sequence of thrusting within a thick skinned tectonic wedge evidence from40ar 39ar and rb sr ages from the austroalpine nappe complex of the eastern alps the sequence of thrusting within a regional scale alpine thick skinned tectonic wedge has been constrained by rb sr and 40ar 39ar dating of whole rock constituent white mica and biotite from structural units of the austroalpine nappe complex eastern alps austria preservation of variscan and pre variscan isotopic signatures in pre alpine basement units indicates a non penetrative metamorphic alpine overprint alpine nappe assembly and accompanying ductile deformation occurred during maintenance of lower greenschist facies metamorphic conditions c 300 450 c analyses from greenschist facies metamorphic mylonites and penetratively ductile deformed cover sequences display decreasing 40ar 39ar plateau ages structurally downward thrusting and accompanied ductile deformation commenced at c 100 90 ma in the uppermost structural levels footwall fault propagation effected internal deformation and out of sequence thrusting within intermediate structural levels at c 95 80 ma this was contemporaneous wit

late cretaceous exhumation history of an extensional extruding wedge graz paleozoic nappe complex austria late cretaceous structures within the eastern graz paleozoic nappe complex define an extruding wedge with north eastward directed thrusting in eastern portions and strike slip shear along the margins stacking structures are overprinted by south westward directed extension with low grade metamorphic rocks in the hangingwall and high grade basement rocks in the footwall pressure temperature and structural data are obtained from successively opening quartz veins that record various stages of progressive deformation and metamorphism fluid inclusion data and related structures show that during extension isothermal decompression from ca 550 c and 8 kbar down to ca 450 c and 2 kbar was related to exhumation of rocks from deep crustal levels the data point to a high geothermal gradient and explain condensed paleo isotherms due to ductile normal faulting in the eastern areas of the graz paleozoic nappe complex the investigated late cretaceous structural elements suggest that the graz paleozoic nappe complex decoupled from the surrounding basement units and operated as a large scale extension extrusion corridor that evolved prior to miocene extrusion tectonics in the eastern alps

prolonged acadian orogenesis revelations from foliation intersection axis fia controlled monazite dating of foliations in porphyroblasts and matrix numerous phases of garnet growth are revealed by detailed studies of foliation inflection intersection axes preserved in porphyroblasts fias in acadian metamorphic rocks in the chester dome region of vermont u s a a regionally consistent succession of four different fia tends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe these monazite inclusions which lie within the various foliations that define the fias provide absolute ages for multiple periods of deformation and episodic phases of garnet growth the monazite inclusions reveal a progression in foliation ages from 431 2 to 349 3 million years within porphyroblasts and from 366 3 to 327 5 million years in pervasive matrix foliations three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts through the medians to the rims and then into the matrix ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the fias the four fia sets began forming prior to 424 3 405 6 386 6 and 366 4 million years ago respectively thus multiple stages of garnet growth occurred throughout acadian deformation and metamorphism in vermont and orogenesis was far more prolonged than previously thought invariably garnet growth occurred early in the accompanying deformation event dating of monazite inclusions without careful separation of phases of garnet growth by fia studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history analysis of monazite grains in the matrix alone will likely only present ages for the youngest events these ages can be amalgamated from grain to grain to yield apparently precise ages but such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis or the overall continuity of the deformation and metamorphism that accompanies plate motion fia and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed this in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories

ordovician trilobites from the tafilalt lagerstatte new data and reappraisal of the bou nemrou assemblage

a new detrital mica 40ar 39ar dating approach for provenance and exhumation of the eastern alps detrital thermochronology can be used as a tool to quantitatively constrain exhumation rates and its spatial variability from active mountain belts commonly used methods for this purpose assume a steady state relationship between tectonic uplift and erosion however this assumption does not account for the transitory response of a dynamic orogenic system to changes in the boundary conditions we propose a different approach that uses the observed detrital age distributions as markers of the past exhumation and of the present day erosion and mixing occurring in a river system in this paper we present new 40ar 39ar biotite and white mica age distributions for 19 modern river sands from the eastern alps north of the periadriatic line the results present three main clusters of ages at 0 5 50 60 120 and 250 350 ma that record the main orogenic phases in this sector of the alps we have applied two numerical methods to the cooling ages to a linearly compute the spatial variability of the relative present day erosion of a set of four detrital mineral samples from drainage basins along the inn river and b quantify the rates of the cooling and erosion in the tauern window during paleocene miocene time of the alpine orogeny our results suggest a 0 34 0 84 mm yr range of exhumation rates for the tauern window since the miocene our estimates of exhumation rates of the western tauern window are higher than those for the eastern tauern window which is consistent with the previous studies

recycling of foliations during folding bedding on the limbs of early formed regional folds generally lies oblique to the major directions of bulk shortening in the crust that is the horizontal and vertical during subsequent deformations matrix foliations on at least one limb of these folds may start to form but then be destroyed by reactivation of the bedding causing decrenulation and rotation into parallelism with the compositional layering consequently the schistosity parallel to bedding s0 s1 in multiply deformed rocks contains the relics of many deformation events and the 2 or 3 oblique foliations record only the very youngest history this lengthy early history is preserved as inclusion trails within porphyroblasts recycling of foliations depends on the shear sense acting on any newly developing foliation and the orientation of this foliation relative to s0 s1 for some orientations and combinations of shear senses both limbs of a pre existing fold can be reactivated from the commencement of a new deformation event this can result in the decrenulation and obliteration of a new foliation that is beginning to form before it shows any significant degree of development for other combinations one limb of a pre existing fold will behave in this manner whereas a new foliation does develop fully on the other limb however subsequent phases of deformation switch which limb shears versus develops an oblique new cleavage rotating the earlier formed oblique foliation into parallelism with s0 s1

shear sense a new approach that resolves conflicts between criteria in metamorphic rocks investigation of microstructural relationships in major movement zones in metamorphic rocks where the sense of displacement is known from regional geological relationships indicates numerous problems with current concepts of shear sense criteria and their application the direction of apparent shearing commonly conflicts from one criterion to another e g from the symmetry of quartz c axis orientation diagrams to the asymmetry of extensional crenulation cleavages this implies that interpretations of shear sense along foliations from some mesoscale and microscale criteria have been erroneous a new approach to interpreting shear sense involving the use of strain fields resolves conflicts in mesoscopic and microscopic criteria and provides a method for determining coherent shear sense histories extending back before the last shearing event for any foliated metamorphic rock it also provides a powerful tool for determining the structural metamorphic path that a rock has followed within an orogen for determination of the shear sense on the last foliation developed in a rock this approach uses geometries developed around competent heterogeneities such as quartz pebbles pegmatite pods veins porphyroclasts porphyroblasts and breccia clasts a shear sense history is derived by applying this approach to earlier foliations preserved within the heterogeneities and their strain shadows

deformation sequence of baltimore gneiss domes usa assessed from porphyroblast foliation intersection axes abstract the ne sw trending gneiss domes around baltimore maryland usa have been cited as classic examples of mantled gneiss domes formed by diapiric rise of migmatitic gneisses eskola p 1949 the problem of mantled gneiss domes quarterly journal of geological society of london 104 416 461 476 however 3 d analysis of porphyroblast matrix foliation relations and porphyroblast inclusion trail geometries suggests that they are the result of interference between multiple refolding of an early formed nappe a succession of six fia foliation intersection axes sets based upon relative timing of inclusion texture in garnet and staurolite porphyroblasts revealed 6 superposed deformation phases the successions of inclusion trail asymmetries formed around these fias document the geometry of deformation associated with folding and fabric development during discrete episodes of bulk shortening exclusive top to nw shear asymmetries of curvature were recorded by inclusion trails associated with the vertical collapsing event within the oldest fia set ne sw trend this strongly indicates a large ne sw striking nw verging nappe had formed early during this deformation sequence this nappe was later folded into ne sw trending up right folds by coaxial shortening indicated by an almost equal proportion of both inclusion trail asymmetries documented by the second n s trending fia set these folds were then amplified by later deformation as the following fia sets showed an almost equal proportion of both inclusion trail asymmetries

the paleoenvironmental distribution of phanerozoic wrinkle structures abstract wrinkle structures are microbially induced sedimentary structures that are known to have formed in ancient marine siliciclastic environments this study reviews the sedimentary context and paleoenvironmental distribution of these sedimentary structures throughout three distinct intervals of the phanerozoic the cambrian the post cambrian paleozoic and the lower triassic during these times wrinkle structures are found primarily within two sedimentary environments storm dominated subtidal environments and the intertidal zone subtidal occurrences of wrinkle structures during the post ordovician phanerozoic only appear to occur during intervals of environmental stress while earlier examples from the cambrian and lowermost ordovician likely formed prior to significant increases in extent and depth of bioturbation that hallmark most of the phanerozoic intertidal examples occur regardless of severe environmental stress and may have formed under low levels of bioturbation due to inherent vagaries of a marginal marine setting these environmental preferences appear to be conservative throughout the phanerozoic and may very well extend throughout earth s history

resolving miss conceptions and misconceptions a geological approach to sedimentary surface textures generated by microbial and abiotic processes the rock record contains a rich variety of sedimentary surface textures on siliciclastic sandstone siltstone and mudstone bedding planes in recent years an increasing number of these textures have been attributed to surficial microbial mats at the time of deposition resulting in their classification as microbially induced sedimentary structures or miss research into miss has developed at a rapid rate resulting in a number of misconceptions in the literature here we attempt to rectify these miss misunderstandings the first part of this paper surveys the stratigraphic and environmental range of reported miss revealing that contrary to popular belief there are more reported miss bearing rock units of phanerozoic than precambrian age furthermore miss exhibit a pan environmental and almost continuous record since the archean claims for the stratigraphic restriction of miss to intervals prior to the evolution of grazing organisms or after mass extinction events as well as claims for the environmental restriction of miss appear to result from sampling bias in the second part of the paper we suggest that raised awareness of miss has come at the cost of a decreasing appreciation of abiotic processes that may create morphologically similar features by introducing the umbrella term sedimentary surface textures of which miss are one subset we suggest a practical methodology for classifying such structures in the geological record we illustrate how elucidating the formative mechanisms of ancient sedimentary surface textures usually requires consideration of a suite of sedimentological evidence from surrounding strata resultant interpretations microbial or non microbial should be couched within a reasonable degree of uncertainty this approach recognizes that morphological similarity alone does not constitute scientific proof of a common origin and reinstates a passive descriptive terminology for sedimentary surface textures that cannot be achieved with the current miss lexicon it is hoped that this new terminology will reduce the number of overly sensational and misleading claims of miss occurrence and permit the means to practically separate initial observation from interpretation furthermore this methodology offers a scientific approach that appreciates the low likelihood of conclusively identifying microbial structures from visual appearance alone informing the search for true miss in earth s geological record and potentially on other planetary bodies such as mars

micrometric detail in palaeoscolecid worms from late ordovician sandstones of the tafilalt konservat lagerstatte morocco abstract the late ordovician tafilalt biota of morocco is a recently discovered konservat lagerstatte that provides abundant paropsonemid eldonioids resembling those from the cambrian burgess shale and chengjiang biotas or other paropsonemid occurrences from the ordovician to silurian of the us and australia large discoidal holdfasts non biomineralised cheloniellid arthropods rare vermiform fossils and articulated skeletons of echinoderms and trilobites exceptional preservation of soft bodied organisms occurs in medium to coarse grained sandstones in a style reminiscent of the soft bodied ediacaran fossils of the white sea assemblage here we describe the first articulated scleritomes of a large palaeoscolecid worm from africa in addition to the mineralised sclerites the specimens also exhibit extensive soft tissue preservation down to micron scale including fine detail of annuli with their plate platelet and microplate arrangement on a reticulate cuticle compression fossils of the new species gamascolex vanroyi are represented by external moulds with remains of phosphatised cuticular structure secondarily weathered into strontian crandallite partial foregut fossilisation is seen in one specimen and phosphatised digestive structures are also reported in three co occurring trilobite genera at the bou nemrou locality from a palaeogeographic point of view these north african palaeoscolecids represent the highest palaeolatitudinal occurrence of this palaeozoic group in gondwana being described for the first time in cold water areas adjacent to the late ordovician south pole it also provides a palaeobiogeographic link to the original distribution of the genus gamascolex in a central european peri gondwanan terrane

the geometry and kinematics of flow perturbation folds abstract minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow using structures associated with wnw directed caledonian thrusting in n scotland we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction these define a series of larger scale fold culminations reflecting surging flow or depressions reflecting slackening flow that are bisected by transport parallel culmination and depression surfaces the fold patterns suggest a dominance of layer normal differential shearing due to gradients in shear strain normal to transport culmination surfaces are marked by along strike reversals in the polarity of structural facing and vergence of minor folds which contrary to classic fold patterns define reverse asymmetric relationships culmination surfaces separate folding in to clockwise z folds and anticlockwise s folds domains relative to the transport lineation the dip of fold axial planes systematically increases as their strike becomes sub parallel to transport resulting in a 3d statistical fanning arrangement centred about the transport direction thus mean s and z fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved culminations display convergent fold patterns with fold hinges becoming sub parallel to transport towards the culmination surface and underlying detachment whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface thus around culminations and depressions there are ordered scale independent relationships between transport direction shear sense fold facing vergence and hinge axial plane orientations the techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow

wrinkle structures microbially mediated sedimentary structures common in subtidal siliciclastic settings at the proterozoic phanerozoic transition sedimentary structures produced by microbial activity are well known from ancient carbonate facies but little is known about equivalent microbial structures in ancient siliciclastic facies wrinkle marks and kinneyia ripples are closely related sedimentary structures that are commonly found in ancient siliciclastics and they may represent microbial activity to evaluate this possibility these structures were studied in vendian cambrian strata of north america and compared to structures formed by modern microbial mat communities in redfish bay texas striking similarities in sedimentologic petrographic and morphologic characteristics of modern and ancient occurrences suggest that the enigmatic but widespread ancient structures could have been formed by microbial processes the relative abundance of these structures in siliciclastic facies of this interval further suggests that prior to the onset of relatively intense bioturbation in the ordovician microbial mats could have played a significant role in the evolution and diversification of early life in a broad variety of sea floor environments

porphyroblast rotation versus nonrotation conflict resolution data on foliation intersection and or inflection axes preserved in porphyroblasts fias indicate that no porphyroblast rotation occurs during ductile deformation relative to spatial coordinates this contrasts with 99 of investigations of rigid objects in non coaxially deforming media where the objects rotate when anastomosing shear zone formation around relatively strong objects in a weaker matrix is modeled no porphyroblast rotation occurs formation of these anastomosing zones controls the development of this phenomenon called gyrostasis if such zones are absent porphyroblasts rotate in weak materials the gyrostatic situation arises because the superposition of simple shearing deformation normal to initial coaxial shortening results in only small rotations of principal axes of stress since shear zones are controlled by the orientations of principal axes of stress initial anastomosing zones retain their orientations and positions during subsequent non coaxial deformation the porphyroblast is isolated from the embedding non coaxially deforming matrix but this material close to the porphyroblast continues to deform coaxially no local rotation occurs this has major tectonic significance because allowing for the effects of rotation due to brittle deformation porphyroblasts can now be routinely used to access lengthy structural and or metamorphic histories destroyed in the matrix by reactivation such as movement directions shear senses and extended pressure temperature time paths

relationship of spaced cleavage to folds and thrusts in the idaho utah wyoming thrust belt the idaho utah wyoming thrust belt of the rocky mountain cordillera preserves structures that developed at very shallow levels of the crust t 200 c the geometry of major structures is well documented and synorogenic conglomerates record the timing of movement on major thrusts spaced cleavage and pencil fracturing are developed within micrites of the jurassic twin creek formation the cleavage developed dominantly by pressure solution early in the deformation history of each thrust sheet aided by elevated temperatures and pressures in areas where increased overburden resulted from emplacement of earlier overriding sheets cleavage within any sheet is well developed in the area covered by the pre erosional extent of the previous sheet forelandward from this area cleavage intensity decreases drastically thermal modelling of the sheets agrees well with temperature estimates from illite crystallinity studies on material from the cleavage planes cleavage is axial planar to small folds and formed concurrently with small scale buckle folds and contraction faults during the early history of movement on each thrust however cleavage strongly fans about large scale folds formed later during movement suggesting passive rotation of cleavage with bedding during the later stages of thrust history the rotated cleavage may also be folded and overprinted by a second weaker cleavage during a second ramping or during movement on a later lower thrust cross folding may also develop as a result of lateral ramping thus complex patterns of interfering folds multiple cleavages and associated fiber filled fractures may be produced during motion on a single thrust fault and so magnify the problem of correlating cleavages and deciphering multiple deformations

significance of transport parallel strain variations in part of the raft river shear zone raft river mountains utah usa this paper examines part of the raft river shear zone rrsz in northwestern utah that exhibits an extreme transport parallel increase in strain intensity coupled with a transition from flattening to constrictional strain detailed geologic mapping and finite strain quartz c axis fabric and kinematic vorticity analyses demonstrate local necking of the shear zone associated with an increase in transport parallel elongation accommodated by a stretching fault at the base of the shear zone the domain of intense deformation and necking of the shear zone is localized where the basal stretching fault cuts rheologically weak rocks this domain is characterized by strain in the constrictional field caused by transport perpendicular flow into the area of high transport parallel elongation where rocks cut by the stretching fault are rheologically strong the rrsz locally records flattening strain and lower strain intensities limiting the amount of stretching needed at the base of the shear zone in any one direction and or recording transport perpendicular flow into adjacent highly extended domains the rheology of rocks cut by the stretching fault directly controlled the amount and style of zone normal shortening and transport parallel elongation and these observations provide an example of deformation partitioning in a crustal scale structure driven by local rheological transitions

wrinkle structures a critical review abstract in this paper a variety of so called wrinkle structures is reviewed in an attempt to help distinguish between crinkly decorations arising from physical processes that acted on siliciclastic bedding surfaces and true microbially induced wrinkle structures two types of small scale microbially induced sedimentary structures are prominent due to their distinct geometry and mode of occurrence 1 elephant skin textures characterized by reticulate patterns of sharp crested ridges forming mm to cm scale polygons occurring on argillite or argillaceous veneers above fine grained sandstone and likely reflecting growth structures of microbial mats 2 kinneyia structures characterized by mm scale flat topped winding ridges and intervening troughs and pits sometimes resembling small scale interference ripples kinneyia structures usually occur on upper surfaces of siltstone sandstone beds themselves frequently event deposits and are thought to have formed beneath microbial mats additionally more linear variations of mat growth structures partly resembling small scale petees may be developed finally some wrinkly structures resulting from tractional mat deformation or mat slumping are occasionally preserved these may appear as arcuate belts of non penetrative small scale folds or as wrinkled bulges on otherwise flat surfaces wrinkle structures as indicators for the former presence of mats gain in importance if other mat related structures are additionally observed in the same clastic succession e g sand chips sandy intraclasts or spindle shaped or sinuously curved to circular sand cracks frequently combined in networks furthermore appropriate lithologies and facies are required for instance if compared with the distribution of modern cohesive microbial mats laminated siltstone argillite with intercalated siltstone sandstone beds representing event deposits in tidal flat successions would be compatible with microbial mat development within a variety of physically induced small scale wrinkly structures miniature load structures may above all be misinterpreted as microbially induced wrinkle structures due to their similar size and appearance and their comparatively frequent occurrence

microbially induced sedimentary structures a new category within the classification of primary sedimentary structures abstract cyanobacterial films and mats syndepositonally influence erosion deposition and deformation of sediments the biomass levels surface morphologies and microbial mats stabilize depositional surfaces and shelter the sediment against erosion or degassing growing microbial mats dredge grains from their substrate upwards whereas cyanobacterial filaments that are oriented perpendicular to the mat surface reach into the supernatant water and baffle trap and bind suspended particles these and similar biotic physical interactions are reflected in syndepositional formation of microbially induced sedimentary structures we distinguish structures on bedding planes leveled bedding surfaces wrinkle structures microbial mat chips erosional remnants and pockets multidirectional ripple marks and mat curls and internal bedding structures sponge pore fabrics gas domes fenestrae structures sinoidal laminae oriented grains benthic ooids biolaminites mat layer bound grain sizes we propose to place this group of microbially mediated structures as a fifth category bedding modified by microbial mats and biofilms in pettijohn and potter s 1964 existing classification of primary sedimentary structures

permian bulk shortening in the narragansett basin of southeastern new england usa pennsylvanian age sediments within the narragansett basin allow the effects of alleghanian deformation to be distinguished from the taconic and acadian orogenies that affected the rocks to the west three dimensional microstructural analyses of both matrix foliations and structures preserved in porphyroblasts define two extended periods of deformation and metamorphism about two differently trending foliation intersection inflection axes in porphyroblasts fias ssw nne trending fias set 1 formed first followed by wsw ene trending fias set 2 the ssw nne fia trends from the south central zone lie parallel to regional folds in the entire southern graben the changes in fia trends reveal that the direction of maximum bulk shortening changed from wnw ese to nnw sse during amphibolite facies metamorphism within the central zone of the basin they reveal a more varied history than the simple kinematic framework of solely w e directed bulk shortening that was previously suggested

regional scale foliation reactivation and re use during formation of a macroscopic fold in the robertson river metamorphics north queensland australia abstract the rocks defining a macroscale antiform 25 km2 in the proterozoic robertson river metamorphics have been affected by four deformations of distinctive style with the fourth deformation d4 being responsible for formation of the macroscale geometry of the fold redistribution of progressive shearing strain due to strain partitioning during progressive d4 deformation resulted in the accommodation of d4 shearing strain along s2 differentiated crenulation cleavages that had been synchronously rotated into favourable orientations on the antiformal limbs this commonly resulted in reactivation of s2 because shearing during d4 was in a sense that was antithetic relative to that on the bulk scale of the fold continued d4 deformation caused unfolding of d2 crenulations resulting in straightening of sigmoidally folded s1 and its rotation toward the axial plane of the synchronously forming macroscopic d4 fold in zones where the sense of shear during d4 was the same as that operating on the bulk scale of the fold i e synthetic d4 shearing strain was accommodated dominantly by the favourably oriented approximately axial planar s1 fissility in zones where the progressive shearing component of d4 deformation was relatively more intense the s2 cleavage was also rotated into parallelism with s4 and was also re used as opposed to reactivated detailed microstructural analysis particularly of porphyroblast matrix relationships combined with field observations have resolved the processes operating during folding and reveal that despite the intensity of d4 deformation a separate cross cutting s4 cleavage has rarely been produced at either the meso or microscale similar processes have probably operated at all scales in other orogenic belts

40ar 39ar muscovite ages from the penninic austroalpine plate boundary eastern alps the radstadt mountains eastern alps expose the tectonic boundary between the base of the austroalpine continental plate hanging wall and the penninic oceanic units foot wall parts of the austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the permian 290 250 ma austroalpine permo mesozoic cover rocks were deformed during the cretaceous 80 ma and the paleogene 55 50 ma these ages are interpreted as representing two distinct events of nappe stacking the younger ages represent the collision of austroalpine and penninic tectonic units the penninic nappe complex displays a successive decrease of ages from 37 to 25 ma from high to deep tectonic levels a second age group of 22 ma was found both in low temperature release steps and as plateau ages close to the penninic austroalpine boundary it is attributed to a thermal overprint due to ductile extension of the overthickened orogenic wedge

deformation partitioning foliation successions and their significance for orogenesis hiding lengthy deformation histories in mylonites although orogenesis commonly lasts 100 million years a maximum of three foliations are usually preserved within any outcrop they record little of the total history due to the effects of preferential partitioning of progressive shearing along s0 such reactivation eventually destroys or rotates multiple successive foliations into parallelism with any layering that is present plate motion subjects a collisional orogen to spatially partitioned episodic but non stop deformation the bulk forces operating are horizontal and intermittently vertical associated with gravitational load due to crustal thickening forming sub vertical and sub horizontal foliations these orientations are preserved in the strain shadows of anything competent that hinders reactivation of the compositional layering deformation partitioning results in portions of rock remaining unaffected for many events gravitational collapse of over thickened orogen cores leads to significant extrusion in orogen rims and a dominance of sub horizontal foliations in the latter and episodically continues throughout orogenesis reactivation causes decrenulation and or rotation of foliations into parallelism with s0 destroying evidence that multiple sub vertical and sub horizontal foliations have developed except where preserved as inclusion trails in porphyroblasts and multiply truncated foliations in mylonites however the same processes operate in both the cores of orogens and ductile portions of their margins

deep structure and kinematics of the northern calcareous alps transalp profile the northern calcareous alps nca of southern bavaria and northern tyrol constitute a carbonate dominated polyphase fold thrust wedge together with its grauwacken zone basement it is the northernmost part of the far travelled upper austroalpine thrust complex of the eastern alps the present geometry developed in several kinematic stages jurassic extensional faults that affected large parts of the nca and their basement originated when the main part of the nca was still located southeast of the central alpine otztal silvretta complex these faults and related facies transitions influenced the later style of detachment of the nca thrust sheets mid to late cretaceous detachment thrust sheet stacking and motion over the central alpine complex are registered in clastic deposits of syntectonic basins the latest cretaceous to cenozoic nne to n directed motion of the nca towards europe in front of the central alpine complex created another set of significant contraction structures which at depth overprinted all previous structures during cretaceous to cenozoic deformation the nca experienced about 80 km of shortening i e about 73 along the transalp profile the european basement and autochthonous mesozoic cover beneath the allochthonous nca thrust sheets and flysch complexes seem to have remained relatively undeformed

upper ordovician planktic and benthic graptolites and a possible hydroid from the tafilalt biota southeastern morocco

distribution of pre folding linear indicators of movement direction around the spring hill synform vermont significance for mechanism of folding in this portion of the appalachians abstract three distinctly oriented sets of pre folding and one set of syn folding axes of curved inclusion trails are preserved in garnet porphyroblasts in 50 samples around the doubly plunging spring hill synform in southeast vermont over one third of the samples contain consistent changes in the trend of these axes from the core to rim since the core grew before the rim this enabled the relative timing of each set of axes to be determined from the oldest to the youngest as ne sw e w nnw sse and nne ssw the youngest trend is parallel to the axial plane of the regional folds only those samples with the latter trend have their inclusion trails connected continuously to the matrix foliation the three pre folding sets of axes have the same orientation on both limbs this consistency in orientation has significant implications for the processes operating during folding and three mechanisms are presented that could potentially explain it these involve the classic card deck model of shear folding de sitter s model of clay bricks shortening as they shear past one another and the progressive bulk inhomogeneous shortening model the relative merits of each of these models are discussed

time constraints on deformation and metamorphism from epma dating of monazite in the proterozoic robertson river metamorphics ne australia epma dating of monazite confirms the relative timing of a succession of foliation intersection axis trends in porphyroblasts fias and two periods of progressive metamorphism intervened by retrogression in the robertson river metamorphics ne australia a complete pressure temperature deformation time p t d t path has been deciphered involving an early clockwise p t loop followed by high temperature low pressure metamorphism successive generations of monazite were identified using microstructure and fia trends and then dated the succession of four fias trending ene wsw fia1 e w fia2 n s fia3 ne sw fia4 plus four matrix foliations s1 s4 were accompanied by three periods of garnet and two periods of staurolite growth monazite grew in response to most of these events isopleth thermobarometery using p t pseudosections in the mnnckfmash system for garnet porphyroblasts selected based on fia trends in their core reveal that pressure increased progressively from 3 4 kb at 530 550 c to 6 7 kb at 600 620 c during medium pressure temperature metamorphism this was accompanied by changing bulk shortening directions from nnw sse to n s to e w perpendicular to fias 1 2 and 3 the development of fia1 and fia2 occurred between 1655 2 2 ma and 1625 15 ma fia3 developed over 30 50 ma periods from 1625 15 ma to 1586 6 ma retrogressive metamorphism decompression and exhumation occurred over a not vert similar45 ma period further metamorphism accompanied regional granite intrusion and generated lower pressure higher temperature metamorphism during the development of fia4 nw se bulk shortening at 1542 8 ma this period possibly extended for another 30 50 ma based on the monazite ages obtained from the youngest foliations in the matrix correlation of these data with other ne australian proterozoic inliers suggests that they were all once part of a single orogen that developed from ca 1650 to 1500 ma

predicting seismic properties from three dimensional microstructures a new look at an old quartzite the mylonitic cambrian quartzites moine thrust zone nw scotland have long been used to study microstructural and petrofabric evolution and to develop understanding of grain scale processes accommodating large scale displacements today structural geology is entering a new age of understanding of the basic processes involved in microstructural evolution due to the emergence of novel instrumental techniques and theoretical models it seems apposite therefore to re evaluate the microstructure of one example of this classic quartz mylonite from the stack of glencoul assynt using arguably the most important of these new techniques electron backscattered diffraction ebsd the three dimensional 3d microstructure and petrofabric of this rock was analysed using ebsd to 1 corroborate previous optical and x ray texture goniometry measurements 2 investigate the potential for sampling and or tectonic sectioning bias that may be introduced inadvertently into any petrofabric analysis and 3 predict its seismic properties it is found that microstructures do differ between orthogonal structural sections leading to variations in strengths of different components in the overall petrofabric that might impact on seismic properties the results emphasize the true 3d nature of microstructures and petrofabrics which can be recognized and accommodated more readily by this new generation of analytical techniques

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

conversion of carbonaceous material to graphite within the greywacke zone of the eastern alps morphology and reflectance of carbonaceous material cm in late carboniferous metasediments of the eastern greywacke zone eastern alps indicate a mixture of vitrinite grainy textured and lamellar shaped particles as imaged by high resolution atomic force microscopy vitrinite and the grainy textured particles show mesophase structures which can be described as facetted nanocrystals within the carbon matrix high resolution transmission electron microscopy has revealed two types of microtextures representing different degrees of graphitization the first type is characterized by elongated ring shaped microtextures whereas the second type is characterized by graphite lamellae and polygonal flakes with long range ordered aromatic layers in spite of the heterogeneity of the cms the geographical distribution of quantitative metamorphic parameter raman spectra parameter x ray diffraction pattern microscopic reflectance suggests a graphitization process which is promoted by advective heat transport during post collisional processes in a tentative pressure time path late cretaceous thrusting results in a turbostratic ordering of the aromatic layer ordering to long range ordered aromatic layers was achieved during the late cretaceous paleogene exhumation of mid crustal rocks beneath the eastern greywacke zone

luminescence color and fission track zoning in apatite crystals of the panasqueira tin tungsten deposit beira baixa portugal

microstructural analysis of the classical spiral garnet porphyroblasts of south east vermont evidence for non rotation new data strongly suggest that the classical spiral garnet porphyroblasts of south east vermont usa generally did not rotate relative to geographical coordinates throughout several stages of non coaxial ductile deformation the continuity of inclusion trails si in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities consisting of truncations and differentiation zones where quartz graphite si bend sharply into more graphitic si discontinuous tight microfold hinges with relatively straight axial planes are also present these microstructures form part of a complete morphological gradation between near orthogonally arranged discontinuous inclusion segments and smoothly curving continuous si spirals some 2700 pitch measurements of well developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike from specimens collected at 28 different locations around the chester and athens domes the results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations irrespective of variations in the external foliation attitude macrostructural geometry and apparent porphyroblast matrix rotation angles combined with evidence for textural zoning this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation dimensionally elongate idioblasts also have a preferred shape orientation with long axes orientated normal to the mica folia within which epitaxial nucleation occurred truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins in association with progressive and selective strain induced dissolution of matrix minerals and locally also the porphyroblast margin non rotation of porphyroblasts relative to geographical coordinates suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed relatively large and rigid heterogeneities which cause the progressive shearing rotational component of deformation to partition around them the spiral garnet porphyroblasts therefore preserve the most complete record of the complex polyphase tectonic and metamorphic history experienced in this area most of which was destroyed in the matrix by progressive foliation rotation and reactivation together with recrystallization

survival of burgess shale type animals in a middle ordovician deep water setting exceptional preservation of non biomineralized arthropods sponges and vermiform taxa occurs in the darriwilian middle ordovician llanfallteg formation of south wales uk the assemblage contains elements typical of ordovician communities juxtaposed with those more commonly associated with cambrian konservat lagerstatten this assemblage is preserved in rocks of a deep marine succession dominated by fine grained siliciclastic and volcaniclastic density current deposits non biomineralized taxa of cambrian aspect are preserved as pyritized carbonaceous compressions on the spectrum of burgess shale type preservation trilobites with phosphatized digestive structures have also been recovered the assemblage of the llanfallteg formation konservat lagerstatte demonstrates that some burgess shale type faunal elements survived into the middle ordovician within cool deep water refugia in the welsh basin offshore from communities dominated by typical ordovician taxa supplementary material assemblage composition data and sedimentological images can be found at https doi org 10 6084 m9 figshare c 2182218

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

early palaeozoic tectonothermal events in basement complexes of the eastern greywacke zone eastern alps evidence from u pb zircon data field observations and u pb zircon data yield evidence that the kaintaleck complex represents a distinct crystalline basement unit of the eastern greywacke zone and contrasts partly with other pre alpine basement complexes of the eastern alps the age data indicate possible late archean rock formation several magmatic and metamorphic events in the early palaeozoic ca 520 490 and ca 400 360 ma and low grade metamorphic overprint in the cretaceous zircons from a garnet gneiss layer in a plagioclase gneiss and amphibolite sequence yield an upper intercept age of ca 2 55 ga which is interpreted as a possible protolith age the lower intercept age at 514 ma represents either 1 a magmatic event or 2 a metamorphic overprint within amphibolite facies conditions magmatic zircons derived from granitic orthogneiss boulders of the kalwang conglomerate which covers the crystalline basement crystallised at ca 500 ma the lower discordia intercepts of the orthogneiss boulders mark the cretaceous low grade metamorphic overprint the lower intercept age of paragneiss zircons from another slice of the kaintaleck complex suggests a likely devonian metamorphic event at ca 390 400 ma the new data demonstrate that the kaintaleck complex experienced several stages of tectonothermal evolutions which are in contrast to the evolution of the main mass of the austroalpine basement the data constrain linkages of this part of the austroalpine basement to the cordillere ligerienne and cadomian block within west european variscides

seismic image of the deep crust at the eastern margin of the alps austria indications for crustal extension in a convergent orogen abstract a 39 km long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the eastern alps austria the ese wnw trending low fold seismic line crosses austroalpine basement units and extends approximately from 20 km west of the penninic window group of rechnitz to 60 km sse of the alpine thrust front the explosive source seismic data reveals a transparent shallow crust down to 5 km depth a complexly reflective upper crust and a highly reflective lowermost crust the upper crust is dominated by three prominent west dipping packages of high amplitude subparallel reflections the upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low angle normal shear zones related to the miocene exhumation of the rechnitz metamorphic core complex in the western portion of the upper crust east dipping and less significant reflections prevail the lowermost package of these reflections is suggested to represent the overall top of the european crystalline basement along the western portion of the line the lower crust is characterised by a 6 8 km thick band of high amplitude reflection lamellae typically observed in extensional provinces the moho can be clearly defined at the base of this band at approximately 32 5 km depth due to insufficient signal penetration outstanding reflections are missing in the central and eastern portion of the lower crust we speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower moho in the east preferable at about 30 5 km depth the high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust the latter interpreted to represent broad extensional mylonite zones emphasises the latest extensional processes in accordance with eastward extrusion

structural evolution of an antiformal window the scheiblingkirchen window eastern alps austria abstract the scheiblingkirchen window a lower austroalpine tectonic window at the eastern margin of the eastern alps austria was formed during late cretaceous continent continent collision structural investigations including structural mapping microstructural studies and texture analysis revealed a decompression related three stage tectonic history of lower austroalpine units during the formation of the scheiblingkirchen window 1 intra lower austroalpine nappe stacking was by top to the n out of sequence thrusting of the kirchberg fold nappe over the wechsel nappe under lower greenschist facies metamorphic conditions the structural expression of the stacking event d1 comprises a penetrative foliation containing a n s trending stretching lineation isoclinal recumbent folds trending subparallel to the stretching lineation and ultramylonites quartz and calcite microstructures indicate that dynamic recrystallization processes accompanied deformation their commonly moderately developed lattice preferred orientation record dominant slip on the prism and rhomb planes parallel to a 2 subsequent exhumation of previously stacked rocks is related to the formation of foliation parallel mylonitic shear zones within an e w extensional regime d2 microstructures and textures suggest similar deformation temperatures during thrusting and extension 3 a superimposed phase of nw se oriented horizontal shortening d3 was accommodated by large and small scale upright folding of the area around ne sw trending axes and by backthrusting leading to the antiformal doming of the scheiblingkirchen window subsequent subvertical flattening resulting from the shortening phase led to the formation of ne sw trending outcrop scale open recumbent folds low temperature deformation conditions as inferred from the low degree of recrystallization of quartz and calcite aggregates and the dominance of glide on the basal planes point to a cooling related deformation event

thermomechanical consequences of cretaceous continent continent collision in the eastern alps austria insights from two dimensional modeling we use two dimensional numerical modeling techniques to investigate the thermomechanical consequences of closure of the meliata hallstatt ocean and consequent cretaceous continent continent collision in the eastern alps austria in the modeling a lower plate position of the austro alpine aa continental block is adopted during collision with the upper juvavic silice block the thermal structure of the lithosphere was calculated for major aa tectonic units upper middle and lower austro alpine by integration of the transient heat flow equation along an approximately nw se cross section east of the tauern window indications of the rheological evolution of the aa were determined by calculating strength profiles at key stages of the cretaceous orogeny making use of the thermal modeling predictions combined with rock mechanics data cooling in the upper plate and lower greenschist facies metamorphism within footwall parts of the lower upper austro alpine ua plate related to se directed underthrusting of the ua beneath the upper juvavic silice block at circa 100 ma were predicted by the numerical model the observed pressure temperature path for deeply buried middle austro alpine ma upper crustal units and their subsequent isothermal exhumation are best reproduced assuming a pressure peak at 95 ma and exhumation rates ranging between 4 and 7 5 mm yr 1 from the modeling results we deduce that the temperature evolution during eclogite exhumation is primarily dependent on rates of tectonic movements and largely independent of the mode of exhumation thrusting versus erosion furthermore very rapid postmetamorphic exhumation of southern lower austro alpine la units is predicted in order to account for subsequent cooling this is constrained by 40ar 39ar data the cooling paths of ma and la rocks appear to be primarily controlled by their near surface positions at the end of the cretaceous rather than by other processes such as concurrent underthrusting upward advection of heat by rapid exhumation induced thermal weakening of the thickened crust the inferred weakness of the central parts of the orogenic system may play an important role during detachment related tectonic unroofing orogenic collapse and concomitant basin formation central alpine gosau basins

the nappe rule why does it work the nappe rule stresses that imbrication emplaces nappes with higher metamorphic grade above those with lower grade this means that the overriding nappes must be exhuming during nappe stacking otherwise metamorphic grade would increase structurally downwards the high grade metamorphic internides of many orogens are characterized by a pervasive nappe emplacement related subhorizontal foliation indicative of vertical ductile shortening thinning by quantifying finite strain in the gran paradiso massif italian alps we show that vertical ductile thinning associated with a nappe emplacement related subhorizontal foliation caused sufficient exhumation of the overriding nappes and can thus explain the occurrence of higher grade rocks above lower grade ones

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

evolution of veins and sub economic ore at strassegg paleozoic of graz eastern alps austria evidence for local fluid transport during metamorphism the north eastern part of the graz paleozoic thrust sheet at strassegg austria is characterised by metamorphic assemblages that reflect increasing temperature conditions from 3008c in hangingwall to around 5508c in footwall units the main quartz andror carbonate vein generation cuts the schistosity at moderate to low angles is folded with the schistosity and is commonly boudinaged there is a correlation between vein and the host rock mineralogy additionally the d 18 o and 13 d c signatures of vein minerals quartz andror carbonates vary systematically with the isotopic signature of the host rock and its minerals within individual units d 18 o of vein quartz has a narrow range of values in contrast with a larger variation in both with d 18 o and d 13 c of carbonates at strassegg the greenschist unit hosts an as au mineralisation arsenopyrite

the role of deformation partitioning in the deformation and recrystallization of plagioclase and k feldspar in the woodroffe thrust mylonite zone central australia abstract in the woodroffe thrust mylonite zone central australia recrystallization in plagioclase and k feldspar involved subgrain rotation assisted by grain boundary or kink band boundary bulging without contribution from a change in the chemical composition from host grains to new grains the size of subgrains and new grains changes across the mylonite zone apparently as a function of the strain rate and the h2o content of the rock the partitioning of deformation into zones of progressive shearing and progressive shortening controls the sites of recovery and recrystallization in feldspar during mylonitization the size of feldspar porphyroclasts in well developed mylonites is governed by the scale of deformation partitioning reached in the earlier stages of mylonitization before the formation of a large proportion of fine grained matrix that can accommodate the progressive shearing component of the deformation recrystallization occurs in microcline apparently without involving a translation to a monoclinic structure as microcline twinned new grains are common adjacent to microcline twinned host grains k feldspar triclinicity values calculated from xrd traces increase from the margins to the interior of the mylonite zone in conjunction with deformation intensity k feldspar host grains locally have cores of orthoclase or untwinned microcline surrounded by mantles of twinned microcline suggesting a relationship between the presence of microcline twinning and the degree of k feldspar triclinicity

structures microfabrics and textures of the cordilleran type rechnitz metamorphic core complex eastern alps rechnitz window group represents a cordilleran style metamorphic core complex which is almost entirely located within nearly contemporaneous neogene sediments at the transition zone between the eastern alps and the neogene pannonian basin two tectonic units are distinguished within the rechnitz metamorphic core complex rmcc 1 a lower unit mainly composed of mesozoic metasediments and 2 an upper unit mainly composed of ophiolite remnants both units are metamorphosed within greenschist facies conditions during earliest miocene followed by exhumation and cooling the internal structure of the rmcc is characterized by the following succession of structure forming events 1 blueschist relics of paleocene eocene age formed as a result of subduction d1 2 ductile nappe stacking d2 of an ophiolite nappe over a distant passive margin succession ca e w to wnw ese oriented stretching lineation 3 greenschist facies grade metamorphism annealing dominant in the lower unit and 4 ductile low angle normal faulting d3 with mainly ne sw oriented stretching lineation and 5 ca e to ne vergent folding d4 the microfabrics are related to mostly ductile nappe stacking to ductile low angle normal faulting paleopiezometry in conjunction with p t estimates yield high strain rates of 10 11 to 10 13 s 1 depending on the temperature 400 350 c and choice of piezometer and flow law calibration progressive microstructures and texture analysis indicate an overprint of the high temperature fabrics d2 by the low temperature deformation d3 phengitic mica from the paleocene eocene high pressure metamorphism remained stable during d2 ductile deformation as well as preserved within late stages of final sub greenschist facies shearing chlorite geothermometry yields two temperature groups 376 328 c and 306 132 c chlorite is seemingly accessible to late stage resetting the rmcc underwent an earlier large scale coaxial deformation accommodated by a late non coaxial shear with ductile low angle normal faulting resulting in subvertical thinning in the extensional deformation regime the rmcc was rapidly exhumed during ca 23 18 ma

porphyroblast rotation versus nonrotation conflict resolution comment fay et al 2008 1 present a few numerical simulations with unspecified code or boundary conditions as evidence for nonrotation of porphyroblasts during non coaxial flow however the large body of work on the behavior of porphyroblasts in deforming ductile rocks is significantly undervalued

on the internal geometry of mylonite zones mylonite zones are generally characterized by abrupt and very large strain transitions which commonly result in excessively anastomosing schistosities on a wide range of scales when compared with nonmylonitic foliations this geometry is very susceptible to remodification during progressive mylonitization resulting in unusual and complex fold lineation and foliation geometries and interrelationships open folds of the mylonitic foliation with axes parallel to the stretching lineation in the surrounding mylonite cannot have formed by the rotation of fold axes through a large angle within their axial planes as has been usually proposed for isoclinal and sheath folds in mylonitic zones open folds initiate with axes parallel or close to the stretching lineation due to the geometric effects of folding a mylonitic foliation which anastomoses around an ellipsoidal pod of less deformed material this initial geometry also allows the generation of fold axes curved within their axial plane through 1800 about the stretching lineation at the time of nucleation successive mylonitic foliations develop during this folding and refolding process with boundaries that truncate and isolate earlier fold hinges and portions of fold limbs as a consequence stretching and intersection lineations can vary from plane to plane through the mylonite zone although careful examination often reveals a weak overprinting stretching lineation parallel to the bulk movement direction for the whole zone fold asymmetry in mylonite zones is a potential indicator of shear sense across a zone if the fold axes lie at an angle to the bulk stretching lineation direction in such circumstances however a single asymmetry projected onto a plane perpendicular to the mylonitic foliation and containing the bulk stretching lineation can indicate either sense of shear depending on a variety of factors these include whether the foliation folded is primary or mylonitic and in the latter case whether the mylonite zone formed with a steep dip and horizontal stretching lineation or in some other orientation the most satisfactory sense of shear indicator is the asymmetry of s and c planes

the internal inclusion trail geometries preserved within a first phase of porphyroblast growth the inclusion trail geometry within a first phase of porphyroblast growth can differ significantly from that preserved by further enlargement because the porphyroblast forms a rigid mass up against which the rock preferentially strains during ensuing events the geometry of the first overgrown inclusion trails is affected by their primary orientation including any pre existing curvature combined with any heterogeneous rotation of this foliation about the developing stretching lineation this can impact the apparent timing of foliation intersection inflection axes preserved within porphyroblasts fias that nucleated during the development of a sub horizontal foliation but is readily resolved 3 d computer analysis of sigmoidal inclusion trails reveals that the asymmetry method for fia determinations is unaffected by the cut location relative to the porphyroblast core significantly perfect spiral inclusion trail geometries can be produced from a sigmoidal shape in cuts up 30 away from the fia therefore since fias in most porphyroblasts bear no relation to matrix structures there is a 17 chance that thin sections cut relative to the foliation lie within 30 of a fia and could contain such an apparent spiral fias maintain consistent trends for the first phase of porphyroblast growth accompanying horizontal bulk shortening but may vary in plunge fias have sub horizontal plunges for porphyroblasts nucleating during gravitational collapse but may vary in trend for all periods of porphyroblast regrowth the data available indicates that fias remain consistently trending and sub horizontal until the relative direction of plate motion causing orogenesis changes

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

thrusting and duplex formation at mount isa queensland australia large scale repetition of mount isa and haslingden group rocks in the anticline north of mount isa is interpreted as the core of a thrust duplex which has been subsequently folded parallel to the direction of thrusting displacement of mount isa group rocks above the duplex is extremely significant to the regional geology and exploration for more mount isa style pb zn and cu orebodies interpretation of the geometry of the thrust belt indicates thrusting on different stratigraphical levels below and above the duplex during its formation horses formed from a heelwards protruding footwall ridge instead of a trough and the complex effects on stratigraphical cross sections through duplexes of sloping lateral footwall ramps on horses

microbial shaping of sedimentary wrinkle structures wrinkle structures in ancient sedimentary environments are enigmatic wave tank experiments suggest that wrinkle structures are shaped by microbial mat fragments that are moved by waves over sandy bed surfaces and thus are morphological biosignatures

multiple deformations with successive subvertical and subhorizontal axial planes in the mount isa region their impact on geometric development and significance for mineralization and exploration the first orogeny to affect most rocks exposed in the mount isa province caused significant repetition of stratigraphy through north south shortening geological maps are dominated however by younger folds with subvertical north south striking axial planes these folds were produced by east west shortening during a second orogeny both orogenies produced several foliations however some foliations have negligible geometric consequences allowing the geometry of the region to be described generally in terms of just the three deformations d 1 d 2 and d 3 which produced macroscopic folds and the younger deformations which produced brittle faults one of the weak events that appears superficially to have had little or no geometric impact played a highly significant role in the generation of structurally controlled mineralization that role has been identified following advances in conceptual understanding of fold development and associated foliation the deformation which occurred between the d 2 and d 3 events produced geometric effects that were masked by overprinting of north south trending d 2 folds by similarly trending d 3 folds significantly these d 3 folds which control the localization or upgrading of much mineralization in the mount isa province did not form in rocks unaffected by this intermediate event because the near parallelism of s 2 and s 3 in such rocks resulted in intensification of s 2 rather than development of a new s 3 the intermediate deformation event called d sub 2 5 locally produced highly asymmetric deflections in bedding and s 2 with subhorizontal axial planes that were subsequently overprinted by d 3 folds and an s 3 cleavage no mesoscopic foliation was produced during d sub 2 5 and its rotational effects on bedding were commonly subsumed within younger d 3 structures thus d 3 disguises the presence of d sub 2 5 except in relatively rare locations where d 2 folds were isoclinal such as in the tommy creek block and on the northern end of the snake creek anticline southeast of cloncurry and northwest of mount isa in these three locations in the latter locations both limbs were affected resulting in large scale overturning of bedding on one limb prior to overprinting by d 3 the asymmetric folds developed during d sub 2 5 created sites where bedding could be pulled apart during d 3 resulting in implosion because of the high fluid pressure in the surrounding country rock with brecciation jostling fluid access and mineralization large scale zones of mineralization form where d sub 2 5 was localized at an earlier formed heterogeneity such as a thrust surface e g at mount isa mine or a granite country rock contact in these locations early formed angular bedding relationships were locally rotated into orientations which created sites that could implode during d 3 this role of d sub 2 5 in the generation of d 3 allows recognition of regional zones for exploration that previously have received little attention recognition of the role of d sub 2 5 reveals a cyclicity in successive subvertical and subhorizontal events not recognized by most workers in this terrain that may occur in other orogens around the world but has similarly gone unnoticed after the first orogeny finished a succession of three near vertical and two subhorizontal events occurred during the east west shortening orogeny at mount isa these were succeeded by younger kinks with east west to northeast southwest trending axial planes that appear to be the youngest ductile deformation to have affected the mount isa province