change in fabric of melange in the shimanto belt japan change in relative convergence systematic changes in fabrics of melanges have been detected in rocks of the shimanto belt japan several shear indicators including s c structures riedel shears and folds in the melanges indicate consistent sense of shear both in outcrop and microscopic scales the deformation mechanisms independent particulate flow and pressure solution suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening after lithification cataclastic deformation occurred locally but most of the earlier deformation features are preserved this process appears to have taken place along the decollement beneath the off scraped accretionary prism sinistral reverse sense of shear is observed in the campanian melange whereas a dextral reverse sense of shear is preserved in the early eocene melange in the shimanto belt this change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates three different plate models have been proposed for the late cretaceous to early tertiary western pacific margin 1 subduction of the kula pacific ridge along the continental margin 2 subduction of the pacific plate since 85 ma and 3 subduction of the aborted kula pacific ridge after 43 ma the observed change in fabric of melange is consistent only with the third model which predicts a change in relative convergence from sinistral to dextral at the appropriate time

large scale pseudotachylytes and fluidized cataclasites from an ancient subduction thrust fault in the kodiak accretionary complex kodiak island alaska pseudotachylyte occurs in black locally vitreous ultrafine grained fault rock microscopic observations show that the pseudotachylytes are composed of glass with vesicles amygdules microlites and flow structures indicating a frictional melt the pseudotachylyte is gradational to cataclasite and shows outcrop scale injection and ductile deformation structures the cataclasite was ductily mobile i e fluidized simultaneous with the formation and emplacement of pseudotachylyte melt the pseudotachylytic rocks postdate the stratal disruption fabric of associated shear zone melanges and show similar direction of thrust transport and have undergone limited subsequent deformation we interpret the stratal disruption as resulting from underthrusting of the subducting plate and pseudotachylyte development as the final activity of this thrust surface the gradational contacts between pseudotachylyte and cataclasite demonstrate that the cataclasite also formed as a seismic product and may represent paleoseismic rupture zones possibly of very great earthquakes with or without accompanying pseudotachylytes the pseudotachylytes are voluminous and many are spatially disconnected from generation surfaces this style is distinct from pseudotachylytes described in other environments and this may explain the rarity of documented examples of subduction thrust pseudotachylyte

structure and organization of the porma melange progressive denudation of a submarine nappe toe by gravitational collapse the porma melange is a unit up to 1 km thick underlying the forcada nappe in the variscan foreland of the iberian peninsula most of the porma melange consists of a highly deformed boudinaged sequence with extension values over 300 percent the boudins or melange blocks are the result of bed parallel extension in two perpendicular directions giving rise to equidimensional boudins in plan view choco late tablet structure in cross section view the boudins show different shapes rectan gular rhomboid triangular trapezoid or with arrowhead edges due to various combinations of extension and shear fractures both the provenance of the melange blocks mostly derived from the upper part of the bodon nappe that overlies the forcada nappe and flattening type strain ellipsoid shapes support the interpretation that hanging wall gravitational collapse of these nappes was the cause of the exten sional deformation the current distribution of the melange blocks from different nappe formations implies progressive unroofing of the nappe stack through subse quent submarine slumps the internal structure of the melange blocks depends on the degree of sediment lithification during slumping and illustrates the significance of the so called native and exotic blocks the source of most of the melange material shaly matrix and native blocks or boudins was the semilithified syntectonic sediments of the upper part of the bodon nappe which underwent brittle ductile deformation boudinage during slumping however a few and usually larger exotic blocks also came into the basin as individual slip blocks from competent well lithified formations originally located at the lower part of the nappe stack with higher shear resistance and less suitable for spreading thus the arrangement of major nappe fragments traditionally named klippes and showing sizes that may reach up to several kilometers does not agree with the geometric rules of thrusting and are reinterpreted as megablocks resulting from gravitational sliding during denudation of advancing nappes

a suggestion for the origin of mesoscopic fabric in accretionary melange based on features observed in the chrystalls beach complex south island new zealand the chrystalls beach complex a probable accretionary melange of triassic age cropping out in south island new zealand contains a distinctive set of mesoscopic structures all of which contribute to the typically chaotic appearance of the melange these structures are 1 soft sediment bedding pull aparts 2 weakly developed slaty cleavage 3 folds in dismembered bedding with slaty cleavage as the axial surface foliation and 4 quartz fiber lineated shear surfaces analysis of overprinting relations indicates that these structures formed during a single progressive deformation rather than in several discrete events as such they record a change in response to incremental strain from initial ductile behavior toward one dominated by brittle failure it is suggested that this variation resulted from progressive dewatering during shallow level accretion at the base of an inner trench slope

the coalification pattern in the northern apennines and its palaeogeothermic and tectonic significance the rocks of the northern apennines predominantly consist of non metamorphic terrigeneous deposits flysches and molasses some of which are preorogenic some synorogenic and others postorogenic with respect to the nappe tectonics miocene as plant fragments frequently occur in these sediments a study of coal rank based on reflectance measurements on vitrinites rm mean value of the random reflectance in non polarized light contributes to the clarification of the relation between the orogenic and the palaeogeothermal development the determination of the rm values of more than 180 samples from outcrops and three deep drillings revealed some important features within the pile of liguride and tuscanide nappes the coal rank increases from the uppermost nappe to the lower nappes until lowgrade metamorphism is reached in the lower tuscanides in the single nappes the rank decreases from the tyrrhenian coast internal zone towards the po plain external zone this regional trend is disturbed only locally by young post coalification tectonics in the uppermost liguride nappe m antola unit a pre oligocene i e pre apenninic thermal event was detected postorogenic heating is connected with the magmatic activity of late miocene to pleistocene age in tuscany except for these preorogenic and postorogenic thermal events the main coalification is generally younger than the emplacement of the nappes in the nappe pile during the apenninic orogeny in the miocene but it is older than the last thrust movements and the final tensional tectonics in the internal zones of the chain for these reasons the main regional thermal event has to be considered as synorogenic or more precisely as late synorogenic

mechanisms of melange formation examples from sw japan and southern scotland abstract the formation of synthetic riedel shears either as discrete faults or ductile shear zones is important in the fragmentation of layers to form melanges the magnitude of longitudinal and shear strain necessary for this fragmentation has been assessed using a simple rigid domino model for layer parallel shear the strain required for layer fragmentation is a function of block aspect ratio and initial fracture dip relative to layering layer stretching can occur during simple shear if folds are developed but a component of extension parallel to the shear zones reduces the strain required for fragmentation simple models of this type break down when there is a component of dilation across the melange block bounding fractures allowing shale matrix infill or mineral vein growth some melanges show evidence in the form of symmetrical structures of a more coaxial deformation history and this may in part reflect deformation partitioning between sandstone and shale layers other mechanisms of melange formation include the development of out of sequence thrusts cutting through already dipping beds the causes of strike parallel extension are tentatively related to large scale evidence of arc parallel forearc stretching

evidence for a weak and overpressured decollement beneath sediment dominated accretionary prisms we present structural data from several underthrust sequences in southwest alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical 1 and regionally extensive layer parallel shearing these results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where 1 plunges gently seaward or toward the deformation front on the basis of mohr circle diagrams we propose that these different stress directions are consistent only with a model in which the two regimes are separated by a very weak decollement and calculate coefficient of friction values that range from 0 25 to 0 08 these values are significantly lower than coefficient of friction values calculated for the basal decollement in the taiwan fold and thrust belt e g 0 2 barr and dahlen 1989a and they are also lower than laboratory friction measurements for most rocks e g 0 6 0 8 byerlee 1978 consequently these low values suggest the presence of rocks with unusual material properties e g montmorillonite logan and rauenzahn 1987 or anomalous fluid pressures along the decollement on the basis of the occurrence of shale diapirs the distribution of early veins and the absence of montmorillonite in the underthrust sequences we propose that there was an anomalously high fluid pressure along the decollement fault instead of there being such a low coefficient of friction these results emphasize the importance of dewatering and fluid flow in controlling the position of the decollement which in turn determines how the accretionary prism grows in essence it is the development of a weak overpressured decollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes

debris flow olistostromes and slumping on a distal passive continental margin the palombini limestone shale sequence of the northern apennines an olistostrome accumulation up to 530 m thick occurs in the casanova area of the northern apennines it lies within or above the calciturbiditic palombini limestone shale sequence and is part of the allochthonous vara complex sediments originally deposited on oceanic crust the olistostromes are poorly sorted monomict matrix supported submarine debris flow deposits with rigid plugs they have a compactional foliation and a compaction modified planar clast fabric created during flow although diachronous in the main part of the area the olistostromes have a vertical gradational contact with the overlying slumped palombini in which recumbent asymmetric fold hinges and trains and slump boudins are present many criteria indicate soft sediment deformation fold asymmetry indicates a uniformly sw dipping palaeoslope the textural gradation from slumps to olistostrome beds plus slump folds and boudins as olistostrome clasts show that the olistostromes are dismembered slumps in vertical sections variations in limestone petrography volume percentage and size of clasts confirm that the olistostromes were derived from the palombini as a series of bed by bed slumps keeping pace with sedimentation of the palombini from olistostrome clast sizes and bed thicknesses a depositional slope of 4 is estimated the olistostromes are not precursor sediments shed from advancing nappes as in the bracco ridge model of some authors rather they were formed at the foot of a distal block faulted passive continental margin long before nappe emplacement

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

simulations of the 2004 lava flow at etna volcano using the magflow cellular automata model since the mechanical properties of lava change over time lava flows represent a challenge for physically based modeling this change is ruled by a temperature field which needs to be modeled magflow cellular automata ca model was developed for physically based simulations of lava flows in near real time we introduced an algorithm based on the monte carlo approach to solve the anisotropic problem as transition rule of ca a steady state solution of navier stokes equations was adopted in the case of isothermal laminar pressure driven bingham fluid for the cooling mechanism we consider only the radiative heat loss from the surface of the flow and the change of the temperature due to mixture of lavas between cells with different temperatures the model was applied to reproduce a real lava flow that occurred during the 2004 2005 etna eruption the simulations were computed using three different empirical relationships between viscosity and temperature

kinematic analysis of melange fabrics examples and applications from the mchugh complex kenai peninsula alaska permian to cretaceous melange of the mchugh complex on the kenai peninsula south central alaska includes blocks and belts of graywacke argillite limestone chert basalt gabbro and ultramafic rocks intruded by a variety of igneous rocks an oceanic plate stratigraphy is repeated hundreds of times across the map area but most structures at the outcrop scale extend lithological layering strong rheological units occur as blocks within a matrix that flowed around the competent blocks during deformation forming broken formation and melange deformation was noncoaxial and disruption of primary layering was a consequence of general strain driven by plate convergence in a relatively narrow zone between the overriding accretionary wedge and the downgoing generally thinly sedimented oceanic plate soft sediment deformation processes do not appear to have played a major role in the formation of the melange a model for deformation at the toe of the wedge is proposed in which layers oriented at low angles to s1 are contracted in both the brittle and ductile regimes layers at 30 458 to s1 are extended in the brittle regime and contracted in the ductile regime and layers at angles greater than 458 to s1 are extended in both the brittle and ductile regimes imbrication in thrust duplexes occurs at deeper levels within the wedge many structures within melange of the mchugh complex are asymmetric and record kinematic information consistent with the inferred structural setting in an accretionary wedge a displacement field for the mchugh complex on the lower kenai peninsula includes three belts an inboard belt of late triassic rocks records west to east directed slip of hanging walls a central belt of predominantly early jurassic rocks records north south directed displacements and early cretaceous rocks in an outboard belt preserve southwest northeast directed slip vectors although precise ages of accretion are unknown slip directions are compatible with inferred plate motions during the general time frame of accretion of the mchugh complex the slip vectors are interpreted to preserve the convergence directions between the overriding and underriding plates which became more oblique with time they are not considered indicative of strain partitioning into belts of orogen parallel and orogen perpendicular displacements because the kinematic data are derived from the earliest preserved structures whereas fabrics related to strain partitioning would be expected to be superimposed on earlier accretion related fabrics 1999 elsevier science ltd all rights reserved

the role of trapped fluids during the development and deformation of a carbonate shale intra wedge tectonic melange mt massico southern apennines italy abstract numerous studies exist on exhumed tectonic melanges along subduction channels whereas in accretionary wedge interiors deformation mechanisms and related fluid circulation in tectonic melanges are still underexplored we combine structural and microstructural observations with geochemical stable and clumped isotopes and isotope composition of noble gases in fluid inclusions of calcite veins and u pb geochronological data to define deformation mechanisms and syn tectonic fluid circulation within the mt massico intra wedge tectonic melange located in the inner part of the central southern apennines accretionary wedge italy this melange developed by shear deformation at the base of a clastic succession deformation was characterized by disruption of the primary bedding mixing and deformation of relicts of competent olistoliths and strata within a weak matrix of deformed clayey and marly interbeds recurrent cycles of mutually overprinting fracturing veining and pressure solution processes generated a block in matrix texture the geochemical signatures of syntectonic calcite veins suggest calcite precipitation in a closed system from warm 108 147 c paleofluids with 18o vlaues between 9 and 14 such as trapped pore waters after extensive 18o exchange with the local limestone host rock and or derived by clay dehydration processes at t 120 c the 3he 4he ratios in fluid inclusions are lower than 0 1 ra indicating that he was exclusively sourced from the crust we conclude that 1 intraformational rheological contrasts inherited trapped fluids and low permeability barriers such as marly shaly matrix can promote the generation of intra wedge tectonic melanges and the development of transient fluid overpressure 2 clay rich tectonic melanges developed along intra wedge decollement layers may generate low permeability barriers hindering the fluid redistribution within accretionary wedges

the tyrrhenian apennines system structural setting and seismotectogenesis abstract the available geological geophysical and seismological data of the tyrrhenian apennines system are briefly reviewed and synthesized in a number of sketch maps integrating and comparing these data a large scale simple shear kinematic model is proposed for the tyrrhenian apennines system that is here interpreted as an oblique asymmetric passive continental margin developed during the progressive counterclockwise rotation of the adriatic foreland from the late miocene to the present in the frame of this working hypothesis the thinning and stretching of the tyrrhenian crust were accommodated by progressive eastward motion of extensional allochthonous slices bounded by low angle west dipping oblique shear zones rotating above a common detachment that penetrates into the mantle lithosphere at any given time the most external eastward moving slice is bounded by a transtensional shear zone at the top and by a transpressional one at the bottom the displacement within the hanging wall of the transtensional shear zone is accommodated by high angle normal faults and en echelon grabens the displacement within the hanging wall of the transpressional shear zone is instead accommodated by co axial en echelon sets of folds thrusts and high angle strike slip faults thus the development of the late miocene pliocene pleistocene contractional strain field of the apennines of italy might not be a consequence of complex subduction processes but subordinate to the deformation processes that lead to the progressive opening of the tyrrhenian sea the seismicity of the crustal apennines of italy is analyzed in the frame of the above history of progressive deformation and a seismotectogenetic model is outlined

an evaporite bearing accretionary complex in the northern front of the betic rif orogen the guadalquivir accretionary complex forms a largely oblique prism at the northern edge of the betic rif orogen where miocene sediments plus allochthonous evaporite bearing units were accreted during the displacement of the alboran domain towards the west traditional interpretations end the tectonic structuring of the betic cordillera at the present topographic front beyond which gravitational and or diapiric processes would predominate however this study shows pervasive tectonic deformation in the outer prism with coherent oblique shortening kinematics which is achieved through an alternation of roughly n s arcuate thrust systems connected by e w transfer fault zones these structures accord well with the geophysical models that propose westward rollback subduction the main stage of tectonic activity occurred in the early middle miocene but deformation lasted until the quaternary with the same kinematics evaporite rocks played a leading role in the deformation as evidenced by the suite of ductile structures in gypsum distributed throughout the area s and l gypsum tectonites scaly clay fabrics and brittle fabrics coexist and consistently indicate westward motion top to 290 with subordinate n s contraction almost perpendicular to the transfer zones this work reveals ductile tectonic fabrics in gypsum as a valuable tool to elucidate the structure and deformational history of complex tectonic melanges involving evaporites above the decollement level of accretionary wedges

thermochronologic evidence for orogen parallel variability in wedge kinematics during extending convergent orogenesis of the northern apennines italy analysis of 146 new apatite u th he ages six new apatite fi ssion track ages and 165 previously published apatite fi ssiontrack aft ages from the northern apennine extending convergent orogen reveals a signifi cant along strike change in post late miocene wedge kinematics and exhumation history east of 11 30 e age patterns and age elevation relationships are diagnostic of ongoing frontal accretion and slab retreat consistent with a northeastward migrating orogenic wave enhanced erosion rates of 1 mm yr over a period of 3 5 ma are recorded on the contractional pro side of the orogen and 0 3 mm yr on the extending retro side west of 11 30 e ongoing exhumation has been restricted to the range core since at least ca 8 ma at rates of 0 4 mm yr increasing to 1 mm yr in the pliocene ca 3 ma accompanied by post pliocene tilting and associated faulting this pattern can be attributed to either continued convergence but a switch in the transfer of material into the wedge to a regime dominated by underplating or out of sequence shortening or a slowdown or cessation of frontal accretion and slab retreat with enhanced pliocene uplift and erosion triggered by a deeper seated process such as lithospheric delamination complete slab detachment or slab tear these fi ndings emphasize that no single model of wedge kinematics is likely appropriate to explain long term northern apennine orogenesis and synconvergent extension but rather that different lithospheric geodynamic processes have acted at different times in different lateral segments of the orogen

experimental ejection angles for oblique impacts implications for the subsurface flow field a simple analytical solution for subsurface particle motions during impact cratering is useful for tracking the evolution of the transient crater shape at late times a specific example of such an analytical solution is maxwells z model which is based on a point source assumption here the parameters for this model are constrained using measured ejection angles from both vertical and oblique experimental impacts at the nasa ames vertical gun range data from experiments reveal that impacts at angles as high as 45 to the targets surface generate subsurface flow fields that are significantly different from those created by vertical impacts the initial momentum of the projectile induces a subsurface momentum driven flow field that evolves in three dimensions of space and in time to an excavation flow field during both vertical and oblique impacts a single stationary pointsource model specifically maxwells z model however is found inadequate to explain this detailed evolution of the subsurface flow field during oblique impacts because 45 is the most likely impact angle on planetary surfaces a new analytical model based on a migrating point source could prove quite useful such a model must address the effects of the subsurface flow field evolution on crater excavation ejecta deposition and transient crater morphometry

the tectonic significance of italian magmatism an alternative view to the popular interpretation the tyrrhenian rift zone has been the site of widespread magmatism since late tortonian times a pronounced asymmetrical distribution reflecting the tectonic structure characterizes italian magmatism sodic basalts occur on the western tyrrhenian flank and transitional morb basalts occur in the tyrhenian sea the eastern flank however is characterized by k alkaline and hk to ultra alkaline e g carbonatites and melilitites rocks major trace elements and isotopic compositions allow two major magmatic lineages to be identified one relating to a non radiogenic basaltic end member and the other to a mantle end member enriched in ca with high lile hfse ratio and high sr isotopic ratios their mantle sources are located within the lithosphere thermal boundary layer tbl and the metasomatized phlogopite carbonate asthenosphere at the base of the tbl respectively the composition and spatial distribution of volcanism and relative mantle sources tend to map the geometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary between the thinned tyrrhenian lithosphere and the adriatic lithosphere a mechanism of intra continental passive rifting which drives mantle upwelling is sufficient to satisfy the petrological and geochemical constraints and the observed tectonic environment without requiring a subduction plane

structural style of the offscraped ligurian oceanic sequences of the northern apennines new hypothesis concerning the development of melange block in matrix fabric shaly dismembered formations crop out extensively on the eastern side of the northern apennines as overthrusted cretaceous oceanic units the lack of metamorphism and the unconformably overlain epi ligurian sequence which represents the trench slope basin sedimentary sequence suggest that these units are accreted sequences accomplished by offscraping and imbrication these units are characterized by stratal disruption lack of internal coherence and complete destruction of the original stratigraphic sequence block in matrix fabric results in symmetric boudins and systematic presence of isolated or intrafolial hinges of isoclinal folds two orthogonal sets of tight to isoclinal folds are always recognizable involving the break up and the boudinage of the competent layers now transposed parallel to the axial planes of the disrupted isoclinal folds the superposition of two generations of orthogonal tight to isoclinal folds may be locally observed either in single bed fragments or in some preserved coherent units disruption and block in matrix fabric is a consequence of polyphase folding and intense shortening this geometry and kinematic interpretation well constrain the tectonic environment where these rocks were deformed to the proximity of the prism toe and within the prism itself thus the apennine broken formations result from the disruption of a sedimentary pile lying on the subducting plate of the cretaceous to eocene trench fore arc system which generated the ligurian accretionary wedge

