diagnostic features and field criteria in recognition of tectonic sedimentary and diapiric melanges in orogenic belts and exhumed subduction accretion complexes abstract multiple episodes of deformation during the tectonic evolution of orogenic belts and ancient subduction accretion complexes cause obfuscation of primary block in matrix fabric of melanges and thereby making the recognition of their tectonic sedimentary or diapiric origin difficult here we present a comprehensive overview and synthesis of a diverse set of field based stratigraphic and structural criteria which are at the base of geological mapping rules to differentiate between various melange types developed by disparate geological processes and mechanisms we first define the current concepts of melange and melange nomenclature and describe the most diagnostic features of tectonic sedimentary and diapiric melanges at different scales we discuss some of the main issues complicating the application of these diagnostic criteria such as i similarities between the block in matrix fabric of different melange types formed in partially lithified sediments at shallow structural levels ii transformation of fabric elements with increased depth due to tectonic reworking and recrystallization processes iii significance of exotic versus native blocks in melange matrix and iv age relationships between blocks and matrix in a melange we introduce two additional criteria in approaching these complexities and in recognizing different processes of polygenetic melanges formation in the field when primary diagnostic fabrics were reworked by multiple deformational events these new criteria are based on i the coherence between lithological compositions of melange components blocks and matrix and characteristics and tectonic evolution of the geodynamic setting of their formation tectonic environment criterion and ii specificity and kinematic coherence in the distribution of deformation between blocks and the matrix deformation criterion the discussed diagnostic criteria can be applied to all field based investigations of melanges and broken formations in orogenic belts and exhumed subduction accretion complexes around the world regardless of their location age and tectonic history

kinematic analysis of deformed structures in a tectonic melange a key unit for the manifestation of transpression along the zagros suture zone iran kinematic analysis of melange fabrics provides critical information concerning tectonic processes and evaluation of the kinematics of ancient relative plate motion systematic kinematic analysis of deformed structures within a tectonic melange exposed along the zagros suture zone elucidates that this zone is an ancient transpressional boundary the melange is composed of a greywacke and mudstone matrix surrounding various lenses blocks and ribbons of radiolarian chert limestone sandstone pillow lava tuff serpentinite shale and marl the deformation fabrics of the melange suggest that the melange units were tectonically accreted at shallow levels within a subduction complex resulting in layer parallel extension and shearing along a nw se trending suture that juxtaposes the afro arabian continent to the south and the central iranian microcontinents to the north the tectonic melange is characterized by subhorizontal layer parallel extension and subsequent heterogeneous non coaxial shear resulting in alternating asymmetric and layer parallel extensional fabrics such as p y fabrics and boudinaged layers kinematic data suggest that the melange formed during oblique subduction of the neo tethys oceanic lithosphere in late cretaceous time kinematic shear sense indicators reveal that the slip direction n9 e to n14 e during accretion related deformations reflects the relative plate motion between the afro arabian continent and central iranian microcontinents during late cretaceous to miocene times

probabilistic rainfall thresholds for landslide occurrence using a bayesian approach 1 various methods have been proposed in the literature to predict the rainfall conditions that are likely to trigger landslides in a given area most of these methods however only consider the rainfall events that resulted in landslides and provide deterministic thresholds with a single possible output landslide or no landslide for a given input rainfall conditions such a deterministic view is not always suited to landslides slope stability in fact is not ruled by rainfall alone and failure conditions are commonly achieved with a combination of numerous relevant factors when different outputs landslide or no landslide can be obtained for the same input a probabilistic approach is preferable in this work we propose a new method for evaluating rainfall thresholds based on bayesian probability the method is simple statistically rigorous and returns a value of landslide probability from 0 to 1 for each combination of the selected rainfall variables the proposed approach was applied to the emilia romagna region of italy taking advantage of the historical landslide archive which includes more than 4000 events for which the date of occurrence is known with daily accuracy the results show that landsliding in the study area is strongly related to rainfall event parameters duration intensity total rainfall while antecedent rainfall seems to be less important the distribution of landslide probability in the rainfall duration intensity shows an abrupt increase at certain duration intensity values which indicates a radical change of state of the system and suggests the existence of a real physical threshold

a wave cisk model of squall lines abstract the wave cisk model of raymond incorporating lag effects in the updraft and downdraft is implemented as an initial value problem in physical space examples of both midlatitude and tropical squall lines are successfully simulated diagnosis of the model shows that condensational heating in the updraft is the primary driving mechanism of a mature squall line with evaporative cooling and convective momentum transfer playing subsidiary roles however an instability involving the displacement of boundary layer air by downdrafts apparently plays an important role in squall line initiation a weakness of the model is its inability to predict the direction of squall line propagation relative to the low level wind shear this is traced to the insensitivity of the convective parameterization to midlevel entrainment however unlike strict two dimensional squall lint models the parameterization allows cross stream mass transfer to occur in the context of overall slab symmetry such transfer is dynamical

observation and analysis of near surface pore pressure measurements in clay shales slopes clay shales largely outcrop in the northern apennines of italy where they are associated with frequent and widespread landslide activity the paper presents the results of monitoring activities carried out in two field sites and focused on the hydrology of clay shales prone to landsliding in the monitored slopes the water table is close to the ground surface throughout the year with seasonal fluctuations of tens of centimetres to a couple of metres shallow weathered soils or clay cover and unaltered material exhibit a much different behaviour although responses to single rainfalls are clearly identifiable as pressure pulses in the clay cover they are never observed in the underlying parental clay shales where downward gradients indicate recharge and only long term seasonal fluctuations are recorded short term variations of pressure head in the clay cover are ruled by downward propagation of a pressure wave in saturated conditions more than 400 pore pressure responses collected in the two sites are analysed in terms of lag time rate and amount of pressure increase and timing of pressure peaking it emerges that the linear diffusion theory can satisfactorily explain the observed behaviour in deeper unaltered material the barometric effect influences the measures similarly to what is observed in confined aquifers after its removal the corrected data exhibit regular seasonal fluctuations which are used to estimate the diffusivity of the material based on phase lag and attenuation of the downward propagating long term periodic pulse copyright 2011 john wiley sons ltd

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

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

sedimentary and structural evolution of a pleistocene small scale push moraine in eastern poland new insight into paleoenvironmental conditions at the margin of an advancing ice lobe abstract recent studies of push moraines have focused on the interplay between the dynamics of ice margins and the environmental variables of the foreland into which they advance these studies showed that the spatial distribution geometry and style of the glaciotectonic deformation of push moraines are controlled by ice induced stresses the strain rate the rheology of the deposits and hydraulic conductivity in this work we provide new insight into this interplay at a small spatio temporal scale specifically the ancient glacial system of the liwiec ice lobe within the younger saalian ice sheet in eastern poland the paleoenvironmental variables that are analysed here refer to the dynamics of the hydrological processes that affected the patterns and sediment deposition rate on the terminoglacial fan and the resulting mechanical stratigraphy and hydraulic conductivity of the foreland we document the progradational sequence of the fan deposits that developed as a result of the ice lobe thickening and the steepening of its stationary front the sedimentary features of the fan the lithology of its basement and the hydraulic conductivity of the foreland strongly influenced the geometry and kinematics of fold growth during the advance of the ice lobe the predominance of flexural slip and the development of fractures including fold accommodation faults were interpreted to be an effect of buckle folding due to horizontal shortening induced by ice advance the partial overriding of the push moraine by the ice lobe and thus the submarginal conditions for deformation were inferred from the significant hinge migration and internal deformation of the strata under undrained conditions in one of the folds the synfolding deposition pattern of the fan growth strata allowed us to suggest that the push moraine was probably formed by a sustained advance rather than surge

on assessing local conditional symmetric instability from atmospheric soundings abstract the standard parcel method of assessing the susceptibility of the atmosphere to moist convection using tephigrams is extended to account for the centrifugal as well as the gravitational potential energy of the displaced air parcel this leads to a measure of the stability of the moist baroclinic atmosphere to finite slantwise reversible displacements of a two dimensional air parcel such a measure differs from previously derived measures of conditional symmetric instability which have considered only infinitesimal displacements in saturated atmospheres it is demonstrated that the combined gravitational and centrifugal potential of a two dimensional air parcel or tube can be assessed by displacing the tube slantwise along a surface of constant angular momentum and that this combined potential energy can be estimated using a single atmospheric sounding several examples of the application of this technique are presented in the context of a case study of apparent slantwise convection the result

solar migrating atmospheric tides in the winds of the polar region of venus we study the effects of migrating solar tides on the winds at the cloud tops of the polar region of venus the winds were measured using cloud tracking on images obtained at wavelengths of 3 9 and 5 0 l mb y the instrument virtis m onboard venus express these wavelengths probe about the same altitude close to the cloud tops allowing for the first time to retrieve winds simultaneously in the day and nightside of the planet we use a dataset with observations from 16 orbits covering a time span of 289 days and a latitude range between 70s and 85s the region where the so called cold collar resides diurnal and quarter diurnal tides wavenumbers 1 and 4 were detected in the wind field with a decoupled influence on the zonal and meridional directions the diurnal tide is the dominant harmonic with amplitudes of about 4 7 m s exclusively affecting the meridional component of the wind and forcing a solar to antisolar circulation at the polar region the quarter diurnal mode is only apparent in the zonal wind in a restricted latitude range with amplitudes 2 2 m s the spatial structure of the diurnal tide has also been investigated obtaining a vertical wavelength of about 8 km in accordance with predictions by models finally a theoretical relation between the amplitudes of tidal temperature and tidal wind has been derived and its validity tested with models and results from previous missions

exhumation and reshaping of far travelled allochthonous tectonic units in mountain belts new insights for the relationships between shortening and coeval extension in the western northern apennines italy abstract coeval extensional and contractional tectonics are a common feature in orogenic belts especially in post collisional phases the attempt to constrain their relative timing and the responsible mechanisms led to several models taking into account different geodynamic contexts of various mountain chains in peri mediterranean collisional orogens extension and compression are commonly associated and coeval to the emplacement of far travelled allochthonous units therefore their internal deformation acquired during the translation across collisional belts is likely imprinted by the extensional and contractional tectonics affecting the orogen in the western portion of the northern apennines of italy one of the northernmost peri mediterranean tertiary orogens the far travelled allochthonous units i e the ligurian and subligurian units are widely exposed the present study integrates published and newly acquired surface geological data geological maps cross sections subsurface boreholes data and paleothermal vitrinite reflectance mixed layer illite smectite and kubler index and thermochronological data apatite fission tracks in order to investigate the deformation history and consequently the record of the extension compression relationships within these far travelled allochthonous units the study of modes and timing of reshaping of the far travelled allochthonous units allowed us to strengthen the hypothesis that extensional and contractional tectonics were coeval in the northern apennines and tightly connected by a cause effect relationship in particular the late orogenic low and high angle extensional tectonics represent the response to an overall contractional regime which induced episodes of gravitational instability in the orogenic wedge

strain partitioning during oblique plate convergence in northern sumatra geodetic and seismologic constraints and numerical modeling global positioning system gps measurements along the subduction zone of northern sumatra 2 s to 3 n reveal that the strain associated with the oblique convergence of the australian plate with eurasia is almost fully partitioned between trench normal contraction within the forearc and trench parallel shear strain within a few tens of kilometers of the sumatran fault kinematic analyses of interplate earthquake slip vectors provide slip rates on the sumatran fault within a few millimeters per year of gps and geologic rates giving us more confidence in the use of slip vectors for inferring slip partitioning elsewhere the inferred slip rate on the sumatran fault is 1 3 less than the full margin parallel component of plate motion an across forearc rotation in the slip vectors suggests that the missing arc parallel shear occurs seaward of the geodetic network between the forearc islands and the trench simple finite element models are used to explore the conditions under which the change in the principal strain rate directions between the forearc and the arc region can occur modeling suggests that neither a preexisting strike slip fault nor a zone of thermally induced lithospheric weakness in the overriding plate is needed for strain partitioning to occur in general forearc slivers form over the region of interplate coupling and are driven along strike by the basal shear a volcanic arc can help the partitioning process by localizing the margin parallel shear strain in the upper plate if its crust and mantle are weaker than its surroundings interplate slip vectors and geodetic results from sumatra together suggest that the highest coupling on the plate boundary occurs beneath and seaward of the forearc islands consistent with inferences about the rupture zones of great nineteenth century earthquakes there the sumatra example suggests that geodetic measurements of interseismic margin parallel shear strain at oblique convergent margins can be used to map the landward extent of the relatively high basal stress beneath the overriding plate if one can correct for strain localization caused by weak upper plate strike slip faults

accretion in the wake of terrane collision the neogene accretionary wedge off kenai peninsula alaska subduction accretion and repeated terrane collision shaped the alaskan convergent margin the yakutat terrane is currently colliding with the continental margin below the central gulf of alaska during the neogene the terrane s western part was subducted after which a sediment wedge accreted along the northeast aleutian trench this wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the pacific plate prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop the lateral extent of the structures is constrained by high resolution swath bathymetry and seismic lines collected along strike accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating toward the northeast the lower slope steepens the wedge narrows and the accreted volume decreases notwithstanding a doubling of sediment thickness in the trench in the northeasternmost transect near the area where the terrane s trailing edge subducts no frontal accretion is observed and the slope is eroded the structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion as the trailing edge of the yakutat terrane is subducting

collisional melange development geologic associations of active melange forming processes with exhumed melange facies in the western banda orogen indonesia analysis of block assemblages matrix clay mineral composition and microfauna and offshore seismic reflection profiles reveal that the bobonaro melange in the timor region is sourced from mud rich australian continental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacement melange facies include broken formation matrix rich mud injections and classic mixed block in clay facies each of these are distinguished by varying degrees of remobilization mixing and dispersion at different structural positions across the orogenic wedge the most important structural control is whether melange was generated beneath or in front of upper plate banda forearc basement banda terrane at the present collisional deformation front in the timor trough seismic reflection profiles show that melange forms mostly by stratal detachment and fluid assisted remobilization above a basal decollement propagating laterally along overpressured jurassic to cretaceous clay rich sequences of the distal australian continental margin the broken clay rich material injects upward through faults to form intrusive bodies at the base of slope cover sediment to form mud ridges at the surface similar patterns of stratal disruption are exposed onshore in the pliocene kolbano fold and thrust wedge of southern timor which is structurally contiguous with the timor trough deformation front melange in the kolbano mountains is mostly broken formation and matrix rich injections of mud from jurassic and cretaceous units deformation mechanisms include intense layer parallel extension associated with emplacement by mud diapirs that rise from near the decollement upward to the surface along fault conduits in the hinterland of the orogenic wedge east timor and northern west timor melange is dominantly of mixed block in clay facies with large blocks derived from roof thrust sheets of intermixed banda terrane and maubisse formation units at the structural base of these thrust sheets is the sonnebait disruption zone sdz which formed the initial suture between the banda terrane and structurally underthrust australian continental margin sequences during the late miocene to early pliocene the thickest accumulations of mixed block in clay melange in the timor region are found at the southern edge outlet of the sdz near the central and viqueque synorogenic basins the extent of block dispersion and mixing in the sdz is indicative of intense shear strains perhaps induced by an oversupply of accretable material when the suture zone was clogged by underthrusting of the australian continental margin

venus cloud level circulation during 1982 as determined from pioneer cloud photopolarimeter images i time and zonally averaged circulation abstract new measurements of cloud motions on venus during 1982 reveal significant changes in the mean cloud level circulation compared to that seen during 1974 s s limaye and v e suomi 1981 j atmos sci 38 1220 1235 as well as 1979 s s rossow a d del genio s s limaye l d travis and p h stone 1980 j geophys res 85 8107 8128 s s limaye c j grund and s p burre 1982 icarus 51 416 439 in order to establish the validity of these changes two individuals measured cloud motions independently in nearly identical sets of images the results of these independent estimates for 1982 are in remarkable agreement for not only the average zonal and meridional components but also for the measured meridional transport of momentum by the eddy circulation unlike the results for 1979 the latitudinal profile of the time and longitudinally averaged zonal component of the cloud motions for 1982 shows pronounced jets near 45 north and south lat while the average zonal component near the equator is weaker than by more than 10 msec 1 the time and longitudinally averaged meridional component is found to be slightly greater than that during 1979 at all measured latitudes the estimated poleward transports of angular momentum by the mean and eddy circulation are qualitatively similar to those estimated from 1979 measurements with the eddy transport being of the same sign as that of the mean circulation an accompanying paper presents the structure seen in the observed motions seen in a solar locked longitude latitude system interpreted to be the signature of the solar thermal tides s s limaye 1988 icarus 73 212 226

structural and tectono stratigraphic review of the sicilian orogen and new insights from analogue modeling abstract the apennines sicilian maghrebian fold and thrust belt originated from the subduction of the alpine tethys and the later collision of drifted continental blocks against the african and apulian paleomargins from north to south the sicilian fold and thrust belt sftb is divided in four main tectono stratigraphic domains 1 the calabro peloritani terrane drifted from the european margin 2 the remnants of the alpine tethys accretionary wedge atw related to the subduction of the tethys 3 the folded and thrusted platform panormide and deep water imerese sicanian series of the offscrapped african margin and 4 the african foreland hyblean unfortunately scarce quality seismic lines and outcrops of key tectono stratigraphic units make the structure and dynamic evolution of the central eastern part of the sftb controversial first this study outlines through a review of the tectono stratigraphic evolution of the central eastern sector of the sicilian orogen the major remaining issues concerning 1 the occurrence of inferred alpine tethys units far from the region where the remnants of the atw outcrop nebrodi mountains both in a forearc position above the peloritani block to the north and in an active foreland context along the present day southern front of the belt and 2 the diverging tectonic styles from stacked large scale tectonic nappes to foreland imbricated thrust systems rooted into a main basal decollement secondly new constraints are given using analogue modeling to test mechanically the hypothesized structural and tectono stratigraphic evolution of the sftb the experiment simulates the orogenic evolution of the sftb at crustal scale from the oligocene tethys subduction to the middle miocene late pliocene continental collision between the european and african paleomargins the tectono stratigraphic synthesis is used to model the first order mechanical stratigraphy of the sedimentary units involved in the sicilian belt as well as the imprint of the african margin structural inheritance the experiments succeed in reproducing the general structure and tectono stratigraphic evolution of the sftb in particular the models support field observations hypothesing a gravity driven origin of the inferred alpine tethys units intercalated within the forearc and foreland syntectonic sedimentation moreover the model testifies of the main tectonic steps that led to the sftb building first a low tapered accretionary wedge was accreted above the alpine tethys oceanic crust from the oligocene to the early miocene the following underthrusting of the stretched african continental margin and its frontal panormide platform shortened and thickened the accretionary wedge this phase provided favorable conditions for significant pulses of reworked alpine tethys units that intercalated within the forearc and foredeep successions during the middle miocene the decollement of the african meso cenozoic cover panormide platform and imerese sicanian deep water basin enhanced a deep seated deformation phase along with duplexing of the panormide platform beneath the alpine tethys wedge leading to its emersion since the late messinian activation of basement faults led to a generalized emersion of the orogenic units through large wavelength fold culminations accompanied by syn tectonic deposition at their southern limbs concurrently the prism front was partly indented to the southeast by the thick and locally already emerged hyblean platform

deformational styles in a sequence of olistostromal melanges pacific rim complex western vancouver island canada the pacific rim complex is exposed in a fault bounded slice along the west coast of vancouver island it is divided into two parts 1 the ucluth formation a basement unit composed of lower mesozoic arc volcanic rocks and 2 a suprajacent sedimentary sequence more than 2 km thick composed of lower cretaceous olistostromal melanges an unconformity is present at least locally at the ucluth melange contact and indicates that the ucluth formation is stratigraphic basement to the melanges melange style deformation is restricted to the sedimentary sequence and has affected virtually all parts of that sequence the melanges formed without the development of a cleavage or pervasive cataclasis indicating that deformation occurred prior to consolidation of the sediments three types of melanges have been recognized the first consists of a matrix of inter bedded mudstone chert sandstone and green tuff which encloses numerous exotic blocks of igneous rocks and limestone beds within the matrix show variable amounts of layer parallel fragmentation which is attributed to in situ liquefaction and lateral flowage within individual beds the blocks in this melange most of which were derived from the underlying ucluth formation are considered to be submarine slide blocks produced by rock falls from nearby basement scarps the matrix sediments near the blocks commonly contain wispy lenses of poorly sorted plutonic and volcanic detritus green tuff interpreted as scree from the rock slides the second and third types of melange are distinguished by the predominance of either mudstone or sandstone but are otherwise quite similar these melanges lack exotic blocks and consist of a contorted assemblage of mudstone turbidite sandstone conglomerate pebbly mudstone and rare chert all of which are depositionally interrelated sedimentary structures and stratal coherence are generally well preserved the internal structure of these melanges consists of a series of dismembered depositional sequences individual sequences are commonly more than 75 m thick some are upright and others are overturned paleoecological evidence indicates that some of these sequences were originally deposited in an infra slope basin thus these melanges are interpreted to have formed mainly by the accumulation of large slump sheets at the base of a submarine slope previous interpretations have considered the pacific rim complex to be a late mesozoic subduction complex constructed along the western margin of the wrangellia terrane vancouver island several factors argue against a subduction melange interpretation 1 the melanges were deposited on an older arc related basement not oceanic crust 2 exotic blocks were introduced as rock slides and not by subsurface faulting and 3 at least some of the strata originated in an infra slope basin the heterogeneous deformational style is more compatible with an origin by near surface mass movement processes including submarine slides rock falls debris flows and in situ liquefaction the amount of mass wasting recorded in the pacific rim melanges probably requires a seismically active setting where frequent ground shaking produced large transients in pore fluid pressure similarities in stratigraphy and metamorphism suggest that during late cretaceous time the pacific rim complex was part of the san juan cascades thrust system located 220 km to the southeast in northwestern washington state if this is correct the pacific rim melanges would have formed on the inboard side of wrangellia about 30 m y prior to the collision of wrangellia with the american continent the present location of the pacific rim outboard of wrangellia is attributed to younger latest cretaceous or early tertiary transcurrent displacement

zonal winds in the middle atmosphere of venus from pioneer venus radio occultation data abstract pioneer venus radio occultation data for four seasons between december 1978 and october 1981 are used to construct meridional cross sections of temperature and zonal wind velocity for the middle atmosphere of venus in the altitude range of 40 80 km and in the latitude range of 1 5 85 wind speeds are derived assuming cyclostrophic balance the wind field contains as many as three jets first a prominent one above the clouds situated between 50 and 55 latitude with a maximum speed of 140 m s 1 a second with a speed of 95 m s 1 centered at 70 and at 60 km altitude and a possible third jet at 15 and 65 km altitude with a speed of 100 m s 1 the notable midlatitude jet is associated with the cold polar collar wind speeds above 70 km generally decrease with height because of the warm pole at these levels however within the jet the speed is 100 m s 1 at 83 km the jets may be barotropically unstable and inertial instability may occur above the clouds on the equatorial side of the main

thermal tides in the atmosphere of venus comparison of model results with observations a linearized primitive equation lpe model is developed to study thermal tides in the atmosphere of venus the lpe model describes diurnal and semidiurnal oscillations of a cyclostrophically balanced atmosphere in which zonal velocity varies with altitude and latitude the numerical algorithm follows staniforth and daley the solar thermal forcing is increased algebraically in time to separate the forced tidal response from free atmospheric oscillations parameters of the basic state and forcing agree with pioneer venus observations results of the model are compared with the solar fixed component of brightness temperature variations measured by taylor et al and elson using data from the pioneer venus orbiter infrared radiometer oir the comparison is made by convolving the computed model radiances with the weighting functions of the oir channels agreement between lpe model results and oir observations is excellent two interesting features of the oir data are accounted for namely the slow variation of phase with altitude and the dominance of the semidiurnal oscillation over the diurnal oscillation success of the lpe model opens the way for calculating tidal transports of heat and momentum and assessing the role of tides in maintaining the venus super rotation

kinematic analysis of shear zones in sandstone and mudstone of the shimanto belt shikoku sw japan abstract i describe kinematic criteria for sense of shear in unmetamorphosed interbedded sandstone and mudstone of the tertiary shimanto accretionary complex at ariigawa k chi prefecture in certain zones parallel to bedding and up to several meters wide sandstone layers with internal bedding parallel turbidite laminations are broken into blocky rectangular fragments or inclusions many inclusions rotated counterclockwise and so indicate a sinistral sense of shear with respect to the present orientation of bedding asymmetric folds corroborate this interpretation amounts of rotation are variable and are inversely proportional to the axial ratios of fragments fragments with axial ratios greater than about 4 6 did not rotate while those with ratios between 4 and 1 record progressively greater maximum rotations these observations accord with theoretical and experimental analyses describing the behavior of rigid inclusions in a matrix subjected to simultaneous simple shear and pure shear the data from the shimanto suggest that 1 the rotation inducing deformation included a coaxial component and departed from strictly non coaxial simple shear and 2 total shear strains were of the order of 3 5

polyorogenic deformation history recognized at very shallow structural levels the case of the antola unit northern apennine italy abstract the antola unit is a ligurian unit occurring at the top of the nappe pile of the northern apennines italy structural analysis indicates that the antola unit has been involved in a complex polyphase deformation history developed at a shallow structural level the successive deformation phases belong to both the alpine and apenninic orogenic cycles therefore the antola unit is a good example of a polyorogenic tectonic unit deformed at a very shallow structural level the older part of the structural evolution is assigned to the meso alpine tectonic stage and consists of two deformation phases d1 and d2 characterized by opposite tectonic transport directions the d1 and d2 structures were sealed by the middle eocene miocene deposits of the tertiary piedmont basin the younger part of the deformation history instead can be referred to the northern apennines tectonics and consists of two phases d3 and d4 both involving the tertiary piedmont basin succession the structural data indicate that the d1 deformations developed in mostly unlithified rocks and that these structures can be related to the westward emplacement of the antola unit in the alpine orogenic wedge the d2 phase is characterized by folds and low angle extensional faults and is referable to a gravity driven deformation both the d3 and d4 phases characterized by folds and thrusts are due to the compressive regime related to the development of the northern apennines

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

pervasive near surface stratal disruption in an accretionary prism setting kaczawa complex sw poland the tectonized and metamorphosed mudrocks within the variscan accretionary prism of the kaczawa mountains in sw poland comprise sedimentary melanges together with more coherent stratigraphic units some represent large olistoliths deposited in a submarine trench we infer a trend of progressive near surface stratal disruption in mud dominated deposits due to dewatering that forms a continuum with subduction related tectonic structures imposed on unconsolidated sediment during deeper burial the assemblage of characters suggests that an accretionary prism environment can influence and leave characteristic traces of the total burial history of a trench succession

growth processes and melange formation in the southern apennines accretionary wedge abstract the southern apennines owe their present geometry to the subduction of an apulian continental lithospheric slab and are thus best interpreted as an accretionary wedge southward the apenninic thrust front branches on to the still active ionian wedge which is related to the present calabro sicilian subduction the interpretation of conventional seismic reflection profiles and exploration wells combined with geologic field studies biostratigraphy and structural geology provide constraints for a geodynamic model for the post messinian growth of the apenninic prism our model for the apennines is a good analogy for submerged oceanic accretionary prisms in both instances features such as out of sequence thrusts underplating and duplexes are viewed as processes to explain the thickening of accretionary wedges syntectonic terrigenous deposits have accumulated in the synclines developed above the flats of pliocene thrust faults these depocenters represent piggy back basins younger accretion at the eastern border of the accretionary wedge has led to a progressive uplifting and tilting of the already emplaced thrust sheets including a continous westward migration of the depocenters in the piggy back basins during pliocene and early quaternary times a contrary west to east migration of the depocenters locally occurred where preexisting flats of pliocene thrust faults have been refolded rather than only being rotated dewatering processes and effects of overpressures on undercompacted sediment deformations can also be studied in the southern apennines melanges and broken formations form an important component of the trench apparently restricted to the base of tectonic nappes listric normal faults are a major structural element which preserves in combination with the erosion of topographic highs and resedimentation in the piggy back basins a uniform geometric shape critical taper for the prism the age and location of these distensive or gravitational effects are also discussed

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

deformation at the toe of an active accretionary prism synopsis of results from odp leg 131 nankai sw japan toes of accretionary prisms record the initial deformation of wet sediments during accretion at a convergent plate margin odp leg 131 succeeded in coring the toe of the nankai prism sw japan together with the basal decollement the underthrust sediments and the basaltic ocean basement an unprecedented structural geological inventory has been compiled gravitational deformation is important in the slope apron deposits but tectonic stresses too are transmitted to these levels core scale deformation bands due to heterogeneous bulk shortening of the prism are exceptionally well developed at nankai and range through kink bands shear zones and faults anomalous frequencies indicate localized high strain zones stress inversion analysis gives a principal compression close to the plate convergence vector the frontal thrust of the prism is a 26 m thick zone of breccia scaly fabric with a 309 m net slip it may well be currently active and accommodating almost half of the present convergence strain the basal decollement a 19 m thick breccia zone separates the deforming overconsolidated sediments of the prism from the virtually undeformed materials below these are highly underconsolidated and probably overpressured anomalous fluid pressures localize several of the prism structures unlike some other prisms drainage of the nankai prism is dominantly pervasive rather than controlled by major structures

a study of mesoscale convective bands behind cold fronts part i mesoscale organization this paper reports observations made over the north atlantic of a system of convective cloud bands found within the cold air behind an occluded front the observations were made from an instrumented aircraft which additionally released a pattern of dropsondes measuring temperature relative humidity and wind as functions of pressure these data are analysed to establish some of the mesoscale cloud scale and microphysical properties of the system here in part i the mesoscale structure is described and compared with the predictions of various theoretical models the convective cloud structure emerges as a consequence of the mesoscale organization but appears to play only a minor role in its creation and maintenance one of the theoretical models considered conditional symmetric instability provides a piausible explanation of the mesoscale observations

semi allochthonous sedimentation in the apenninic thrust belt abstract on thrust sedimentation is not so common in mobile belts outside the apennines one of the relevant contributions to apenninic geology by the florentine giovanni merla in the 1950 s is the recognition of these traveller or floating sequences this concept of semi allochthon as merla called it is reviewed here and discussed in the light of recent advances in stratigraphy and basin analysis several unconformities are detected in the sequence which make it possible to split it into cycles or depositional sequences at variance with classical formations correlations have been attempted with sediments deposited behind and in front of the thrust belt in comparison with foredeep flysch sequences semi allochthonous ones show better preserved stratigraphical relations vertical stacking and less deformation which emphasizes their possible role as reference sections for syn orogenic sedimentation in the apennines

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

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

the relevance of conditional symmetric instability to the prediction of mesoscale frontal rainbands this paper examines the theory of conditional symmetric instability bennetts and hoskins 1979 and considers its relevance to meso scale frontal rainbands the theoretical growth rate of the instability is evaluated from synoptic scale data obtained from a numerical forecast model and compared with observations of the horizontal structure of rainfall observed by radar one case study is presented in detail and a further 44 cases summarized rainbands were observed on 80 of the occasions on which they were predicted

stratal disruption and development of melange western newfoundland effect of high fluid pressure in an accretionary terrain during ophiolite emplacement abstract the bay of islands ophiolite was emplaced onto the continental margin of north america during the mid ordovician taconic orogeny when tectonic slices of continental margin sediments were accreted to the moving allochthon tectonic slices grade into and are surrounded by melange early fracture in sandstones formed without grain breakage and allowed penetration of liquid petroleum along fracture planes other fractures involved cataclastic flow and were sometimes re activated during formation of later pressure solution cleavage shear fracture and extension fracture boudinage affect competent strata extensional veins cut cement in limestone beds and are filled by shale quartz calcite and bitumen folds also formed at a time when siltstone and sandstone were at least partially lithified melange matrix shows abundant shear and extension fractures in a variety of orientations coaxial extension responsible for disruption of bedding can be explained by a brittle accretionary wedge model in which high fluid pressures resulted from tectonic dewatering of shales surface slope decreased as fluid pressure rose beneath the ophiolite causing horizontal extension of the wedge after escape of excess water the surface slope steepened again as renewed stacking occurred

fluid flow during accretion in sediment dominated margins evidence of a high permeability fossil fault zone from the internal ligurian accretionary units of the northern apennines italy abstract we report here a detailed structural study carried out in the internal ligurian units of the northern apennines italy formed during the building of the alpine accretionary complex through subduction of the sediment filled ligure piemontese oceanic basin the deformation mechanisms associated with fluid migration across an accretion related fault zone have been studied through a detailed analysis of different generations of syn tectonic veins hydrofracturing occurred mainly sub parallel to bedding in unlithified to semi lithified sediments transient upward directed fluid injection locally connected the decollement parallel veins through bedding normal hydrofractures of lithified sandstone layers a third vein system comprises fibrous hydrofractures developed on the limbs of accretion related folds crosscutting vein sets and the peculiar features of each identified vein set suggest that deformation was intricately associated with lithification and diagenetic processes dehydration produced fluids transiently injected the lithifying sediments leading to local stress permutations the proposed model provides a ramp flat migration of fluids in which fluid flow is enhanced along high permeability less cohesive layers leading to the development of regional dilated hydrofracture channels like those recognized along the decollement zone of modern margins the more competent layers are truncated by high angle fractures representing the transient connectivity that existed between horizontal conduits

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

improved automatic estimation of winds at the cloud top of venus using superposition of cross correlation surfaces abstract accurate wind observation is a key to study atmospheric dynamics a new automated cloud tracking method for the dayside of venus is proposed and evaluated by using the ultraviolet images obtained by the venus monitoring camera onboard the venus express orbiter it uses multiple images obtained successively over a few hours cross correlations are computed from the pair combinations of the images and are superposed to identify cloud advection it is shown that the superposition improves the accuracy of velocity estimation and significantly reduces false pattern matches that cause large errors two methods to evaluate the accuracy of each of the obtained cloud motion vectors are proposed one relies on the confidence bounds of cross correlation with consideration of anisotropic cloud morphology the other relies on the comparison of two independent estimations obtained by separating the successive images into two groups the two evaluations can be combined to screen the results it is shown that the accuracy of the screened vectors are very high to the equatorward of 30 degree while it is relatively low at higher latitudes analysis of them supports the previously reported existence of day to day large scale variability at the cloud deck of venus and it further suggests smaller scale features the product of this study is expected to advance the dynamics of venusian atmosphere

small scale polygenetic melanges in the ligurian accretionary complex northern apennines italy and the role of shale diapirism in superposed melange evolution in orogenic belts abstract the argille varicolori varicolored scaly clays of the external ligurian units of the northern apennines have been widely described as a typical unmetamorphosed broken formation i e a chaotic unit without exotic blocks produced by offscraping and tectonic imbrication during the evolution of the ligurian accretionary wedge geological mapping and integrated structural and stratigraphic observations show that the argille varicolori consist of diverse types of small scale melanges non mappable at a 1 25 000 scale forming a composite chaotic unit in which the superposition of tectonic sedimentary and diapiric processes resulted in the occurrence of polygenetic chaotic bodies at different scales these melange units record the evolution of the ligurian accretionary wedge from subduction to collision and intracontinental deformation tectonically disrupted body 1 tdb1 comprises boudinage and pinch and swell structures formed by layer parallel extension contraction at the wedge front of the ligurian accretionary complex during the late cretaceous middle eocene it is interleaved with non mappable gravity driven chaotic bodies gcb developed during alternating episodes of accretion and removal of material at the wedge front the late oligocene early miocene out of sequence thrusting related to the collisional episodes in the apennines overprinted the previously formed chaotic bodies and formed a polygenetic tectonic melange tectonically disrupted body 2 tdb2 this unit is characterized by a structurally ordered block in matrix fabric and by the gradual decrease of stratal disruption away from the regional thrust overpressurized fluids concentrated along the shear surfaces and the scaly cleavage planes facilitated the diapiric upward movement of unconsolidated sediments in the early miocene this process produced non mappable shale dike injections ddb1 and mappable diapirically disrupted bodies ddb2 which show an internal zonation of deformation this deformation reworked the previously formed chaotic bodies although some of these polygenetic melanges cannot be mapped at a 1 25 000 scale their careful documentation provides a better understanding of time progressive scale independent melange forming processes

the structure and kinematics of an imbricate stack of oceanic rocks in the jurassic accretionary complex of central japan an oblique subduction model the jurassic accretionary complex of the mino belt in the takayama area central japan includes a major unit characterized by abundant bedded chert and basalt slabs this unit has an imbricate structure represented by the repetition of stratigraphic layers composed of sandstone black mudstone felsic tuff bedded chert siliceous claystone limestone and basalts shear zones subparallel to the trend of the slabs occur in the siliceous claystones and basalts deformation structures in the shear zones suggest that phyllosilicate rich layers within the accretionary complex were preferentially weakened due to grain size reduction by cataclasis and pressure solution accommodated deformation restored shear directions in the siliceous claystones and basalts indicate dextral kinematics subparallel to the trench axis in contrast mesoscopic asymmetric folding of the bedded chert indicated from the restored shear was oblique or normal to the trench axis to interpret the two distinct shear directions a model is proposed in which the constituent rocks of this unit were obliquely subducted dextrally along the east asian continental margin due to the oblique subduction slip vectors were apparently partitioned into trench orthogonal components in the shallower part of the subduction zone and trench parallel components in the deeper part

maintenance of strong rotational winds in venus middle atmosphere by thermal tides the cloud level atmosphere of venus takes little more than 4 days to complete one rotation whereas the solid planet below has a 243 day period computer simulations of the circulation of the venus middle atmosphere between 40 and 85 kilometers as driven by solar radiation absorbed in the clouds reproduce i the observed cloud level rotation rate ii strong vertical shears above and below the cloud tops and iii midlatitude jets and strong poleward flow on the day side simulated circulations converge to yield nearly the same zonal winds when initialized with both stronger or weaker rotation rates these results support the hypothesis that the observed cloud top rotation rate is maintained by statistical balance between fluxes of momentum by thermal tides and momentum advection by mean meridional circulation

dynamical effect of thermal tides in the lower venus atmosphere 1 the thermal tides in the lower atmosphere of venus are examined it is shown that both the diurnal and semidiurnal tides excited in the cloud layer propagate to the ground and the existence of thermal tides below the cloud bottom can be attributed almost to the solar heating in the cloud layer at altitudes of 0 10 km the atmospheric superrotation is accelerated and decelerated by the momentum transport associated with the diurnal and semidiurnal tides respectively the effect of diurnal tide is much smaller than that of the semidiurnal tide so that the mean zonal flow is accelerated in the direction opposite to the venus rotation there it is argued that this momentum transport by the semidiurnal tide is balanced with the surface friction in the atmospheric layer adjacent to the ground and the net momentum is supplied from the solid venus to maintain the atmospheric superrotation

underplated units in an accretionary complex melange of the shimanto belt of eastern shikoku southwest japan imbricated thrust stacks composed of melange and oceanic slabs occur in the shimanto belt eastern shikoku sw japan the melanges are divided into two types melange i consists of lenticular sandstone blocks surrounded by scaly shale matrix and melange ii is composed of blocks of oceanic material such as basalts cherts and red shale in a scaly matrix of shale melange ii is observed only near the boundaries between melange i and oceanic slabs the fabric of melange i indicates progressive deformation during underthrusting of originally coherent trench fill turbidites layer parallel extension characterizes early deformation and is divided into two stages an early stage formed during normal faulting and a subseguent stage formed perpendicular to the early extension the early extension is interpreted in terms of riedel shear associated with layer parallel shearing and tectonically induced loading normal to the layering the second extension is due to sticking thrust movement late deformation of melange i is represented by folding in association with layer parallel shearing this appears to be related to compression just before or during underplating of melange i beneath the accretionary prism layer parallel shear extends downward due to strain hardening of sheared sediments and then reaches to the base of the sediments subsequently the shear zone penetrates pelagic sediments and finally oceanic basement at this time melange ii is formed by the shearing an imbricated stack of melanges i ii and oceanic slabs forms duplexes during the underplating process the weak metamorphism of the melange complex in the study area suggests that underplating occurs at shallow depths where trench fill is thin

insights into shallow level processes of mountain building from the northern apennines italy recent observations from submarine convergent margins reveal that sediments at shallow levels of burial undergo intricate and irregular fluctuations in physical conditions as they progressively deform and lithify the early stages of mountain building similar on land evidence is comparatively sparse as a result of difficulties of exposure and of early features having been obscured here we describe aspects of three structures from the northern apennines of italy that corroborate these notions of complex shallow level orogenic processes the upper cretaceous castiglioncello melange formed during the subduction phase of apennine orogenesis the major cervarola thrust and the smaller intra formational balduini thrust both of tortonian age represent the ensuing collisional phase all three structures formed at burial depths less than 2 or 3 km and are syn diagenetic although differing in detail all three show features such as clay fabrics and mineralized veins that record complex histories of fluid flow overpressure and deformation the clays show varying types of scaliness some of which had to involve pore dilation and collapse operating in cycles the veins typically evolved from networks of bedding parallel and bedding perpendicular veinlets through dilation breccias and mosaic textures to shear and extensional fibre growth during hydrofracture such observations imply fluctuating physical conditions and dynamic tectonic processes operating on shallow level sediments in a way that is highly heterogeneous with depth areally and through time

numerical simulation of the large amplitude mesoscale gravity wave event of 15 december 1987 in the central united states abstract an observational study employing spectral methods is first made to establish a background for a modeling effort of the mesoscale gravity wave event of 15 december 1987 the waves are found to have wavelengths of 100 160 km phase speeds of approximately 30 m s 1 and lifetimes of over 6 h conditions for their maintenance are evaluated indicating the presence of a wave duct and a supportive role for wave cisk convection shearing instability and geostrophic adjustment are all implicated as possible source mechanisms for the observed waves the case is then simulated with the pennsylvania state university national center for atmospheric research mm4 mesoscale forecast model with the following primary objectives i to test the model s ability to simulate a mesoscale gravity wave event ii to examine in detail the environments of mesoscale gravity wave development and iii to investigate the mechanisms of mesoscale gravity wave generation and maintenance the full physics control experim

band formation in a new england winter storm abstract this case study addresses mechanisms of band formation in a new england winter storm the structure of the bands and their environment are documented with synoptic observations radar data and analyses of instrumented aircraft flights through the bands we postulate that processes on three scales are responsible for the bands observed that the bands are probably a manifestation of mesoscale symmetric instability and that potential energy for the instability is generated by synoptic scale differential lapse rate advection and converted to kinetic energy by symmetric overturning frontogenetical forcing results in an intermediate or subsynoptic scale region of ascent which brings the atmosphere to saturation and initiates the release of potential energy

melanges through time life cycle of the world s largest archean melange compared with mesozoic and paleozoic subduction accretion collision melanges abstract melanges represent a significant component of collisional and accretionary orogenic belts we present a comparison of subduction accretion melanges of different ages to test how melange forming processes at convergent margins changed through time we describe the mesozoic mchugh melange in alaska and the paleozoic proto tethyan laohushan melange of the qilian orogenic belt of the alpine himalayan chain together with documentation of a neoarchean melange that marks the suture between the eastern block and central orogenic belt of the north china craton ncc the mchugh is a classic accretionary melange that has not yet experienced a collision and the laohushan is an accretionary collisional melange with structures related to both stages of its evolution we compare the characteristics of these melanges with the world s largest archean melange the newly documented circa 2 5 ga 1600 km long jianping zunhua zanhuang dengfeng jzzd melange of the north china craton the jzzd is a classical melange in terms of map patterns lithological relations structural characteristics including mixed rock types of different ages origins and metamorphic histories and preserves both paired metamorphic features and local blocks preserving ultra high pressure uhp assemblages it shows high stratal disruption with both block in matrix and highly mixed thrust bounded tectonic slices types of map patterns the melange is part of an arc accretionary complex that was emplaced over a 2 7 2 5 ga passive margin sequence on the western edge of the eastern block of the ncc at 2 5 ga the frontal zone of the accretionary complex marks the suture between the eastern block and an oceanic arc terrane in the central orogenic belt cob and exhibits significant along strike n s variations in the north the jianping pyroxenite melange contains blocks of 2 52 ga metapyroxenite metabasalt tonalite trondhjemite granodiorite diorite ttgd gneisses in a metasedimentary and serpentinite matrix intruded by 2 49 2 47 ga plutons metamorphosed to amphibolite granulite conditions at 2 47 ga the zunhua shangying ophiolitic melange contains relatively intact 2 5 ga ophiolitic sheets in a 2 5 ga metasedimentary melange intruded by 2 48 ga granitic dikes the zunhua ultra high pressure uhp podiform chromite melange has blocks of pillow basalt bif dikes gabbros pyroxenite harzburgite some with 2 55 ga podiform chromites in dunite envelopes enclosed in a 2 52 2 46 ga metasedimentary matrix metamorphosed and intruded by granitic dikes at 2 46 ga the chromites and dunite pods formed through reaction of a boninitic melt and harzburgite during subduction initiation bewtween 2 6 2 55 ga uhp conditions are documented by tio2 ii inclusions in chromites indicating subduction to at least 270 km depth the zanhuang melange includes blocks of metapsammites metagabbros metabasalts and ultramafic rocks in a metapelitic matrix intruded by 2 49 2 54 ga granites then imbricated with 2 65 ga marble metapelite quartzite of the passive margin of the eastern block the dengfeng melange includes upper plate blocks of metagabbro metabasalt bif and felsic rocks in a sandy to pelitic matrix thrust over a turbiditic broken formation melange unit in the east circa 2 5 ga metamorphic conditions record a relatively high t p anatexis 950 c gpa in the west whereas the central part of the belt records intermediate t p metamorphism

on the tectonic evolution of the ligurian accretionary complex in southern italy unraveling the deformation pattern characterizing the transition from final oceanic subduction stages to early stages of deformation of a foreland continental margin is crucial for a better understanding of the geodynamic processes taking place at convergent plate boundaries in particular the combined role of internal wedge dynamics and continental margin architecture in controlling the tectonic evolution of an accretionary complex during its final emplacement onto the foreland continent is discussed in this study to this purpose we conducted integrated structural stratigraphic and low temperature thermochronometric analyses on the ligurian accretionary complex units exposed in the campania region italy and on continental margin successions located in their footwall as well as on related foredeep and wedge top basin deposits our results point out a series of late early miocene burdigalian shortening events also involving buttressing of the accretionary wedge against the crustal ramp of the foreland continental margin emplacement of the overthickened accretionary complex onto the distal part of the continental margin was followed by horizontal extension and wedge thinning aiding the development of wedge top depocenters extension may have been either related to reduced subduction rates during the middle miocene or to a period of subduction erosion known to have occurred in the northern apennines in the same time frame early miocene nw se shortening recorded by ligurian accretionary complex units was completely unrelated to later late miocene to pleistocene ne directed thrusting in the apennines which was coeval with backarc extension in the tyrrhenian sea therefore our results emphasize the occurrence of a major discontinuity in the neogene geodynamic evolution of the southern apennines the tectonic history of which may be clearly subdivided from a kinematic point of view into pre and syn tyrrhenian backarc extension stages

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

underplating process from melange formation to duplexing example from the cretaceous shimanto belt kii peninsula southwest japan accretionary complexes are considered to form through off scraping and underplating which results in the lateral and vertical growth of an accretionary prism accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process although the origin of melange is considered controversial in order to better understand the origin of melange and its role in accretion in particular during underplating we have studied the relationship between the formation of melange and duplexing in the miyama assemblage here the melange is composed dominantly of shale basalt chert and sandstone this study stresses the following features 1 the structure of the miyama assemblage appears to be a composite suite of landward and seaward dipping duplexes 2 the melange fabric displays a systematic asymmetry which may have resulted through shearing along the decollement and 3 facing of horses within duplexes changes from north to south coinciding with a change in the sense of shear inferred from the melange fabric which suggests that it formed prior to duplexing

gravitational signatures of atmospheric thermal tides on venus abstract thermal tides in the atmosphere of venus transport a considerable amount of mass on a wide range of spatial and temporal scales we use simulated values of the surface atmospheric pressure distribution to estimate the time varying gravitational potential exterior to venus the expected amplitudes of gravitational signatures of thermal tides are below the errors from existing magellan mission doppler data and have not yet been detected however they are large enough that we expect them to be well resolved by any of several measurement configurations for future venus gravity missions

mesoscale dynamics in the palm sunday tornado outbreak abstract radar and satellite imagery suggest that strong mesoscale forcing occurred in the palm sunday tornado outbreak on 27 march 1994 parallel lines of severe thunderstorms within each of three mesoscale convective systems developed just ahead of a cold front in mississippi and alabama on this date analyses of routine meteorological observations barograph data and forecasts from the eta and ngm models and a mesoscale research model mass are used to examine the relative roles of large scale dynamics and mesoscale processes in triggering and organizing the mesoscale convection quasigeostrophic forcing was absent in the outbreak region likewise a thermally direct circulation system transverse to the upper level jet that was present to the northwest of the outbreak region was decoupled from the strong low level ascent occurring in northern alabama and mississippi at the time of the outbreak strong ageostrophic frontogenesis in the presence of conditional symmetric instability csi was the chief

large scale variscan shearing at the southeastern margin of the eastern rhenohercynian belt a reinterpretation of chaotic rock fabrics in the harz mountains germany for the past 50 years large parts of the harz mountains eastern rhenohercynian zone germany have been considered to consist of olistostromes of early carboniferous age which were later deformed by variscan tectonics the olistostrome model was associated with the assumption that sliding of coherent nappe units occurred contemporaneously with the submarine mass flow processes a sedimentary origin of olistostromes was derived mainly from the block in matrix fabric of the rocks assumed soft rock deformation fabrics and the composition and age of blocks based on available biostratigraphic data block composition and fabric studies of outcrops and drilling cores it can be shown that the fabric of chaotic rocks is mainly caused by variscan tectonic deformation at late carboniferous without any indication of soft rock deformation processes and structures indicating a tectonic origin are boudinage extensive cataclasis and transposition of folded rocks commonly associated with intense mineralization as well as the regular spatial distribution and orientation patterns of blocks the chaotic rock fabric mainly evolved in the brittle ductile transitional zone late to post kinematic relative to the synmetamorphic first cleavage widespread chaotic rocks are localized in the allochthonous domain which itself is characterized by generally strong tectonic rock deformation and thrust faulting our results suggest that significant mass flow deposits do not occur the previously proposed olistostromes are melanges and broken formations of tectonic origin which form a large scale network of anastomosing shear zones that penetrate and juxtapose more or less intact coherent rock units of different stratigraphy and provenance due to the tectonic origin the entire early paleozoic sequences may exhibit chaotic rock fabrics

development of arc continent collision melanges linking onshore geological and offshore geophysical observations of the pliocene lichi melange southern taiwan and northern luzon arc western pacific abstract although the lichi melange has been studied intensively over the last decade its enigmatic nature has generated debates regarding its origin and evolution two prominent models have been proposed to explain the lichi melange the olistostrome model with slumping sediments and the tectonic collision model with intensive shearing neither model can explain what causes the interpreted slumping and complex faulting processes for the sediments here we study the lichi melange using a time space equivalence approach in which the tectonics of the offshore continuation of the lichi melange should represent an earlier stage in its evolution our study of marine multichannel seismic data suggested that the backthrusts in the accretionary prism propagate arcward above and within the deforming forearc and arc basement to incorporate the sediment and basement materials into the rear of the accretionary prism therefore we proposed the retrowedge evolution model whose novel key feature is that the irregular topography of the arc basement affects the taper angle of the retrowedge the retrowedge has a greater taper than the minimum taper of the prowedge so the slope can be steeper and thus favors gravitational failure such processes would generate complex faulting and slumping processes in the backthrusted forearc ridge and basin in sum this retrowedge model reconciles the mixture of slumping and faulting processes found in the lichi melange

early exhumation of underthrust units near the toe of an ancient erosive subduction zone a case study from the northern apennines of italy apatite fission track aft analyses were performed on 16 sandstone samples from a tectonic melange unit exposed in three tectonic windows located near the inferred front of the early miocene subduction system of the northern apennines of italy the tectonic windows display a block on block tectonic melange present under the ligurian units the melange is formed by portions of the upper plate incorporated in the plate boundary shear zone as a consequence of a mechanism of frontal tectonic erosion during the aquitanian early miocene aft and structural data together with stratigraphic constraints allow the reconstruction of a complete deformation cycle with a phase of underthrusting followed by underplating and early exhumation of the tectonic melange the exhumation in particular took place at 10 20 km from the original subduction front moreover the analysis suggests that multiple faults were active at the same time in the frontal part of the subduction zone

a case study of gravity waves convective storms interaction 9 may 1979 abstract an analysis is presented of a series of severe storms which occurred in the north central united states on 9 may 1979 and whose spatial distribution and movement correlate well with observed gravity waves two gravity wave trains of 2 1 3 mb amplitude 2 5 3 3 h period and 240 265 km horizontal wavelength were isolated through power spectra analysis and cross correlation techniques applied to national weather service barograph traces the wave trains propagated in the 200 direction which coincided with the jet axis with a phase velocity of 20 30 m s 1c and within a 300 km wide band the storms were identified on enhanced infrared goes satellite pictures with the help of radar summaries these convective systems initially developed in nebraska and propagated north northeast at 25 m s 1 revealing wave like characteristics with a separation of 300 400 km the convective systems were closely linked to the observed wave trains with cell intensity height and associated rainfall maximized at the wa

topographical and local time dependence of large stationary gravity waves observed at the cloud top of venus the existence of large stationary gravity waves was discovered during akatsuki s first observation sequence in 2015 in this study the further detection of large stationary gravity waves in brightness temperature images over a 1 5 year period is reported the waves periodically appeared mostly above four specific highland regions in the low latitudes when these regions were in the local afternoon the wave amplitudes attenuated after the wave locations passed beyond the evening terminator and the locations of the waves tended to slowly drift eastward over their lifetimes the appearances of stationary waves depend not only on surface topography but also on latitude and local time suggesting that solar heating during the daytime and atmospheric structure affected by solar heating may control the excitation and propagation of stationary waves

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

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

sensitivity of thermal tides in the venus atmosphere to basic zonal flow and newtonian cooling 1 a numerical model is constructed to investigate sensitivity of thermal tides in the venus atmosphere to distributions of basic mean zonal wind and newtonian cooling rate it is found that the amplitude and spatial structure of the diurnal tide are strongly influenced by the midlatitudinal jets and the newtonian cooling rates in all cases examined in this study the diurnal tide propagates downward in the lower atmosphere below the cloud levels and reaches the ground the semidiurnal tide is generally less dependent on these parameters but its zonal winds are increased in the upper jet regions

structural evolution of the rides prerifaines morocco structural and seismic interpretation and analogue modelling experiments the rides prerifaines rp of morocco constitute the leading edge of the rif chain they involve a triassic palaeocene succession deposited on a peneplained palaeozoic fold belt and accumulated in basins delimited by ne sw trending normal fault systems a significant hiatus separates an overlying middle miocene upper miocene foredeep sequence the reconstruction of the complex structural evolution of the rp during the later compressive phases that affected the rif chain since middle miocene time has been the aim of this paper we integrated field structural analyses seismic line interpretation and analogue modelling in order to evaluate the control exerted by the late triassic jurassic normal fault systems onto the later compressive tectonics the maximum compression direction associated with the first compressive phase is roughly ne sw to ene wsw oriented during this phase the mesozoic basin fill was scooped out from the graben and the main decollement level were the triassic evaporites since pliocene times the maximum compression direction was oriented roughly n s during this phase the rp assumed their present structural setting the earlier normal faults delimiting the mesozoic graben were reactivated in a strike slip mode also involving the palaeozoic basement the analogue modelling experiments demonstrated that the basement reactivation promoted salt tectonics and favoured fluid circulation

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

using 3d kinematic models in subduction channels the case of the chanaral tectonic melange coastal cordillera northern chile abstract the chanaral tectonic melange northern chile is a local unit within the late paleozoic accretionary complex formed at the southwestern margin of gondwana the structural characteristics of the studied melange were mostly developed during a first deformation phase d1 and include a block in matrix fabric lineations l1 and foliations s1 tight to intrafoliar folds s c and s c c composite planar fabrics and a conspicuous spatial separation of domains with predominantly linear and linear planar fabrics folding during a second stage d2 modified the orientation of the previous fabrics and structures the eastern boundary of the chanaral melange is n s to nnw sse oriented moderately dipping to the east its western boundary is not exposed the plane showing the maximum structural asymmetry the vorticity normal section is ene wsw directed and sub vertical kinematic criteria consistently reveal top to the wsw displacement a kinematic model of triclinic transpression with inclined extrusion has been applied to evaluate the d1 structural features of the chanaral melange the pitch of the simple shear direction on the deformation plane ranged from 60 n to 90 the pitch of the estimated extrusion direction was of 30 40 s the coaxial component was clearly constrictional logarithmic k value of 2 to 5 the vorticity number has not been constrained by the model but its spatial variation can explain the domainal distribution of the fabrics in the melange the simulated particle paths show the predominance of material displacement parallel to the margin with low to moderate down dip displacements which is in accordance with the low pressure metamorphic assemblages found in the melange the convergence direction between the blocks separated by the melange unit was n50 60 e kinematic blocking of the chanaral melange probably related to the accretion of an oceanic volcanic domain allowed the d2 folding of the previous structures a process that at least initially proceeded without a change in the convergence direction

cloud tracked winds from pioneer venus ocpp images abstract analysis of ultraviolet image sequences obtained from the pioneer venus orbiter cloud photopolarimeter and covering five 80 day periods from 1979 1985 provides the first climatological description of the cloud top circulation on venus the average zonal winds can be characterized as a 5 day retrograde rotation of the whole cloud level atmosphere with weak jets at middle to high latitudes both the midlatitude and equatorial zonal winds vary by about 5 8 m s 1 over time spans of 1 6 years the average meridional circulation is poleward in both hemispheres up to at least 60 latitude consistent with the presence of a thermally direct hadley circulation associated with the clouds the strength of the hadley circulation also varies with time four wave modes are clearly identified a diurnal solar tide a semi diurnal solar tide a 4 day equatorial wave and a 5 day midlatitude wave the semidiurnal tide appears to have an amplitude of about 5 m s 1 and to be approximately constant with time

from seamount accretion to tectonic erosion formation of osa melange and the effects of cocos ridge subduction in southern costa rica 1 the costa rica portion of the middle america trench mat is characterized by active tectonic erosion a process that causes the removal of material from the base of the upper plate as the plate boundary migrates upward offshore studies demonstrate accelerated subduction erosion starting at the miocene pliocene boundary as the result of subduction of thickened galapagos related crust as presently represented by the cocos ridge the subduction of the cocos ridge also caused uplift and exposure of the outer forearc on the osa peninsula which offers a window to explore the tectonic evolution of the area the rocks outcropping on osa peninsula are a middle eocene middle miocene melange dominated by basalt chert and limestone resulting from accretion of seamounts the accretion dominated period of the mat evolution ended at the miocene pliocene boundary when thickened crust a paleo cocos ridge produced at the galapagos hot spot arrived at the trench the thick crust caused uplift and severe tectonic erosion of the accretionary edifice allowing exhumation of the osa melange the change from accretion to erosion caused the outer forearc to be offset along subvertical faults that define small kilometer to tens of kilometers size blocks that are going through differential vertical movements in response to the morphology of the subducting ridge subduction accretion and erosion are two processes that can alternate in time and space or coexist along the same margin so that mass removal can develop on a previously growing margin and completely remove an accretionary prism

structural anatomy of the ligurian accretionary wedge monferrato nw italy and evolution of superposed melanges we document in this study the internal structure of the late cretaceous late oligocene ligurian accretionary wedge in northwestern italy and the occurrence in this exhumed wedge of broken formation and three different types of melanges that formed sequentially through time the broken formation is the oldest unit in the accretionary wedge and shows bedding parallel boudinage structures which developed as a result of layer parallel extension at the toe of the internal part of the alpine wedge front during the late cretaceous middle eocene this broken formation experienced an overprint of tectonic diapiric and sedimentary processes as a result of continental collision in the late oligocene the ne vergent thrusting and associated shortening produced a structurally ordered block in matrix fabric through mixing of both native and exotic blocks forming the tectonic melange the concentration of overpressurized fluids along the thrust fault planes triggered the upward rise of shaly material producing the diapiric melange which in turn provided the source material for the downslope emplacement of the youngest late oligocene sedimentary melange the sedimentary melange units unconformably cover the collisional thrust faults constraining the timing of both this episode of contractional deformation related to continental collision and the combination and overlap of tectonic diapiric and sedimentary processes our multiscale structural analysis of the ligurian accretionary wedge shows that tectonic diapiric and sedimentary processes played a significant role in its evolution and that the interplay between and the superposition of these different processes strongly controlled the dynamic equilibrium of the accretionary wedge in the nw apennines western alps this kind of polygenetic melange development may be common in many modern and ancient accretionary complexes and the processes involved in their formation are likely to be responsible for major tsunamic events in convergent margins

structural evolution of underthrusted sediments kodiak islands alaska the late cretaceous to early tertiary sediments of the kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin because of the thick trench fill this early deformation resulted in the formation of two distinct structural terranes 1 coherent terranes of layered turbidites and 2 melange terranes the earliest deformation d1 in the coherent terranes is marked by normal faults mineralized layer perpendicular veins in sandstones and microfaults in shales these features suggest that 1 was vertical and that 3 was horizontal and nw trending during d1 we interpret d1 structures as hydrofractures that formed in flat lying sediments with high fluid pressures caused by rapid tectonic loading in contrast the melanges appear to record a more protracted and complex history that culminated in structures similar to the d1 structures above we have recognized three successive transitional stages in the melanges i breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix resulting in the blocks in matrix character of the melange ii development of ductile shear bands that crosscut the melange fabric at a low angle and iii hydrofracturing with the formation of calcite and quartz filled veins in sandstone inclusions and microfaults in the shales the sequence of microstructures associated with stages i iii records increased cohesion of the sediments during deformation dominated by layer parallel shear intense mineralization and microfaulting during stage iii suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability stage iii structures are texturally indistinguishable and contemporaneous with the less pervasive d1 structures in the coherent terranes based on these observations we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement the coherent terranes represent the structurally and stratigraphically lowest sediments and they bypassed the stage i and ii deformations of the melanges because they were the greatest distance from the decollement when they were deformed moreover both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism