are vulnerable to erosive overdeepening of sub-glacial channels by piping of overpressured sediment-laden meltwater. Substrates with fracture permeabilities dilated by rapid rebound may be susceptible to hydraulic lift at depths approaching waste isolation levels. Overpressure of fluids along existing subhorizontal fractures (or panels of mined cavities) dilated by unloading stress differentials may lift large slaps of bedrock that can be transported as megablocks by ice, or perhaps, even water. Substrates with Darcian permeabilities appear to be too impermeable to allow significant internal pore pressure to develop. Rather than the outward escape of fluids, the potential threat here is high-

pressure hydraulic jets incising huge cavities downward into frozen substrates under the margins of lobes fed by grounded ice streams. Any major glacial incisions eroded deep into rocksalt might become cryptic after they close. Waste isolation facilities throughout the lowlands of Europe that are not either ventilated or rendered gas-proof are prone to the combined potential threats of gasblowouts in the floors of deep glacial incisions.

Acknowledgments Drs Veijo Pohjola, Udo Hunsche, Robin Wingfield and Martin Ekman are thanked for discussions and suggested improvements, any remaining mistakes are the author’s, not theirs. SKB, the Swedish Nuclear Fuels and Waste Management Company, is thanked for past funding and access to data relating to some of the problems discussed here, and NFR, the Swedish Natural Science Foundation is also acknowledged for past funding.

References Aber, J.S., 1985. The character of glaciotectonism. Geologie en Mijnbouw 64, 389–395. Anndreassen, K., 1995. Seismic reflections associated with submarine gas hydrates. PhD thesis, Department of Geology, University of Tromso¨, Norway. Arvidsson, R., 1996. Fennoscandian earthquakes: whole crustal rupturing related to postglacial rebound. Science 274, 744–746. Ba¨ckblom, G., Stanfors, R. (Eds.), 1989. Interdisciplinary study of post-glacial faulting in the Lansja¨rve area, northern Sweden. Swedish Nuclear Fuels and Waste Management Company, Progress Report 89-31, Stockholm. Ben, D.I., Evans, D.J.A., 1998. Glaciers and Glaciation, Arnold, London, p. 724. Bjo¨rk, 1989. Personal communication. Bjo¨rnsson, H., 1992. Jo¨kulhlaups in Iceland: prediction, characteristics and simulation. Ann. Glaciol. 16, 95–106. Boulton, G.S., Payn, A., 1993. Simulation of the European ice sheet through the last glacial cycle and prediction of future glaciation. Swedish Nuclear Fuels and Waste Management Company, Progress Report 14-93. Stockholm, p. 138. Boulton, G.S., Smith, G.D., Jones, A.S., Newsom, J., 1955. Glacial Geology and glaciology of the last mid-latitude ice sheets. Geol. Soc. Lond. J. 142, 447–474.

C.J. Talbot / Engineering Geology 52 (1999) 177–192 Carlsson, A., 1979. Characteristic features of a superficial rock mass in south central Sweden. Striae 11, 79 Christiensen, E.A., 1971. Geology and groundwater resources of the Melville area (62K,L) Saskatchewan, Saskatchewan Research Council, Geology Division, Map No. 12, Saskatchewan. Clark, P.U., Licciardi, J.M., MacAyeall, D.R., Jenson, J.W., 1996. Numerical reconstruction of a soft-bedded Laurenide Ice Sheet during the last glacial maximum. Geology 24, 670–682. Denton, G.H., Hughes, T.J., 1981. The Last Great Ice Sheets. Wiley, New York. Ehlers, J. ( Ed.), 1981. Some aspects of glacial erosion and deposition in north Germany. Ann. Glaciol. 2, 143–146. Ekman, M., 1988. Gaussian curvature of postglacial rebound and the discovery of caves created by major earthquakes in Fennoscandia. Geophysics 24, 47–56. Ekman, M., 1996. A consistent map of the postglacial uplift of Fennoscandia. Terra Nova 8, 158–165. Folwer, A.C., Ng, F.S.L., 1996. The role of sediment transport in the mechanics of jo¨kulhlaups. Ann. Glaciol. 22, 255–259. Hovland, M., Judd, A.G., 1988. Seabed Pockmarks and Seapages. Graham & Trotman, London, p. 293. Johnston, A.C., 1987. Suppression of earthquakes by large continental ice sheets. Nature 330, 467–469. Johnston, P., Wu, P., Lambert, K., 1998. Dependence of horizontal stress magnitude on load dimensions in glacial rebound models. Geophys. J. Int. 132, 41–60. Kirkbride, M.P., Matthews, D., 1997. The role of fluvial and glacial erosion in landscape evolution: the Ben Ohau Range, New Zealand. Earth Surface Processes Landforms 22, 317–327. Kleman, J., Borgstro¨m, I., 1994. Glacial landforms indicative of a partly frozen bed. J. Glaciol. 40, 255–264. Kuijpers, A., Duin, E.J.Th., 1986. Boundary current and controlled turbidity deposition; a sedimentation model for the Southern Nares Abyssal plain, Western North Atlantic. Geol. Mar. Lett. 61, 21–28. Kujansuu, R., 1964. Nuorista siirroksistta Lapissa. Summary: recent faults in Lapland. Geologi 16, 30–36. Lagerba¨ck, R., 1979. Neotectonic structures in northern Sweden. Geol. Foren. Stockholm Forh. 100, 263–269. Lagerba¨ck, R., 1990. Late Quaternary faulting and paleoseismicity in northern Fennoscandia with particular reference to the Lansja¨rve area, northern Sweden. Geol. Foren. Stockholm Forh. 112, 333–355. Milnes, A.J., 1985. Geology and Radwastes, Academic Press, London, p. 32. Mo¨rner, M.A., 1990. The Swedish failure in defining an acceptable bedrock repository for nuclear waste deposition. Geol. Foren. Stockholm forh. 112, 375–380. Muir-Wood, R., 1989, Extraordinary deglaciation reverse faulting in northern Fennoscandia. In: Gregerson, S., Basham, P.W. (Eds.): Earthquakes and North-Atlantic Passive Margins. Kluwer Academic, Netherlands, pp. 143–174. Muller, B., Werhrle, V., Zeyen, H., Fuchs, K., 1997. Short-scale variations of tectonic regimes in the western European stress

191

province north of the Alps and Pyrenees. Tectonophysics 275, 199–219. Munier, R., 1993. Segmentation, fragmentation and jostling of the Baltic shield with time. Ph.D. Thesis 37. Acta Universtatis Upsaliensis, Uppsala. Munier, R., Talbot, C.J., 1993. Segementation, fragmentation ˚ spo¨, southand jostling of cratonic basement in and near A east Sweden. Tectonics 12, 713–727. Nye, J.F., 1976. Water flow in glaciers, jo¨kulhlaups, tunnels and veins. J. Glaciol. 17, 181–207. Olvmo, M., 1989. Meltwater canyons in Sweden. GUNI report 27. Ph.D. thesis, University of Go¨teborg, Go¨teborg, Sweden. Payne, A.J., Donglemans, P.W., 1997. Self-organization in the thermomechanical flow of ice sheets. J. Geophys. Res 102 (B6), 12219–12233. Pa˚sse, T., 1996. A mathematical model of the shore level displacement in Fennoscanida. Progress Report 96-24. Swedish Nuclear Fuels and Waste Management Company, Stockholm. Piotrowski, J.A., 1997. Subglacial hysdrology in north-western Germany during the last glaciation; groundwater flow, tunnel valleys and hydrological cycles. Q. Sci. Rev. 16, 169–185. Plenefisch, T., Bonjer, K.-P., 1997. The stress field in the Rhine Graben area inferred from earthquake focal mechanisms and estimations of frictional parameters. Tectonophysics 275, 71–97. Pusch, R., Borgesson, L., Knuttson, S., 1990. Origin of silty fracture fillings in crystalline bedrock. Geol. Foren. Stockholm Forh. 112, 209–213. Renshaw, C.E., 1997. Mechanical controls on the spatial density of open-mode fracture networks. Geology 25, 923–926. Ringberg, B., 1988. Later Weichselian geology of southeastern Sweden. Boreas 17, 243–263. Ruszczynska-Szenajch, H., 1976. The origin of glacial rafts; detachment, transport, deposition. Boreas 16, 101–112. Sauer, E.K., 1978. The engineering significance of glacier icethrusting. Cana. Geotechnical J. 15, 457–472. Sissons, J.B., 1961. Some aspects of glacial drainage systems by the Tinto Hills, Lanarkshire. Transact. Edinburgh Geol. Soc. 18, 175–193. Sjo¨berg, R., 1987. Caves as indicators of neotectonics in Sweden. Z. Geomorphol. N.F. Suppl-Bd. 63, 141–148. Solheim, A., Elerhoi, A., 1993. Gas related floor craters in the Barents Sea. Geo-Mar. Lett. 13, 235–243. Stephasson, O., Ericksson, B., 1975. Pre-Holocene joint fillings at Forsmark, Uppland, Sweden. Geol. Foren. Stockholm Forh. 97, 91–95. Sugden, D.E., 1978. Glacial erosion by the Laurentide ice sheet. J. Glaciol. 20, 361–391. Talbot, C.J., 1986. A preliminary structural analysis of the patterns of post-glacial faults in northern Sweden. Progress Report 86-20. Swedish Nuclear Fuels and Waste Management Company, Stockholm. Talbot, C.J., 1990. Problems posed to a bedrock radwaste repository gently dipping fracture zones. Geol. Foren. Stockholm Forh 112, 355–359.

192

C.J. Talbot / Engineering Geology 52 (1999) 177–192

Talbot, C.J., 1999. Salt extrusion in the Zagros. Geological Society of London Special Publication No. 143, Geological Society of London, London, pp. 315–334. Wingfield, R.T.R., 1989. Glacial incisions indicating Middle Pleistocene and Upper Pleistocene ice limits off Britain. Terra Nova 1, 538–548. Wingfield, R.T.R., 1990. The origin of major incisions within

the Pleistocene deposits of the North Sea. Mar. Geol. 91, 31–52. Zirngast, M., 1996. The development of Gorleben salt dome (northwest Germany) based on qualitative analysis of peripheral sinks. Geological Society of London Special publication No. 100, Geological Society of London, London, pp. 203–226.