the interrelation of glaciotectonic and glaciodepositional processes within the glacial environment abstract in recent years it has been recognised that ice sediment coupling occurred beneath the quaternary ice sheets that advanced over the soft sediments of lowland areas this paper looks in detail at the effects of this coupling on the sediments which results in glaciotectonic deformation and also discusses the interaction of deformation and deposition within the subglacial environment two types of glaciotectonic deformation are discussed that are produced by the active movement of ice 1 proglacial tectonics at the margin which include compressive fold styles such as listric thrusts and faults and open folding 2 subglacial tectonics formed beneath the glacier which include fold styles resulting from simple shear and represent a soft rock purely dynamic shear zone styles of deformation associated with stagnant ice are also investigated we argue that glaciotectonic deformation is a common phenomenon and an integral part of geological record from continental ice sheets it is suggested that zones of deformation within the sediment are related to similar zones of strain in the ice sheet and complex deformation sequences are produced by the superimposition of these differing styles upon one another as the ice sheet advances and retreats it is also argued that subglacial deposition and deformation are related and that on soft beds underformed till is rare whilst deformed till is very common

microtextural analysis of a glacially deformed bedrock implications for inheritance of preferred clast orientations in diamictons although analysis of clast macrofabrics have been used to differentiate between different types of glacial diamictons and to determine palaeo ice flow directions no account appears to have been made of preferred clast orientations inherited from the parental source material clast macrofabrics in tills are typically interpreted as having developed in response to an imposed subglacial deformation and as such provide a link between the sedimentary record and glacier dynamics they rely on the assumption that any preferred clast orientation is a result of deformation flow the results of the micromorphological study of the langholm till exposed at north corbelly near dumfries sw scotland clearly demonstrates that bedrock structure can influence clast orientation macrofabric within diamictons in the lower part of the till the orientation of elongate clasts preserves the geometry of the tectonic cleavage present within the underlying bedrock the intensity of this steeply inclined inherited clast fabric decreases upward through the till to be replaced by a more complex pattern of successive generations of clast microfabrics developed in response to deformation flow these results indicate potential limitations of applying clast macrofabric or microfabric analysis in isolation to establish till genesis or palaeo ice flow directions consequently due account should be made of other glacial palaeoenvironmental and ice flow indicators as well as rockhead depth and morphology in relation to the selection of fabric measurements sites

styles of subglacial glaciotectonic deformation within the context of the anglian ice sheet a substantial proportion of the glacial sediments of east anglia england are chaotic and highly deformed this has been attributed to a direct glaciotectonic deformation or b slumping of glaciomarine sediments we show that interpretation a leads to a classification of subglacial glaciotectonic deposits into sites of constructional deformation where little or no erosion of the preglacial deposits has occurred and styles of deformation are preserved moving up in the sequence and excavational deformation where substantial erosion of preglacial sediments has occurred and deformational styles are superimposed on each other by using glaciological and geotechnical theory we show that the expression of these deformational styles in east anglia is a natural response to the stress fields imposed by the ice sheet and that in consequence their spatial relationships are explicable the depth of deformation is related to the geotechnical properties of till and substrate the stress and effective stress fields and found to be consistent with observations

lobal interactions rheologic superposition and implications for a pleistocene ice stream on the continental shelf of british columbia abstract data on structures stone striae clast pavements fabrics morphology provenance and matrix grain size of four pre late wisconsinan diamictons are presented from the lower accessible part of an 855 m long sea cliff at cape ball east coast of graham island queen charlotte archipelago british columbia the data indicate that an ice stream crossed the continental shelf from the british columbian mainland to the queen charlotte islands local queen charlotte piedmont ice coalesced with the western edge of the cordilleran ice sheet resulting in northward flow deflection along the east coast of graham island generally consistent structural and stone data indicate that diamictons were deposited as primary subglacial tills and that subglacial dynamics and rheologic conditions were complex two of the tills can be traced continuously over the cape ball section which provides a north south transect oblique to ice flow each of the two tills reveals five major changes in rheology and genesis ranging from a strong lodgement component to dominantly deformation mainly by subglacial squeeze flow true end member tills are rare and in almost all places the tills are hybrids that formed within a continuum of till forming processes commonly involving stone rotation and superposition of structures and rheologic states this could have occurred mainly in response to fluctuating till pore water content along the transect two dimensional stretches of contrasting till types probably represent a three dimensional patchwork of contrasting till rheologies and pore waters if patches of ductily deforming till were common and or widespread they could have accounted for much of the movement of cordilleran ice over glaciomarine mud on the continental shelf when sea level was much lower than present deforming probably saturated muddy till reconstituted glaciomarine would have possessed negligible shear strength resulting in a thin rapidly moving glacier most likely an ice stream that reached cape ball but failed to remove the record of previous glacial events the ice stream probably issued out of the mainland s skeena valley which is aligned with cape ball and is approximately the size of valleys in antarctica that today conduct small outlet glaciers and ice streams evidence for this and other ice streams on the northwestern continental shelf of north america imply that destabilization and rapid decay of the western edge of the cordilleran ice sheet may have been accomplished by a combination of ice streams sea level rise and extensive calving

archaean rocks from southeastern karelia karelian granite greenstone terrain abstract we subdivide southeastern karelia vodlozero into two different areas on geological grounds 1 the vodla river area in which the metamorphic complex contains gneisses and amphibolites which yield u pb zircon ages of 3 14 0 03 ga and 3 13 0 09 ga respectively they were affected by polyphase deformation and polymetamorphism during 3 1 and 2 8 ga events 2 the lay ruchey river and vyg river area where two groups of tonalites with u pb ages of 3 17 and 3 14 ga can be distinguished by their major and trace element compositions the first group is similar to some tonalites from the ttg suite in eastern finland martin et al 1983 the second group is similar to tonalites ttg suite from lapland jahn et al 1984 two groups of amphibolites can also be distinguished in the vodlozero region yielding u pb ages of 3 13 and 2 86 ga geological relationships together with geochemical features show the younger group to be comparable to metabasalts from greenstone belts tonalites gneisses and the older amphibolites have model nd ages which are older than the u pb zircon ages from the same suites

glacitectonized lake sediments barrier lake kananaskis country canadian rocky mountains stratigraphic and sedimentological analyses of exposures through a glacilacustrine sedimentary sequence along the south shore of barrier lake kananaskis country reveal evidence of glacitectonic disturbance relating to a readvance of the bow valley glacier at the end of the last glaciation prior to disturbance palaeocurrent measurements in gravel and sand foreset beds record the deposition of subaqueous fans deltas from a glacier lobe retreating eastwards along the barrier lake depression the fan delta sediments fine upwards into ripple and cross bedded sands and laminated muds with dropstones documenting progressively distal sedimentation palaeostress directions measured from large scale folds shear zones and glacitectonites and deformation tills indicate that glacier ice readvanced southwards from a glacier lobe located over the barrier lake depression these stress directions are used to reconstruct the flow lines within the southern margin of a low profile glacier lobe that terminated halfway up lower barrier lake a more extensive readvance than previously envisaged in the area for this period comparisons of diamicton and glacitectonite fabric shapes with similar sediments elsewhere indicate that the subglacially deformed material that caps some of the sections is immature and has undergone short travel distances although the exact age of the readvance is unknown it probably represents the canmore readvance of the late wisconsinan glaciation

the effect of grain texture on the occurrence of microstructural properties in subglacial till in this paper we examine whether till grain size affects the range and occurrence of micromorphological features associated with subglacial shear our till samples were collected from two glaciers in iceland and varied in texture from a coarse sandy clast rich till fjallsjokull to a fine grained silty sandy till vestari hagafellsjokull we found a wide range of deformational microstructures that included skelsepic plasmic fabric intraclasts of pre existing eroded bedrock basalt and weathered clay and mini shear zones between clasts we classified our micromorphological data into three classes rotational intermediate and linear in addition to these observations we performed extensive microfabric analysis at different scales on all of our samples we found that the coarse grained till contained a greater number and variety of microstructures than the fine grained till in addition the fine grained till showed a distinct lack of rotational structures that we attribute to the lack of significantly sized clasts in the matrix we argue that the varied texture of the coarse grained till provides a greater degree of perturbation within the shearing layer and so more distinct microstructures form in a more fine grained till shearing is more homogeneous since there are less perturbations in the matrix and this leads to a more singular kind of microstructure our observations suggest that subglacial shear occurs within a multi layered patchwork of different grain sizes competence and pore water pressures it is these factors that are so crucial in determining the occurrence and type of microstructural evidence we see in subglacial tills

geological evolution of the karelian granite greenstone terrain abstract the karelian craton 2 3 ga has been recognized within the baltic shield the basement of the craton is a late archaean granite greenstone terrain the greenstone belts from different regions of karelia show variations in age and composition of volcanic rocks two older belts 2 9 ga one with multimodal volcanism and the other with bimodal volcanism constitute the eastern part of the terrain central and eastern karelia two younger belts 2 8 2 6 ga with the same modal distributions of volcanic rocks make up the western part of the terrain western karelia and eastern finland according to magnetic anomaly patterns there are four types of granite gneiss areas that separate the greenstone belts three types represent the greenstone basement reworked at different degrees the fourth type corresponds to tonalites and granites surrounding the greenstone belts the emplacement and evolution of the tonalites and granites are related to the evolution of the greenstone belts geochronological data on the granites of the basement demonstrate a decrease in age from e to w late archaean low pressure granulite metamorphism took place almost synchronously 2 65 ga ago in both the western and eastern domains

the saamian of the belomorian mobile belt new geochronological constraints abstract most gneisses and amphibolites of the belomorian belt in the white sea region have previously been considered to represent the oldest saamian crustal component in the baltic shield which has been referred to the early archaean new u pb zircon ages of some of the most typical saamian rocks from the northern segment of the belomorian belt are reported these are 1 2 65 2 60 ga for the main rebolian event of folding high p metamorphism and migmatization 2 2 8 ga for the protolith of an orthogneiss in a granulite facies relic area within the region of rebolian folding on the eastern shore of lake kovdozero 3 2 74 ga for tonalitic orthogneisses that dominate the belomorian gneiss terrain around kandalaksha bay of the white sea 4 2 4 ga for a gabbro granite intrusion on the tolstik peninsula of the white sea this intrusion has previously been considered to be one of the type rock units of the early archaean these and other age determinations previously only reported in russian constrain the age of the assumed saamian in the belomorian belt in conjunction with new field observations they place most of the dated rock units within the same age group as the late archaean lopian rocks as a result of the age determinations the concept of a single array of late archaean geodynamic events in the baltic shield is advanced the belomorian belt is interpreted as an edge of the late archaean granite greenstone terrain in karelia which had been involved in strong structural reworking during an event of tectonic stacking and thrusting 2 65 2 60 ga ago part of the belomorian belt was subsequently also affected by palaeoproterozoic deformation the new results throw doubt on the previously assumed late archaean lopian age of some metavolcanic rock units occurring within the belomorian belt but overlying the presently dated orthogneisses these metavolcanics which comprise for instance the so called kolvitsa belt and its continuation towards the northwest are assigned tentatively to the palaeoproterozoic they may be age equivalents of lapponian sumian 2 5 2 4 ga old rocks underlying the lapland granulite belt

geochemistry and petrology of 2 40 2 45ga magmatic rocks in the north western belomorian belt fennoscandian shield russia abstract widespread mafic and ultramafic rocks drusites and minor granites of 2 40 2 45 ga age occur within the archaean gneisses of the belomorian fold belt several of these mafic ultramafic and acid intrusions in the north western part of that belt have been studied in detail the mafic and ultramafic rocks are rich in si and mg and also show lree enrichment they have nd 2 45 values varying between 0 2 and 1 8 assimilation fractional crystallization afc and mixing models in terms of nd t la sm ratios have been evaluated for the origin of the mafic rocks the results show that an afc model is preferable it involves 20 30 crustal rock assimilation by undepleted picrite magma with fractional crystallization the granites are similar in composition to a type within plate granites the tupaya bay granite pluton is characterized by geochemical layering which supports an anorogenic origin the granites and mafic ultramafic rocks drusites in the belomorian belt are similar in composition to coeval granites acid and mafic volcanic rocks mafic dykes and mafic layered intrusions within the kola and the karelian terrains and can be interpreted as part of the palaeoproterozoic igneous province connected with ascent of a mantle plume the small sizes corona textures and evidence for ductile deformation in the drusites are connected with their deeper level of emplacement p 8 12 kbar than that of the mafic layered intrusions 2 7 kbar

the last glacial maximum in the north sea basin micromorphological evidence of extensive glaciation despite a long history of investigation critical issues regarding the last glacial cycle in northwest europe remain unresolved one of these refers to the extent timing and dynamics of late devensian weichselian glaciation of the north sea basin and whether the british and scandinavian ice sheets were confluent at any time during this period this has been the result of the lack of the detailed sedimentological data required to reconstruct processes and environment of sediments recovered through coring this study presents the results of seismic sedimentological and micromorphological evidence used to reconstruct the depositional processes of regionally extensive seismic units across the north sea basin thin section micromorphology is used here to provide an effective means of discriminating between subglacial and glacimarine sediments from cored samples and deriving process based interpretations from sediment cores on the basis of micromorphology critical formations from the basin have been reinterpreted with consequent stratigraphic implications within the current stratigraphic understanding of the north sea basin a complex reconstruction is suggested with a minimum of three major glacial episodes inferred on at least two occasions during the weichselian devensian the british and scandinavian ice sheets were confluent in the central north sea whilst micromorphology can provide much greater confidence in the interpretation of late quaternary offshore stratigraphic sequences it is noted that a much better geochronology is required to resolve key stratigraphic issues between the onshore and offshore stratigraphic records copyright 2006 john wiley sons ltd

geochronology of an archaean tonalitic gneiss dome in northern finland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone archaean gneiss greenstone relationships are still unresolved in many ancient cratonic terrains although there is growing evidence that most of the late archaean greenstone assemblages were deposited on older tonalitic crust

micromorphological evidence for polyphase deformation of glaciolacustrine sediments from strathspey scotland a combination of field investigation and micromorphological analysis has been applied to polydeformed late devensian rhythmites and glacigenic diamicton exposed in strathspey scotland this provided information on the geometry kinematics and relative ages of ductile and brittle structures and records a complex subglacial deformation history the deformation is interpreted as resulting from a single progressive event associated with over riding of proglacial lake sediments by wet based ice the earliest deformation d1 resulted from compaction loading pure shear and imposed a bedding parallel s1 fabric throughout the rhythmites s1 was subsequently deformed by kink bands and minor ductile shearing during d2 a later d3 event characterized by soft sediment deformation and fluidization of matrix poor sands was accompanied by an increase in pore water pressure this lead to hydrofracturing of the rhythmites the most intense deformation d4 which resulted from simple shear was partitioned into the upper part of the sequence it produced folding thrusting and brittle microfaulting in response to nnw directed ice push these findings indicate that in general subglacial deformation is not homogeneous and can extend to depths of 3 m below the presumed ice sediment interface

evidence from microstructures of deformable bed conditions within drumlins chimney bluffs new york state abstract discussion on the process of drumlin formation has been ongoing for over a century at present two dominant schools of thought prevail viz formation by a subglacial hydraulic mechanisms and b due to subglacial sediment deformation the latter mechanism depends on the existence of a subglacial deformable bed evidence from part of the large central new york drumlin field reveals evidence of syn depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in chimney bluffs state park these diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange style deposition under subglacial deformable bed conditions the evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement it is likely that formation of the drumlins occurred under these same subglacial bed conditions

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

titanite rutile thermochronometry across the boundary between the archaean craton in karelia and the belomorian mobile belt eastern baltic shield abstract u pb isotopic dating has been carried out on titanites and rutiles from the karelian protocraton the belomorian mobile belt and the intervening junction zone these are some of the principal archaean crustal units in the baltic shield which have undergone regeneration to various degrees during the palaeoproterozoic palaeoproterozoic resetting of u pb titanite ages was complete in the belomorian belt and almost complete in the junction zone while it hardly affected the karelian protocraton in the latter major crustal cooling occurred at 2 71 2 69 ga after a major igneous event at 2 74 2 72 ga but a tectonothermal event at 2 65 2 64 ga was less comprehensive in the belomorian belt a northeastern marginal zone immediately underlying the collisional thrusting suture of the lapland kola orogen has somewhat higher titanite ages of ca 1 94 1 87 ga than the central zone where these ages range between 1 87 and 1 82 ga comparison between the titanite and rutile u pb ages suggests a postorogenic cooling rate between 2 and 4 ma in these parts of the belt the neoarchaean junction zone between the karelian and belomorian provinces was a zone of particularly intense tectonic magmatic and hydrothermal activity during or after the palaeoproterozoic lapland kola orogeny dominant newly grown titanites in that zone have ages as young as 1 78 1 75 ga and the age differences between the titanite and rutile u pb ages are substantially smaller than elsewhere

agpaitic magmatism in the northeastern baltic shield a study of the niva intrusion kola peninsula russia abstract the recently discovered niva agpaitic syenite intrusion is located 50 km southwest of the khibina complex central kola peninsula it forms a lense like body 1 5 2 km in size that outcrops over an area of 300 200 m on the southern bank of the imandra lake rb sr 379 21 ma and sm nd 383 58 ma isochron ages of the intrusion coincide with the age of the khibina and lovozero complexes a characteristic feature of the niva rocks is their extremely high agpaicity and enrichment in sr ba y zr nb rb lree and volatiles the mineral association in the agpaitic syenite is k feldspar aegirine diopside biotite lamprophyllite aenigmatite and shcherbakovite isotopic compositions indicate the absence of contamination by continental crust during formation of the niva alkaline magmas a depleted asthenospheric mantle reservoir mixed with a slightly enriched mantle component is suggested to be the main source of the niva magma a metasomatically altered garnet granulite xenolith from the western contact of the niva intrusion contains a primary paragenesis of diopside garnet and plagioclase with chemical characteristics similar to those in the granulite suite of the adjacent lapland zone and lower crustal granulite xenoliths from devonian pipes in the kandalaksha area additionally the granulite contains vinogradovite neptunite pectolite and natrolite which partially replace both primary minerals of the granulite assemblage and secondary minerals which were formed during a later regional metamorphism pt estimates support the lower crustal origin of the xenolith the u pb zircon age of the niva garnet granulite is 2711 7 ma

holocene palaeo oceanography of the northern iceland sea indicated by radiolaria and sponge spicules the micropalaeontological analysis of 49 samples from three box cores collected from the northern iceland plateau indicates that radiolarian and sponge remains are rare in glacial sediments but are common after deglaciation mostly during the first half of the holocene the record trend of the abundance of oxycalthrop sponge spicules is regional in character fluctuations in radiolarian abundance and the first occurrence of each of the species inhabiting the iceland sea at present coincide with changes in oceanographic conditions that occurred during the holocene at the younger dryas chronozone the downcore record of cycladophora davisiana ehrenberg depicts a maximum peak because the sea surface was colder than it has been recently though still seasonally free of ice cover during the preboreal boreal and atlantic chronozones amphimelissa setosa cleve was highly abundant as a consequence of the increase in mixing between the arctic water and the north atlantic water during the subboreal chronozone the east icelandic current became better developed and initiated the onset of the arctic front in the iceland sea as is suggested by the lithomitra lineata ehrenberg record this cooled environment also allowed an increase in the occurrence of artobotrys borealis cleve the reappearance of stylodictya validispina jorgensen in the iceland sea during the sub atlantic chronozone indicates a larger input of atlantic water via the norwegian current

fabric signature of subglacial till deformation breidamerkurjokull iceland the foreland of breidamerkurjokull iceland is the only locality where tills known to have undergone subglacial deformation are exposed till on the foreland has a two tiered structure consisting of a dilatant upper horizon c 0 5 m thick and a compact lower till these horizons correspond to the ductile deforming a horizon and the brittle ductile b horizon observed below the glacier by g s boulton and co workers the relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till the upper horizon exhibits variable a axis fabrics and abundant evidence for clast re alignment reflecting ductile flow and rapid clast response to transient strains in contrast the lower horizon has consistently well organized a axis fabrics with a narrow range of dip values recording clast rotation into parallel with strain axes during brittle or brittle ductile shear the data indicate that till strain history imparts identifiable macrofabric signatures providing important analogues to guide the interpretation of pleistocene tills

till fabric associated with deformable beds this paper investigates the development of till fabric associated with glaciers overlying deformable beds first the till fabrics from both modern and ancient soft bed and hard bed subglacial environments are investigated and then soft bedded sites with specific strain histories are examined the results indicate that lodgement tills had a strong fabric whilst there is a wide range of much weaker fabric strengths associated with soft bed tills it is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development field sites were chosen to isolate these two factors and fabrics investigated in areas of known relative shear strain and known relative thickness of the deforming layer the results indicate that 1 for a fixed deforming layer fabric strength does increase with increasing deformation except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain and 2 lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength whilst soft bed tills associated with a thick deforming layer have a low fabric strength where the deforming layer is thin clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed however where the deforming layer is thicker and shear strains are high melt out processes encourage a parallel fabric but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric these processes combine to reduce the overall fabric strength till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till

smnd evidence for late archaean crust formation in the lapland kola mobile belt kola peninsula russia and norway abstract a sm nd model age traverse across the kola peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early archaean saamian crust in the region granitoid gneisses and subordinate metasediments have been sampled from the main archaean geological units i e from northeast to southwest the murmansk domain model ages of 2 68 2 96 ga central kola domain including samples from the superdeep borehole 2 74 2 92 ga belomorian domain including the eastern part of the tersk area 2 70 2 93 ga the narrow range of depleted mantle model ages from 2 68 to 2 96 ga for archaean granitoid orthogneisses close to the oldest crystallization ages from the region indicate a major episode of late archaean crustal growth and reveal no indication of ancient saamian material in keeping with other geochronological evidence similar results 2 86 to 2 93 ga for archaean metasediments support this conclusion proterozoic model ages for supracrustals from the western part of the tersk area 2 16 to 2 23 ga indicate that the crust in this area is much younger than previously recognized initial nd values for late archaean samples with reasonable age constraints fall within a range of 2 3 to 2 5 two well dated samples from the belomorian domain have the highest initial nd values nd 2 81 ga 2 0 and nd 2 78 ga 2 3 and suggest that a depleted mantle source was present in the kola region in late archaean times dated samples from the central kola domain have lower initial nd between 2 5 and 1 2 suggesting a possible regional difference in mantle character in contrast to the depleted signature of the archaean orthogneisses between 2 7 ga and 2 8 ga previous studies of earliest palaeoproterozoic rift related majic intrusives in the kola and karelia regions report negative nd values at 2 4 2 5 ga possibly these data reflect a change to more enriched mantle values

on the interpretation of subglacial till fabric abstract the modal distribution of stone long axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till in this paper we offer a two axis diagram that compares fabric modality to fabric isotropy s3 s1 and addresses the problem of eigenvectors falling between the modes of some well developed till fabrics with low eigenvalues our simple five fold scheme of modality categories includes 1 unimodal clusters 2 spread unimodal 3 bimodal clusters 4 spread bimodal and 5 polymodal to girdle like fabrics and requires the analyst to study equal area lower hemisphere schmidt plots of the fabric data after assigning the fabric to a morality category isotropy is calculated and both results are plotted on the graph whic helps to separate two main fields of subglacial till 1 lodgement and subglacial meltout tills and 2 deformation fill on the basis of selected published fabrics from tills at modern glaciers as well as our own pleistocene till data lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics in contrast deformation tills and tills that experienced multiple processes tend to have polymodal to girdle like fabrics some overlap occurs between fields be cause of the complex nature of till formation i e because pure end member till facies are rare and most tills are hybrids we strongly recommend that schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till however fabric data alone is not enough multiple criteria including structural lithologic and stone morphologic data from the till must also be considered before drawing conclusions on till genesis furthermore if eigenvectors fall between fabric modes then they cannot be used to indicate former ice movement directions finally our new modality isotropy diagram may have wider applications

characterization of pebble fabrics in modern terrestrial glacigenic sediments pebble fabric data are available from several facies of glacigenic sediments deposited by modern glaciers where sedimentary processes can be observed or inferred with relatively little ambiguity over 100 samples from contemporary environments illustrate fabrics characterizing melt out till deformed and undeformed lodgement till sediment flow deposits and ice slope colluvium lodgement till fabric variability is related to the two layer structure of these sediments a structureless friable upper layer with low shear strength and high consolidation coefficient overlying a very compact material of horizontal platy structure fabric strength assessed by eigenvalue analysis is weaker and pebble dip is more dispersed in the upper structureless horizon stronger fabrics in the lower platy horizon may be primary depositional fabrics which are destroyed by subglacial shearing to give weaker fabrics in the upper horizon alternatively upper horizon fabrics may be characteristic of all recently deposited lodgement tills with stronger fabrics developing at depth by dewatering and consolidation there is a general reduction in fabric strength and an increase in particle dip associated with the transition from melt out tills through undeformed and deformed lodgement tills to sediment flow deposits and ice slope colluvium there is however considerable overlap in the fabric strengths characteristic of sediment flow deposits and deformed lodgement tills fabric data from modern glacial sedimentary facies are used to assist in interpreting the mode of deposition of some quaternary glacial sediments relatively strong fabrics characteristic of melt out tills and undeformed lodgement tills are more likely to be diagnostic of genesis than weaker fabrics associated with deformed sediments

age tectonic setting and provenance of ostfold marstrand belt supracrustals westward crustal growth of the baltic shield at 1760 ma the ostfold marstrand paragneiss belt is the oldest part of the sw scandinavian domain of the baltic shield in the swedish part of the belt minor mafic volcanics now amphibolites in the metagreywacke dominated stora le marstrand slm supracrustals are chemically similar to primitive modern oceanic arcs or back arc regions a wide compositional range is attributed mainly to fractional crystallisation yielding fe rich andesitic amphibolites diverging lree patterns between different localities are consistent with variations in the degree of partial melting in the source a nine point sm nd errorchron of 1758 78 ma mswd 9 4 for the amphibolites is interpreted to date the volcanism and coeval sedimentation and hence directly dates crustal growth in this part of the baltic shield the exclusion of two pillow lava samples which may have been contaminated yields an isochron with an identical though more precise age an nd value of 4 3 indicates a depleted mantle source similar to that of canada sw u s a s greenland and parts of finland but rather more depleted than that reported hitherto from eastern sweden at the same time nd isotopic data suggest a two component provenance for the slm sediments involving acid derivatives from the mafic volcanics and a continental source similar to the exposed 2 0 1 75 ga old svecofennian crust of eastern sweden with minimal contribution from substantially older crust the mafic volcanism took place during subduction of oceanic crust outboard of the svecofennian part of the baltic shield after substantial addition of 1650 ma old granitoids accretion to the baltic shield possibly took place at 1580 ma

juvenile helium in ancient rocks ii u he k ar sm nd and rb sr systematics in the monche pluton ratios frozen in uranium free ultramafic rocks abstract the important geodynamic parameter the 3 he 4 he ratio in rocks and fluids of the continental crust is generally decreasing from the mantle values 10 5 to the radiogenic ratio 10 8 on the time scale of about 1 ga or less however the ratios observed in some ancient rocks and minerals are much higher than the radiogenic value due to a preferential retention of trapped he when compared with radiogenic helium and or a low u 3 he ratio in a sample the distribution of he ar nd and sr isotopes k rb sm and u in ultrabasic rocks in rock forming minerals in ores from the 2 49 ga monche pluton and in basic rocks of the main range the kola peninsula enables us to distinguish sources of the rocks and trapped fluids and outline some peculiarities of petrogenetic and fluid processes the initial values of nd t 0 9 0 5 87 sr 86 sr t 0 7021 0 0002 for the 2 49 ga monche pluton are rather similar to these for other layered intrusions of the baltic shield they differ considerably from the model values for the depleted 2 5 ga old mantle probably the melts originated in the specific enriched subcontinental mantle 4he concentrations of 10 480 10 6 cm 3 g and u concentrations of 0 01 0 001 ppm in rocks of the pluton are translated into atomic ratio of 4 he u up to 500 and u he age up to 10 ga these rocks contain dominantly trapped he despite the fact that u in gabbros from the main range is about two orders of magnitude higher this conclusion appears to be valid because of better retention of trapped he as compared with radiogenic one the 3 he 4 he ratios 0 16 0 74 10 6 exceed considerably the radiogenic value but are much lower than those in the mantle due to a contribution of crustal he isotopic and geochemical evidence implies that a melt from which the ultrabasic rocks were formed originated from the mantle thus crust mantle fluid occurred in the mantle melt geological observations and the relationship between 3 he 4 he and mg fe mg values show that the high magnesium divines formed during melt ascent i e under higher temperatures and probably at slightly greater depth as compared with those parameters for the magma chamber the observed and rather different sources of3 he and 4he as well as the con siderable constancy of their ratio in different minerals separated from both the ultramafic rocks and gabbros implies 1 1 an intensive process of mixing between mantle and crustal components a melt convection in the chamber may have occurred 2 2 the two types of rocks could originate by crystallization differentiation of one and the same melt moreover similar ratios of 3 he 4 he 0 2 10 6 are observed in the enclosing gneisses of the kola belomorian series practically all 3he and 4he are concentrated in secondary amphiboles hence the fluid which stimulated the metamorphic process was probably released from the ultramafite bearing melt the source of trapped 4he and 40ar corrected for air appears to be the continental crust but their ratio varies over a wide range in contrast to approximately constant 3 he 4 he ratio degassing of the melt is probably responsible for extremely high 4 he ar values in some minerals

multi chronometric ages and origin of archaean tonalitic gneisses in finnish lapland a case for long crustal residence time the tojottamanselka gneisses of the koitelainen region northern finland have been dated by the sm nd and the common pb methods the sm nd data of seven samples from a small area 100 m 100 m define an isochron oft 3 06 0 12 2 ga with correspondingind 0 50848 9 2 orend t 3 7 1 8 this age is in good agreement with the zircon u pb discordia age 3 1 ga reported by kroner et al 1981 and is interpreted as the time of magmatic emplacement the distinctly negativeend t value is found for the first time for archean tonalitic gneisses and implies derivation of these magmas by remelting of continental material with a long 200 500 ma crustal residence time a few samples on the other hand possessend t values close to zero hence they are thought to be derived by partial melting of basaltic sources with nearchondritic ree distribution patterns

particle and aggregate mobility in till microscopic evidence of subglacial processes abstract microscopic studies of tills show that the deforming bed of glaciers is made up of minute rotational elements or wheels consisting of a nucleus alone or a nucleus with spiralling arms of fine grains these fine grains can move from one wheel to the next providing a process of high grain mobility within a deforming subglacial till recognition of this process explains a variety of glacial distribution patterns i e the mineralogical reflection of subjacent lithologies in the till matrix the side by side occurrence of far travelled erratics and locally derived matrix and the shape of geochemical anomalies

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

approaches to the study of glacier bed deformation investigation of glacier bed conditions and subglacial processes is an obligatory requirement for the modelling and understanding of contemporary glacier behaviour past glacier behaviour glacier response to both spatial and temporal variations in bed material and glacier response to external forcing factors such as climate change the recognition of the importance of coupling between the glacier and its bed resulted in a paradigm shift in the subject and drew attention to the critical importance of subglacial deformation as opposed to simply subglacial deposition and sliding a number of methods have been adopted to measure glacier bed deformation ranging from direct observation and instrumentation the interpretation of macroscale and microscale sedimentary structures to the interpretation of glacier bedforms and glacier bed modelling this paper is concerned to describe the methods used by the two authors and co workers and attention is given to field methods the type and arrangement of sedimentary structures thin section micromorphology and microfabric and clast microfabric and glacier bedform morphology as evidence of subglacial deformation sedimentary structures macro and mesoscale are used to identify constructional deformation where deformational units are preserved one on top of another due to the formation of a thin deforming layer by low basal stresses and excavational deformation in which the deformational style is superimposed on pre existing styles due to increasing thicknesses of glacier deformation caused by increasing basal shear stresses thin section micromorphology has been used to define a range of glacial processes of which subglacial deformation is just one but has drawn attention to critical criteria such as discrete microscale shears orientated matrix kink structures and fractured grains as well as deformed intraclasts that can be used to recognise deformational processes more recently individual aggregates taking the form of rounded pebbles within till matrix have been shown to reflect rotational processes during glacial deformation and provide an additional mechanism to discrete shear within subglacial deformation and an explanation for the transport paths of subglacial materials finally measurements of sediment flow directions using clast macrofabric characteristics coupled with measurement of quantities of sediment transfered and detailed palaeoglacial properties have been used to determine the rates and scales of subglacial deformation associated with the formation of small scale glacier bedforms

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

nd isotopic evidence for the antiquity of the wyoming province sm nd isotopic data on late archean age metasedimentary rocks from around the wyoming province yield nd values at 2 6 ga from 12 9 to 3 3 which correspond to depleted mantle model ages of 2 7 to 3 8 ga these results indicate that crust of a wide variety of ages was exposed and eroding in late archean time the predominance of middle archean model ages suggests that continental crust was extensive in the wyoming province by this time no resolvable correlation exists between initial nd value and geographic location or depositional environment of the metasedimentary rock samples thus there is no indication that any part of the province was established at a time different from the rest the nd isotopic characteristics of archean rocks from the wyoming province contrast with those of the superior province confirming that although their proterozoic evolution has been parallel the two cratons did not share a common archean history

a sequential back arc and foreland basin thrust duplex model for the southern uplands of scotland thrust imbrication of ordovician and silurian submarine fan sequences overlapping pelagic deposits in the southern uplands has been interpreted in terms of an accretionary prism formed above a nw directed subduction zone structural features invoked to support accretion are not definitive and could be explained in terms of a thin skinned thrusting model new palaeocurrent and compositional evidence from llandeilo to mid llandovery age turbidites in the northern part of the southern uplands proves interdigitation of sediments with strongly contrasting petrography turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz rich formations this implies a back arc situation the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south oblique collision of the opposing continental margins of the iapetus ocean during the llandovery caused the cessation of subduction underthrusting of the southern margin initiated a se propagating thrust stack which deformed the back arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc a southward migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late llandovery hawick group and wenlock sequences mid to end silurian sinistral strike slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep seated structures allowed the intrusion of lamprophyre dykes and granites

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

does clast size influence fabric strength abstract clast fabric strength is commonly used as a quantitative tool to determine the depositional history of subglacial tills this paper emphasizes the variance in fabric strength relative to clast size data on 46 clast fabric analyses were sampled from two basal till localities in southeast denmark and another eleven analyses from a basal till locality beyond a glacier in south iceland it is demonstrated that clasts longer than 2 3 cm show a stronger preference for parallel orientation than do shorter clasts this implies that two operators sampling fabric data a few meters apart within the same till unit but selecting clasts longer or shorter than 20 mm may classify the till as two different types of basal till because of differences in clast size alone additionally this study demonstrates considerable variations in fabric strength over short lateral distances within the same basal till bed it is concluded that caution should be exercised in using eigenvalues to infer depositional processes and strain histories in subglacial environments sedimentological criteria other than clast fabric must also be considered before drawing final conclusions on subglacial deposition

diffusive mixing between shearing granular layers constraints on bed deformation from till contacts shearing of subglacial till has been invoked widely as a mechanism of glacier motion and sediment transport but standard indicators for determining shear strain from the geologic record are not adequate for estimating the very high strains required of the bed deformation model here we describe a laboratory study of mixing between shearing granular layers that allows an upper limit to be placed on bed shear strain in the vicinity of till contacts owing to random vertical motions of particles induced by shearing mixing can be modeled as a linearly diffusive process and so can be characterized with a single mixing coefficient d ring shear experiments with equigranular beads and lithologically distinct tills provide the value of d although in experiments with till d decreases systematically with strain to a minimum value of 0 0045 mm 2 kinetic gas theory provides an estimate of the dimensionless mixing coefficient which is within an order of magnitude of laboratory values knowing the minimum value of d the distribution of index lithologies measured across till contacts in the geologic record can be used to estimate the maximum shear strain that has occurred across till contacts application of this technique to the contact between the des moines and superior lobe tills in east central minnesota u s a indicates that shear strain did not exceed 15 000 at the depth of the contact

till fabric patterns and significance particle response to subglacial stress abstract the analysis of particle arrangements within glacial sediments fabric has long been considered to reflect the behaviour of the ice that transported and deposited or deformed the sediment particles can develop specific fabric arrangements as a response to bulk sediment strain driven by effective stress at the bed of a glacier as such fabric analysis has been used to try and understand the complex relationship between glacier and ice sheet dynamics and the geological evidence of former glaciation developments have focused on the use of stereonet patterns and statistical measures vector and eigenvector analysis of clast fabric to infer the process of sediment deposition deformation and consequent ice flow directions and dynamics however as this study identifies fundamental processes of particle orientation march jeffery and taylor rotation mechanisms would suggest more complex explanations than have been previously reported this study reports the results of a detailed field and laboratory investigation of macro scale clast and microfabrics from a till sequence in central scotland the results suggest a systematic behaviour of different particle size fractions which may be interpreted in terms of the strain response of sediment three fabric patterns are identified that illustrate consistent progressively shifting and more complex particle behaviour it is suggested these orientation patterns indicate the degree to which subglacial sediment has undergone strain and the nature of the deforming bed erosional or accretionary it is concluded that this approach may yield fundamental information about the strain conditions underlying different ice masses providing critical data linking glaciological modelling with the sedimentary evidence of glaciation

petrology of nattanen type granite complexes northern finland abstract the nattanen type granites form a group of post tectonic and post metamorphic 1 77 ga old epizonal multiple granite intrusions in the granulite belt granitic gneiss surroundings in northern finland the granite complexes the tepasto pomovaara and nattanen stocks and the riestovaara batholith in finland and the juvoaivi stock in the u s s r are composed of various slightly peraluminous microcline plagioclasebiotite granites monzogranites that contain sphene and magnetite as typical minor constituents geochemical evolution within the complexes is demonstrated by the increase of sio2 and rb and decrease of tio2 feo mgo cao sr ba and zr from the oldest to the youngest granite varieties molybdenum mineralization is associated with the latest intrusive phases of the tepasto and juvoaivi stocks the isotope geochemical studies indicate a major archaean crustal component in the source of the nattanen stock that is situated within the early proterozoic granulite belt

a revised geodynamic model for the lapland kola orogen abstract the paleoproterozoic lapland kola orogen in fennoscandia has been studied for decades and several plate tectonic models have been proposed including one sided subduction zone either towards sw or nw or two opposite verging subduction zones before the collision based on new structural and isotope data from finland and recently published data from russia we propose a revised tectonic model for the paleoproterozoic lapland kola orogen the main components are foreland in the ne followed by cryptic suture inari arc retro arc basin and retro arc foreland in the sw the latter three constitute the inari orocline subduction towards present sw and subsequent arc magmatism inari arc started at ca 1 98 ga followed by voluminous sedimentation in the deepening retro arc basin underplating of a mid ocean ridge caused flat subduction and magmatic flare at 1 92 ga over a broad distance in the retro arc basin rapid heating led to melting of the retro arc basin sediments and voluminous amounts of granulite facies diatexites formed during collision d1 at 1915 1910 ma large thrust nappes formed on the foreland deformation in the retro arc basin is seen as recumbent folding and shearing of diatexites in the lower parts of the basin and thrusting of metatextite diatexite packages in the upper parts a post collisional stage is seen as 1904 ma appinites and decompression derived granites at 1 90 1 89 ga renewed shortening d2 due to far field effects in sw at 1 88 1 87 ga led to thick skin shortening of the archean middle crust large scale crustal duplexing of already cooled granulites towards the retro arc foreland and inclined upright folding of granulites in the opposite direction towards the inari arc a switch in the stress field from ne sw to nw se led to orogen parallel contraction and buckling started along a dextral strike slip fault zone to form the inari orocline buckling is seen in the bending of pre orocline fabrics and formation of syn orocline fabrics radial conical folds d3 radial fractures a strike slip fault zone and thrusting at the hinge zone the end result is a mega scale parallel multi layer fold composed of the inari arc retro arc basin and possibly also the heated retro arc foreland

apparent partial loss age spectra of neoarchean hornblende murmansk terrane kola peninsula russia the role of biotite inclusions revealed by 40ar 39ar laserprobe analysis metamorphic hornblende frequently yields spectra with progressively increasing 40ar 39ar age steps often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid state diffusion yet in many cases rising ca k ratio spectra for such samples imply the presence of minor inclusions of k contaminant minerals to avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope laser step heating of drilled biotite free hornblende discs yielded flat age and ratio spectra in contrast furnace step heated hornblende separates from the same samples produced apparent loss age spectra moreover biotite free samples yielded flat spectra by laser and furnace dating consequently apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step heating c 2640 ma hornblende ages constrain the murmansk terrane s cooling

isotope data on the age and genesis of layered basic ultrabasic intrusions in the kola peninsula and northern karelia northeastern baltic shield abstract new u pb zircon ages and sm nd data are presented for layered basic ultrabasic intrusions of the kola peninsula and northern karelia in the northeastern part of the baltic shield all ages are in the range between 2 5 and 2 4 ga but there appear to exist three phases of intrusion thus the monchegorsk intrusion and the fedorovy pansky massif have ages between 2493 and 2470 ma the kivakka and lukkulaisvaara massifs of the olanga group of intrusions between 2445 and 2435 ma while the imandra lopolith is 2396 7 ma old a narrow range of negative nd t values between 2 3 and 1 2 suggests formation by partial melting of a semi homogeneous enriched mantle source

microstructures in diamictons evidence of subglacial bed conditions abstract examination of microstructures within diamictons from southern ontario canada and southern bavaria and baden wurttemburg germany reveal evidence illustrative of certain subglacial bed conditions the microstructures from ontario were obtained from a complex glacial melange unit that forms the internal structure of an isolated drumlin megaflute on the north shore of lake erie the evidence from germany was taken from individual drumlins within drumlin fields in the inn chiemsee iller rhein and salzach glacier areas the structures within the diamicton from ontario reveal clear evidence of massive shear associated with high externally applied strain there are kink band arrays high angle shear zones crenulation and sedimentary dyke and water escapes structures in the german diamictons there is a lack of evidence of high levels of bulk deformation there is however a very high carbonate content some evidence points to the possibility that these diamictons may contain intraclasts of paleosols interpretation of both sets of diamictons suggests that the ontario sediments were subject to conditions at the bed of an ice mass best typified as a deformable bed the german sediments may be interpreted as 1 being so saturated that any deformation must have been by inter grain slip pervasive deformation and thus no structures are recorded or 2 being already cemented by the carbonates and sufficiently rigid to cause strain to be taken up elsewhere or 3 were frozen prior to transposition and were again too rigid to suffer deformation or 4 suffered no deformation whatsoever the analyses of these two sets of sediments have important implications for subglacial conditions before and during drumlin and other subglacial bedform development

polyphase deformation at an oscillating ice margin following the loch lomond readvance central scotland uk the sequence of glacitectonic disturbance of an ice contact delta during the initial stages of deglaciation following the loch lomond readvance is examined an ice marginal gilbert type deltaic sequence exposed in drumbeg quarry drymen scotland displays a polyphase deformation history that is punctuated by periods of erosion and deposition progradation of the delta into ice dammed lake blane led to a temporary stabilisation of the loch lomond glacier during recession from its loch lomond readvance maximum position this was followed by a ne directed readvance into the ice contact slope of the delta resulting in proglacial deformation d1 and the formation of a thrust block ridge in the middle to outer parts of the delta complex sedimentation continued uninterrupted subsequent retreat of the ice was accompanied by erosion and deposition as the ice contact slope was re established a second phase of readvance resulted in subglacial ductile shearing d2 and deposition of a diamicton during ne directed over riding of the delta sediments by the ice the direction of ice flow subsequently changed towards the se this was followed by a further retreat of the ice and re establishment of the fan delta complex the complexity of the glacitectonic sequence at drumbeg records the impact of an oscillating ice margin responding to either relative deepening of the lake waters in contact with the receding snout or climatic controls it provides further evidence that many of the scottish highland glaciers were subject to active recession rather than stagnation at the end of the loch lomond readvance

late archaean smnd model ages from the voche lambina area implications for the age distribution of archaean crust in the kola peninsula russia abstract although an early archaean age for the saamian gneisses has been generally accepted in the former soviet union partly on the basis of their commonly granulite facies metamorphic grade and partly on account of their structural complexity and complex intrusion history these rocks have yielded no precise isotopic ages in excess of 2 9 ga sm nd isotopic analyses are used to test the possible presence of early archaean 3 5 ga saamian gneisses at voche lambina in the kola peninsula here tonalitic trondhjemitic and granodioritic gneisses presumed to be early archaean saamian in age are overlain by a sequence of late archaean lopian supracrustals a sample of the structurally oldest trondhjemitic gneiss with a minimum age of 2 64 ga has a depleted mantle model age tdm of 2875 ma tonalitic gneiss also assumed on structural grounds to be early archaean has a tdm age of 2808 ma identical to its u pb zircon age of 2807 9 ma this suggests that a depleted mantle model may be appropriate for at least this part of the kola region in the late archaean lopian rhyodacite with a u pb zircon age of 2800 ma and a rb sr isochron age of 2540 83 ma has a tdm age of 2756 ma in each case the close agreement between the sm nd model age and the time of intrusion or eruption leaves little scope for a significantly older crustal history for these rocks the data provide no evidence for a saamian component in this part of the kola peninsula

sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system abstract the extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed new evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments it is concluded that in most experimental sites beneath soft bedded modern glaciers deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process particular modes of deformation facilitate incorporation of underlying material into the till whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation leading to the commonly found relationship where till overlies other strata with a sharp planar interface it is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them with important implications for the coupling of ice sheet flow and bed properties it is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes they show that rapid glacier advances can produce undrained loading of sediments that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation

microscopic evidence of subglacial deformation abstract subglacial deformation is by now a well established process however recognition of this process in pleistocene tills is not always clear studies of thin sections of pleistocene tills show that there are a variety of microscopic features caused by deformation this paper gives a review of microscopic evidence for subglacial deformation and starts with a case where deformation could be established macroscopically then traces the microscopic features through planar and rotational movements which are described and separated accordingly attention is also given to features caused by crushing of grains and sediment dewatering these studies indicate that almost all basal tills show evidence of subglacial deformation rather than sedimentary derivation this implies that if we want to understand subglacial processes such tills should be described in tectonic rather than sedimentary terminology

glacigenic clast fabrics genetic fingerprint or wishful thinking the interpretation of glacigenic diamictons is a subjective process for which quantitative support is frequently sought from parameters such as clast shape and fabric it has been widely suggested that different glacigenic diamicton facies possess distinct clast fabric signatures this paper examines this concept using a data set of 111 clast fabrics and a synthesis of published results eigenvalues are calculated and compared for a variety sedimentary facies it is concluded that clast fabric alone is not able to discriminate between different glacigenic facies and it is argued that clast fabric offers little quantitative support in the interpretation of glacigenic sediments it is suggested therefore that although clast fabric may continue to have a role as an indicator of relative strain at specific sites its use in the discrimination of glacigenic facies is limited consequently we should be much more selective in undertaking such analyses in the future copyright 1999 john wiley sons ltd

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

discussion and reply clast fabric development in a shearing granular material implications for subglacial till and fault gouge discussion hooyer and iverson 2000 1 presented the results of shear box experiments performed on till matrix material obtained from below storglaciaren and they discussed their implications for mechanisms of fabric development in subglacial deforming layers and fault gouge the results of their well

archaean tonalitic gneiss of finnish lapland revisited zircon ion microprobe ages we report single grain and grain domain u pb zircon ages for the tojottamanselka tonalitic gneiss previously investigated by the whole rock rb sr pb pb and sm nd methods by conventional u pb zircon density size fraction analysis and by hf isotopes kroner et al 1981 patchett et al 1981 jahn et al 1984 and established as one of the oldest known rocks of the baltic shield our data confirm the intrusive age as 3115 29 ma standard error but we also found slightly older xenocrystic zircon cores with 207pb 206pb ages between 3161 19 and 3248 10 ma that may either be derived from earlier phases of the tonalite melt or from pre tonalite sialic crust new magmatic zircon growth probably during a metamorphic event that led to migmatization is recorded by an age of 2836 30 ma and may be coeval with widespread tonalite emplacement elsewhere in the northern baltic shield at about this time

evolution of the precambrian continental crust in eastern china abstract based on 1143 sm nd analytical data the precambrian continental crustal accretion and evolution in eastern china are discussed in this paper eastern china is generally subdivided into five geological units from north to south these are the xiang an fold belt north china block qinling fold belt yangtze block and cathaysian block the most important crust mantle fractionation of the north china block occurred in the archaean whereas that of the other terrains of eastern china took place in the proterozoic the nd isotopic character in the north china block is similar to that of the tarim block of western china and the aldan shield of arabia but is different from the other terrains of eastern china there may be several micronucleii in the north china block which are eoarchaean and palaeoarchaean in age they are the oldest crustal remnants and around these micronucleii the lateral and vertical growth including underplating and overplating occurred at least four important stages of precambrian accretion of the north china block are identified we propose here a multi cycle model of precambrian continental accretion

effect of mechanical interactions on the development of shape preferred orientations a two dimensional experimental approach abstract modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method for increased concentration of rigid particles the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours in coaxial flow the fabric intensity is consequently reduced in simple shear flow the fabric evolution is no longer cyclic the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane in the case of preferred orientations of particles having a low aspect ratio 2 5 the fabric rotates through the shear plane then undergoes a reverse rotation and rotates toward the shear plane again in the light of these experimental results we emphasize that quantification of finite strain based on such fabrics may lead to significant underestimates and calculated values should be taken as minimum estimates the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows the concentrated population fabrics show an angular behaviour close to that of passive markers they tend to align parallel to the shear plane in simple shear flow