evidence of isotopic equilibration between microgranitoid enclaves and host granodiorite warburton granodiorite lachlan fold belt australia abstract microgranitoid enclaves in the s type warburton granodiorite lachlan fold belt australia display a range in mineralogical and isotopic compositions many show obvious similarities to the host granodiorite they contain biotite as the sole ferromagnesian phase and have isotopic signatures similar to the host other enclaves contain cummingtonite actinolite and rare orthopyroxene mainly in the core of the enclaves these enclaves typically have higher nd i and lower sol 87 sr 86 sr i than the host rocks reversely zoned plagioclase megacrysts in some enclaves contain anhedral cores which are similar in composition to an poor plagioclase crystals in the host rock an45 these megacrysts are jacketed by an rich plagioclase an70 an80 the enclaves are interpreted to be globules of a more mafic magma which mingled with the host while it was still partially liquid megacrysts within the enclaves are thought to have been derived from the granodioritic magma the mafic enclave magma is likely to have been derived from a mantle source but equilibration between enclaves and host rock has changed the mineralogy and isotope geochemistry of all enclaves the equilibration process was incomplete for some enclaves resulting in mineralogical zoning and distinctions in isotopic signature between these enclaves and the host rock this equilibration process is likely to have been arrested as a result of rapid cooling of the pluton due to its small size and shallow emplacement level it is uncertain whether the compositions of the least equilibrated enclaves are truly representative of the original mafic enclave magma or whether their trace element signature has been affected by diffusional processes within a stratified magma chamber

evidence for crustal extension and inversion in eastern tasmania australia during the neoproterozoic and early palaeozoic abstract the island of tasmania in southeast australia consists of a number of stratotectonic elements the relationships between these elements are largely obscured by younger cover of the tasmania basin which contains extensive dolerite sills that limit the ability of potential field techniques to map basement therefore the development of a robust tectonic model for tasmania has been inhibited to assist in the development of a tectonic model a deep seismic reflection program undertaken offshore around the entire island was designed to map the large scale structures of tasmania at depth the airgun seismic energy was also recorded at a number of seismographs deployed across the island allowing low resolution 3d tomographic imaging short reflection profiles were recorded onshore across structures which could not be imaged by the offshore profiling this paper focuses on eastern tasmania in the seismic sections the proterozoic basement in the southeast is mostly featureless except for large rotated blocks with weakly reflective boundary faults indicating extension of the tyennan element by block faulting the deposition of the sedimentary succession of the adamsfield jubilee element was related to this extensional event in the northeast a reflective lower crust is interpreted to represent thrust slices of previously highly extended continental crust and possibly fragments of oceanic crust the early palaeozoic sedimentary succession of the northeast tasmania element formed across the inverted margin the apparently complex geology of eastern tasmania therefore fits into an extensional model where continental extension eventually led to the formation of very thin continental crust and possibly oceanic crust to the east the extension was probably related to late neoproterozoic extension recorded elsewhere in australia the region was subsequently shortened probably in a northeast southwest direction with most shortening accommodated in the seismically reflective probably oceanic part of the crust and little or no shortening in the block faulted and extended continental crust

structural evolution of the texas orocline eastern australia the texas orocline with a half wavelength of similar to 120 km is the largest and most obvious orocline in the southern new england orogen and is clearly recognised in geological maps and geophysical images in the area of the orocline there is a major unconformity between devonian carboniferous metasedimentary rocks texas beds and the overlying early permian rift related basins detailed structural mapping shows that units both above and below the unconformity are folded around the orocline indicating that at least part of the oroclinal deformation has occurred after the deposition of the early permian rocks in addition early permian 298290 ma granitoids are aligned parallel to the oroclinal structure further indicating that deformation partly occurred during or after the early permian pre oroclinal isoclinal folds f 1 and related axial plane cleavage si are well developed in the texas beds and are curved around the oroclinal structure syn oroclinal structures are characterised by minor kink folds and disharmonic folding f 2 in the core of the orocline a later phase of kink folding corresponding to a similar to n s shortening direction does not seem to be related to the orocline and is interpreted as post oroclinal deformation a secondary penetrative fabric parallel to the axial plane of the orocline was not observed indicating low contractional shortening across the orocline 30 we demonstrate that the observed strain is too low to account for oroclinal bending during dextral transpression as previously proposed we suggest an alternative model involving an initial curved structure probably related to subduction rollback or a pre existing curvature in the palaeomargin of eastern australia which was amplified by dextral transpression and subsequent e w contraction c 2011 international association for gondwana research published by elsevier b v all rights reserved

evaluation of the role of cambrian granites in the genesis of world class vhms deposits in tasmania abstract an analysis of the distribution composition and alteration zonation of cambrian granites which intrude the mt read volcanics of western tasmania provides evidence that there may have been a direct input of magmatic fluids containing fe cu au and p to form the copper gold volcanic hosted massive sulphide vhms mineralisation in the mt lyell district interpretation of regional gravity and magnetic data indicates that a narrow discontinuous body of cambrian granite 2 4 km wide extends along the eastern margin of the mt read volcanic belt for over 60 km the cambrian granites are altered magnetite series types which show enrichment in barium and potassium and contrast markedly with the fractionated ilmenite series devonian granites related to tin mineralisation elsewhere in the dundas trough copper mineralisation occurs in a linear zone above the apex of the buried cambrian granite body at the southern end of the belt from mt darwin to the mt lyell district over a strike length of 25 km gold and zinc mineralisation are concentrated higher in the volcanic stratigraphy more distant from the granite overlapping zones of alteration extend from the granite into the surrounding volcanic rocks an inner zone of k feldspar alteration is overprinted by chlorite alteration which passes outwards into sericite alteration magnetite pyrite chalcopyrite apatite mineralisation is concentrated in the chlorite alteration zone as veins and low grade disseminations the mt lyell copper gold stringer and disseminated mineralisation is hosted in felsic volcanic rocks 1 to 2 km west of the interpreted buried granite position magnetite apatite pyrite veins in the prince lyell deposit at mt lyell are very similar to the veins in the halo of the granite further south and provide evidence for magmatic fluid input during the formation of the copper gold vhms deposits a model involving deeply penetrating convective seawater mixing with a magmatic fluid released from the cambrian granites best explains the features of vhms mineralisation in the mt lyell district

assessing the origin of old apparent ages derived by pb stepwise leaching of vein hosted epidote from mount isa northwest queensland australia epidote metasomatism affected large areas of tholeiitic metabasalts of the 1 780 ma eastern creek volcanics in the western fold belt of the proterozoic mount isa inlier hydrothermal epidote generally occurs in quartz veins parallel to or boudinaged within the dominant s2 fabrics which formed during the regional metamorphic peak at 1 570 ma associated with the isan orogeny previously published stable isotopic and halogen data suggest that the fluids responsible for epidote formation are metamorphic in origin with an evaporitic component application of the pb stepwise leaching technique to the epidote does not separate radiogenic pb4 and common pb2 generating little spread in 206pb 204pb between 16 0 and 30 5 the causes for this relatively low range are twofold there is little radiogenic pb in the epidotes the most radiogenic steps account for 1 of pb released and both pb2 and uranogenic pb4 substitute into the same site in the epidote crystal lattice consequently age regressions using the pb stepwise leaching data give ages between 150 and 1 500 myrs older than the host rocks and over 450 myrs older than the thermal metamorphic peak these old ages are attributed to chemical inheritance from the host metabasalts via radiogenic pb release by breakdown of phases such as zircon monazite titanomagnetite and ilmenite during metamorphism this idea is supported by trace element data and chrondrite normalized rare earth element patterns that are similar to both the metabasalts and epidotes except for a variable eu anomaly in the latter relatively high fo2 during vein formation fe3 dominates in the epidote crystal lattice would allow the incorporation of th4 and exclusion of u6 and would explain elevated th u ratios up to 12 in epidote compared with the host metabasalts non incorporation of u would explain the relatively low u pb ratios and non radiogenic character of the epidote this process may provide a source of metal for the small u deposits around mount isa and may also suggest a relationship between u mineralization and regional cu mobilization during the isan orogeny our work suggests that non conventional geochronometers should be used only if additional geological information and geochemical data e g mineral chemistry trace elements are available to evaluate any resulting age calculations

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

mafic volcanic rocks on king island tasmania evidence for 579 ma break up in east gondwana abstract the eastern coast of king island in southeastern australia exposes a thick well preserved sequence of latest neoproterozoic volcanic and related shallow intrusive rocks these rocks are associated with shallow marine carbonates and siltstones and pass up into massive conglomerates representing a marine flooding event and unconformity during continental break up and subsequent volcanic passive margin formation unusual differentiated sills grimes intrusive suite with extreme internal variation wehrlite to andesite compositions intrude deformed proterozoic metasediments of the rodinian basement a thin basal tholeiitic basaltic volcanic unit city of melbourne volcanics is less contaminated than the underlying sills and preceded eruption of a thick sequence of highly depleted picritic pillows sub aerial flows and hyaloclastites shower droplet volcanics the picrite sequence is overlain by thick tholeiitic basalts and reworked volcanogenic conglomerates bold head formation that show a strong compositional similarity to enriched mid ocean ridge basalts both the picrites and the upper tholeiitic basalts are not crustally contaminated and have an nd sm isochron age of 579 16 ma with initial nd of 4 2 the lithostratigraphy and range of compositions represented are analogous to early magmatism associated with continental break up and volcanic passive margin formation including voluminous seaward dipping reflector sequences in the mesozoic north atlantic volcanic margins

oroclines in the tasmanides abstract oroclines are features of orogenic belts that exhibit curvature in plan on a map scale with the additional requirement that this curvature resulted from relative rotation of the limbs around a vertical or near vertical axis much debate has centred on contrasts between thin skinned and thick skinned systems and on the question of whether the structural fabric was imposed prior to or synchronous with the development of curvature leading to a variety of inconsistent terminology a simple hierarchy in which all map scale curved orogenic features which pass a test for rotation are termed oroclines as distinct from primary arcs and in which these are then subdivided by timing into progressive synchronous fabric and curvature and secondary pre existing fabric is then rotated and by mechanism into thin or thick skinned is applied to a suite of curved orogenic features in the tasmanides of eastern australia this analysis includes the first formal application of the orocline test palaeomagnetic or palaeocurrent directions plotted against structural strike to the tasmanides the neoproterozoic to early mesozoic tasmanides comprise four orogens each of which contains curved orogenic features which are highlighted by extensive aeromagnetic datasets the delamerian orogen developed on the gondwana margin exhibits two curved structures in the neoproterozoic to cambrian adelaide fold belt termed the fleurieu and nackara arcs previous structural interpretations have indicated that the nackara arc is non rotational and hence not oroclinal re examination of existing palaeomagnetic data including a positive palaeomagnetic orocline test shows the nackara arc to in fact be a hinge of the adelaide orocline curvature in the adelaide orocline was dominantly progressive and thin skinned but may have been influenced by secondary rotation of the pre existing partly underlying curnamona craton mega kinks in a buried delamerian arc volcanic chain revealed in aeromagnetic images follow the curvature of the nackara arc suggesting an accompanying thick skinned secondary oroclinal deformation further from the continental margin orogenic curvature in the palaeozoic lachlan orogen has only been widely recognised since the advent of tilt filtered aeromagnetic images and a model of oroclinal folding with two hinges riverina and tambo and strike slip offset during the silurian bindian orogeny has recently been proposed to explain perplexing repetitions of sequence and inversions of structural vergence and directions of sediment provenance palaeomagnetic data indicates rotation of elements within the proposed orocline and a preliminary study indicates a positive palaeomagnetic orocline test for the riverina hinge palaeocurrent data around the tambo hinge provide a positive if scattered orocline test an independently conceived numeric model of the roll back response to congestion of a subduction zone has generated an oroclinal structure which shares many features with the proposed lachlan orocline and deep crustal ambient noise seismic data confirms that the major curved and strike slip features persist throughout the crust the lachlan orocline appears to be secondary and thick skinned orogenic curvature in the largely covered thomson orogen occurs along its southern border with the lachlan orogen the putative olepoloko orocline and around its geometrically complex boundary with the mossman orogen in northern queensland the charters towers orocline rotation of the olepoloko structure cannot be confirmed and competing models for its formation leave open the question of whether this is a primary arc a thin skinned progressive orocline or a thick skinned secondary orocline palaeomagnetic data from the charters towers orocline are limited to two poles but a change in declination between the silurian pole and the devonian pole matches the apparent rotation of the orocline which appears to have been thick skinned and secondary in contrast to the other orogens consensus that at least part of the curvature in the palaeozoic to triassic new england orogen is oroclinal has been broad although not universal there remains substantial debate over the number of hinges from two to four and mechanism palaeomagnetic poles have previously been cited as evidence of rotation of blocks within the orocline but this paper presents the first formal palaeomagnetic orocline test which is positive for the manning and texas hinges a palaeocurrent orocline test of the manning hinge in younger rocks than the palaeomagnetic sample is negative constraining rotation in the southern manning hinge to the carboniferous before 322 ma while rotation in the northern texas hinge appears to be latest carboniferous or permian the existence of a hastings hinge is questionable but if real its rotation also appears to be younger than that of the manning hinge thick skinned secondary rotation of the hinges of the new england orocline appears to have been diachronous

structure of the crust and upper mantle beneath bass strait southeast australia from teleseismic body wave tomography acknowledgments we thank many land owners and field team members from mainland australia and tasmania particular thanks to armando arcidiaco and qi li from anu for assistance with the collection and archiving of the data used in this study arc grants dp120103673 le120100061 lp110100256 and dp0986750 were instrumental in supporting the wombat and bass deployments

granitic magmas with i type affinities from mainly metasedimentary sources the harcourt batholith of southeastern australia the high k calcalkaline granitic rocks of the 370 ma post orogenic harcourt batholith in southeastern australia have i type affinities but are mildly peraluminous and have remarkably radiogenic isotope characteristics with 87sr 86srt in the range 0 70807 to 0 714121 and endt in the range 5 6 to 4 3 this batholith appears to be a good example of magmas that were derived through partial melting of distinctly heterogeneous source rocks that vary from intermediate meta igneous to mildly aluminous metasedimentary rocks with the balance between the two rock types on the metasedimentary side such transitional s i type magmas formed from mainly metasedimentary source rocks may be more common than is generally realised the harcourt batholith also contains mainly granodioritic igneous microgranular enclaves imes like their host rocks the imes are peraluminous and have rather radiogenic isotope signatures 87sr 86srt of 0 71257 0 71435 and endt of 7 3 to 4 3 though some are hornblende bearing origins of these imes by mixing a putative mantle end member with the host granitic magma can be excluded because of the variability in whole rock isotope ratios and for the same reason the ime magmas cannot represent quench cumulates autoliths from the host magmas less abundant monzonitic to monzosyenitic imes cannot represent accumulations of magmatic biotite and or alkali feldspar because k feldspar is absent and there is no co enrichment of k2o and feo mgo nor can they be mixtures of anything plausible with the host rock magma the granodioritic imes probably originated through high degrees of assimilation of a range of crustal materials partial melts by basaltic magmas in the deep crust and the monzonitic imes as melts of enriched subcontinental mantle such enclave suites provide little or no information on the chemical evolution of their host granitic rocks

ore shoots in folded and fractured rocks insights from 3d modelling of the fosterville gold deposit victoria australia abstract at the fosterville gold deposit high concentrations of visible and refractory gold are hosted in arrays of quartz and quartz carbonate veins they are distributed in elongate zones to form south plunging ore shoots within a folded and fractured turbidite sequence here we distinguish the influence of faults versus folds at fosterville by assessing the deposit scale structural framework and hydrothermal fluid flow using a combination of 3d implicit modelling structural analysis and 3d fault slip dilation tendency analyses gold assay and bedding trend data from a large drillhole dataset were spatially interpolated and used to analyse and measure the orientation of ore bodies and folded host rocks field measurements of fold axis orientations were compared to bedding measurements in oriented drillcore bingham axial analysis of bedding trends and orientation data from a 3d implicit model both field and drill core structural data show that ore shoots at fosterville do not parallel the first order gently 10 south plunging folds instead the orientation of the ore shoots is coincident with an intersection lineation between steeply west dipping bedding parallel faults and moderately dipping linkage faults that obliquely cut across the main structural grain resulting in ore shoot plunges larger than 20 slip and dilation tendencies suggest that linkage faults had a higher likelihood of activity in the inferred far field stress regimes at the time of mineralisation relative to other structures at fosterville these linkage faults are therefore interpreted to have been preferential fluid flux conduits in the system despite appearing to be subordinate structures at the scale of the deposit this new structural model for fosterville highlights that fluid flow and ore shoot formation are strongly controlled by obliquely oriented structures that are unlike the main structural grain of first order folds and faults

kinematic history of the narragansett basin massachusetts and rhode island constraints on late paleozoic plate reconstructions the formation of the narragansett basin of rhode island and massachusetts and the polyphase deformation geometry of the pennsylvanian age basin sediments can be explained by collision and strike slip interactions between an avalonian microcontinent the european north american plate and the african plate during the late paleozoic sinistral movement between an avalonian microcontinent and the european north american plate resulted in formation of pull apart basins subsequently africa moved toward north america along an east west trending dextral megashear zone and collided with the eastern margin of southern north america the narragansett basin is located on the avalonian microcontinent near the suture with the north american continent and in the transition zone between the megashear and the collision tectonic regimes the basin shape stratigraphy and sedimentary relationships can be explained by the coalescence of two composite grabens which formed at different times the thicker northern graben is only affected by one deformation whereas the southern graben shows evidence of four distinct phases of postrifting deformation the first two coaxial folding deformations of the basin sediments are attributed to the closure of the basin complex during collision of africa and north america the third and fourth deformations are interpreted as sinistral r riedal and dextral shears respectively associated with the east west trending megashear system a prograde barrovian metamorphic front started in the western portion of the southern graben slightly after the first deformation and migrated northward and eastward where it is synchronous with the second deformation

timing of intrusion of the narragansett pier granite relative to deformation in the southwestern narragansett basin rhode island the pennsylvanian sediments of the narragansett basin rhode island were deformed during the al leghanian orogeny and record the effects of alleghanian deformation in southeast new england without the complication of earlier structures during this deformation the basin was intruded at its southwestern corner by the narragansett pier granite which has a crystallization age of 273 ma mapping of basin metasediments along the narragansett bay shore in the southwestern portion of the basin reveals features formed by several episodes of deformation two generations of isoclinal folds along north trending axes f1a and f1b are the earliest structures in the area followed by a set of small closed folds f2 also on north trending axes the fla f1b and f2 folds are the result of east west closure of the basin minor sinistral and more significant dextral motion on faults within the beaverhead shear zone produced local east trending crenulations s3 and open folds with north trending axes f4 in the study

forced folding above shallow magma intrusions insights on supercritical fluid flow from analogue modelling abstract magma emplacing at shallow crustal levels may cause significant deformation in the overlying country rock i e forced folding fracturing and faulting both at a local and or regional scale to get insights into these processes we investigated in the laboratory the development of forced folds and associated fracture fault networks an analogue magma simulated by polyglycerols was intruded into a sand pack representing the brittle crust the scaled analogue models reproduced different 3d deformation structures depending on the model parameters e g magma viscosity injection rate volumetric flux and the rheology and thickness of the host and cover rocks however all models support the observation that the emplacement of shallow magmatic bodies may result in the growth of dome shaped forced folds and associated development of tensional and compressional deformation in the host rock although the models involve simplifications these results provide useful hints for geothermal research as fractures and faults associated with magma emplacement can significantly influence the distribution and migration of superhot geothermal fluids these structures can therefore be considered potential targets for geothermal and or ore deposit exploration in this perspective the results of analogue models may provide useful geometric constraints for field work numerical modelling and particularly seismic interpretation allowing production and a better understanding of integrated conceptual models concerning the circulation of supercritical fluids

scale matters the influence of structural inheritance on fracture patterns abstract fracture systems are often geometrically invariant across a range of scales but the impact of structural inheritance on this relationship is poorly understood this paper shows how fracture orientations in sedimentary rocks vary at different scales when influenced by pre rift basement structures we use high resolution unmanned aerial vehicle uav orthophotos to map folds and fractures in the basement and cover rocks of the gippsland basin southeast australia outcrop scale observations are compared with 1 km long faults previously interpreted from potential field data we use length coloured rose diagrams of fracture traces to compare trends in fracture orientations early cretaceous syn rift normal faults exhibit the same ene wsw trend at basin 1 km and outcrop meters scales pervasive outcrop scale subvertical nnw sse striking joints record a subsequent regional shortening event but at the basin scale this is only expressed as reverse reactivated ene wsw striking faults thus fabrics and or faults in the underlying basement exert significant control on the orientation of basin scale fractures in the cover but appear to have limited influence on outcrop scale fracture orientations our observations show that fracture systems influenced by structural inheritance are not scale invariant and that a proper understanding of structural architecture can only be achieved by analysing data that span multiple scales

timing of gold mineralisation in western and central victoria australia new constraints from shrimp ii analysis of zircon grains from felsic intrusive rocks abstract age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central victoria australia occurred over a prolonged period of time ranging from prior to the late silurian to the late devonian an early phase of gold mineralisation associated with regional deformation pre dated the emplacement of late silurian felsic porphyry dykes at 413 3 ma in the central lode system of the magdala deposit stawell gold mineralisation associated with the final stages of the main regional deformation also pre dated the intrusion of early ca 400 ma to late devonian ca 370 ma plutons at stawell wonga deposit mt bute tarnagulla tallangalook maldon and heathcote in both the stawell pyrenees and ballarat metallogenic provinces consistent with preliminary dating studies of hydrothermal micas from major gold deposits later phases of gold mineralisation throughout western and central victoria were either temporally associated with or post dated the intrusion of late silurian to late devonian felsic dykes examples of this style of mineralisation include the wonga deposit at stawell mafeking fosterville maryborough minor occurrences near linton and the woods point region preliminary constraints from ballarat west indicate that late devonian dykes intruded at ca 386 ma were pervasively altered by interaction with an auriferous fluid similar to that responsible for gold deposition within the guiding star lode but that this was probably related to remobilisation during emplacement of the dykes into a pre existing mineralised structure with the exception of the woods point region and ballarat west these later phases often have a magmatic affiliation indicated by an association with cu mo bi sb and w these complex timing relationships reflect the diachronous nature of deformation metamorphism and magmatism in the western lachlan fold belt of southeast australia and suggest at least two discrete periods of gold mineralisation in the region

slab pull mantle convection and pangaean assembly and dispersal two global scale mantle convection cells presently exist on earth centred on upwelling zones in the south pacific ocean and northeast africa one cell panthalassan contains only oceanic plates the other pangaean contains all the continental plates they have remained fixed relative to one another for 400 ma a transverse rheic tethyian subduction system splits the pangaean cell poloidal plate motion in the oceanic cell reflects circumferential pull of panthalassan slabs but toroidal flow in the pangaean cell reflected by vortex type motion of continents toward the altaids of central east asia throughout the phanerozoic has resulted from the competing slab pull forces of both cells the combined slab pull effects from both cells also controlled pangaean assembly and dispersal assembly occurred during palaeozoic clockwise toroidal motion in the pangaean cell when gondwana was pulled into pangaea by the ne trending rheic subduction zone forming the appalachian variscide altaid chain pangaean dispersal occurred when the rheic trench re aligned in the jurassic to form the nw trending tethyside subduction system which pulled east gondwanan fragments in the opposite direction to form the cimmerian himalayan alpine chain this re alignment also generated a new set of indian mid ocean ridge systems which dissected east gondwana and facilitated breakup 100 200 myr long phanerozoic wilson cycles reflect rifting and northerly migration of gondwanan fragments across the pangaean cell into the rheic tethyian trench pangaean dispersal was amplified by retreat of the panthalassan slab away from europe and africa which generated mantle counterflow currents capable of pulling the americas westward to create the atlantic ocean thermal blanketing beneath pangaea and related hotspot activity were part of a complex feedback mechanism that established the breakup pattern but slab retreat is considered to have been the main driving force the size and longevity of the two cells organised and maintained by long lived slab pull forces favours deep mantle convection as the dominant circulation process during the phanerozoic

ophiolite accretion in the lachlan orogen southeastern australia slivers of dismembered cambrian ophiolites are preserved in major fault zones of the turbidite dominated western and central lachlan orogen of southeastern australia geometrical chronological and metamorphic constraints indicate that these slivers did not undergo classic tethyan style obduction but were incorporated into evolving turbidite wedges as offscraped slices imbricated fault slivers or duplexes and as blocks in melange the processes involved include decoupling of the oceanic crustal stratigraphy predominantly above the moho formation and metamorphism of serpentinite talc matrix melange up to blueschist conditions and accretion via underplating or offscraping during underthrusting these processes have produced major large scale fault zones within the chevron folded turbidites that show marked along strike variation in structural complexity that relate to magnitude of displacement the fault zones may have formed preferentially where topographic highs occurred in the oceanic crust promoting fold and fault nucleation within irregularities in the layering these fault zones have predominantly brittle character due to their formation under a low geothermal gradient duplex like structures in the fault zones and the inferred development through buckling of then faulting of the oceanic lithosphere have analogues in modern oceanic environments e g indian ocean and young subduction systems e g sulu sea southeast asia they are interpreted to have formed due to convergence in a non collisional intra oceanic setting during closure of a cambrian backarc basin

spatial analysis of an intra plate basaltic volcanic field in a compressional tectonic setting south eastern australia abstract the newer volcanics province nvp is a pliocene to recent intra plate basaltic volcanic field bvf that has formed in a compressive tectonic setting v

structural evolution of the orogenic gold deposits in central victoria australia the role of regional stress change and the tectonic regime abstract this review synthesizes the main characteristics of the orogenic gold deposits in western and central victoria australia and presents new insights into the relationships between orogen scale deformation and the 12 hard rock gold deposits that have each produced 30 t au we examine five of these large deposits stawell ballarat bendigo fosterville and costerfield from the start of the delamerian orogeny at ca 515 ma to the end of the tabberabberan orogeny at ca 370 ma the eastern continental margin of gondwanan australia experienced both accretionary growth and the development of the lachlan orocline central western victoria lies on the western edge of this oroclinal structure and is partly underlain by a proterozoic continental fragment the selwyn block all of the 30 t orogenic gold deposits in the region lie either on or to the west of the selwyn block from the delamerian orogeny onwards the deformation front progressively moved eastwards and the principal stress direction rotated clockwise from ne sw through e w and ultimately to n s so that the initial faulting and folding created pathways and traps for the subsequent mineralizing fluids through the brittle to brittle ductile crust thus the 455 440 ma benambran orogeny was the major mineralizing event at stawell and resulted in some early mineralization in the bendigo zone the 420 410 ma bindian orogeny gave rise to significant additional mineralization at ballarat bendigo and fosterville while the 385 370 ma tabberabberan orogeny drove mineralization at costerfield and a second bonanza event at fosterville the largest gold deposits formed where the fluid pathways either remained open or reopened in this rotating stress field allowing multiple pulses of gold rich fluids

assembly of a zoned volcanic magma chamber from multiple magma batches the cerberean cauldron marysville igneous complex australia abstract the late devonian 374 ma cerberean cauldron forms the northern part of the marysville igneous complex in central victoria australia filled with around 900 km 3 of intra caldera ignimbrites the basal volcanic formation is the rhyolitic high al rubicon ignimbrite overlain by a larger volume of crystal rich rhyolitic low al rubicon ignimbrite which grades upward into the voluminous rhyodacitic lake mountain ignimbrite the rocks are s type in character with initial 87 sr 86 sr around 0 709 to 0 710 and end t varying from 4 7 to 6 0 suggesting metagreywacke protoliths the chemistry of the volcanic rocks is incompatible with formation by a differentiation mechanism experimentally determined phase relations of a low al rubicon ignimbrite and a lake mountain ignimbrite show that early crystallisation of the lake mountain magma began at 450 mpa and at 875 c possibly up to 940 c with an initial magma h 2 o content of 4 1 to 5 3 wt in the pre eruption magma chamber the rubicon ignimbrite magma had a temperature of 780 c and contained 4 wt h 2 o each formation and indeed smaller volumes of rock appears to have been produced by partial melting of slightly contrasting greywackes in a protolith with spatial variations in its chemistry and mineralogy with the magma delivered in batches to a high level chamber the rubicon ignimbrite magmas underwent some internal differentiation probably by crystal settling prior to eruption and variations in the lake mountain ignimbrite are most probably due to small but variable degrees of peritectic phase entrainment the limited gradation between the rubicon ignimbrite and lake mountain ignimbrite is due to minor pre eruption mixing across the magma interface such limited mixing between individual magma batches appears typical of anatectic granitic magmas

australian antarctic breakup and seafloor spreading balancing geological and geophysical constraints abstract the motion of diverging tectonic plates is typically constrained by geophysical data from preserved ocean crust however constraining plate motions during continental rifting and the breakup process relies on balancing evidence from a diverse range of geological and geophysical observations often subject to differing interpretations reconstructing the evolution of rifting and breakup between australia and antarctica epitomizes the challenges involved in creating detailed models of pangea breakup in this example differing degrees of emphasis on and alternative interpretations of offshore geophysical data in particular magnetic anomalies and seismic reflection profiles and onshore geological data lead to starkly contrasting views of how the continents were configured at the onset of mesozoic rifting here we critically review reconstructions of rifting and breakup in the light of all available geological and geophysical data including magnetic anomalies fracture zones conjugate crustal domains amounts of continental extension continental geology plate boundary locations break up ages and stratigraphy we identify the most viable plate tectonic reconstructions both with and without the input of the oldest more controversial magnetic anomaly interpretations and discuss implications for reconstructions of other margin pairs our analysis highlights key discrepancies between reconstructions based solely on geological piercing points and those based on a range of constraints these insights provide a powerful framework for reducing the range of viable models for australian antarctic rifting and provide key lessons for future efforts aimed at constraining pre and syn rift plate tectonic reconstructions

the mount wright arc a cambrian subduction system developed on the continental margin of east gondwana koonenberry belt eastern australia abstract the mount wright arc in the koonenberry belt in eastern australia is associated with two early to middle cambrian lithostratigraphic groups developed onto the late neoproterozoic volcanic passive margin of east gondwana the gnalta group includes a calc alkaline basalt andesite dacite suite mount wright volcanics interpreted to represent the volcanic component of the arc volcaniclastic gnalta group rocks now buried in the bancannia trough represent the continental back arc developed immediately behind the arc in a manner analogous to the modern taupo volcanic zone of new zealand east of the gnalta group is the ponto group a deep marine sedimentary package that includes tholeiitic lavas bittles tank volcanics and felsic tuffs interpreted as part of a fore arc sequence the configuration of these units suggests the mount wright arc developed on continental crust in response to west dipping subduction along the east gondwana margin in contrast with some models for cambrian convergence on other sections of the delamerian orogen which invoke east dipping subduction and arc accretion by arc continent collision this convergent margin was deformed by the middle cambrian delamerian orogeny which involved initial co axial shortening followed by sinistral transpression and oroclinal folding around the edge of the curnamona province

mesoproterozoic tasmania witness to the east antarctica laurentia connection within nuna most recent paleogeographic reconstructions of the supercontinent nuna juxtapose the north australian craton mawson continent south australia east antarctica and laurentia between 1 6 ga and 1 3 ga but differ in their relative positioning the 10 km thick siliciclastic rocky cape group of tasmania was deposited in an opening marine basin on the margin of east antarctica during nuna breakup based on a similar detrital zircon signature and depositional age the rocky cape group has been correlated with the upper belt purcell supergroup in laurentia thus representing a key tie point within nuna here the detrital zircon age signature of mesoproterozoic rocky cape group quartzites is investigated by comparing new detrital zircon u pb hf isotopic data to an extensive compilation of zircon isotopic data from australia east antarctica and laurentia our analysis demonstrates that the rocky cape group is unlikely to have been sourced from any geological terrane exposed in presentday australia instead zircon u pb hf isotopic data from basement terranes in laurentia and east antarctica show striking similarities to the rocky cape group detrital signature paleocurrent data indicate that the majority of sediment in the rocky cape group was sourced from laurentia which was to the southeast present day coordinates of tasmania supporting a sweat like southwest united states east antarctica configuration for nuna we suggest that rifting left a thinned continental connection between east antarctica and laurentia onto which the lower middle rocky cape group was deposited between 1 45 and 1 30 ga

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

geometry and kinematics of a major crustal shear zone segment in the appalachians of southern new brunswick in southern new brunswick the kennebecasis fault follows the northern boundary of a crystalline portion of the late precambrian cambrian avalon terrane this low grade crystalline complex forms the basement to a series of carboniferous through triassic basins this complex also contains a major shear zone relic that is largely flat lying but upturned adjacent to the fault and that with it defines the pocologan kennebecasis fault zone the orientation of the main composite foliation s1 and the included mineral stretching lineation l1 indicate that this geometry is a primary feature of the shear zone representing a linked pair of horizontal and vertical detachments bounding an allochthonous unit kinematic indicators show that this allochthon moved parallel to the strike of the north appalachian orogen with top towards the west or west southwest the bounding shear zone is not uniform but consists of a mylonite phyllonite adjacent to the detachment the upturned segment of the shear zone has bee

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

constraining the vandieland microcontinent at the edge of east gondwana australia abstract using airborne magnetic and marine gravity data the geological subdivisions of western tasmania have been interpreted north across bass strait into victoria the three westernmost tasmanian zones the king island rocky cape and burnie zones are inferred to form the largely concealed selwyn block in victoria the eastern tasmania zone correlates with the victorian tabberabbera zone thus the tasmanian tamar fracture zone corresponds with the victorian governor fault the victorian ceres gabbro is correlated with magnetic rocks west of king island that are tentatively considered to be neoproterozoic most of the cambrian felsic volcanic rocks of the tasmanian mount read volcanics lie above the burnie zone as do the similar rocks exposed in the jamieson licola and glen creek windows in central victoria reinterpretation of a victorian deep seismic reflection line indicates burnie zone equivalent rocks were thrust south west over rocky cape zone equivalents a link between western tasmania and central victoria is evident from upper devonian granites intruded into the selwyn block region the eastern end of the upper devonian cobaw complex and the warburton granodiorite contain calcsilicate enclaves interpreted to be derived from a northern equivalent to the smithton basin the mount disappointment granodiorite has high ni and cr contents and pseudomorphs after orthopyroxenes consistent with having been partly sourced from the underlying basaltic rocks like those on the eastern margin of the king island zone the magnetic responses under the strathbogie complex the cerberean caldera and the lysterfield granodiorite are attributed to metamorphism of part of an extension of the smithton basin probably equivalents to the 580 ma spinks creek volcanics quartzite cobbles in a devonian conglomerate in the south eastern melbourne zone may be derived from rocky cape group equivalents when integrated with the geological interpretation of tasmania we provide a stratotectonic map of the vandieland micro continent

deep seismic images and the tectonic framework of early rifting in the otway basin australian southern margin abstract the otway basin is one of a number of basins formed along australia s southern continental margin during the late jurassic early cretaceous rifting of australia from antarctica it lies on a transitional area of extended lithosphere between the margin to the west where continental separation took place close to the australian craton and the region to the east where separation failed to develop through bass strait but instead switched to the south of tasmania seven agso regional deep seismic profiles recorded during 1992 across onshore parts of the basin provide images of synrift basin bounding faults that dip predominantly towards the continent basin sequences in half graben rift segments at two way times greater than 4 s and mid crustal detachments with ramp and flat geometry at mid crustal levels when combined with industry seismic data trend variations on faults indicate that extensional strain direction varied along the rift system probably locally controlled by pre existing palaeozoic geology but consistent with general north south lithospheric extension basement highs between some of the rift segments are interpreted as accommodation or transfer zones multiple basin bounding faults in some places mirror features within basement and may indicate a component of local strike slip during early rifting the syn rift segments form part of the first stage early cretaceous failed rift system in the otway bass strait region crustal thickness is interpreted to be 31 km 10 3 10 5 s twt near the northern basin margin thinning to about 25 km 9 s twt 40 reduction in thickness of palaeozoic crust seaward of the tartwaup fault zone this fault zone and the timboon fault system farther east are interpreted to be headwall faults landward limit of a late cretaceous second stage rifting system along a lower plate margin which ultimately separated australia from antarctica

alleghanian polyphase deformation of the hope valley shear zone southeastern new england structural petrographical and rb sr isotopic studies in the highly deformed basement rocks of southern rhode island have allowed the identification of the southern continuation of the hope valley shear zone hvsz previous work recognized the hvsz as an alleghanian tectonic boundary separating the distinct late precambrian esmond dedham and hope valley terranes in southeastern new england two important components of the esmond dedham terrane the late precambrian ponaganset gneiss and the devonian scituate granite gneiss now have been traced with structural continuity into southern rhode island the distribution of these gneissic units and their structures shows that the southern segment of the hvsz has been strongly deformed abruptly deflecting it from a nw se orientation to a n s trend the deflection of the hvsz marks an abrupt structural boundary between two distinct regionally developed mineral lineation domains a northwestern domain with north to nnw trends characteristic of most of the hvsz and a southeastern domain with ne trends structural relations suggest that all mineral lineations were originally parallel along the hvsz with a north to nnw trend the deflection formed during a subsequent phase of ductile right lateral shear producing a clockwise rotation of both lineations and the hvsz in southern rhode island thus basement gneisses in southeastern new england have suffered polyphase alleghanian deformation at high metamorphic grades it has been argued that during the late paleozoic southern new england was located in a structural transition zone between convergent tectonics in the central and southern appalachians and wrench tectonics in the northern appalachians the structural link between these two distinct tectonic regimes is an e w right lateral transcurrent fault the late dextral shear deformation described in southern rhode island basement rocks appears to have resulted from right lateral movement at depth within this transcurrent fault zone

a shear zone origin for alleghanian permian multiple deformation in eastern massachusetts the area around worcester massachusetts has been used to determine which deformational and metamorphic features are due to the alleghanian permian orogeny and which are pre alleghanian isolated fault bounded inliers of carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages the first cleavage formed synchronously with metamorphism pre carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages the initial cleavage formed during the first metamorphism and the second cleavage formed during the second retrogressive metamorphism thus the second two cleavages and the second metamorphism in pre carboniferous rocks are interpreted as alleghanian normal displacements on two distinct faults are bracketed by the formation of the first and second alleghanian cleavages the initial faulting along the newly defined wachusett mylonite zone wmz formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest the second faulting along the clinton newbury fault cnf occurred along a steeply inclined plane which cuts the wmz mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined this zone is interpreted as a ductile shear zone which moved twice forming the first and second alleghanian cleavages early formed pre alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates and new hydrous minerals formed in the shear zone indicating that it was a pathway for fluid flow the initial motion was left lateral with a thrust component the second cleavage formed during top to the northwest normal displacements thus the history of alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones this suggests that the strain history included two distinct episodes with the second experiencing slightly different strain orientations at different times resulting in first wmz normal motion then cnf normal motion and finally normal displacements related to the second alleghanian cleavages this history agrees well with other work on the nature of the alleghanian orogeny in the northeast united states

high precision u pb zircon chronostratigraphy of the mount read volcanic belt in western tasmania australia implications for vhms deposit formation the middle cambrian mount read volcanics of western tasmania australia host several world class volcanic hosted massive sulfide vhms deposits representing a wide range of deposit styles although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures rapid lateral and vertical facies changes and faults with uncertain sense and magnitude of displacement have made it impossible to correlate stratigraphy across the belt previous dating studies in the area have yielded relatively imprecise crystallization ages we have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy id tims u pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the mount read volcanics and underlying mafic ultramafic complexes the new data permit detailed resolution of age relationships within the belt the study establishes an age of 516 0 0 9 ma for the mcivor hill gabbro which is part of a mafic ultramafic complex interpreted to underlie the mount read volcanics magmatism in the central mount read volcanics lasted at least 12 7 m y from 506 8 1 0 ma for a massive dacite unit in the lower part of the central volcanic complex to 496 0 0 9 ma for a welded ignimbrite in the lower tyndall group together with previous age constraints results of the study provide a precise chronostratigraphic framework for magmatism and vhms deposit formation within the mount read volcanics the precise age data indicate that north of the henty fault magmatism occurred in three discrete pulses at least two of which were separated by periods of sedimentation we demonstrate that vhms deposits in the mount lyell roseberry hercules and que hellyer districts comprising the majority of the known significant vhms deposits in the belt formed within a narrow time interval at 500 1 ma at a relatively late stage in the evolution of the belt some of the larger intrusions in the belt e g the bonds range porphyry 500 4 0 8 ma were emplaced contemporaneously with vhms deposit formation however other bodies such as the murchison granite 497 3 0 9 ma are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested these conclusions will aid explorers in targeting vhms mineralization in the mount read volcanics

small scale structures formed during progressive shortening and subsequent collapse in the navia alto sil slate belt hercynian fold belt nw spain abstract a prominent unit of the hercynian orogen in north western spain is the navia alto sil slate belt which is formed predominantly by llanvirn caradocian slates lower upper ordovician from a structural point of view the belt forms an infolded syncline between two uplifted areas the narcea antiform and the mondonedo fold nappe both uplifted areas are hangingwall antiforms the thrusts below these structures ramp downwards towards the hinterland and flatten towards the foreland the first deformation events were controlled by low angle shears which at the cover level produced recumbent and asymmetric folds associated with a slaty cleavage in the west and the detachment of the cantabrian zone in the frontal part of the orogen eastwards of the narcea antiform these structures were modified as deformation progressed and two generations of steep dipping crenulation cleavages were locally formed these crenulations occur in belts separated by others in which the slaty cleavage remains undisturbed the youngest structures are flat lying crenulations small chevron folds and kink bands which develop where the slaty cleavage has not been previously crenulated the flat lying small scale structures end in general against the bands with vertical crenulations although in some cases they cross the boundary the alternation of belts with vertical and horizontal crenulations is found in all scales from the kilometre to the microscopic scale the flat lying structures indicate near vertical shortening and together with a few w dipping normal faults can be regarded as the result of orogenic collapse

tectonic affinity of the mathinna group in the lachlan fold belt abstract pre carboniferous rocks of tasmania comprise two disparate terranes which were brought together during the middle palaeozoic the eastern tasmania terrane consists of middle palaeozoic granitoids and the mathinna group a turbidite succession of ordovician to early devonian age the older part of the adjacent western tasmania terrane the only part of the southern tasmanides with exposed precambrian basement is related to the kanmantoo fold belt and older tectonic provinces located northwest of tasmania the younger part of the western tasmania terrane comprises upper cambrian to lower devonian terrestrial to shallow marine siliciclastics and carbonates a marked contrast to the coeval deep water deposits of the mathinna group the contact between the two terranes is concealed by the tamar valley in northern tasmania the main folds in the eastern tasmania terrane verge to the northeast whereas structures in the adjacent western tasmania terrane indicate tectonic transport to the southwest with the break in structural facing coinciding with the tamar valley reinvestigation of the structure of the westernmost eastern tasmania terrane where there is a large recumbent f1 fold 10 km extent in profile view raises the possibility that a second generation of structures previously regarded as of only local importance could have caused rotation of the upright to steeply west dipping axial surfaces of f1 folds to recumbent attitudes during southwesterly directed thrusting in this interpretation recumbent or near recumbent zones could reflect ramps in a sole thrust and the mid devonian imbricate thrust zone near beaconsfield on the west bank of the tamar river would be either the frontal thrust system or a splay off the sole thrust which continues westward beneath the exposed precambrian badger head block the implications of this new interpretation are that docking of the eastern and western tasmania terranes must have occurred after the ordovician but before the middle devonian during the consolidation phase there were two deformations of the mathinna group the first which occurred in the early devonian prior to emplacement of the st marys porphyrite 388 1 ma produced upright to locally overturned folds verging to the northeast the second which took place during the early middle devonian before deposition of givetian cave deposits resulted in the mathinna group being thrust southwestward this sense of thrusting is opposite to the direction of thrusting at the same time in the melbourne zone of victoria where the fold and thrust belt verges eastward away from the craton a concealed devonian transfer fault zone in bass strait could account for this change of polarity

timing of orogenic gold mineralisation in northeastern tasmania implications for the tectonic and metallogenetic evolution of palaeozoic se australia new 40ar 39ar data from sedimentary rock hosted orogenic gold deposits in northeastern tasmania constrain most ore formation to between 395 ma and 385 ma these 385 395 ma ages for the formation of orogenic gold agree well with an inferred early to middle devonian timing for peak deformation and folding across much of northeastern tasmania data from micas within alteration halos in some deposits give dates of 420 430 ma these dates confirm the occurrence of an earlier silurian phase of deformation and suggest that at least some of the mineralisation was possibly generated during this event gold mineralisation hosted by middle devonian post tectonic granites may be genetically related to magmatism following orogeny but these deposits formed virtually synchronously with peak deformation related systems early to middle devonian deformation in northeastern tasmania also reactivated older structures in western tasmania and the formation of quartz vein hosted gold mineralisation there based on geological structural tectonic and metallogenetic similarities northeastern tasmania is interpreted as a lateral equivalent of the turbidite dominated fold thrust belt of the western lachlan orogen however unlike victoria where the sedimentary rock sequence developed on oceanic crust northeastern tasmania was probably underlain by thinned proterozoic crust either as part of a promontory along the gondwana margin or as a microcontinental fragment this may have protected the palaeozoic succession from large scale pre devonian orogeny with collision not beginning until the middle devonian these variations in the structural and tectonic evolution and the timing of deformation and ore formation can explain the difference in contained gold and the distribution and number of major orogenic gold deposits within the palaeozoic of northeastern tasmania

implications for proterozoic plate margin evolution from geophysical analysis and crustal scale modeling within the western gawler craton australia abstract complexly deformed late archean to mesoproterozoic rocks of the western gawler craton are poorly exposed thus reducing the ability to extract meaningful geological structural and tectonic information this study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and bouguer gravity data interpretation with quantitative two dimensional forward modeling this integration provides a means to interpret the crustal architecture as well as constrain a sequence of events which aid the interpretation of a tectonic history for the western gawler craton results indicate a polyphase shearing history dominated by a crustal scale array of predominantly west dipping shear zones the initiation of east west trending sz1 structures was likely coincident with ca 1750 1720 ma crustal extension high heat flow and deposition on thinned areas of archean crust with the development of a dense possibly underplated lower crustal component sz1 r2 dextral transpressional movements reactivated sz1 during the ca 1720 1670 ma kimban orogeny which is interpreted to record collision between the gawler craton and the north australian craton during this event basins marginal to and in the interior of the archean crustal block of the western gawler craton were inverted and the dense lower crust was vertically offset causing some of the long wavelength 30 50 km bouguer gravity anomalies presently observed shear zone activity is constrained until ca 1680 ma during which time subduction rollback established a new arc position at the southern margin of the archean crust with magmatism persisting until ca 1670 ma sz3 structures overprint and offset the interpreted axis of the kimban orogeny within the archean crust these structures record a major phase of crustal scale sinistral strike slip movement associated with north south shortening between ca 1630 and 1540 ma prior to their reactivation at ca 1450 ma this sz3 r4 reactivation is interpreted to cause southwest directed crustal transport which was focused as west side up movements internal to the archean continent whereas dextral strike slip was focused at a major rheological boundary between thick archean crust to the south and thick paleoproterozoic basins to the north

towards understanding the early gondwanan margin in southeastern australia abstract this review synthesizes the proterozoic and early paleozoic geology of tasmania bass strait and western and central victoria we examine the many different conflicting hypotheses that have been proposed to solve the paradoxical relationships between tasmanian geology and that of mainland australia most notably the prevalence of proterozoic basement of western and central tasmania while immediately across bass strait evidence of proterozoic rocks is much more cryptic we conclude that the selwyn block model is the most satisfactory hypothesis to date since it fits best with the obvious patterns in the magnetic and gravity data this model proposes that the central victorian melbourne zone is underlain by the northern extension of thin tasmanian proterozoic and cambrian crust under bass strait and that the silurian to middle devonian melbourne zone was shortened along a decollement during the tabberabberan orogeny the ordovician rocks of eastern tasmania correlate more closely with the tabberabbera zone than the melbourne zone in victoria however the silurian and devonian correlations are less certain major unresolved issues are the origins of the proterozoic and early cambrian lithostratigraphic packages tectonic models for their assembly during the tyennan orogeny and how these models fit with those for mainland australia

costerfield antimony gold deposit southeast australia coupling between brittle deformation and dissolution precipitation reactions in the melbourne zone abstract the antimony gold deposits at costerfield are a swarm of dyke like antimony gold lodes within a brittle transpressional fault system they are hosted in the lower silurian sedimentary rocks of the melbourne zone of the lachlan orogen individual lodes are dominated by stibnite or stibnite gold and widths vary from 1 cm to 200 cm averaging 30 cm and lengths can exceed 1 km lodes occur as fault parallel layers or lenses mantled by deformed quartz rich margins or overgrowths of hydrothermal quartz and carbonate inclusions in the stibnite rich lodes include vuggy randomly oriented prismatic crystals crack sealed and stretched quartz with sugary mosaics of equant quartz grains and quartz breccias randomly oriented quartz and stibnite textures common in the centre of the lodes indicate free growth from a fluid throughout the mineralized system the hydrothermal stibnite has replaced and overgrown the earlier quartz deformed lode margins preserve sub horizontal lineations and evidence of oblique crack seal re opening that reflect emplacement associated with late sinistral strike slip movements in bounding faults crystallographic orientation data collected from the marginal quartz using neutron diffraction and supported by microstructural observations reveal that lode emplacement also involved dissolution precipitation creep and pressure solution both the marginal quartz and the lodes are overprinted by later microfractures within which there has been precipitation of stibnite pyrite arsenopyrite sphalerite and minor gold 3 d neutron tomography revealed individual gold crystals with gold voxel sizes ranging from 140 m to 580 m were located in late arsenopyrite gold domains that overprint the stibnite the mechanical evolution of the lodes involved short lived brittle processes in combination with dissolution precipitation crystallization in active fault zones these ore bodies provide insights about the dynamics of fluid flow and flow velocities when faults breach over pressured reservoirs of hydrothermal fluid our results also suggest that the costerfield deposit was formed directly from metal rich metamorphic fluids produced during tabberabberan 375 ma prograde metamorphism of sedimentary rocks near the base of the lower paleozoic melbourne zone which lie above the proterozoic selwyn block from a reinterpretation of seismic reflection data the deformation in the melbourne zone sedimentary wedge is characterized by thrusts isolating relatively undeformed packages of sedimentary units which were moved in a piggyback manner up west dipping faults the ore bearing fluids migrated upwards at least in part along these footwall thrusts from the base of the heathcote fault zone in the upper levels of the sedimentary package out of sequence strike slip sub vertical splays from these second order faults carried the mineralizing fluids to the orebodies east west compression during the tabberabberan orogeny preceded a northwest southeast oriented stress field which reflects the collision of vandieland against the gondwanan margin this produced the sinistral strike slip faults which in turn controlled the overall permeability of the hydrothermal system

inferring a deep crustal source terrane from a high level granitic pluton the strathbogie batholith australia the strathbogie igneous complex is comprised of the ignimbritic rocks of the violet town volcanics and the granitic rocks of the strathbogie batholith it is late devonian in age and postorogenic extensional in tectonic setting the batholith was constructed from peraluminous metasediment derived magmas emplaced as several internally heterogeneous plutons chemical variation in the magmas was largely inherited from the protolith rather than having been produced by differentiation crystal liquid separation or magma mixing the strathbogie magmas formed during a granulite facies metamorphic event that caused partial melting of the rocks of the proterozoic selwyn block which forms the basement in this region the chemistry of the strathbogie batholith the violet town volcanics and various other felsic complexes of similar age implies that the selwyn block here originally consisted of andesite dacite greywacke and pelite probably deposited in a back arc extensional setting the sedimentary components of this protolith may have been deposited in a basin that was extending and deepening with time so that the sediments contained progressively higher ratios of clay to volcanic materials much later in the late devonian extensional tectonics allowed the emplacement of mantle magmas into the deep and middle crust causing the low pressure granulite facies metamorphic event that was responsible for the production of the crustal components in the granitic magmas of central victoria

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

structural analysis of progressive deformation within complex transcurrent shear zone systems southern narragansett basin rhode island abstract complex progressive ductile deformation on dutch island narragansett basin rhode island is localized along a system of non parallel intersecting shear zones with opposite senses of strike and oblique slip motion the structural intensity and complexity of this area can be attributed in part to the proximity of nne ne and ene trending pre existing basement faults bounding early horsts and grabens reactivation of these faults caused the formation of shear zones within the overlying metasediments structural superposition of non coaxial mesoscopic folds and crenulation cleavages formed as a result of progressive movement along these shear zones provide unusual and accurate kinematic indicators that track movement along individual zones through time sinistral motion on nne trending zones preceded dextral motion on ne trending zones dextral oblique slip motion on ene trending zones followed deformation became increasingly brittle with time the deformation is the result of two transcurrent shear systems with opposite senses of motion that formed adjacent to the north american and african plate boundary after plate collision

a tectonic model for the late paleozoic of southeastern new england hornblende and biotite sup 40 ar sup 39 ar age spectra from rocks in south central connecticut help define a permian triassic cooling curve for the area together with petrologic and structural information a time temperature pressure strain path is established similar data for the narragansett basin in rhode island and massachusetts allow correlation of the late paleozoic histories of the two areas together these data suggest that in the late paleozoic south central new england was part of a fold thrust belt and the narragansett basin was a retroarc foreland basin nw se compression during the final assembly of pangaea resulted in se directed thrusting causing the development of clastic wedges in adjacent rhode island and massachusetts in the late carboniferous early permian a clockwise rotation of this deformation from nw to nne led to northward underthrusting and concomitant uplift of both eastern connecticut and rhode island in the permian and triassic

sims u pb zircon ages for the upper devonian snobs creek and cerberean volcanics from victoria with age uncertainty based on uo2 uo v uo u precision sensitive high resolution ion microprobe shrimp zircon 206 pb 238 u ages for the snobs creek volcanics and the overlying cerberean volcanics in central victoria are analytically indistinguishable at 373 2 1 8 and 375 0 2 0 ma 2 the shrimp data provide a refined numerical age for interbedded freshwater fish fauna of the taggerty formation and together with its recently revised givetian age based on independent faunal correlations provide a new calibration point on the devonian time scale the use of pb uo 2 rather than pb u in secondary ionization mass spectrometry sims dating reduces the range of secondary ion discrimination from zircon between spot correlations observed between uo u and uo 2 uo ratios give quantitative estimates for external sims uncertainty per spot age that are independent of assumed uniformity in zircon pb u they can exceed internal uncertainties by up to 1 5 dependent on the analytical session this has important effects on the calibration of sims ages at the sub percent level using imperfect reference zircons subject to small scale perturbations of the u pb system

authigenic monazite and detrital zircon dating from the proterozoic rocky cape group tasmania links to the belt purcell supergroup north america abstract the oldest known rocks in tasmania occur in the proterozoic rocky cape group a 10 km thick quartzarenite siltstone pelite dominated succession previously constrained to have been deposited between 1450 ma and 750 ma the rocky cape group contains the enigmatic fossil horodyskia string of beads and has the potential to place tasmania within supercontinent reconstructions detrital zircon and authigenic monazite grains dated via u pb laser ablation inductively coupled plasma mass spectrometry la icpms analysis yield a deposition window between c 1450 ma youngest zircon populations and c 1330 ma oldest authigenic monazite population for the 9 km thick lower middle units pedder river siltstone lagoon river quartzite balfour subgroup which hosts horodyskia detention subgroup the upper units 1 km include the irby siltstone which is younger than c 1310 ma this unit is likely separated from both the lower middle units and the overlying c 1900 ma grains were identified authigenic monazite age distributions are complex with multiple age domains within most samples the common pb corrected 206u 238pb ages defined by oldest grains in each sample identify three statistically significant groups 1 1358 1292 ma inclusive of two sigma errors lagoon river quartzite and pedder river siltstone 2 1283 1239 ma cowrie siltstone and balfour subgroup and 3 1085 9 ma detention subgroup we suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin the original source for ree bearing fluids could be detrital monazite which is rarely preserved and or organic matter from the interbedded carbonaceous shales the rocky cape group has a shared provenance with the higher grade metasediments surprise bay and fraser formations of nearby king island the newly derived depositional ages also overlap and support the correlation of these rock associations on the basis of current datasets there are no obvious correlations that can be made with mesoproterozoic basins preserved in mainland australia instead an overlap in the timing of deposition similarities in detrital zircon signatures and analogous depositional environment suggests the c 1 45 1 37 ga upper belt purcell supergroup missoula and lemhi groups of western north america constitutes a plausible correlation with the tasmanian mesoproterozoic succession if the unexposed palaeoproterozoic basement of tasmania correlates with the transantarctic mountains region of east antarctica as previously proposed we suggest that the overlying mesoproterozoic sequences were deposited during rifting of the supercontinent nuna between proto australia including the mawson craton of antarctica and laurentia as predicted by the most recent palaeogeographic reconstructions both the tasmanian and western laurentian packages were affected by episodic post depositional fluid flow events between c 1 35 and 1 05 ga possible thermotectonic imprints of the subsequent assembly of rodinia

thrust kinematics and transposition fabrics from a basal detachment zone eastern australia abstract emplacement of an upper crustal leading imbricate fan thrust belt in the lachlan fold belt of eastern australia was accomplished along a 0 5 1 km thick zone of heterogeneously deformed low grade phyllonite in pelitic rock continuous recrystallization and neocrystallization of mica in a zone of transposition layering has provided a weak zone at the base of a 100 km wide 150 km exposed length 10 km thick thrust system the basal deformation zone is characterized by a low moderately dipping strong intense transposition foliation enclosing elongate fault bounded slices up to 20 km long 5 km wide in map view of disrupted cambrian metavolcanics and upper ordovician black shales and slates these are derived from a structurally lower zone of duplexing or from the overturned limbs of anticlinorial structures the detachment zone is a 10 15 km wide zone of intense deformation showing a transition from open upright folds with weak cleavage to inclined tightisoclinal folds with strong axial surface cleavage the intensity of minor faults also increases into the zone leading imbricate fan thrust belts show maximum deformation effects along the basal detachment which forms the frontal or leading fault the leading imbricate geometry is due to emplacement of the basal detachment zone up the lowest and last formed imbricate thrust movement is along a relatively ductile low viscosity layer at the base where strain softening occurs with development of transposition layering this enables confined flow along the basal zone with transport and emplacement of the fold system and duplex zone to higher structural levels reaction enhanced ductility and grain boundary sliding may be important deformation mechanisms responsible for this flow localized polydeformation marked by mesofolds and crenulation cleavage reflects the interaction between thrust sheets and the movement on faults

isotopic evidence for rapid continental growth in an extensional accretionary orogen the tasmanides eastern australia the trace element signature of earth s continental crust resembles that of arc lavas but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra subduction zone magmatism we explore the role of coupled arc back arc accretionary processes in crust generation by considering the tectonic context of whole rock nd isotope and zircon hf o isotope data from igneous rocks of the australian tasmanides 515 230 ma which is thought to have evolved by the repeated opening and closure of sediment filled back arc basins behind a long lived subduction zone the significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective granitic rocks in this area define striking secular end ehf 18o trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity these trends reveal that juvenile magmatic input was enhanced during extensional back arc rifting episodes that immediately followed crustal thickening suggesting a relationship between slab rollback and continental growth interaction between juvenile magma and sedimentary units deposited during a preceding back arc rifting cycle was integral to the formation of stable continental material this highlights the importance of back arc environments for both the generation and differentiation of continental crust the juvenile component within the tasmanide igneous rocks increased from the cambrian to the triassic consistent with a diminished input from cratonderived metasedimentary material as the subduction zone migrated outboard of the gondwana margin subduction zone retreat formed large tracts of new crust in eastern australia at comparable rates to crust generation at modern island arcs providing a mechanism for rapid continental growth at convergent margins using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation