the world s second oldest strophomenoid dominated benthic assemblage in the first dapingian middle ordovician brachiopod fauna identified from iran abstract rocks previously mapped as devonian at gerd kuh in the eastern alborz mountains of iran have yielded middle ordovician faunas including the brachiopods described here the first rhynchonelliform brachiopods of dapingian age ever described from iran the shells described are defined as a new dirafinesquina benthic association and which is the second oldest strophomenoid dominated assemblage from anywhere in the world there are six brachiopods three of which are newly named as the strophomenoid dirafinesquina antiqua sp nov the orthoid zhanorthis gerdkuhensis gen et sp nov and the porambonitoid eoporambonites raziabadensis sp nov

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

deformation paths in the shallow levels of an accretionary prism the eocene shimanto belt of southwest japan detailed structural analysis of coherent turbidites in a part of the eocene shimanto belt on the muroto peninsula shikoku island japan reveals a structural evolution that is typical of the shallow levels of accretionary prisms in style and sequence of deformational fabrics but anomalous in that it includes both a marked rotation in shortening direction and a major out of sequence thrust deformation throughout a history of progressive lithification and burial was dominated by subhorizontal shortening and seaward vergent shear structures evolved from predominantly mesoscale thrusts and folds to larger more brittle faults and finally to kilometer scale amplified fault bend folds with an associated pressure solution cleavage this structural progression is consistent with increasing strength of the rocks decreasing fluid pressures and increasing temperature and pressure as the sediments were dewatered lithified and accreted to the prism in contrast to this straightforward structural evolution the prism scale deformation of the eocene shimanto belt is difficult to reconcile with simple models of prism developmentthat assume orthogonal subduction and seaward stepping imbrication a 50 counterclockwise rotation in the shortening direction and a major out of sequence or intraprism thrust are recorded by two distinct structural domains in both the earliest structures are clastic dikes and faults indicative of north south shortening one domain is characterized by east striking regional scale folds and cleavage that are transected by an east northeast trending cleavage however the other is characterized by northeast striking folds and cleavage that are transected by a slightly more north striking cleavage the different deformation histories of the two do mains require that they be separated by a major intraprism fault this fault carries tectonic melange containing blocks of pillow basalt and chert indicating that it rooted in the decollement zone beneath the prism the tectonostratigraphy and deformation histories of the two domains suggest that both before and after the out of sequence fault developed higher structural levels of the prism experienced significant subhorizontal shortening whereas lower structural levels bypassed this zone of deformation

chronology of deformation within the turbidite dominated lachlan orogen implications for the tectonic evolution of eastern australia and gondwana ar ar data from fabric forming white mica in slates syntectonic quartz veins and granitic mylonites constrain the timing of metamorphism deformation and exhumation in the lachlan orogen australia these data also help define the tectonic evolution of the tasmanides during paleozoic time the lachlan orogen formed by the progressive accretion of a thick turbidite fan sequence and volcanic terrains to gondwana during the closing of a small marginal ocean or back arc basin this tectonic setting was similar to the present western and southwestern pacific region accretion of the lachlan orogen to gondwana occurred by closing of the basin system by subduction accretion processes and some translation the process is typified in the western lachlan orogen by a major eastward migrating deformation involving chevron folding and faulting over an eastward propagating decollement melange zone and is recorded by ar ar mica ages ranging from 455 ma in the western part to 390 ma in the eastern part in the central lachlan orogen deformation migrated southwestward from 440 430 to 405 ma away from the high temperature wagga omeo metamorphic complex where deformation metamorphism occurred between 440 and 400 ma in the north 400 ma mica ages record deformation and inversion of structures in the cobar basin in the eastern lachlan orogen ar ar mica dates range from 450 to 340 ma ages of 455 445 ma are yielded by the narooma accretionary complex 405 390 ma ages are found along the major thrust faults bounding high grade metamorphic complexes and 360 340 ma cooling ages are found in the inverted extensional basins e g hill end and related structural zones the ar ar results also document periods of reactivation on early formed structures during later deformation elsewhere in the orogen

synkinematic microscopy of transparent polycrystals abstract deformation of weak polycrystalline materials in thin section on the stage of a petrographic microscope offers insight into the origin of optical microstructures similar to microstructures in naturally deformed rocks sample materials used to date include several soft minerals ice carnallite bischofite nitratite and various organic plastic materials they can be simple sheared or pure sheared at temperatures to 300 c at a variety of strain rates using relatively simple and inexpensive apparatus the complete microstructural evolution can be recorded photographically along with the complete kinematic history research applications have focussed on dynamic recrystallization and progress has been made towards understanding how the average grain size is maintained constant in steady state recrystallization flow records have been obtained of the development of various crystal plastic microstructures and of lattice reorientation trajectories during flow teaching applications have begun to be developed and several videotapes are available future technical developments are expected to permit stress measurement or control during experiments and synkinematic recording of lattice reorientation numerous research areas invite attention these include investigation of the microstructural developments that precede faulting the behaviour of deforming crystal melt systems and the behaviour of dynamically reactive multi phase systems

structural evolution of the shimanto belt accretionary complex in the area of the gokase river kyushu sw japan the shimanto belt of sw japan is an accretionary complex which developed between late cretaceous and late miocene the deeper levels of this complex are exposed along the gokase river of eastern kyushu deformation in this region took place as a result of southeasterly directed accretionary thrusting resulting in the development of folds fabrics and melanges all these structures may be attributed to progressive top to the se shearing and the melanges show many similarities to other rock types in fault or shear zones it is also possible to apply shear zone models to explain extensional structures developed perpendicular to thrust transport direction it is suggested that deformation took place during underplating beneath the developing prism

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

crater floor topography and impact melt sheet geometry of the sudbury impact structure canada terra nova 00 000 000 2010 abstract based on high resolution topography data and the geometry of geological contact traces at the surface we determined the orientation of layer interfaces of the sudbury igneous complex sic our analysis provides for the first time quantitative structural evidence for the thickness variation of its layers which supports an impact melt origin of the sic the present crater floor topography varies up to 400 m over distances of hundreds of metres to a few kilometres and up to 1500 m over a distance of about 25 km crater floor depressions are spatially associated with economically important sulphide mineral deposits which points to a viable exploration strategy finally observed variations in the width of the thermal aureole imparted by the sic on its host rocks are due to primary thickness variations of the sic rather than post impact deformation

early cleavage development in the late ordovician of northeast victoria australia abstract a pervasive cleavage s that subparallels bedding in the upper ordovician rocks of northeast victoria can be related to an incremental strain distribution in a shear environment during the establishment of the first set of regional tectonic folds and faults the cleavage is a strong preferred orientation of phyllosilicates which overprints sedimentary features is contemporaneous with early formed extension fractures and postdates the formation of clastic dykes the cleavage is attributed to tectonic and metamorphic processes rather than a static mimetic recrystallization of clay minerals during burial metamorphism there is little evidence to suggest that this is an early deformation originating during imbricate stacking of an ordovician accretionary prism

progressive deformation in an accretionary complex an example from the shimanto belt of eastern kyushu southwest japan the shimanto accretionary complex of eastern kyushu records a progressive deformational history that reflects the gradual underthrusting of sediments beneath the prism and their eventual accretion at depth to the base of the complex initial deformational style is controlled by grain size and porosity of the sediments stratal disruption of coarser grained highly porous sandstones is accommodated by ductile intergranular flow whereas finer grained less porous sandstones and mudstones deform by localized shearing and brittle failure as further reductions in porosity accompany lithification both sandstones and mudstones continue to deform in a brittle manner with progressive increases in pressure and temperature accompanying tectonic burial shear bands and minor isoclinal folds are generated deforming earlier formed fabrics and indicating a south southeasterly shear direction with increasing strain these folds undergo rotation and their axes approach that of the transport direction a pronounced stretching lineation is typically associated with regions where these folds have been rotated suggesting intense noncoaxial layer parallel shear these zones of higher shear strains may be related to thrust fault propagation during accretion of material to the base of the prism

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

timing constraints on deformation history of the sudbury impact structure herein we present new high resolution seismic images of the sudbury impact structure acquired across the sudbury igneous complex and its environs which provide evidence for the relative timing of the deformation events that reshaped the initial sudbury structure the seismic images show that the lower unit of the sudbury basinal fill sediments the onwatin argillite is penetrated by a set of blind imbricated thrusts whereas the overlying chelmsford turbidites are unaffected by faulting we interpret this observation to mean that the deposition of the chelmsford sediments postdates the latest major deformation of the sudbury structure suggesting that the uniform paleocurrent trends observed in the chelmsford turbidites are not related to the initial shape of the sudbury structure

mid crustal deformation at the southern flank of the sudbury basin central ontario canada a field based structural analysis was made in the southern footwall of the sudbury basin to facilitate a reassessment of the igneous emplacement mechanism of the 1 85 ga sudbury norite i e synfold intrusion impact melt ponding more specifically we wanted to discern whether attitude and curvature of the norite shell were primary or produced mainly by noncylindrical folding the southern footwall of the sudbury basin is composed of paleoproterozoic metavolcanic rocks of the lower huronian supergroup that host the 2 3 ga creighton and murray plutons the geometry of foliation trajectories and other structural evidence reveal that the plutons were emplaced into ductilely deforming host rocks during the blezardian tectonic pulse 2 4 2 2 ga this deformation led to a southward overturn of huronian metavolcanic rocks and was accompanied by amphibolite facies metamorphism static greenschist facies metamorphism in metavolcanic and granitoid rocks was prompted by penokean northward thrusting after subconcordant emplacement of the norite the lack of evidence for penokean folding of huronian footwall rocks suggests that the curvilinear shape of the norite shell is primary this in turn constitutes evidence against impact melt ponding and lends indirect support to an intrusive origin

backflow and margin parallel shear within an ancient subduction complex lineated and foliated tectonites of the teslin suture zone yukon represent the deep seated part of a west dipping subduction complex that formed seaward of western north america in permian triassic time the tectonites were progressively underplated onto the hanging wall plate during b type subduction with continued subduction and underplating the boundary between the hanging wall and subduction channel migrated oceanward resulting in growth of the hanging wail plate at depth these tectonites preserve a progressive record of backflow dextral margin parallel shear and downflow within the subduction channel early jurassic a type subduction of the leading edge of western north america resulted in eastward overthrusting of the tectonites as a coherent block onto north american strata these tectonites provide empirical evidence that backflow margin parallel displacement and downflow can all occur within the deeper parts of subduction zones

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

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

ordovician marginal basin evolution near the palaeo pacific east gondwana margin australia although the ordovician macquarie arc part of the eastern lachlan orogen of southeastern australia has long been considered to be an intra oceanic arc within an accretionary orogen key characteristics contrast with more typical examples of accreted arcs significantly multiple stacked phases of mafic to intermediate volcanic rocks with suprasubduction zone chemistry are flanked to the east and west by extensive coeval continental margin turbidite chert and black shale sequences by analysing stratigraphic and contact relationships within and between the volcanic rocks ophiolitic components sensu lato and turbidite sequences constrained by precise biostratigraphy we document a repeated cycle of uplift upper plate extension and collapse common to all sequences this cycle is interpreted as resulting from localized extension rifting or hyper extension and lower crustal delamination within a continent margin sequence developed upon an already established marginal or back arc basin of probable middle to late cambrian age our interpretation provides a counter example to prevailing arc accretion models by inferring extensional tectonics at the palaeo pacific east gondwana margin during the ordovician with development of alkalic and calc alkalic cu au porphyry deposits away from an active arc system

