accretion related deformation of an ancient early paleozoic trench fill deposit central appalachian orogen the greenwich slice of the hamburg klippe a taconic allochthon in eastern pennsylvania is part of an ancient subduction complex that formed during early paleozoic crustal convergence in the central appalachian orogen the dominant lithologies of the greenwich slice are arranged in fault bounded coarsening upward sequences of pelagic deposits lower to middle ordovician overlain by hemipelagic and turbidite deposits middle ordovician these sequences record advance of a site on oceanic lithosphere from a pelagic realm toward and ultimately into the trench in middle ordovician time the trench fill deposits of the greenwich slice are characterized by structurally coherent sandstone rich zones adjacent to mudstone rich horizons of mesoscopically deformed sandstone and scaly mudstone deformation of sandstone ranges from minor variations in bed thickness to disruption so severe that beds cannot be traced for any appreciable distance across the outcrop deformed sandstone beds typically contain minor but conspicuous dewatering structures and lack internal primary sedimentary structures all sandstone beds display various proportions of microgouge and pressure solution zones microscopic analysis of the scaly mudstone reveals 1 reorientation of platy grains along scaly cleavage surfaces 2 the presence of microstriations and micro steps on scaly cleavage surfaces and 3 microfractures filled with sparry calcite and quartz some of which truncate scaly cleavage surfaces consideration of the structural evolution of these deposits suggests that initial accretion related deformation and dewatering were localized along weak mud rich horizons whereas stronger sand rich zones remained coherent increased pore fluid pressures of sand beds within mud rich zones resulted in intergranular ductile flow and stratal disruption of the sand interlayered mud became the stronger sediment by virtue of its greater cohesive component of strength and failed brittlely resulting in development of scaly cleavage subsequently pore fluid pressures within the deforming mud rich zones became sufficient enough to generate a fracture porosity progressive deformation resulted in less of a strength contrast between the strong sand rich and deformed mud rich horizons eventually both early deformed and coherent sand beds failed brittlely by grain granulation and pressure solution the variations in style of deformation of the greenwich slice trench fill deposits are probably reflective of variations in porosity and state of compaction of the accreted sediment

structure of franciscan complex in the stanley mountain window southern coast ranges california three sets of deformational events are recognized in the franciscan complex of the stanley mt area s coast ranges california first in pre melange time shortening of the relatively cohesive sequence of interbedded graywacke and mudstone formed isoclinal folds and an axial plane slaty cleavage second fragmentation of the once cohesive sequence probably over a considerable period of time produced the configuration now considered a melange third after the melange developed the franciscan complex was deformed along with the surrounding upper mesozoic great valley sequence into the stanley mt antiform in the cohesive upper cretaceous carrie creek formation macroscopic and mesoscopic folds have 2 predominant orientations the less cohesive franciscan complex attempted to fold as shown by the distribution of shear foliations on stereographic projections but lack of lithologic continuity and slip along previously formed shear fractures prevents the recognition of macroscopic folds hence in the franciscan complex of the stanley mt window several lines of evidence show that the melange structure is tectonic in origin not just a tectonic imprint superimposed upon already chaotic rocks of sedimentary origin olistostromes 43 references

the origin of the upper palaeozoic chanaral melange of n chile the fragmented and mixed strata of the chanaral melange extend for some 220 km from n to s in the coastal ranges of n chile 26 20 to 28 20 s the melange was produced in an accretionary wedge resulting from the ne subduction of the ancestral pacific oceanic plate beneath the gondwana land margin during carboniferous times the melange consists essentially of blocks of sandstone in a pelitic matrix these sediments were deposited as deep sea basin plain turbidites prior to their fragmentation other blocks in the melange include mafic volcanic rocks with the geochemical characteristics of oceanic island basalts and rare shallow marine limestones structures in the melange suggest an origin by two distinct deformation processes involving unlithified sediments initial boudinage and break up of the strata was apparently accomplished by intrastratal movement resulting from imbricate thrusting within the accretionary wedge the second process produced cross cutting zones of breccia which are interpreted as fluid escape conduits resulting from the high pore pressures produced during underthrusting

accretion and post accretion metamorphism in subduction complex terranes of the new england fold belt eastern australia regional metamorphic rocks that form late palaeozoic subduction complexes in central queensland australia are products of two metamorphic episodes synaccretion metamorphism m1 gave rise to prehnite pumpellyite and greenschist facies rocks whereas a subsequent episode m2 at about 250 ma formed upper greenschist to upper amphibolite facies rocks of both intermediate and low pressure type probably in a compressive arc or back arc setting a similar pattern can be recognized for 1000 km along the new england fold belt although at several localities where higher grade rocks are exposed metamorphism was essentially continuous over the m1 m2 interval with a rapid rise in geothermal gradient at the end of accretion where out stepping of tectonic elements has occurred at long lived convergent margins elsewhere similar overprinting of high by lower pressure facies series is anticipated complicating the tectonic interpretation of metamorphism the discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out stepping are followed by the subduction of a major heat source

quaternary scale evolution of sequences of talus flatirons in the hyperarid negev article i nfo talus flatiron sequences are ubiquitous landforms in arid and semiarid regions characterized by horizontal erodible rocks capped by more resistant rocks alternating phases of deposition and erosion lead to the formation of generations of talus flatirons in which ancient ones are located farther from the source cliff the existing conceptual model of this systematic spatial distribution of talus flatirons is related to glacial interglacial climatic cycles and to relatively high rates of cliff retreat three groups of talus flatirons were analyzed in the northeastern hyperarid negev desert the analyses include mapping age determination using optically stimulated luminescence and cosmogenic nuclide exposure dating electrical resistivity tomography and sedimentological and soil analyses all talus flatirons contain gypsic salic soil catena typical to hyperarid climate b80 mm yr 1 no pedogenic indicators of past wetter environments such as buried calcic soil 10 be exposure ages of the talus flatirons suggest that they were deposited during the middle pleistocene 610 ka and 170 ka for the oldest and intermediate aged talus flatiron groups respectively these ages combined with the present location of the talus flatirons relative to the source cliff yield retreat rates of 6 12 and 200 m ma 1 for the bedrock cliff and talus flatiron apex respectively and in opposite directions our results show that a climatic changes at glacial interglacial time scales are not the main controls over the formation of talus flatiron generation and b that significant cliff retreat is not the main cause for the systematic spatial distribution of talus flatiron generations relative to the cliff our results indicate that the process of talus flatiron formation in the negev desert must be associated with the balance between production deposition and removal of clasts under hyperarid conditions with only a minor cliff retreat

from pull apart to the rifting process the formation of the pannonian basin abstract during the middle and the late miocene intra carpathian basins have developed south and west of the carpathian arc these basins are bounded by normal faults and connected by conjugate shear zones sinistral strike slip fault systems trend ne sw whereas dextral ones trend nw se by means of geometrical analyses of fault system and paleostress tensor determinations it has been possible to consistently reconstruct the succession of tectonic events in the hungarian mountains the two main stages in the structural development of the intra carpathian basins are 1 1 the initiation of pull apart basins along conjugate shear zones in a n s compressional and e w extensional regime and 2 2 the development of these basins in a tectonic regime characterized by a dominant e w trending extension

structural style and kinematics of an underplated slate belt kodiak and adjacent islands alaska the kodiak formation composed of coherent maastrichtian turbidites is a slate belt whose dominant structures developed during underplating to an accretionary wedge in the latest cretaceous it consists of about 80 coherent landward dipping thrust packets zones of disrupted sandstone associated with a scaly argillite matrix constitute the remainder about half of these disrupted sandstone zones are related to pre accretion deformation and the rest formed along late stage strike slip faults that postdate development of slaty cleavage the formation is divided into three structural belts the landward and seaward belts include steeply dipping structures and the central belt contains shallowly dipping structures and rocks that have experienced the highest strain the central belt probably acted as a low angle southeast verging floor thrust zone beneath the landward belt the structural history of the kodiak formation includes 1 early soft sediment disruption 2 tectonic stratal disruption 3 thrust faulting slaty cleavage s 1 development and folding f 1 4 intrusion of granodioritic plutons dikes and sills and associated normal faulting 5 development of crenulations f 2 and crenulation cleavage s 2 6 thrust faulting and 7 development of right lateral strike slip faults event 3 produced the dominant northwest dipping structural grain and caused the greatest amount of shortening the timing of event 7 relative to events 5 and 6 is not certain the dominant structures of the kodiak formation are interpreted as developing during underthrusting underplating and intra wedge shortening during latest cretaceous time within an accretionary wedge the maximum time from deposition to emplacement was about 12 m y and the timing is constrained by the maximum depositional age 74 m y and the age of plutonic rocks 62 m y that crosscut the dominant fabric pre cleavage zones of stratal disruption reflect deformation on the lower plate during underthrusting slaty cleavage thrusts and f 1 folds developed during underplating that also resulted in the formation of duplexes crenulations are probably related to post underplating subhorizontal shortening of sediments within the accretionary wedge syn deformation dynamic recrystallization of quartz minimum syn accretion metamorphic temperatures between 205 and 250 c and pressures of at least 2 65 kb recorded during the earliest stages of deformation suggest underplating at 10 km

large scale sediment underplating in the makran accretionary prism southwest pakistan field relations in the emergent part of the makran accretionary prism show that a mid miocene to early pliocene slope and shelf sedimentary sequence was deposited directly on abyssal plain turbidites without any detectable stratigraphic or structural discordance sedimentological evidence for rapid shoaling however indicates that the underlying sediment column was tectonically thickened by a factor of between two and three during this period this can be explained by large scale underthrusting and underplating of sediment a mode of accretion that is also favored by mass balance considerations the visible deformation in the coastal makran occurred when the region was 70 100 km north of the contemporary prism front

variscan to alpine heterogeneous palaeo stress field above a major palaeozoic suture in the carpathian foreland southeastern poland abstract the tectonic evolution of the region adjacent to the sw border of the east european craton is crucial for understanding the palaeozoic amalgamation of europe and its later evolution we present a structural analysis of the brittle deformation fault slip data inversion and folding carried out in the cambrian to cenozoic rocks of the european platform in se poland to characterize the late palaeozoic maastrichtian palaeocene and miocene tectonics of this region the variscan late carboniferous palaeo stress pattern was characterized by n s to nne ssw oriented compression as inferred from the folding of the palaeozoic mainly devonian rocks the compression was associated with the northwesterly present day coordinates motion of gondwana towards the northerly situated laurussia subsequently the mid polish trough mpt developed in an extensional regime the stress regime changed to compressional at the maastrichtian palaeocene transition in response to the africa europe plate kinematics the compressional regime induced the tectonic inversion of the mid polish trough the stress pattern was characterized by ne sw oriented compression in the holy cross mts hcm area and in the radomsko high folds and faults in the mesozoic cover revealed a sinistral reactivation of the holy cross fault hcf and of nw se trending faults the alpine stress patterns were related to the subduction of the european platform beneath the carpathian orogen the flexural bending of the subducted lithosphere induced a homogeneous middle miocene n s extension in the carpathian foreland subsequently the northward propagation of the carpathian front resulted in late middle miocene n s compression restricted to the proximal area of the frontal thrusts implied by a strain partitioning at a low friction contact between the carpathian nappes and the european platform since the palaeozoic the teisseyre tornquist zone ttz behaved as lithospheric weak zone which has been reactivated during the maastrichtian palaeocene change in stress regime it resulted in the tectonic inversion of the polish trough inherited faults conditioned superficial stress perturbations and focused the deformation

porosity of sediments in accretionary prisms and some implications for dewatering processes rapid dewatering largely through loss of porosity is one of the most striking changes affecting deformed sediments in accretionary prisms analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology more specifically the porosity depth gradient increases arcward across the prism until porosities less than 10 occur within a few kilometers of the surface in deeper parts of the prism dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactions calculations involving volume change lead us to define a function which reflects total volume reduction the distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements an initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths several lines of evidence suggest that in addition to gradual diffuse dewatering localized zones of strong dewatering are associated with zones of intense shear this model of shear dewatering based on critical state soil mechanics suggests that sediments adjacent to the basal decollement should be strongly dewatered

diverse melanges of an ancient subduction complex abstract three lithologically and structurally diverse melanges occur within an early paleozoic early middle ordovician subduction complex in the central appalachian orogen type i melange characterized by horizon of variabls y deformed sandstone and scaly mudstone that alternate with coherent sandstone rich sequences is interpreted to reflect accretion related deformation of water saturated trench deposits type ii melange composed of exotic radiolaria bearing mudstone clasts in a scaly mudstone matrix can be explained by remo biliza tion and mixing of inner trench slope sediments type iii melange is a poorly sorted polymict assemblage of native lithology clasts in a scaly mudstone matrix evidence o f forceful injection of matrix mud into clasts and inferred discordant contacts between melange and surrounding bedded deposits suggest that the type iii melange formed from mud diapirism the close association o f these melanges points out the diversity of tectonic and sedimentary processes previously documented from modern convergent margins that may be reflected in older sub duction complexes introduction

shearing of partially consolidated sediments in a lower trench slope setting shimanto belt sw japan abstract detailed mapping of a coastal platform in shikoku sw japan provides evidence for progressive deformation in partially lithified sediments the eocene sediments involved are interpreted as lower slope basin deposits an assemblage of listric normal faults sheath folds broken formations and late stage faulting has developed during the sediments burial and uplift history these structures are typical of many other areas in the shimanto belt of shikoku despite the soft sediment style of deformation the consistency of the fold orientations relative to the regional foliation suggests that they are valid kinematic indicators a sequence of extensional faulting overprinted by synchronous folding and shearing is recognized this is interpreted as the response of the sediments to shape changes in the accretionary basement induced by shortening a general model has been constructed for the evolution of the structures it is proposed that early listric normal faults are subsequently deformed either by shearing along planar surfaces or by motion over frontal and lateral ramps back rotation of sediments during progressive shortening near the front of the prism tightens the fold hinges and rotates the fold axes towards the local shear direction alternative sequences which could account for the observed geometries are also discussed

a rift drift delamination model of continental evolution palaeozoic tectonic development of eastern australia the adelaide lachlan and new england fold belts of eastern australia record the continental growth of eastern gondwanaland during the palaeozoic the new england fold belt nefb represents a tectonic collage formed by subduction accretion during the late paleozoic but contains remnants of subduction related rocks that date from the cambrian the lachlan fold belt lfb developed inboard from the nefb initially as an amalgam of proterozoic continental and cambrian oceanic fragments that formed by rift and drift at the leading edge of eastern gondwanaland convergent tectonism at 500 ma welded the fragments to the craton and formed the adelaide fold belt oceanward dispersal of detritus across the lfb produced an overlap assemblage of quartzose ordovician turbidites and a new subduction zone developed at the eastern margin the tectonic setting was similar to the modern philippines plate of the western pacific behind the arc silurian devonian convergent tectonics converted the 1700 2000 km wide continental margin of the proto lfb into a 750 km wide thin skinned fold magmatic belt within 60 ma the driving force was delamination which produced a ubiquitous basaltic underplate that generated 1 regional low p metamorphism 2 local anticlockwise p t t paths and crustal scale isobaric cooling 3 voluminous syn to late tectonic granitoids emplaced at rates approximately twice that of modern arcs and 4 normal thickness crust despite an average of 60 shortening delamination occurred in two separate areas originally 1000 km apart first in the eastern then in the western part of the lfb the lfb and its northern counterpart the thomson fold belt represent approximately one fifth of the australian continent and evolved from dispersed fragments into stable continental crust in 300 ma such rapid growth rates are typical of the 2 7 2 5 ga and 1 9 1 7 ga periods when voluminous widespread granitoids and large turbidite dominated low p metamorphic belts were produced in settings that are not obviously subduction related therefore a similar process of rift drift delamination riddel tectonics may have periodically operated throughout earth history

structural relations in the subduction complex of the paleozoic new england fold belt eastern australia the new england fold belt of southeastern australia is divided by the peel fault system into a western magmatic arc frontal arc succession and an eastern subduction complex assemblage the western part of the subduction complex consists of repeated slices of a sequence which where most complete comprises dolerite overlain by basalt sometimes pillowed radiolarian chert siltstone and in the younger thicker slices sandstone the basal contact of this sequence is always faulted a planar veined surface separates overlying basic igneous rocks from a sheared mass composed mostly of sedimentary material in which are imbedded tectonically derived blocks of basic igneous rock this sheared zone passes gradationally down into less deformed sedimentary rocks of the underlying sequence slices each of which is usually a few hundred meters thick young constantly westward toward the magmatic arc forming a stacked steeplydipping sequence tens of kilometers thick the distribution and inter relationship of li

perspectives on the roles of melanges in subduction accretionary complexes a review abstract melanges play three principal overlapping roles in the architecture of subduction accretionary complexes sacs during and after sac formation first tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction accretion process these melanges facilitate preservation of inter melange less deformed accretionary units aus beneath the trench side of the sac the initial deformation zone along the decollement at the top of the down going plate is marked by non melange tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below olistostromal rocks may be incorporated into the decollement here in the mid arc to inner arc areas of the sac exotic block bearing melanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy in mud diapirs and along out of sequence faults some of which facilitate uplift of high pressure rocks and serpentinite matrix melanges second after accretion the sedimentary tectonic diapiric and polygenetic melange units serve as single block or sheet architectural aus or subunits within larger accreted aus of the sac diapiric melanges and sedimentary olistostromal melanges formed in and above sac fault blocks respectively may become incorporated into the sac during ongoing deformation and become architectural units as well third melanges serve as post subduction stress guides that focus shear strain during continuing and post accretion deformation of sacs allowing ongoing modification of the sac during progressive deformation of the orogen examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts

structure of a growing accretionary prism hikurangi margin new zealand the hikurangi margin of eastern north island new zealand represents the feather edge of the indian plate at its convergent boundary with the subducting pacific plate a migrated seismic reflection profile across this margin clearly displays the structural evolution of an accretionary prism a 25 km wide band of protothrusts is delineated between the toe of the slope and a converging seamount this illustrates an early stage in the seaward propagation of a deformation front landward tilted trench slope basins are separated by ridges that have clearly defined thrusts which appear to sole out at a decollement the decollement continues at an angle of only 3 beneath the 150 km wide margin to a depth of 14 km near the coast where it coincides with an onshore zone of high seismicity

thin skinned thrusting in the northern new england orogen central queensland australia regional cross sections of the northern new england orogen and eastern bowen basin in eastern queensland indicate that the climactic permian triassic deformation involved thin skinned tectonics with the development of a mountain front represented by a syncline formed by crustal delamination with wedging of a thrust sheet from the east the thrust sheet is exposed in the folded zone of the bowen basin and is characterized by upright nnw trending folds in late permian sediments the folds have a chevron to flattened chevron style and formed mainly by flexural slip at about 60 shortening with detachment from an underlying basement of paleozoic rocks farther east the basement is overlain by pervasively cleaved rocks of the gogango overfolded zone in which structures show a consistent westerly vergence these rocks overlie an inferred passive roof duplex in the underlying basement together the folded zone of the bowen basin and the gogango overfolded zone constitute a foreland fold thrust zone with a shortening of 50 90 km this implies that the new england orogen has been thrust west along a major east dipping detachment

crustal structure of the mid polish trough beneath the teisseyre tornquist zone seismic profile abstract a new seismic refraction and wide angle reflection experiment in the teisseyre tornquist zone ttz in poland was conducting during july 1993 as an international polish finnish german and swedish co operation the ttz profile running in an se nw direction was exactly located in the central part of the mid polish trough in the zone of maximum subsidence from upper permian to upper cretaceous 19 shot points were placed along the 450 km profile the interval between the shot points was about 25 km 31 explosions 90 to 1000 kg of dynamite were recorded in two deployments using 135 modern three component seismic stations with spacing of the recording sites of about 1 7 km in this area the depth of the consolidated basement with a velocity of about 5 7 5 8 km s is 5 to 12 km deep nevertheless the p wave velocity is very low vp

structural signature of sediment accretion in a palaeozoic accretionary complex southeastern australia abstract chaotic block in matrix melange broken formation along high strain zones associated with large scale imbrication early bedding parallel cleavage recumbent folds in turbidites and structural complexity in cherts are key elements of a mid palaeozoic subduction complex in the eastern part of the lachlan fold belt southeastern australia the complex consists of an imbricated turbidite chert and basalt sequence of mid cambrian to late ordovician age structural and biostratigraphic controls require a complexly imbricated sequence with reversals in younging seawards towards the inferred former trench subduction zone as in young subduction accretionary complexes such as the kodiak accretionary complex in the aleutians and the shimanto complex of japan subduction accretion or underplating in the narooma accretionary complex is not typified by duplexing but shows a strain dependent transition from an inland belt of chevron folding cut by reverse faults to a coastal belt with an early bedding parallel fabric isoclinal recumbent folding poly deformation and differentiated layering in quartzrich turbiditic greywackes greywacke units show multiple cleavage fabrics formed in one folding event cherts below the turbidites show multiple folding events and zones of broken formation with intense veining and stylolitisation the structurally lowest units including deformed pillow basalts and block in matrix melange show strong planar linear fabrics in the block in matrix melange prolate shaped pods of greywacke and chert are aligned sub parallel to the bulk extension direction defined by mica pull aparts and pressure shadows on pyrite within the mudstone matrix mica neocrystallisation and pressure solution are the dominant deformation mechanisms at the base of the complex

genesis and deformation of mud injections containing chaotic basalt limestone chert associations examples from the southwest japan forearc muddy chaotic deposits now melanges are common throughout the shimanto belt southwest japan a cretaceous neogene forearc extending for more than 1800 km along strike and also in the older more northerly arc related belts many zones within these melanges show features that are consistent with an early localized genesis as mud injections seepages or mud diapirs synchronous with early slope and accretion related deformation these have subsequently undergone considerable postlithification deformation melanges containing basalt limestone and chert associations may have resulted partly from the explosive fragmentation and fluidization of parts of accreted ocean seamounts or ridges as pore fluids escaping from overpressured shales moved along faults and fractures within the basalt limestone chert units to form large mud injections using field examples from the shimanto and related belts we review criteria for recognition of a variety of mud injections in the light of previous work these criteria include the nature of contacts the geometry and internal deformation of clasts or blocks the matrix fabric and orientation of clasts or blocks in the matrix and anomalous age relations the incorporation of basalt limestone chert associations in some mud injections in southwest japan is thought to result from the penetration of overpressured fluidized muds through accreted seamount or ridge lithologies this generates a variety of textural relations which may be further complicated by subsequent tectonic shearing

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

the gundahl complex of the new england fold belt eastern australia a tectonic melange formed in a palaeozoic subduction complex abstract the new england fold belt of eastern australia preserves a palaeozoic fore arc terrain with a magmatic arc fore arc basin and a subduction complex the gundahl complex is a tectonic melange of regional extent in the subduction complex the matrix and slabs of the gundahl complex have six mappable lithofacies argillite greywacke argillite greywacke argillite tuff bedded chert and greenstone the argillite matrix is pervasively sheared with many slickensided shear fractures locally the matrix is formed by highly sheared greenstone greywacke and greenstone blocks are affected by internal shear zones and the blocks themselves pinch and swell folds in places with axial surface spaced cleavage are common within those slabs comprised of well bedded sequences bedding plane shear and faulting at a high angle to bedding also occur in these slabs on a map scale much of the gundahl complex comprises slabs up to 10 km long in imbricate fault bounded slices which repeat the disrupted pre melange stratigraphic sequence elsewhere there are lithologically distinctive blocks containing thick coherent sequences which are structurally incorporated into the gundahl complex the unit is believed to have formed by accretion imbrication and subsequent tectonic disruption of arc derived sediments and less abundant pelagic sediment and greenstone in an ancient subduction complex

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

variety of subduction and accretion processes in cretaceous to recent plate boundaries around southwest and central japan abstract the shimanto and miura boso terranes in southwest and central japan are analyzed for the understanding of variation of the subduction accretion process in convergent plate boundaries the shimanto is a cretaceous to recent honshu accretionary prism of 200 km width in the honshu arc side and is made up of melange flysch and olistostrome units these are repeated by many thrust faults and local broad and tight folds to make schuppen structures shallow marine and olistostromal bodies overlie part of the schuppen structures unconformably or are included within the structures normal subduction tectonics prevail for the formation of the accretionary prism but the effects of probable transform tectonics during late oligocene to early miocene are considerable the miura boso region is a terrane newly accreted from the izu arc to the honshu arc this terrane is made up of miocene and younger volcaniclastic sedimentary rocks dismembered ophiolitic materials and some of the rocks from the shimanto supergroup the miura boso terrane is characterized by transpressional tectonics associated with eduction during the sedimentation on the highly oblique subduction zone between the eurasia later north america and philippine sea plates several cases for the emplacement of the basaltic and other oceanic rocks to the landward side of the accretionary prism are discussed to account for the basic problem of the subduction accretion tectonics

tertiary extension development and extension compression interplay in the west carpathian mountain belt abstract this paper describes palaeostresses calculated from fault striae data the inferred palaeostrain patterns and determines the inter relation of the compression and extension during the tertiary development of the west carpathians the calculated stress and inferred strain patterns for the palaeogene burdigalian indicate that the ancestral west carpathians formed a more or less straight orogenic belt this belt underwent contraction and uplift in its narrow frontal zone stretching along its strike during the paleocene chattian and regional contraction and uplift during the chattian burdigalian the strain stress pattern is similar to the collision related pattern known from the eastern alps for the paleocene burdigalian during the burdigalian tortonian the calculated and inferred west carpathian stress and strain patterns indicate narrow frontal contractional and lateral sinistral transpressional zones in the orogenic front and broad extensional and dextral transtensional zones in the orogenic interior the stress strain pattern is similar to the subduction related pattern known from areas such as the hellenic or sunda banda arcs

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

diagenetic controls on the structural evolution of siliceous sediments in the golconda allochthon nevada u s a abstract a model is proposed that links the diagenesis of siliceous sedimentary rocks with deformation to explain the heterogeneous structural fabric of radiolarian cherts in the upper paleozoic golconda allochthon of nevada u s a numerous thrust faults slice the cherts into packets each with a unique set of internal structures fold geometries boudin profiles pressure solution features etc vary from packet to packet and layer to layer analogous variations in the diagenetically mixed siliceous sediments of the miocene monterey formation california suggest the cherts of the golconda allochthon were similarly mixed when they were deformed radiolarian sediments composed of biogenic silica opal a developed ductile structures and pressure solution features because of their high porosity weak lithification and the high solubility of disordered silica lowporosity strongly lithified relatively insoluble quartz cherts the final product of silica diagenesis deformed in a brittle fashion and developed fewer pressure solution features diagenetically intermediate ct cherts and ct porcelanites exhibited transitional behavior mixed diagenetic zones produced structures with layer by layer changes in structural style the dehydration of opal a and opal ct helped create the excess pore fluid pressures responsible for thrust faults hydraulic fractures dilation breccias and elastic intrusions the presence of ductile structures and numerous pressure solution features in the oldest cherts of the havallah sequence suggests that these cherts were deformed while diagenetically immature possibly within a long lived upper paleozoic accretionary prism

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

subduction complex melange of the wandilla terrane palaeozoic new england orogen central queensland australia devonian and carboniferous rocks of the northern new england orogen developed in a forearc setting in which the wandilla terrane was part of the subduction complex the wandilla terrane shows three episodes of deformation the first was associated with accretionary processes and formed lenticular and mudseam melange lenticular melange was derived from interbedded greywacke and mudstone bedded chert and greenstone and shows progressive bedding parallel extension with pinch and swell and boudinage it is characterized by an alternation between zones of semi coherent beds and zones with total bed disruption mudseam melange formed from thick bedded greywacke units and is characterized by abundant mud injection features and scalloped greywacke contacts indicative of deformation of unconsolidated sediment and mobilization of overpressured mud both melange types formed at shallow structural levels in offscraped and imbricated successions lenticular melange resulted from stratal extension whereas mud seam rocks were produced by the injection of overpressured mud into massive sands the second and third deformations post dated formation of the subduction complex and gave rise to a variably developed cleavage s2 and steeply dipping strike slip faults respectively

melange production and the importance of shale diapirism in accretionary terrains although melanges are widely believed to form in subduction zones there is little agreement as to the mechanism by which the characteristic chaotic aspect of a melange develops1 2 debate centres on the relative effects of gravity sliding down the inner trench slope and shearing along thrust faults during offscraping or underplating2 3 the results of deep sea drilling and mapping of modern accretionary terrains4 combined with geophysical investigation of outer arc regions favour the thrust or fold packet model1 4 8 of accretion although some melange deposits exposed on land appear to fit an olistostrome model better1 9 shale diapirs and mud volcanoes are abundant in many modern accretionary terrains10 12 and probably form as a result of rapid overloading caused by tectonic thickening4 13 an investigation of extensive melange zones in northern irian jaya has now shown that they are the product of shale diapirism which suggests that shale diapirism is an important process in the tectonic mixing of accretionary terrains

structural metamorphic evolution of the tia complex new england fold belt thermal overprint of an accretion subduction complex in a compressional back arc setting rocks in the tia complex underwent a simple clockwise p t t deformation path moving from mid crustal levels at blueschist facies conditions d1 d2 6 kbar 200 c to upper crustal levels at high t amphibolite facies conditions d4 d5 2 5 kbar 600 c in a relatively short time 15 ma the rocks remained under these conditions for approximately 40 ma d6 before they cooled and were brought to the surface d7 d8 during d6 permian rift basins were opening as thermal gradients in the tia complex reached a maximum d1 2 deformation was associated with accretion subduction processes and was followed by progressively less intense mainly e w oriented compression and uplift during d3 5 bringing the rocks to within 2 kbars of the surface d6 is characterized by limited extensional deformation and final uplift took place during d7 8 along shear zones that have orientations similar to d3 5 thrusts the shift from subduction dominated processes to thermally dominated processes occurred during d3 4 uplift of 4 kbar around 310 300 ma this coincided with a change in tectonic setting of the tia complex from an accretionary prism to a back arc position as the subduction zone stepped eastward an increased heat flow associated with back arc processes gradually caused thermal weakening of the lithosphere which due to external subduction related compressive forces resulted in uplift d3 with continued heating of the crust thermal doming resulted and uplift stopped d4 5 whilst thermal weakening led to limited crustal collapse as permian basins opened d6 the extremely high heat flow that was maintained for 40 ma suggests a heat source external to the crust possibly associated with thermal relaxation dehydration and detachment of the remnant stab left beneath the new england orogen after the subduction zone shifted east a stable crustal thermal structure was re established by 260 ma as the crust strengthened and could further support subduction induced compressional structures d7 8