a newly recognized type of slickenside striation abstract this note summarizes current views on the origin of slickenside striations and points to the existence of a type of striation not readily explained by established genetic ideas such striations have been seen on fault surfaces in naturally deformed shale and in experimentally deformed paraffin wax they are defined by arrays of shallow slip parallel ridges and grooves of millimetric width and below on any given slip surface the ridges are as common as the grooves and display similar dimensions and profile shapes across any pair of slip surfaces in contact at the end of faulting ridges protruding from one surface fit into or nest with grooves on the other surface the origin of these features is not yet understood

late quaternary movement of aspy fault nova scotia an emerged sangamonian 125 ka intertidal rock platform at 7 m asl is apparently upthrown to 22 m asl along aspy fault on cape breton island the 15 m displacement may be an isostatic response to rapid ice retreat or erosional unloading or it may be a tectonic effect due to release of crustal stress stored during wisconsinan glacier cover

relationship between non cylindrical fold geometry and the shear direction in monoclinic and triclinic shear zones we investigate the relationship between non cylindrical fold geometry and the shear direction of the hosting high strain zone by numerical modeling and show that apical axes of non cylindrical folds may develop into directions highly oblique to the shear direction if the zone has a pure shear component the common practice of using well developed sheath folds as indicators for the shear direction is not reliable and the use of immature non cylindrical or sheath folds appears more reliable hinge lines of mature sheath folds approach parallelism with the fabric attractor to which all material lines rotate which can have variable angles with respect to the shear direction in thinning zones the fabric attractor is the direction of the maximum principal strain rate of the pure shear component and is parallel to the shear zone boundary in thickening zones it lies somewhere in the quadrant between the direction of the maximum principal strain rate of the pure shear component perpendicular to the shear zone boundary and the simple shear direction and is generally oblique to the shear zone boundary the exact location depends on the flow geometry of the shear zone 2007 elsevier ltd all rights reserved

alleghanian reactivation of the acadian fold belt meguma zone southwest nova scotia shear zones and northwest verging folds define a 30 km wide belt of deformation that overprints the acadian fold belt and telescopes isograds in the meguma zone in southwest nova scotia the shear zones appear to form a linked system that accommodated convergence dominated transpression of the meguma zone against an irregular avalon boundary available geochronological data indicate a mid carboniferous alleghanian variscan age for the overprinting deformation the mid carboniferous basement reactivation in southwest nova scotia is likely coeval with deformation of carboniferous strata and reactivation of basement structures meguma group in the northern meguma zone together these mid carboniferous structures may define a wide belt of alleghanian variscan deformation across the northwest cratonward margin of the meguma zone

geophysical studies of the structure of the appalachian orogen in the atlantic borderlands of canada results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the appalachian orogen in eastern canada wide angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins high velocity lower crust attributed to underplating is found below the former laurentian continental margin in newfoundland and below the magdalen basin potential field data are used to trace the surface zones of the orogen from the northeast newfoundland shelf to cape breton but extrapolation to new brunswick and quebec is unclear because of late paleozoic basin development the central mobile belt of the orogen is only a few tens of kilometres wide in southwest newfoundland and cape breton but broadens substantially to around 200 km elsewhere reflection images show a strong deep crustal fabric that runs along the orogen with a margin that crosses into the avalon zone in southern newfoundland but coincides with the avalon gander zone boundary el

drivers of explosivity and elevated hazard in basaltic fissure eruptions the 1913 eruption of ambrym volcano vanuatu sw pacific abstract fissure eruptions along linear structures can extend for several tens of kilometres with distinct separate manifestations of volcanism along their length they typically involve low viscosity mafic magmas forming long lava flows and cinder cones eruptions in 1894 and 1913 on ambrym volcano vanuatu showed how these mildly explosive eruptions can rapidly transform into violent explosive events producing significant hazard and widespread volcanic ash clouds during the 1913 episode a fissure began in the central caldera and basaltic magmas broke out in a series of locations down the island s western flank in all sites over 100 m in elevation fissure outbreaks produced vigorous lava fountains and highly fluid lava flows that travelled rapidly to the shoreline when the outbreaks propagated along the island s axis into coastal plain areas a climactic series of explosive eruptions occurred producing a 1 2 km long by 600 m wide maar and tephra ring a further small tuff ring was formed later creating a temporary island 400 m offshore the onshore tephra ring destroyed a hospital and associated buildings its last evacuating occupants were close witnesses to the eruption processes deposits exposed in the lower portion of the tephra ring show that this part of the eruption began with a mild phreatomagmatic explosive eruption from a narrow vent followed by a magmatic scoria producing phase subsequently a complex sequence of highly explosive phreatomagmatic eruptions occurred producing pyroclastic surges along with repeated distinctive breccia horizons rich in coral and lava country rock these features tally with eye witness accounts to indicate that the main eruption phase was produced by a periodically shifting locus of phreatomagmatic fragmentation and eruption along a single e w fissure the glassy and vesicle poor pyroclasts produced during this eruption phase were dominantly fragmented in a brittle manner by magma water interaction low volatile content of the magma upon fragmentation is confirmed by ftir analysis showing

stratigraphy structure and lode gold system at the central manitoba mine trend rice lake greenstone belt archean superior province manitoba canada abstract the central manitoba mine trend is one of the most important lode gold districts in the rice lake greenstone granitoid belt of the western uchi subprovince within western superior province manitoba canada neoarchean host rocks consist of a south facing metavolcano sedimentary succession 2 75 2 73 ga intruded by voluminous gabbroic sills and tonalitic granodioritic plutons 2 73 2 72 ga as well as late aplite dikes 2 73 2 72 ga and quartz feldspar porphyry dikes 2 73 2 71 ga five generations g1 g5 of deformation structures have been recognized through detailed geological mapping the entire succession was folded during g1 and g2 prior to rare late aplite dike emplacement all fault fill veins and extension veins cut all lithologic units and are structurally governed by late g3 and g4 conjugate shear zones main gold mineralization occurs within fault fill veins hosted by west trending steeply dipping g4 dextral brittle ductile and ductile shear zones which occur along or across contacts of metabasalt metagreywacke and metagabbro or entirely within metagabbro microstructural and paragenetic analyses on main gold bearing veins have revealed that gold is intimately associated with quartz pyrrhotite and tellurobismuthite main gold introduction is interpreted to have taken place contemporaneously with pyrrhotite and tellurobismuthite deposition early during g4 shearing these results in structural and mineralogical aspects at the central manitoba mine trend provide new constraints on structural evolution and gold mineralization of the southeastern rice lake belt and can be used to improve exploration models for gold deposits in archean terranes

subaqueous basaltic magmatic explosions trigger phreatomagmatism a case study from askja iceland abstract sequences of basaltic pillow lavas that transition upward with systematic gradation from pillow fragment breccias to fluidal bomb bearing breccias to bomb bearing lapilli tuffs are common at askja volcano iceland based on the detailed textural investigation of three of these sequences we argue that they record temporally continuous transitions from effusive to explosive products that were erupted from and deposited at or near a single subaqueous vent the recognition of such sequences is important as they provide evidence for controls on the onset of explosive activity in subaqueous environments such investigations are complicated by the interplay of magmatic gas expansion and phreatomagmatic and mechanical granulation fragmentation mechanisms in the subaqueous eruptive environment all of the sequences studied at askja have textural componentry and sedimentological characteristics suggestive of a close genetic and spatial relationship between the pillow lavas and all of the overlying glassy clastic deposits the identification of magma fragmentation signatures in pyroclasts was accomplished through detailed textural studies of pyroclasts within the full range of grain sizes of a given deposit i e bomb blocks lapilli and fine ash these textural characteristics were compared and evaluated as discriminators of fragmentation in pyroclastic deposits the presence of angular vitric clasts within the breccia and lapilli tuff displaying fragile glassy projections indicates little or no post depositional textural modification a shift in vesicle and clast textures between the pillow lavas and the large concentration of fluidal bombs in the breccia indicates that the phreatomagmatic explosions were initially triggered by magmatic vesiculation the initial magmatic gas expansion may have been triggered by depressurization caused by the drainage of the ice confined lake surrounding askja the fuel coolant interactions fcis of the more efficient phreatomagmatic explosions were enabled by the increase in the surface area to volume ratio of the fluidal bombs in the water producing a premix of magma and water the onset and increasing influence of phreatomagmatic fragmentation is preserved in the presence of very fine blocky ash particles and diminished presence of larger particles such as fluidal bombs the textural sedimentological and environmental characteristics of these deposits suggest that phreatomagmatic explosions can be triggered by initial magmatic gas expansion but that it is likely one of many mechanisms for triggering such explosions

mechanisms of hydrovolcanic pyroclast formation grain size scanning electron microscopy and experimental studies pyroclasts produced by explosive magma water interactions are of various sizes and shapes data from analysis of over 200 samples of hydrovolcanic ash are interpreted by comparison with experimentally produced ash grain size and scanning electron microscopy sem reveal information on the formation of hydrovolcanic pyroclasts strombolian explosions result from limited water interaction with magma and the pyroclasts produced are dominantly centimeter sized with increasing water interaction hydrovolcanism increases in explosivity to surtseyan and vulcanian activity these eruptions produce millimeter to micron sized pyroclasts the abundance of fine ash 63 m diameter increases from 5 to over 30 percent as water interaction reaches an explosive maximum this maximum occurs with interactions of virtually equal volumes of melt and water five dominant pyroclast shape types determined by sem result from hydrovolcanic fragmentation 1 blocky and equant 2 vesicular and irregular with smooth surfaces 3 moss like and convoluted 4 spherical or drop like and 5 plate like types 1 and 2 dominate pyroclasts greater than 100 m in diameter types 3 and 4 are typical of fine ash type 5 pyroclasts characterize ash less than 100 m in diameter resulting from hydrovolcanic fragmentation after strong vesiculation fragmentation mechanisms observed in experimental melt water interactions result from vapor film generation expansion and collapse fragments of congealed melt are products of several alternative mechanisms including stress wave cavitation detonation waves and fluid instability mixing all result in rapid heat transfer these mechanisms can explain the five observed pyroclast shapes stress wave fracturing cavitation of the melt results from high pressure and temperature gradients at the melt water interface simultaneous brittle fracture and quenching produces type 1 pyroclasts type 2 develops smooth fused surfaces due to turbulent mixing with water after fracture and before quenching fluid instabilities promote turbulent mixing of melt and water and produce fine ash this kind of fragmentation occurs during high energy explosions the increased melt surface area due to fine fragmentation promotes high efficiency heat exchange between the melt and water shapes of resulting pyroclasts are determined by maximum surface area type 3 and surface tension effects type 4 type 5 pyroclasts result from nearly simultaneous vesicle burst and melt water fragmentation

large explosive basaltic eruptions at katla volcano iceland fragmentation grain size and eruption dynamics abstract katla volcano in iceland produces hazardous large explosive basaltic eruptions on a regular basis but very little quantitative data for future hazard assessments exist here details on fragmentation mechanism and eruption dynamics are derived from a study of deposit stratigraphy with detailed granulometry and grain morphology analysis granulometric modeling componentry and the new quantitative regularity index model of fragmentation mechanism we show that magma water interaction is important in the ash generation process but to a variable extent by investigating the large explosive basaltic eruptions from 1755 and 1625 we document that eruptions of similar size and magma geochemistry can have very different fragmentation dynamics our models show that fragmentation in the 1755 eruption was a combination of magmatic degassing and magma water interaction with the most magma water interaction at the beginning of the eruption the fragmentation of the 1625 eruption was initially also a combination of both magmatic and phreatomagmatic processes but magma water interaction diminished progressively during the later stages of the eruption however intense magma water interaction was reintroduced during the final stages of the eruption dominating the fine fragmentation at the end this detailed study of fragmentation changes documents that subglacial eruptions have highly variable interaction with the melt water showing that the amount and access to melt water changes significantly during eruptions while it is often difficult to reconstruct the progression of eruptions that have no quantitative observational record this study shows that integrating field observations and granulometry with the new regularity index can form a coherent model of eruption evolution

slickenside and fault surface kinematic indicators on active normal faults of the alpine betic cordilleras granada southern spain abstract twenty four mesoscopic slickenside kinematic indicators are described here for the pliocene quaternary active normal faults of the alpine betic cordilleras granada southern spain the indicators are classified into twelve groups depending on the down or up slope asymmetric orientation of features such as concavities damaged versus sharp grain borders or step edges v shaped markings fractures and trailed material seventeen of the indicators were not previously described in the literature the new kinematic indicators include two types of asymmetric grains a carrot shaped marking six substructures of congruous and incongruous steps trailed fault material a drop shaped figure a v shaped marking three types of synthetic hybrid fractures with congruous steps and two varieties of pluck holes we think that some of these kinematic indicators provide information on the amount of displacement aseismic versus seismic slip rates cataclastic flow frictional wear or surface polishing

silurian u pb zircon intrusive ages for the red river anorthosite northern cape breton island implications for the laurentia avalonia boundary in atlantic canada abstract current interpretations of the geology of cape breton island suggest that it exposes a complete cross section of the appalachians from laurentia across iapetan vestiges to avalonia crucial to this view is the presence of ca 1 ga plutons including anorthosites which have been regarded as correlatives of grenvillian basement a correlation that overlooks the fact that avalonia is also underlain by a ca 1 ga basement we analyzed zircons from the red river anorthosite blair river complex northwestern cape breton island previously dated as ca 1 1 ga they yielded 421 3 ma intrusive ages with older ages between 865 18 ma and 1044 20 ma inferred to be either xenocrysts derived from the country rock or from the source implications of these data suggest that the accompanying low pressure granulite amphibolite facies metamorphism of the blair river complex is either the root of a 440 410 ma magmatic belt produced during slab break off or relict ca 1 ga basement the blair river complex occurs in a nne ssw sinistral positive flower structure that progresses upwards from a neoproterozoic rifted arc through a low grade upper ordovician silurian overstep sequence to amphibolite facies fault slices capped by the low pressure granulite facies rocks blair river complex the correlation of neoproterozoic rifted arc units across most of cape breton island suggests it represents the deformed northwestern margin of avalonia intruded by a silurian lower devonian magmatic belt as the geological record in the blair river complex is similar to both grenvillian and avalonian basements its provenance is equivocal however pb isotopic data suggest the blair river complex has amazonian avalonia affinities thus cape breton island rather than representing a complete cross section of the appalachian orogen is part of pristine deformed avalonia with a positive flower structure exposing a cross section of avalonian crust

source of ash zone 1 in the north atlantic geochemical evidence shows that the silicic component of the widespread ash zone 1 in the north atlantic is derived from a major ignimbrite forming eruption which occurred at the katla caldera in southern iceland during the transition from glacial to interglacial conditions in younger dryas time both trace and major element evidence of the rhyolitic products excludes the oraefajokull volcano as a source the high ti basaltic component in the marine ash zone can also be attributed to contemporaneous eruption in the katla volcanic complex dispersal of tephra from this event is primarily attributed to the generation of co ignimbrite ash columns in the atmosphere with ash fallout on both sea ice and on the ocean floor north and east of iceland owing to the changing ocean circulation characteristics of the glacial regime including suppression of the irminger current and a stronger north atlantic current tephra was rafted on sea ice south into the central north atlantic and deposited as dispersed ash zone 1 sediments south of iceland also show evidence of the formation of ash turbidites generated either by the entrance of pyroclastic flows into the sea or during discharge of jokulhlaups or glacier bursts from this subglacial eruption

slickenside kinematic indicators abstract a new classification of slickenside kinematic indicators is presented based on 61 criteria these slickensides have been subdivided into eleven major groups v or crescentic markings steps fractures trains of inclined planar structures trailed material asymmetric elevations deformed elements mineralogical crystallographic orientations asymmetric plan view features asymmetric cavities and asymmetric folds this classification constitutes a useful tool for geologists interested in the determination of the shear sense in fault surfaces bearing slickensides examples of application of this classification to natural fault surfaces at different scales are presented

contrasting nature of deformation along an intra arc shear zone the liquine ofqui fault zone southern chilean andes abstract the liquine ofquifault zone lofz cuts the patagonian batholith and the modern volcanic arc of southern chile for ca 1000 km the rock fabric along three transects of the lofz combined with new and published geochronological data suggest marked differences in the nature and timing of deformation along strike in the liquine transect 39 s a 1 km wide northeast striking subvertical mylonitic zone shows north plunging stretching lineations this mylonitic zone has been juxtaposed by high angle reverse faulting against a nearly undeformed miocene granitoid metamorphic assemblages and microstructures in the mylonites indicate greenschist facies conditions and sinistral reverse displacement deformation pre dates a 100 2 ma undeformed porphyritic dike hornblende 40ar 39ar mean age in the reloncavi transect 41 42 s deformation in cretaceous and miocene plutons is predominantly brittle kinematic analysis of two fault populations yields compressional and dextral strike slip stress regimes interpreted as late miocene in age in the hornopiren transect 42 43 s a 4 km wide mylonitic zone developed in plutons and wallrocks shows subhorizontal stretching lineations and dextral displacement a single fault population overprinting the mylonites supports a dextral strike slip stress regime available u pb k ar and 40ar 39ar dates on deformed cenozoic plutons and wallrock range from 9 to 13 ma on hornblende and from 6 to 3 ma on biotite microstructures and mineral assemblages indicate that the youngest ductile fabrics in the plutons formed at greenschist facies similar to the biotite ar closure temperature subparallel magmatic and solid state fabrics combined with geochronology suggest that dextral displacement was continuous during emplacement and cooling of the plutons dextral oblique subduction of the nazca plate beneath western south america has driven long term intra arc deformation at the southern andes plate boundary zone ridge collision in turn has favored dextral displacement at the leading edge of the continent since the pliocene

the role of bubbles in generating fine ash during hydromagmatic eruptions the abundant fine ash produced in the 2011 subglacial eruption of grimsvotn iceland highlights the fragmentation efficiency of mafic hydromagmatic eruptions which is considerably higher than for comparable dry eruptions ash from the 2011 eruption can be divided into three morphological components vesicular particles shards and dense fragments distinguished by the size and abundance of constituent vesicles we use the vesicle characteristics to define a new shape factor the concavity index which provides an unbiased way to classify individual ash particles as either bubbly vesicular particles and shards or dense the relative proportion of bubbly and dense particles varies systematically with grain size with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from hawaiian fountains and suggest a comparable degassing history during magma ascent yet concordance between the size distributions of ash and of bubbles in the grimsvotn samples stands in contrast to the size distributions in hawaiian fountains where pyroclasts are orders of magnitude larger than individual bubbles we propose that the grimsvotn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water the strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation

the effect of landform variation on vegetation patterning and related sediment dynamics semi arid ecosystems are often spatially self organized in typical patterns of vegetation bands with high plant cover interspersed with bare soil areas also known as tiger bush in modelling studies most often straight planar slopes were used to analyse vegetation patterning the effect of slope steepness has been investigated widely and some studies investigated the effects of microtopography and hillslope orientation however at the larger catchment scale the overall form of the landscape may affect vegetation patterning and these more complex landscapes are much more prevalent than straight slopes hence our objective was to determine the effect of landform variation on vegetation patterning and sediment dynamics we linked two well established models that simulate a plant growth death and dispersal of vegetation and b erosion and sedimentation dynamics the model was tested on a straight planar hillslope and then applied to i a set of simple synthetic topographies with varying curvature and ii three more complex real world landscapes of distinct morphology results show banded vegetation patterning on all synthetic topographies always perpendicular to the slope gradient interestingly we also found that movement of bands a debated phenomenon seems to be dependent on curvature vegetation banding was simulated on the slopes of the alluvial fan and along the valley slopes of the dissected and rolling landscapes in all landscapes local valleys developed a full vegetation cover induced by water concentration which is consistent with observations worldwide finally banded vegetation patterns were found to reduce erosion significantly as compared to other vegetation configurations

neutral turbulent flow over forested hills numerical simulations of neutral turbulent flow over periodic forested hills are carried out using a first order turbulence closure scheme the forest is represented by the inclusion in the model of a canopy near the surface where additional drag is exerted on the flow as a result of the trees simulations with a more traditional roughness length parametrization of the surface which does not explicitly model the flow within the forest are also carried out for comparison comparison of simulations with existing wind tunnel data shows that the simple mixing length canopy closure scheme provides a good estimate of the mean flow over a forested hill and is a significant improvement on a traditional roughness length parametrization in this case the numerical results are seen to be in close agreement with existing linear analytical solutions for flow over a low and wide forested hill the model is used to investigate the breakdown of the analytical solution when the hill becomes steep and the perturbations to the flow are no longer linear even in the case of a gentle slope the analytical solution can break down when the hill is narrow or the canopy is deep and the vertical velocities near the top of the canopy become large this leads to a downwind shift in the surface pressure perturbation and an enhanced drag compared to a roughness length model model results also show enhanced flow separation in the presence of a canopy these results are important for a range of meteorological problems including the interpretation of flux measurements parametrization of orographic drag predicting wind damage to trees and calculating the yield from wind turbines royal meteorological society 2005 s b vosper s contribution is crown copyright

eruptive mechanisms of the neapolitan yellow tuff interpreted from stratigraphie chemical and granulometric data abstract the neapolitan yellow tuff 12 ka is the second largest pyroclastic deposit of the campanian volcanic area covering at least 1000 km 2 with conservative estimates of volume placed at 40 km 3 previous studies showed that this mainly trachytic deposit composed of two members was erupted by 1 a central vent mostly phreatoplinian phase lower member that generated pyroclastic surges and fallout reaching 34 km from the vent followed by 2 a multiple vent phreatomagmatic and magmatic phase upper member associated with onset of caldera collapse that produced surges extending 14 km from the vent the lower member is well bedded and comprises 13 subunits that alternate between phreatoplinian surges flows and plinian pumice and ash fallout the upper member is relatively lithic rich and more massive in character for both members magma compositions vary from alkali trachyte through trachyte to latite which does not fit a simple inversion of magma chamber gradients calculations based on magma chemistry show an increase in magma density a decrease followed by an increase in viscosity and a general decrease of gas fraction during the course of both eruptive phases after migrating to a depth of about 400 m calculated fragmentation depths gradually rise during each phase application of sequential fragmentation transport analysis to granulometric data shows for the lower member an average ratio of phreatomagmatic to magmatic components of 70 30 while the upper member shows an average ratio of 80 20 however considering the tephra volume represented by samples computed water magma interaction ratios r are shown to fluctuate but generally decrease from about 0 65 to 0 05 during eruption of the lower member while the upper member shows r fairly constant at about 0 1 furthermore surge flow runout distances and estimates of eruptive velocities from r values show that column collapse heights were extremely high 6 to 7 km during the first phase and were substantially lower during the second phase 2 to 3 km vent radii required for calculated eruption velocities of 180 to 370 m s are between 70 and 300 m suggesting a cumulative eruption duration of over 10 hours perhaps spanning of one to several days

late devonian carboniferous detachment faulting and extensional tectonics in western cape breton island nova scotia canada abstract shallow dipping detachment faults have played a key role in the evolution of the late devonian to carboniferous maritimes basin of western cape breton island the faults are responsible for large scale post orogenic crustal thinning and denudation of mid crustal levels tournaisian and westphalian clastic sedimentation can be related to detachment faulting along the margaree shear zone msz and the ainslie detachment ad respectively the msz consists of thick shallow dipping retrogressive mylonite overprinted by cataclastic horizons brittle detachment faults and chloritic breccia the shear zone crosscuts and transports low grade late devonian bimodal volcanic rocks and redbeds of the fisset brook formation in its hanging wall towards the west and juxtaposes them against exhumed medium to high grade metamorphic rocks and basement in its footwall locally the mylonite is 200 700 m thick and outcrops along strike from north to south for approximately 60 km tournaisian coarse clastic units and debris flow deposits of the horton group unconformably overlie ductile mylonite of the msz due to incision of brittle faults into the msz regional transgression and marine flooding of the extensional complex and exhumed units are marked by the thick visean carbonate and evaporite deposits of the windsor group the carbonates reflect a relatively quiescent tectonic regime and widespread subsidence was likely due to the cessation of extension related thermally induced buoyancy in a tectonically thinned crust the ainslie detachment nucleated as a regional flat lying fault along the carbonate evaporite contact at the top of the macumber formation forming calc mylonite near the base of the windsor group significant stratigraphic omissions in the windsor group above the detachment provide clear evidence of an extensional regime during faulting clastic deposits of the westphalian cumberland group are bounded by the detachment and sedimentation is related to listric normal faults which cut down section towards the west southwest the documentation of detachment faults has widespread implications for the interpretation of the post orogenic evolution of the northern appalachians an extensional regime can now be recognized deep reflection seismic data are interpreted in light of the new findings and can be related to late devonian carboniferous basin formation

intracrystalline vorticity record of flow kinematics during shear zone reactivation abstract the vorticity axis of deformation of a shear zone is a rotational axis controlled by the relative movement of its boundaries when characterized it is a reliable tool to determine the ductile flow geometry monoclinic general monoclinic transpressive or triclinic and an asset to identify the kinematics of complex oblique collisions advancements in analysis of electron backscattered diffraction ebsd maps now permit inference of vorticity axes known as crystallographic vorticity axes cvas from the intragranular lattice orientation dispersion of strain bearing minerals however natural shear zones may preserve a record of multiple shear events with contrasting kinematics and it is not understood to which extent the quartz lattice may preserve a kinematic record through multiple deformation events herein we characterize the finite flow kinematic record produced during reactivation of the eastern highlands shear zone ehsz in an oblique collision within the canadian appalachians we integrate field and thin section observations quartz lattice preferred orientations ebsd maps and grain classification leading to a new method to determine and compare cva within relict and recrystallized quartz grains using this method we were able to conclude that the ehsz was reactivated in an heterogenous monoclinic flow environment

regular and irregular patterns in semiarid vegetation vegetation in many semiarid regions is strikingly patterned forming regular stripes on hillsides and irregular mosaics on flat ground a simple model of plant and water dynamics based on ecologically realistic assumptions and with reasonable parameter values captures both of these types of patterns the regular patterns result from a turing like instability the irregular patterns arise when the ecological dynamics amplify slight small scale topographic variability because of the close agreement between observations and these theoretical results this system provides a clear example of how nonlinear mechanisms can be important in determining the spatial structure of plant communities

secondary seed dispersal and its role in landscape organization 1 mathematical models of banded vegetation patterns predict rapid upslope migration of vegetated patches not realized in field observations a key point of disagreement between theory and observation it is shown that the disagreement between model results and field observations can arise from seed dispersal dynamics two representations of biomass movement are used to test the hypothesis that secondary seed dispersal in overland flow inhibits band migration the first is based on coupling down slope water transport and seed advection the second uses a kernel based representation of seed transport where an anisotropic dispersal kernel combines the effects of isotropic primary and downslope secondary seed dispersal and ensures that conclusions about secondary dispersal are independent of diffusive representations of biomass movement the analysis demonstrates that secondary seed dispersal can retard upward movement of banded vegetation irrespective of the precise representation of biomass movement as long as the anisotropic effects are accounted for

stepped fibres in sillimanite bearing veins valid shear sense indicators in high grade rocks abstract sillimanite bearing veins with striking lineated and stepped fibre surfaces occur in migmatitic amphibolite and granulite facies sillimanite bearing gneisses in the betic cordillera southern spain the quartz dominated and peraluminous leucogranite veins occupy brittle to semi brittle hydraulic fractures and shear fractures that cross cut the main foliation of the gneisses the shear sense suggested by fibre steps concurs with independent criteria where observed e g foliation deflection at vein margins petrographic study suggests that oriented quartz sillimanite fibres were formed during vein dilation and experienced later ductile and brittle strain with microboudinage and microfracturing of sillimanite aggregates perpendicular to the long axes of the fibres due to their pronounced anisotropy the geometry of the steps is thus controlled by the initial fibre orientations reflecting the original sense of shear during vein formation

u pb zircon titanite and monazite ages in the bras d or and aspy terranes of cape breton island nova scotia implications for igneous and metamorphic history new u pb ages from zircon titanite or monazite separates from 12 units in the aspy and bras d or terranes nova scotia range from late precambrian to devonian and generally support the interpretation that the 2 terranes had differing histories until juxtaposition in the devonian five plutons of dioritic tonalitic and granodioritic compositions in the northern bras d or terrane yielded zircon ages between 575 maximum and 556 4 m y younger titanite ages of 548 2 and 521 2 m y from two of these plutons may indicate younger metamorphism previously undocumented early ordovician plutonism 493 2 ma zircon and metamorphism 496 5 ma titanite also occurred in the bras d or terrane in the aspy terrane a zircon age of 433 7 4 m y was obtained for a rhyolite unit in the upper part of a widespread metavolcanic metasedimentary sequence which together with higher grade probably correlative units is characteristic of the terrane the age is interpreted to indicate that most metamorphic rocks in the terrane are ordovician to early silurian in age in contrast to precambrian ages in the bras d or terrane devonian plutonism was widespread in the aspy terrane whereas devonian plutonism in the bras d or terrane appears restricted to the vicinity of the boundary with the aspy terrane zircon titanite or monazite ages from 4 plutonic and orthogneissic units in the aspy terrane and in the bras d or terrane adjacent to the terrane boundary range from 403 3 to 375 5 4 m y

pressure solution structures in a granite abstract syntectonic two mica granites in northwest spain have suffered regional late variscan shear deformation the deformed corcoesto granite shows a marked planar and linear fabric resulting from ductile behaviour and contains quartz veins at about 60 to the foliation offsets of these quartz veins along shear planes are commonly contrary to the general sense of shear and thus provide evidence for pressure solution processes

dynamics stratigraphy and proximal dispersal of supraglacial tephra during the ice confined 2004 eruption at grimsvotn volcano iceland the basaltic phreatomagmatic eruption of grimsvotn volcano iceland in november 2004 g2004 lasted for 5 days during which time two separate vents were active significant deposition of tephra occurred in the first 45 h only we have subdivided the deposit into seven units a g on the basis of differences in texture grain size and componentry and the presence of sharp contacts between the layers the distribution of tephra lobes was used to infer the vent of origin for each unit the g2004 deposit is poorly sorted overall and consists of non vesicular to highly vesicular juvenile components units a and b comprise almost exclusively non to poorly vesicular glass fragments whereas units c g contain at least 30 vol highly vesicular pumice the proportion of non juvenile fragments increases significantly in the final unit unit f of the main phase non juvenile fragments are restricted to the coarse 0 fraction of the deposit main phase units c and e account for 80 of the total deposit volume including the entire distal portion and are interpreted to represent a mixture of 1 a widely dispersed component that fell from the upper margins of a strongly inclined 45 6 10 km high plume and 2 a locally dispersed 3 km from source component originating from pyroclastic density currents and minor tephra jets

the geometrical relationship between the stretching lineation and the movement direction of shear zones abstract the geometrical relationship between foliations and lineations in shear zones is considered quantitatively assuming that the internal structures of shear zones have monoclinic symmetry the common practice of directly correlating the pitch or plunge of the stretching lineation with the obliquity of the shear direction is challenged it is the orthogonal projection of the stretching lineation on the shear zone boundary that is parallel to the shear direction a shear zone is parallel to a plane containing both the pole to the symmetry plane and the strike of the shear zone and the shear direction is parallel to the intersection of the symmetry plane and the shear zone boundary the symmetry plane is determined by the stretching lineation and the poles to the s foliation the implications of this study for the interpretation of shear zones are discussed with reference to an actual example

a succession of near orthogonal horizontal tectonic shortenings in the paleoproterozoic central lapland greenstone belt of fennoscandia constraints from the world class suurikuusikko gold deposit the relative and absolute timing of orogenic gold deposits in complex structural settings are active and challenging topics of research especially in precambrian greenstone belts the suurikuusikko gold deposit in central lapland greenstone belt is currently the largest primary gold producing deposit in europe located on a slight bend of the strike slip kiistala shear zone kisz gold is refractory and locked inside arsenopyrite and pyrite in this study different structural features were investigated along the kisz from the recently stripped etela pit which is the southern extension of the suurikuusikko ore body our data source ranges from aeromagnetic to high resolution aerial images x ray computed tomography scans of selected rock samples and regional geological and geophysical datasets the kisz has recorded five discrete deformation phases spanning between ca 1 92 and 1 76 ga the refractory gold at the suurikuusikko deposit formed during e w contraction related to the d1 thrusting phase this was followed by a n s shortening event d2 where most of the strain was taken up by the northern and southern thrusts tectonic vectors then switched from n s to ne sw and as a result dextral strike slip regime d3 commenced along the kisz this event exsolved invisible gold from the sulfides and remobilized it along with fractures a near orthogonal switch of the regional stress regime from ne sw to nw se flipped the kinematics of the kisz from dextral to sinistral d4 the last deformation phase d5 produced widespread veining under e w contraction and secured gold mineralization at the iso kuotko gold deposit within the kisz

the influence of porosity and vesicle size on the brittle strength of volcanic rocks and magma volcanic rocks and magma display a wide range of porosity and vesicle size a result of their complex genesis while the role of porosity is known to exert a fundamental control on strength in the brittle field less is known as to the influence of vesicle size to help resolve this issue here we lean on a combination of micromechanical sammis and ashby s pore emanating crack model and stochastic rock failure and process analysis code modelling the models show for a homogenous vesicle size that an increase in porosity in the form of circular vesicles from 0 to 40 and or vesicle diameter from 0 1 to 2 0 mm results in a dramatic reduction in strength for example uniaxial compressive strength can be reduced by about a factor of 5 as porosity is increased from 0 to 40 the presence of vesicles locally amplifies the stress within the groundmass and promotes the nucleation of vesicle emanating microcracks that grow in the direction of the applied macroscopic stress as strain increases these microcracks continue to grow and eventually coalesce leading to macroscopic failure vesicle clustering which promotes the overlap and interaction of the tensile stress lobes at the north and south poles of neighbouring vesicles and the increased ease of microcrack interaction is encouraged at higher porosity and reduces sample strength once a microcrack nucleates at the vesicle wall larger vesicles impart higher stress intensities at the crack tips allowing microcracks to propagate at a lower applied macroscopic stress larger vesicles also permit a shorter route through the groundmass for the macroscopic shear fracture this explains the reduction in strength at higher vesicle diameters at a constant porosity the modelling highlights that the reduction in strength as porosity or vesicle size increases is nonlinear the largest reductions are observed at low porosity and small vesicle diameters in detail we find that vesicle diameter can play an important role in dictating strength at low porosity but is largely inconsequential above 15 porosity vesicle clustering and stress lobe interaction are implicit at high porosity regardless of the vesicle diameter in the case of an inhomogeneous vesicle size the microcracks grow from the largest vesicles and brittle strength is closer to that of the largest vesicle end member the results of this study highlight the important role of vesicle size and the complex interplay between porosity and vesicle size in controlling the brittle strength of volcanic rocks and magma

kilometer scale refolded folds caused by strike slip reversal and intraplate shortening in the beishan region china 1 spectacular refolded folds clearly visible from space deform permian turbidite successions in a 180 km2 fault bounded compartment within the beishan region of northwestern china the folds formed after final terrane amalgamation of the central asian orogenic belt and record an important sequence of intraplate deformation events in central asia during the late permian jurassic the fold interference patterns are dominantly type 2 refolds with kilometer scale wavelengths they are bounded to the north and south by regional e w faults with multiple displacement histories as revealed by crosscutting ductile and brittle fabrics and kinematic indicators indicating opposing senses of motion the proximity of the refolded folds to the main bounding faults strongly suggests that the fold interference structures were caused by strike slip fault reactivation and shear related bending of f1 folds combined with components of n s compression across the compartment faulting within the folded compartment is limited to an array of right lateral strike slip faults which appear to have helped accommodate room problems during the refolding event the driving force for the deformation was likely to be distant including regional block collision to the south large scale rotations between the siberian and north china cratons and plate boundary convergence along the eastern chinese margin our study demonstrates that continental interior regions characterized by terrane collages are susceptible to reactivation by distant plate margin effects and that polyphase reactivation of major strike slip and oblique slip faults can generate refolding within upper crustal basinal compartments in addition the fault bounded basin containing the refolded folds in the beishan region is surrounded by older more complexly deformed crystalline basement therefore the deformed basin sequence represents a unique compartmental archive of the permian triassic history of intraplate deformation north of tibet

soil and water components of banded vegetation patterns abstract banded landscapes are comprised of alternating bands of vegetation and bare ground aligned along the contours in arid and semi arid regions 50 750 mm rainfall on very gentle and uniform slopes 0 2 2 vegetated bands can be perpendicular to the direction of the dominant wind or more frequently of the slope under given climatic conditions slope gradient is the controlling factor of the type of pattern spotted broadly or typically banded for a given slope gradient mean annual rainfall determines the contrast between the vegetated and bare phase as well as the band length and the interband width band width ratio a typical transect through such two phase mosaic includes a dynamic succession of surface crusts which generates a run off run on system this favours the capture of limited water resources and thus a biomass production greater for banded patterns than spotted patterns or uniformly scattered vegetation moreover vegetated bands act as natural bench structures that greatly limit soil erosion as suggested by modelling banded vegetation patterns can equally be derived from nearly bare areas or from dense vegetation patterns because banded vegetation patterns have often been considered as a form of degradation of previous continuous vegetation cover many attempts have been made by foresters to restore this initial cover by reforestation of the bare interbands however the numerous failures of reforestation of the bare interbands illustrate the key role of these mosaic components in the maintenance of the tiger bush ecosystem banded vegetation patterns are more resilient to climate change than to human disturbances the loss of landscape patchiness due to the clearing of the bands either for fuelwood or for ephemeral cropping induces a rapid decline in soil fertility and water infiltration potential long term monitoring shows that the interband width band width ratio adapts to rainfall variations that are intrinsic of semi arid zones moreover climatic changes might also induce a change in the type of patterning suggesting that banded vegetation patterns are highly self sustainable the ecological differences between the pioneer and the decaying edges of the vegetated bands the temporal spatial succession of soil crusts as well as other indices such as the spatial distribution of soil organic matter and termite nests suggest the upslope migration of the bands nevertheless field evidence of such a shift remains scarce profound lessons can be learnt from the study of banded landscapes in terms of ecosystems functions when designing water harvesting systems or restoring degraded arid and semi arid land banded vegetation patterns should be imitated to optimize biomass production and limit land degradation

the crustal magma chamber of the katla volcano in south iceland revealed by 2 d seismic undershooting summary results of a 2 d seismic undershooting experiment on the katla central volcano in south iceland are reported large localized traveltime anomalies 0 4 s are observed on an array within the katla caldera the traveltimes are forward modelled using a wavefront tracker developed in appendix a thus non linear effects encountered in traveltime tomography are avoided as well as common problems with ray tracing in the presence of strong lateral heterogeneity an extreme variation in compressional velocity is required to extend over a significant volume in order to model the data the resulting model is not unique but constraints on the allowable range of velocities 2 5 6 0 km s render the basic features well constrained a clear s wave shadow is closely associated with delays in traveltime due to a shallow slow anomaly low amplitude p waves go hand in hand with early arrivals due to thin structural features flanking the slow anomaly the model is interpreted in terms of a magma chamber containing extensively molten rock the magma chamber is shallow with a bottom at a depth of about 1 5 km below sea level 3 0 km below surface and measures about 5 km across the depth of the chamber is roughly at the level of buoyant equilibrium for basaltic melt in the crust owing to poor vertical resolution at shallow depths in the undershooting geometry the top of this shallow magma chamber is not well resolved on the other hand the bottom of the chamber is well resolved the chamber is underlain by rocks of average or high velocity for that depth the magma chamber is a persistent feature big enough 10 km3 to supply magma for large eruptions and to supply heat to permit remelting of hydrated basaltic crust to produce silicic magmas at shallow levels the chamber is fed by magma fracturing from below the model agrees with phenomenological models of magma chambers in iceland based on geological observations and provides a quantification of those models in terms of depth and size on the other hand it is fundamentally different from recent models of magma chambers at mid ocean ridges which may be more akin to the pervasive region of partial melt at depth beneath iceland this underlines the important effect of the icelandic hotspot on tectonics and volcanism in iceland and implies a substantially different crustal and thermal structure in iceland from that of normal mid ocean ridges

experimentally produced slickenside lineations in pyrophyllitic clay abstract during shearing of pyrophyllitic clay fault planes bearing slickenlines were generated on previously initiated c and c planes these slickenlines have excess lengths with respect to the measured slip displacement on the fault plane and are of the ridge in groove type with the hangingwall nesting into the footwall and vice versa these features cannot be explained in terms of either frictional asperity ploughing or dissolution reprecipitation but are produced during the development of a complex ductile shear zone that contains the fault plane this initial shear zone nucleates from localized shear bands that link up to form a throughgoing shear plane the nucleation and propagation of the fault post dates the formation of the shear plane and depends on the development of a zone of highly oriented layer silicates within the pyrophyllitic clay the fault is confined to this zone the lineations observed on the fault surfaces are produced while the material behaves plastically prior to the brittle faulting with increasing shear strain the brittle ductile transition is crossed and sliding continues under brittle conditions along the former shear zone

revisiting the hubbert rubey pore pressure model for overthrust faulting inferences from bedding parallel detachment surfaces within middle devonian gas shale the appalachian basin usa abstract both bedding parallel slickensides and cleavage duplexes are forms of mesoscopic scale detachment faulting populating black marcellus and geneseo burket and intervening gray mahantango shales of the middle devonian a section known for abnormal pore pressure below the appalachian plateau the abundance and the orientation of slickensides and cleavage duplexes in the more organic rich black shale relative to gray shale suggests that maturation related abnormal pore pressure facilitates detachment a mesoscopic manifestation of the hubbert rubey pore pressure model for overthrust faulting the former are discrete slip surfaces whereas the latter consists of nested anastomosing slip surfaces either cutting through bedding or on disrupted bedding surfaces stacked as mesoscopic versions of thrust duplexes cleavage duplexes are between a few cm and over 1 m thick with their hanging walls commonly transported toward the appalachian foreland regardless of local limb dip cleavage duplexes are most common near the stratigraphic maximum flooding surface the organic rich section most prone to develop maturation related pore pressure in the middle devonian gas shales bedding parallel slickensides are somewhat more evenly distributed in the black shale but also found in overlying gray shale in both black and gray shales slickensides are more abundant on the limbs of folds an indication of pore pressure related flexural slip folding on the macroscopic scale the pine mountain block of the southern appalachian mountains was enabled by a basal detachment cutting along the upper devonian chattanooga black shale which has a thermal maturity sufficient for the generation of abnormal pore pressure the pine mountain block is a large scale overthrust showing little evidence of collapse of the hinterland side a credible example of a pore pressure aided overthrust fault block of the type envisioned by the hubbert rubey model

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

tectono stratigraphic terranes in cape breton island nova scotia implications for the configuration of the northern appalachian orogen cape breton island is divided into four terranes on the basis of contrasts in pre carboniferous geology the blair river complex in the north is an exposure of north american grenvillian basement analogous to the humber zone basement in newfoundland ordovician to devonian metavolcanic metasedimentary gneissic and granitic rocks of the aspy terrane are correlative with parts of the gander terrane of newfoundland and new brunswick the bras d9or terrane characterized by low pressure gneisses a carbonate clastic platform sequence and late precambrian early cambrian plutons may be correlative with units previously included in the gander terrane in southern newfoundland and the avalon terrane in southern new brunswick the mira terrane in southeastern cape breton island including late precambrian to early cambrian volcanic and sedimentary sequences and fossiliferous cambrian ordovician units is clearly part of the avalon terrane therefore with the exception of the dunnage terrane which is not represented in cape breton island the terranes of newfoundland continue through cape breton island they are offset to the northwest to the mainland part of the appalachian orogen in new brunswick

grain shape of basaltic ash populations implications for fragmentation here we introduce a new quantitative method to produce grain shape data of bulk samples of volcanic ash and we correlate the bulk average grain shape with magma fragmentation mechanisms the method is based on automatic shape analysis of 2d projection ash grains in the size range 125 63 m loose bulk samples from the deposits of six different basaltic eruptions were analyzed and 20 000 shape measurements for each were obtained within 45 min using the particle insight dynamic shape analyzer pidsa we used principal component analysis on a reference grain dataset to show that circularity rectangularity form factor and elongation best discriminate between the grain shapes when combined the grain population data show that the studied eruptive environments produce nearly the same range of grain shapes although to different extents our new shape index the regularity index ri places an eruption on a spectrum between phreatomagmatic and dry magmatic fragmentation almost vesicle free surtseyan ash has an ri of 0 207 0 002 2 whereas vesiculated hawaiian ash has an ri of 0 134 0 001 2 these two samples define the end member ri while two subglacial one lacustrine and another submarine ash sample show intermediate ris of 0 168 0 002 2 0 175 0 002 2 0 187 0 002 2 and 0 191 0 002 2 respectively the systematic change in ri between wet and dry eruptions suggests that the ri can be used to assess the relative roles of magmatic vs phreatomagmatic fragmentation we infer that both magmatic and phreatomagmatic fragmentation processes played a role in the subglacial eruptions

the origin of ridge in groove slickenside striae and associated steps in an s c mylonite abstract a three dimensional study of an s c mylonite reveals that the s surfaces anastomose around porphyro clasts and c surfaces are not truly planar but curvi planar with the linear generator parallel to the movement direction it is suggested that it may be common for shear surfaces at all scales to take this form in the material studied surfaces exposed by parting primarily along c surfaces which are a product of ductile deformation and are penetrative have all the characteristics of classic slickensides on the slickensides striae of the ridge in groove type and steps of both congruous and incongruous types are developed the ridge in groove shape of the striae is directly related to the curvi planar shape of the c surfaces incongruous steps are smooth they are the result of parting along s surfaces congruous steps are rough first order congruous steps are formed by fracturing between individual c surfaces whereas second order congruous steps form by breaking off the tips of the dihedra between s and c surfaces thus the smoothness roughness technique can be used to determine the sense of shear in strongly deformed rocks where the angle between s and c surfaces is too small to be recognized in the field this may prove to be a useful shear sense indicator

late proterozoic tectonic model for the avalon terrane in maritime canada the avalon terrane in maritime canada extends from cape breton island through northern nova scotia antigonish and cobequid highlands to southern new brunswick the terrane contains abundant late proterozoic volcanic rocks 570 680 m y old the geochemistry of the volcanic rocks of southern cape breton island and southern new brunswick indicates that they were erupted in a volcanic arc environment in both regions the mafic rocks display a remarkably similar compositional zonation which resembles the across arc variations observed in modern volcanic arcs the progressive compositional changes that include a transition from island arc tholeiites along the southeastern coasts to calc alkaline rocks inland to the northwest are probably due to a northwest dipping subduction zone with a trench located to the southeast in the antigonish and cobequid highlands continental tholeiites and calc alkaline volcanic rocks are interbedded and overlain by turbidites that have probably been deposited in a back arc or intra arc environment the superimposed positive gravity and magnetic anomalies that may be traced discontinuously from the grand banks through nova scotia collector anomaly and along the northern side of the gulf of maine to near boston probably represent late proterozoic ophiolitic remnants preserved by incomplete suturing along a late proterozoic plate boundary northeast trending segments are inferred to mark subduction zones and the e w portions could correspond to transform faults thus the present geographical distribution of the late proterozoic volcanic rocks is explained by a tectonic model involving a single subduction zone offset by two late proterozoic transform faults located approximately along the minas fault and the scatarie ridge

when do dragfolds not develop into sheath folds in shear zones abstract using a unified model for high strain zones rotation paths of material lines in different flow types are investigated and presented as stereographic projections a material fabric element such as a mineral lineation or a fold axis can be plotted on such stereograms and the rotation path revealed using this method we examine the rotation of folds and the development of sheath folds in general three dimensional zonal deformation we show that the evolution of dragfold geometry in a high strain zone depends on the flow type and the initial perturbation of the fold axis in simple shear zones and thickening zones monoclinic or triclinic dragfolds will evolve into sheath folds in monoclinic thinning zones with biaxially stretching boundaries if the shear direction is parallel to the maximum stretching direction of the boundary sheath folds will develop if the shear direction is parallel to the minimum stretching direction of the boundary dragfolds become more cylindrical as deformation advances in thinning zones with boundaries equally stretched in all directions the likelihood of sheath fold development depends on the ratio of the shear strain rate to the thinning rate in triclinic thinning zones fold evolution depends on the details of the flow and the value of initial fold axis perturbation

local properties of patterned vegetation quantifying endogenous and exogenous effects dryland ecosystems commonly exhibit periodic bands of vegetation thought to form due to competition between individual plants for heterogeneously distributed water in this paper we develop a fourier method for locally identifying the pattern wavenumber and orientation and apply it to aerial images from a region of vegetation patterning near fort stockton tx usa we find that the local pattern wavelength and orientation are typically coherent but exhibit both rapid and gradual variation driven by changes in hillslope gradient and orientation the potential for water accumulation or soil type endogenous pattern dynamics when simulated for spatially homogeneous topographic and vegetation conditions predict pattern properties that are much less variable than the orientation and wavelength observed in natural systems our local pattern analysis combined with ancillary datasets describing soil and topographic variation highlights a largely unexplored correlation between soil depth pattern coherence vegetation cover and pattern wavelength it also surprisingly suggests that downslope accumulation of water may play a role in changing vegetation pattern properties

analytical models for the mean flow inside dense canopies on gentle hilly terrain simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments laser doppler anemometry lda measurements are conducted in a neutrally stratified boundary layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy the hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy h l 1 and lc l 1 where h is the hill height l the half length and lc the canopy adjustment length scale the lda data suggest that the often criticized linearizations of the advective terms turbulent shear stress gradients and drag force appear reasonable except in the deep layers of the canopy as predicted by a previous analytical model the lda data reveal a recirculation region within the lower canopy on the lee slope adjusting the outer layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model s performance for the velocity perturbations in the deeper layers of the canopy a new analytical model which retains a balance between mean horizontal advection mean pressure gradient and mean drag force but neglects the turbulent shear stress gradient is developed the proposed model reproduces the lda measurements better than the earlier analytical model which neglected advection but retained the turbulent shear stress gradient in the lower layers of the canopy and near the hill top this finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small in essence the newly proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean momentum equation we briefly discuss how to integrate all these findings with recent advances in canopy lidar remote sensing measurements of general topography and canopy height copyright 2008 royal meteorological society

a new view of an old suture zone evidence for sinistral transpression in the cheyenne belt the type locality of the archean paleoproterozoic suture zone in the southern rocky mountains is marked by a series of subvertical shear zones collectively called the cheyenne belt the cheyenne belt is a key structure for developing models for 1780 1740 ma tectonism along the southern margin of the archean wyoming province which heralded a rapid period of continental amalgamation this paper tests existing structural and plate tectonic models for the cheyenne belt with detailed geologic mapping kinematic analyses quartz crystallographic fabric analyses and deformation mechanism analyses of the northern mylonite zone of the eastern medicine bow mountains mylonites of this zone record a complex deformation history but the main deformation phase was sinistral northwest side up oblique transpression evidence for southeast side up dip slip motion that characterizes many other areas of the belt is confined to ultramylonites immediately adjacent to the terrane boundary hence fabrics related to sinistral transpression were likely overprinted by southeast side up motion sinistral strike slip motion is recorded in at least two other localities in the cheyenne belt because synmetamorphic fabrics on both sides of the suture zone record sinistral strike slip and northwest side up motion this was probably the dominant deformation style in the field area and may have been the dominant deformation style throughout the cheyenne belt based on these data and regional constraints we interpret the cheyenne belt as a subvertical transpressional stretching fault system that simultaneously accommodated sinistral strike slip motion penetrative horizontal shortening and dip slip motion related to differential crustal thickening between the relatively cold wyoming province and younger hotter rocks to the south

explosive silicic volcanism in iceland and the jan mayen area during the last 6 ma sources and timing of major eruptions abstract fifty three major explosive eruptions on iceland and jan mayen island were identified in 0 6 ma old sediments of the north atlantic and arctic oceans by the age and the chemical composition of silicic tephra the depositional age of the tephra was estimated using the continuous record in sediment of paleomagnetic reversals for the last 6 ma and paleoclimatic proxies 18 o ice rafted debris for the last 1 ma major element and normative compositions of glasses were used to assign the sources of the tephra to the rift and off rift volcanic zones in iceland and to the jan mayen volcanic system the tholeiitic central volcanoes along the iceland rift zones were steadily active with the longest interruption in activity recorded between 4 and 4 9 ma they were the source of at least 26 eruptions of dominant rhyolitic magma composition including the late pleistocene explosive eruption of krafla volcano of the eastern rift zone at about 201 ka the central volcanoes along the off rift volcanic zones in iceland were the source of at least 19 eruptions of dominant alkali rhyolitic composition with three distinct episodes recorded at 4 6 5 3 3 5 3 6 and 0 1 8 ma the longest and last episode recorded 11 pleistocene major events including the two explosive eruptions of tindfjallajokull volcano thorsmork ca 54 5 ka and katla volcano solheimar ca 11 9 ka of the southeastern transgressive zone eight major explosive eruptions from the jan mayen volcanic system are recorded in terms of the distinctive grain size mineralogy and chemistry of the tephra the tephra contain k rich glasses k 2 o sio 2 0 06 ranging from trachytic to alkali rhyolitic composition their normative trends ab q or and their depleted concentrations of ba eu and heavy ree reflect fractional crystallisation of k feldspar biotite and hornblende in contrast their enrichment in highly incompatible and water mobile trace elements such as rb th nb and ta most likely reflect crustal contamination one late pleistocene tephra from jan mayen was recorded in the marine sequence its age estimated between 617 and 620 ka and its composition support a common source with the borga pumice formation at sor jan in the south of the island

geological evolution of the northwestern superior province clues from geology kinematics and geochronology in the gods lake narrows area oxford stull terrane manitoba the study of lithology geochronology and structure in the oxford stull terrane in particular in the gods lake narrows area has led to the recognition of three distinct supracrustal sequences 2 8 2 9 ga volcanic rocks a 2720 ma fault bounded package of volcanics and sandstones and 2705 ma conglomerate and alkaline volcanic rocks of the oxford lake group detrital zircon as old as 3647 ma is present in the oxford lake group an early generation of folding and shearing occurred prior to deposition of the oxford lake group and was probably synchronous with emplace ment of 2721 ma tonalite dykes the second generation of deformation caused south over north thrusting of volcanic rocks over the oxford lake group the youngest fabric resulted from east southeast trending dextral south over north shearing the youngest rock dated in the area is the 2668 1 ma magill lake pluton which records crustal melting following deformation the pattern of sedimentation and deformation in this area is similar to

amplified hazard of small volume monogenetic eruptions due to environmental controls orakei basin auckland volcanic field new zealand orakei maar and tuff ring in the auckland volcanic field is an example of a basaltic volcano in which the style and impacts of the eruption of a small volume of magma were modulated by a fine balance between magma flux and groundwater availability these conditions were optimised by the pre 85 ka eruption being hosted in a zone of fractured and variably permeable plio pleistocene mudstones and sandstones orakei maar represents an end member in the spectrum of short lived basaltic volcanoes where substrate conditions rather than the magmatic volatile content was the dominant factor controlling explosivity and eruption styles the eruption excavated a crater 80 m deep that was subsequently filled by slumped crater wall material followed by lacustrine and marine sediments the explosion crater may have been less than 800 m in diameter but wall collapse and wave erosion has left a 1 000 m diameter roughly circular basin a tuff ring around part of the maar comprises dominantly base surge deposits along with subordinate fall units grain size texture and shape characteristics indicate a strong influence of magma water and magma mud interactions that controlled explosivity throughout the eruption but also an ongoing secondary role of magmatic gas driven expansion and fragmentation the tuff contains 70 of material recycled from the underlying plio pliestocene sediments which is strongly predominant in the 2 fraction the magmatic clasts are evolved alkali basalt consistent with the eruption of a very small batch of magma the environmental impact of this eruption was disproportionally large when considering the low volume of magma involved dre 0 003 km3 hence this eruption exemplifies one of the worst case scenarios for an eruption within the densely populated auckland city destroying an area of 3 km2 by crater formation and base surge impact an equivalent scenario for the same magma conditions without groundwater interaction would yield a scoria spatter cone with a diameter of 400 550 m destroying less than a tenth of the area affected by the orakei event

regional 18o depletion of neoproterozoic igneous rocks from avalonia cape breton island and southern new brunswick canada neoproterozoic igneous rocks of the avalonian mira terrane cape breton island and caledonia terrane southern new brunswick have experienced regional 18 o depletion the majority of these rocks have 18 o wr values between 1 and 6 markedly different from igneous rocks of the inboard ganderian terranes which have normal high 18 o wr values of 7 to 12 the 18 o depletion of these avalonian terranes resulted from pervasive hydrothermal alteration all of the igneous rocks contain classic propylitic alteration assemblages of turbid feldspar chlorite epidote sericite and calcite and host multiple generations of veins containing the same phases the 18 o values of the rock forming minerals show strong disequilibrium with qtz fs ranging from 2 5 to 11 0 and kfs plg from 0 5 to 1 0 such values are indicative of post magmatic hydrothermal alteration the hydrothermal alteration occurred at ca 560 550 ma during initial transcurrent rifting of avalonia at the gondwanan margin the development of rift wrench basins with associated bimodal volcanism allowed large volumes of hydrothermal fluids to penetrate and circulate within mira and caledonia terrane crust before its submergence in the early cambrian ca 540 530 ma the hydrothermal fluids were predominantly meteoric although the presence of a significant seawater component cannot be ruled out the almost ubiquitous 18 o depletion exhibited by the neoproterozoic rocks in these avalonian terranes is absent in the associated ganderian terranes suggesting that avalonia remained separate from ganderia until at least the cambrian ordovician this 18 o depletion provides an additional geochemical tool for identification of avalonian crust in other areas of the appalachians and europe

strain localization in homogeneous granite near the brittle ductile transition a case study of the kellyland fault zone maine usa abstract we present a detailed case study of a strike slip shear zone that cut homogeneous granite near the brittle ductile transition this zone contains three distinct strain facies 1 a 2 3 km wide belt of foliated granite 2 a 100 300 m wide belt of small localized shear zones and 3 a 200 400 m wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites the foliated granite formed via dislocation creep of quartz and minor dissolution precipitation creep and dislocation creep of feldspar under amphibolite facies conditions shortly after granite crystallization the localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite and they deformed by granular flow grain boundary sliding coupled with dissolution precipitation creep these deformation features reveal a three phase rheologic evolution in granite near the brittle ductile transition phase 1 is recorded by the foliated granite and the rheology was governed by dislocation creep of quartz phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone phase 3 was a long lived period of ductile deformation and the rheology was governed by the granular flow mechanism strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation

do principal surfaces of stress and strain always exist variations of stress and strain are commonly expressed by patterns of stress or strain trajectories three mutually orthogonal families of continuous lines parallel to maximum intermediate and minimum stress or strain axes it might be assumed that there are equivalent continuous principal surfaces of stress or strain for any state of continuously varying stress or strain we demonstrate that this will not generally be the case for three dimensionally varying states of stress or strain whether or not principal surfaces of stress or strain exist is governed by the abnormality of the vector field of principal trajectories we consider the z vector fields for examples of many types of three dimensional heterogeneous deformation and show that most of these do not lead to definable principal xy strain surfaces an alternative geometric test is presented termed the continuity loop for simply demonstrating the existence or not of principal surfaces using geometrical and orientational information it is important to the understanding of geological structures to know which kinds of heterogeneous deformation give rise to principal surfaces of stress or strain we conclude with examples of structures which might be indicative of the absence of continuous principal surfaces of stress segmented faults echelon veins and dykes a discussion of the implication for strain fabrics and foliations and a warning that foliation trace trajectories on maps or sections may not necessarily indicate the existence of real foliation surfaces in three dimensions

geochemistry of the neoproterozoic metasedimentary gamble brook formation avalon terrane nova scotia evidence for a rifted arc environment along the west gondwanan margin of rodinia abstract the neoproterozoic gamble brook formation in the avalon terrane of nova scotia consists of quartzitic pelitic and psammitic metasedimentary rocks the formation is generally interpreted to have been deposited in an atlantic type passive margin setting along the amazonian margin of rodinia before the main ca 650 550 ma phase of avalonian subduction related magmatism the geochemical and sm nd isotopic signature of these metasedimentary rocks however shows that although the quartzites are predominantly derived from ancient cratonic basement the pelites and psammites were derived from moderately fractionated mafic to intermediate proto avalonian crust the two contrasting sources for the gamble brook rocks are incompatible with a passive margin setting and are instead interpreted to reflect deposition along a basin margin within a rifted arc environment taken as a whole the amazonian margin of gondwana preserves evidence of episodic subduction in a western pacific like setting between 1000 a

a new simple shear deformation apparatus for rocks and soils abstract we present a new design for a simple shear type deformation apparatus which has the capacity to deform medium strength anisotropic rocks as well as soils and similar strength materials the special feature of the design is that the deformation is achieved by the application of a particular set of boundary displacements so that a specimen is constrained to deform by a progressive simple shearing history these boundary displacements are applied through three pairs of steel platens each of which are in contact with opposite sides of the specimen which is initially a right rectangular prism one pair of platens is fixed so that deformations are plane strain a second pair of platens remains horizontal but they are free to slide against the reaction frame in the shear direction while at the same time they can apply a vertical normal force through hydraulic actuators the third pair of platens are vertical faces cut longitudinally through two steel shafts so that the shaft axes lie horizontally through the centre of the faces shear is applied by rotation of the two shafts so that the two vertical end faces of the specimen are rotated about the shaft axes this rotation means that horizontal planes in the specimen remain horizontal and suffer no shape change while vertical planes normal to the shaft axes remain vertical but suffer a shearing type change in shape the combination of these boundary displacements is a progressive simple shearing of the specimen we show typical stress strain responses for a number of different material types and discuss the degree of strain homogeneity throughout the specimen examples are presented of the microstructural changes which occur in specimens deformed in the apparatus

the effects of gentle topographic variation on dispersal kernels of inertial particles 1 seed dispersal kernels of wind dispersed species imprint the initial spatial template over which later demographic processes such as establishment or re colonization operate from a major knowledge gap in seed dispersal modeling by wind is the role of complex topography in modifying the kernel shape when referenced to the flat world case how complex topography modifies the dispersal kernels of inertial particles such as seeds is explored here via novel flume experiments a train of gentle cosine hills and a canopy composed of densely arrayed rods with a roughness density resembling the leaf area density profile of hardwood species at maximum leaf area were used to represent a canopy hill system spherical inertial particles having a coefficient of variation in terminal velocity commensurate with values reported from field studies were employed as model seeds it was demonstrated that the seed dispersal kernel maintained its canonical wald form as derived from simplified turbulent dispersion theories for flat terrain and vertically uniform flow field seeds released near the canopy top and from the top of the hill have a dispersal kernel mean distance that was about 35 times larger than its counterpart for releases from the bottom of the hill moreover the probability of occurrence of long distance dispersal ldd events defined here as seeds traversing a longitudinal distance exceeding 10 times the canopy height were one order of magnitude larger for seeds released near the canopy top and from the hill top when compared to their counterpart released from the hill bottom

40ar 39ar age pattern associated with differential uplift along the eastern highlands shear zone cape breton island canadian appalachians abstract the eastern highlands shear zone in cape breton island is a crustal scale thrust it is characterized by an amphibolite facies deformation zone 5 km wide formed deep in the crust that is overprinted by a greenschist facies mylonite zone 1 km wide that formed at a more shallow level hornblende 40ar 39ar plateau ages on the hanging wall decrease towards the centre of the shear zone in the older zone over 7 8 km from the centre the ages are between 565 and 545 ma in the younger zone within 4 5 km of the centre they are between 425 and 415 ma and in the transitional zone in between they decrease abruptly from 545 to 425 ma pressures of crystallization of plutons in the hanging wall based on the al in hornblende barometer and corresponding to depth of emplacement increase towards the centre of the shear zone and indicate a differential uplift of up to 28 km associated with movement along the shear zone the age pattern is interpreted to have resulted from the differential uplift the pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende ar blocking temperature 500 c until differential uplift occurred the 40ar 39ar ages in the zone 425 415 ma thus date shear zone movement or the last stage of it in contrast rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of 565 555 ma plutons in the area long before shear zone movement the transitional zone corresponds to the ar partial retention zone before differential uplift the 40ar 39ar age pattern thus reflects a neoproterozoic to silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone a similar k ar age pattern has been reported for the alpine fault in new zealand it is suggested that such isotopic age patterns can be used to help constrain the ages kinematics displacements and depth of penetration of shear zones

late cambrian middle furongian shallow marine dysoxic mudstone with calcrete and brachiopod olenid lotagnostus faunas in avalonian cape breton island nova scotia the common belief that organic rich mudstones formed in quiescent distal settings is further weakened by study of an upper cambrian leptoplastus lower peltura superzones succession in the chesley drive group in avalonian cape breton island that is comparable to alum shale successions in baltica dramatic sea level likely eustatic changes are now recognized by punctuation of deposition of shallow wave influenced black mudstone with brachiopod orusia lenticularis and olenid trilobite bearing limestones by offlap and formation of a subaerially cemented calcrete clast conglomerate subaerial exposure was followed by transgression and accumulation of clastic pyrite sand and phosphatic granules with leptoplastus superzone l ovatus zone trilobite sclerites dynamic processes are shown by wave ripples in the mudstone and limestone sorting and winnowing of fossil rudstones and pre compactional fracture of the conglomerate and rudstones orusia rudstones in the succession below the conglomerate are regarded as analogues of eoorthis and billingsella rudstones in the biomere extinction intervals of the laurentian basal sunwaptan the lowest orusia rich beds are no older than the p spinulosa zone but as elsewhere in avalonia they range into the higher leptoplastus cape breton and even the peltura britain new brunswick superzones rare agnostoid sclerites in lower peltura superzone ctenopyge tumida zone olenid rudstone resemble those traditionally assigned to lotagnostus trisectus in avalonian britain and sweden and are distinct from laurentian l americanus an l americanus zone cannot be identified in avalonia or baltica and the first appearance datum fad of purported l americanus is not suitable as a standard for the base of the highest cambrian stage

altering avalonia oxygen isotopes and terrane distinction in the appalachian peri gondwanan realmlaboratory for stable isotope science lsis the university of western ontario contribution 236 distinct 18o depletion is characteristic of a large majority of the 620 550 ma felsic igneous rocks of avalonia in the northern appalachian orogen neoproterozoic rocks in the boston avalon terrane have the lowest 18owr values 3 1 followed by the mira terrane in cape breton island and the caledonia terrane in new brunswick 1 2 the avalon terrane in newfoundland 2 8 and the antigonish highlands in nova scotia 5 3 in contrast this depletion of 18o is observed in very few of the paleozoic felsic igneous rocks from these avalonian terranes and also in very few of the neoproterozoic and paleozoic felsic igneous rocks from the inboard ganderian terranes the low 18o character of the neoproterozoic igneous rocks is related to regional pervasive post magmatic alteration by predominantly meteoric hydrothermal fluids 18oh2o 6 to 4 at 200 450 c the alteration likely occurred during late neoproterozoic transtensional extension of avalonia large scale fluid infiltration and circu

evidence for early mesozoic extensional faulting in carboniferous rocks southern new brunswick canada the high grade sylvinite orebody 28 k2o mined by the potacan mining company formerly the denison potacan potash company near sussex new brunswick canada has undergone intense internal folding the folds vary in size from centimetres to hundreds of metres are mostly tight to isoclinal recumbent and asymmetrically overfolded towards the southeast and belong to a single varied style group suggesting one deformation event folds are noncylindrical to sheath like locally and have highly variable cross sectional morphologies slickensides are locally common and penetrative slickenline trends in all parts of the mine are northwest southeast all the evidence is consistent with a southeast directed movement across the top of the evaporite body having caused extensive ductile drag folding within both the sylvinite and halite lithologies the evaporites in fact have all the characteristics of a near horizontal mylonite zone variscan deformation during the late carboniferous is generally cited as bein

structural setting and geochronology of auriferous quartz veins at the high rock island gold deposit northwestern superior province manitoba canada the high rock island gold deposit occurs across the intrusive contact between a granitoid pluton and a mafic metavolcanic rock at the southern margin of the island lake greenstone belt the main structure at the deposit is a 5 m wide north trending and steeply dipping dextral ductile brittle fault zone the main fault that cuts at a right angle across the pluton greenstone contact and the contact parallel foliation total horizontal displacement along the fault is 9 m of which 3 m is accomplished by ductile shearing and 6 m by brittle faulting the gold deposit consists of steeply dipping auriferous quartz veins that are hosted in and were emplaced during the dextral shearing along the main fault they include 1 the main vein a fault fill vein that occurs in the center of and is parallel to the main fault 2 en echelon veins extensional veins that occur on both sides of the main vein and cut across the foliation and 3 foliation parallel veins that occur locally detailed analysis of the geometry of the en echelon veins partly based on a new mohr circle construction method shows that the deformation in the main fault was transtensional the faulting and quartz vein emplacement were controlled by the contacts of a north trending mafic dike they took place under greenschist facies conditions postdating amphibolite facies metamorphism in the host rock the mineralized quartz veins contain both monazite and titanite which yield u pb ages of 2687 2 ma and 2658 5 ma respectively although the exact interpretation of the two ages remains somewhat uncertain it is likely that gold could have been introduced during a secondary event at 2658 ma in a vein originally formed at 2687 ma or earlier this is supported by the isotopic affinity of pb in titanite with that in galena that is spatially closely associated with gold the interpretation that gold was introduced after the emplacement of the quartz veins is also consistent with the observation that the main vein only contains significant gold and sulfide where it is cut by a vein parallel fault in any case the mineralization at the deposit took place at the latest stages of orogenic development in the northwestern superior province

late early permian 266 ma n s compressional deformation of the turfan basin nw china the cause of the change in basin pattern previous studies suggested an important but yet poorly understood tectonic transition in the altaids also termed the central asian orogenic belt caob in the permian this tectonic transition clearly documented by published stratigraphic data and provenance analyses suggested a unified junger turfan basin in northwest china in permian time and it further indicated that extension dominated early permian tectonics in the region whereas flexural foreland subsidence controlled late permian basin evolution our new structural observations microtectonic analyses and 40ar 39ar geochronological data from southwest of the turfan basin reveal that in the late early permian 266 ma a ns directed contractional deformation operated along the southern border of the unified junger turfan basin which was probably related to the transition in basin evolution the contraction gave rise to a nw striking right lateral transpressional rather than simple shear dextral ductile shear zone along the southwestern border of the turfan basin and to an interference fold pattern together with closely spaced concentrated cleavage and thrusts in a constrictional strain regime in the basin interior after the late permian the tectonic evolution of the caob changed from paleozoic continental amalgamation to mesozoic cenozoic intracontinental orogenic reactivation

importance of differentiating ductile slickenside striations from stretching lineations and variation of shear direction across a high strain zone shear direction is an important parameter in the kinematic interpretation of high strain zones recent developments in the study of high strain zones show that there is no simple relationship between the orientation of stretching lineations and the shear direction and it is difficult to use the former to determine the latter in contrast striations on c surfaces a product of ductile deformation form parallel to the shear direction it is therefore important for the kinematic interpretation of high strain zones to differentiate such striations from stretching lineations ductile striations are much more common in natural high strain zones than reported in the literature we discuss how to differentiate the two types of lineations and describe a natural high strain zone example from the superior craton of canada the striation data show that the shear direction of the high strain zone varies from subhorizontal to moderately plunging across the strike of the zone we conclude that well developed ridgein groove type striations are a reliable indicator of the shear direction we also discuss situations where the orientation of stretching lineations can potentially be used as an indicator of shear direction 2007 elsevier ltd all rights reserved

the maritsa strike slip shear zone between kostenets and krichim towns south bulgaria structural petrographic and isotope geochronology study abstract the present study describes the characteristics of the maritsa shear zone msz a major tectonic element in the balkanides in south central bulgaria metamorphic rocks of four lithotectonic units madan chepinska asenitsa and thrace units crop out in the study area strike slip ductile deformation in msz affects the thrace lithotectonic unit tlu for up to 15 km the stratigraphy of this unit is divided in two parvenets succession and variegated succession u pb zircon dating reveals late jurassic protolith age for metagranitoids and metagabbros of the variegated succession for its metasedimentary part triassic to upper jurassic age is suggested based on the strontium isotope signature of the marbles the parvenets succession affiliates to the variscan metamorphic basement of europe the metamorphic evolution of the zone is subdivided into synmetamorphic strike slip deformations and annealing stages the ductile shearing occurred in greenschist to lower amphibolite facies between 130 ma discordant u pb ages and 82 78 ma late syntectonic granites this stage is connected with the oblique collision of the rhodope late jurassic arc with the european platform with the docking of the arc and the triggering of the strike slip movements msz represents an orogen scale border between the rhodope south vergent thrust complex and the north vergent deformations in the srednogorie and sakar strandzha zones during the late cretaceous msz is the contact between the srednogorie magmatic arc part of the apuseni banat timok srednogorie belt and the rhodopean metamorphic core complexes nw se dextral faulting characterized the brittle tectonics along the zone strike slip faults of the southern border of the tlu are transferred into reverse faults along which the tlu overthrusted oligocene sediments msz is an orogen scale transpressional shear zone and an important border in the structure of the balkanides this multidisciplinary research emphasizes its role as a major tectonic element by presenting new structural petrographic and isotope geochronology data

can a simple dynamical system describe the interplay between drag and buoyancy in terrain induced canopy flows abstractunder nonneutral stratification and in the presence of topography the dynamics of turbulent flow within a canopy is not yet completely understood this has among other consequences serio

deformation path partitioning within the transpressional white mountain shear zone california and nevada most numerical simulations of transpression zones predict a change in finite stretching direction from subhorizontal to subvertical for simple shear dominated zones we provide a detailed description of a dextral transpression zone the white mountain shear zone wmsz with a range of lineation orientations and compare these natural data to numerical models the wmsz is characterized by steeply dipping foliations with dominant shallowly plunging lineations and coeval subordinate domains of steeply plunging lineations within shallowly lineated domains foliation geometry shear sense indicators and quartz c axis fabrics indicate a large component of simple shear while microstructural and quartz c axis fabric data from steeply lineated domains indicate a large component of pure shear geometric relationships between foliations and lineations and quartz c axis fabrics demonstrate that lineation orientation has remained constant during much of the deformation history comparison of numerical models with the data collected from wmsz shows that the shear zone geometry and the observed strain path partitioning do not match any of these models we propose a conceptual kinematic model for the wmsz involving stable segregated coeval kinematic domains of simple shear dominated fabrics and pure shear dominated fabrics that accommodate the transcurrent and contractional components of deformation respectively

new radiocarbon dates for the vedde ash and the saksunarvatn ash from western norway abstract the vedde ash bed mid younger dryas and the saksunarvatn ash early holocene are important regional stratigraphic event markers in the north atlantic the norwegian sea and the adjacent land area it is thus essential to date them as precisely as possible the occurrence of the saksunarvatn ash is reported for the first time from western norway and both tephras are dated precisely by ams analyses of terrestrial plant material and lake sediment at krakenes the vedde ash has been previously dated at sites in western norway to about 10 600 yr b p it is obvious in the younger dryas sediments at krakenes and its identity is confirmed geochemically the mean of four ams dates of samples ofsalix herbacealeaves adjacent to the tephra is 10 310 50 yr b p the saksunarvatn ash is not visible in the early holocene lake sediment at krakenes after removal of organic material and diatoms the identity of the tephra particles was confirmed geochemically and their stratigraphic concentration was estimated from curve matching of a series of seven ams dates of terrestrial plant macrofossils and whole sediment the radiocarbon age of the ash is 8930 9060 yr b p corresponding to an age of 9930 10 010 cal yr b p 7980 8060 cal yr b c

high strain zones a unified model abstract where a shear zone is bounded by undeformed wall rocks and there is no volume change the deformation path within the zone on the bulk scale approximates a simple shear where the zone boundaries are stretched and the shear direction between the two boundaries is parallel to one of the principal stretching directions of the boundaries the deformation path within the zone is not a simple shear but still has monoclinic symmetry both simple shear and general monoclinic deformation paths should be regarded as special cases in nature because generally not only are the zone boundaries stretched but also the shear is oblique to the principal stretching directions of the boundaries in this general case the deformation path is triclinic we establish a model which includes this general case also taking into consideration volume change five independent parameters are required to characterize the rate of deformation of the flow four are sufficient to characterize the flow kinematics the model is generally triclinic and orthorhombic and monoclinic deformations are subgroups by varying the values of the characterizing parameters the various types of high strain zones can be represented the kinematics of flow within such zones and the accumulated finite deformation geometry are investigated many field observations such as for example lineations in transcurrent shear zones that vary between vertical and horizontal cleavage transected folds and the absence of sheath folds in some shear zones where folds have been significantly rotated can sometimes be better interpreted in the context of our general model

the 2011 unrest at katla volcano characterization and interpretation of the tremor sources abstract a 23 hour tremor burst was recorded on july 8 9th 2011 at the katla subglacial volcano one of the most active and hazardous volcanoes in iceland this was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the southern flank no visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes or by the flood the tremor signal consisted of bursts with varying amplitude and duration we have identified and described three different tremor phases based on amplitude and frequency features a tremor phase associated with the flood was recorded only at stations closest to the river that flooded correlating in time with rising water level observed at gauging stations using back projection of double cross correlations two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes the greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub glacial eruption a less plausible interpretation is that the tremor was generated by hydrothermal boiling and or explosions with no magma involved this may have been induced by pressure drop triggered by the release of water when the glacial flood started all interpretations require an increase of heat released by the volcano

the role of bedding in the evolution of meso and microstructural fabrics in fault zones abstract to investigate the role of bedding in the evolution of meso and microstructural fabrics in fault zones detailed microscopic mineralogical and geochemical analyses were conducted on bedding oblique and bedding parallel faults that cut a folded neogene siliceous mudstone that contains opal ct smectite and illite an analysis of asymmetric structures in the fault gouges indicates that the secondary fractures associated with each fault exhibit contrasting characteristics those of the bedding oblique fault are r 1 shears whereas those of the bedding parallel fault are reactivated s foliation the bedding oblique fault shows the pervasive development of s foliation lacks opal ct and has low sio 2 tio 2 ratios only in gouge whereas the bedding parallel fault exhibits these characteristics in both gouge and wall rocks the development of s foliation and the lack of silica can result from local ductile deformation involving the sliding of phyllosilicates coupled with pressure solution of opal ct although such deformation can occur in gouge the above results indicate that it may occur preferentially along bedding planes preceding the formation of a gouge slip surface thus in sedimentary rocks that contain phyllosilicates and soluble minerals bedding can influence the rheological evolution of meso and microstructural fabrics in fault zones

flow over a hill covered with a plant canopy we develop an analytical model for atmospheric boundary layer flow over a hill that is covered with a vegetation canopy the slope of the hill is assumed to be small enough that the flow above the canopy can be treated within the linear framework of hunt perturbations to the flow within the canopy are driven by the pressure gradient associated with the flow over the hill in the upper canopy this pressure gradient is balanced by downwards turbulent transport of momentum and the canopy drag the flow there can be calculated from linearized dynamics which show that the maximum streamwise winds are where the perturbation pressure is at a minimum i e near the crest of the hill deep within the canopy the pressure gradient associated with the flow over the hill is balanced by the canopy drag here the nonlinear canopy drag this nonlinear balance shows how the streamwise winds are largest where the perturbation pressure gradient is largest i e on the upwind slope of the hill in the lee of the hill this nonlinear solution shows how the pressure gradient decelerates the wind deep within the canopy leading to separation with a region of reversed flow when the canopy is sufficiently deep coupling between the out of phase flows within and above the canopy means that the maximum velocity is further upwind of the hill crest than in flow over a rough hill while the extra turbulent mixing caused by the canopy significantly reduces the magnitude of the velocity speed up over the hill finally we find that there is no formal limit process where the solutions with a canopy yield the well known solutions for flow over a rough hill this finding calls into question the very use of a roughness length in accelerating or decelerating turbulent boundary layers copyright 2004 royal meteorological society

air flow over a two dimensional hill studies of velocity speed up roughness effects and turbulence wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough bell shaped two dimensional hill with height h and maximum slope 0 26 placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo 0 02 h close agreement is found between the mean velocity and predictions obtained from taylor s 1977 computational model and jackson and hunt s 1975 analytical linearized model for locations at or upwind of the hill top but not in the wake examination of the models shows that the shear stresses are only important in an inner region close to the hill surface so that as suggested by jackson and hunt 1975 the perturbation to the mean flow outside this region is essentially inviscid the theory shows that over very rough surfaces such as wooded or urban terrain the height of this inner region can be of the same order as the height of the roughness elements so that in our experiments no measurements could be made in this region in a second experiment flow over a smooth hill on a rough surface was studied the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness in this case rough to smooth in the lee of a hill however the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field

kinematic indicators in fault gouge tectonic analog for soft bedded ice sheets abstract polished slip planes resulting from basin and range detachment faulting of mid tertiary volcaniclastics are well exposed at lake pleasant arizona u s a slip planes show erosional features formed by abrasion below moving sheets of gouge coarse grained diamict during repeated faulting and shearing tectonically generated forms are identical to those found in glaciated terrains slip planes show rat tail ridges and grooves ridge in groove structures nail head striations and wear tracks a prominent morphological element consists of flute ridges up to 30 cm wide and at least 7 m long formed of gouge preserved in the lee of boulders projecting as obstacles above slip surfaces crescentic scours around the stoss side of such boulders record enhanced abrasion by streams of gouge debris clasts are striated and show shapes comparable to flat iron clasts produced by glacial abrasion kinematic indicators at lake pleasant are directly comparable in form to flutes and drumlins in glaciated terrains the data presented here not support a recent hypothesis arguing that glacial striations flutes and drumlins are the product of turbulent subglacial meltwater instead diamict produced along fault planes gouge by low grade shearing is directly comparable to diamict facies deformation till formed by pervasive shear at the base of soft bedded ice sheets both facies are tectonically generated cataclastites produced in low grade shear zones and display genetically related but differently scaled streamlined bedforms

the influences of ecological factors on tiger bush and dotted bush patterns along a gradient from mali to northern burkina faso seven catenas across contracted vegetation patterns including tiger bush and dotted bush were studied in the gourma region in mali and in northern burkina faso mean annual rainfall 200 550 mm the tiger bush encompasses an alternate succession of bands oriented nearly perpendicularly to the wind direction ne sw sandy grassy microdune sloped band of bare crusted ground and woody depressive band a similar succession without wind orientation is found for the dotted bush in this region tiger bush develops under specific conditions 1 same orientation but opposite direction of wind and slope 2 sedimentary and metamorphic rocks sandstone and schist ironcrust 3 shallow 1 m and impervious soils under such circumstances the dynamics of banded patterns result from alternate and successive action of water and wind erosion field evidences showed that vegetation bands migrate upwards with a velocity of 0 2 0 7 m yr 1

how do vegetation bands form in dry lands insights from numerical modeling and field studies in southern nevada usa 1 vegetation bands are periodic bands of vegetation separated by interband spaces devoid of vegetation oriented parallel to the topographic contour in some gently sloping arid to semiarid environments models of vegetation band formation attribute their formation to positive feedbacks among vegetation density soil porosity permeability and infiltration rates here we present an alternative model based on field measurements at our study sites in southern nevada in this model interband spaces between vegetation bands form because topographic mounds beneath vegetation bands detain water upslope from vegetation bands leading to hydrologic and sedimentologic conditions that inhibit the survival of plants in interband spaces we used terrestrial laser scanning tls to create high resolution 10 cm2 pixel raster data sets of bare earth topography and canopy height for four study sites analyses of the tls data in addition to measurements of soil shear strength and particle size document the potential for detention in interband spaces and a near inverse proportionality between band spacing and regional slope we describe a cellular automaton model herein called model 1 for vegetation band formation that includes just two user defined parameters and that generates vegetation bands similar to those at our field sites including the inverse proportionality between spacing and regional slope a second model model 2 accurately predicts the width of vegetation bands in terms of the number and spacing of plants and the geometry of individual plant mounds we also present a gis based analysis that predicts where bands occur within a region based on topographic and hydroclimatic controls

vertical and horizontal strain partitioning of the central tianshan nw china evidence from structures and 40ar 39ar geochronology abstract as a major tectonic collage within the central asia orogen belt caob the chinese tianshan provides a natural laboratory for studying structures resulting from the amalgamation of different tectonic units detailed field and micro tectonic observations along two orogen perpendicular transects identify several structural domains with greatly variable structural styles that were generated at different structural levels but are now juxtaposed geological features combined with 40 ar 39 ar geochronologic data suggest the variable structural styles of the domains developed simultaneously during ca 320 300 ma kinematic reconstruction and deformation temperature estimation taking 40 ar 39 ar thermochronological data into account suggest both horizontal and vertical extrusion of material of the central tianshan magmatic arc assemblage ctma crust which is the expression of heterogeneous strain partitioning during orthogonal continental convergence such a kinematic scenario is quite compatible with and satisfactorily interpreted in the light of the results of available analogue deformation experiments and numerical modeling the structure styles with orthogonal symmetry of both the ctma and the southern tianshan fold and thrust belt revealed in this study in conjunction with previously published paleomagnetic data from the tarim plate suggest that the diachronous collision of the illi central tianshan micro continent with the tarim plate may have caused the tarim block rotation

late quaternary terrestrial tephrochronology of iceland frequency of explosive eruptions type and volume of tephra deposits close to 100 silicic tephra layers have been identified in icelandic terrestrial soils of holocene age the majority of these tephras were erupted at the hekla torfajokull oraefajokull askja snaefellsjokull eyjafjallajokull and katla central volcanoes by far the most active is hekla with close to 50 identified silicic tephra layers in 8000 years showing an inverse relationship between eruption frequency and volume of erupted tephra volumes of uncompacted silicic tephra layers range from 10 km3 with nearly 50 of known tephra volumes lying between 0 1 and 0 5 km3 seven closely spaced hekla eruptions in the period 950 550 bc with identical major element composition illustrate the need of stratigraphic control in short and long distance correlations a 2600 year old tephra geochemically indistinguishable from the borrobol tephra is a reminder that no tephra is geochemically unique the major producers of basaltic tephra are the volcanic systems that are partly covered by ice or partly lying within areas of high groundwater or extending offshore i e the grimsvotn katla veidivotn bardarbunga reykjanes kverkfjoll and vestmannaeyjar volcanic systems the best known eruption record is that of the katla system with over 170 identified basaltic tephra layers and more than 300 estimated in 8400 years the grimsvotn system is currently the most active volcanic system with over 70 eruptions during the last 1100 years volumes of uncompacted basaltic tephra layers range from 20 km3 the majority of known tephra volumes lying between 0 1 and 1 km3 the most voluminous basaltic tephra deposit the 10 200 yr old saksunarvatn tephra may however represent more than one eruption on the grimsvotn system deposition of approximately 800 basaltic tephra layers during the last 9000 14c years is estimated but many of those erupted from volcanoes within ice caps have not been preserved copyright 2007 john wiley sons ltd