nucleation of ductile shear zones on dilatant fractures small shear zones in granitic rocks of the sierra nevada california and near roses rosas northeast spain display features which indicate that dilatant fracturing preceded the localization of ductile shear deformation many zones have sharp nearly planar boundaries between their highly deformed interiors and the undeformed wall rock the roses shear zones narrow continuously to hairline fractures at their ends mineralized microcracks oriented parallel to the shear zones cluster near the zone boundaries and are interpreted as relicts of the earlier fracturing episode dynamic recrystallization of minerals filling these microcracks demonstrates that fracturing predated the ductile deformation gradients in ductile strain suggest that deformation spread laterally into the wall rocks after nucleation of the shear zones we suggest that cracks enhance the wall rock ductility by increasing local fluid fluxes thereby promoting chemical alteration and or hydrolytic weakening

sheath like folds and progressive fold deformation in tertiary sedimentary rocks of the shimanto accretionary complex japan abstract the upper oligocene lower miocene portion of the cretaceous lower miocene shimanto accretionary complex on the muroto peninsula southwest japan comprises alternating belts of coherent clastic rocks and shaly melange sheath like folds formed locally during a fold thrust deformation that affected a 10 km structural thickness of these rocks in a subsequent event confined to the melange and adjoining areas the axes of these folds were rotated nearly 90 within the plane of their axial surfaces the widespread uniform development of the folds and an accompanying axial planar pressure solution cleavage strongly suggest that these deformations were tectonically induced rather than gravity driven slump features the sheath like folds indicate that the most plausible environment for these rocks was in an accreted packet of trench fill behind the imbricate toe of the shimanto accretionary complex the timing of the deformation is constrained by depositional ages as well as by the probable early miocene age of an overlying superficial basin and early middle miocene igneous dikes that intrude the sequence this chronology overlaps with the late oligocene early miocene opening of the shikoku back arc basin bordering the shimanto belt to the southwest thus accretion and subduction were active during the basin opening event these conclusions conflict sharphy with earlier held notions of strike slip and olistostrome tectonics along this plate margin during the early miocene opening of the shikoku basin

eocene underplating along the kodiak shelf alaska implications and regional correlations structural geology and geophysical data from the kodiak shelf suggest that the mesozoic rocks exposed on the shelf are structurally underlain at about 12 km depth by several kms of eocene age strata kinematic data from the late cretaceous to paleocene ghost rocks formation indicate that this formation and probably all of the kodiak islands were uplifted vertically to nearly their present elevations landward tilting and imbrication are not indicated the age of uplift is indicated by a regional angular unconformity of early eocene to early oligocene age that separates deep sea rocks from shallow water to non marine rocks the uplift of the accretionary prism is believed to have been caused by underplating of an eocene sedimentary sequence because 1 a band of seismic reflections that occur 12 to 20 km beneath the shelf is interpreted as the top of the underplated material and 2 an obductively offscraped sequence of eocene deep sea rocks crops out on the seaward side of the kodiak shelf suggesting that a thick trench fill sequence may have been present prior to uplift of the prism the underplated material is interpreted to be part of either a previously unrecognized turbidite fan of early eocene age or a proximal equivalent of the zodiac fan of late eocene to early oligocene age other possible on land remnants of the underplated material may be present in prince william sound the montague belt the gulf of alaska lower sections of the yakutat block and in the coast ranges of oregon and washington the large volume of underplated material beneath the kodiak shelf suggests that underplating may be the dominant process in the growth of convergent margins

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

the dynamic evolution of the palaeozoic geography of eastern asia abstract new palaeogeographical reconstructions are presented for eleven time intervals through the palaeozoic of the eastern asia region from the middle cambrian at 510 ma to the end of the permian at 250 ma they centre on the continental blocks of north china south china and annamia indochina and their relationships with northeastern gondwana which was united to form part of pangea from the late carboniferous onwards also shown is the continent of tarim during the lower palaeozoic as well as the hutag uul songliao and khanka jiamasu bureya terranes both of which straddle the russian mongolian and chinese borders today from silurian times onwards we conclude that annamia and south china were united as a single continent throughout the lower palaeozoic and early devonian and were translocated by major strike slip faulting along the northeastern gondwana margin during that period from off afghanistan to outboard of the sibumasu and australian sectors of the superterrane they left the gondwana marginal area together during the lower devonian opening of the palaeotethys ocean but very shortly afterwards they themselves divided into the two separate continental blocks that we recognise today not to reunite until the triassic the various cambrian to permian rocks found in japan largely represent active volcanic arcs which originally lay to the southeast of south china although the carboniferous was more quiescent there the neotethys ocean opened during the permian dividing sibumasu and the tibetan terranes from gondwana and the palaeotethys ocean started to close progressively in the upper palaeozoic as most of the east asian continents and smaller terranes moved towards siberia the positions of the various continents and terranes have been deduced from a mixture of palaeomagnetic and faunal data the positions of large igneous provinces and kimberlites and the need to provide kinematic continuity between maps of successive ages however many uncertainties remain

middle palaeozoic thrusting in the eastern lachlan fold belt southeastern australia abstract early to middle palaeozoic clastic and volcanic strata in the goulburn bungonia region 150 km southwest of sydney are divided by the n trending yarralaw fault into two domains in the western domain major e w shortening formed folds axial planar cleavage and w dipping contraction faults in all pre upper devonian units this deformation formed an anticlinorium cored by a w dipping thrust system with an overturned e younging limb in the eastern domain upright flattened chevron folds occur in ordovician strata with stratal repetition along steep contraction faults spaced at intervals of 1 km or less zones of tectonic melange occur in the hangingwalls of these faults and have a well developed scaly fabric with a steeply plunging striation overprinting a bedding parallel cleavage in mudstone in interbedded silurian shale and limestone a classical stair step trajectory is recognized for the frome hill fault which duplicates the succession and has undergone back rotation to its present steep dip overall thrusting progressed from east to west with deformation in the eastern domain pre dating intrusion of the early devonian marulan batholith whereas farther west deformation continued into the middle devonian the folds and thrusts in the goulburn bungonia region are part of a major fold thrust zone that extends throughout the eastern lachlan fold belt and which was formed in the middle silurian to middle devonian during a period of plate convergence between gondwana and the palaeo pacific ocean the fold thrust zone is inferred to have an arc frontal arc setting above a w dipping subduction zone and the deformation relates to underthrusting of an allochthonous terrane with major shortening in the hangingwall block

