contrasting modes of reactivation in the outer hebrides fault zone northern barra scotland the gently ese dipping outer hebrides fault zone may be traced for c 170 km through the lewisian gneisses of northwestern scotland and defines broad arcuate swings in strike from ne sw to almost north south this study evaluates the kinematic evolution of the fault zone within the hinge region of such a strike swing exposed in the small isles north of barra new structural mapping of this area has concentrated on the geometry kinematic significance and the relative age of different structural elements of the fault zone and provides a comparison with the better known tectonics of the northern portions of the outer hebrides fault zone hitherto unrecognized top to the nw ductile thrusting in this southern portion of the fault zone coupled with abundant evidence of extensional reactivation and lack of sinistral strike slip in phyllonites as reported further north suggests that the present segmented and arcuate geometry is a primary feature of the fault system the original thrust template governed the subsequent reactivation and relative importance of both late caledonian strike slip and extensional collapse components inherited structural architectures may thus provide geometric constraints on the nature and patterns of subsequent reactivation and tectonism

time variation in coarse materials from lake bottom sediments and secular paleoclimatic change variations in the percentage of materials coarser than 4 5 phi in sediment from lake biwa are closely related to global climatic change and are indicative of large fluctuations in sediment discharge into the lake coarse materials increased markedly during the interglacials postglacial and last interglacial periods the variations also are linked with solar insolation and have distinct periods of about 20 000 and 40 000 years which correspond to the periods of the obliquitical and processional parameters periods of 60 000 and 120 000 years are major in the variations

relationships between the loch ailsh and borralan alkaline intrusions and thrusting in the moine thrust zone southern assynt culmination nw scotland

microstructures mineral chemistry and geochronology of white micas along a retrograde evolution an example from the aar massif central alps switzerland abstract this study investigates gneissic tectonites of different mechanical origins and aims at unravelling the link between microstructures white mica chemistry and k ar data the highest temperature mylonitic deformation which is investigated is dominated by dislocation creep in monomineralic quartz and viscous granular flow in polymineralic domains these mylonites show homogenous mineral chemistry resulting in robust and consistent k ar age data the lowest temperature deformation investigated is that of a retrograde brittle deformation producing cohesionless fault gouges within the cohesionless fault gouges frictional granular flow is considered as the main deformation mechanism the k bearing phases inside the fault gouges record no chemical resetting during this brittle overprint but contain mechanically fragmented k bearing micas and significant amounts of newly produced smectite this yields k ar ages inside of a gouge that are in the range of adjacent mylonite ages and may not represent the timing of gouge formation in an effort to understand these uncertainties in k ar ages detailed observations of the microstructural context of k bearing white mica were made microstructural criteria in combination with quantitative element mapping allowed for the discrimination of five important mica related processes 1 fracturing recrystallization of white mica 2 re precipitation of new fine grained white mica inside the deforming rock matrix 3 pseudomorphic replacement of precursor minerals by white mica and chlorite 4 pseudomorphic replacement of pre existing white mica without grain size reduction and 5 remnant grains the combination of these different processes led to a high variability in mineral chemistry and isotope data for tectonites which relates directly to variations in the size of the equilibrium volumes despite the samples being highly deformed in the presence of fluids inherited sheet silicates may survive if inherited sheet silicates are present they contribute to an increase in both the chemical heterogeneity and the apparent k ar ages of a sample

understanding kinematic data from the moine thrust zone in terms of a kinematics based mathematical model of deforming thrust wedges abstract a salient recess pair along the moine thrust zone exhibits significant lateral kinematic variations specifically the more steeply dipping portion of the thrust surface associated with the recess shows evidence for greater strain magnitudes and more out of the plane motion than the adjoining salient additionally a subset of the grain shape data has grain shape approximations with their long axes oriented perpendicular to the regional transport direction to more fully understand these kinematic features we developed a kinematics based mathematical model of thrust wedge deformation our numerical model predicts an increase in strain magnitude as well as non plane strain flattening in the region of a thrust ramp consistent with the strain patterns observed within the recess along the moine thrust zone furthermore in the absence of a lateral confining boundary condition much of the model thrust wedge experiences transport perpendicular maximum extension except for the region closest to the thrust where simple shear deformation is dominant using our numerical model we predict the amount of lateral confining strain needed to induce plane strain deformation ps as well as the amount of lateral confining strain necessary to make the long axis of the strain ellipsoid parallel with the transport direction parallel we have found that the incremental parallel can be as much as 42 of the incremental strain magnitude thus we consider transport perpendicular lineations like those seen along the moine thrust zone to be the result of lateral confining boundary conditions that are weak enough to allow transport perpendicular maximum extension

detrital muscovite 40ar 39ar ages from carboniferous sandstones of the british isles provenance and implications for the uplift history of orogenic belts major progradations of clastic sediments are recorded in the sedimentary record of the famennian and the visean namurian of the united kingdom and surrounding waters we have determined 40ar 39ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 ma spanning both progradations detrital mica ages are dominated by a peak at 415 ma with minor peaks at 440 ma and 390 ma the 415 ma muscovites are derived from the unroofing of the scandian nappes during the compressional phase of the caledonian orogeny in scandinavia the 440 ma muscovites record pre scandian orogenic activity which is rarely preserved in the orogenic record thermochronological evidence suggests that episodic postorogenic uplift and exhumation events kept the scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations the near total absence of detrital muscovites with ages 415 ma suggests that the scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension the river s which supplied the sediments probably ran parallel to the strike of the major scandian thrusts along the length of the caledonian orogen in a manner analogous to the major river systems of contemporary orogenic highlands

a model for the deep structure of the moine thrust zone seismic refraction and electrical conductivity profiles across northern scotland together with geological and other geophysical data are interpreted in terms of the deep structure of this part of the orthotectonic caledonides our preferred interpretation is that the moine thrust zone extends to the base of the crust and the caledonian front is steep this contrasts with recently proposed ultra thin skinned solutions the results of the recently completed moist deep seismic reflection experiment should inter alia test our hypothesis in order to test the geometrical feasibility of our interpretation balanced sections have been constructed to illustrate the crustal evolution of the region during the caledonian orogeny these sections permit estimates to be made of crustal shortening thickening and erosion our model requires that the n highlands were once overlain by major exotic nappes which by analogy with nw newfoundland may have been ophiolitic the proposed regional structural pattern comprises a duplex of crustal dimensions we suggest that crustal duplex development is an important mechanism in the orogenic deformation of previously thinned and extended passive continental margins

dating deformation and cooling in the caledonian thrust nappes of north sutherland scotland insights from 40ar 39ar and rb sr chronology 40 ar 39 ar and rb sr mineral ages have been determined from various lithologies exposed in the caledonian foreland and structurally overlying thrust nappes of north sutherland scotland rb sr muscovite ages of c 428 c 421 and c 413 ma obtained from moine thrust zone mylonites are interpreted to date closely regional thrusting during the late silurian to early devonian 40 ar 39 ar muscovite ages within the lower parts of the moine nappe are mostly anomalously old with respect to rb sr analyses of muscovites from the same samples it is likely that this discrepancy results from a component of extraneous or excess argon 40 ar 39 ar hornblende ages and rb sr and 40 ar 39 ar muscovite ages obtained from structurally higher metamorphic units in the caledonian thrust nappes generally range between c 440 ma and c 410 ma these ages are interpreted to date cooling during and following d 2 regional thrusting and folding within internal sectors of the nappe sequence a possible tectonic model involves the silurian collision of baltica with scottish segments of laurentia resulting in the scandian orogeny and broadly coeval moine thrust zone d 2 structures were superimposed on structures and metamorphic fabrics formed during a regional mid ordovician tectonothermal event dated previously at c 470 460 ma syn d 2 temperatures were generally 600 c and sufficient to achieve more or less complete thermal rejuvenation of rb sr and 40 ar 39 ar isotopic systems in muscovite and hornblende even in areas of low d 2 strain

statistical analysis of grain size distribution in pleistocene sediments from lake biwa japan abstract time series of grain size distributions from pleistocene sediments deposited in lake biwa during the past 550 millennia show dominant periods of 40 000 and 20 000 yr that are very close to those predicted by the milankovitch theory as well as a period of about 70 000 yr not directly predicted by this theory the 70 000 yr period is strongest followed by the 20 000 yr period the sequences also show that coarser particles were deposited in general during strong solar insolation whereas finer particles were deposited during weak insolation

dating mylonitic deformation by the 40ar 39ar method an example from the norumbega fault zone maine strongly mylonitic rocks associated with the regionally extensive norumbega fault zone in south central maine provide an excellent opportunity for testing the effects of mylonitization on argon isotopic systems in muscovite 40ar 39ar muscovite age spectra from samples outside the zone of mylonitization are relatively undisturbed and have well defined early carboniferous plateau ages in contrast to these nonmylonitized samples all age spectra for muscovite from the mylonites are highly discordant they are characterized by young ages at low extraction temperatures which systematically increase to ages that equal the plateau ages for muscovite collected outside the mylonite zone detailed petrographic observations suggest that these systematic discordances reflect a mixing of argon components from older relict muscovite porphyroclasts and fine grained white mica aggregates that recrystallized during mylonitic deformation total gas ages of five different grain size fractions separated from the same mylonite sample become progressively younger with decreasing grain size indicating a larger component of the recrystallized grains in the finer grain size fractions although the three finest grain size fractions give different total gas ages and do not overlap in age for most of their release spectra their initial increments do coincide at approximately 290 ma this indicates a minimal older age contribution from the relict porphyroclasts in the initial increments and suggests the 290 ma age provides a good estimate for the time of mineral growth associated with mylonitic deformation these data combined with kinematic analysis reveal that the segment of the norumbega fault zone studied the sandhill corner fault is a late carboniferous early permian dextral strike slip fault a lack of significant offset in regional early carboniferous mineral age patterns across the fault suggests that displacement was probably less than 30 km this study demonstrates that 40ar 39ar dating methods can be used to date deformational events effectively as long as several important criteria are met first and foremost samples must be well characterized prior to analysis dynamic recrystallization must have occurred at or below the closure temperature of the mineral to be analyzed regional cooling patterns must also be established through detailed thermochronology so that mineral ages and age spectra from the deformed rocks can be compared to regional cooling ages of the same mineral finally the effects of excess argon must be negligible

astronomical theory of the pleistocene ice ages a brief historical review abstract this paper traces the evolution of the astronomical theory of the pleistocene ice ages from its origin in the work of adhemar and croll through its quantitative transformation in the hands of milankovitch to its current renaissance emphasis is given to the interplay between climatic theory and geologic observation from 1941 when milankovitch published his final version of the theory until the early 1970s most attempts to test the theory empirically were carried out in the time domain by comparing the geologic record of climate with predicted climate curves these attempts ran into serious difficulties particularly the inaccuracy of the geologic time scale and the ambiguity of climate predictions early attempts to evaluate the theory with simple radiation balance models appeared to indicate that the climatic response to secular changes in the orbit would be too small to be observed geologically but by the mid 1970s the advent of an improved time scale for pleistocene marine deposits and the availability of long closely sampled records made it possible to arrange generalized but unambiguous tests of the theory in the frequency domain by comparing orbital and climatic spectra these results decisively confirmed the idea that orbital variations significantly influenced the earth s climate during the pleistocene but they did not tell us how this influence was exercised the emphasis of current work has therefore shifted to the identification and quantitative modeling of physical mechanisms by which the climate system responds so dramatically to the predicted rather subtle changes in radiative boundary conditions

hornblende k ar ages and the climax of tertiary metamorphism in the lepontine alps south central switzerland an old problem reassessed abstract twenty five new k ar ages for amphiboles from the tertiary barrovian type metamorphic terrain of the central alps were determined these results lead to the conclusion that the meso alpine metamorphism reached its peak condition and maximum expanse about 26 3 m y ago along a profile of increasing grade of alpine metamorphism in both basement and cover rocks the following observations can be made 1 pre alpine hornblendes in the zone of greenschist facies show ages ranging from 639 to 152 m y the implied open system behavior may be related to the observed mineralogical instability of these hornblendes during the alpine overprint in contrast two greenschist facies hornblendes which grew in pennine mesozoic rocks during the alpine metamorphism yield ages of 23 2 and 29 3 m y 2 on both sides of the boundary marking the first occurrence of alpine staurolite pre or early alpine hornblende k ar ages are not fully reset unless the rocks were completely recrystallized for one rock specimen which clearly exhibits two generations of hornblende an age two times greater was obtained for the smaller crystals oriented parallel to the foliation than for the structurally younger hornblende porphyroblasts which overgrew the preexisting texture 3 in the high grade area of alpine metamorphism only young alpine ages between 23 and 28 m y were found for hornblendes which grew syn to postkinematically with respect to the last penetrative deformation one amphibolitic inclusion not affected by this deformation yielded a significantly older hornblende age of 32 1 m y these results suggest that temperature controlled ar diffusion is not the main factor governing the k ar system of metamorphic hornblendes we conclude that these hornblende ages should not be interpreted as cooling ages from a structurally ill defined earlier event instead they indicate that the high grade metamorphism responsible for the well known isograd pattern in the lepontine area persisted until some 23 m y ago

direct dating of mylonite evolution a multi disciplinary geochronological study from the moine thrust zone nw scotland rb sr dating of sub closure temperature syn deformationly crystallized white micas from the moine mylonites knockan and dundonnell has yielded ages which vary from 437 ma to 408 ma morphological and major element analyses of the micas indicate that all the micas within the analysed samples were re crystallized during the moine thrust shearing the ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the moine thrust the variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone alternatively it may indicate flow of strontium rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist facies shear zones however the rb sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal the general cessation of ductile deformation at c 430 ma probably dates the general initiation of imbrication within footwall to the moine thrust the age of 430 ma is therefore likely to be a time of significant brittle displacement on the underlying ben more thrust k ar dating of the re crystallized moine mylonite micas yielded anomalously old ages in comparison to rb sr values as a result of incorporation of excess ar during shearing the degree of incorporated excess ar decreases away from the base of the shear zone into the hanging wall becoming indiscernible at c 1 5 km structurally above the base of the shear zone the distance c 1 5 km may represent the distance that fluid can have flowed out of the shear zone during deformation

constraining deformation stages in brittle ductile shear zones from combined field mapping and 40ar 39ar dating the structural evolution of the grimsel pass area aar massif swiss alps abstract 40 ar 39 ar dating of syn kinematic white mica phengite and biotite is used to constrain the age of shear zones within the granodiorites of the grimsel pass area aar massif central alps three sets of steeply dipping shear zones have been mapped stage 1 shear zones are very broad 200 m wide zones of biotite bearing rocks with a progressive increase in intensity of ductile deformation from rim to core and have dip slip kinematics subsequent stage 2 shear zones are narrower contain phengite rich mylonites and show a gradual change from dextral strike slip to dip slip kinematics across strike strain localisation during the formation of stage 2 shear zones is interpreted to have been aided by reaction weakening during extensive fluid rock interaction brittle precursors of stage 2 shear zones are marked by biotite rich cataclasites preserved along the northern contact of the overprinting main stage 2 shear zone biotite from a stage 1 shear zone yields an age of 21 1 0 2 ma phengites in stage 2 mylonites in the grimsel pass area have a very narrow age range 13 8 12 2 ma which is interpreted to bracket the duration of ductile deformation during stage 2 deformation at mid crustal levels however the biotite rich cataclasites at the margin of a stage 2 shear zone may have localised fluid flow up to 2 ma prior to the onset of main stage 2 ductile deformation stage 3 shear zones are brittle strike slip faults containing cataclasites breccias and clay fault gouges earlier studies have established that these late faults formed at

thermal effects and timing of thrusting in the moine thrust zone illite crystallinity studies in the foreland to the moine thrust belt show a widespread heating event which is interpreted as a thermal response to thrust sheet emplacement temperatures reached 275 c approximately 24 ma after the main thrusting events based on the relationships between syn thrusting intrusions and the thrust planes combined with radiometric dates from the moine sheet it is argued that movement on the moine thrust started between 440 and 430 ma ago contractional slip in the moine thrust belt ceased at about 425 ma ago and it is proposed that illite metamorphism attained temperatures of 275 c at about 408 ma ago

theoretical investigation of the time variation of drainage density a theoretical equation was developed to express the time variation of drainage density in a basin or geomorphic surface di t t is the drainage density at time t on the i th basin or geomorphic surface which was formed at time t is a factor related to the erosional force causing the development of the rivers of the basin or surface at time i is the maximum drainage density and di is the initial drainage density on the i th geomorphic surface or basin the equation is based on the assumption that the drainage density increases with time until it reaches a specific upper limit i t the maximum drainage density which is related to certain physical properties of the basin the equations for various dated basins or geomorphic surfaces can be combined into one modified equation if the same relative erosional forces have acted on those basins or surfaces t t and if the basins or surfaces have the same physical properties i t i t di d0 the application of this equation to coastal terraces and glacial tills shows that the model is compatible with observed drainage densities on various dated basins or surfaces

a reassessment appraised comment on hornblende kar ages and the climax of tertiary metamorphism in the lepontine alps south central switzerland an old problem reassessed by alexander deutsch and rudolf h steiger

40ar39ar chronology of variscan tectono metamorphic events in an exhumed crustal nappe the monts du lyonnais complex massif central france abstract the monts du lyonnais metamorphic complex is part of the high grade nappes of the massif central france retrogression of metamorphic conditions occurred synchronously with evolution of deformation from penetrative thrust structures at high t conditions stage p1 2 700 50 c to strike slip ductile shear at lower p t conditions stage p3 t 600 c this evolution distinguishes progressive exhumation and cooling of this previously buried variscan segment in this study 40ar 39ar chronologic investigations are addressed to representative samples of each tectono metamorphic stage associated with the exhumation of the nappe the two mineral generations yield indistinguishable dates of 350 340 ma which are interpreted as post metamorphic cooling ages however the minerals which crystallized during the late intra continental stage p3 yield concordant plateau dates while the minerals which grew during the p1 2 stage and escaped later retrogressive deformation systematically display disturbed age spectra these heterogeneous profiles could result from the presence of a minor excess ar component incorporated at the time of the p3 event when intense deformation took place under decreasing metamorphic temperature and synkinematic granite emplaced along ductile wrench faults this tectono metamorphic event could have been responsible for a local redistribution of ar from the deformed rock volumes into the less or not deformed ones or for a redistribution of ar at the grain scale in the monts du lyonnais complex the effects of intense deformation and dynamic crystallization were probably associated with retrograde cooling to control the contrasting behaviour of the two mineral generations as a regional consequence these radiometric data imply that complete retrogression of the monts du lyonnais complex was achieved 340 ma ago the sharp cluster of ages obtained for the amphiboles muscovites and biotites used in this study suggests relatively rapid cooling at least 50 c ma 1 of the complex after attainment of metamorphic conditions associated with the p3 event the younger values of 325 315 and 250 ma produced by k feldspar and plagioclase respectively may reflect that 1 the metamorphic complex experienced slow cooling at most 10 c ma 1 between 325 and 315 ma ago and 2 a late weak thermal event

orbital signature of interglacials a specific orbital configuration high obliquity combined with the june perihelion marked the beginning of the past three interglacials this suggests that the primary cause of the glacial cycle may be astronomical an astronomical climate index aclin is introduced which combines the three orbital variables in the time lag bivariant model designed to predict the major climate changes in the late and middle pleistocene and in the near future aclin closely correlates with the major climatic events revealed by independently dated proxy climate indicators of the past 130 000 yr it successfully differentiates the interglacials and displays a 100 000 yr periodicity it predicts an early end of the present inter glacial and the start of a new one in 114 000 yr

the crystal chemical basis for ar retention in micas inferences from interlayer partitioning and implications for geochronology cation partitioning data for coexisting muscovite and biotite are shown to be useful indicators of relative interlayer bond length strength in these minerals these data therefore provide a useful crystal chemical perspective on relative mass transfer kinetics of radiogenic isotopes and account for the observation that biotite is generally less retentive of 40ar and 87sr than coexisting muscovite partitioning behavior of trace elements underscores three reasons why overall interlayer bonding in biotite is weaker than in muscovite first the preferences of large rb cs in biotite and of small la3 and na in muscovite indicate a relatively spacious interlayer volume in biotite suggesting a longer mean k o bond second the preference of interlayer vacancies in biotite with some all possibly h2o h3o filled suggests that its adjacent 2 1 sheets are connected by fewer interlayer bonds per unit cell than those of muscovite third the relative exclusion of large ba2 from biotite despite its large interlayer sites is attributed to o h bonds pointing into the interlayer cavity sub normal to 001 k ba2 h repulsion thereby induced by the bare proton both destabilizes ba2 and weakens k o bonds in contrast muscovite offers a more favorable electrostatic environment for ba2 substitution since its o h bonds are directed into the vacant m1 octahedral site sub parallel to 001 this hypothesis is supported by the observation that progressive f oh 1 exchange enhances ba2 partitioning into biotite phlogopite relative to coexisting muscovite these crystal chemical differences between biotite and muscovite are mirrored in calculated values of ionic porosity z i defined here as the percentage of their interlayer unit cell volume not occupied by ions a monitor of ionic packing density and geometry z i is inversely correlated with k o bond strength which appears to be the rate determining kinetic common denominator for a variety of processes affecting micas including those responsible for loss of radiogenic isotopes in biotite and muscovite accordingly the relatively longer weaker k o bonds of biotite are envisioned as being more easily stretched during volume diffusion or broken during recrystallization or retrograde alteration this in turn accounts for common observations of enhanced radiogenic ar sr loss and younger 40ar 39ar and rb sr ages in natural biotite high z i relative to coexisting muscovite lower z i significantly this pattern may arise irrespective of isotopic loss mechanism diffusion or recrystallization etc and it follows that any age discordance observed between muscovite and biotite cannot be ascribed uniquely to one mechanism or the other without appropriate field petrographic and petrologic constraints extension of this partitioning porosity based synthesis leads to prediction of corollary age retentivity composition effects among chemically diverse trioctahedral and dioctahedral micas which are best field tested in terranes that cooled slowly under dry static conditions pressure effects on argon retention are also inferred from the porosity model

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

paleozoic deformation and isotopic disturbance in the southeastern arunta block central australia proterozoic metamorphic rocks of the arunta block central australia were thrusted southward over the northern margin of the neoproterozoic to mid paleozoic amadeus basin during the 400 300 ma alice springs orogeny the extent and nature of mid paleozoic ductile deformation in the southeastern arunta block is examined in light of available u pb 40ar39ar k ar rb sr and sm nd isotopic age data most of the proterozoic amphibolite and granulite facies rocks north of the zone of basement cover interaction were exhumed and cooled from above 500 to below 125 c as a result of mid paleozoic nappe formation this suggests that deformation early in the cooling history occurred at amphibolite facies conditions and progressed to greenschist facies conditions later in the cooling history in general shear zone rocks yield the youngest isotopic ages indicating that deformation induced recrystallization grain size reduction or localized thermal perturbations are responsible for the relatively young ages we conclude that shear zones of mid paleozoic age were active under greenschist and amphibolite facies conditions though further work is needed to differentiate whether amphibolite facies shear zones are paleozoic or proterozoic some proterozoic shear zones were uplifted passively in the mid paleozoic resulting in final closure and mixed ages in key minerals like hornblende

high resolution stable isotope stratigraphy of the upper cambrian and ordovician in the argentine precordillera carbon isotope excursions and correlations abstract we report the occurrence of important carbon isotope excursions in early paleozoic formations of the eastern and central argentine precordillera the steptoean positive isotope carbon excursion spice is known from north america kazakhstan south china australia and south america and the negative isotope carbon excursion snice has been described for the first time in south america we report here the record of the spice and snice in a single section in the eastern precordillera in the central precordillera a minor middle darriwilian positive carbon isotope excursion mdice and a late sandbian positive isotope carbon excursion the gice 3 c bicornis biozone are reported from two sections one pre gice positive carbon isotope excursion sandbian sa1 n gracilis biozone in the central precordillera with a 13c peak of 2 is perhaps equivalent to the positive spechts ferry 13c excursion of north america a positive 13c excursion 6 n persculptus biozone recorded at the base of the late hirnantian la chilca formation probably corresponds to hice these carbon isotope excursions resulted from more than one factor where paleoceanographic events probably played a major role a sea level fall and vigorous fluctuations in the steptoean spice b sea level rise in the sunwaptan snice c important transgression in the sandbian pre gice and gice and d sea level fall in the late hirnantian hice in the darriwilian and sandbian stages organic burial has led to a large 12c sequestration in the deep anoxic ocean with saline bottom water recorded by the graptoliferous black shales of the gualcamayo and los azules formations in central precordillera helped the building of the mdice and gice anomalies 18o values for the upper cambrian carbonates are likely near primary isotope signals that point to progressive cooling from the spice to the snice whereas for sandbian carbonates they suggest strong temperature fluctuations the 13c peak of the gice coincides with a cooler period with temperatures warming up towards the late hirnantian to rhuddanian the upper cambrian to the middle ordovician of the precordillera recorded a decrease of seawater 87sr 86sr ratios in accordance to the global picture this decrease probably reflects the influence of widespread volcanic activity from arc terranes in low latitude settings along eastern margins of laurentia and in the argentine precordillera in the early to middle ordovician five carbonate samples from san juan gualcamayo and los azules formations yielded end values that along seven already published values seem to plot along the nd isotopic evolution trend of the iapetus ocean together the record of global upper cambrian and ordovician carbon isotope excursions in the argentine precordillera is a valuable proxy in refining early paleozoic stratigraphy establishing of regional global high resolution correlations and sea level change history in south america

periodic variations of grain size in pleistocene sediments in lake biwa and earth orbital cycles a time sequence of grain size distributions from the pleistocene sediments in lake biwa located in the central part of japan 35 n 136 e has dominant periodicities of 41 000 23 000 and 19 000 years which are very close to those predicted from the astronomical theory of paleoclimate introduced by milankovitch the time variation of the grain size is very similar to that of insolation of caloric summer in the northern hemisphere

relationships between marginal thrusting and movement on major internal shear zones in the northern highland caledonides scotland abstract the development of the syn metamorphic sgurr beag slide zone a major ductile shear zone of initially low dip caused at least 50 km north western thrust displacement of part of the internal metamorphic complex of the northern highland caledonides of scotland initiation of the zone and movements upon it were earlier than formation of the marginal moine thrust zone movement on the zone followed but overlapped the peak caledonian metamorphism and the mid to high amphibolite facies mineral assemblages fabrics and structures produced during the development of the slide zone and those surviving from earlier events were reworked under greenschist facies conditions during mylonitization associated with initiation of the moine thrust zone displacements on the slide zone and thrust movements were separated by emplacement of a regional suite of pegmatites and a considerable change of metamorphic grade thus they may not constitute members of a progressive sequence of caledonian thrusts formed over a short time interval rather preliminary isotopic data may imply an interval of c 25 ma between movement on the slide zone and final ductile translation along the moine thrust zone

40ar 39ar thermochronology of the kampa dome southern tibet implications for tectonic evolution of the north himalayan gneiss domes structural and thermochronological studies of the kampa dome provide constraints on timing and mechanisms of gneiss dome formation in southern tibet the core of kampa dome contains the kampa granite a cambrian orthogneiss that was deformed under high temperature sub solidus conditions during himalayan orogenesis the kampa granite is intruded by syn tectonic leucogranite dikes and sills of probable oligocene to miocene age overlying paleozoic to mesozoic metasedimentary rocks decrease in peak metamorphic grade from kyanite staurolite grade at the base of the sequence to unmetamorphosed at the top the kampa shear zone traverses the kampa granite metasediment contact and contains evidence for high temperature to lowtemperature ductile deformation and brittle faulting the shear zone is interpreted to represent an exhumed portion of the south tibetan detachment system biotite and muscovite 40 ar 39 ar thermochronology from the metasedimentary sequence yields disturbed spectra with 14 22 0 18 to 15 54 0 39 ma cooling ages and concordant spectra with 14 64 0 15 to 14 68 0 07 ma cooling ages petrographic investigations suggest disturbed samples are associated with excess argon intracrystalline deformation mineral and fluid inclusions and or chloritization that led to variations in argon systematics we conclude that the entire metasedimentary sequence cooled rapidly through mica closure temperatures at 14 6 ma the kampa granite yields the youngest biotite 40 ar 39 ar ages of 13 7 ma immediately below the granite metasediment contact we suggest that this age variation reflects either varying mica closure temperatures re heating of the kampa granite biotites above closure temperatures between 14 6 ma and 13 7 ma or juxtaposition of rocks with different thermal histories our data do not corroborate the inverse mica cooling gradient observed in adjacent north himalayan gneiss domes instead we infer that mica cooling occurred in response to exhumation and conduction related to top to north normal faulting in the overlying sequence top to south thrusting at depth and coeval surface denudation

the sensitivity of stationary waves to variations in the basic state zonal flow abstract a linear primitive equation stationary wave model having high vertical and meridional resolution is used to examine the sensitivity of orographically forced primarily by himalayas stationary waves at middle and high latitudes to variations in the basic state zonal wind distribution we find relatively little sensitivity to the winds in high latitude but remarkable sensitivity to small variations in the subtropical jet fluctuations well within the range of observed variability in the jet can lead to large variations in the stationary waves of the high latitude stratosphere and to large changes even in tropospheric stationery waves implications for both sudden warmings and large scale weather are discussed

an 40ar 39ar study of a polymetamorphic complex in the arunta block central australia a polymetamorphic complex in the arunta block central australia which has undergone metasomatism involving introduction of rb and loss of sr has been investigated by 40ar 39ar step and total degassing techniques in an attempt to date five metamorphic events m1 m5 identified microstructurally the complex isotopic data and apparent age spectra obtained have been interpreted in the light of constraints provided by an associated tectonometamorphic study together with independent geochronological controls from the immediate study area and elsewhere in the arunta block this has led to the delineation of ages of 1 780 1 820 m y for m1 1 690 1 720 m y for m2 1 450 1 500 m y for m3 1 030 1 060 m y for m4 and 370 510 for m5 the polyphase m5 event is correlated with the alice springs orogeny previously accorded younger age limits of 330 370 m y the latter are interpreted to reflect closure of mica k ar systems as uplift continued subsequent to the close of the event possibly culminating in minor high level brittle deformation and the development of pseudotachylytes in the study area

a reassessment appraised comment on hornblende kar ages and the climax of tertiary metamorphism in the lepontine alps south central switzerland an old problem reassessed reply to peter k zeitler and jan r wijbrans

the relationship between k ar mineral ages mica grainsizes and movement on the moine thrust zone nw highlands scotland muscovite and biotite separated from moine schists close to the moine thrust zone yield a range of k ar ages incompatible with the simple cooling regime previously proposed for the caledonides of northern scotland differences of only 1 ma between muscovite and biotite of the same grainsize contrast with differences of 12 13 ma between the 125 200 m and 250 500 m grainsize fractions this is contrary to the normally accepted dominance of mica type over grainsize effects dynamic recrystallization of the micas during early deformation associated with the moine thrust zone is thought to play a role as important as temperature in determining the k ar ages the conclusions drawn have important implications for the use of uplift and cooling rates derived from the absolute k ar closure temperatures of micas close to thrust zones although the k ar ages from micas with diameters greater than 200 m are not reproducible the data can be used to show that deformation associated with thrusting commenced between c 443 ma and c 435 ma and that the thrust zone was active until at least 422 ma ago

time variations of erosional force and grain size in pleistocene lake sediments abstract theoretical investigations on models of the developmental processes of erosional features show that the time variations in erosional force have more sensitive influence on the time fluctuation of eroded material such as sediments in lakes than that of the quantity representing erosional features such as volume of mountains time sequences of grain size distributions from the pleistocene sediments in lake biwa central japan indicate dominant periodicities of about 40 000 and 20 000 yr very close to those predicted from the milankovitch theory the time variations of the grain size parameters which may be indicators of erosional forces are very similar to that of the caloric summer insolation in the northern hemisphere

late caledonian extensional reactivation of a ductile thrust in nw scotland evidence is given for ductile extension across a fault zone located on the western seaboard of the northern highlands of scotland the fault zone lies within the metamorphic caledonides and extensional movement occurred during the waning stages of caledonian orogenic activity an earlier phase of westerly directed overthrusting took place on the same fault zone at higher metamorphic temperatures but after the peak of caledonian metamorphism the compression may be broadly coeval with early displacements on the moine thrust zone which probably lies offshore some 10 km to the northwest the extensional reactivation could relate to local crustal instability resulting directly from overthrusting it may have been due to local gravitational collapse following overthickening of the crust alternatively the extensional movement may witness the onset of regional gravitational collapse of the orogen during the late silurian to early devonian reactivation accords with interpretations of the movement histories of similarly eastward dipping offshore faults recognized on seismic reflection profiles

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

isotope systematics in minerals biotite rejuvenation and exchange during alpine metamorphism abstract the hercynian schists and granites of the helvetic basement aar massif in the central swiss alps are overprinted by low grade alpine metamorphism and associated deformation rb sr and k ar ages of biotites are systematically reset by the later greenschist facies event this rejuvenation is apparently controlled by temperature mineral chemistry and grain size and independent of deformation and exsolution thermally activated volume diffusion best explains this behaviour with ar and sr having similar activation energies and diffusion rates for sr showing a greater dependence on biotite composition feldspars and apatite also show increasing equilibration to the alpine event and together with alpine equilibria in the micas reveal important changes in the isotopic compositions of exchanging reservoirs these spatial and temporal variations must be assessed before true mineral ages can be calculated generally the higher grade equilibria show less radiogenic exchange reflecting increased dilution by external fluids of radiogenic components released at the main front of isotopic resetting rb sr ages of alpine muscovites in the granites suggest a maximum possible age of metamorphism of 25 ma in this low grade area this implies significant diachroneity in the timing of peak metamorphism in the central alps with the relatively delayed metamorphism in this area a reflection of the later initial thickening and delayed uplift of the helvetic basement

the thrust and shear zones of the moine thrust zone and the nw scottish caledonides the caledonian thrust belts of nw scotland exhibit many features typical of foreland fold and thrust belts the major structures propagated westwards towards the foreland the thin carapace of cambro ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets on a broad scale the rules of thrust geometry are obeyed but many local anomalies are found particularly in the assynt region in assynt several large fault bounded gravity spreading structures surge zones have been identified extensional faults have been traced into strike slip and then contractional faults there are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow similar structures have been found along the length of the moine thrust belt suggesting that gravity spreading was an important mechanism during the later stages of movement within the main caledonian fold belt there are several major tectonic slides which are recognized by a local increase in intensity of deformation the majority are nw verging and were originally gently dipping in the highlands se of the great glen there are early major se verging recumbent folds associated with considerable crustal thickening reflected in high pressure 12kbar metamorphism many of the major slides may have formed by spreading away from this thickened crust the caledonian deformation and metamorphism occurred later in the moines and moine thrust zone than in the dalradian and grampian moines to the se of the great glen this may reflect different levels of deformation alternatively it may argue for a large strike slip displacement on the great glen fault juxtaposing crustal blocks which had experienced different ages of caledonian deformation and metamorphism the late movement on the moine thrust involved spreading from an area of thickened caledonian crust formed either by the stair step trajectory of the basal thrust to the moines and or by thickening due to intrusion of large masses of late caledonian granites

rb sr and k ar ages on minerals at temperatures of 300 400 c from deep wells in the larderello geothermal field italy rb sr and k ar ages have been obtained on six biotites two muscovites and one hornblende from samples of micaschist gneiss and amphibolite of lower paleozoic to precambrian age at a depth exceeding 2 000 m in basement rocks of the larderello travale geothermal region most of the data cluster in the range 2 5 3 7 ma revealing the existence of a pliocene thermal event to which the origin of the field may be attributed the resulting duration of the larderello geothermal field is unexpectedly long in the basement levels of the two wells examined unstabilized minimum temperatures of 290 and 380 c were measured all the biotites show almost complete 40ar and 87sr retention at the measured well temperatures petrologic evidence stilpnomelane stability and experimental data activation energies and diffusion coefficients also favour a closure temperature above 400 c for rb sr and k ar in biotites in agreement with recent direct experimental determinations

chronology of tectonic events in the crystalline core of the himalaya langtang national park central nepal the main central thrust mct is an important intracontinental subduction zone that accommodated a significant proportion of shortening between india and asia during tertiary himalayan orogenesis argon 40 argon 39 geochronology indicates at least two distinct periods of thrust movement on the mct in the langtang national park region of central nepal ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40ar 39ar dating of muscovites to have occurred sometime before 5 8 ma a later period of brittle deformation resulting in the juxtaposition of rocks of different lithology within the mct zone occurred at approximately 2 3 ma based on 40ar 39ar dating of neoblastic muscovites from the brittle fault zones the hanging wall of the mct contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the greater himalayan sequence argon 40 argon 39 cooling ages of muscovite and biotite from the gneisses range from 4 6 to 9 7 ma these dates contrast with previously obtained 16 21 ma u pb monazite and zircon ages for metasedimentary rocks from the same structural levels parrish et al 1992 indicating relatively slow cooling over the early to middle miocene interval for much of the mct hanging wall however one 19 3 ma biotite 40ar 39ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than u pb monazite ages for nearby anatexites possibly suggesting rapid cooling of the uppermost greater himalayan sequence by tectonic denudation associated with the structurally higher south tibetan detachment system the general consistency of 40ar 39ar ages throughout the 11 km thick greater himalayan sequence suggests rapid cooling in late miocene time probably due to an increase in erosion rate related to ramping on the structurally lower main boundary thrust an alternative possibility would be massive hydrothermal resetting in late miocene time of the entire sequence

periodic variations in late pleistocene speleothem abundance in britain abstract temporal change in the growth of speleothems in the united kingdom during the past 150 000 years is shown to be related to the insolation variation by the milankovitch theory the speleothem data compiled by gordon et al 1989 quarternary research 31 14 26 have two milanokovitch frequencies the ca 40 000 yr period related to change in the earth s obliquity and the ca 20 000 yr period related to the precession of the equinoxes the abundance of speleothem growth was in general large during the last interglaciation and small during the last glaciation in both periods however speleothem abundance was greater during periods of strong insolation and less during weak insolation

the metamorphic environment of the sgurr beag slide a major crustal displacement zone in proterozoic moine rocks of scotland new evidence from the sw northern highlands of scotland establishes the southerly extension of the sgurr beag slide interpretation of changes in the mineralogy of calcsilicate rocks particularly the use of the anorthite content of plagioclase feldspar as an index of metamorphic grade together with consideration of the texture and mineralogy of pelitic rocks suggests that the slide juxtaposes crustal segments of different metamorphic grade and history movement on the slide post dates a major phase of regional metamorphism and two phases of regional deformation of precambrian age was synchronous with a regional fold forming event and regional metamorphism at or before c 467 ma caledonian and was followed by regional folding models of the thermal effects of displacement across syn metamorphic shear zones are investigated with particular reference to the roles of thermal relaxation and levels of shear strain a model based on the development of a ductile syn metamorphic asymmetric shear zone best explains the metamorphic patterns associated with the sgurr beag slide an original low to moderate easterly dip is implied with north westerly upthrusting of the eastern crustal block

uplift patterns and orogenic evolution in the scottish dalradian rb sr and k ar mineral ages from the dalradian rocks of angus and perthshire indicate considerable diachronism of cooling following the caledonian regional metamorphism the oldest mineral ages 515 ma are similar to the independent estimates of peak metamorphic age and hence indicate very rapid cooling they occur in the highest grade samples of the angus region and are associated with localized uplift regional rapid uplift events at 460 440 ma and 410 390 ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the scottish highlands thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the p t paths they suggest that the major diachronous metamorphism and granite production in the scottish highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources localized uplift may exercise considerable control on mineral facies variation within orogenic belts the local uplift in angus introduces hot high pressure tarfside nappe rocks into higher level tay nappe rocks thus the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

the relation between grain size and 40ar 39ar date for alpine white mica from the siviez mischabel nappe switzerland abstract variations in alpine white mica 40 ar 39 ar dates from the cover units of the siviez mischabel nappe relate to regional variations in the thermal history of the nappe we focus on three regions within the nappe the central siviez mischabel csm the southern siviez mischabel ssm and the eastern siviez mischabel esm our approach weaves together observations of quartz and mica textures in thin section the variation of 40 ar 39 ar date with grain size considerations of the effective diffusion dimension edd of argon in white mica and a comparison of dates with diffusion model results in the csm pressure solution of quartz and dislocation glide in mica accommodated alpine deformation dates record mica growth during nappe emplacement from 40 to 36 ma and do not vary with grain size in the ssm and esm both mica and quartz show textures associated with dynamic recrystallization and dates decrease with grain size in the ssm dates also agree with the timing of nappe emplacement but in the esm dates significantly post date the timing of nappe emplacement a comparison of dates with diffusion model results supports inferences from rock fabrics that the ssm experienced higher peak temperatures than the csm even though dates from both units approximate the timing of mica growth dates obtained from the esm however do not compare well with simple models and the thermal evolution of this region of the nappe in the neighborhood of the simplon fault zone is not well understood

evidence for non plane strain flattening along the moine thrust loch srath nan aisinnin north west scotland we report quartz c axis patterns grain shape fabrics and microstructures for 11 mylonitic quartzites and quartz phyllosilicate schists from a transect across the moine thrust at loch srath nan aisinnin north west scotland in the footwall samples collected more than 42 m normal distance from the thrust surface quartz c axis textures indicate a general flattening strain i e 0 k 1 samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c axis patterns recrystallized hanging wall fault rocks exhibit random c axis patterns on the scale of a standard thin section relict footwall grains provide the closest approximation of finite strain they have octahedral shear strains s between 1 10 and 1 47 and exhibit general flattening k values 0 0524 0 659 the long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ese in contrast recrystallized footwall grains have a mean grain shape with the longest axis oriented nearly perpendicular to the transport direction furthermore these samples have grain shape k value ranges from 0 157 to 0 295 recrystallized hanging wall grain shapes exhibit the lowest octahedral shear strains s 0 532 0 733 and largest mean k values 0 351 0 961 of this sample set the long axes of the mean recrystallized hanging wall grain shapes are parallel to transport similar to that of relict footwall grains unrecrystallized quartz overgrowths about opaque mineral grains suggest concurrent elongation in all directions within the mylonitic foliation and support the inference of general flattening deformation the mylonitic foliation and penetrative lineation are consistent with a wnw shearing direction however both were folded during later deformation increments recrystallized grains in footwall quartzites suggest a 305 320 azimuth for the shearing direction the best fit axis of the poles to the foliation is 18 145 close to the mean trend plunge of mesoscopic fold hinges at this site 18 151 both suggest a nnw azimuth for the shearing direction thus structural microstructural and fabric indicators suggest a change in the azimuth of the shearing direction from 286 to 331

40ar 39ar dating of synkinematic white mica insights from fluid rock reaction in low grade shear zones mont blanc massif and constraints on timing of deformation in the nw external alps this paper highlights the use of synkinematic white mica biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low grade metamorphic conditions the mont blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow resulting in substantial differences in mineralogy and whole rock geochemistry on the basis of their synkinematic alteration assemblages and geographic distribution within the mont blanc massif three main metamorphic zones are distinguished within the network of shear zones these are i epidote white mica bearing assemblages ii chlorite phlogopite bearing assemblages and iii white mica biotite calcite actinolite epidote bearing assemblages 40ar 39ar age spectra of biotite and phlogopite are complex and reflect significant variations in chemical composition in biotite this is partly due to inheritance from precursor variscan magmatic biotite in contrast new white mica grew at the expense of feldspar during alpine deformation and its ar spectra do not show any excess 40ar on the se side of mont blanc ages of shear zone phengites have a narrow range of 15 8 16 0 0 2 ma which is in the same age range as 40ar 39ar ages of minerals from kinematically related veins the top to se sense of shear is consistent with initiation of a mont blanc flower structure within a dextral transpressional system by 16 ma on the nw side mini plateaux ages of 14 5 0 3 and 23 4 0 4 ma are preserved in the same sample suggesting the possibility of two phases of deformation this is also supported by partly preserved ages of 18 36 6 ma in biotites and phlogopites ages between 36 and 18 ma might reflect ongoing top to nw thrusting following penninic front activation in a context of nappe stacking and crustal thickening nw directed thrusting on the nw side of mont blanc continued after 18 ma synchronous with se directed thrusting on the se side of the massif these divergent movements produced the overall pop up geometry of the mont blanc massif which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending sw from the rhone valley into the mont blanc area

k ar and ar ar dating the aim of this chapter is to present the k ar and ar ar dating techniques in the context of noble gas studies since there are already several recent texts on k ar and ar ar dating dickin 1995 mcdougall and harrison 1999 the focus of this section will be aspects of argon transport and storage in the crust which affect k ar and ar ar dating including ar loss from minerals by diffusion and ar gain by minerals or excess argon the k ar dating technique was one of the earliest isotope dating techniques developed soon after the discovery of radioactive potassium and provided an important adjunct to u pb and u he dating techniques the ease of measurement and ideal half life 1250 million years see table 2 below for dating geological events has made this the most popular of isotopic dating techniques aldrich and nier 1948 first demonstrated that 40ar was the product of the decay of 40k and soon after k ar ages were being measured in several laboratories most often using an absolute method such as a mcleod gauge to measure argon concentrations the first published k ar results by such a technique were those of smits and gentner 1950 who analyzed sylvite from the buggingen oligocene evaporite deposits obtaining an age of 20 million years mass spectrometers which simultaneously measured very small amounts of gas and the isotope ratios necessary to make corrections for atmospheric contamination quickly replaced manometric techniques crucially the use of static vacuum techniques pioneered by john reynolds at the university of california berkeley meant that mass spectrometers were sufficiently sensitive to analyse the small amounts of gas released from common rocks and minerals although the earliest mass spectrometers were built in house the introduction of the commercially available ms10 farrar et al 1964 a small 180 metal mass spectrometer built for

regional scale lateral variation and linkage in ductile thrust architecture the oykel transverse zone and mullions in the moine nappe nw scotland sharp lateral changes in structural geometry of ductile thrust stacks are not widely reported a regional scale lateral culmination wall forms the southern boundary of the cassley culmination in moine rocks in the caledonides of sutherland northern scotland this culmination wall is part of the oykel transverse zone otz a kilometre scale shear zone characterized by constrictional finite strain fabrics aligned sub parallel to the regional wnw directed thrust transport direction main phase folds and fabrics in the transverse zone hanging wall are folded by main phase folds and fabrics in the footwall thus recording foreland propagating ductile deformation south of the cassley culmination shortening occurred uniformly without development of discrete subsidiary thrusts distributed deformation fold development alternated with localized thrusting within the culmination the classic ese plunging mullions at oykel bridge are an integral part of the otz and were generated by constriction aligned sub parallel to the transport direction constriction is attributed to differential transtensional movement across the otz during culmination development subsequent formation of the underlying assynt culmination further accentuated upward bulging of the cassley culmination amplifying the lateral change across the transverse zone the otz aligns with a pronounced gravity gradient on the south western side of the lairg gravity low interpretive modelling relates this gradient to a buried basement ramp that possibly controlled the location of the transverse zone