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

hidden terranes in the northern apennines italy a record of late cretaceous oligocene transpressional tectonics we propose that a very oblique or transpressional tectonic regime was dominant during the early pre collisional orogenic evolution of the northern apennines late cretaceous to early oligocene this hypothesis resolves many inconsistencies in the previous reconstructions of this orogenic belt which were based on a classic model of orthogonal convergence between the european and adriatic plates the crucial lines of evidence that point to a major role of oblique tectonics in the structuring of the northern apennines are 1 the plate tectonics framework that indicates left lateral oblique convergence along the europe adria plate margin 2 the lack of a magmatic arc during the entire pre collisional convergent history of the chain a time span 45 m y from late cretaceous to early oligocene 3 the long 20 m y residence time of turbidite sequences in the trench the dormant trench 4 the multiple source areas of turbidites from both sides of the basin and the associated coarse gravity d

the scaling of complex craters the empirical relation between the transient crater diameter dg and final crater diameter dr of complex craters and basins is estimated using cumulative terrace widths central uplift diameters continuous ejecta radii and transient crater reconstructions determined from lunar and terrestrial impact structures the ratio dg dr is a power law function of dr decreasing uniformly from unity at the diameter of the simple complex crater morphology transition to about 0 5 for large multiring basins like imbrium on the moon the empirical constants in the dg dr relation are interpreted physically to mean that the position of the final rim relative to the transient crater and hence the extent of collapse is controlled or greatly influenced by the properties of the zone of dissociated material produced by the impact shock the continuity of the dg dr relation over the entire spectrum of morphologic types from complex craters to multiring basins implies that the rims of all these structures form in the same tectonic environment despite morphologic differences

geological record of fluid flow and seismogenesis along an erosive subducting plate boundary a fossil erosive subduction channel the shear zone marking the plate boundary preserved in the northern apennines of italy is identified the fossil zone records the presence of two decollements which seem to have been simultaneously active at the top and base of the subduction channel in modern subduction zones the onset of seismic activity is believed to occur at 150 c however in the fossil channel the onset seems to have occurred at colder temperatures tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the earth s subduction zones1 2 in situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone until now a fossil erosive subduction channel the shear zone marking the plate boundary3 has not been recognized in the field so that seismic observations have provided the only information on plate boundary processes at erosive margins here we show that a fossil erosive margin is preserved in the northern apennines of italy it formed during the tertiary transition from oceanic subduction to continental collision and was preserved by the late deactivation and fossilization of the plate boundary the outcropping erosive subduction channel is 500 m thick it is representative of the first 5 km of depth with its deeper portions reaching 150 c the fossil zone records several surprises two decollements were simultaneously active at the top and base of the subduction channel both deeper basal erosion and near surface frontal erosion occurred at shallow depths extension was a key deformation component within this erosive convergent plate boundary and slip occurred without an observable fluid pressure cycle at depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast co seismic and slow inter seismic slip in the deepest portions of the outcropping subduction channel extension is finally overprinted by compressional structures in modern subduction zones the onset of seismic activity is believed to occur at 150 c but in the fossil channel the onset occurred at cooler palaeo temperatures

kinematic evolution of thrust related structures in the umbro romagnan parautochthon northern apennines italy in the internal part of the umbro marchean romagnan apennines the foredeep clastic wedge constituting the neogene part of the sedimentary cover is completely detached from the underlying mesozoic palaeogene succession the resulting umbro romagnan parautochthon consists of tectonostratigraphic units with a general geometry of broad synclinal blocks separated by narrow faulted anticlines thrust related structures observed in the field require thrust ramp propagation to have occurred within already folded rocks therefore they cannot be restored using simple fault bend fold or fault propagation folding models evidence for a passive fold origin in the studied rocks suggests that an early detachment folding episode preceded ramp propagation the latter was facilitated by the enhanced thickness of incompetent material in the cores of detachment anticlines which became the preferential sites where thrust ramps cut up section depending on the trajectory of such thrust ramps different types of fault related structures could develop hanging wall anticlines which give way to monoclinal structures higher up in the section are associated with listric thrust ramps whereas hanging wall monoclines approximately parallel to the underlying fault surface are associated with straight trajectory ramps this kinematic evolution which occurred partly during syn depositional compression also accounts for the observed lithofacies distribution the latter reflects an early differentiation of the foredeep trough into sub basins that are progressively younger towards the foreland the detachment anticlines that originally bounded such sub basins were the site of later thrust propagation leading to a tectonic juxtaposition of different tectonostratigraphic units consisting of broad nw se elongate synclinal blocks

criteria helpful in recognizing shear zone and diapiric melanges examples from the hoh accretionary complex olympic peninsula washington a study of the middle miocene hoh melanges exposed on the western olympic peninsula washington provides criteria for differentiating between shear zone and diapiric melanges the shear zone melange is characterized by a consistent foliation orientation throughout the outcrop intense pervasive scaly foliation uniform fold vergence directions exotic clasts predominantly phacoidal clasts and consistent clast long axis lineations the diapiric melange is characterized by radial foliation in map view well developed scaly foliation at the contacts and poorly developed scaly foliation in the center of the melange opposite fold vergence directions across the melange and rare exotic clasts additionally clasts vary in shape from phacoidal at the contacts to angular in the center in the diapiric melange large clasts are found only in the center of the melange and clast long axes vary from tightly clustered at the contacts to near random orientations in the center the systematic variation in clast size in the diapir may be due to increased shear at the margins and or clast migration to the center of the diapir caused by a velocity viscosity contrast interpretation of field data indicates that some clasts rotate continuously in the matrix yet clast long axes have a definite preferred orientation this contradiction may be resolved by theoretical models indicating that clasts rotate fastest at high angles to the shear plane thus although clasts above a critical aspect ratio asymptotically approach stability parallel to the shear plane the long axes of rotating clasts spend a proportionally greater amount of time close to the shear plane parasitic folds in the matrix indicate that clast long axes align parallel to transport direction small scale folds in the matrix clast long axis lineations and the mean foliation indicate that the hogsback section of the hoh melange was emplaced by a high angle oblique thrust directed to n10 e

the architecture of an incipient oceanic basin a tentative reconstruction of the jurassic liguria piemonte basin along the northern apennines alpine corsica transect in this paper a scenario for the early evolution of the jurassic oceanic liguria piemonte basin is sketched for this purpose four selected examples of ophiolite sequences from the northern apennines and corsica are described and analyzed in the external ligurian units northern apennines the ocean continent transition of the adria plate was characterized by a basement made up of subcontinental mantle and lower continental crust covered by extensional allochthons of upper crust both the basement rocks and the extensional allochthons are cut by basaltic dikes and covered by basalts and pelagic deposits the conjugate ocean continent transition of the corsica margin represented by the balagne nappe corsica was composed of mantle peridotites and gabbros covered by basaltic flows and minor breccias that in addition include continent derived clasts by contrast the innermost i e closest to the ocean preserved area observed in the internal ligurian northern apennines and inzecca corsica units consists of former morphological highs of mantle peridotites and gabbros bordered by small basins where the basement is covered by a volcano sedimentary complex characterized by ophiolitic breccias and cherts interlayered with basaltic flows the overall picture resulting from our reconstructions suggests an asymmetric architecture for the liguria piemonte basin with a central area bounded by two different transition zones toward the continental margins this architecture can be interpreted as the result of a rifting process whose development includes a final stage characterized by passive asymmetric extension of the lithosphere along an east dipping detachment fault system

polygenetic melanges a glimpse on tectonic sedimentary and diapiric recycling in convergent margins a significant part of melanges recognized in exhumed convergent margins around the world has been recently documented to have chiefly originated from masse transport and subsurface remobilization and disruption i e melanges from sedimentary and mud serpentinite diapiric processes and from in situ fluidification disruption tectonic and or sedimentary processes occurring during subsequent multiple deformational events of convergent margin evolution commonly overprint and significantly rework the primary sedimentary or diapiric melange fabric forming polygenetic melanges this ultimately complicates their distinction from true tectonic melanges masking part of the recorded tectono sedimentary evolution of the associated convergent margin the contributions gathered in this thematic collection explore with different approaches from field structural and stratigraphic observations to geophysical analyses different types of polygenetic melange at various scales around the world these studies conclude that the understanding of this type of melange may provide crucial insights for a more detailed interpretation of the evolution of ancient and modern convergent margins and of processes and mechanisms triggering potential natural hazards earthquakes and tsunamis case studies include the apennines in the central mediterranean region the carpathians in central europe and the nankai prism in japan thematic collection this article is part of the polygenetic melanges a glimpse on tectonic sedimentary and diapiric recycling in convergent margins collection available at https www lyellcollection org cc polygenetic melanges

fluid history related to the early eocene middle miocene convergent system of the northern apennines italy constraints from structural and isotopic studies 1 the late eocene middle miocene erosive plate boundary between the european and the adriatic plates is exhumed in the northern apennines of italy the fossil fault zone is 500 m thick and the outcropping portion exposes the first 5 km of its depth at this plate boundary basal and frontal tectonic erosion incorporated unlithified fluid rich sediments into the fault zone the deformation and nature of the material along the plate boundary define a fossil subduction channel here we couple a detailed structural analysis of the apennine subduction channel focusing in particular on calcite veins with a stable isotope analysis to characterize the fluid regime along an active subduction channel the 13c and 18o composition of calcite vein and host rock samples within the fault zone indicates that there is a deep metamorphic source of fluids migrating upward along the subduction channel in addition to locally derived fluid components dewatering of subducting turbidites contributes significantly only in the shallowest part of the channel structural observations indicate fluid flow along and across the subduction channel at deep levels fluid flow is associated with discrete deformation events on shear faults offset by dilational jogs filled with implosion breccias at intermediate levels deformation is still cyclic and associated with repeated crack and seal events at the shallowest levels deformation occurred while portions of the subducting material were still unlithified here the deformation was quasicontinuous without associated vein development both isotope and structural analyses indicate that this erosive subduction channel behaved as a weak fault with a vertical maximum principal stress

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

a new active tectonic model for the construction of the northern apennines mountain front near bologna italy 1 we integrate existing and new geologic data retreating trench extension and accretion tectonics retreat project particularly on the origin growth and activity of the mountain front at bologna italy into a new model that explains apennine orogenesis in the context of a slab rollback upper plate retreat process the bologna mountain front is an actively growing structure driving rock uplift 1 mm year cored by a midcrustal flat ramp structure that accommodates ongoing shortening driven by adria subduction at a rate of 2 5 mm year the data we use are assembled from river terraces and associated pleistocene growth strata geodesy including releveling surveys reinterpretation of published reflection lines and a new high resolution reflection line these data constrain a simple trishear model that inverts for blind thrust ramp depth dip and slip apennine extension is recognized both in the foreland as high angle normal faults and modest stretching in the carapace of the growing mountain front and in the hinterland with larger normal faults that accomplish some crustal thinning as the upper plate retreats this coevolution of extension and shortening shares some notable characteristics with other basement involved collisional orogens including the early tertiary laramide orogeny in the american west and the oligocene to miocene evolution of the alps we propose a possible relationship between underplating and the development of the po as a sag basin as a quaternary phenomenon that may also apply to past periods of apennine deformation tortonian continued shortening on the structure beneath the bologna mountain front represents by far the most important and underappreciated seismogenic source in the front of the northern apennines

shale diapirism in the quaternary tectonic evolution of the northern apennine bologna italy 1 we use morphologic stratigraphic and structural data to identify a set of diapirs just southward uphill of the northern margin of the northern apennine range the diapirs consist mainly of ligurid chaotic shaley units that reach a maximum thickness of over 3 km we also reconstruct marine sedimentation abrasion surfaces that record a 1 cm a differential uplift induced by diapirism on the basis of this data we use the lubrication approximation of the navier stokes equations and analogue experiments to show that thrusting of the epiligurid units can be generated by diapirism of the underlying ligurid units in addition to regional compression owing to a gravity component parallel to the strata the structures formed by diapirism are similar to those generated by compressive tectonics differing in that thrusting is lubricated by diapiric rise using the analogue experiments we show that deformation begins with folding eventually evolving to thrusting and we derive an empirical relation to infer the time remaining to the end of deformation in our area diapiric deformation is still in the folding regime but it should eventually evolve to the thrusting regime and terminate in about 105 a we suggest that the whole northeastern slope of the northern apennine is controlled by diapiric tectonics our interpretation contrasts with the more traditional one which views regional tectonic compression as the only responsible for the structures observed in the field since our model is based on first principles it could be applied to other areas with similar geology and topography

multistage melange formation in the franciscan complex northernmost california matrix induration and deformation style denote relative time and process of melange origin related to subduction mappable melange units contain chiefly blocks of greenstone chert and graywacke dispersed in weakly indurated argillite emplacement of these units as olistostromes is indicated by depositional contacts and lateral inter tonguing with turbidites some blocks in sheared olistostromes are in themselves complexly mixed these include breccias of gabbro plagiogranite diabase basalt tuff chert graywacke and argillite variously combined and brittlely deformed blocks of intercalated tuff and argillite and of diamictite containing rounded clasts of tuff chert and graywacke dispersed in highly indurated foliated argillite exhibit primary ductile and secondary brittle deformation all block material was indurated brecciated and or foliated during underplating at the base of a trench slope and later uplifted to provide olistostrome detritus the elk head and klamath coast terranes are highly indurated and compositionally similar to the mixed lithology blocks in olistostromes they are interpreted to form an accretionary prism consisting of tectonic melange slabs separating slope basin rocks

modeling impact ejecta plume evolution a comparison to laboratory studies this study demonstrates the efficacy of a recently developed impact crater excavation flow properties model efpm that accomplishes the following goals 1 uses hydrodynamic streamline theory to develop the expressions that extend theclassic impact ejecta scaling relationships into regions near the crater rim wheretarget strength and or gravity bring crater excavation flow to a halt 2 links this new general ejecta position velocity scaling relationship to the existing general cratersize volume scaling relationship including the transition region between gravity andstrength dominated cratering 3 provides a means for estimating impact ejecta plumemass density and ejecta blanket thickness as a function of azimuth and distance from theimpact site in conditions ranging from low to high target strength and 4 brings inour experimentally derived knowledge of impact ejection angles and the effects of obliqueimpact to develop a useful 2 d and 3 d model of both leading edge and trailing edgeejecta plume behavior in this work this excavation flow properties model is used tosimulate the images and data produced by three laboratory impact studies which utilizedmodern laser based non intrusive means to investigate ejecta plume formation expansion and fallout from three different perspectives

block in matrix fabrics that lack shearing but possess composite cleavage planes a sedimentary melange origin for the yuwan accretionary complex in the ryukyu island arc japan while melanges have been taken to be a significant component of accretionary complexes the use of this term has been mostly descriptive given the lack of consensus concerning their genesis however many workers have interpreted asymmetric shear fabrics in melanges as evidence of a tectonic origin i e giving rise to tectonic melanges in contrast to olistostromes which are considered to be purely sedimentary in origin due at least in part to the lack of such shear fabrics it is still uncertain whether accretionary processes include those that involve the intense shearing of oceanic lithologies along with for example the incorporation and entrapment of exotic blocks by muddy matrices during sediment deposition and deformation in axial deep sea trench settings here we argue that the so called block in matrix fabrics that characterize accretionary complexes are primarily sedimentary in origin and are distinct from shear fabrics represented by tectonic melanges we also propose that following their accretion subsequent recycling of such oceanic materials is a common feature within evolving accretionary prisms lastly we draw attention to the observation that d1 pressure solution cleavage planes notably widespread in the lower cretaceous yuwan complex amami oshima island in the ryukyu arc sw japan are absent from the minor asymmetric shear structures that often characterize tectonic melanges in contrast we suggest that the yuwan complex represents the effects of broadly symmetric flattening this scenario is supported by our observation that the number of sedimentary and deformation events differs significantly compared to the single phase of shearing expected in tectonic melanges d minus 1 d 1 pressure solution and axial planar cleavages are only observed in exotic blocks and exhibit older accretionary deformation events within a preexisting accretionary prism of the existing structural associations sd1 is the oldest predating the d 1 sedimentary episode and it represents an oceanic sedimentary phase associated with the formation of a distinct type of block in matrix fabric produced by the accumulation of debris flows at seamount margins sd2 represents a succeeding sedimentary episode characterized by trench axis debris flow including oceanic inclusions derived from collapsed material from an older accretionary prism the resulting sd2 block in matrix fabric is one of the main lithologic components of the amami oshima yuwan complex here the sd2 episode typically includes an igneous fabric consisting of nonexotic intrusive and extrusive basalts within a terrigenous matrix suggesting that the yuwan prism may have also been associated with a trench trench ridge triple junction d1 coaxial deformation is observed to overprint all of the episodes represented although it has not destroyed or significantly sheared their respective fabrics especially in regard to the main sd2 block in matrix structures which are therefore considered to represent original sedimentary structures

pseudotachylyte from an ancient accretionary complex evidence for melt generation during seismic slip along a master decollement pseudotachylyte is present in the shimanto accretionary complex in shikoku japan the pseudotachylyte occurs as a vein in a fault zone and displays flow structure tio 2 microlites and other melting related textures the mineral assemblage of the pseudotachylyte suggests that the temperature was raised at least 450 c above the background levels in the host rocks this is the first direct evidence of seismogenic fault rock from an accretionary complex the pseudotachylyte bearing fault is a roof thrust above underthrusted melange which is overlain by an offscraped package of coherent units the geologic setting of the pseudotachylyte is consistent with melt generation during seismic slip along the master decollement in the subduction zone

a crater and its ejecta an interpretation of deep impact we apply recently updated scaling laws for impact cratering and ejecta to interpret observations of the deep impact event an important question is whether the cratering event was gravity or strength dominated the answer gives important clues about the properties of the surface material of tempel 1 gravity scaling was assumed in pre event calculations and has been asserted in initial studies of the mission results because the gravity field of tempel 1 is extremely weak a gravity dominated event necessarily implies a surface with essentially zero strength the conclusion of gravity scaling was based mainly on the interpretation that the impact ejecta plume remained attached to the comet during its evolution we address that feature here and conclude that even strength dominated craters would result in a plume that appeared to remain attached to the surface we then calculate the plume characteristics from scaling laws for a variety of material types and for gravity and strength dominated cases we find that no model of cratering alone can match the reported observation of plume mass and brightness history instead comet like acceleration mechanisms such as expanding vapor clouds are required to move the ejected mass to the far field in a few hour time frame with such mechanisms and to within the large uncertainties either gravity or strength craters can provide the levels of estimated observed mass thus the observations are unlikely to answer the questions about the mechanical nature of the tempel 1 surface

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

pseudotachylytes in an ancient accretionary complex and implications for melt lubrication during subduction zone earthquakes abstract pseudotachylyte bearing fault zones found in the shimanto accretionary complex southwest japan developed during subduction or underplating at seismogenic depths the pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a melange the microstructures of pseudotachylytes are represented by a fragment laden glass supported texture resulting from the rapid cooling of the frictional melt transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed the compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite rich slip zone with a minimum melting temperature of 1100 c the viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as from the volume fraction and aspect ratio of the unmelted grains the viscosities at 1100 c range from 85 to 290 pa s and the corresponding shear resistance along a 1 mm thick melt layer at a slip rate of 1 m s was 0 1 0 3 mpa the formation of a melt layer in an illite rich slip zone can possibly induce large stress drops increase the slip rate and enhance rupture propagation which together could affect the earthquake magnitude in a subduction zone

deformation and hydrofracture in a subduction thrust at seismogenic depths the rodeo cove thrust zone marin headlands california we have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths the rodeo cove thrust zone which outcrops north of the golden gate bridge of san francisco imbricates two basalt chertsandstone sequences belonging to the marin headlands terrane franciscan complex the thrust outcrop is a 200 m thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop dominated by basaltic lithologies where zones of concentration of deformation have been mapped disruption is made by variably dense discrete fault systems synthetic to the main thrust r and p fractures these faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defi ned by chlorite and pumpellyite which also constrain the depth of faulting 8 10 km t 200 250 c within the seismogenic zone the central part of the fault also features the densest system of carbonate fi lled veins veins occur in the broken formation matrix and fragments in both cases parallel to the foliation the veins are either folded truncated or pressure solved along the cleavage cementation and hardening of shear surfaces of the fault core may have caused the distribution as opposed to localization of subsequent slip events the fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fl uids occurring through systems of multiple dilatant fractures grossly parallel to the decollement zone the crosscutting relationships between veining and foliation suggest that fl uid injection is cyclic and consequently that large transient variations in permeability and cohesion may occur the repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress or by difference in tensional strength parallel to and perpendicular to the foliation both of which would require extremely high fl uid pressure we interpret the features of the rodeo cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress fi eld in the interseismic phase and an extensional stress fi eld in the immediately postseismic phase

field evidence of pore pressure diffusion in clayey soils prone to landsliding 1 the hydrologic behavior of shallow weathered soils commonly determines the propensity for slope failure here we use laboratory data and field data collected by an automated monitoring system to assess the character of pore water pressure responses in a natural clay slope subject to intermittent rainfall although we did not measure pore pressure distributions that triggered slope failure we obtained three years of field data that provided reliable and largely reproducible documentation of transient pore pressure responses at depths of tens of centimeters to a few meters below the ground surface moisture and pressure sensors recorded relatively fast transient responses to precipitation the speeds of pore pressure pulses advancing downward in the saturated zone were much larger than those of advective fronts driven by gravity and the amplitudes of the pulses attenuated with depth statistical assessment of 129 pressure head responses demonstrates that this behavior is consistent with predictions of a linear one dimensional pore pressure diffusion model however the model best simulates measurements if diffusivity is treated as a calibration parameter and if initial moisture conditions match model assumptions for regional assessment of slope stability the predictive accuracy of the linear diffusion model is limited by inherent uncertainties in defining the initial conditions and in assigning the values of hydraulic parameters

syn diagenetic evolution of shear structures in superficial nappes an example from the northern apennines nw italy abstract in the superficial nappes of the northern apennines meso and microstructures in sheared mudstones and fine grained turbidites consist mainly of networks of striated faults and calcite vein arrays the striated faults correspond to the first generation of structures they form two sets respectively synthetic and antithetic to the general shearing locally a sigmoidal microfabric is associated with the synthetic faults in scaly deformation bands up to a few metres thick late laterally extensive calcite veins are superposed on these structures they result from the opening of multiple releasing oversteps between shear surfaces the crack seal mechanism of deformation played a major role in their formation structures of the first generation result from heterogeneous ductile deformation of initially poorly lithified and water rich sediments the late formation of the calcite veins corresponds to a transitional ductile brittle behaviour in sufficiently lithified sediment evolution of structures probably depended on a strain hardening process and was dependent on the progressive compaction and dewatering of the sediment the gently dipping faults and veins probably formed the main pathway for the interstitial fluids possibly overpressured expelled from the sediment we emphasize that the geometry of the structures described provides important shear sense criteria for the study of superficial nappe tectonics

pressure fluctuation during uplift of the northern apennines italy a fluid inclusions study abstract p t conditions existing after the main syn collision tectonic phase in the western part of the northern apenninic chain italy e g in the tuscan nappe outcropping in the la spezia area were estimated on the basis of a detailed microstructural structural petrographic and fluid inclusion study of quartz of syn tectonic d2 veins developed in the tertiary flysch at the top of the tuscan nappe macigno formation three main fluid events have been distinguished as follows during retrograde metamorphism d1 to d2 phase fluids in equilibrium with turbidites from the tuscan nappe were h2o ch4 mixtures issued from water organic matter interactions in temperatures conditions that may have reached at least 260 c or more 280 c depending on the considered depth estimates and maximum pressures around 210 250 mpa evidence of strong fluid pressure fluctuation between lithostatic and hydrostatic within the metamorphic formations up to 100 150 mpa possibly linked to fault valve activity at the beginning of the uplift triggered phase separation of the water methane fluids and production of methane rich and water rich fluids fluctuations in pressure during these events played a crucial role in quartz crystallization especially in extensional fissures formed perpendicular to the d2 folds axial foliation changes in the fluid regime and sources with time are evidenced by the input of brines which mix to distinct degrees and are trapped in healing microfissures during retrograde fluid evolution such mixing processes are an indication of the connection between separate fluid reservoirs with different temperature conditions consequently mass and heat transfer have to be taken into account as the downward percolation of cooler fluids probably accelerated the rate of cooling of the exhumed formations these processes are certainly common to most orogenic terrains and can be quantitatively studied through p v t x reconstruction using fluid inclusion data on the drainage zones where the mixing processes occurred this work confirms that fluid inclusion studies can provide accurate quantitative estimates of fluid pressure variations during the evolution of orogenic terrains and could therefore constitute a useful tool in tectonic interpretations at the light of the models developed for pressure variations in the upper crust

tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion the case of the ligurian wedge of the northern apennines italy subduction can be associated either with accretion or with removal of material from the overriding plate these two processes can either coexist or alternate in time along the same margin their inception has the potential to change the dynamic equilibrium of a marginal wedge resulting in the development of out of sequence thrusts normal and strike slip faults or large submarine landslides in the frontal part of the subduction zone in this work we investigate the effects of the transition from frontal accretion to frontal erosion on the stability of a subduction complex through the study of an ancient example from the northern apennines ligurian subduction complex new structural data suggest that in the early neogene aquitanian the removal and underthrusting of the toe of the wedge formed by both the accreted oceanic sediments and the overlying wedge top basin fill i e the subligurian units implied a process of frontal tectonic erosion the presence on top of the subduction complex of a complete succession of mid late eocene to late miocene slope apron sediments i e the epiligurian succession facilitates a reconstruction of the sedimentary response to this event in the aquitanian large areas of the wedge were denuded from the lower slope sedimentary cover through extensive gravitational mass movements the subsequent deposition of a thick body of submarine debris flows is documented the mass wasting deposits are interpreted as the sedimentary response to the underthrusting of the frontal part of the ligurian subduction complex formed by the subligurian units

estimating the uniaxial compressive strength of a volcanic bimrock abstract preparation of the standard sized cores from geological mixtures or fragmented rocks such as melanges fault rocks coarse pyroclastic rocks breccias and sheared serpentinites is often extremely difficult therefore determination of the mechanical parameters such as cohesion friction angle and ucs is extraordinarily difficult for these types of rocks this paper presents the results of studies using ankara agglomerate to develop an empirical approach for the determination of the ucs for a volcaniclastic mixture of strong andesite blocks within weak tuff matrix formed from the aggregation of volcanically exploded rock debris mixed with ash fall tuff i e a bock in matrix rock or bimrock the uniaxial compressive strength of a volcanic bimrock can be predicted using the conceptual approach within as yet poorly understood error limits therefore the approach introduced is open to improvement depending on the number of data and cases including different type of bimrocks

deformation and diagenesis in an ancient mud diapir southwest japan sandstone blocks in an ancient mud diapir in the tertiary shimanto belt record isotropic dilation in the form of a three dimensional 3 d disaggregation texture that is similar to textures documented in ancient and active mud diapirs in many locations worldwide we attribute this unusual texture to the combination of porosity loss in the margins of the sandstone block due to diagenetic reactions and decompression as the diapir was emplaced the expansion and degassing of pore filling fluids coupled with the low porosity resulted in the isotropic dilation of the blocks similar porosity reducing diagenetic reactions may seal off permeable wall rock units penetrated by mud diapirs and thus aid in the generation and maintenance of overpressures in addition the textures we document may be useful in recognizing mud diapirs in the rock record

progressive development of block in matrix fabric in a shale dominated shear zone insights from the bobbio tectonic window northern apennines italy 1 block in matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary tectonic and mud diapiric processes in many exposed orogenic belts a true melange originates when this fabric is associated with mixing of rocks of different ages and provenance as that characterizing the main decollement shear zone developed between the highly allochthonous ligurian nappe and its substratum of foredeep deposits at the margins of the bobbio tectonic window trebbia valley northern apennines mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement evidence of polyphased deformations spanning from liquefaction related and hydroplastic structures to pseudo hydrofracturing features and mineralized veining have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure a novel evolutionary scheme for block in matrix fabric development is here proposed involving the progressive dismembering of already highly deformed units originally developed in non to poorly lithified conditions through a generalized simple shearing achieved during the evolution of the shear zone coupled with strain hardening due to tectonic compaction synkinematic diagenesis and fluid expulsion

evolution and kinematics of an ancient decollement zone melange in the shimanto accretionary complex of okinawa island ryukyu arc detailed structural analysis of the melange in the shimanto accretionary complex of okinawa island in the ryukyu arc elucidates the spatial distribution and temporal progression of decollement related deformation early deformation took place in the footwall of the decollement and is characterized by subhorizontal layer parallel extension and subsequent heterogeneous shear of partially lithified sediments resulting in alternation of asymmetric e g s c fabrics and layer parallel extensional boudined layers fabrics mud intrusions suggest that high fluid pressure was locally generated in the footwall of the decollement late deformation is marked by the partitioning of deformation in the melange the lower structural level of the melange was pervasively sheared along microfaults whereas the upper structural level was flattened with development of planar pressure solution cleavage because shear is concentrated in the lower structural level late deformation is considered to have occurred in the hanging wall of the decollement kinematic indicators suggest that shear directions during decollement related deformation reflect the relative plate motion during early tertiary after the melange was incorporated into the accretionary prism crenulation cleavage and cylindrical upright folds were formed under subhorizontal shortening

record of mega earthquakes in subduction thrusts the black fault rocks of pasagshak point kodiak island alaska on kodiak island alaska decimeterthick black fault rocks are at the core of foliated cataclasites that are tens of meters thick the cataclasites belong to melange zones that are regarded as paleodecollements active at 12 14 km depth and 230 260 c each black layer is mappable for tens of meters along strike the black fault rocks feature a complex layering made at microscale by alternation of granular and crystalline micro textures both composed of micronscale subrounded quartz and plagioclase in an ultrafi ne phyllosilicate rich matrix in the crystalline microlayers tabular zoned micro lites of plagioclase make up much of the matrix no such feldspars have been found in the cataclasite we interpret these crystalline microlayers as pseudotachylytes the granular microlayers show higher grainsize variability crushed microlites and textures typical of fl uidization and granular fl ow deformation crosscutting relationships between granular and crystalline microlayers include fl ow and intrusion structures and mutual brittle truncation this suggests that each decimeters thick composite black fault rock layer records multiple pulses of seismic slip in each pulse ultracomminuted fl uidized material and friction melt formed and deformed together in a ductile fashion brittle truncation by another pulse occurred after solidifi cation of the friction melt and the fl uidized rock x ray powder diffraction xrpd and x ray fl uorescence xrf analyses show that black fault rocks have similar mineral composition and chemical content as the cataclasites the observed systematic chemical differences cannot be explained by bulk or preferential melting of any of the cataclasite components the presence of an open fl uidinfi ltrated system with later alteration of black fault rocks is suggested the geochemical results indicate that these subductionrelated pseudotachylytes differ from those typically described in crystalline rocks and other tectonic settings

non intrusive measurements of crater growth abstract an experimental technique to measure crater growth is presented whereby a high speed video captures profiles of a crater forming after impact obtained using a vertical laser sheet centered on the impact point unlike previous so called quarter space experiments where projectiles were launched along a transparent plexiglas sheet so that growth of half a crater could be viewed the use of the laser sheet permits viewing changes in crater shape without any physical interference to the cratering process this technique indicates that for low velocity impacts 300 m s into 220 m glass beads that are without cohesion and where the projectile is not disrupted craters initially grow somewhat proportionally but that later their depths remain essentially constant while their diameters continue to expand in addition these experiments indicate that as the impact velocity increases the rate of growth and the transient depth to diameter ratio at the end of ejecta excavation decreases these last two observations are probably due to the large time of penetration of the projectile which becomes a significant fraction of the time of crater formation this is contrary to the expectations for the scaling rules which assumes a point source very high curtain angles 45 are also seen and could be due to the low friction angle of the target significant crater modification which is rarely seen in quarter space experiments is also observed and appears to be controlled by the dynamic angle of repose of the target these latter observations indicate that differences in target friction angles may need to be considered when determining near rim ejecta mass distributions and large scale crater modification processes on the planets