strain variations in an ancient accretionary complex implications for forearc evolution strain magnitudes and orientations and incremental strain histories are characterized throughout the kodiak formation a portion of a late cretaceous early tertiary accretionary prism the results of these analyses are used to evaluate the history of underplating penetrative strain and unroofing in the forearc of the southwest alaskan convergent margin five major deformation events are recognized in the kodiak formation 1 layer parallel extension and layer parallel shear d1 related to underthrusting 2 seaward verging fold and thrust deformation d2 with development of both a slaty cleavage s2 and a downdip stretching lineation l2 related to underplating and early penetrative strain within the wedge 3 development of a regional scale antiform d3 4 development of a shear zone along the northwest limb of the d3 antiform d4 and 5 right lateral strike slip faulting and penetrative right lateral simple shear in the seaward half of the kodiak formation d5 based on sharp differences in structural style and the magnitude and orientation of strain associated with d2 the kodiak formation has been subdivided into three structural units informally called the landward central and seaward belts the structural variations observed in these three belts correspond to differences in structural relief of 1 5 km associated with the d3 antiform the lowest d2 structural level is in the core of the antiform central belt and progressively higher structural levels are exposed on both limbs landward and seaward belts the regional distribution of incremental strain histories associated with d2 d3 d4 and d5 has been quantified by using syntectonic fibers in pressure shadows around spherical pyrite framboids from the highest to the lowest structural level d2 strain is characterized by 1 an increase in the magnitude of finite strain 2 a change in the orientation of finite extension from nearly vertical to gently northwest plunging 3 a change in the incremental extension direction from nearly vertical to moderately northwest plunging and finally 4 a change in the strain history from coaxial to noncoaxial with a sense of rotation consistent with southeast directed thrusting we interpret these d2 strain patterns to reflect a strain field that was fixed relative to the basal decollement with coaxial subhorizontal shortening in the interior of the prism and noncoaxial strain probably simple shear near the base subsequent variations in the orientation of incremental extension associated with d3 are consistent with relatively rapid vertical translation of the accreted sequences relative to the d2 strain field we specifically propose two periods of wedge thickening in response to a combination of underplating and penetrative strain these two wedge thickening events correspond with episodes of subaerial exposure and erosional unroofing in the late cretaceous and middle tertiary

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

structural analysis of the central and southwestern sudbury structure southern province canadian shield the sudbury structure ss is an unusual crater structure which acquired its present oval surface shape during northwest directed ductile thrusting lower amphibolite facies metamorphism accompanied the thrusting which generated a major reverse shear zone at least 50 km long the south range shear zone srsz transects the south range of the sudbury structure and exhumes a low level of the sudbury igneous complex sic assuming heterogeneous simple shear in the northwest southeast vertical plane on northeasterly striking glide surfaces minimal estimates of net displacement across the srsz exceed 8 km this displacement magnitude and the map pattern of the sic require the southwest closure of the ss to be steeply plunging in accord with a hypothetical funnel shape of the sic the rocks of the metasedimentary core of the ss are deformed into a family of second order buckle folds the tangent surface of which forms an upright open flexure within the first order structure of the sudbury synclinorium the

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

strain pattern in the aar granite central alps orthogneiss developed by bulk inhomogeneous flattening the central part of the aar granite aar valley shows lens shaped domains of low strain separated by anastomosing domains of high strain a comparable pattern is found on all scales of observation deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites ultramylonites are concentrated within conjugate ductile shear zones where moderate the strain is concentrated in very narrow small scale faults which affect relatively undeformed granite within foliated domains finite strain measurements on deformed xenoliths indicate that i strain intensity increases gradually with grain size reduction and ii finite strain ellipsoids are of flattening type irrespective of the degree of mylonitization the pattern of small scale faulting the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

geometric consequences of ductile fabric development from brittle shear faults in mafic melt sheets evidence from the sudbury igneous complex canada abstract compared to felsic igneous rocks the genetic relationship between brittle and ductile fabric development and its influence on the geometry of deformed mafic melt sheets has received little attention in structural analyses we explore these relationships using the sudbury igneous complex sic as an example the sic is the relic of a layered impact melt sheet that was transformed into a fold basin the sudbury basin during paleoproterozoic deformation at the southern margin of the archean superior province we studied brittle and ductile strain fabrics on the outcrop and map scales in the southern sudbury basin notably in the norite and quartz gabbro layers of the sic here deformation is heterogeneous and occurred under variable rheological conditions evident by the development of brittle shear fractures brittle ductile shear zones and pervasive ductile strain the mineral fabrics formed under low to middle greenschist facies metamorphism whereby brittle deformation caused hydrolytic weakening and ductile fabric development principal strain axes inferred from all structural elements are collinear and point to a single deformation regime that led to thinning of sic layers during progressive deformation ductile fabric development profoundly influenced the orientation of sic material planes such as lithological contacts and magmatic mineral fabrics more specifically these planar structural elements are steep where the sic underwent large magnitudes of thinning i e in the south limb of the sudbury basin here the actual tilt component of material planes is likely smaller than its maximum total rotation 60 inferred from inclined igneous layering in the norite our field based study shows that ductile fabric development from brittle faults can have a profound influence on the rotational components of primary material planes in deformed igneous melt sheets

a review of the metallogeny and tectonics of the lachlan orogen future mineral exploration within eastern australia will be enhanced by resolving the tectonic evolution of the lachlan orogen to establish the spatial and temporal terrane distribution of the various mineral deposits the lachlan orogen from north eastern tasmania through to central and eastern new south wales is host to a number of major mineral deposit styles including orogenic gold e g stawell ballarat bendigo volcanic hosted massive sulphide e g woodlawn currawong sediment hosted cu au e g cobar basin deposits porphyry au cu e g cadia parkes cowal and granite related sn e g ardlethan beechworth each of these mineral deposit styles is a sensitive and diagnostic indicator of the prevalent tectonic environment during their formation in this review we briefly summarise the deposit to large scale factors that define the diverse metallogenic evolution of the lachlan orogen this overview is intended to set the scene for subsequent specialist papers published in this thematic issue on the metallogeny and tectonics of the lachlan orogen in south east australia

character and kinematics of faults within the turbidite dominated lachlan orogen implications for tectonic evolution of eastern australia reply fault zones within turbidite dominated orogenic systems typified by the lachlan orogen of eastern australia are characterised by higher than average strain and intense mica fabrics transposition foliation and isoclinal folds poly deformation with overprinting crenulation cleavages and steeply to moderately plunging meso and micro folds they have a different character compared to the brittle ductile fault zones of classic foreland fold and thrust belts such as the appalachians and the canadian rocky mountains multiple cleavages and transposition layering record a progressive shear related deformation history an intense mica fabric evolves initially during shortening of the overlying sedimentary wedge but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults the deformed wedge outside the fault zones generally undergoes one phase of deformation shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright subhorizontally plunging chevron folds other faults within the turbidites of the lachlan orogen include the steep zones of ductile strike slip deformation that bound a centrally located high t low p metamorphic complex characterised by s c mylonites these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge with emplacement to higher structural levels along a leading edge ductile thrust fault ar ar dating constrains the timing of regional deformation to be mostly late ordovician through silurian across the lachlan orogen faults in the low grade turbidite sequences record the kinematic evolution of the developing lachlan orogen and indicate progressive deformation associated with simultaneous eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt these deformation fronts are related to accretionary style deformation at the leading edges of overriding plates in an inferred southwest pacific type subduction setting from the late ordovician to the mid devonian along the former gondwana margin the fault zones effectively accommodate and preserve movements within the structurally thickening migrating and prograding accretionary wedge

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

tectonic implications of the nan suture zone and its relationship to the sukhothai fold belt northern thailand abstract the nan suture and the sukhothai fold belt reflect the processes associated with the collision between the shan thai and indochina terranes in southeast asia the shan thai terrane rifted from gondwana in the early permian as it drifted north a subduction complex developed along its northern margin the nan serpentinitic melange is a thrust slice within the pha som metamorphic complex and in total this unit is a late permian accretionary complex containing offscraped blocks from subducted oceanic crust of carboniferous and permian age the deformational style within the pha som metamorphic complex supports a west dipping subduction zone the late permian to late triassic fore arc basin sediments are preserved in the sukhothai fold belt and include a near continuous sedimentary record at least locally the whole sequence was folded and complexly thrust in the late triassic as a result of the collision late syn to post kinematic granites place an upper limit of 200 ma on the time of collision post orogenic sediments prograded across the suture in the jurassic

chronology and tectonic framework of turbidite hosted gold deposits in the western lachlan fold belt victoria results abstract 40 ar 39 ar data constrain the history of metamorphism deformation and mineralization in the western subprovince of the lachlan fold belt rocks in the mount stavely volcanic complex and the bushy creek pluton give 40 ar 39 ar ages of about 500 ma for hornblende and biotite the dates from the mount stavely volcanic complex limit the age of mineralization in the metavolcanic rocks of the stavely belt whereas the date for the bushy creek pluton extends the area of cambrian ordovician plutons of the delamerian orogeny to east of the grampians mountains and helps define the surface expression of the western border of the paleozoic lachlan fold belt 40 ar 39 ar dating of metamorphic mica growth phengite in cleaved slates and phyllites within the landsborough fault avoca fault zone heathcote fault zone and mount wellington fault zone gives ages of 453 2 440 2 426 4 and 410 390 ma respectively these data indicate a progression of deformation from west to east starting in late ordovician early silurian time in the stawell and bendigo ballarat structural zones the major phase of deformation in the western subprovince of the lachlan fold belt is therefore silurian and not devonian sericites from major gold deposits in the stawell zone and bendigo ballarat zone give 40 ar 39 ar dates of about 440 ma that are coincident with folding and thrusting a second episode of mineralization and fault reactivation at about 420 to 410 ma is found in some deposits in these zones based on data from a previous study in the melbourne zone including the woods point dike swarm and locally in the bendigo ballarat zone some gold deposits give sericite dates of about 380 360 ma these ages are consistent with the close association of many melbourne zone gold deposits with devonian dikes and plutons the results indicate that gold mineralization in the stawell zone bendigo ballarat zone and melbourne zone was episodic and both temporally and spatially associated with crustal heating linked to regional metamorphism and plutonism initial stages of mineralization in the province comprising many of the large deposits in western and central victoria e g bendigo ballarat stawell wattle gully tarnagulla accompanied metamorphism folding and thrusting subsequent mineralization is widespread but produced mainly smaller deposits

synchronous compression and extension in east gondwana tectonic controls on world class gold deposits at 440 ma the collision of a seamount or microcontinental block with an arc may produce complex strain distributions and changes in the tectono magmatic setting slab rollback and associated upwelling of asthenosphere occurred ca 440 ma near the margin of east gondwana in response to subduction zone lock up following arc collision ca 455 ma world class porphyry copper gold deposits e g cadia formed in the backarc during extension and magmatism associated with slab rollback upwelling of the asthenosphere also provided the heat engine for generation of hydrothermal fluids the result was elevated fluid pressures and compressional brittle deformation associated with orogenic gold mineralization e g bendigo inboard of the east gondwana margin

