development of structures under the influence of heterogeneous flow field around rigid inclusions insights from theoretical and numerical models rocks that are mechanically heterogeneous due to the presence of stiff or rigid inclusions floating in a ductile matrix commonly show a variety of micro to macro scale structures developing under the influence of heterogeneous flow field in the neighbourhood of the inclusions it is of fundamental importance to apprehend the nature of strain heterogeneity around inclusions to understand progressive development of structures associated with rigid inclusions such as strain shadow foliation drag porphyroclast mantle porphyroblast inclusion trails intragranular fractures etc the development of these diverse types of structures can be analyzed with the help of a suitable hydrodynamic theory in this paper we review different continuum models that have been proposed to characterize the heterogeneous flow field around rigid inclusions focusing on recent developments recent studies reveal that jeffery s proc r soc lond a 120 1922 161 theory dealing with the motion of ellipsoidal rigid bodies in an infinitely extended viscous medium is more general in nature and applicable for modeling the heterogeneous flow around both equant and inequant shapes of inclusions and ideal or non ideal shear deformation of the matrix the application of this theory therefore has advantages over other models based on lamb s lamb h 1932 hydrodynamics cambridge university press cambridge theory dealing with spherical inclusions the review finally illustrates numerical simulations based on hydrodynamic theories highlighting the controls of physical and kinematic factors on the progressive development of the structures mentioned above

tethyan and indian subduction viewed from the himalayan high to ultrahigh pressure metamorphic rocks abstract the himalayan range is one of the best documented continent collisional belts and provides a natural laboratory for studying subduction processes high pressure and ultrahigh pressure rocks with origins in a variety of protoliths occur in various settings accretionary wedge oceanic subduction zone subducted continental margin and continental collisional zone ages and locations of these high pressure and ultrahigh pressure rocks along the himalayan belt allow us to evaluate the evolution of this major convergent zone 1 cretaceous 80 100 ma blueschists and possibly amphibolites in the indus tsangpo suture zone represent an accretionary wedge developed during the northward subduction of the tethys ocean beneath the asian margin their exhumation occurred during the subduction of the tethys prior to the collision between the indian and asian continents 2 eclogitic rocks with unknown age are reported at one location in the indus tsangpo suture zone east of the nanga parbat syntaxis they may represent subducted tethyan oceanic lithosphere 3 ultrahigh pressure rocks on both sides of the western syntaxis kaghan and tso morari massifs formed during the early stage of subduction exhumation of the indian northern margin at the time of the paleocene eocene boundary 4 granulitized eclogites in the lesser himalaya sequence in southern tibet formed during the paleogene underthrusting of the indian margin beneath southern tibet and were exhumed in the miocene these metamorphic rocks provide important constraints on the geometry and evolution of the india asia convergent zone during the closure of the tethys ocean the timing of the ultrahigh pressure metamorphism in the tso morari massif indicates that the initial contact between the indian and asian continents likely occurred in the western syntaxis at 57 1 ma west of the western syntaxis the higher himalayan crystallines were thinned rocks equivalent to the lesser himalayan sequence are present north of the main central thrust moreover the pressure metamorphism in the kaghan massif in the western part of the syntaxis took place later 7 m y after the metamorphism in the eastern part suggesting that the geometry of the initial contact between the indian and asian continents was not linear the northern edge of the indian continent in the western part was 300 to 350 km farther south than the area east of the nanga parbat syntaxis such en baionnette geometry is probably produced by north trending transform faults that initially formed during the late paleozoic to cretaceous gondwana rifting farther east in the southern tibet the collision occurred before 50 6 0 2 ma finally high pressure to ultrahigh pressure rocks in the western himalaya formed and exhumed in steep subduction compared to what is now shown in tomographic images and seismologic data

precise eclogitization ages deduced from rb sr mineral systematics the maksyutov complex southern urals russia abstract the maksyutov complex southern urals russia is a well preserved example of subduction related high pressure metamorphism one of its two litho tectonic units consists of rocks that experienced eclogite facies conditions published 40ar 39ar data on phengite u pb data on rutile and sm nd mineral data define a cluster of ages around 370 to 380 ma nevertheless no consensus exists as to the detailed interpretation of data and the exact age of eclogitization we present new high precision internal mineral rb sr isochrons for eclogite facies metabasites felsic eclogites and eclogite facies quartz veins nine isochrons mainly controlled by omphacite and white mica phases give concordant ages with an average value of 375 2 ma 2 microtextural features such as prograde growth zoning in eclogite facies phases suggest that the assemblages dated formed at a stage of prograde metamorphism sr isotopic equilibria among eclogite facies phases and among eclogite facies fluid veins and the host rocks indicate that our ages reflect crystallization ages related to the prograde metamorphic probably fluid mediated eclogitization reactions this interpretation is reinforced by data from fluid precipitated quartzitic eclogites whose modal composition together with intergrowth relationships conclusively imply closed system behavior after crystallization the possible occurrence of a pre 375 ma event of ultra high pressure metamorphism uhpm in the maksyutov complex is disproved by isotope systematics microtextures and mineral zoning patterns

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

insights into u hp metamorphism of the western gneiss region norway a high spatial resolution and high precision zircon study abstract combining high spatial resolution and high precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh pressure uhp metamorphism resulting from the collision of baltica avalonia and laurentia during the scandian orogeny in the western gneiss region of norway zircons were extracted from a layered eclogite in the saltaneset region southern uhp domain and from an eclogite in the ulsteinvik region central uhp domain zircons were first analyzed for u pb and trace element compositions by laser ablation split stream lass inductively coupled plasma mass spectrometry icp ms followed by analysis of those same zircons that yielded scandian dates by integrated u pb isotope dilution thermal ionization mass spectrometry and trace element analysis tims tea lass results from a garnet quartz layer within the saltaneset eclogite give scandian dates of ca 413 397 ma with subsequent id tims analyses ranging from 408 9 0 4 ma to 401 4 0 2 ma 2 an omphacite rich layer from the same eclogite yields lass dates of ca 414 398 ma and a single id tims date of 396 7 1 4 ma in comparison the ulsteinvik eclogite lass results give dates spanning ca 413 397 ma whereas id tims analyses range from 409 6 0 6 ma to 401 3 0 4 ma id tims zircon data from the eclogites reveals two age populations 1 ca 409 407 ma and 2 ca 402 ma both in situ and solution trace element data show a distinct pattern for scandian zircons with strongly depleted hree and weakly negative eu anomalies eu eu whereas inherited zircon ree patterns are distinguished by steep hree slopes and marked negative eu eu when coupled with partition coefficients calculated for zircon and garnet these ree patterns indicate that zircon re crystallized during eclogite facies metamorphism at ca 409 407 ma and ca 402 ma at two widely separated uhp localities

himalayan metamorphism and its tectonic implications the himalayan range exposes a spectacular assemblage of metamorphic rocks from the mid and deep crust that have fostered numerous models of how the crust responds to continental collisions recent petrogenetically based petrologic and geochronologic studies elucidate processes with unprecedented resolution and critically test models that range from continuum processes to one time events the pronounced metamorphic inversion across the main central thrust reflects continuum thrusting between ca 15 and 20 ma whereas exposure of ultrahigh pressure rocks in northwestern massifs and syntaxis granulites reflects singular early 45 ma and late 10 ma exhumation events multiple mechanisms including wedge collapse and flow of melt weakened midcrust are debated to explain pressure temperature trajectories patterns of thinning and thermal overprinting a geochronologic revolution is under way in which spatially resolved compositions and ages of accessory minerals are combined in a petrogenetically valid co

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

early to mid silurian extrusion wedge tectonics in the central scandinavian caledonides we present evidence for an extrusion wedge in the scandian fold thrust belt of the central scandinavian caledonides seve nappe complex rb sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal sense roof shear zone and at the reverse sense floor shear zone between 434 ma and 429 ma a sm nd age of 462 ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction pressure temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at 670 50 c from 17 5 to 14 5 kbar in garnet kyanite mica schists during reverse sense shearing and from 15 to 11 kbar in garnet mica schists during normal sense shearing these data and the presence of decompression related pegmatites dated at 434 ma and 429 ma indicate that the seve nappes form a large scale extrusion wedge this wedge extends along strike for at least 150 km and marks an early stage of ultrahigh pressure metamorphism exhumation and orogenic wedge formation in this part of the scandinavian caledonides predating the major post 415 ma ultrahigh pressure exhumation processes in southwestern norway

ultra high pressure metamorphism and orogenesis an alpine perspective article i nfo the pressure temperature time deformation evolution of the high to ultra high pressure units of the western alps has been progressively refined in the last 40 years leading to several paradigm shifts in the understanding of orogenesis this set of information combined with cretaceous tertiary plate kinematic reconstructions and mesozoic palaeogeography indicates that the western alps represent the amalgamation of a cretaceous and an eocene orogen which developed at the expense of the adriatic and european rifted margins respectively in the cretaceous nne directed drift of the adriatic plate parallel to the inherited jurassic structural trends led to the development of a highly oblique subduction boundary in this setting the canavese zone and parts of the sesia zone derived from the hyper extended adriatic margin underwent deformation and metamorphism at conditions ranging from sub greenschist to eclogite facies a lower eocene switch to nnw directed motion perpendicular to the rift related jurassic structural trends culminated in the collision between the proto alps and the brianconnais block at ca 44 ma as a result the brianconnais domain and parts of its hyper extended margin preserved in the u hp piemonte units were accreted to the orogen continued convergence was subsequently accommodated by subduction of the more external valaisan basin and of the thinned european margin the arrival of thick european continental crust at the subduction zone at ca 35 30 ma marked the onset of the final continental collision to a first order the orogen grew through the progressive episodic accretion of units located towards north westerly positions accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units locally resulting in renewed deformation metamorphism

geochronology of fluid induced eclogite and amphibolite facies metamorphic reactions in a subduction collision system bergen arcs norway rb sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes granulite facies rocks of the lindas nappe bergen arcs norway were subducted and exhumed during the caledonian orogeny the rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation along fractures and shear zones the first occurred at eclogite facies 15 21 kbar 650 750 c and a later one at amphibolite facies conditions 8 10 kbar 600 c the presence of fluids enabled local metamorphic equilibration only near fluid pathways in fluid absent domains preexisting assemblages were metastably preserved this resulted in a heterogeneity of metamorphic signatures on meter to m scales well preserved granulite facies rocks preserve their proterozoic rb sr mineral ages as does the u pb system of zircon in most lithologies six rb sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid induced eclogitization at 429 9 3 5 ma 2 weighted average mswd 0 39 an eclogite facies vein has yielded metamorphic zircon with concordant u pb ages of 429 3 ma identical to the u pb age of 427 4 0 9 ma for zircon xenocrysts in an amphibolite facies vein seven rb sr mineral isochron ages date amphibolite facies fluid infiltration at 414 2 2 8 ma mswd 1 5 an age value testifying to residence of the rocks in the deep orogenic crust at temperatures 600 c for nearly 15 ma the new data show that rb sr mineral isochron ages effectively date fluid induced re crystallization events rather than stages of cooling the direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the p t reaction time space which is essential for understanding exhumation histories and the internal dynamics of orogens in general

exhumation of the high pressure tsakkok lens swedish caledonides insights from the structural and white mica 40ar 39ar geochronological record

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

timing and kinematics of caledonian thrusting and extensional collapse southern norway evidence from 40ar 39ar thermochronology 40ar 39ar dating methods have been applied to rocks across the caledonian orogen in southwestern norway in the eastern nappe area k feldspar thermochronological modeling microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time two groups of muscovite and biotite ages from this area 415 408 for most samples with top to the se fabrics and 402 394 for samples with top to the nw fabrics are therefore interpreted as ages of contractional thrusting and extensional hinterland directed nappe translation deformation respectively in the west hinterland peak caledonian temperatures were higher and 40ar 39ar plateau ages are generally interpreted as cooling ages the western basement and lower nappes cooled rapidly through 500 c basement at 404 ma and 350 c basement and lower nappes shortly after i e during extensional top to the nw transport of the orogenic wedge in addition tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling probably following ordovician orogenic activity prior to the main collisional stage the new 40ar 39ar data conform to kinematic observations that contraction and extension in the caledonian nappe region were sequential and that the change from contraction convergence to extension divergence was quick between 408 and 402 ma

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

the geometry of listric growth faults in the devonian basins of sunnfjord w norway thick devonian clastic sequences accumulated in fault bounded basins in w norway at the close of the caledonian orogeny and mark the beginning of a phase of crustal extension traditionally the eastern boundary fault to the devonian basins has been regarded as a thrust but cross sections based on the use of a branch line reveal it to be an extension fault there is clear omission of structural section across the boundary and by matching up metamorphic rocks in the hanging and foot walls of the boundary fault over 40 km of extensional dip slip can be measured at the n end of the fault the boundary fault is a flat lying listric fault with rollover anticlines in the hanging wall the fault also cuts up and down section along strike to form lateral ramps parallel to the movement direction these ramps appear as wrench faults in outcrop the ramp flat geometry of the fault may have been inherited from a precursor caledonian thrust fault which reversed its sense of slip in early middle devonian times to form the basins over 25 km of onlapping sediment accumulated in the largest basin during faulting in 14 ma with an average slip rate on the fault of 3 mm yr 1

seasonal patterns of runoff and erosion responses to simulated rainfall in a badland area in mediterranean mountain conditions vallcebre southeastern pyrenees regolith surface characteristics and response were examined over a three year period in a badland area in a mediterranean middle mountain zone near vallcebre eastern pyrenees preliminary work carried out in this area indicated clear seasonal patterns of regolith properties driven by frost heaving in winter and crusting and erosion in the rest of the year rainfall simulations were performed with a small portable nozzle simulator in order to study seasonal changes in runoff generation erosion rates and raindrop effect on bulk density changes the results showed large seasonal variations in runoff and erosion responses in ltration rates after runoff start were correlated with precipitation depth before runoff start runoff generation was therefore related to regolith saturation only to a very limited extent erosion rates were more controlled by runoff rates than by the weakness of regolith against raindrop splash and sediment grain size increased with concentration the combined role of antecedent regolith moisture and bulk density explained most of the seasonal variability in in ltration bulk density changes during rainfall and erosion rates but some seasonal differences in sediment detachability were not explained by these variables and may be attributed to changes in roughness overall runoff and erosion responses were relatively stable during spring and autumn whereas wide variations in in ltration rates and sediment detachment occurred in winter and summer respectively experiments conducted in a single season would have produced poorly representative if not erroneous results copyright 2004 john wiley sons ltd

the western alps from the jurassic to oligocene spatio temporal constraints and evolutionary reconstructions abstract despite extensive research in the last 150 years the regional tectonic reconstruction of the western alps has remained controversial the curved orogenic belt consists of several ribbon like continental terranes sesia austroalpine internal crystalline massifs brianconnais which are separated by two or more ophiolitic sutures piemonte valais antrona lanzo canavese high pressure hp metamorphism of each terrane occurred during distinct orogenic episodes at 65 ma in the sesia austroalpine at 45 ma in the piemonte zone and at 35 ma in the internal crystalline massifs it is suggested that these events reflect individual accretionary episodes which together with kinematic indicators and the speed and direction of plate motions provide constraints for the discussed reconstruction model the model involves a prolonged orogenic history that took place during relative convergence of europe and adria here considered as a promontory of the african plate the first accretionary event involved the sesia austroalpine terrane final closure of the piemonte ocean occurred during the eocene 45 ma and involved ultra high pressure uhp metamorphism of the piemonte oceanic crust incorporation of the brianconnais terrane in the accretionary wedge occurred thereafter possibly during or after subduction of the valais ocean in the late eocene 45 35 ma this subduction was terminated at ca 35 ma when the internal crystalline massifs i e the assumed internal parts of the brianconnais terrane were buried into great depths and underwent hp and uhp metamorphism

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

from passive margins to orogens the link between ocean continent transition zones and ultra high pressure metamorphism a lithostratigraphic association consisting of serpentinized mantle rocks continent derived allochthons mid oceanic ridge gabbros of jurassic age and post rift sediments typical of an ocean continent transition is found in the eclogitic piemonte units in the western alps in situ u pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement in contact with serpentinites primary magmatic zircons of permian age were overgrown by a second generation of zircon at ca 166 150 ma likely related to melt infiltration associated with the intrusion of the underlying gabbroic body this indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the jurassic and they were probably part of hyper extended crust related to the opening of the tethys therefore the complex lithological association described here which is also characteristic of several ultra high pressure melange zones worldwide was acquired prior to the orogenic event during which it was only partly reworked ocean continent transitions are in positions favorable to reach ultra high pressure conditions following negatively buoyant oceanic lithosphere into subduction and then being accreted to the orogen in response to the arrival of more buoyant continental lithosphere resisting subduction the ocean continent transition is now found in the immediate footwall of a 500 m thick shear zone which accommodated multiple episodes of deformation during eocene oligocene time suggesting an important link between alpine deformation and rift related structures

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

protracted shearing at midcrustal conditions during large scale thrusting in the scandinavian caledonides during continental collision large tracts of crust are mobilised along major shear zones the metamorphic conditions at which these zones operate the duration of thrusting and the deformation processes that facilitated hundreds of km of tectonic transport are still unclear in the scandinavian caledonides the lower seve nappe displays a main mylonitic foliation with thickness of 1 km this foliation is overprinted by a brittle to ductile deformation pattern localized in c and c type shear bands proximal to the tectonic contact with the underlying sarv nappe thermobarometry of amphibolites and micaschists suggest a first high pressure stage at 400 500 c and 1 1 3 gpa recorded in mineral relics the main mylonitic foliation developed under epidote amphibolite facies conditions along the retrograde path from 600 c and 1 gpa to 500 c and 0 5 gpa age dating of synkinematic titanite grains in the amphibolites indicates that this mylonitic fabric formed at around 417 9 ma but older ages spanning 460 430 ma could represent earlier stages of mylonitization the shear bands developed at lower metamorphic conditions of 300 400 c and 0 3 gpa in the micaschists the recrystallized grain size of quartz decreases towards the shear bands monomineralic quartz layers are eventually dismembered to form polyphase aggregates deforming by dominant grain size sensitive creep accompanied by slip in muscovite and chlorite plagioclase zoning truncations suggest that the shear bands originated by fracturing followed by ductile deformation the results suggest protracted and long lasting shearing under amphibolite to greenschist facies conditions during the juxtaposition stacking and exhumation of the lower seve nappe

hydraulic conditions in experimental rill confluences and scour in erodible soils 1 few studies have examined processes active within rill networks or the effect of confluence geometry on rill sediment flux and it is not clear if observations from river confluences also apply at rill scale laboratory experiments were carried out in artificial rectangular rill channels with fixed and erodible beds to identify the effects of confluence geometry on hydraulic conditions and scour patterns symmetrical and asymmetrical confluences with angles from 19 to 90 were used hydraulic changes in the confluence zone in fixed bed experiments included decreased flow velocity increased shear velocity transition from supercritical to subcritical and from transitional to turbulent flow bed scour in the confluence zone resulted generally increasing with confluence angle but a more important influence was junction symmetry or asymmetry symmetrical confluences produced a symmetrical scour pattern mirroring the original channels but in asymmetrical confluences scour was more complex evolving to a symmetrical pattern with confluence angles higher than the original system results are consistent with channel evolution to an optimally branched system controlled by minimum power criteria but validation requires testing with more precise velocity measurement

microdiamond on areskutan confirms regional uhp metamorphism in the seve nappe complex of the scandinavian caledonides metamorphic diamond in crustal rocks provides important information on the deep subduction of continental crust here we present a new occurrence of diamond within the seve nappe complex snc of the scandinavian caledonides on areskutan in jamtland county sweden microdiamond is found in situ as single and composite diamond carbonate inclusions within garnet in kyanite bearing paragneisses the rocks preserve the primary peak pressure assemblage of ca mg rich garnet phengite kyanite rutile with polycrystalline quartz surrounded by radial cracks indicating breakdown of coesite calculated p t conditions for this stage are 830 840 c and 4 1 4 2 gpa in the diamond stability field the ultrahigh pressure uhp assemblage has been variably overprinted under granulite facies conditions of 850 860 c and 1 0 1 1 gpa leading to formation of ca mg poor garnet biotite plagioclase k feldspar sillimanite ilmenite quartz this overprint was the result of nearly isothermal decompression which is corroborated by ti in quartz thermometry chemical th u pb dating of monazite yields ages between 445 and 435 ma which are interpreted to record post uhp exhumation of the diamond bearing rocks the new discovery of microdiamond on areskutan together with other evidence of ultrahigh pressure metamorphism uhpm within gneisses eclogites and peridotites elsewhere in the snc provide compelling arguments for regional at least 200 km along strike of the unit uhpm of substantial parts of this far travelled allochthon the occurrence of uhpm in both rheologically weak gneisses and strong lithologies eclogites peridotites speaks against the presence of large tectonic overpressure during metamorphism

lithological properties and weathering response on badland hillslopes rills and rill networks are important geomorphological features in clay rich badland areas rill network geometry varies with climate and topography but the lithological properties are also critical rill systems on mudrock and sandstone in alberta badlands were studied to examine progressive changes in material properties as smectite rich mudrocks and sandstones undergo weathering and the impact of seasonal changes in precipitation patterns three plots with well developed rill systems on each unit were examined in may 2001 and 2003 geometric properties were determined along a dense grid and surface and subsurface samples were analyzed for texture crust structure sem mineralogical composition xrd content of major elements xrf aggregate stability and moisture content the rill width depth ratio decreased from 2001 to 2003 on mudrock rill networks the popcorn crust found in 2001 changed to a thin dense crust in 2003 2001 materials had higher na and ca concentrations and the 2003 materials have higher al k concentrations unweathered mudrock shards were subjected to incremental weathering in controlled laboratory conditions depending on moisture input different crust types developed on materials after 10 wetting drying cycles material responded to different amounts of precipitation by alteration from swelling to dispersion

late caledonian extension in the scandinavian caledonides the roragen detachment revisited abstract this paper concerns the interplay of compression and extension during palaeozoic collisional orogeny in scandinavia and focuses on a small old red sandstone basin in the central scandes at roragen early devonian sediments are preserved there in a half graben located in the western limb of a major late upright antiform the latter the vigelen antiform exposes proterozoic crystalline rocks in the core and folds the entire caledonian tectonostratigraphy of the lower middle and upper allochthons mylonites related to the eastward directed thrust assembly of these allochthons have been mapped over the antiform they occur in the western limb of the fold in the footwall of a high angle w dipping normal fault the latter separating the old red sandstones from the caledonian nappes these thrust generated mylonites exhibit a suite of superimposed down to nw shear sense indicators in the footwall of the fault displacement on this roragen detachment is inferred to be about ten kilometers reflection seismic profiling across the northern extension of the vigelen antiform in the storlien area indicates that this major longitudinal fold developed in the hanging wall of a sole thrust that dips very gently westwards beneath this part of the mountain belt it is argued here that the early devonian roragen half graben may have developed during thrust stacking of the vigelen antiform providing evidence of scandian upper crustal extension accompanying the emplacement of the thrust sheets onto the baltoscandian platform in the southern scandes full crustal extension would have occurred thereafter leading to the development of the mid devonian basins in the hinterland of westernmost norway

absolute timing of caledonian orogenic wedge assembly central sweden constrained by rb sr multi mineral isochron data abstract multi mineral rb sr isochron ages for mylonites of the caledonian nappe pile jamtland sweden constrain the orogenic evolution of the central scandinavian caledonides an age of 437 8 3 9 ma from a metamorphic leucosome in the central nappe stack dates crystallization of migmatized gneiss thirteen isotopic ages derived from mylonites across all structural levels are between 434 6 6 9 ma and 426 3 3 1 ma these results complemented by a dense network of kinematic field data are interpreted to reflect the timing of protracted top to the ese general shear under decreasing amphibolite to upper greenschist facies conditions across all nappes three samples derived from the top middle and bottom of the nappe pile yield additional deformation ages between 416 0 3 6 ma and 410 1 3 0 ma these ages are interpreted to represent post assembly imbrication of the nappe stack under decreasing metamorphic conditions the new tectonochronologic data are consistent with a tectonic model that explains nappe stacking in jamtland by three stages of ese directed in and out of sequence thrusting in the first stage postdating earlier subduction exhumation of the high grade metamorphic seve nappe complex the latter was emplaced in sequence in the lower mid crust above nappes in its footwall foreland at c 430 ma this event is related to the collision of baltica with an island arc concurrently at c 430 ma determined by paleogeography and structurally unrelated to the first stage second stage out of sequence thrusting emplaced the koli nappe complex atop the developing nappe stack during final baltica laurentia collision eventually imbrication and exhumation of the composite jamtland nappe stack in the uppermost middle crust during the third stage at c 414 ma was caused by continuous underthrusting of baltica beneath laurentia this three stage thrust model reflects the complex interactions of baltica arc laurentia collisions that were accommodated by two merging subduction zones

p t paths in the handl area central scandinavia record of caledonian accretion of outboard rocks to the baltoscandian margin the seve koli nappe complex is widespread in the scandinavian caledonides and is composed of units representing parts of the baltoscandian margin seve nappes now overlain by magmatic sedimentary rocks koli nappes derived from west of this margin the metamorphic evolution of koli and seve units has been studied in the handol area central scandinavian caledonides where a fragmented ophiolite with cover sequence in the lower koli units is thrust over the higher grade seve units thermobarometry constrains metamorphic conditions to 490 570 c 950 600 mpa with a slight downwards increase in grade for the lower koli bunnerviken lens 520 620 c 1000 600 mpa for the upper seve taljstensvalen complex 630 740 c 750 650 mpa for the middle seve snasahogarna nappe and 480 600 c 1150 1000 mpa for the lower seve blahammarfjallet nappe p t paths during garnet growth have been constructed for all units except the highest grade middle seve these paths record heating at the base of the koli and cooling in the underlying seve units pressure increase during garnet growth is indicated for all units leading to anticlockwise p t paths in the seve the results imply thermal convergence with time for all units and spatial convergence in metamorphic grade in the koli it is suggested that the contrasting metamorphic histories on either side of the seve koli boundary resulted from the emplacement of relatively colder koli rocks on top of relatively hotter seve rocks and that emplacement of structurally higher units contributed to the increase in pressure

evidence from rb sr microsampling geochronology for the timing of alpine deformation in the sonnblick dome se tauern window austria

tectonic stratigraphic and sedimentologic significance of a regional discontinuity in the upper judith river group belly river wedge of southern alberta saskatchewan and northern montana the lithostratigraphic interval between the taber and lethbridge coal zones in the upper portion of the nonmarine judith river group of southeastern alberta is divisible into two lithostratigraphic units separated by a regionally extensive and diachronous discontinuity the lower unit referred to here as the oldman formation is characterized by very fine grained to fine grained sandstones that contain fewer than 2 volcanic rock fragments sandstone bodies with numerous sets of horizontally stratified sandstone showing little or no evidence of lateral accretion siliceous paleosols ganisters and a relatively high gamma ray signal in the upper half of the formation the oldman formation comprises deposits of a low sinuosity perhaps ephemeral fluvial system that originated in the southern cordillera of canada and northern montana and flowed northeastward perpendicular to the axis of the alberta basin the upper unit is assigned to a new formation the dinosaur park formation and is characterized by

geomorphic agents versus vegetation spreading as causes of badland occurrence in a mediterranean subhumid mountainous area abstract previous evidence on the chief role of physical weathering on badland development in the vallcebre area south eastern pyrenees led to the study of the amounts of energy available for the main geomorphic agents and to analyze the results in the light of the patterns of vegetation cover and species distribution in order to explain both the occurrence of these badland forms and their preference for north facing shady hillslopes results showed that the weathering potential provided by freezing is about two orders of magnitude greater than rainfall kinetic energy and that the former strongly depends on hillslope aspect the study of vegetation demonstrated that vegetation is poorer in cover and species composition on shady hillslopes than on sunny ones spontaneous revegetation of these surfaces being restrained by cold thermal conditions rather than by water availability these results show that the occurrence of these badland areas depends on montane climatic attributes that control geomorphic and biological processes and that are different from those that promote the occurrence of badlands in dry mediterranean areas the preference for northern aspects provides evidence for these differences

u pb ages of ophiolites and arc related plutons of the norwegian caledonides implications for the development of iapetus u pb zircon ages are reported for four ophiolites and three crosscutting arc related plutons from the norwegian caledonides plagiogranite differentiated from gabbro of the karmoy ophiolite is dated at 493 7 4 ma whereas arc related trondhjemite cutting this ophiolite crystallized at 485 2 ma a crosscutting clinopyroxene phyric gabbro intrusion is dated at 470 9 5 ma by near concordant magmatic titanite sphene and discordant u rich 2903 6677 ppm zircon lower intercepts of 247 68 and 191 70 ma defined by the plagiogranite and clinopyroxene phyric gabbro best fit lines may reflect a real low t alteration rift related event

deformation induced resetting of rb sr and 40ar 39ar mineral systems in a low grade polymetamorphic terrane eastern alps austria a paradox in dating metamorphic events in low grade polymetamorphic terranes is exemplified by the eastern lower austroalpine nappes of the eastern alps here the last metamorphic event best recorded in post variscan cover rocks is dated as late cretaceous in age c 80 85 ma by white mica rb sr and 40 ar 39 ar systems within the underlying polymetamorphic basement 40 ar 39 ar and rb sr ages of phengitic white mica record only early and or late variscan ages 375 270 ma indicating that the alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of variscan mineral ages based on these results the timing of a penetrative ductile top to wnw simple shear deformation recorded within both basement and cover rocks was contradictory deformation within the post variscan cover rocks had to be alpine in age whereas phengite 40 ar 39 ar ages from basement mylonites yield variscan ages to date this deformation directly we isolated different mineral size fractions 63 30 and 30 10 m from highly strained shearbands within the wechsel basement nappe a resulting rb sr errorchron pointed to an upper cretaceous age c 85 ma for the deformation consistent with the timing of alpine metamorphism in the cover rocks coarser grained white mica 150 300 im from similar basement mylonites do not reflect any alpine overprint of either k ar and or rb sr systems it follows that dynamic re and or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the rb sr system locally virtually re equilibrated this is valid even for overprinting metamorphic conditions below the temperatures required for ar diffusional loss in phengitic white mica c 40 ar 39 ar should be considered with caution

the role of some site geochemical processes in the development and stabilisation of three badland sites in almeria southern spain samples of both surface and subsurface soils were collected on various surfaces in three badland areas of differing ages tabernas vera and mocatan in almeria province southern spain data were explored with the aim of identifying any diagnostic geochemical relationships between site ec ph sediment size and sar which might constitute typical signatures for site materials with some success data for the three sites are further used to explore the hypothesis that changes in site geochemistry through time are involved in badland stabilisation it was found that 1 the physico chemical properties of the three sites can be differentiated by the functional relationship between ec and sar and this may represent a useful tool in characterising sites where piping predominates 2 there is some equivocal evidence that sediment size distribution may play an additional role in site diagnosis and evolution and 3 surfaces appear to lose sodium over time the suggestion being that sodium is either progressively buffered by calcium during leaching or exchanges with hydrogen on the cation exchange sites under a vegetation cover since other research suggests that smectite can also relocate down the profile on some marl sites reducing hydraulic conductivity and suppressing pipes we argue that the surface at tabernas may be capable of long term autostabilisation in which piping intensity progressively reduces however this possibility really applies only to fine grained densely structured materials and evidence from the silty triassic rich unit in the mocatan basin suggests that without a high clay content piping may be expected to increase through time from this standpoint contemporary badland morphology at mocatan does not represent an early stage of the forms seen in vera and tabernas

accretion and lateral extension in an orogenic wedge evidence from a segment of the seve ko li terrane boundary central scandinavian caledonides abstract the seve ko li boundary in the central scandinavian caledonides represents the transition from baltica continental margin related rocks seve to outboard westerly derived terranes ko li in this study we present and discuss structural and kinematic data recording the syn and post accretionary evolution of this boundary in west central sweden amphibolite facies thrusting mylonitization imbrication and folding occurred before and during the establishment of the seve ko li boundary subsequent asymmetric lateral extension considerably modified earlier structures and the boundary at lower grade contractional ramps and extensional synforms developed and translation continued during progressive deformation kinematic analysis indicates dominant movement towards the east or southeast local movement in the opposite direction may be due to stretching and rotation in the hanging wall above extensional shear zones a pattern of minor shear zones and shear bands demonstrate stretching in an approximately e w direction the presence of extensional structures developed during top to the east movement suggests alternative mechanisms for the formation of some characteristic caledonian structures e g late synforms and thin lenticular tectonic units these features may be due to asymmetric extension during scandian top to the east motion of the nappe pile contemporaneously basement antiforms may have been stacked at lower tectonic levels

low temperature recrystallization in calcite mechanisms and consequences vein calcite dominated fault rocks collected from several locations show evidence for intense intracrystalline plasticity and interface twin and grain boundary mobility leading to dynamic recrystallization of calcite at temperatures 150 250 c significantly below those at which these features are commonly anticipated these observations require a reappraisal of calcite deformation at low temperature particularly the capability for dynamic recrystallization in the apparent absence of significant thermally activated recovery processes the cyclic introduction of coarse grained calcite veins is observed to be essential for the initiation of intracrystalline deformation and associated dynamic recrystallization the introduction of veins generates an essentially monomineralic rock of a grain size larger than the protolith as a result the mylonitization does not occur within a given protolith but rather in the introduced secondary calcite through hall petch type grain size dependent dislocation interactions stress is locally increased and the resulting increase in dislocation densities promotes grain boundary migration the recognition that nominal high temperature creep processes and associated microstructures can occur outside their expected temperature range has implications for fault rheology strength and fault permeability and porosity

replacement reactions and deformation by dissolution and precipitation processes in amphibolites the deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition fluid content pressure and temperature these mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation here we show that fracturing and fluid flow triggered coupled dissolution precipitation and dissolution precipitation creep processes which were responsible for the formation of a mylonitic microstructure in amphibolites this fabric is developed over a crustal thickness of 500 m in the lower seve nappe scandinavian caledonides amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence the albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the 001 cleavage planes and display lobate edges with embayments and peninsular features two albite oligoclase generations bright in cathodoluminescence resorbed and overgrew the porphyroclasts sealing the fractures electron backscattered diffraction shows that the two albite oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them these two albite oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation the amphibole crystals experienced a similar microstructural evolution as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c axis no crystal plasticity is observed in either mineral species plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies 600 c 0 75 0 97 gpa our results demonstrate that at middle to lower crustal levels the presence of h2o rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution precipitation and favours deformation by dissolution precipitation creep over dislocation creep in plagioclase and amphibole

erosion and sediment transport on steep marly hillslopes draix haute provence france an experimental field study abstract despite a thin regolith layer measured sediment yields indicate denudation rates of more than 10 to 20 mm yr 1 on strongly denuded slopes in black marl terres noires near draix alpes de haute provence overland flow and rainfall simulation experiments were conducted on three slopes in the draix erosional research basin to determine the nature and intensity of hillslope erosion processes the experiments showed that during a single storm sediment transport includes water solid mixtures with a wide range of concentrations from highly charged rill flow up to 400 g l 1 to hyperconcentrated flow up to 800 g l 1 to debris flows up to 1400 g l 1 miniature debris flows mdfs strongly controlled sediment transport as long as the sediment store was not depleted slope surfaces were completely reworked in the winter and the regolith layer was rebuilt temperature records from nine recent winter periods indicate that conditions are favourable for soil frost processes

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

rb sr microchrons of synkinematic mica in mylonites an example from the dav fault of the eastern alps abstract a technique for texturally controlled microsampling from rock thick sections and subsequent rb sr dating of g sized samples has been developed it allows direct dating of minerals unequivocally related to deformation the technique has been applied to strictly synkinematically grown white mica prepared from mylonites related to the defereggen antholz vals dav fault south of the tauern window eastern alps control of deformation ages is provided by field relationships with independently dated plutons and dykes dated here by single mineral rb sr the field observations and isotopic ages are fully consistent and demonstrate that this is a reliable method for sub mm scale geochronology the ages obtained establish that sinistral strike slip shearing associated with the dav fault lasted from at least 33 ma to 30 ma followed by an abrupt change to dextral transpression related to the adjacent periadriatic fault

rill network development and sediment budgets threshold conditions for rill incision are well known but few studies have examined the effect of network geometry on water and sediment fluxes within an evolving rill system this paper reports the first in a series of studies designed to identify the influence of soil properties on rill network and confluence geometry and on water and sediment fluxes in rill systems the object of this study was to prepare detailed rill network water and sediment budgets identifying important sources and sinks simulated rainfall experiments were carried out on a canadian silt loam soil in a 7 1 m 2 4 m flume on a 5 slope rill networks of varying complexity developed which were ultimately constrained by flume boundaries sediment and water fluxes and hydraulic conditions were measured within networks and at a terminal weir networks evolved by initial knickpoint incision in the lower flume as flow shear velocities reached critical levels of 4 5 5 cm s 1 followed by headward migration and tributary development microtopography determined tributary location but the timing and intensity of development were controlled by the incision and migration rate of the main channel which changed local thalwegs and base levels raising shear velocities on side slopes above critical levels sediment discharge at the weir broadly reflected rill incision intensity and transport limited conditions but as active incision moved headward the linkage became attenuated and identification of discrete erosional incidents e g local bank collapse in the signal of the weir record became very difficult detailed water and sediment budgets showed much more complex patterns of localized incision and deposition within networks strongly influenced by local changes in thalweg and in water discharge due to seepage or return flow and by the effect of confluences on hydraulic conditions results indicate the value of detailed sediment budgets in interpreting weir water and sediment flux records and the necessity of linking such measurements for erosion plots and hillslope segments to rill network characterization experimental results are consistent with a simple model of rill system evolution based on channel incision headward migration and the critical shear velocities for rill initiation copyright 2000 john wiley sons ltd

evolution of a gneiss in the seve nappe complex of central sweden hints at an early caledonian medium pressure metamorphism abstract ultrahigh pressure uhp metamorphism has recently been reported from various crustal rocks in the seve nappe complex snc in which microdiamonds were found we studied a gneiss from the areskutan nappe in central sweden this nappe of the snc was recently interpreted to have experienced uhp metamorphism the studied medium grained and well foliated gneiss contains quartz biotite garnet k feldspar plagioclase and potassic white mica with accessory kyanite sillimanite rutile ilmenite and monazite pressure p temperature t pseudosections were mainly used to decipher the metamorphic evolution of the gneiss according to polyphase inclusions such as biotite chlorite potassic white mica in the garnet core the prograde p t conditions were approximately 600 c and 8 8 5 kbar an isobaric heating to 700 c followed constrained by homogenous garnet with xca 0 03 and xmg 0 295 the highest si content of 3 15 per formula unit in potassic white mica and zr in rutile thermometry a late migmatitic stage occurred at about 725 c and 6 5 kbar u th pb dating of monazite included in garnet with the electron microprobe yielded two populations at 463 12 2 ma y2o3 2 0 wt and 458 7 2 ma y2o3

crusted microtopography on badland slopes in southeast spain summary badland slopes developed in soft miocene marine marls of the guadix baza depression of southeast spain have four crust types developed on them the bare crusted surfaces are confined to slopes with southerly aspects the north facing slopes are sparsely vegetated and these crusts are generally absent the crusts and the weathered mantle on which they occur have been tested for soil ec aggregate stability and investigated in thin section the distribution of crust types in a representative badland gully has been mapped one of the crust types the pediment crust is a depositional feature found on low angled pediments flanking the steep gully side slopes the other three crust types domed tower and pinnacle represent a sequence of increasing age and increasing stability associated with progressive colonisation by the blue green algae microcoleus vaginatus and scytonema and the lichen dermatocarpon lachneum while this is accompanied by a progressive reduction in soil ec the evidence suggests that the algae and lichen play an important role in stabilising the soil surface the development sequence of these crust types is outlined

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

thrust related metamorphism in basement windows of the central scandinavian caledonides the late caledonian metamorphic evolution of the basement beneath the caledonian allochthons of the central scandes has been determined by studying mafic schists and phyllonites of the crystalline basement and the overlying vendian and or cambrian sedimentary rocks the studied rocks occur along the caledonian front and in tectonic windows within the scandinavian caledonides a gradual westwards increase in metamorphic grade is recorded which varies from upper diagenetic conditions in the autochthonous rocks along the eastern thrust front of the caledonides to middle amphibolite facies in the western parts of the nasafjall and tomeras windows locally strain related deviations from this pattern occur further west the grade of caledonian metamorphism of the basement increases and reaches the eclogite and granulite facies the scandian metamorphic conditions in the basement rocks studied prevailed in the mid to late silurian at pressures of 8 2 kbar and temperatures of 550 50 c in the tommeras window and 7 2 kbar at 550 50 c in the nasafjall window uplift and cooling followed thereafter continuing into the devonian the final retrograde metamorphism was characterized by temperatures of 450 50 c at pressures of about 3 kbar in the nasafjall and tommeras windows the occurence of syn schistosity amphibolite facies assemblages implies heating of the basement rocks during nappe emplacement available time constraints suggest that this was a relatively rapid process in late silurian to early devonian the prograde phase was followed by uplift and erosion the metamorphic evolution also demonstrates that early devonian thrust movements were associated with a decrease in the thickness of the nappe pile

microdiamond discovered in the seve nappe scandinavian caledonides and its exhumation by the vacuum cleaner mechanism when a continent collides with an island arc or other continent continental crust of the subducted continent may be buried to depths exceeding 100 km and exposed to pressures that can cause formation of coesite and diamond this process leads to substantial density increase in sio2 rich rocks and in turn to a reduction of the buoyancy of the subducted material which should inhibit exhumation nevertheless coesite and diamond bearing continental crustal rocks are known from several occurrences worldwide we report on the discovery of microdiamond in kyanite garnet gneiss from allochthonous metasediments of the seve nappe complex in the scandinavian caledonides our discovery calls for general reconsideration of existing exhumation models of deeply subducted continental crust we propose that the diamond bearing rocks were subducted in an arc continent collision setting and their exhumation was facilitated by local pressure reduction resulting from extraction of the forearc lithospheric block

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

40 ar 39 ar mineral age record of early caledonian tectonothermal activity in the baltoscandian miogeocline central scandinavia in central portions of the scandinavian caledonides greenschist facies volcanosedimentary successions within the koeli nappe complex have been thrust several hundred kilometers eastward onto the baltoscandian platform these were derived from eugeoclinal terranes situated outboard west of the baltica continent during the early paleozoic the koeli nappe complex is tectonically underlain by higher grade units within the seve nappe complex these are composed of amphibolite and granulite facies rocks and locally contain eclogites the seve nappes tectonically separate koeli units from structurally lower allochthons derived from more inboard environments along the baltoscandian miogeocline previous mineral isotopic age determinations from seve and koeli units have been in the 430 to 390 ma range and have been interpreted to presumably date cooling following scandian middle silurian to early devonian metamorphism however incremental release sup 40 ar sup 39 ar dates recorded by minerals within some of the koeli and seve nappes exposed in jaemtland sweden taennforsen and are districts provide evidence of earlier tectonothermal activity hornblendes from the seve and koeli nappe complexes display variably discordant age spectra as a result of low temperature experimental evolution of loosely bound extraneous argon components however in most analyses plateau ages of 510 to 475 ma koeli and 465 to 455more ma seve are defined in contrast muscovite and biotite from all tectonic units record scandian cooling ages between 245 and 410 ma the older events recorded by hornblende within these seve and koeli units are evidence of early caledonian tectonothermal activity and subsequent diachronous cooling during the early middle ordovician less

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

timing of metamorphism in the tauern window eastern alps rb sr ages and fabric formation the peripheral schieferhulle of the tauern window of the eastern alps represents post hercynian penninic cover sequences and preserves a record of metamorphism in the alpine orogeny without the inherited remnants of hercynian events that are retained in basement rocks the temperature time deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose p t evolution and structural setting are already well understood the eclogite zone of the central tauern formed from protoliths with penninic cover affinities and suffered early alpine eclogite facies metamorphism before tectonic interposition between basement and cover it then shared a common metamorphic history with these units experiencing blueschist facies and subsequent greenschist facies conditions in the alpine orogeny the greenschist facies phase associated with penetrative deformation in the cover and the influx of aqueous fluids reset sr isotopes in metasediments throughout the eclogite zone and cover schists recording deformation and peak metamorphism at 28 30 ma the peripheral schieferhulle of the south east tauern window yields rb sr white mica ages which can be tied to the structural evolution of the metamorphic pile early prograde fabrics pre date 31 ma and were reworked by the formation of the large north east vergent sonnblick fold structure at 28 ma peak metamorphism post dated this deformation but by contrast to the equivalent levels in the central tauern peak metamorphic conditions did not lead to widespread homogenization of the sr isotopes localized deformation continued into the cooling path until at least 23 ma partially or wholly resetting sr white mica ages in some samples these isotopic ages may be integrated with structural data in regional tectonic models and may constrain changes in the style of crustal deformation and plate interaction however such interpretations must accommodate the demonstrable variation in thermal histories over small distances

calcite twin morphology a low temperature deformation geothermometer abstract twinning of the e plane is the dominant crystal plastic deformation mechanism in calcite deformed below about 400 c calcite in a twin domain has a different crystallographic orientation from the host calcite grain so called thin twins appear as thin black lines when viewed parallel to the twin plane at 200 320 magnification under a petrographic microscope thick twins viewed in the same way have a microscopically visible width of twinned material between black lines calcite e twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse grained calcite in this paper we present a compilation and analysis of data from limestones of the frontal alps france and switzerland and the appalachian valley and ridge and plateau provinces eastern united states to document this temperature dependence mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 c and thick twins dominate above 200 c above 250 c dynamic recrystallization is an important deformation mechanism in calcite mean twin intensity twin planes mm correlates negatively with temperature and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation these relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data

onset of seafloor spreading in the iapetus ocean at 608 ma precise age of the sarek dyke swarm northern swedish caledonides abstract the sarek dyke swarm sds crops out in the sarektjakka nappe sn of the seve kalak superterrane in the northern swedish caledonides the sn has two main components 1 a 4 5 km thick succession of rift related sedimentary rocks which is intruded by 2 a suite of tholeiitic dykes the sds constituting 70 80 of the nappe the nappe was deformed during caledonian thrusting but dykes and sedimentary rocks in the interior of the eastern parts of the sn are preserved in a pristine state the tholeiitic dykes of the sds commonly occur in sheeted dyke complexes and up to 11 successive generations can be identified from crosscutting relations the sn represents the fossil continent ocean transition between the baltic craton and the iapetus ocean marking the initiation of seafloor spreading bubble shaped pods and veinlets of diorite are present in the sds sheeted dyke complexes the pods are absent in the oldest dykes but the younger a dyke the more frequent the pods the diorite pods are the equivalent of gabbro pegmatites and both cogenetic and coeval with the dykes the rapid successive emplacement of tholeiitic magma raised the ambient temperature in the dyke complex so that crystallization in the youngest dykes mimicked similar processes in gabbro plutons six zircon fractions from the diorite pods including two single grains were analysed geochronologically by the u pb thermal ionization mass spectrometry method the data yield a linear array of points that are 0 4 0 8 normally discordant indicating a crystallization age of 608 1 ma 207pb 206pb 607 9 0 7 ma mswd 0 33 this age is inferred to date the onset of seafloor spreading in the iapetus ocean along the baltoscandian margin