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

sandstone matrix olistostrome deposited on intra subduction complex serpentinite franciscan complex western marin county california abstract detailed field mapping in the franciscan complex in western marin county california offers insight into the origins and development of melanges as well as accretionary complex architecture from structurally high to low the study area units comprise a structurally high sandstone matrix olistostrome serpentinite a structurally intermediate sandstone matrix olistostrome early cretaceous nicasio reservoir terrane greenstone and coherent cretaceous greywacke all exhibiting prehnite pumpellyite burial metamorphism the structurally high sandstone matrix olistostrome includes volcanic blocks up to 700 m in diameter and chert blocks up to 100 m in diameter whereas the structurally intermediate olistostrome unit comprises a lithic rich sandstone matrix enclosing volcanic blocks 1 20 m in diameter the youngest detrital zircons in the high olistostrome and the coherent greywacke have u pb ages of 20 m y older than the inferred depositional ages of the units themselves suggesting that the bulk of the detritus comprising these units was derived by exposure and sedimentary recycling of significantly older previously accreted sediments the matrix of both olistostrome units exhibits minimal strain with weak local development of pressure solution cleavage these findings demonstrate introduction of franciscan exotic blocks and development of franciscan block in matrix fabrics through sedimentary sliding and support sedimentary interpretations for more highly strained shale and serpentinite matrix melanges additionally these data suggest that sandstone matrix olistostromes melange may constitute a larger part of the franciscan and other accretionary complexes than previously believed the structurally high olistostrome preserves a depositional contact on serpentinite verifying an intra subduction complex origin for the serpentinite the serpentinite lacks block in matrix structure and likely represents a tectonically emplaced unit derived from the subducting plate as a coherent nappe rather than a sedimentary serpentinite deposit multi kilometer scale west vergent overturned folds with subhorizontal fold axes deform the nappes of the field area and a later generation of open northeast trending folds overprints the earlier folds

ophiolite melange zone records exhumation in a fossil subduction channel recent models propose that the exhumation of high pressure rocks occurs by means of return flow inside a low viscosity channel of serpentinite situated between the plates to test this hypothesis we investigated a serpentinite melange in the western alps which contains exotic mafic and metasedimentary tectonic blocks recording heterogeneous metamorphic evolutions and variable high pressure ages the peak metamorphic conditions range from eclogite to garnet blueschist facies the structural evidence and the pressure temperature paths of the different blocks suggest coupling between blocks and matrix at least in the blue schist facies 39 ar 40 ar dating indicates eclogite facies peak at ca 43 ma and blueschist facies peak at ca 43 and 40 ma in different blocks respectively these data point to diachronous metamorphic paths resulting from independent tectonic evolutions of the different slices we therefore propose that this melange formed during exhumation of subducted rocks equilibrated at different depths inside a subduction channel this mechanism can be extended to other serpentinite melanges in the alps and other orogens e g the cyclades the coast ranges of california for which a growing heterogeneity in the timing of metamorphic equilibration and of pressure temperature paths can be expected with further investigations

the troodos cyprus and kizildag s turkey ophiolites as structural models for slow spreading ridge segments the troodos cyprus and kizildag southern turkey ophiolites in the eastern mediterranean region represent remnants of the neotethyan ocean which evolved as a result of seafloor spreading north of afro arabia in mesozoic time both ophiolites display numerous planar to listric normal faults and structural grabens in their sheeted dike complexes suggestive of tectonic extension and complex intrusive relations in their plutonic units indicate recurring and intermittent magmatic activities detachment surfaces within the lower crustal sequence troodos or at the boundary between the crustal and mantle sequences kizildag define a brittle plastic transition zone along which tectonic thinning and extension within the crustal units above was accommodated by mylonitization and denudation in lower crustal units or mantle rocks below the variation in depth to these accommodation zones is inferred to be a function of the vertical temperature gradient in the oceanic lithosphere which may in turn point to the

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

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

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

ophiolites in earth history introduction ophiolites record significant evidence for tectonic and magmatic processes from rift drift through accrefionary and collisional stages of continental margin evolution in various tectonic settings structural petrological and geochemical features of ophiolites and associated rock units provide essential information on mantle flow field effects including plume activities collision induced aesthenospheric extrusion crustal growth via magmatism and tectonic accretion in subductionaccretion cycles changes in the structure and composition of the crust and mantle reservoirs through time and evolution of global geochemical cycles and seawater compositions ophiolite studies over the years have played a major role in better understanding of mid ocean ridge and subduction zone processes mantle dynanlics and heterogeneity magma chamber processes fluid flow mechanisms and fluid rock interactions in oceanic lithosphere the evolution of deep biosphere the role of plate tectonics and plume tectonics in crustal evolution during the precambrian and the phanerozoic and mechanisms of continental growth in accretionary and coltisional mountain belts through multi disciplinary investigations and comparative studies of ophiolites and modern oceanic crust and using advanced instrumentation and computational facilities the international ophiolite community has gathered a wealth of new data and syntheses from ophiolites around the world during the last 10 years the purpose of this book is to present the most recent data observations and ideas on different aspects of ophiolite science through case studies and to document the mode and nature of igneous metamorphic tectonic sedimentological and or biological processes associated with the evolution of oceanic crust in different tectonic settings in earth s history it comprises 32 papers collected in six sections on temporal relations anaongst ophiolite genesis mantle plume events and orogeny in earth history tethyan ophiolites in the alpinehimalayan orogenic system magmatic metamorphic and tectonic processes in ophiolite genesis hydrothermal and biogenic alteration of oceanic crust mechanisms of ophiolite emplacement and regional occurrences of ophiolites and their geodynamic implications

melange diapirs into the cambria slab a franciscan trench slope basin near cambria california near cambria california a franciscan trench slope basin of late cretaceous age is intruded by two diapirs of blueschist block bearing mud matrix melange the rise of the diapirs caused major upturning of the bedded strata within a few hundred meters of the contact and complex interfingering of mud matrix melange with basin sediments near the contacts at one locality a fragment of basin sandstone is observed partially detached from the wall rock the mud matrix melange is highly mobile and the intrusive character during diapirism is clearly analogous to that of magma in igneous intrusions diapirism may be responsible for large scale intercalation of slope and slope basin sediments with subduction generated melange at actively convergent plate margins

seismic versus sonic velocities a vertical seismic profiling study vertical seismic profiling vsp techniques provide a method to measure accurately the seismic velocity and lithologic structure near the borehole the analysis of a vsp survey can also provide insight into seismic wave propagation especially when related to sonic measurements but vsp and sonic log velocities or traveltimes are often found to disagree recent field evidence of these differences suggests that the vsp traveltimes are delayed with respect to the integrated sonic times especially in the deep section 3000 ft by about 2 0 ms 1000 ft on the average the vsp has numerous applications in exploration geophysics such as calibrating the sonic log it is thus important to understand why the two measurements differ differences in the geometries source frequencies and instrumental errors of the two surveys are reviewed more detailed analysis of seismic wave propagation in the vsp shows that short path multiples and velocity dispersion can have a significant delaying effect on the seismic tr

the giant ruatoria debris avalanche on the northern hikurangi margin new zealand result of oblique seamount subduction despite convergent margins being unstable systems most reports of huge submarine slope failure have come from oceanic volcanoes and passive margins swath bathymetry and seismic profiles of the northern hikurangi subduction system new zealand show a tapering 65 30 km wide by 65 km deep margin indentation with a giant 3150 630 km3 blocky debris avalanche deposit projecting 40 km out across horizontal trench fill and a debris flow deposit projecting over 100 km slide blocks are well bedded up to 18 km across and 1 2 km high the largest being at the avalanche deposit s leading edge samples dredged from them are mainly miocene shelf calc mudstones similar to those outcropping around the indentation cores from cover beds suggest that failure occurred 170 40 ka possibly synchronously with a major extension collapse in the upper indentation however the northern part of the indentation is much older the steep straight northern wall is close to the direction of plate convergence and probably formed around 2 0 0 16 ma as a large seamount subducted leaving in its wake a deep groove obliquely across the margin and an unstable triangle of fractured rock in the 60 angle between groove and oversteepened margin front the triangle collapsed as a blocky avalanche leaving a scalloped southern wall and probably causing a large tsunami tentative calculations of compacted volumes suggest that the indentation is over 600 km3 larger than the avalanche supporting a two stage origin that includes subduction erosion since failure convergence has carried the deposits 9 km back toward the margin causing internal compression the eventual subduction accretion of the ruatoria avalanche explains the scarcity of such features on active margins and perhaps the nature of olistostromes in fold belts

fronts and shelf circulation models satellite imagery shows that fronts and frontal eddies are widespread on the northwest european continental shelf the implications for the numerical modelling of transports for example of pollutants are discussed a brief review of some models of shelf circulation is given it is argued that to include fronts in models of shelf circulation requires a better understanding of dynamics on the frontal scale a three dimensional numerical model of eddy formation in a coastal front is then presented that reproduces many of the observed features

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

growing folds and sedimentation of the gosau group muttekopf northern calcareous alps austria analysis of the three dimensional geometry of upper cretaceous clastics in the muttekopf area northern calcareous alps austria indicate fold and fault structures active during deposition coniacian continental to neritic sedimentation lower gosau subgroup was contemporaneous with displacements on nw trending faults and minor folding along ne trending axes from the santonian onwards sedimentation of the deep marine upper gosau subgroup the nw trending faults were sealed and large folds with wsw trending axes developed the direction of contraction changed to n s after the end of gosau deposition in the danian paleocene synorogenic sedimentation patterns indicate continuous contraction from the coniacian to the late maastrichtian danian therefore large scale extension as observed in the central part of the eastern alps cannot be documented in the western parts of the northern calcareous alps a combination of subduction tectonic erosion for the frontal parts and gravitational adjustment of an unstable orogen after nappe stacking for the internal parts possibly accounts for the different development of gosau basins in the frontal and trailing regions of the austroalpine wedge

magmatic origin of low t mafic blueschist and greenstone blocks from the franciscan melange san simeon california abstract the franciscan melange exposure near san simeon contains abundant greenstone and minor blueschist blocks that were tectonically boudinaged while encased in the shale matrix tectonic deformation of the blueschists is evident from variable amounts of cataclastic flow along their margins and in pinched tails major trace and rare earth elemental analyses indicate that blueschist and greenstone blocks in this area of the melange were derived from sources with morb like composition along with some having trace element and ree patterns similar to oib compositions most blocks are low lree basalts that probably formed in an open ocean ridge setting but some blocks have high lree contents similar to off axis seamounts linear trends of incompatible elements for both blueschist and greenstone blocks indicate that both lithologies were probably derived from a similar variably fractionated tholeiitic magma blueschist blocks with sodic amphibole lawsonite epidote were pervasively recrystallized at 300 350 c and foliated during ductile deformation that included folding their protolith can only be identified as mafic a few blocks contain very small amounts of metasedimentary materials indicating some were probably seafloor basalts but some may have been diabase or even gabbro where interlayered sediment was present the mafic protolith was enriched in k rb and na greenstones on the other hand contain abundant pseudomorphic evidence of magmatic textures alteration to albite chlorite and pumpellyite at temperatures of 100 200 c is intense especially in cataclastic margins and pinched tails some of the basaltic greenstones have attached radiolarian chert and a few have relict diabasic textures the mafic blueschists and greenstones in the melange near san simeon are probably fragments from the uppermost part of the farallon plate the blueschists may be mostly mafic slabs uprooted from the subducting crust underplated to the base of the north american plate and later detached and carried back towards the surface in upwelling melange the greenstones are most likely fragments detached from seamounts subducted during times of melange upwelling

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

the origin of melanges cautionary tales from indonesia abstract the origin of block in matrix melanges has been the subject of intense speculation by structural and tectonic geologists working in accretionary complexes since their first recognition in the early twentieth century because of their enigmatic nature a number of important international meetings and a large number of publications have been devoted to the problem of the origin of melanges as melanges show the effects of the disruption of lithological units to form separate blocks and also apparently show the effects shearing in the scaly fabric of the matrix a tectonic origin has often been preferred then it was suggested that the disruption to form the blocks in melanges could also occur in a sedimentary environment due to the collapse of submarine fault scarps to form olistostromes upon which deformation could be superimposed tectonically subsequently it has proposed that some melanges have originated by overpressured clays rising buoyantly towards the surface incorporating blocks of the overlying rocks in mud or shale diapirs and mud volcanoes two well known examples of melanges from the banda and sunda arcs are described to which tectonic and sedimentary origins were confidently ascribed which proved on subsequent examination to have been formed due to mud diapirism in a dynamically active environment as the result of tectonism only indirectly evidence from the australian continental shelf to the south of sumba shows that large quantities of diapiric melange were generated before the diapirs were incorporated in the accretionary complex comparable diapirs can be recognised in timor accreted at an earlier stage evidence from both timor and nias shows that diapiric melange can be generated well after the initial accretion process was completed the problem is why when diapirism is so abundantly found in present convergent margins is it so rarely reported from older orogenic belts many occurrences of melanges throughout the world to which tectonic and or sedimentary and origins have been ascribed may in future investigations prove to have had a diapiric origin it is emphasised that although the examples of diapiric melange described here may contain ophiolitic blocks they were developed in shelf or continental margin environments and do not contain blocks of high grade metamorphic rocks in a serpentinous matrix such melanges originate diapirically during subduction in a mantle environment as previous authors have suggested

formation of taconic melanges and broken formations in the hamburg klippe central appalachian orogenic belt eastern pennsylvania abstract the hamburg klippe of the central appalachian orogenic belt exposed in eastern pennsylvania displays a complex record of poly phase melange and broken formation development in a convergent margin setting it includes an imbricate stack of tectonic slices which consist of upper cambrian to upper ordovician deep water and continental slope sedimentary rocks emplaced by gravity sliding onto the laurentian passive margin during deposition of the upper ordovician martinsburg formation based on their internal structure and stratigraphy the block matrix ratios and relations and the inferred tectonic settings of origin we have differentiated the following melanges and broken formations whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision i sedimentary broken formations without exotic blocks formed by in situ and local down slope remobilization during the early stages of closure of the octoraro sea basin in the early ordovician ii two types of sedimentary melanges with exotic blocks formed at the front of the advancing accretionary wedge during the middle ordovician subduction of the laurentian continental margin beneath a microcontinent magmatic arc tectonic assembly iii layer parallel extensional broken formation and a diapiric melange formed in the outer trench and at the toe of the accretionary wedge respectively during the early late ordovician iv precursory olistostromes formed during the late ordovician collisional episodes as the hamburg klippe was emplaced in the martinsburg formation on the downgoing laurentian continental margin and v contractional deformation related broken formations formed at the base of main thrust faults overprinting the previously formed melanges and broken formations this sequential development of different melange types in the central appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences the structural level of melange formation within the accretionary wedge and the kinematics of deformational processes

matrix deformation mechanisms in hp lt tectonic melanges microstructural record of jadeite blueschist from the franciscan complex california abstract exhumed high pressure low temperature metamorphic melanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction rigid blocks of widely varying size are embedded in a weak matrix which takes up the deformation and controls the rheology of the composite the microfabrics of a highly deformed jadeite blueschist from the franciscan complex california are investigated to help understand deformation mechanisms at depth the specimen shows a transposed foliation with dismembered fold hinges and boudinage structures several generations of open fractures have been sealed to become veins at high pressure metamorphic conditions the shape of these veins frequently restricted to specific layers indicates distributed host rock deformation during and after sealing small cracks in jadeite and lawsonite are healed with tiny quartz inclusions aligned along the former fracture surface large jadeite porphyroblasts show strain caps and strain shadows open fractures are sealed by quartz and new jadeite epitactically grown on the broken host monophase glaucophane aggregates consist of undeformed needles with a diameter between 0 1 and 2 m grown after formation of isoclinal folds only quartz microfabrics indicate some stage of crystal plastic deformation followed by annealing and grain growth aragonite in the latest vein generation shows retrogression to calcite along its rims the entire deformation happened under hp lt metamorphic conditions in the stability field of jadeite and quartz at temperatures between 300 and 450 c and pressures exceeding 1 1 4 gpa the microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism accompanied by brittle failure and vein formation at quasi lithostatic pore fluid pressure this indicates low flow strength and combined with high strain rates expected for localized deformation between the plates a very low viscosity of material in the interplate shear zone at a depth 30 45 km

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

the thickness of subduction plate boundary faults from the seafloor into the seismogenic zone the thickness of an active plate boundary fault is an important parameter for understanding the strength and spatial heterogeneity of fault behavior we have compiled direct measurements of the thickness of subduction thrust faults from active and ancient examples observed by ocean drilling and fi eld studies in accretionary wedges we describe a general geometric model for subduction thrust decollements which includes multiple simultaneously active anastomosing fault strands tens of meters thick the total thickness encompassing all simultaneously active strands increases to 100 350 m at 1 2 km below seafl oor and this thickness is maintained down to a depth of 15 km thin sharp faults representing earthquake slip surfaces or other discrete slip events are found within and along the edges of the tens ofmeters thick fault strands although fl attening primary inherited chaotic fabrics and fault migration through subducting sediments or the frontal prism may build melange sections that are much thicker to several kilometers this thickness does not describe the active fault at any depth these observations suggest that models should treat the subduction thrust plate boundary fault as 1 20 cm thick during earthquakes with a concentration of postseismic and interseismic creep in single to several strands 5 35 m thick with lesser distributed interseismic deformation in stratally disrupted rocks surrounding the fault strands

role of late jurassic intra oceanic structural inheritance in the alpine tectonic evolution of the monviso meta ophiolite complex western alps

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

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

diverse sources for igneous blocks in franciscan melanges california coast ranges igneous blocks in franciscan melanges are of three chemical petrologic types 1 tholeiitic basalts of both arc and spreading center origin with depletions in light relative to heavy rare earth elements 3 tio 2 1 high y zr and y ti ratios and relict augites that generally have low al and ti and well defined iron enrichment trends 2 basalts of probable seamount origin with marked enrichments in light relative to heavy rare earth elements 5 tio 2 1 lower y zr and y ti than 1 and ti al rich augites showing little if any iron enrichment trends and 3 hypabyssal intrusives having sio 2 52 tio 2 1 flat or only slightly fractionated rare earth abundance patterns and diopsidic augites that are very low in ti and al and show no iron enrichment trends all of the blocks are metamorphosed most are undeformed pumpellyite bearing greenstones and a few contain sodic amphibole lawsonite sodic pyroxene the melanges are probably olistostromal in origin deriving the

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

significance of mud volcanism 1 mud volcanism and diapirism have puzzled geoscientists for 2 centuries they have been described onshore and offshore in many places on earth and although they occur in various tectonic settings the majority of the features known to date are located in compressional tectonic scenarios this paper summarizes the main thrusts in mud volcano research as well as the various regions in which mud volcanism has been described mud volcanoes show variable geometry up to tens of kilometers in diameter and several hundred meters in height and a great diversity regarding the origin of the fluid and solid phases gas predominantly methane water and mud may be mobilized at subbottom depth of only a few meters but in places can originate from several kilometers depth with minor crustal or mantle input the possible contribution of mud extrusion to global budgets both from quiescent fluid emission and from the extrusive processes themselves is important in regions where mud volcanoes are abundant such as the collision zones between africa and eurasia fluid flux through mud extrusion exceeds the compaction driven pore fluid expulsion of the accretionary wedge also quiescent degassing of mud volcanoes may contribute significantly to volatile budgets and hence to greenhouse climate

tectonic significance of low temperature blueschist blocks in the franciscan melange at san simeon california abstract the blueschist bearing franciscan melange exposed near san simeon is very similar to the melange terrane forming the central belt north of san francisco epidote bearing low t blueschist blocks at san simeon n 3 are the same age as high t garnet bearing blocks in the northern and diablo range melange terranes 155 150 ma this age approximates the time of initiation of franciscan subduction during the initial stages of franciscan subduction part of the top volcanic sequence from the subducting oceanic plate and small volumes of overlying sediments were imbricated at the bottom of the overriding plate and metamorphosed under high p conditions this underplated material was variably retrograded as subduction refrigerated the forearc blocks actinolitic rinds formed at the contacts between blueschists and intercalated mantle under relatively static conditions as the base of the overriding plate became serpentinized a model is proposed in which exhumation was facilitated by gravity driven seaward oriented extensional thinning of the forearc prism caused by the decrease in subducting plate dip starting around 80 ma that caused the laramide orogeny as this occurred blueschist and graphite schist blocks were plucked from the bottom of the hanging wall incorporated into the shale and water rich shear zone at the plate interface subduction channel shear zone and exhumed during the upward flow of melange driven by the movement of the downgoing plate while subduction was still active

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

a giant catastrophic mud and debris flow in the miocene makran we challenge the former interpretation of the sedimentary me lange of the makran accretionary complex as a tectonic me lange diapirically emplaced from below and provide evidence for its sedimentary gravitational emplacement from the north during tortonianmessinian times between 11 8 and 5 8 ma it is an olistostrome that includes blocks of ophiolites and oceanic sediments derived from the coloured me lange to the north and reworked chunks of the turbidites on which it rests with an erosional truncation the chaotic scattering of blocks of any size and lithology and the weak soft sediment deformation of the matrix argue against a tectonic emplacement of the chaotic formation its size and internal structure together with the size of the individual blocks make the olistostrome a fossil equivalent of the large gravitationally emplaced debris flows observed along present day continental margins and unstable volcanic edifices

diapirism and structural thickening in an early palaeozoic subduction complex southeastern new south wales australia abstract a major problem in the study of subduction complexes has been to distinguish between processes of mud diapirism deformation of unlithified sediments frontal accretion and underplating in the batemans bay area cambro ordovician rocks of the south coast of new south wales eastern australia contain common mud rich melanges that occur amongst an imbricated succession of early to middle ordovician quartz turbidites late ordovician black mudstone and chert cambrian limestone and mafic volcanics an early set of fluidal obliquely trending folds are restricted to the late ordovician black mudstone and chert and probably formed by gravitational down slope movement on a lower trench slope features indicative of deformation of unlithified sediments are still preserved and include mud injections delicate small scale faulting and the absence of grain scale deformation tight to isoclinal north south folds with axial planar west dipping cleavage overprint all units and formed during the younger main phase deformation that post dated early deformation in the prism the melange has been strongly overprinted by this main phase deformation early melange formation is attributed to mud diapirism involving intrusion along early accretionary faults bedding planes and in pipe like features resulting in chaotic outcrop and map patterns

extensional tectonics and gravitational collapse in an ordovician passive margin the western argentine precordillera abstract the paper describes ubiquitous extensional structures developed in a passive margin of ordovician age in the argentine precordillera these extensional structures include normal faults and boudinaged sequences in some places the boudinage reaches very high extension values giving rise to block in matrix formations most of these extensional structures developed when sediments were not well lithified as recorded by hydroplastic fractures slump folds and pinch and swell structures the presence of slump folds coeval with the extensional deformation the variable extension directions obtained from the kinematic analysis and a weak cleavage recording layer perpendicular shortening support the interpretation that gravitational collapse related to submarine sliding was the cause for extensional deformation well consolidated rocks located at the lower part of the stratigraphic sequence also display scarce extensional faults these extensional faults predate folding because they were breached by flexural slip faults and as a result of their passive rotation in fold limbs these initial normal faults may now appear as reverse faults particularly in steep and overturned limbs the truncation of extensional faults by flexural slip faults produces typical bed thickness changes across the extensional faults giving rise to apparent synsedimentary faults these normal faults can be attributed to the crustal extension that generated the passive continental margin or may represent deep parts of faults related to gravitational collapse

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

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

geodynamic evolution of ophiolites from albania and greece dinaric hellenic belt one two or more oceanic basins all the geological constraints for an exhaustive reconstruction of the triassic to tertiary tectonic history of the southern dinaric hellenic belt can be found in albania and greece this article aims to schematically reconstruct this long tectonic evolution primarily based on a detailed analysis of the tectonic setting the stratigraphy the geochemistry and the age of the ophiolites in contrast to what was previously reported in the literature we propose a new subdivision on a regional scale of the ophiolite complexes cropping out in albania and greece this new subdivision includes six types of ophiolite occurrences each corresponding to different tectonic units derived from a single obducted sheet these units are represented by 1 sub ophiolite melange 2 triassic ocean floor ophiolites 3 metamorphic soles 4 jurassic fore arc ophiolites 5 jurassic intra oceanic arc ophiolites and 6 jurassic back arc basin ophiolites the overall features of these ophiolites are coherent with the existence of a single though composite oceanic basin located east of the adria pelagonian continental margin this oceanic basin was originated during the middle triassic and was subsequently early jurassic affected by an east dipping intra oceanic subduction this subduction was responsible for the birth of intra oceanic arc and back arc oceanic basins separated by a continental volcanic arc during the early to middle jurassic from the uppermost middle jurassic to the early cretaceous an obduction developed during which the ophiolites were thrust westwards firstly onto the neighboring oceanic lithosphere and then onto the adria margin

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

mechanisms of subduction accretion as implied from the broken formations in the apennines italy mesoscale structural analysis of the ligurian broken formations of the northern apennines and the regional occurrence of subduction related structures indicate polyphase folding as the mechanism responsible for progressive disruption the evidence for direct correlation between regionally extensive noncylindrical folds and tectonic disruption has a strong bearing on determining how the ligurian accretionary prism evolved for example the block in matrix structure of the ligurian tectonic melanges is produced through extension developed along fold limbs in a compressive stress field as expected from offscraping and intraprism deformation the model implied by the mesoscale and macroscale analyses of the ligurian tectonic melanges provides one mechanism of accretion reconciling two deformation styles i e stratal disruption and folding commonly seen in ancient accretionary prisms furthermore this model could be relevant in active accretionary prisms especially in the intraprism part where the seismic and drilling data are sparse

the role of tectonic shear strain on the illitization mechanism of mixed layers illite smectite a case study from a fault zone in the northern apennines italy the influence of tectonic strain on the diagenetic degree and illitization process of mixed layers illite smectite at shallow crustal conditions was studied for this purpose the modal composition of clay fraction and illite fwhm parameters of argillites deformed by a regional scale fault zone were studied in detail by xrd chemical analyses and by sem observations analyses were performed on deformed samples of the fault rock and compared with the non deformed rocks off the fault zone in addition this paper reports a detailed comparative analysis of deformed shear surfaces and cleavage domains and non deformed domains lithon cores of a scaly fabric in the fault rock a systematic increase in illite concentration a decrease of kubler index and fwhm 002 values and an enrichment of k ions were observed in cleavage domains with respect to the non deformed sediments off the fault zone and the lithon cores within the fault rock migration of k rich fluids along scaly cleavage domains causes progressive conversion of smectite rich i s to illite rich i s and thickening of illite crystallites along the c direction changes in mineralogical and crystallographic parameters therefore seem to be strongly controlled by shear plane development in highly sheared rocks

trench floor sedimentary sequences in a palaeozoic subduction complex eastern australia abstract late palaeozoic rocks of the new england fold belt in eastern australia preserve from west to east a continental volcanic chain a forearc basin and a subduction complex the gundahl complex and the coffs harbour sequence are two tectono stratigraphic units in the subduction complex that consist of contrasting trench floor sedimentary sequences the gundahl complex is a tectonic melange containing slabs and blocks of greywacke greywacke argillite argillite argillite tuff bedded chert and greenstone some slabs have representative sections of the pre melange stratigraphic sequence that resemble trench floor deposits other slabs have thick sequences of argillite and argillite tuff and are probably trench slope basins kneaded into the subduction complex the coffs harbour sequence is a faulted folded assemblage of turbidites the structural style regional extent and near strike parallel sediment movement directions suggest it was probably deposited in a trench probable environments of turbidite deposition are starved trench non channelized trench and outer fan submarine settings the facies differences between the two units reflect slower subduction rates and higher sedimentation rates for the coffs harbour sequence

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

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

thickening of fault zones a mechanism of melange formation in accreting sediments sediments accreted at subduction zones undergo stratal disruption and form a type of melange the thickness of the disrupted zones grows with progressive deformation this suggests that initial fault surfaces are abandoned and deformation propagates into adjacent undeformed sediment factors causing the abandonment of fault surfaces during continuing deformation include 1 strengthening owing to porosity loss during consolidation 2 localized drops in fluid pressure on fault surfaces that act as dewaterinig conduits and 3 reorientation of fault surfaces the disruptive processes occurring in accretionary prisms result principally from the deformation of a consolidating sediment mass

fabric based criteria to distinguish tectonic from sedimentary melanges in the shimanto accretionary complex yakushima island sw japan abstract fabric analyses based on the anisotropy of magnetic susceptibility ams provide a criterion with which to distinguish tectonic from sedimentary melanges in the tertiary shimanto accretionary complex yakushima island sw japan sedimentary melanges are characterized by fabric overprinting associated with compaction and diagenesis the magnetic ellipsoids for such melanges represent a cleavage bedding parallel magnetic foliation similar to that obtained for coherent sedimentary layers in nearby rocks the tectonic melanges and tectonically formed duplex structure are characterized by grain rotation whereby the grain short axis is rotated about axes oriented parallel to the intersection between s and c planes and the long axis of pencil cleavage perpendicular to the shear direction the ams data obtained for tectonic melanges and duplex structure show a less intense magnetic foliation compared with the sedimentary melanges with kmax the maximum magnetic susceptibility axes oriented parallel to the intersection between s and c planes and the long axis of pencil cleavage and kmin the minimum magnetic susceptibility axes showing a girdle distribution about kmax these contrasting paths of fabric development may reflect different stages of melange formation during the overall history of sediment deposition burial accretion and uplift sedimentary melanges are likely to be strongly compacted given that they formed soon after sedimentation in contrast tectonic melanges and duplex structure contain a shear induced fabric because they develop at a relatively late stage once the sediments have been sufficiently compacted for such a fabric to develop possibly at levels near the maximum burial depth

order from chaos a field based estimate on bulk rheology of tectonic melanges formed in subduction zones abstract many tectonic hp lt melanges exhibit block in matrix structures in contrast to the pervasively deformed matrix the blocks of various lithologies and pt histories usually appear undeformed if we accept that an hp lt melange reflects persistent high strain deformation in a deep 30 km subduction channel the record of these rock associations can provide insight into stress state rheology and flow patterns along a plate interface the bulk rheology of a block in matrix structure is controlled by a the dominant deformation mechanism of the matrix b the volume fraction of blocks engulfed in that matrix and c the geometry of the blocks the microstructures of the matrix indicate distributed deformation by dissolution precipitation creep dpc while crystal plastic deformation is subordinate dpc is grain size sensitive controlled by the type of interfaces and requires an aqueous pore fluid at quasi lithostatic pressure the rheology of a rock body undergoing deformation by dpc is believed to be newtonian viscous matrix viscosities are estimated to be on the order of 10 19 pa s or less here we explore the effect of rigid blocks embedded in a weak matrix on the bulk rheology using stereology we determine the block size distributions as well as the total block volume fractions for block in matrix structures from several regions the volume fractions are found to range from 2 to 70 with typical values between 5 and 50 using published mixing laws we model the subduction channel as a suspension of rigid spheres with a linear viscous matrix and predict that for the typical values of 5 to 50 the viscosity increase is less than one order of magnitude consequently the influence of blocks on the bulk viscosity of a subduction channel is minor instead bulk rheology is primarily controlled by the deformation behaviour of the matrix material

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

unmetamorphosed sedimentary melange with high pressure metamorphic blocks in a nascent forearc basin setting abstract melanges crop out within unmetamorphosed basal great valley group gvg forearc basin strata and between gvg and the underlying coast range ophiolite cro in the san francisco bay region of coastal california these melanges include high pressure hp metamorphic blocks of the franciscan subduction complex that structurally underlies the unmetamorphosed cro as well as blocks of gvg and cro the matrix consists of foliated shale and serpentinite locally interleaved at centimeter scale the melanges strike and dip parallel to bounding gvg sandstones and conglomerates the matrix locally consists of sedimentary breccia and conglomerate made up of clasts of serpentinite and shale gvg sandstones within and bounding the melanges have detrital serpentinite clasts the field relationships indicate a sedimentary origin of the melanges as olistostromal deposits within latest jurassic to earliest cretaceous basal gvg the melanges correlate to units along the eastern margin of the northern coast ranges about 250 km to the north with restoration of post subduction dextral faulting and differ from the latter in the higher proportion of shale stronger matrix foliation and common occurrence of hp blocks similar units may have mistakenly been assigned to the franciscan owing to the foliated nature of the matrix and occurrence of hp blocks this and the broad distribution of localities indicate that these deposits are more widespread than previously believed exhumation rates of coarse grained hp melange blocks may have been 2 to 10 mm yr or higher based on the ages of similar blocks in the coast ranges burial depth of the blocks and depositional age of the enclosing strata exhumation and deposition of franciscan blocks in these gvg melanges predates preserved accretion of similar materials in the adjacent franciscan by at least 30 m y suggesting subduction erosion of previously accreted material or exhumation of the blocks in forearc serpentinite shale diapirs

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

neoproterozoic to early cambrian franciscan type melanges in the tepla barrandian unit bohemian massif evidence of modern style accretionary processes along the cadomian active margin of gondwana abstract melanges have been reported from a variety of modern tectonic settings and are a hallmark of the franciscan type subduction accretion complexes in the precambrian however well documented examples of melanges become extremely scarce this paper describes in detail neoproterozoic to early cambrian melanges recently recognized in the cadomian basement of the tepla barrandian unit bohemian massif the melanges are of sedimentary and tectonic origin and include voluminous successions of graywackes interpreted to represent mass transport deposits presumably along the upper slope of an accretionary wedge the clast compositions indicate multiple episodes of mixing of terrigenous arc derived and deep water pelagic material moreover abundant bodies of meta basalts interpreted as dismembered seamounts or pillow volcanoes were emplaced tectonically into various structural levels of this sedimentary melange by two different mechanisms at an upper level the irregular to elongated bodies of basalts were offscraped only weakly strained and incorporated in the sedimentary matrix that exhibits steep cleavage indicating horizontal pure shear shortening and oblate strain in contrast at a lower level disc shaped and closely spaced basalt bodies are ductilely sheared and reworked into a flat lying prolate to plane strain fabric suggesting horizontal ductile flow presumably along the base of the accretionary wedge during flat slab oceanic subduction our new u pb ages also reveal that the melange formation continued until the early cambrian

paleomagnetism and paleogeography of the northern calcareous alps austria prefolding magnetization components have been resolved at 38 sites of liassic adnet limestones in the central northern calcareous alps nca and at one site in the drauzug the site mean directions are northeasterly and down and are closer to coeval european directions than to southern alpine african directions the western end of both the nca and of the drauzug yield different site mean directions and negative fold tests indicating remagnetization the sampling localities have been rotated relative to one another during thrusting however comparison of paleomagnetic poles with the european apparent polar wander path implies an average clockwise rotation of about 30 for the nca relative to europe larger scale clockwise rotations in excess of 80 relative to the southern alps are implied by comparison with the southern alpine african polar wander path facies analysis of jurassic and lower cretaceous basinal sediments in the central part of the nca suggests that liassic subsidence was related to presently e w sinistral strike slip faults whereas malm berriasian subsidence was controlled by presently e w south dipping normal faults both paleomagnetic and facies data tend to indicate that the nca did not undergo the african rotation seen in southern alpine paleomagnetic data this implies the existence of a short lived tract of triassic jurassic ocean floor between the nca and the southern alps which linked the pindos ocean to the ligurian trough and allowed adria to rotate independently from the nca during the late jurassic eo alpine deformation and metamorphism are associated with the early cretaceous closure of this hallstatt trough

tectonic evolution of the troodos ophiolite within the tethyan framework a new tectonic model reconciles conflicting structural and geochemical evidence for the origin of the troodos ophiolite a well preserved remnant of neotethyan oceanic crust grabens and normal faults within the sheeted dike complex and the extrusive sequence of the troodos ophiolite resemble those of oceanic spreading centers diverse intrusive and tectonic contact relationships between the sheeted dike complex and the underlying plutonic sequence indicate multiple and episodic intrusion of magma and along and across strike variation in volcanic and tectonic activity during development of oceanic crust coupled with the existence of the arakapas transform fault to the south these structural and intrusive relationships suggest origin at an intersection between a spreading center and a transform fault the arclike chemistry of sheeted dikes and related extrusive rocks and the inferred highly depleted and hydrous nature of the mantle source of the late stage intrusive and extrusive rocks argue however for generation of part of the ophiolite within a subduction zone environment regional reconstructions suggest that the mesozoic neotethys may have evolved as a marginal basin both to the afro arabian continent and the paleotethyan ocean over an active or recently active south dipping subduction zone the troodos ophiolite and other eastern mediterranean ophiolites whose magma compositions were affected by the subducted paleotethyan slab may have formed along east west trending spreading centers separated by north south trending transform faults within this marginal basin a rapid change in relative plate motion in late cretaceous time between eurasia and afro arabia created a regional compressive regime that may have resulted in plate boundary reorganizations within the neotethyan realm and in initiation of north dipping subduction zone s beneath the troodos and other ophiolites in the region the apparent forearc setting of the troodos ophiolite is a consequence of this intraoceanic displacement after its formation and thus is unrelated to its generation

the franciscan complex california usa the model case for return flow in a subduction channel put to the test the model of return flow in a subduction channel rfsc is put to the test by opposing key assumptions of the model to an integrated review of field data petrology geochronology and structures of high p melanges of the franciscan subduction complex fsc to which rfsc has been first applied key assumptions are i the subduction channel sc is a low viscosity zone of finite width restrained by the rigid subducted and overriding plates ii forced return flow in the sc is associated with formation and km scale displacements of small tectonic blocks 1 km in a highly deformable matrix iii this results in apparently random distributions and close associations of blocks exhumed from different depths hence the block in matrix structures and exotic blocks in high p melanges would reflect the structural state at depth of the sc on the basis of the predictions ensuing from the key assumptions of the rfsc model on the rock record it is detailed why the actual record of the high p melanges of the fsc does not comply with this perception the record rather reflects recurring subduction underplating associated decollement erosion and mingling of high p rocks by sedimentary processes in the trench all occurring in an 80 m y lasting period distributed deformation of the melange matrix is unrelated to formation of apparently chaotic block in matrix structures since i creation subduction ascent and cooling of exotic blocks on counterclockwise p t paths predates deposition of the bulk of the metasediments constituting the matrix by 20 40 m y ii deformation of the matrix occurs during a second loop of high p metamorphism postdating creation and ascent of the exotic blocks by at least 20 40 m y iii the magnitudes of distributed deformation of the matrix are too low to cause juxtaposition of blocks different in p t paths against each other from km scale vertical distances instead narrow few tens of metres wide fault shear and decollement zones formed at locally high pore fluid pressures accumulate the major tectonic transport during high p metamorphism they bound several 100s of metres wide tabular tectonic bodies that are coherent sedimentary processes at the active continental margin formed the bulk of the melanges tectonically formed block in matrix structures within the shear and decollement zones are unrelated to juxtaposition of exotic blocks universal diagnostic criteria are developed to identify return rfsc as a mechanism that potentially amalgamates heterogeneous metamorphic terrains by using field data petrology and geochronology

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

structure and petrology of tauride ophiolites and mafic dike intrusions turkey implications for the neotethyan ocean cretaceous neotethyan ophiolites occur in four east west trending subparallel zones within the tauride tectonic belt in southern turkey the ophiolites of the inner intermediate and outer zones tectonically overlie the mesozoic platform carbonates of the tauride belt and are commonly underlain by a cenomanian ophiolitic melange these ophiolites consist mainly of tectonized mantle rocks mafic ultramafic cumulates and gabbros and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence metamorphic soles that are several hundred meters thick occur as thrust faulted slices beneath these ophiolites and show well developed metamorphic field gradients ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit dike rocks are made of subalkalic basalt to andesite typical of evolved island arc tholeiites they display large compositional variations with sio 2 content between 50 and 60 wt and mgo between 8 and 4 wt and contain higher ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates the majority of the analyzed dike rocks show a slight depletion in light rare earth elements ree with low la smn ratios and are depleted in both high field strength hfs and heavy rees while enriched in large ion lithophile elements lile relative to normal mid ocean ridge basalt morb these characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by lile and light ree enriched fluids geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field or in a transitional zone between the garnet and spinel stability fields at about 60 km depth this asthenospheric window was probably created during subduction of a neotethyan ridge system magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere the new 40 ar 39 ar hornblende dates of 92 90 ma and 90 91 ma from the metamorphic soles and dike swarms respectively show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the inner tauride seaway these data suggest that the tauride ophiolites within the three zones to the north originated from the same root zone situated north of the tauride carbonate platform and that they constitute remnants of a single ophiolitic nappe sheet derived from the inner tauride seaway within the neotethyan ocean

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

unraveling the schneeberg garnet puzzle a numerical model of multiple nucleation and coalescence a quantitative kinetic model for the growth of the different garnet porphyroblast microstructures type 1 and type 2 of the western schneeberg complex wsc is presented these porphyroblasts formed by a multiple nucleation and coalescence mechanism our numerical simulation shows that at constant diffusion rates 1 low interface reaction rates result in a fully amalgamated porphyroblast type 2 2 intermediate reaction rates result in a porphyroblast where coalescence of grains closer to the margin prevented amalgamation of those in the centre similar to type 1 porphyroblasts and 3 high interface reaction rates result in a porphyroblast microstructure with an atoll form all three microstructures are characterised by distinctive cluster size distributions a 2 d cluster size distribution analysis of type 1 porphyroblasts of wsc shows that these did not form because of intermediate interface reaction rates but because the diffusion rate of nutrients was too low to keep pace with the interface reaction rate

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

evolution of a conjugate passive margin pair in mesozoic southern turkey in the western tauride belt of southern turkey a north pointing cusp isparta angle occurs along the mesozoic carbonate axis and encompasses passive continental margin sequences rift assemblages mantle peridotites an ophiolite complex and platform fragments the contacts between these tectonostratigraphic units are commonly tectonic with the carbonate platforms forming the autochthonous basement to the west and east directed thrust sheets both in the west and the east rift assemblages consist of chert pelagic limestone mudstone sandstone and alkaline lavas ranging in age from upper triassic to upper cretaceous and locally include serpentinite lenses mantle peridotites are commonly harzburgitic cut by numerous doleritic dikes and underlain by a syntectonic melange the ophiolite complex occurs in the south adjacent to and overlying the western platform bey daglari and consits mainly of peridotites and cumulate gabbros with a poorly developed dike complex and rare extrusive rocks platform fragments are represented by north south trending slivers of limestones that have affinity with the western platform they are technically overlain by the rift assemblages and or mantle rocks within the isparta angle rock types and associated structures within the isparta angle indicate a progressive evolution from continental rifting and passive margin development to ocean basin formation in the tauride belt of the neotethyan realm during triassic to late cretaceous time the pronounced asymmetry in the crustal architecture of the conjugate margins suggests that initial stages of rifting and subsequent extension were probably facilitated by crustal failure along an east dipping in present coordinates low angle fault continued rifting and associated crustal thinning resulted in upwelling and emplacement of the mantle beneath the lower plate and in partial melting of this elevated mantle generating alkaline lavas further extension in the south established a spreading system along which cretaceous oceanic crust now represented by the tekirova ophiolite was created contractional deformation in late cretaceous to late tertiary time caused telescoping of rock units within the isparta angle and emplacement of thrust sheets onto the platform carbonates both in the east and the west the opening and closure stages of the ocean basin within the isparta angle were diachronous while the bounding carbonate platforms were probably revolved around a pivoting pole located at or near the apex of this sharp bend in the tauride belt

subducted sedimentary serpentinite melanges record of multiple burial exhumation cycles and subduction erosion abstract serpentinite matrix melanges give insight into large scale convergent plate margin processes particularly because of the derivation of the serpentinite from oceanic mantle similar to shale matrix melanges a field geologist may easily recognize the sedimentary origins of little deformed serpentinite matrix melanges but melanges within accretionary prisms have undergone significant deformation and recrystallization of matrix serpentinite melanges of the franciscan subduction complex of california have a seemingly intact and foliated matrix such exposures contrast sharply with the granular undeformed sedimentary serpentinite melanges of the coeval great valley group gvg forearc basin deposits that depositionally overlie coast range ophiolite that structurally overlies the franciscan nonetheless franciscan serpentinite melanges display evidence of sedimentary origins including sedimentary breccia composed of exotic block material tolay ridge sedimentary serpentinite breccia panoche pass road basal serpentinite conglomerate with exotic clasts sunol regional wilderness and serpentinite sandstones and conglomerates including a basal conglomerate overlying coherent metagraywacke tiburon peninsula these examples record two burial exhumation cycles to blueschist facies depths in addition a melange breccia in the panoche pass area may have components that record three burial exhumation cycles to blueschist or greater depth exhumation rates for various cycles ranged from about 1 2 to 10 mm year the tiburon peninsula serpentinite melange occupies the structurally highest horizon in the franciscan of the san francisco bay area and regional field relationships indicate deposition at ca 100 ma apparently about 65 ma of subduction erosion non accretion followed initiation of franciscan subduction in this region the oldest franciscan serpentinite melanges are at least 35 ma younger than sedimentary serpentinites of the gvg subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench evidence of multiple burial exhumation cycles indicates reworking of subduction complex material consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages

tectonostratigraphy of the glen app area southern uplands scotland anatomy of an ordovician accretionary complex the stratigraphy and structure of the glen app coastal area northwestern southern uplands are characterized by structural repetition of both chaotic and coherent units of the middle ordovician caradocian tappins group the chaotic unit the currarie formation comprises blocks including basalt chert and terrigenous sedimentary rocks set in folded and in places sheared red to grey siliceous mudstone the coherent unit is turbiditic and is divided into the dalreoch formation in the north and the corsewall formation in the south the former is characterized by inclined isoclinal folds and thrusts whereas the latter shows a homoclinal dip although both are largely overturned northwards small scale duplexes are observed within the coherent turbidites of the dalreoch formation and chaotic units larger duplexes identified on the basis of mapping are oriented at an oblique angle to the general trend of regional structures the chemistry of basalt blocks within the currarie formation shows a within plate type suggesting an origin as seamounts the tectono stratigraphy of the area is interpreted as the result of accretion of a substantial seamount at a subduction zone during mid ordovician time thrust interleaving of chaotic and coherent units is inferred to have taken place in and above the decollement zone while thick turbidite beds without strong folding were probably preserved below the decollement and emplaced below a large scale nappe interpretation of the glen app area as part of an accretionary complex confirms that an ocean basin remained open in part of the iapetus system at least until caradocian times

the casanova complex of the northern apennines a melange formed on a distal passive continental margin abstract the cretaceous palaeocene casanova complex occurs in two thrust sheets of the eugeosynclinal ligurids of the northern apennines it is a sedimentary melange with ophiolitic and quartzose turbidites or limestone shale olistostrome submarine debris flows as matrix exotic blocks of ophiolite and granite serpentinite breccias and lenticular ophiolitic breccias and olistostromes contribute to the melange character of the complex deformational structures include soft sediment slump folds indicating a sw dipping palaeoslope and boudins a gradational slumped top to the melange small scale faults in chert blocks and deformation associated with the emplacement of the exotic slide blocks the blocks were shed as rotational slides from submarine fault scarps and are surrounded by haloes of debris created by submarine weathering the stacking pattern of the blocks with the originally stratigraphically highest ophiolite lithologies lowest in the pile of blocks is explained by a diverticulation model with progressively deeper erosion mechanical analysis shows that the blocks were stable when partly exposed resting on a soft sediment substratum criteria which distinguish the casanova complex from a tectonic melange and which may be of value in other melanges are discussed previous interpretations of the complex as a precursor olistostrome to northeastward nappe emplacement the bracco ridge model are rejected the melange is believed to have formed on ocean crust as a result of turbidite and debris flow sedimentation soft sediment deformation block faulting gravity sliding and submarine erosion at the distal edge of a uniformly sw dipping continental margin

partial eclogitisation of gabbroic rocks in a late precambrian subduction zone zambia prograde metamorphism triggered by fluid infiltration gabbros and eclogites occur closely associated in a 200 km long and up to 40 km wide area of the zambezi belt in central zambia this area is interpreted to represent part of a late precambrian suture zone with the mafic rocks being relics of subducted oceanic crust gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions this resulted in kyanite omphacite bearing assemblages for eclogites that have al poor bulk compositions undeformed eclogites typically preserve features of a former gabbroic texture reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre scale domains in most of the rocks no evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites in contrast the fine grained intergrowth of omphacite garnet kyanite and quartz which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions eclogitisation took place under p t conditions of 630 690 degreesc and 26 28 kbar suggesting a large overstepping 10 kbar of reaction boundaries eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large subhedral grains of omphacite kyanite and garnet the gabbro to eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution precipitation mechanism that characterises the metamorphic mineral replacements the process of eclogitisation was limited by reaction kinetics and dissolution precipitation rates rather than by the metamorphic p t conditions even though ductile deformation occurred and equilibrium phase boundaries were overstepped the infiltration of fluids was necessary for triggering the gabbro to eclogite transformation

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