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

the early devonian palaeobiogeography of eastern australasia abstract biogeographical analyses of devonian australasia australia and new zealand have previously presented a narrative approach to the interrelationships of continental faunal provinces useful for the creation of biotic hypotheses similarities and differences between biotic provinces remains untested this study seeks to propose the first regional faunal provinces allowing a test of geological and biotic hypotheses using parsimony analysis of endemicity the relationships of eastern australasian terranes were tested through the hierarchical analysis of faunal composition biochorologic sub provinces were proposed using geo spatial computer software and analysed through phylogenetic software that resulted in the formalisation of 20 biochorologic sub provinces the relationships between these units show a significant holdover of silurian species the interactions between convergent terranes and the continental margin the distinct history of the lachlan fold belt terranes and offers biotic support for the existence of the tasmanian microcontinent vandieland

accretionary complex structure and kinematics during paleozoic arc continent collision in the southern urals abstract the southern urals contain a well preserved accretionary complex that has overthrust the continental margin during arc continent collision between the east european craton eec and the magnitogorsk island arc in the late devonian within the accretionary complex we study three tectonic units that differ in deformation style and each provides a unique geodynamic implication the zilair nappe the largest and best exposed unit consists of 5 6 km of syncollisional arc derived upper devonian to lower carboniferous polymictic and graywacke turbidites that were deposited across the continental margin and incorporated by frontal accretion into the accretionary complex the zilair nappe is a bivergent thrust imbricate where the west vergent thrusts dominate and have associated kilometer scale ramp anticlines with well developed east dipping axial planar cleavage along its eastern contact however the cleavage fans until it dips moderately westward and the folds are east vergent following its emplacement west vergent basement involved thrusting that breached the whole accretionary complex imbricated the zilair nappe the timirovo duplex is structurally beneath the zilair nappe and outcrops for several tens of kilometers along its northwestern margin the duplex forms a west vergent thrust stack composed of a highly deformed and sheared lower and middle devonian reef carbonates of the former eec margin platform these rocks were shallowly underplated at the base of the accretionary complex during emplacement over the margin the suvanyak complex outcrops along the eastern contact of the zilair nappe and consists of polydeformed greenschist facies metasediments of the former eec slope that were offscraped underplated and incorporated at the rear of the accretionary complex

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

the evolution of the onaping formation at the sudbury impact structure the 1 4 1 6 km thick onaping formation consists of a complex series of breccias and melt bodies lying above the sudbury igneous complex sic at the sudbury impact structure based on the presence of shocked lithic clasts and various glassy phases the onaping has been described as a suevitic breccia with an origin at least in part as fallback material recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases both as clast types and discrete intrusive bodies the nature of the onaping and that of other suevitic breccias overlying impact melt sheets is reviewed the relative thickness internal stratigraphic and lithological character and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the onaping the sudbury structure formed in a foreland basin and water played an essential role in the evolution of the onaping as indicated by a major hydrothermal system generated during its formation taken together observations and interpretations of the onaping suggest a working hypothesis for the origin of the onaping that includes not only impact but also the interaction of sea water with the impact melt resulting in repeated explosive interactions involving proto sic materials and mixing with pre existing lithologies this is complicated by additional brecciation events due to the intrusion of proto sic materials into the evolving and thickening onaping fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two thirds of the onaping

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

nucleation growth and structural development of mylonitic shear zones in granitic rock abstract well exposed shear zones in granitic rock of the sierra nevada are preserved in various degrees of development facilitating interpretation of the mechanisms of shear zone nucleation and growth field relations suggest that these shear zones formed by shear localization within pre existing aplite dikes that served as tabular zones of inherent weakness crossing dikes provide a measure of the offset at several points along the shear zones apparently the shear zones grew in width as they accumulated offset and propagated longitudinally by end to end linkage of approximately coplanar aplite dike segments propagation caused discontinuous dikes or dike segments to be joined giving the shear zones a forked or eye shaped trace pattern some shear zones have developed opening fractures at their tips analogous to the splay fractures that form near the tips of small faults these shear zones provide an excellent opportunity to study the processes involved in natural shear zone formation in granitic rocks where shear is localized along pre existing zones of inherent weakness they may serve as useful analogs for the growth and development of larger scale faults that involve ductile shearing

significance of the early foliation at bermagui n s w australia the first generation mesoscopic folds f1 in the ordovician turbidites at bermagui n s w australia are tight to locally isoclinal and have a convergently fanning axial surface foliation s1 which is a differentiated crenulation cleavage s1 overprints an earlier foliation herein called s in psammites s is a strong alignment of mineral grains and rough micaceous folia and in pelites s is a microbanding of quartzose and micaceous layers depending on rock type and position in the f1 fold profile s may be 1 parallel to bedding 2 rotated out of parallelism with bedding by the s1 cleavage and 3 truly cross cutting bedding in many places s cuts across bedding demonstrating that it is a superimposed tectonic foliation and not simply an inherited depositional fabric in outcrops south of bermagui s changes vergence with apparent congruence with f1 folds in other outcrops near bermagui however s maintains a constant vergence around f1 folds the geometry indicating a pre f1 anticline to the east since there is no evidence for large scale pre f1 folds the various relations of s to s1 and f1 are best explained by the rarely described process of late stage hinge nucleation during a single protracted f1 folding sequence other possibilities such as fold hinge migration and pre fold imbricate stacking are discussed but considered less likely

seismic images and three dimensional architecture of a proterozoic shear zone in the sudbury structure superior province canada a broad zone of intense ductile to brittle deformation transects the paleo proterozoic sudbury igneous complex and likely formed during the final stages of the penokean orogeny on the southern margin of the superior province herein we present a three dimensional image of this deformation zone derived from four seismic reflection profiles additional information from rock property measurements drill results and published structural mapping suggests that the seismic data define the three dimensional disposition of listric shear zones the seismic image of the deformation is best developed within the onaping formation however the images of the deformation zone envelope are imperfect being most clear where truncating known contacts within the sudbury igneous complex and its contact with huronian host rocks the southern edge of the seismically imaged shear zones does not extend significantly beyond the exposed sudbury igneous complex suggesting limits to the extent of deformation in the sudbury structure

tectonic and economic implications of trace element 40ar 39ar and sm nd data from mafic dykes associated with orogenic gold minerals in central victoria australia comment

strain cleavage and microstructure variations in sandstone implications for stiff layer behaviour in chevron folding sandstone layers within tightly chevron folded quartz rich turbidites of the lachlan fold belt southeastern australia show a fanning spaced cleavage overprinted by a non fanning grain alignment cleavage low internal strains recorded by quartz grain separations xz 1 4 1 8 and low fold flattening strains xz 1 8 angular relations between spaced cleavage and bedding in folds with varying tightness suggest that sandstone layers initially act as stress supporting members with spaced cleavage development due to pressure solution taking place at limb dips up to 15 during fold development sandstone layers undergo rigid body rotation involving interlayer slip without marked internal deformation except at fold hinges after fold lock up bedding spaced cleavage angles change due to inner arc collapse by pressure solution and subsequent flattening with formation of a grain alignment cleavage and mica beard growth internal deformation of sandstone is characterized by microstructures related to solution redeposition processes and growth of mica chevron folds form by homogeneous shortening and pressure solution due to layer parallel shortening tangential longitudinal strain due to buckling fold tightening by inner arc collapse due to pressure solution and fold flattening at metamorphic peak with growth of mica geometric modelling of internal strains and bedding cleavage angles requires a significant component of interlayer slip during chevron fold development under greenschist facies conditions

accordion vs quantum tectonics insights into continental growth processes from the paleozoic of eastern gondwana the early paleozoic lachlan fold belt of eastern australia is widely regarded as an ancient convergent plate margin beneath which paleo pacific panthalassic oceanic lithosphere was continuously subducted it is cited as the type example of a retreating accretionary orogeny however sandstone compositions the sedimentological nature and timing of chert accumulation and overall stratigraphic architecture are not necessarily consistent with this model we suggest an alternative explanation for growth of gondwanan continental margin oceanic lithosphere outboard of the passive gondwana continental margin was subducted beneath an extensive intra oceanic island arc that now crops out as an allochthonous element macquarie arc within the fold belt once intervening oceanic lithosphere was eliminated this arc collided with and was emplaced upon the gondwana margin recognition of four such events along this margin through the phanerozoic suggests it is a significant mechanism for continental growth

ordovician terranes of the southeastern lachlan fold belt stratigraphy structure and palaeogeographic reconstruction abstract the ordovician of the southeastern lachlan fold belt lfb is represented by two contrasting provinces the molong volcanic province with shoshonitic basalts and shallow marine sediments in the north and a very extensive but relatively thin quartzose flysch chert and black shale sequence elsewhere this paper concentrates on the latter and reviews its stratigraphy correlation sedimentary facies palaeocurrent directions and structure the molong volcanic province contributed insignificant sediment to the surrounding turbidites and there is no evidence that the rise was a fragmented arc as is postulated by most previous workers the rise may have been a hot spot volcano and the southeastern lfb appears to have been a passive margin in the ordovician within the flysch province two terranes are recognized the melbourne terrane in the west and benambra terrane in the east both showing overall upward and eastward fining in the ordovician the boundary is the mount wellington fault zone along which a post ordovician dextral strike slip of up to 400 km is postulated

ordovician continental margin terranes in the lachlan orogen australia implications for tectonics in an accretionary orogen along the east gondwana margin 1 four continental margin turbidite black shale terranes of the lachlan orogen in the southern tasmanides of eastern australia formed in two major systems along the east gondwana margin and constrain the ordovician assembly of this accretionary orogen key features are the dissimilar stratigraphies of the adjacent bendigo and melbourne terranes in the western system the dissimilar stratigraphies of the adjacent melbourne and albury bega terranes that reflect juxtaposition of the two systems during the middle devonian and the presence of the albury bega terrane both west and east of the macquarie arc in the eastern system that also includes the ocean floor narooma terrane and igneous ocean crust terrane s repetition of the albury bega terrane either side of the arc requires either rifting or orogen parallel strike slip duplication of a once contiguous package terrane interactions began in the earliest gisbornian with early docking uplift deformation and exchange of detritus amalgamation occurred in the earliest silurian benambran orogeny with accretion in the middle devonian over 40 myr discrete turbidite terranes aligned along the gondwana margin in two systems were converted into a very wide orogen characterized by the along strike juxtaposition of superficially similar terranes

low temperature deformation mechanisms at a lithotectonic interface near the sudbury basin eastern penokean orogen canada abstract southwest of the sudbury basin variations in macroscopic strain microstructure and deformation mechanisms within archaean granitoid basement and overlying proterozoic rocks suggest that rock strength was strongly influenced by large amounts of fluids ductilely deformed granitoids with strong shape fabrics are concentrated in a 150 200 m wide zone parallel to the subvertical basement cover interface high fluid activity in this zone is indicated by 1 thickness and close spacing of foliation parallel quartz veins 2 abundant fluid inclusion trails in quartz 3 quartz c axis fabrics and 4 transformation of feldspar into phyllosilicates under middle greenschist facies metamorphic conditions away from this zone brittle ductile deformation in archaean granitoids caused grain size reduction of feldspar by fracturing and crystal plastic deformation of quartz indicating largely dry conditions strain was concentrated at the interface in a region of bulk horizontal shortening and linked kinematically to large scale shortening of the sudbury igneous complex and thrusting in the south range shear zone after early open buckling of the sudbury igneous complex reverse shearing in this zone may have been controlled by ductile deformation at the basement cover interface physical continuity of and age constraints on strain fabrics suggest that the south range shear zone represents a segment of a large ductile deformation zone which is most likely linked to the murray fault and on which higher grade rocks ascended between 1 85 and 1 23 ga ago

deep geometry of the sudbury structure from seismic reflection profiling seismic reflection data show the deep geometry of the sudbury structure to be markedly asymmetric the sudbury north range comprises shallowly south dipping sedimentary strata overlying a thick unit of heterolithic breccia and melt bodies layered rocks of the sudbury igneous complex and footwall gneiss reflections from the upper layers are interrupted by faults near the center of the sudbury basin whereas the basal mafic units and footwall gneiss continue dipping southward and appear to be tightly folded or truncated near the creighton fault in contrast the seismic image of the south range is dominated by a distinctive series of moderately south dipping reflections interpreted as thrust faults or shear zones on which severe telescoping and imbrication of lithological units and considerable northwest southeast shortening of the sudbury structure have occurred such shortening explains the noncircular shape of the structure and thus removes a critical objection to the sudbury impact crater hypothesis the new structural model is shown to be consistent with regional magnetic and gravity fields a large hidden ultramfic mafic mass is no longer required to explain the potential field data

styles of cenozoic collisions in the western and southwestern pacific and their applications to palaeozoic collisions in the tasmanides of eastern australia abstract the western and southwestern pacific preserve evidence of cenozoic collisions that guide our understanding of processes and geometries involved in collisions in ancient orogens in particular in this case the palaeozoic tasmanides of southeastern australia although several styles of collisions are present in the pacific ranging from arc arc collision to arc plateau collision the dominant two are oblique and strike slip collisions between island arcs and rifted continental fragments and collisions between forearc lithosphere and continental fragments the 58 ma collision along the northern margin of the australian plate in new guinea the 44 34 ma collision preserved in new caledonia and the 26 25 ma collision in the north island of new zealand may be parts of a single plate boundary collision that migrated southwards along the plate boundary they characterize the main style of deformation in which a collision between forearc crust and continental fragment produces subduction flip or rollback thereby avoiding a classic arc continent collision processes involved in and geometries that have developed from sw and w pacific style collisions have been applied to the interpretation of the evolution of the delamerian orogen and lachlan orogen in the southern tasmanides with varying degrees of success the ophiolite obduction model has been successfully applied to the western tasmania part of the delamerian orogen although there is discussion about its applicability to the mainland the best example of an arc accretion that of the ordovician macquarie arc in the eastern lachlan orogen developed from rare geometry in the western pacific wherein with the constraint that no forearc or subduction complex has been identified the arc lies on the continental plate above a continental dipping subduction zone the multiple subduction zone model of halmahera has been widely applied to the back arc of the lachlan orogen but evidence for clear subduction zones or arcs is not present

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

mapping mars geochemically using multivariate cluster analysis we identify six distinct geochemical provinces on mars from the concentrations of k th fe si ca cl and h 2 o determined by the mars odyssey gamma ray spectrometer the results show that the martian surface is dominated by basaltic rocks that vary in their abundance of incompatible k th and major elements fe si and ca these chemically distinct geochemical provinces are in large contiguous regions comprising a mixture of geologic units the k th ratios are uniform among the geochemical provinces to prevent measurable fractionation of k from th aqueous events must have been brief and or the total throughput of water small the muted weathering effects led to deposition of younger sedimentary deposits with the same compositions as older igneous units explaining why a geochemical province may contain mapped units with the same composition but a range of ages

conodont biostratigraphy and faunal assemblages in radiolarian ribbon banded cherts of the burubaital formation west balkhash region kazakhstan biostratigraphical study of the early to mid ordovician conodont fauna from ribbon banded radiolarian cherts of the middle burubaital formation in central kazakhstan reveals an almost complete succession of conodont biozones from the late tremadocian to the early darriwilian during this interval biosiliceous sediments were deposited in basinal environments inhabited by lingulate brachiopods sponges pterobranchs and caryocaridids in conditions of high fertility and primary productivity of surface water the community structure of taxonomically diverse conodont assemblages typifying open oceanic environments is not significantly different from that of epicratonic basins of the north atlantic conodont province the regional increase of oxygenated bottom waters at the base of the oepikodus evae biozone is possibly related to considerable changes in palaeo oceanographical circulation patterns the finds of three natural clusters of prioniodus oepiki mctavish enable us to propose an emended diagnosis of this species

tectonic processes along the front of modern convergent margins research of the past decade at convergent margins oceanic crust descends beneath continental crust to form a complex crustal juncture this juncture begins at an ocean trench and can be followed deep into the earth as a zone of high seismicity created as the two crustal plates move past one another the places where oceanic crust descends into the earth known as subduction zones are highly dynamic features modern examples contain analogues for processes that created the now static remains of ancient margins exposed on land at the fronts of subduction zones where the descending oceanic crust is still relatively shallow exploration geophysical techniques can define the structure of sedimentary strata modern seismic reflection techniques can resolve 100 m intervals in the upper 2 6 km of a sedimentary section at trench depths drill cores of such sediment from trenches have been successfully recovered between about 0 5 and 1 2 km below the seafloor this review focuses on the advances in knowledge of the fronts of modern convergent margins where the structure and tectonic history can be so observed the review is in part historical because new findings of the past decade became increasingly difficult to explain without extensive modifi

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

convergence and intraplate deformation in the lachlan fold belt of southeastern australia abstract the lachlan fold belt of southeastern australia is dominated by a widespread oceanic association including cambrian submarine mafic volcanics and an overlying widespread ordovician quartz rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres these rocks are overlain by or in fault contact with deep marine to continental silurian to early carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks major deformation achieved by thrusting and folding affected the fold belt during the silurian to middle devonian and the early carboniferous shortening estimates vary throughout the belt but the widespread ordovician quartz turbidite succession usually has at least 60 shortening which implies an original width of the fold belt of about 1700 km strike slip faulting was important on a local to regional scale but large scale strike slip displacements appear unlikely the large values of shortening are consistent with development of the belt in a convergent margin setting at least for the silurian to carboniferous the nature of the lower crust of the lachlan fold belt has always presented a problem of interpretation but we favour a late proterozoic quasi continental lower crustal layer that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during silurian carboniferous convergence and provided a source for silurian devonian granites

contrasting modes of eclogite and blueschist exhumation in a retreating subduction system the tasmanides australia abstract blueschists and eclogites located in the tasmanides of eastern australia preserve evidence of contrasting modes of exhumation a review of structural metamorphic geochronological and geochemical data indicates that these hp metamorphic rocks can be sub divided into three main groups i eclogite blueschists with calc alkaline and tholeiitic affinities contained within thick sedimentary sequences called continental hp rocks ii moderate pressure thermobarometric and geochronological data from eclogites and blueschists in the peel manning fault system new england orogen also provide evidence for discontinuous exhumation of subducted oceanic rocks these data indicate that eclogites were exhumed from depths of 70 km to 30 km during the ordovician 490 470 ma with terminal exhumation and exposure along the peel manning fault system probably occurring during the permian based on these timing constraints we suggest a model where hp rocks reside between depth dependant exhumation circuits for considerable lengths of time

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

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization