exhumation of late paleozoic blueschists in queensland australia by extensional faulting blueschists in southeastern queensland australia record a carboniferous history of subduction and metamorphism at 6 kbar a later thermal overprint associated with intrusion of s type granitoids ca 306 ma and regional greenschist facies metamorphism accompanied formation of ductile fabrics in the lower plate of an oceanic protolith metamorphic core complex late carboniferous extensional deformation in this part of the new england orogen may have accompanied outstepping of the subduction trench or rollback and steepening of the paleo pacific plate during development of a wide zone of asthenospheric upwelling and continental back arc extension authors

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

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

a review of the western part of the altaids a key to understanding the architecture of accretionary orogens abstract the altaids is one of the largest accretionary collages in the world and the tectonic styles of the accretionary processes have been interpreted in several ways including as an amalgamation of multiple terranes as a result of oroclinal bending of a long single arc or as a caledonian continental collision based on recent tectonostratigraphic analyses together with paleomagnetic data the tectonic styles of the neoproterozoic to paleozoic accretionary processes of the altaids are discussed the western altaids is the main focus of the study which was mainly composed of several independent linear components such as arcs and microcontinents with proterozoic basement and cover rocks various kinds of arcs existed in the paleo asian ocean including a complicated type of arc alaskan type which is a combination of the japan and mariana type intra oceanic arcs and the cordillera type continental arcs these linear components rotated and collided with each other with multiple subduction polarities which could have been an important result of multiple linear element amalgamation and which has contributed greatly to the architecture of the eurasian continent the basic tectonic styles of the altaids can be summarized as arc arc collision oroclinal bending and large scale rotation and multiple subductions with a complicated archipelago paleogeography these basic features of accretionary orogens in general can be attributed to the amalgamation of complicated multiple linear elements some mesozoic to cenozoic accretionary orogens in the world are also characterized by processes of multiple linear element amalgamation more attention should be paid to the multiple linear element amalgamation of ancient accretionary orogens which will shed light on lateral and vertical continental growth

mantle dynamics in the mediterranean the mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt lithospheric dynamics are affected by slab rollback and collision of two large slowly moving plates forcing fragments of continental and oceanic lithosphere to interact this paper reviews the rich and growing set of constraints from geological reconstructions geodetic data and crustal and upper mantle heterogeneity imaged by structural seismology we proceed to discuss a conceptual and quantitative framework for the causes of surface deformation exploring existing and newly developed tectonic and numerical geodynamic models we illustrate the role of mantle convection on surface geology a coherent picture emerges which can be outlined by two almost symmetric upper mantle convection cells the downwellings are found in the center of the mediterranean and are associated with the descent of the tyrrhenian and the hellenic slabs during plate convergence these slabs migrated backward with respect to the eurasian upper plate inducing a return flow of the asthenosphere from the backarc regions towards the subduction zones this flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath anatolia and eastern iberia this convection system provides an explanation for the general pattern of seismic anisotropy in the mediterranean first order anatolia and adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as anatolia and eastern iberia more generally the mediterranean is an illustration of how upper mantle small scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the tethyan collision

magmatic changes during the stabilisation of a cordilleran fold belt the late carboniferous triassic igneous history of eastern new south wales australia abstract in a 60 ma interval between the late carboniferous and the late permian the magmatic arc associated with the cordilleran type new england fold belt in northeast new south wales shifted eastward and changed in trend from north northwest to north the eastern margin of the earlier devonian late carboniferous arc is marked by a sequence of calcalkaline lava flows tuffs and coarse volcaniclastic sedimentary rocks preserved in the west of the fold belt the younger arc late permian triassic is marked by i type calcalkaline granitoids and comagmatic volcanic rocks emplaced mostly in the earlier forearc but extending into the southern sydney basin in the former backarc region the growth of the younger arc was accompanied by widespread compressional deformation that stabilised the new england fold belt during the transitional interval two suites of s type granitoids were emplaced the hillgrove suite at about 305 ma during an episode of compressive deformation and regional metamorphism and the bundarra suite at about 280 ma during the later stages of an extensional episode isotopic and ree data indicate that both suites resulted from the partial melting of young silicic sedimentary rocks probably part of the carboniferous accretionary subduction complex with heat supplied by the rise of asthenospheric material both mafic and silicic volcanic activity were widespread within and behind the fold belt from the onset of rifting ca 295 ma until the reestablishment of the arc these volcanic rocks range in composition from morb like to calcalkaline and alkaline the termination of the earlier arc and the subsequent widespread and diverse igneous activity are considered to have resulted from the shallow breakoff of the downgoing plate which allowed the rise of asthenosphere through a widening lithospheric gap in this setting division of the igneous rocks into pre syn and post collisional groups is of limited value

structural evolution of the early permian nambucca block new england orogen eastern australia and implications for oroclinal bending the paleozoic to early mesozoic southern new england orogen of eastern australia exhibits a remarkable ear shaped curvature orocline but the geodynamic processes responsible for its formation are unclear oroclinal bending took place during the early permian simultaneously with the deposition of the rift related sydney gunnedah and bowen basins which bound the oroclines to the west the nambucca block is another early permian rift basin but it is situated in the core of the oroclinal structure here we present new stratigraphic structural and geochronological data from the nambucca block in an attempt to better understand its tectonic history and relationships to the formation of the oroclines we recognized four phases of folding and associated structural fabrics s1 4 with the second phase s2 dated at 275 265 ma by 40ar 39ar geochronology of muscovite this age overlaps with independent constraints on the timing of oroclinal bending suggesting that the earlier two phases of deformation in the nambucca block f1 and f2 were associated with orocline formation we propose that oroclinal bending involved three major stages the first stage 300 ma was associated with variations in rates of trench rollback and formation of rift basins in a hot extensional back arc setting this was followed by a second stage of oroclinal bending possibly linked to dextral wrench faulting which involved n s contraction f1 subsequent deformation at 275 265 ma involved formation of nappe style structures f2 this phase of contractional deformation may have resulted from an increased plate coupling that was possibly linked to flat slab subduction

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

paleozoic terranes of eastern australia and the drift history of gondwana abstract critical assessment of paleozoic paleomagnetic results from australia shows that paleopoles from locations on the main craton and in the various terranes of the tasman fold belt of eastern australia follow the same path since 400 ma for the lachlan and thomson superterranes but not until 250 ma or younger for the new england superterrane most of the paleopoles from the tasman fold belt are derived from the lolworth ravenswood terrane of the thomson superterrane and the molong monaro terrane of the lachlan superterrane consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic australia by the late early devonian the lolworth ravenswood terrane is interpreted to have undergone a 90 clockwise rotation between 425 and 380 ma although the tamworth terrane of the western new england superterrane is thought to have amalgamated with the lachlan superterrane by the late carboniferous geological syntheses suggest that movements between these regions may have persisted until the middle triassic this view is supported by the available paleomagnetic data with these constraints an apparent polar wander path for gondwana during the paleozoic has been constructed after review of the gondwana paleomagnetic data the drift history of gondwana with respect to laurentia and baltica during the paleozoic is shown in a series of paleogeographic maps

a 125 post early serravallian counterclockwise rotation of the gorgoglione formation southern apennines italy new constraints for the formation of the calabrian arc abstract the southern apennines calabro peloritane block and sicilian maghrebides form a 700 km long orogenic bend known as calabrian arc cifelli et al 2007 the bending of this orogenic system was realized progressively through opposite sense rotation of the two limbs counterclockwise ccw in the southern apennines and clockwise cw in the sicilian maghrebides synchronous to the miocene to present opening of the tyrrhenian sea despite the wealth of paleomagnetic data from the southern apennines the main miocene rotational phase still remains poorly constrained in time and more importantly data from the most internal paleogeographic domains of the belt are completely lacking the gorgoglione formation a middle miocene piggy back deposit of the southern apennines unconformably resting over the internal sicilide unit offers the unique opportunity to document the deformation pattern of the most internal units and reconstruct the incipient tectonic phases leading to the formation of the calabrian arc new paleomagnetic and biostratigraphic data from the gorgoglione fm reveal a post early serravallian 125 ccw rotation with respect to stable africa such a large rotation affecting the gorgoglione fm and consequently the underneath allochthonous sicilide nappe exceeds by 45 the maximum mean ccw rotation previously reported for the southern apennines we propose that the additional 45 ccw rotation measured in the sicilide unit is the result of an earlier late miocene phase of deformation related to the onset of the tyrrhenian sea opening and affecting the most internal paleogeographic domains of the southern apennines our reconstructed tectonic scenario confirms and emphasizes the central role of the ionian slab in the geodynamic evolution of the central mediterranean

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

the contorted new england orogen eastern australia new evidence from u pb geochronology of early permian granitoids 1 a series of sharp bends oroclines are recognized in the paleozoic to early mesozoic new england orogen of eastern australia the exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated here we present zircon u pb ages that confirm the lateral continuation of early permian 296 288 ma granitoids and shed new light on the oroclinal structure orogenic curvature is defined by the alignment of early permian granitoids parallel to the structural grain of the orogen as well as the curved geometry of sub vertical deformation fabrics forearc basin terranes and serpentinite outcrops alternative geometrical interpretations may involve two bends texas and coffs harbour oroclines three bends manning orocline or even four bends nambucca orocline we argue that the model involving four bends is most consistent with available data although further kinematic constraints are required to confirm the existence of the manning and nambucca oroclines a subsequent phase of younger magmatism 260 ma cuts across the curved structural grain providing a minimum age constraint for orocline development assuming a structure of four oroclines we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300 285 ma followed by bending associated with dextral transpression a final tightening of the curved structures was possibly obtained by e w shortening during the late permian to triassic 265 230 ma hunter bowen orogeny

does the manning orocline exist new structural evidence from the inner hinge of the manning orocline eastern australia the map view structure of the southern new england orogen in the eastern gondwanan margin is characterised by four tight orogenic scale curvatures texas coffs harbour manning and nambucca oroclines here we focus on the geometry of the manning orocline and examine whether the inner arc area of the oroclinal structure is expressed within the accretionary wedge rocks of the tablelands complex our observations from the tablelands complex armidale walcha area show that rocks were subjected to penetrative deformation d1 which resulted in a regional slaty cleavage s1 and related isoclinal folds this was followed by subsequent deformation d2 associated with minor gentle folds in a larger scale the steeply dipping s1 structural fabric shows a continuous map view curvature thus defining a macroscopic fold structure we interpret this macroscopic fold as the expression of the manning orocline within the accretionary wedge complex this interpretation is consistent with the contorted spatial distribution of other tectonic elements serpentinite belt forearc basin terranes and early permian granitoids which independently define the structure of the manning orocline our new structural data support the existence of the manning orocline and the quadruple oroclinal geometry of the whole southern new england orogen the origin of these oroclines is attributed to multiple stages of bending possibly associated with an earlier phase of curvature during slab rollback in the early permian followed by a subsequent middle late permian episode of contractional deformation that tightened the oroclinal structure

submarine mass wasting deposits as an indicator of the onset of foreland thrust loading late permian bowen basin queensland australia in the eastern part of the permo triassic bowen basin of queensland australia a transition from passive thermal subsidence to flexural foreland basin subsidence is recorded within the upper permian stratigraphy two coarse grained intervals containing deposits of mass wasting processes occur within an otherwise siltstone dominated succession over 1500 m thick the moah creek beds and equivalents these intervals can be traced over at least 350 km north south along the structural eastern margin of the basin the lower of the coarse grained intervals is spectacularly exposed in the banks of the fitzroy river west of rockhampton here interbedded sandstones and siltstones of marine shelf origin are abruptly truncated by a mudrock succession containing evidence of slumping and contemporaneous magmatic activity this unit passes up section into packages of mass flow conglomerates and diamictites interpreted to have formed on an unstable submarine slope the character of the mass flow deposits their stratigraphic position and lateral extent are interpreted in terms of destabilization of a sloping marine surface by pulsed subsurface thrust propagation

unraveling the geometry of the new england oroclines eastern australia constraints from magnetic fabrics the southern new england orogen neo in eastern australia is characterized by tight curvatures oroclines but the exact geometry of the oroclines and their kinematic evolution are controversial here we present new data on the anisotropy of magnetic susceptibility ams which provide a petrofabric proxy for the finite strain associated with the oroclines we focus on a series of preoroclinal devonian carboniferous fore arc basin rocks which are aligned parallel to the oroclinal structure and by examining structural domains we test whether or not the magnetic fabric is consistent with the strain axes ams data show a first order consistency with the shape of the oroclines characterized in most of structural domains by subparallelism between magnetic lineations structural axis and bedding with the exception of the gresford and west hastings domains our results are relatively consistent with the existence of the manning and nambucca hastings oroclines reconstruction of magnetic lineations to a prerotation i e pre late carboniferous stage considering available paleomagnetic results yields a consistent and rather rectilinear ne sw predeformation fore arc basin this supports the validity of ams as a strain proxy in complex orogens such as the neo in the hastings block magnetic lineations are suborthogonal to bedding possibly indicating a different deformational history with respect to the rest of the neo

base of the kiaman its definition and global stratigraphic significance defining and dating the base of the kiaman the long period of reverse polarity that spans the time from the late carboniferous to the middle permian have proven difficult in an attempt to do this an integrated stratigraphic paleomagnetic and geochronologic study was undertaken in carboniferous rocks in the northern tamworth belt of the new england orogen eastern australia new paleomagnetic measurements at 145 horizons sites in the new england orogen of eastern australia provide an accurate record of the late namurian to westphalian geomagnetic field they have high unblocking temperatures satisfy tilt conglomerate and slump tests and show excellent agreement between a wide variety of igneous and sedimentary rock types indicating that magnetism was acquired at or soon after deposition magnetite and to a lesser degree hematite are the carriers of magnetization the record is contained in sequences of glaciogene sediments and arc derived volcano sedimentary units folded into broad structures in the late permian to triassic new isotopic dates for the volcanic units indicate ages from 321 to 306 ma late namurian to westphalian magnetizations are steeply inclined downward reversed polarity in the southern hemisphere except within the clifden formation of the rocky creek syncline where a change from normal to reverse polarity referred to as the wanganui reversal records the onset of the kiaman superchron integrating complex stratigraphic isotopic and paleomagnetic data we correlate the wanganui reversal to the top of n6 in the joggins section nova scotia canada and estimate that its age is between 318 and 316 ma further work may substantiate a younger normal zone in australia that would support a slightly younger age but within these limits these considerations indicate that the wanganui reversal occurs in the late namurian marsdenian and after the time of the mississippian pennsylvania boundary the mean direction of magnetization of the upper carboniferous australian sequences is declination d 195 3 inclination i 76 9 25 95 3 1 with a paleopole at 51 9 s 141 2 e 95 5 7 and a paleolatitude of 65 0 5 6 s p 0 05 this paleolatitude is consistent with the glacial origin of some of the sedimentary units

disentangling interglacial sea level and global dynamic topography analysis of madagascar abstract global inventories of stable sea level markers for the peak of the last interglacial period marine isotopic stage mis 5e play a pivotal role in determining sea level changes and in testing models of glacial isostatic adjustment here we present surveying and radiometric dating results for emergent terraces from northern madagascar which is generally regarded as a stable equatorial site fossil coral specimens were dated using conventional and open system corrected uranium series methods elevation of the upper undated terrace decreases from 33 8 m to 29 5 m over a distance of 35 km an intermediate terrace has an average radiometric age of 130 7 13 2 ka i e mis 5e its elevation decreases from 9 3 m to 2 8 m over a distance of 80 km the record of the lowest terrace is fragmentary and consists of beach rock containing rare corals with ages of 1 6 3 8 ka the spatial gradient of the mis 5e marker is inconsistent with glacio isostatic adjustment calculations instead we propose that variable elevations of this marker around madagascar and possibly throughout the indian ocean at least partly reflect spatial patterns of dynamic topography generated by sub plate mantle convection

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

40ar 39ar thermochronology of epidote blueschists from the north d aguilar block queensland australia timing and kinematics of subduction complex unroofing mechanisms proposed for exhumation of blueschists include whole crust uplift and erosion denudation by crustal scale extensional faulting or ductile strain and transport of blocks entrained in rising diapiric masses insight into the unroofing process and the role of granitic plutonism can be gleaned from study of north d9aguilar block in the new england orogen of southeastern queensland australia epidote blueschist facies rocks occur as structurally coherent schists and as blocks in serpentinite matrix melange these subduction underplated rocks formed at depths 18 km and occur in the lower plate of a metamorphic core complex a slate from the upper plate has a whole rock total gas age of 315 ma interpreted as a minimum age for subduction along this part of the paleo pacific margin exhumation of lower plate schists was coeval with overprinting of early high pressure m 1 metamorphic fabrics by a greenschist facies fabric m 2 and was accomplished in part by ductile stretching and normal faulting phengites from most lower plate schists yield 40 ar 39 ar plateau ages of ca 299 296 ma recording the time elapsed since the schists cooled below 350 50 c following m 2 plateau shaped age spectra and small structural depth related age gradients indicate rapid cooling of schists in the footwall of a present day low angle normal fault samples from deeper levels of the lower plate remained at 350 c for 35 m y longer than the other samples and contain extraneous ar possibly absorbed from fluids originating at depth at ca 307 ma a granodiorite pluton was intruded into the lower plate near the peak of m 2 a synkinematically intruded in part mylonitic pluton increased the regional m 2 fabric to amphibolite facies in its aureole heat from the granitic pluton and perhaps others like it at depth may have played a strain softening role in formation of the core complex similar 40 ar 39 ar cooling ages for different blueschist and greenschist blocks in serpentinite matrix melange support the view that the australian melange was uplifted by extensional tectonic processes unrelated to serpentinite diapirism

paleomagnetic data from the new england orogen eastern australia and implications for oroclinal bending orogenic curvatures oroclines are common in modern and ancient orogens but the geodynamic driving forces of many oroclines remain controversial here we focus on the new england oroclines of eastern australia the formation of which had been previously broadly constrained to the early middle permian this time interval encompasses periods of both back arc extension at 300 280 ma and subsequent contractional deformation hunter bowen orogeny that commenced at 270 ma along the paleo pacific and gondwanan subduction plate boundary we present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction at 272 ma and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks our results thus indicate that oroclinal bending in the southernmost new england orogen has been completed prior to the onset of middle permian contractional deformation it is therefore concluded that the oroclines have likely formed during back arc extension and that a major contribution to the orogenic curvature was driven by trench retreat

chronological constraints on the permian geodynamic evolution of eastern australia abstract the new england orogen in eastern australia developed as a subduction related orogen in the late devonian to carboniferous and was modified in the permian by deformation magmatism and oroclinal bending the geodynamics associated with the development of the new england oroclines and the exact timing of major tectonic events is still enigmatic here we present new 40ar 39ar results from metasedimentary and volcanic rocks from the southern new england orogen eight grains from four metasedimentary samples texas beds that originated in the late devonian to carboniferous accretionary wedge yielded reproducible plateau ages of 293 280 270 and 260 ma these results suggest a complex thermal history associated with multiple thermal events possibly due to the proximity to permian intrusions two samples from mafic volcanic rocks in the southernmost new england orogen alum mountain volcanics and werrie basalt yielded eruption ages of 271 8 1 8 and 266 4 3 0 ma the origin of these rocks was previously attributed to slab breakoff following a period of widespread extension in the early permian we suggest that this phase of volcanism marked the transition from backarc extension assisted by trench retreat to overriding plate contraction the main phase of oroclinal bending has likely occurred during backarc extension in the early permian and terminated at 271 266 ma with the processes of slab segmentation and breakoff

curvature of oceanic arcs oceanic arcs and deep sea trenches are the surface expressions of oceanic plates subducting into the earth s mantle we use a new numerical technique for simulating the dynamical evolution of the lithosphere mantle interaction in order to assess the causes of arc curvature we group the possible causes into two classes external feedback between the migrating lithosphere and the secondary induced mantle flow and internal heterogeneities within the lithosphere e g owing to differences in cooling ages of the plate at the trench we statistically assess that almost all arcs on the earth can be described by these hypotheses the method is also directly applied to the tonga and aleutian arcs bringing new insights on the origin of their shapes