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

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

evolution of the northern belt southern uplands implications for the southern uplands controversy conodont and graptolite dating provide a high resolution biostratigraphical framework for correlation across the southern upland fault the establishment of an early caradoc age for the red chert and volcanic sequences within the northern belt of the southern uplands confounds the accretionary prism model and our data support the view that following the obduction of the ballantrae ophiolite fan complexes prograded southeastwards across a narrow fault bounded shelf into the deep basin of the northern belt the fan complexes were initiated by uplift in the hinterland caused by the intrusion of plutons into the midland valley terrane sedimentation was controlled by regional extension subsidence and eustasy through the llanvirn and caradoc basic lavas with attenuated within plate geochemical signatures were erupted during the early caradoc peak of extension and may have just post dated the initiation of southwards subduction beneath the lake district

offscraping and underthrusting of sediment at the deformation front of the barbados ridge deep sea drilling project leg 78a on leg 78a we drilled sites 541 and 542 into the seaward edge of the barbados ridge complex and site 543 into the adjacent oceanic crust the calcareous ooze marls and muds at sites 541 and 542 are lithologically and paleontologically similar to the upper strata at site 543 and are apparently offscraped from the down going plate a repetition of miocene over pliocene sediments at site 541 documents major thrust or reverse faulting during offscraping the hemipelagic to pelagic deposits offscraped in the leg 78a area include no terrigenous sand beds but they contain numerous neogene ash layers derived from the lesser antilles arc hence this sequence is quite unlike the siliciclastic dominated terranes on land that are inferred to be accretionary complexes the structural fabric of the offscraped deposits at sites 541 and 542 is disharmonic probably along a decollement with an underlying acoustically layered sequence suggesting selective underthrusting of the latter the acoustically layered sequence correlates seismically with pelagic strata cored at site 543 on the incoming oceanic plate cores recovered from the possible decollement surface at both sites 541 and 542 show scaly foliation and stratal disruption approximately lithostatic fluid pressure measured in the possible decollement zone probably facilitates the underthrusting of the pelagic sediments beneath the offscraped deposits in the incoming section a transition from smectitic to radiolarian mud with associated increases in density and strength probably controls the structural break between offscraped and underthrust strata in the leg 78a area the underthrust pelagic section can be traced seismically at least 30 km arcward of the deformation front beneath the barbados ridge complex

tectonics and hydrogeology of accretionary prisms role of the decollement zone abstract at convergent margins the decollement zone comprises the plate boundary and is marked by profound structural disharmony by changes in stress orientation and by a discontinuity in plate velocity the decollement zone initiates in a weak sediment layer typically a low permeability hemipelagic mud lying below a more rapidly deposited stronger more permeable trench turbidite sequence at the deformation front trench turbidites tend to be offscraped whereas the finer grained hemipelagic and pelagic sediments are more likely to be underthrust offscraped materials may undergo only limited burial whereas underthrust deposits can be deeply buried undergo high pressure metamorphism and are more likely to be preserved in the stratigraphic record burial rates associated with underthrusting are high exceeding 20 km my with high rates of underthrusting sediment descending below the decollement zone is probably buried faster than it can dewater resulting in emplacement of relatively high porosity deposits at depth fluids flowing from modern decollement zones have migrated substantial distances laterally and tapped deep sources the episodicity of flow suggests pumping of fluid by cycles of dilation and flow to the decollement zone followed by failure and fluid expansion along the decollement zone possible pressure gradients below the decollement zone allow flow upward into it while maintaining minimum effective stress along it the thickness of sediment underthrust beneath the decollement zone determines whether these deposits are emplaced by diffusive underplating with stratal disruption and efficient dewatering or by coherent underplating with the formation of macroscopic duplexes and transfer of fluid to the base of the accretionary prism deformation mechanisms affecting accreted sediment depend on depth of entrainment into the decollement zone and duration of residence

ordovician ocean plate stratigraphy and thrust duplexes of the ballantrae complex sw scotland implications for the pelagic deposition rate and forearc accretion in the closing iapetus ocean abstract the ballantrae complex at bennane lea in sw scotland contains important ocean plate stratigraphy basalt chert mudstone sandstone in an accretionary prism that is associated with a classic ordovician ophiolite we used the ocean plate stratigraphy to sub divide the prism into 11 tectonic units to determine the depositional age of bedded cherts zircons were separated from 9 tuff beds from 6 different units all the tuffs have early to middle ordovician ages even though their present positions are mutually distant these ages are consistent with microfossil records of radiolaria and graptolites the stratigraphic structural relationships demonstrate that the ocean plate stratigraphy has been repeated by bedding parallel thrusts this is typical of a modern accretionary duplex we calculated the sedimentation rate of early to middle ordovician bedded cherts at bennane lea on the basis of u pb zircon ages obtained from several tuff beds the data indicate that the depositional rate 0 6 3 m myr was as slow as that of mesozoic cenozoic equivalents defined by radiolaria the age spectra of detrital zircons from ballantrae sandstones show prominent single peaks at ca 467 and 478 ma and a lack of precambrian zircons integration of our new zircon ages with published isotopic data and palaeo geographic maps indicates that the sandstones were deposited near an intra oceanic arc and far from any continent containing precambrian rocks the pelagic to clastic sediments at bennane lea were deposited in the closing iapetus ocean from ca 477 ma to ca 464 ma when they were accreted with the intra oceanic arc of ballantrae

sedimentology volcanism structure and metamorphism of the northern margin of a lower palaeozoic accretionary complex bail hill abington area of the southern uplands of scotland

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

magnetic and tectonostratigraphic correlation at a terrane boundary the tappins group of the southern uplands prominent aeromagnetic anomalies around ballantrae in southwest scotland have long been associated with extensive outcrops of basic ultrabasic igneous rocks in the midland valley terrane however recent reprocessing of data from a 1978 detailed aeromagnetic survey has demonstrated excellent correlation between discrete aeromagnetic features and certain magnetic greywacke formations within the early caradoc tappins group this heterogeneous unit forms the oldest and most northwesterly tectonostratigraphic tract in the southern uplands terrane and is characterized by relatively high detrital magnetite and serpentinite content and elevated values for cr and ni this suggests original close sedimentary links with an ophiolitic terrane not unlike the presently adjacent ballantrae complex or its extensive along strike correlatives a consideration which has important implications for terrane reconstructions in this region

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

the northern belt of the longford down inlier ireland and southern uplands scotland an ordovician back arc basin the northern belt of the southern uplands longford down zone contains two compositionally distinct clastic petrofacies suites each ranging from mid to late ordovician age they extend throughout the length of the belt and are generally separated by a strike fault the northwestern metaclast petrofacies suite is characterized by felsic igneous and metamorphic detritus derived from a continental source to the nw whereas the southeastern basic clast petrofacies suite contains abundant calc alkaline detritus derived from an island arc and plutonic metamorphic root zone complex to the s or se a similar source but located to the north is indicated for younger clastic sequences of similar composition in the northern part of the central belt immediately south of a major sinistral shear zone which defines the boundary between the northern and central belts it is concluded in light of other facets of regional geology and possible palaeogeographic configurations that this pattern reflects a back arc basin in the northern belt margined by a volcanic arc massif which once occupied the space now represented by the shear zone and a fore arc basin to the se central and southern belts the northern belt back arc basin is postulated to have been closed and deformed at the end ordovician and subsequently overthrust northwestward by the allochthonous thrust stack imbricated central and southern belts fore arc sequence at the end silurian this in the northern belt resulted in the development of a major recumbent structure downward facing on regional end silurian cleavage

the southern uplands controversy report of a discussion meeting held at burlington house on 12 february 1986 the idea for a discussion meeting followed from remarks made by professor c h holland 1986 in his presidential address to this society in 1985 when he expressed concern about the uncritical application of the accretionary prism model to the southern uplands of scotland it has since become apparent that divergent views are held both among those against the model and those in favour of it this meeting served to air some of these different opinions even if it has not immediately resolved many of the arguments most of the contributors to the meeting see the society newsletter for january 1985 have produced written papers for this volume and so far only one or two have modified their views between writing their abstract and submitting their paper the concept of an accretionary prism model developed gradually a comparison of scotland with burma suggested that the southern uplands were like western burma where trench and oceanic sediments are being underthrust by subduction processes mitchell mckerrow 1975 with the development of modern analogues from the pacific seeley et al 1974 karig sharman 1975 the model was applied to the southern uplands with more emphasis mckerrow et al 1977 leggett et al 1979 a b 1982 moseley 1977 1978 was one of the first to express doubts about this model he believed that the iapetus ocean had closed by the end of the ordovician and thus like many others

mappable features of melanges derived from ocean plate stratigraphy in the jurassic accretionary complexes of mino and chichibu terranes in southwest japan abstract the melanges of the jurassic accretionary complexes of japan have very complicated structures which were formed by the progressive disruption and mixing of ocean plate stratigraphy ops during various stages of accretionary processes along the ancient arc trench system the ops is composed of permian basalt limestone chert permian triassic boundary claystone early triassic early jurassic chert early jurassic earliest cretaceous siliceous shale and jurassic to early cretaceous turbidite in ascending order the ops was detached from the oceanic plate along a decollement which developed within the p t boundary claystone during the offscraping and underplating processes disruption and mixing first occurred along the decollement then along successively developed out of sequence thrusts neighboring rocks along the faults are mixed to become various sized clasts and matrices of melanges mappable slabs in the melanges were survivors from tectonic disruption of the accreted ops on the surface of accretionary wedge a part of the ops was collapse by submarine sliding and mud diaprisim to form chaotic sediments on the top of the ops thus the full extent of accretionary complex was analyzed by age stage development of melange formation in the melanges some of the large tectonic slabs in the melanges kept the original order of the ops even though they were deformed in various degrees this suggests that the ops was tectonically stacked during the offscraping process and was gradually disrupted along the decollement and out of sequence thrusts during the accretionary processes that form the melange structures the melanges structures mentioned above are shown in the geological maps at the scale of 1 50 000 recently published by the geological survey of japan these geological maps clearly show the melange formation processes during ocean plate subduction along the mesozoic asian continental margin

the southern uplands as an accretionary prism the importance of analogues in reconstructing palaeogeography fore arc regions whether accretionary erosional or composite vary greatly both along strike and through time in the geometries which they assume in response to subduction there is no well studied modern neogene margin which can be established as the 9type9 accretionary prism all are individual in some respect or other nor is there an example of a well studied modern neogene accretionary margin which can be compared closely to the southern uplands though there are several good analogues for particular stages of its history accretion took place along the southern uplands margin for c 50 million years the range of geometries on modern neogene accretionary active margins and the extent to which the behaviour of those margins can change during periods as long as 50 million years is reviewed emphasizing the accretionary prisms of mexico sw japan and the makran the interpretation of the southern uplands as an accretionary prism as presented in the late 1970s and early 1980s is reassessed in the light of advances made subsequently in understanding modern neogene prisms the recent alternative model of stone et al for the southern uplands envisages that the terrane evolved by the opening and closure by arc subduction of a marginal basin the new model draws heavily on arc type detritus derived from the oceanward side of the ancient southern uplands trough there are areas along the sw japan fore arc where oceanward derived tubidites are accumulating in a trench nankai trough and where active submarine arc type volcanism is occurring outboard of a currently growing accretionary prism such features in an ancient terrane like the southern uplands do not necessarily mean that an alternative model must be sought involving whole new arc systems furthermore analysis of the life spans and sedimentary fills of modern neogene marginal basins begs the conclusion that the southern uplands9 strata were not deposited in a marginal basin it is most unlikely that the c 50 ma history of pelagic sedimentation preserved there can be manifesting anything other than the closure of a sizeable ocean basin of a scale exceeding any known western pacific type marginal basin and with a different sedimentation history from any such marginal basin

re interpretation of the age and environment of deposition of paleogene turbidites in the andaman and nicobar islands western sunda arc abstract turbidites composed of sandstone shale alternations on the kalipur shibpur coast north andaman island classified under the palaeocene eocene mithakhari group in several recent papers are identified as and compared with the type section of the oligocene andaman flysch exposed at corbyn s cove south andaman island the kalipur turbidites were interpreted as the inner fan and the latter as the distal mid fan facies of a forearc submarine fan the turbidites of these two locations are separated by 250 km detailed studies concerning the identification and comparison have however revealed significant differences and the turbidite outcrops are not continuous between these two localities turbidites at kalipur shibpur and adjacent areas are part of a melange terrane normally gritty and coarse grained massive to locally graded bedded calcareous intercalated interstratified with conglomerates and reefoidal limestones and show a framework composition varying from volcanolithic to lithic poor arkosic sandstones deposited in several isolated basins fed by transverse supply of detritus from an accreted and uplifted ophiolite and arc massif the kalipur shibpur coast exposes marginally deformed sandstone dominated turbidites coherent units containing abundant ichnotraces late palaeocene foraminifera and rip up shale clasts indicating deposition in shallow water accretionary slope basins south of kalipur the ramnagar coast exposes mud rich turbidites showing accretion related deformation these deformed turbidites are interpreted as offscraped trench deposits further south on the rampur coast turbidites intercalated with reefoidal limestones containing late palaeocene foraminifera indicate deposition on the upper slope or on top of the accretionary slope basin in contrast classical bouma sequence bearing sandstone shale turbidites at corbyn s cove are part of a continuous outcrop belt of siliciclastic turbidites lack fossils carbonate facies and conglomerates and consist of compositionally uniform greywackes deposited in an open deep sea fan fed by axially transported detritus derived from the continental blocks of western burma these key differences together with previous mapping and stratigraphic studies confirm the incorrect identification of andaman flysch in north andaman island in recent papers the turbidites of these two locations were neither produced by the same sediment gravity flows nor deposited in the same forearc fan during the oligocene instead they were derived from different palaeographic domains deposited in different tectonic and sedimentary environments and also at different times this provides new insights into the paleogene turbidite deposition in this part of western sunda arc

a model for the crustal evolution of southern scotland in the late silurian southern scotland was a mature accretionary active margin of sumatra type on the southeastern edge of north america while england was on a passive margin on the opposite side of the iapetus ocean southward subduction below england during the middle ordovician produced a magmatic arc which now lies close to the iapetus suture and thus argues for a missing forearc terrane tectonic erosion or strike slip removal may be responsible for some of this missing terrane to the north of the lake district but we now suggest that underthrusting below southern scotland may also be an important cause if the ordovician forearc terrane of northern england is now below the southern uplands we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the iapetus suture velocity structure from the lispb study sr u pb isotopic signatures and geophysical attributes of the c 400 ma granites the under thrusting model still leaves room for considerable modification of both margins by large strike slip faults seismic refraction profiles of modern accretionary active margins sumatra sw japan allow us to propose a likely velocity profile for late silurian southern scotland we choose a mesozoic forearc truncated by strike slip faulting the queen charlotte islands margin of the ne pacific as an analogue for the late silurian southern iapetus margin following previous suggestions of highly oblique and slow convergence during the final stages of closure of iapetus we combine the two profiles by under thrusting the southern margin to produce a model which involves minimal shortening in the collision zone the crustal profile resulting from such a collision would be very similar to the modern southern scotland profile both in terms of the scale of the tectono stratigraphic terranes involved and of the resulting crustal velocity structure the cessation of accretion after the mid wenlock may be related to impingement of the english continental crust against the scottish trench subsequent uplift of the southern uplands may be connected with the appearance of coarse detritus in the lake district during the silurian

accretion and collision related deformation in the southern uplands accretionary wedge southwestern scotland recent models for the tectonic development of the southern scottish caledonides are reviewed in terms of the deformation sequence observed within the southern uplands of scotland the early stages of this history are consistent with deformation within an accretionary prism formed above a northwest directed subduction zone later stages show a reversal in transport direction and also evidence for sinistral shear a deformational history for the area is proposed in terms of recent data on the deep structure of the southern uplands and adjacent regions the southern uplands is considered to be a large scale popup structure between a forethrust the iapetus suture and a backthrust which partially obducted it toward the northwest

tectonic development of the southern belt of the southern uplands accretionary complex detailed structural studies in the wenlock age southern belt of the southern uplands have identified major controls on tectonic development original thickness and composition of sediment control the style of faulting thin poorly consolidated interbedded sandstone mudstone sequences form broad sheared zones while thicker well consolidated sandstone dominated sequences fail along thin bedding parallel brittle fractures with vein infill the structural evolution of the sheared zones provides an insight into melange development depth of burial within the accretionary pile governs the development of cleavage and intensity of post fl deformational phases the intensity of imbrication within accreted material probably relates to subduction rate with early wenlock sequences being highly imbricated and mid wenlock sediments preserved in internally coherent fault bounded packets sinistral strike slip movement controls the development of steeply plunging folds sub horizontal stretching directions and sideways facing bedding cleavage relationships and strongly influences fabric development in sheared zones determination of the relative importance of these factors permits a fuller understanding of the deformational history

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

comment and reply on age and origin of ballantrae ophiolite and its significance to the caledonian orogeny and the ordovician time scale comment

pb sr and nd isotopes in basalts and sulfides from the juan de fuca ridge pb sr and nd isotopes of seven basalt glasses collected by the submersible alvin from the southern juan de fuca ridge sjfr are almost identical 206pb 204pb 18 45 207pb 204pb 15 47 208pb 204pb 37 81 87sr 86sr 0 70249 143nd 144nd 0 51315 whereas all basalts appear cogenetic four of the samples have uniform abundances of u th rb nd sm pb and sr indicating that they are also comagmatic two basalt glasses dredged previously at the sjfr have similiar isotopic compositions but higher concentrations of u th and pb the 206pb 204pb ratios are intermediate between generally less radiogenic ridge basalts from south of the juan de fuca ridge jfr and often more radiogenic basalts from the northern jfr and ne pacific seamounts sr and nd isotopic compositions closely resemble data of other ridge basalts from the northernmost east pacific rise and are intermediate between isotopically more diverse seamount basalts produced nearby eight sulfides collected from three hydrothermal vent sites at sjfr have very similar pb isotopic compositions to that of the associated basalts sr and nd isotopic ratios of a single sulfide sample lie on a mixing curve between seawater and basalt data indicating that the sulfide precipitated after mixing of hydrothermal solutions and ambient seawater

style scale and significance of sand bodies in the northern and central belts southwest southern uplands sedimentological and biostratigraphical data from the rhinns of galloway and adjacent areas in sw scotland confirm that deep water depositional systems consistently operated along and were sourced from the northwestern margin of an asymmetrical basin during the late ordovician and early silurian while pelagic facies accumulated simultaneously to the se providing a scenario analogous to many modern trench systems most of the observed sedimentological anomalies with regard to this general model can be explained within the context of the varied styles of trench filling depositional systems briefly reviewed here and the major stratigraphic and sedimentologic features can be best explained in terms of a geotectonically evolving fore arc region two main phases of development are recognized a llandeilo late ashgill during this time interval the fore arc trench region was tectonically juxtaposed against an active continental margin arc small to medium scale se prograding sand rich fans were formed within a relatively narrow trench leading to axial diversion of the fans initially to ne but later mainly to sw simultaneously a coarse volcanilithic sediment apron flanking the arc migrated gradually northeastwards probably in response to relative fault displacement of the arc and trench b llandovery during this time interval the fore arc trench region was dominated by a variety of mainly fan type depositional systems which were exclusively sourced at least until the uppermost llandovery from the northwestern margin the juxtaposition of coeval depositional systems differing significantly in sedimentological style and composition strongly suggests that the llandovery sedimentation in this region occurred in several structurally separated sub basins within the main trench system

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

evolution of structures and fabrics in the barbados accretionary prism insights from leg 110 of the ocean drilling program the microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo feldspathic mylonitic shear zone torridon nw scotland have been investigated using sem electron channelling our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history microstructural variation is particularly pronounced at low shear strains significant intragranular deformation occurs via a domino faulting style process whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures a sudden reduction in grain size defines the transition to medium shear strains but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin grains within these bands have somewhat steeper preferred dimensional orientations these domains persist into the high shear strain mylonitic region where they are oriented subparallel to the shear zone margin and consist of sub 20 m grains the microstructures suggest that the principal deformation mechanism was intracrystalline plasticity with contributions from grain size reduction via dynamic recrystallization grain boundary migration and grain boundary sliding crystal fabrics measured from the shear zone vary with position depending on the shear strain involved and are consistent with the operation of several crystal slip systems e g prism basal rhomb and acute rhomb planes in a consistent direction probably parallel to a and or m they also reveal the presence of dauphine twinning and suggest that this may be a significant process in quartz deformation a single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed rather the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation the mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution the stability of some microstructures to higher shear strains the exploitation of others at lower shear strains and a continuously evolving crystal fabric mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient

formation of the late palaeozoic early mesozoic karakaya complex and related ophiolites in nw turkey by palaeotethyan subduction accretion several pre jurassic tectonic units in nw turkey are crucial to the current debate regarding the timing and direction of subduction of palaeotethys a major ocean that separated eurasia and gondwana in late palaeozoic early mesozoic times the most critical unit is the karakaya complex a deformed low grade assemblage of oceanic origin which comprises a nw verging se dipping stack of tectonostratigraphic units here interpreted as a palaeotethyan accretionary complex the units display lithologies consistent with origins in seamount trench abyssal and rifted carbonate platform settings clastic basin sequences developed on top of the complex other relevant tectonostratigraphic units in the area include ultrabasic rocks of supra subduction zone affinity which tectonically overlie permian carbonate platform units with intervening metamorphic soles and melanges restored structural trends suggest the presence of a southward dipping palaeotethyan subduction zone bordering gondwana related units during the late permian triassic this was probably additional to more important regional northward subduction along the southern margin of eurasia from the eastern mediterranean to the himalayas

lycian melange southwestern turkey an emplaced late cretaceous accretionary complex the lycian thrust belt is an important part of the tethyan orogenic belt in the eastern mediterranean region it includes a unit of internally disrupted thin skinned thrust sheets layered tectonic melange and a melange rich in ophiolitic material ophiolitic melange the layered tectonic melange is dominated by mesozoic distal deep water sedimentary rocks and the ophiolitic melange includes disrupted thrust sheets of early mesozoic shallow water limestone and blocks of basalt of both mid ocean ridge and within plate geochemical affinities a complete record of closure of a tethyan oceanic basin began with late cretaceous intraoceanic subduction followed by latest cretaceous trench continental collision and ended with mid tertiary continental collision and orogenic collapse

a sequential back arc and foreland basin thrust duplex model for the southern uplands of scotland thrust imbrication of ordovician and silurian submarine fan sequences overlapping pelagic deposits in the southern uplands has been interpreted in terms of an accretionary prism formed above a nw directed subduction zone structural features invoked to support accretion are not definitive and could be explained in terms of a thin skinned thrusting model new palaeocurrent and compositional evidence from llandeilo to mid llandovery age turbidites in the northern part of the southern uplands proves interdigitation of sediments with strongly contrasting petrography turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz rich formations this implies a back arc situation the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south oblique collision of the opposing continental margins of the iapetus ocean during the llandovery caused the cessation of subduction underthrusting of the southern margin initiated a se propagating thrust stack which deformed the back arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc a southward migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late llandovery hawick group and wenlock sequences mid to end silurian sinistral strike slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep seated structures allowed the intrusion of lamprophyre dykes and granites

imbricated ocean plate stratigraphy and u pb zircon ages from tuff beds in cherts in the ballantrae complex sw scotland ocean plate stratigraphy basalt chert clastics of the balcreuchan group in the ballantrae complex is repeated by layer parallel thrusts to form duplex structures south of bennane head 5 km north of ballantrae five tuff beds all in chert have similar u pb zircon ages of ca 470 10 ma the geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the ballantrae complex northwestward thrusting of the duplexes suggests that subduction was from the northwest to the southeast and this polarity at the leading edge of the main ophiolite body is consistent with the northward younging of the volcanic arc rocks of the ballantrae complex

constraining seismic parameters with a controlled source audio magnetotelluric method csamt summary precise seismic two way traveltime twt determinations are often limited by the knowledge of the overburden thickness and its seismic velocity values between 500 and 1800 m s 1 are conceivable according to the water content the csamt method has been successfully applied to determine the thickness of the quaternary fluvio glacial deposits and tertiary molasse interbedded marls and siltstones overlying mesozoic carbonates along a 3 km profile through a syncline in the swiss jura fold and thrust belt the homoclinal structure within the syncline is well constrained by a petroleum exploration vibroseis profile which indicates a sse dipping mesozoic series and a similarly oriented increasing thickness of molasse and quaternary the csamt places tight constraints on the apparent resistivities of the three formations quaternary 300 1000 m molasse 5 15 m mesozoic 100 1000 m furthermore modelling of the csamt sounding data provides absolute thicknesses for both the quaternary 80 20 m and the molasse 10 210 m layers revealing a true dip of the top mesozoic substratum of 7 to the sse the seismic velocities of the quaternary weathering layer and the molasse can thus be determined at 800 and 2100 m s 1 respectively

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 structure of the western part of the southern uplands of scotland the structures of two areas one in the northern belt and one in the central belt of the southern uplands are described and compared the main structural histories are the same both show a main deformation phase related to se directed thrusting with associated fold and cleavage formation both areas exhibit late minor nw directed thrusting with related refolding and crenulation cleavage development in the case of the central belt both areas also show evidence of sinistral strike slip or transpressive deformation which reactivates steepened thrust faults when these deformation histories are reviewed in the context of geophysical radiometric and biostratigraphical data from the region a forearc accretionary setting for the whole of the southern uplands appears the most likely the strike slip deformation has internally disrupted an accretionary prism but has not juxtaposed originally unrelated terranes the effects of the late silurian closure of iapetus and subsequent events are also represented in the southern uplands

duplex accretion and underplating in an ancient accretionary complex kodiak islands alaska the landward belt of the kodiak formation represents a sequence of lower maastrichtian turbidites that was underplated to the kodiak accretionary complex by duplex accretion evidence for duplexes exists on the mesoscopic and map scale at the microscopic scale incremental strain histories are consistent with early shear during underthrusting abrupt landward rotation of individual thrust slices over fault ramps and locking of thrust packets in steeper orientations duplex geometries at the mesoscopic scale are represented by high angle reverse faults bounded either above or below by low angle southeast verging detachments at the map scale different structural belts can be related to a low angle detachment zone central belt flooring a more landward duplex landward belt duplex underplating can result in the uplift and thickening of accretionary prisms without necessarily deforming the overlying previously accreted material and slope sediments

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