paleoproterozoic tectonic transition from collision to extension in the eastern cathaysia block south china evidence from geochemistry zircon u pb geochronology and nd hf isotopes of a granite charnockite suite in southwestern zhejiang abstract the badu complex and associated paleoproterozoic granitoids are among the oldest known rocks in the cathaysia block in south china the paleoproterozoic units of the badu complex are dominantly composed of metapelitic rocks and meta greywackes here we report la ss icp ms laser ablation split stream inductively coupled plasma mass spectrometry zircon u pb data from a newly discovered garnet bearing granite which show an emplacement age of 1929 15 ma and metamorphism at 1872 34 ma we also report u pb ages of 1886 16 ma 1858 7 ma 1848 11 ma from a gneissic granodiorite and two charnockites respectively the garnet bearing granite is peraluminous with a cnk range from 1 1 to 1 3 the rock shows relatively high sio2 k2o and rb contents and low total ree sr cao and ferromagnesian components typical of leucogranites the whole rock nd two stage model age tdm2 nd of this rock is ca 2 7 ga zircon hf crustal model ages tdmc hf peak at about 2 7 ga and abundant inherited zircons occur with u pb ages in the range of 2044 to 2803 ma evidences from zircon u pb age and hf isotope compositions whole rock nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith and that the garnet bearing granite leucogranite was derived by partial melting in a thickened crust at about 1 93 ga the gneissic granodiorite and charnockites show indistinguishable major and trace element features as well as zircon hf and whole rock nd isotope compositions indicating that they were generated from the same source rocks the gneissic granodiorite and charnockites are ferroan calc alkalic and metaluminous with a cnk range from 0 80 to 0 98 they display relatively low sio2 contents and ga al ratios suggesting their a type affinity their zircon tdmc hf age peak is 2 9 ga and whole rock tdm2 nd ages range from 2 8 to 2 9 ga these high temperature rocks were generated possibly through the partial melting of ancient amphibolites 2 9 ga with heat input from asthenosphere upwelling and magmatic underplating in an extensional setting at about 1 85 1 88 ga combined with data from previous studies we suggest three episodes of major crustal growth in this region at 1 8 1 9 ga 2 7 ga and 2 9 ga during the 1 8 1 9 ga event extensive reworking also took place from the 1 93 ga syn collision leucogranite garnet bearing granite the 1 88 1 85 ga post collision a type granites and charnockites the granite charnockite suite and the1 85 1 77 ga intraplate basaltic rocks li 1997 and xiang et al 2008 in the region a paleoproterozoic orogenic cycle can be deciphered in the cathaysia block the rapid transition of tectonic regime from collision to post collisional extension can be correlated to the assembly of the supercontinent columbia

structural inversion of the tamworth belt insights into the development of orogenic curvature in the southern new england orogen australia abstract the middle to late permian hunter bowen event is credited with the development of orogenic curvature in the southern new england orogen yet contention surrounds the structural dynamics responsible for the development of this curvature debate is largely centred on the roles of orogen parallel strike slip and orogen normal extension and contraction to explain the development of curvature to evaluate the dynamic history of the hunter bowen event we present new kinematic reconstructions of the tamworth belt the tamworth belt formed as a carboniferous forearc basin and was subsequently inverted during the hunter bowen event kinematic reconstructions of the tamworth belt are based on new maps and cross sections built from a synthesis of best available mapping chronostratigraphic data and new interpretations of depth converted seismic data the following conclusions are made from our study i the hunter bowen event was dominantly driven by margin normal contraction east west shortening present day coordinates and ii variations in structural style along the strike of the tamworth belt can be explained by orthogonal vs oblique inversion which reflects the angular relationship between the principal shortening vector and continental arc margin given these conclusions we suggest that curvature around the controversial manning bend was influenced by the presence of primary curvature in the continental margin and that the hastings block was translated along a sinistral strike slip fault system that formed along this oblique with respect to the regional east west extension and convergence direction part of the margin given the available temporal data the translation of the hastings block took place in the early permian asselian and therefore preceded the hunter bowen event accordingly we suggest that the hunter bowen event was dominantly associated with enhancing curvature that was either primary in origin or associated with fault block translation during the early permian this model differs to previously proposed reconstructions where curvature largely formed by orogen parallel strike slip transportation during the hunter bowen event

linking collision slab break off and subduction polarity reversal in the evolution of the central indian tectonic zonetectonic evolution of the central indian tectonic zonet deshmukh n prabhakar the central indian tectonic zone demarcates the zone of amalgamation between the north indian craton and the south indian craton presently the major controversies in the existing tectonic models of the central indian tectonic zone revolve around the direction of subduction and the precise timing of accretion between the north indian craton and the south indian craton a new model for the tectonic evolution of the central indian tectonic zone is postulated in this contribution based on recent geological and geophysical evidence combined with previously documented tectonic configurations the present study employs the slab break off hypothesis and subsequent polarity reversal to explain the tectonic processes involved in the evolution of the central indian tectonic zone we propose that the subduction initiated c 2 5 ga in a s directed system producing island arc sequences on the south indian craton the southward subduction regime culminated with slab break off underneath the south indian craton between c 1 65 ga and 1 55 ga which subsequently induced subduction polarity reversal and set the course for n directed subduction

correlation between magnetic fabrics strain and biotite microstructure with increasing mylonitisation in the pretectonic wyangala granite australia abstract the wyangala granite is a foliated porphyritic silurian granite from the palaeozoic circum pacific type eastern lachlan orogen elo of australia it is a paramagnetic ilmenite bearing s marginal i type two mica to mainly biotite granite with different biotite contents and local chlorite alteration very highly strained quartz epidote bands are present in this contribution anisotropy of magnetic susceptibility ams is compared with independently measured intensity and 3d style of strain biotite microstructure and degree of mylonitisation for low strain granites with weak s foliations through medium strain protomylonitic granites with moderate s and c foliations to a high strain altered granite with a strong single foliation the samples are further analysed for possible contributions from sample heterogeneity magmatic flow and sub magmatic deformation a good correlation p ams 1 02 0 04 ln p e qtz is obtained between site average degree of ams p ams in the granite and degree of finite strain anisotropy p e qtz from aspect ratios of quartz aggregates in s foliations in hand specimen and outcrop p ams 1 03 1 14 p e qtz 1 4 19 the magnetic fabric ellipsoids agree with a kinematic regime between neutral and pure oblate predicted by the march model the observed quartz strains however exceed the ams march strains and are near neutral plano linear character the geological factors that may have contributed to these differences include intra and inter crystalline deformation of biotite and bimodality in s and c magmatic fabric is not clearly evident in either the ams or the biotite data new data for synkinematic oligoclase low titanium biotite and low sodium k feldspar show that conditions during deformation were approximately transitional greenschist amphibolite facies i e well below solidus this agrees with published age data that put the granite emplacement in an extensional back arc setting in already deformed country rocks and before later convergent regional deformation in middle devonian tabberabberan event to early carboniferous kanimblan event time

the role of arc arc collision in accretionary orogenesis insights from 320 ma tectono sedimentary transition in the karamaili area nw china

tonalite granodiorite suites as cotectic systems a review of experimental studies with applications to granitoid petrogenesis abstract granodiorites and tonalites granotonalites in this review paper form large batholiths at active continental margins and collisional orogens although the relation with subduction is obvious the mechanisms of magma generation remain among the most debated topics in earth sciences today the most outstanding features of these calc alkaline batholiths are summarized in this review the problem has been addressed by laboratory experiments along decades dispersed data from experimental studies are compiled here in order to give phase equilibria constraints of general application it is apparent from chemical data that major element compositional trends follow cotectic patterns whose composition is largely dependent on intensive variables the comparison between experiments and batholith chemical trends produces an empirical thermodynamic framework useful to understand in a first approach the origin and evolution of magmas forming granite batholiths a new projection scheme of rock compositions onto the pseudoternary plane defined by the fe mg anorthite orthoclase is given this can be taken as first approach to phase equilibria model of batholiths it has been revealed useful to distinguish processes controlled by increasing melting or decreasing crystallization magma temperature from processes controlled by assimilation or magma mixing the available experimental data although scarce can be used to tentatively depict phase equilibria in the projected pseudoternary f an or diagram although important limitations are imposed by the projective procedure phase equilibria relations in equilibrium with liquids can be drafted in the diagram the stability fields of mineral in areas close to the liquid are defined according to reported phase assemblages in the experimental studies of reference the diagram is robust to identify cotectic and non cotectic relations phase equilibrium constraints and rock textures have revealed that one of the most important parameter controlling phase equilibria in granite magmas is the initial water content typically calc alkaline granitoids from qtz diorite to granotonalite and granite have crystallized from strongly undersaturated water magmas low initial water contents w i in the range 1 2 wt h 2 o may account for residual saturated melt fractions as low as 0 1 0 2 for plutons emplaced at 400 mpa the water content of these residual melt fractions is enough to stabilize hydrous minerals namely hbl and bt as predicted by experiments the initial water content is a primary feature of the magma and it is directly related to the melting reaction and more precisely to the temperature of magma generation the study of compositional trends supports magma fractionation by crystal liquid separation as a relevant role as a first approach the pressure composition relations in the melts can be seen as a response to the composition of the solid saturating assemblage and particularly related to the ratio grt pl in the solid residue

complex evolution of volcanic arcs the lithofacies and palaeogeography of the cambrian stavely arc delamerian fold belt western victoria abstract the mount stavely volcanic complex is a cambrian greenstone belt in western victoria australia which formed as a continental arc during subduction of the proto pacific plate under gondwana while the msvc has been broadly described in literature its lithofacies and palaeogeography remains unconstrained the lithofacies that occur throughout the msvc were characterised using field work and diamond drill core from the geological survey of victoria the majority of the msvc consists of fragmental facies that were deposited in a deep marine environment these deposits were subsequently intruded by melts which formed the coherent facies some became either shallow intrusive or even extrusive the volcanic processes that formed the mount stavely volcanic complex are similar to those currently occurring in the present day kermadec arc furthermore the submarine nature of these volcanic facies is similar to the cambrian mount read volcanics in tasmania however in timing and cause of volcanic activity not all of these ancient volcanic terranes are related these findings help constrain the depositional environment in which the mount stavely volcanic complex was emplaced which in turn provides important constraints for the tectonic reconstruction of gondwana

mid crustal deformation in a continental margin orogen structural evolution and timing of the famatinian orogeny nw argentina new structural data from a mid crustal segment in the eastern sierras pampeanas coupled with geochronological methods and p t estimates reveal polyphase contractional deformation and metamorphism during the famatinian orogeny over a long period of time peaks of metamorphic monazite and zircon ages are recorded at c 500 ma between 484 and 465 ma and at c 440 ma between 484 and 465 ma the region attained high temperature ht low pressure lp conditions that resulted in widespread partial melting peak at c 470 ma a contractional phase occurred during this event as suggested by syn anatectic structures as well as folding at subsolidus conditions renewed contraction under subsolidus conditions is evidenced by reverse ductile shearing and folding at c 440 ma thrusting along the la chilca shear zone caused metamorphic inversion the consistent orientation of folds in the quebrada del molle metamorphic complex el portezuelo metamorphic igneous complex and the la chilca shear zone as well as wsw directed thrusting at the later shear zone indicate uniform wsw ene directed shortening this late deformation records the ocloyic tectonic phase and brought the famatinian orogeny to an end in the late ordovician to early silurian supplementary material analytical methods methods and description of the calculated p t pseudosection diagrams bulk compositions and electron microprobe u pb and 40ar 39ar data are available at https doi org 10 6084 m9 figshare c 4697189

metamorphism and deformation of mafic and felsic rocks in a magma transfer zone stewart island new zealand in the mingled mafic felsic halfmoon pluton at the neck stewart island part of the median batholith of new zealand some hornblende gabbros and diorites retain magmatic structures whereas others show evidence of major changes in grain and inclusion shapes and still others are amphibolite facies granofelses with few or no igneous relicts these mafic to intermediate magmas crystallized in felsic magma relatively quickly with the result that most deformation occurred at subsolidus conditions it is suggested that mafic intermediate rocks with predominantly igneous microstructures spent less time in the magmatic system the metamorphism of the mafic rocks appears to be autometamorphic in the sense that elevated temperatures were maintained by magmatic heat during subsolidus cooling elevated temperatures were maintained because of repeated sheet injection and subconcordant dyke injection of hot basaltic and composite mafic felsic magmas into a dominantly transtensional km scale outboard migrating magmatic shear zone that operated semi continuously for between c 140 and c 130 ma complete cooling occurred only when the system evolved to transpressional and the locus of magmatism migrated inboard southward between c 130 and c 120 ma associated with solid state mylonitic deformation intermingled granitic rocks escaped metamorphism because they remained magmatic to lower temperatures and experienced shorter and lower temperature subsolidus cooling intervals however the felsic rocks underwent relatively high temperature solid state deformation as indicated by myrmekite replacing k feldspar and chess board subgrain patterns in quartz locally they developed felsic mylonites the felsic rocks were deformed in the solid state because of their high proportion of relatively weak minerals quartz and biotite whereas the mafic rocks mostly escaped subsolidus deformation except in local high strain zones of hornblende plagioclase schist because of their high proportion of relatively strong minerals hornblende and plagioclase we suggest that such contrasting microstructural features are diagnostic of long lived syntectonic magma transfer zones and contrast with the more typical complex batholith scale magma chambers of magmatic arcs

thermally softened continental extensional zones arcs and rifts as precursors to thickened orogenic belts abstract intra continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence this enables significant thickening with formation of an orogenic root we have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra continental extension with associated heating geologically this situation is appropriate to intra continental rifts and back arc basins if thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak soft lower crust and sub arc rift mantle result this weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot these regions are associated with formation of granulites and metamorphic assemblages typical of high temperature low pressure ht lp if convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc rift domain such sites are ideal for the formation and for the exhumation of metamorphic core complexes subsequent thickening during convergence leads to ht eclogites when the previous arc rift was hot and to medium t eclogites for a thickened standard geotherm these p t paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm if the compression starts long after cessation of the extensional event and associated thermal anomaly then the geotherm of the extended area relaxes and the whole region hardens in this case no homogeneous thickening occurs and deep continental roots cannot form

constraints of zircon hf isotopes on ancient crustal reworking in the early paleozoic altai accretionary wedge central asian orogenic belt abstract a debate exists on whether the deep crust of the altai orogenic belt contains ancient crustal components to further address this issue geochemical analyses and zircon hafnium isotopic measurements were conducted on five devonian magmatic rocks from the southern chinese altai three volcanic rocks have rhyolitic compositions and two other intrusions have granite and granodiorite compositions respectively these samples all have calc alkaline peraluminous compositions and arc related trace element signatures marked by enrichment in large ion lithophile elements relative to many of the high field strength elements hf in zircon isotopic data for these rocks show variable ehf t values ranging from 25 to 5 with corresponding tdmc model ages varying from ca 2 7 to ca 0 9 ga these findings represent the first report describing negative zircon ehf t values for the devonian magmatic rocks of the region documenting magmatic reworking of ancient crustal components the very similar trace elemental patterns and indistinguishable zircon hf isotopic characteristics between the studied magmatic rocks and the pre existing ordovician accretionary wedge sediments the habahe group supports the notion that the magmatic rocks are derived from magmatic reworking of the wedge that contains ancient crustal components the studied magmatism is synchronous with middle devonian anatexis of the ordovician wedge at low pressure high temperature conditions and extrusion of bimodal volcanic rocks occurred in devonian extensional basins these events have been collectively taken as proxies of devonian lithospheric extension in the chinese altai integrated mapping of hf isotopic data for the bulk s type devonian magmatism of the chinese altai shows different domains with dissimilar hf isotopic signatures this further indicates that the magma source i e the ordovician wedge is compositionally heterogeneous and contains ancient crustal components

the 89 ma tortugal komatiitic suite costa rica implications for a common geological origin of the caribbean and eastern pacific region from a mantle plume komatiites are reported for the first time in the northern part of the gulf of nicoya costa rica these rocks dated at 89 7 1 4 ma turonian by 40 ar 39 ar methods occur as a large elongated 14 km long 1 5 km wide n60 w striking body in the ophiolitic nicoya complex these lavas have high mgo 26 29 ni and cr have high cao al 2 o 3 0 98 1 08 and moderate al 2 o 3 tio 2 5 55 8 44 ratios and are depleted in al 2 o 3 4 5 5 k 2 o 0 02 0 37 and tio 2 0 59 0 9 although these lavas are cumulates their geochemical composition indicates an origin from a primary komatiitic magma with a melting temperature of 1700 c at a depth of 150 km similarities in the petrology and age 88 90 ma of gorgona curaiao and nicoya tortugal mafic and ultramafic volcanic rocks suggest that these rocks had a common origin these occurrences suggest a single hotspot center over a large area of the caribbean and eastern pacific mesozoic region due to a major thermal anomaly in the mantle such as a hot rising convective plume

prolonged 100 ma ultrahigh temperature metamorphism in the napier complex east antarctica a petrochronological investigation of earth s hottest crust

melting of continental crust during subduction initiation a case study from the chaidanuo peraluminous granite in the north qilian suture zone the chaidanuo granite batholith is a similar to 500 km 2 homogeneous high level intrusion in the north qilian oceanic suture zone three types of enclaves have been recognized including 1 supracrustal xenoliths 2 biotite gneiss that represents restite of the upper crustal melting and 3 coeval mantle derived mafic magmatic enclave mme the batholith consists dominantly of peraluminous biotite monzogranite with sio2 69 73 wt and a cnk 1 05 1 28 and shows geochemical affinity with the upper continental crust e g enrichment of large ion lithophile elements liles k rb u th pb depletion of high field strength elements hfses nb ta ti p eu and sr and enriched sr sr 87 sr 86 i 0 731 to 0 744 nd epsilon nd t 6 0 to 7 1 isotopes zircon u pb dating indicates that this batholith formed at 516 505 ma coeval with the mmes 510 ma which represent the early stage of seafloor subduction in the north qilian suture zone the hf isotopic composition of the mme epsilon hf t 6 0 to 2 1 is more depleted than that of the host granite epsilon hf t 14 8 to 4 7 suggesting mixing of mantle and crust derived melts the inherited zircon cores in the host granite yield an age peak at similar to 750 ma with a few detritals of 591 1683 ma zircons from a biotite gneiss enclave yield a protolith age of 744 ma and a metamorphic melting event at 503 ma sr nd isotopic modeling reveals that the batholith was generated by melting of neoproterozoic granitic rocks with the addition of 10 18 mantle derived magma therefore peraluminous granite can be produced by melting of crustal materials heated by mantle derived magmas during the early stage of subduction initiation at the site of a prior passive continental margin c 2014 elsevier ltd all rights reserved

reinterpretation of the tectonic context of high temperature metamorphism in the broken hill block nsw and implications on the palaeo to meso proterozoic evolution abstract the origin of high temperatures during regional low pressure granulite facies metamorphism within the proterozoic broken hill block australia has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca 1 71 1 67 ga and then maintained during a ca 1 62 ga short lived mid crustal extensional event mid crustal extension at ca 1 62 ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients and occurred prior to activity along d2 high temperature shear zones peak low pressure granulite facies metamorphism ca 1 60 ga and crustal shortening during the olarian orogeny ca 1 60 1 59 ga this reinterpretation places the broken hill block within an environment in which multiple episodes of transient extension ca 1 71 1 62 ga were followed by a switch to shortening at ca 1 60 ga the interpreted tectonic environment is a continental back arc setting located in the over riding plate of a subduction zone along the southern margin of palaeoproterozoic australia

structural geology and the seismotectonics of the 2004 great sumatran earthquake y the authors acknowledge funding support from the australian research council discovery project dp120103554 a unified model for the closure dynamics of ancient tethys constrained by geodesy structural geology argon geochronology and tectonic reconstruction and linkage project lp130100134 where to find giant porphyry and epithermal gold and copper deposits the research was also supported by the satellites seismometers and mass spectrometers initiative within the research school of earth sciences at anu bob engdahl is thanked for his support during earlier versions and for providing data

deep subduction of continental crust in accretionary orogen evidence from u pb dating on diamond bearing zircons from the qinling orogen central china abstract accretionary orogenic belts are considered the most important sites for the generation of continental crust however it is still not very clear whether in such settings the continental crust has been significantly transported back to the mantle accompanied with its lateral growth continental uhp rocks the best indicators of the recycling of the continental crust have rarely been discovered in accretionary orogenic belts which hinder the exploration of this essential process the possible occurrence of early paleozoic uhp metamorphism in the qinling orogen provides an excellent opportunity to address this question this article reports an integrated study of u pb age trace element and mineral inclusion of zircon from an amphibolite sample in the qinling orogen the zircon crystals show typical metamorphic growth zoning low th u ratios flat hree patterns and insignificant eu anomalies they give a weighted mean u pb age of 490 4 5 8 ma most importantly an in situ diamond inclusion was identified from one of the zircon crystals therefore the obtained 490 4 5 8 ma age was taken as registering the peak uhp metamorphism the discovery of in situ diamond inclusion provides important evidence for the uhp metamorphism of the qinling orogen and indicates that the north qinling microcontinent was subducted to mantle depths of 120 km when it collided with the erlangping arc attached to the southern margin of the north china block combining with previous results we suggested that the north qinling uhp terrane underwent fast exhumation from 120 km to ca 30 km within 10 myr intervals as a few examples of uhp metamorphism have been discovered in some accretionary orogens more attention needs to be paid to the occurrence of continental uhp metamorphism during accretionary orogenic process furthermore evident crustal growth has been documented in the north qinling orogen during the early paleozoic it is inferred that the accretionary orogenic belts may play important roles in recycling of continental crust into the mantle besides generation of continental crust

the vaasa migmatitic complex svecofennian orogen finland buildup of a lp ht dome during nuna assembly tectonic evolution of the paleoproterozoic vaasa migmatitic complex vmc in the central part of the svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps deep seismic

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

a hybrid origin for lachlan s type granites the murrumbidgee batholith example the murrumbidgee batholith is typical of other composite s type granite batholiths in the lachlan fold belt lfb of southeastern australia consisting of discrete peraluminous cogenetic granite suites however it contains a unique suite murrumbucka at its southern extremity that has chemical and petrological features transitional with metaluminous i type granites previous detailed structural and metamorphic studies have shown that the batholith is tilted northward exposing subvolcanic plutons in the north and root zones in the south originally located at depths of not vert similar10 km a transition from mafic foliated sheet like granites in the south to felsic generally less foliated homogeneous granites in the north is consistent with magma ascent via subvertical structurally controlled sheets to emplacement in an overlying magma chamber in the inferred root zones the murrumbucka suite hosts migmatitic metasedimentary and gabbroic rocks both of which have transitional contacts and show evidence for mafic felsic interaction with the murrumbucka suite the migmatites extend southward to become part of the high t low p cooma metamorphic complex which contains a core of heterogeneous remobilised diatexitic granite cooma suite lenses of which also occur throughout the southern deeper parts of the murrumbidgee batholith the composition of the most mafic rocks from the batholith lies on a chemical tie line between cooma suite granites and the gabbros for almost all elements this chemical coincidence is interpreted to reflect derivation of parental murrumbidgee s type granite magmas by bulk mixing between a felsic crustal and mafic mantle derived component consistent with sr and nd isotopic results and field observations based on the tie lines and simple numerical modelling the murrumbucka suite is estimated to be a 50 50 mix of mantle and crustal derived magmas whereas the more peraluminous and widespread clear range suite which is very typical of lachlan s type granites is modelled as a 40 60 mix chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing mixing occurred within sheets during the ascent of the magmas whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber generation of the s type granites probably occurred in an extensional tectonic environment

the use of pollen floras as a tool to estimate palaeoaltitude of mountains the eastern pyrenees in the late neogene a case study northern hemisphere vegetation is generally zoned as a consequence of latitudinal and altitudinal gradients temperature reconstruction from three successive pollen floral assemblages of tortonian messinian and zanclean age from the eastern pyrenees has been used to estimate the altitudinal evolution of the uplifted cerdanya basin and its surrounding massifs a new method has been developed for the uplifted basins which completes that already applied to pollen floras from coastal marine deposits close to mountains in contrast to previous hypotheses it seems that the eastern pyrenees occurred at a lower altitude in the late miocene before continuous uplift began 10 ma ago the rate of this uplift ranged between 0 06 and 0 12 mm yr

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

low pressure regional metamorphism in the omeo metamorphic complex victoria australia the omeo metamorphic complex forms the southern end of the wagga metamorphic belt which is the main locus of palaeozoic low pressure metamorphism in the lachlan fold belt south eastern australia it comprises metamorphosed ordovician quartz rich turbidites originally derived from precambrian cratonic rocks prograde regional metamorphism occurred in the early silurian very soon after sedimentation had ceased the sequence of metamorphic zones with increasing grade is chlorite biotite cordierite andalusite k feldspar and sillimanite k feldspar migmatites occur in the sillimanite k feldspar zone but large bodies of s type granite were derived from rocks underlying the exposed ordovician sequence p and t estimates for the highest grade rocks are t 700 c and p 3 5 kbar indicating a very high p t gradient of 65 c km the high heat flow during prograde metamorphism probably resulted from a combination of a thermal anomaly persisting from a pre metamorphic back arc basin environment and intrusion of hot mantle derived magmas into the lower and middle crust regional retrograde metamorphism coincided with a general reheating of the crust in the siluro devonian accompanied by intrusion of many i type plutons and resetting of the k ar dates of some earlier plutons the omeo metamorphic complex was exposed to erosion at this time

stepping stones and pitfalls in the determination of an anticlockwise p t t deformation path the low p high t cooma complex australia low pressure high temperature low p high t metamorphic rocks of the cooma complex southeastern australia show evidence of an anticlockwise pressure temperature time deformation p t t d path similar to those of some other low p high t metamorphic areas of australia prograde paths are reasonably well constrained in cordierite andalusite schists cordierite k feldspar gneisses and andalusite k feldspar gneisses these paths are inferred to be convex to the temperature axis involving increase in pressure with increase in temperature evidence of the retrograde path is inconclusive but is consistent with approximately isobaric cooling as are available isotopic data on the cooma granodiorite which indicate initially rapid cooling following attainment of peak temperatures the retrograde path is inconsistent with either a clockwise p t t d path involving rapid or even moderate decompression immediately post dating the peak of metamorphism or a path in which the retrograde component simply reverses the prograde component because both these paths should cross reactions forming cordierite from aluminosilicate for which no evidence has been observed determination of the deformational metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples evidence necessary for successful elucidation of the prograde and part of the retrograde deformational metamorphic history in the cooma complex includes 1 sequentially grown porphyroblasts that can be timed relative to surrounding foliations 2 partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development 3 a tectonic marker foliation s4 at cooma that allows correlation of foliations from one location to another and 4 single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone the latest two or three foliations involve low strain accumulation allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined sequential porphyroblast growth and foliation development in the cordierite andalusite schists is examined for situations involving rotation and non rotation of porphyroblasts relative to geographically fixed coordinates although the number of foliations developed varies in the rotational situation depending on the deformation history proposed the sequential order of porphyroblast growths does not differ from the non rotational situation thus whether or not porphyroblasts rotated in the cooma rocks the sequence of reactions and therefore p t t paths inferred from the relative timing of porphyroblast growths remain the same for the deformational histories evaluated

distinguishing between in situ and accretionary growth of continents along active margins abstract active continental margins represent both sites of crustal loss by tectonic erosion and gain by juvenile magmatism and oceanic arc plateau accretion estimating the gain component depends on distinguishing between growth of margins by either accretion of oceanic crust or juvenile magmatic addition during extension distinguishing between these allochthonous and autochthonous models is frequently ambiguous due to difficulties in interpreting the tectonic origin of mafic rocks we present a detailed geochronological and isotopic study constraining the evolution of the continental margin of northwest south america during 190 113 ma semi continuous slab rollback resulted in crustal thinning juvenile magmatism and ultimately the removal of continental slivers negating previous hypotheses that the margin evolved by the accretion of an island arc however basic crustal growth estimates along with comparisons between rocks exposed in northern peru and the northern andes imply that uneven losses of continental crust occurred along northwest south america for mesozoic exposures and that the accretion of the caribbean large igneous province clip onto nw south america probably played a crucial role in maintaining and preserving mesozoic continental crust north of the huancabamba deflection

geochronological and geochemical study of the high grade orthogneissic rocks from the northern fuyun complex implications for crustal architecture and early middle paleozoic evolution of the chinese altai abstract high grade metamorphic complexes are critical for the understanding of regional crustal configuration and accretionary history of the altai orogenic belt the orthogneissic rocks from the northern fuyun complex southern chinese altai are investigated in this study zircon u pb isotopic analysis yields ages of ca 411 398 ma arguing against previous views that these rocks represent old precambrian basement all the samples are calc alkaline metaluminous to weakly peraluminous asi 0 80 1 06 and can be classified as i type granitoids they have low to medium al2o3 and relatively high ferromagnesian compositions and most of them exhibit weak heavy rare earth element hree fractionation and variably negative eu anomalies the above geochemical compositions are consistent with dehydration melting of meta basaltic or amphibolitic crustal rocks followed by later fractional crystallization for their derivation in addition their positive zircon ehf t average 2 58 8 38 and mantle to chur chondritic uniform reservoir like whole rock end t 1 6 to 2 9 values indicate a relatively juvenile signature of their source rocks which suggests that the deep crustal section of the chinese altai may be dominated by juvenile mantle derived materials the high mgo contents 2 06 3 66 wt of some samples and the occurrence of the mafic microgranular enclave may reflect the involvement of mantle materials in their petrogenesis implying possible anatexis of the lower crust driven by basaltic underplating by synthesizing magmatic metamorphic and structural geological evidence we interpret that the chinese altai experienced a lithospheric scale extensional event in the devonian which resulted in massive heat influx emplacement of juvenile magmas syn orogenic crustal anatexis and formation of mature and layered continental crust the available data further suggest that alternating contraction and extension have probably operated on the chinese altai in the early middle paleozoic which is possibly critical for the evolution of accretionary systems worldwide

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

modelling the extension of heterogeneous hot lithosphere abstract the consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models the experiments examined the effects of i the location of heterogeneities in the ductile crust and or in the lithospheric mantle and ii their orientation perpendicular or oblique to the direction of bulk extension the observed deformation patterns are all relevant to the so called wide rifting mode of extension weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area this latter behaviour depends in turn upon the decoupling provided by the ductile crust localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle this contrasts with cold and strong lithospheres whose high strength sub moho mantle triggers a necking instability at the lithosphere scale this also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes thus for the given lithospheric rheology the models have relevance to backarc type extensional systems such as the aegean and the tyrrhenian domains

a hafnium isotopic record of magmatic arcs and continental growth in the iapetus ocean the contrasting evolution of ganderia and the peri laurentian margin abstract we test the sensitivity of combined u pb hf isotopic analyses of detrital and magmatic zircon to document complex processes in accretionary orogens using ganderia from the canadian appalachian orogen as an example ganderia hosted a long lived magmatic arc that began at least by 650 ma and continued until 450 ma during which time it drifted northward from the amazonian margin its isotopic record suggests arc activity began earlier possibly at 750 ma proto ganderia within an oceanic domain the u pb hf zircon isotopic array suggests that ganderia formed on a sliver of paleoproterozoic mesoproterozoic basement tdmc 2 2 1 1 ga a transition towards more evolved hf isotopic compositions between 650 and 600 ma is interpreted to coincide with its accretion to the amazonian margin of gondwana consistent with other geological evidence accretion was followed by increasing amounts of juvenile crustal inputs between 550 and 500 ma which is interpreted to reflect subduction roll back and the onset of drift from the gondwanan margin trench retreat culminated in the separation of the ganderia arc and formation of the exclusively oceanic 510 450 ma penobscot popelogan victoria arcs on ganderia accretion of ganderia to the active notre dame subzone along the laurentian margin occurred in the late ordovician and is marked by the abrupt termination of ganderian arc magmatism the notre dame arc evolved along the laurentian margin between 515 and 425 ma and was built on paleoproterozoic archean t dm c 2 9 2 5 ga crust supra subduction zone magmatism occurred at 490 460 ma during the closure of the taconic seaway and was dominated by the recycling of laurentian crust with progressive mixing of evolved and juvenile arc magmas over time the preservation of very evolved notre dame arc zircon in ganderian overstep sequences confirms the arrival of the leading edge of ganderia to laurentia by 450 ma the hafnium isotopic method is proven here to be a powerful tool when used in conjunction with geological constraints and is able to identify the changing nature of magmatism during the evolution of complex accretionary systems

a review of the famatinian ordovician magmatism in southern south america evidence of lithosphere reworking and continental subduction in the early proto andean margin of gondwana abstract along the proto pacific margin of gondwana from venezuela to northeastern patagonia the early middle ordovician famatinian orogeny was the first orogenic event following assembly of the supercontinent previous isotope studies of the igneous and meta sedimentary rocks of southwestern gondwana yield ambiguous implications for the role of juvenile mantle addition during the early crustal growth at the supercontinental margin to interpret the geological and tectonic evolution of the orogen and the magma sources in different episodes we look at evidence from a large area of southern south america including the 700 600 km type sector of the orogen in the sierras pampeanas 27 33 s the precordillera and northeastern patagonia previous geological geochemical and geochronological results are reviewed together with new u pb shrimp crystallization ages 177hf 176hf and 18o 16o data for dated zircon and whole rock sr and nd isotope compositions four geological domains are recognized in the sierras pampeanas western central eastern and foreland famatinian domains magmatism is mostly restricted to the interval 463 4 to 486 7 ma with the most intense period of emplacement between 468 and 472 ma constituting a magmatic flare up granitoid emplacement in both northeastern patagonia and the cordon de lila puna altiplano chile was effectively synchronous with that in the sierras pampeanas defining a continuous belt combined geochemical and isotopic data whole rock sr nd hf o in zircon indicate that the source of calcic metaluminous suites is the subcontinental lithosphere both mantle and mafic lower crust with variable contamination by the early paleozoic metasedimentary country rocks the lithospheric mantle involved is assumed to underlie the outcropping 1330 1030 ma age basement of the western domain which exhibits tectonic characteristics of active continental margin in the north and oceanic arc back arc in the south the latter sector is the potential source of some minor famatinian igneous rocks with less evolved isotopic compositions although a restricted asthenospheric addition cannot be discarded in this case minor peraluminous granites are spatially associated with the metaluminous sequence but major highly peraluminous batholiths occur on the eastern flank of the central domain field relations and geochemical isotopic evidence indicate that the most obvious source of these crustal melts was the very thick post early cambrian metasedimentary sequence comprising the host country rocks episodic tectono magmatic evolution of the famatinian magmatic belt in two overlapping stages is invoked to explain different characteristics in the four recognized domains in the type sector ca 474 486 ma roll back stage this is a mainly extensional interval involving asthenospheric upwelling and thinning of the subcontinental mantle full development of the marine ensialic basins and early emplacement of both metaluminous granites and highly peraluminous batholiths in the central and eastern famatinian domains trondhjemite plutons with an adakitic signature were emplaced in the foreland domain ca 468 472 ma slab break off stage steepening of the oceanic slab and arc migration to the southwest ended with slab break off due to subduction of continental crust during continental collision with the precordillera terrane this stage produced voluminous metaluminous magmatism at the western edge of the central domain the flare up episode k bentonites in the precordillera leucogranites in the western domain and scattered metaluminous and peraluminous plutons in all famatinian domains both slab roll back and break off stages developed during a high t regime typical of hot orogens although asthenospheric mantle was a necessary heat source for lithospheric melting its material contribution to the growth of early paleozoic crust was apparently very minor recycling of mesoproterozoic lithosphere including the subcontinental mantle coupled with crustal melting of early paleozoic metasedimentary sequences accounts for most famatinian magmatism comparable results from the central andes and east antarctica confirm that the early stages of the terra australis orogen in sw gondwana were dominated by lithospheric reworking processes

palaeozoic arc growth deformation and migration across the lachlan fold belt southeastern australia abstract the palaeozoic lachlan fold belt lfb is a low pressure metamorphic belt characterized by greenschist facies rocks and infolded volcanic sequences which indicate that the belt has not undergone substantial uplift at any stage porphyroblast matrix relationships in one of the highest grade metamorphic zones of the lfb the omeo complex indicate that the thermal peak was reached before or was synchronous with the earliest deformation similar to that in high grade metamorphic zones of the northern arunta inlier central australia in both areas the metamorphism is of low p high t type centred on major granite intrusions as regional aureoles and characterized by anti clockwise p t t paths however in the outer lower grade parts of the aureoles peak metamorphic conditions post date the earliest deformation this diachronous relationship indicates that 1 outward migration of a thermal front was centred on major granitoid intrusions batholiths 2 deformation propagated outward more rapidly than migration of the thermal front and 3 plastic and ductile deformation began only after heating around the batholiths thus orogeny was initiated after and localized by heat focusing in the mid crust associated with batholith emplacement therefore the early deformation is a result of thermal softening it is typically subhorizontal ductile shear at mid crustal levels but is characterized by upright to inclined folds at upper crustal levels in the latter environment the typically discordant contact aureole plutons are often interpreted as post tectonic granitoids but they were part of the ongoing mid crustal thermal perturbation that induced regional greenschist facies metamorphism which overprinted the early formed upright folds as it migrated to upper crustal levels orogeny migrated generally eastward through the lfb it began with the early silurian benambran orogeny centred on the wagga metamorphic belt wmb and terminated with the carboniferous kanimblan orogeny which was most intense in the hill end trough eastward migration of orogeny across central victoria part of the west lfb began in the ballarat bendigo zone during the early devonian and terminated in the melbourne zone in the middle devonian orogeny was centred on the meridional batholiths of the lfb each of which are considered to represent the transitory axis of an ancient magmatic arc stepwise but generally eastward arc migration caused the eastward migration of orogeny a tectonic model based on the modern southwest pacific arc system can be applied to the lfb following cambrian intra oceanic arc growth associated with west dipping subduction an ordovician marginal sea developed partly on stretched cambrian crust and was flooded by an extensive turbiditic wedge closure of the eastern part the east lfb began in the early silurian after a magmatic arc developed over the wmb possibly associated with an east dipping subduction zone after re establishment of west dipping subduction in the middle silurian absolute motion retreat of the upper australian plate caused by oblique plate convergence resulted in dextral transtension and the generation of siluro devonian inter arc and back arc basins transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard eastward arc system periodic compression within the evolving arc and back arc system resulted in a series of east younging east verging linear siluro devonian fold and thrust belts which were localized adjacent to elongate batholiths that represent the relict arc systems local westward jumps in deformation and plutonism occurred the most significant is the 600 km westward jump into the west lfb caused by closure of the remainder of the passive ordovician back arc basin melbourne and bendigo ballarat zones west dipping subduction beneath the basin produced a low pressure fold thrust belt similar to the east lfb though the granitoids have a more primitive isotopic character indicating closure of thin dominantly oceanic crust in the early to middle devonian localized deformation induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins is likely to produce anticlockwise p 7 t paths and may be a typical feature of southwest pacific style tectonism

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

timing and heat sources for the barrovian metamorphism scotland abstract new shrimp u pb zircon ages of 472 2 5 8 ma and 471 2 5 9 ma are presented for the age of peak metamorphism of barrovian migmatites 40 ar 39 ar ages for white mica from the barrovian metamorphic series are presented and are recalculated using recently proposed revisions to the 40 k decay constants to allow more precise and accurate comparison with u pb ages the 40 ar 39 ar ages are found to vary systematically with increasing metamorphic grade between c 465 ma for the biotite zone and c 461 ma for the sillimanite zone there is no evidence for any significant metamorphic heating during the first 15 myr of the grampian orogeny before c 473 ma or the final 4 myr after c 465 ma the barrovian metamorphism occurred over a period of 8 myr within the 27 myr grampian orogeny the barrovian metamorphism records punctuated heating was temporally and spatially associated with large scale bimodal magmatism and developed within crust that was not overthickened the temporally distinct nature of the barrovian metamorphic episode within the grampian orogeny and its heating pattern and tectonic context are not consistent with significant heat contribution from thermal equilibration of overthickened crust rather the barrovian metamorphism records a transient phase of crustal thermal disequilibrium during the grampian orogeny temporal and spatial association with grampian bimodal magmatism is consistent with production of the barrovian metamorphic series within the middle crust as the result of advection of heat from the lower crust and or mantle the barrovian metamorphic series the classic example of orogenic regional metamorphism did not form in response to crustal thickening and thermal relaxation but appears to record large scale contact metamorphism

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

late jurassic to early early cretaceous tectonic nature on the ne asian continental margin constraints from mesozoic accretionary complexes abstract it is widely accepted that subduction of the paleo pacific plate beneath eurasia began in the early mesozoic however the tectonic nature and processes of the ne asian continental margin in the late jurassic to early early cretaceous remain unclear in this contribution lithological provenance and metamorphic deformation data are used to constrain the formation of mesozoic accretionary complexes ac along the ne asian continental margin and determine the tectonic relationship between the ne asian continental margin and the paleo pacific plate in the late jurassic to early early cretaceous the mesozoic ac formed during the late jurassic including the raohe ac in ne china the khabarovsk samarka and taukha acs in the russian far east and the mino tamba ac in japan and early cretaceous including the shimanto ac in japan and the kiselevka manoma ac in the russian far east the jurassic ac underwent a complex evolution including the formation of seamounts middle late jurassic terrigenous clastic sedimentation adjacent to the trench collision of seamounts with eurasia northward migration of the ac early early cretaceous terrigenous clastic sedimentation a second period of accretion and final emplacement the earliest mesozoic ac arrived at the trench at ca 195 ma combined with the occurrence of early mesozoic calc alkaline igneous rocks along the ne asian continental margin we suggest that subduction of the paleo pacific plate beneath eurasia began in the early jurassic jurassic ac in japan was affected by a regional high p t metamorphic event at 160 ma which might have been related to subduction collision of seamounts with eurasia at the trench ages and hf isotopic compositions of detrital zircon grains from the middle upper jurassic and cretaceous ac sandstones together with paleontological and paleomagnetic data indicate that jurassic clastic sediments were sourced mainly from the south china block scb whereas the lower cretaceous sandstones were sourced from the eastern central asian orogenic belt caob the inferred northward migration of jurassic ac in the late jurassic to early early cretaceous indicates that the boundary between the ne asian continental margin and the paleo pacific plate was characterized by strike slip tectonics at this time likely due to a small angle of obliquity in the subduction of the paleo pacific plate beneath eurasia in the late early cretaceous westward subduction of the paleo pacific plate beneath eurasia resulted in the emplacement of the raohe and khabarovsk acs at 137 130 ma and the formation of acs in the russian far east and japan

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

on the evolution of orogens pressure cycles and deformation mode switches abstract evidence of two burial exhumation cycles that took place during a single orogeny has been found in rocks belonging to the piemonte unit of the western alps an early high pressure event which resulted from tectonic burial down to pressures of 1 5 gpa was followed by exhumation to ca 0 20 0 35 gpa as a result of extensional deformation renewed shortening culminated in a second burial episode down to pressures of 0 65 0 80 gpa before the final exhumation took place existing geochronological data allow only ca 13 19 ma for the completion of both burial exhumation cycles therefore we suggest that the evolution of orogens is characterized by multiple short lived burial exhumation cycles related to orogen scale alternance between shortening and extensional deformation

the pangea conundrum geodynamic models for supercontinent assembly whereby the dispersing continental fragments of a supercontinent break up and migrate from geoid highs to reassemble at geoid lows fail to account for the amalgamation of pangea such models would predict that the oceans created by continental breakup in the early paleozoic e g iapetus rheic would have continued to expand as the continents migrated toward sites of mantle downwelling in the paleo pacific reassembling an extroverted supercontinent as this ocean closed instead pangea assembled as a result of the closure of the younger iapetus and rheic oceans geodynamic linkages between these three oceans preserved in the rock record suggest that the reversal in continental motion may have coincided with the ordovician emergence of a super plume that produced a geoid high in the paleo pacific if so the top down geodynamics used to account for the breakup and dispersal of a supercontinent at ca 600 540 ma may have been overpowered by bottom up geodynamics during the amalgamation of pangea

the stratigraphy and deformation history of the tumut region implications for the development of the lachlan fold belt abstract the tumut region forms the southern part of the tumut synclinorial zone in the southeastern part of the lachlan fold belt lfb the area contains two major stratigraphie packages ordovician to early silurian quartz rich to quartz intermediate flysch and volcanics package a and overlying fossiliferous early late silurian volcanics and flysch package b rhyolite within the older package a yields a u pb zircon age of 428 6 ma both packages were meridionally folded during the siluro devonian bowning orogeny following intrusion of the gocup granite at 411 5 ma an earlier deformation characterised by thrust faulting e w recumbent folding and later local coaxial upright folding is present only in the older flysch and volcanics package a this earlier deformation is part of the benambran orogeny that affects the ordovician metamorphics of the wagga metamorphic belt and other parts of the lfb and is constrained to about 425 ma fold characteristics of this deformation are indicative of thin skinned intraplate transpressional deformation rather than classical collisional tectonics as envisaged by some workers for the benambran orogeny here and elsewhere in the lfb the composition and deformational history of the older flysch indicates that it is part of the ordovician to early silurian molong volcanic belt consequently the older concept of an early silurian tumut trough incorporating both packages of rocks is rejected instead it is proposed that up to 2500 m of early late silurian rocks formed in a pull apart basin the tumut basin thus the silurian depositional history of the tumut region previously considered unique in the lfb is little different from other basins of similar age throughout the lfb

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

new models for paleoproterozoic orogenesis in the cheyenne belt region evidence from the geology and u pb geochronology of the big creek gneiss southeastern wyoming the disputed age of the deep crust of the colorado province is central to hypotheses for paleoproterozoic crustal growth in the region we studied the high grade big creek gneiss southeastern wyoming as a potential exposure of pre 1780 ma basement rocks new geologic mapping and u pb geochronological data indicate that the big creek gneiss exposes a deeper but coeval level of the green mountain arc relative to the predominantly supracrustal section to the west the big creek gneiss is composed of supracrustal rocks a ca 1780 ma green mountain arc correlative bimodal intrusive suite a ca 1763 ma extensional bimodal intrusive suite and widespread ca 1630 ma pegmatitic leucogranite the mafic member of the younger bimodal suite is documented here for the first time u pb zircon ages from migmatite leucosomes indicate penetrative deformation of the big creek gneiss at ca 1750 ma we find that the postarc intrusive suite is mantle involved implying a second period of crustal growth shortening postdates arc magmatism by 20 m y implying that termination of arc magmatism and accretion were separate events finally criteria previously used to constrain the polarity of subduction for the green mountain arc are not reliable we propose two competing models 1 southward dipping green mountain arc subduction present coordinates with slab breakoff related magmatism following arc accretion or 2 northward dipping subduction with extensional postarc magmatism in both models high temperature deformation coincides with accretion along the cheyenne belt and extensional magmatism is an important component of crustal growth we prefer the northward dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks

paleozoic to triassic ocean opening and closure preserved in central iran constraints from the geochemistry of meta igneous rocks of the anarak area the anarak area belongs to an ophiolitic belt along the northern border of the central east iranian microcontinent and is thought to contain fragments of the former paleotethys and neotethys oceans a wide range of meta igneous rocks from the late paleozoic to triassic anarak metamorphic complex amc and nearby meraji area have been studied to constrain the origins and modes of emplacement of oceanic remnants in central iran our samples occur as layers and lenses embedded in extensive sequences of deformed meta sediments and smaller bodies of serpentinized ultramafic rocks petrographical and geochemical data combined with field and satellite observations allow recognition of seven types of meta igneous rocks preserved from low grade to blueschist facies conditions their origins based on relative abundances of immobile trace elements include subduction zone mid ocean ridge ocean intraplate and continental rift settings these data and existing geochronological constraints show the amc formed an accretionary complex formed exhumed incrementally during the carboniferous permo triassic and triassic igneous rocks from meraji formed in the early devonian due to opening of the paleotethys and belong to a rift sequence extending over 300 km along the edge of the central east iranian microcontinent the amc and nearby rock associations record the evolution of the paleotethys during a complete wilson cycle between ca 450 and 225 ma with implications for 1 continental rifting 2 ocean opening 3 subduction initiation 4 ocean intraplate and continued mid ocean volcanism 5 ridge subduction and 6 final closure of the ocean during continent continent collision alternate interpretations of the anarak metabasites are possible but require radical departures from the widely accepted model for tectonic evolution of the paleotethys with the existence of paleotethyan backarc basin s and permian or earlier collision of continental blocks in central iran in any case our results show accretionary complexes preserved along suture zones contain an important record of the evolution of oceanic crust from ancient ocean basins

accretionary orogenesis of the chinese altai insights from paleozoic granitoids zircon u pb dating and whole rock major oxide trace element and nd sr isotope compositions have been determined for four representative granitic intrusions in the sw chinese altai in order to understand the orogenesis and history of crustal growth in the central asian orogenic belt caob the ashile and halatas bodiesare relatively small intrusions formed in the late carboniferous 318 6ma andpermian 267 4ma respectively thelargertarlang tb andkeketuohaibatholiths kb havemagmaticagesof 359 to 412 ma the granitoids consist chiefly of tonalite granodiorite and granite and are metaluminous to weakly peraluminous in composition asi 0 8 1 1 the ashile pluton is characterized by relatively high mgo 2 73 3 54 wt and sr 395 456 ppm contents with low k2o na2o ratios 0 31 0 42 and mantle like nd sr isotopic compositions endt 2 85 to 3 26 initial 87 sr 86 sr 0 7047 0 7051 it probably formed by fractional crystallization of a mantle derived magma with limited assimilation of oldcrustalmaterial theotherintrusionshavenear zeroornegative endtvalues 0 67to 4 41 andhighersrisotopiccompositions initial 87 sr 86 sr 0 7067 0 7092 indicating a mixture of juvenile and old components in their genesis these intrusions are characterized by relatively low k rb cs contents and low rb sr ratios mostly b0 6 and generally exhibit moderate to high heavy rare earth element hree concentrations ybn 8 26 and weak hree fractionation dy ybnb1 5 with or without negative europium anomalies eu eu 0 5 1 0 these characteristics are consistent with an origin by dehydration melting of a hornblendebearing mid crustal source above the garnet stability field dominated by mafic to intermediate rocks this suggests that the deep crust of the chinese altai may contain a considerable proportion of juvenile material mantle derived magmatism probably in an active extensional continental margin setting played an important role in the formation of the chinese altai acting not only as a heat sourceforcrustalmeltingbutasasourceofjuvenilecomponents therelativelysmallvolumesanddistinctgeochemicalcompositions of the ashile and halatas plutons may imply a substantial change in the geodynamic setting in the late paleozoic 2007 elsevier b v all rights reserved

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

the capilla del monte pluton sierras de cordoba argentina the easternmost early carboniferous magmatism in the pre andean sw gondwana margin new geochronological geochemical and isotopic data are reported for the capilla del monte two mica granite pluton in the northeastern sierras de cordoba an early carboniferous age is established by a u pb zircon concordia 336 3 ma and a rb sr whole rock isochron 337 2 ma zircon saturation geothermometry indicates relatively high temperatures 735 800 c the granites have high average sio2 74 2 na2o k2o 7 8 and high field strength elements high k2o na2o 1 7 and feo mgo ratios 5 1 with low cao content 0 71 ree patterns with marked negative eu anomalies eu eu 0 14 0 56 indicate crystal fractionation dominantly of plagioclase and k feldspar from a peraluminous magma enriched in f isotope data 87sr 86srinitial 0 7086 e nd336 5 5 to 4 4 with t dm 1 5 ga zircon e hf336 0 8 to 6 1 mean t dm 1 5 ga suggest a mesoproterozoic continental source albeit with some younger or more juvenile material indicated by the hf data the pluton is the easternmost member of a carboniferous a type magmatic suite which shows an increase in juvenile input toward the west in this part of the pre andean margin the petrological and geochemical data strongly suggest a similar intraplate geodynamic setting to that of the nearby but much larger late devonian achala batholith although hf isotope signatures of zircon suggest a more uniformly crustal origin for the latter further studies are required to understand whether these bodies represent two independent magmatic episodes or more continuous activity

the neoproterozoic evolution of the western siberian craton margin u pb hf isotopic records of detrital zircons from the yenisey ridge and the prisayan uplift abstract we present new la icp ms u pb hf detrital zircon data from neoproterozoic sedimentary basins located along the south western margin of the siberian craton in the yenisey ridge the taseeva basin siliciclastic sedimentary units contain abundant 2 7 2 5 ga 2 0 1 75 ga and 0 95 0 57 ga detrital zircon populations whereas for the prisayan uplift slightly different detrital zircon ages of 3 0 2 5 ga 2 2 1 8 ga and 0 95 0 65 ma are common mid neoproterozoic sandstones were deposited in a rift setting or along a rifted cratonic margin facing a back arc basin whereas the immature to submature upper ediacaran to lower cambrian fluvial successions were probably deposited in the foredeep setting at the expense of proximal neoproterozoic igneous suites and the underlying metasedimentary basement a range of ehf values from 10 to 14 indicates intense mixing between juvenile and evolved magma components in a continental arc setting at ca 950 570 ma geological evidence persistent input of detrital zircon material derived from ancient crustal units of western siberian cratonic margin and evolution of hf isotopic compositions suggest that the neoproterozoic arc system of the yenisey ridge and partly sayan orogens developed along the active margin of siberia in the neoproterozoic the arc system probably extended along the periphery of rodinia into the taimyr orogen of northern siberia and valhalla orogen of north eastern laurentia

identification of early carboniferous granitoids from southern tibet and implications for terrane assembly related to the paleo tethyan evolution abstractthis article presents zircon u pb and hf isotope data together with the whole rock major and trace element composition of early carboniferous granitoids newly identified from the jiacha and langxian areas in the southern lhasa terrane southern tibet the jiacha rocks are monzogranites that yield zircon u pb ages of 347 345 ma and ehf t values from 5 4 to 4 9 the langxian rocks are granodiorites with slightly older zircon u pb ages of 355 352 ma and lower ehf t values from 6 8 to 6 5 our data suggest that these granitoids were generated largely by reworking of paleoproterozoic ga basement materials in conjunction with literature data it is further argued that the southern and central parts of the lhasa terrane separated by the sumdo eclogite belt should have been an integrated block before the late paleozoic our study supports the notion that the lhasa terrane was derived from the northern margin of gondwanaland in association with formation of at least two stages of tethyan oc

hot orogens tectonic switching and creation of continental crust comment and reply comment in a stimulating paper collins 2002 1 p 535 asserted that many granulite terrains were too hot to have formed during continental collision in spite of evidence to the contrary e g harley 1989 2 o brien and rotzler 2003 3 based on this assertion collins 2002 1 p

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

ordovician convergent margin volcanism and tectonism in the lachlan sector of east gondwana the enigmatic early paleozoic tectonic setting of the lachlan orogen in australia has obscured reconstructions of east gondwana recognition of an intraoceanic mafic island arc in this paper establishes the presence of a convergent plate boundary in the tasmanides from the early ordovician to the early silurian this in turn facilitates the updating of supercontinent correlations

fractionation and incipient self granulitization during deep crust emplacement of lower ordovician valle fertil batholith at the gondwana active margin of south america abstract large granite batholiths were emplaced at the gondwana active margin during lower ordovician in south america these have contributed to crustal growth by net addition of silicic rocks to the continental crust new u pb shrimp zircon age determinations together with thermobarometric and geochemical data yield that batholith magma intrusion is the responsible of heating and self granulitization of early gabbro pulses partially molten granulitic gabbros which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith forming qtz diorites contain opx bearing trondhjemitic leucosomes surrounding hbl opx pl mafic mesosomes forming typical agmatitic structures hornblende plagioclase equilibria applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes yield temperatures in the range 850 910 c for core to core pairs and in the range 1000 1075 c for rim to rim pairs at pressures of about 0 7 gpa shrimp zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20 ma from 465 to 485 ma with most ages clustered at about 470 ma the age of metagabbros is 473 7 ma for older zircons and 454 4 for younger zircons these ages are almost coincident within error with the age of host migmatites 477 5 ma and those of batholith intrusion of 476 9 ma and 475 3 ma for qtz diorites and 475 5 ma for granites zircon overgrowths of these intrusive rocks yield sages clustered around 450 ma revealing a protracted thermal history more complex than previously believed the geochemical study reveals that qtz diorites tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma a weak adakitic tendency with sr y 15 in several samples implies the presence of grt in the source or magma chamber at a minimum pressure of 1 0 gpa higher than the depth of emplacement at 0 7 gpa the high temperature of magma emplacement which induced the incipient self granulitization of early magmatic pulses together with the cotectic like fractionation linking coeval qtz diorites tonalites and granodiorites is compatible with fractionation at the lower crust of a deep generated infracrustal sublithospheric intermediate magma

episodic behavior of gondwanide deformation in eastern australia insights from the gympie terrane the mechanisms that drove permian triassic orogenesis in australia and throughout the cordilleran type gondwanan margin is a subject of debate here we present field based results on the structural evolution of the gympie terrane eastern australia with the aim of evaluating its possible role in triggering widespread orogenesis we document several deformation events d d in the gympie terrane and show that the earliest deformation d occurred only during the final pulse of orogenesis 235 230 ma within the broader gondwanide orogeny in addition we found no evidence for a crustal suture suggesting that terrane accretion was not the main mechanism behind deformation rather the similar spatiotemporal evolution of permian triassic orogenic belts in australia antarctica south africa and south america suggest that the gondwanide orogeny was more likely linked to large scale tectonic processes such as plate reorganization in the context of previous work our results highlight a number of spatial and temporal variations in pulses of deformation in eastern australia suggesting that shorter cycles of deformation occurred at a regional scale within the broader episode of the gondwanide orogeny similarly to the cenozoic evolution of the central and southern andes we suggest that plate coupling and orogenic cycles in the late paleozoic to early mesozoic gondwanide orogeny have resulted from the superposition of mechanisms acting at a range of scales perhaps contributing to the observed variations in the intensity timing and duration of deformation phases within the orogenic belt

orogen perpendicular structures in the central tasmanides and implications for the paleozoic tectonic evolution of eastern australia the curvilinear e w structures of the southern thomson orogen are approximately orthogonal to the general n s structural trend of the tasmanides of eastern australia the origin of these orogen perpendicular structures and their implications to tectonic reconstructions of eastern gondwana are not fully understood here we use geophysical data to unravel the geometry kinematics and possible timing of major structures along the boundary between the thomson orogen and the southern tasmanides delamerian and lachlan orogens aeromagnetic data from the southern thomson orogen show wnw e w and or ene trending structural grains corresponding to relatively long wavelength linear geophysical anomalies kinematic analyses indicate strike slip and transpressional deformation along these geophysically defined faults structural relationships indicate that faulting took place during the benambran late ordovician to middle silurian and tabberabberan late early to middle devonian orogenies however some of the described crustal scale structures may have developed in the cambrian during the delamerian orogeny interpretation of deep seismic data shows that the crust of the southern thomson orogen is substantially thicker than the lachlan orogen crust which is separated from the thomson orogen by the north dipping olepoloko fault a major lithospheric scale change across this boundary is also indicated by a contrast in seismic velocities together with evidence for the occurrence of delamerian deformation in both the koonenberry belt and northeastern thomson orogen and a significant contrast in the width of the northern tasmanides versus the southern tasmanides it appears that the southern thomson orogen may represent the locus of orogen perpendicular segmentation which may have occurred in response to along strike plate boundary variations

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

new insights into the history and origin of the southern maya block se mexico u pb shrimp zircon geochronology from metamorphic rocks of the chiapas massif the histories of the pre mesozoic landmasses in southern mexico and their connections with laurentia gondwana and among themselves are crucial for the understanding of the late paleozoic assembly of pangea the permian igneous and metamorphic rocks from the chiapas massif as part of the southern maya block mexico were dated by u pb zircon geochronology employing the shrimp sensitive high resolution ion microprobe facility at stanford university the chiapas massif is composed of deformed granitoids and orthogneisses with inliers of metasedimentary rocks shrimp data from an anatectic orthogneiss demonstrate that the chiapas massif was part of a permian 272 ma active continental margin established on the pacific margin of gondwana after the ouachita orogeny latest permian 252 254 ma medium to high grade metamorphism and deformation affected the entire chiapas massif resulting in anatexis and intrusion of syntectonic granitoids this unique orogenic event is interpreted as the result of compression due to flat subduction and accretionary tectonics shrimp data of zircon cores from a metapelite from the ne chiapas massif yielded a single grenvillian source for sediments the majority of the zircon cores from a para amphibolite from the se part of the massif yielded either 1 0 1 2 or 1 4 1 5 ga sources indicating provenance from south american sunsas and rondonian san ignacio provinces

permo triassic anatexis continental rifting and the disassembly of western pangaea crustal anatectites are frequently observed along ocean continent active margins although their origins are disputed with interpretations varying between rift related and collisional we report geochemical isotopic and geochronological data that define an 1500 km long belt of s type meta granites along the andes of colombia and ecuador which formed during 275 223 ma these are accompanied by amphibolitized tholeiitic basaltic dykes that yield concordant zircon u pb dates ranging between 240 and 223 ma a model is presented which places these rocks within a compressive permian arc setting that existed during the amalgamation of westernmost pangaea anatexis and mafic intrusion during 240 223 ma are interpreted to have occurred during continental rifting which culminated in the formation of oceanic crust and initiated the break up of western pangaea compression during 275 240 ma generated small volumes of crustal melting rifting during 240 225 ma was characterized by basaltic underplating the intrusion of tholeiitic basalts and a peak in crustal melting tholeiitic intrusions during 225 216 ma isotopically resemble depleted mantle and yield no evidence for contamination by continental crust and we assign this period to the onset of continental drift dissected ophiolitic sequences in northern colombia yield zircon u pb dates of 216 ma the permo triassic margin of ecuador and colombia exhibits close temporal faunal and geochemical similarities with various crustal blocks that form the basement to parts of mexico and thus these may represent the relict conjugate margin to nw gondwana the magmatic record of the early disassembly of pangaea spans 20 ma 240 216 ma and the duration of rifting and rift drift transition is similar to that documented in cretaceous tertiary rift settings such as the west iberia newfoundland conjugate margins and the taupo lau havre system where rifting and continental disassembly also occurred over periods lasting 20 ma

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

slow cooling and crystallization of the roots of the neoproterozoic aracuai hot orogen se brazil implications for rheology strain distribution and deformation analysis the aracuai ribeira belt formed during the amalgamation of west gondwana in the late neoproterozoic its evolution included a main tectonometamorphic peak at 600 580 ma and a minor peak associated with the final collision with the western congo at 540 530 ma this belt has the characteristics of a hot orogen including a high thermal gradient 30 c km pervasive partial melting of the middle crust emplacement of large volumes of magmas resulting from partial melting of the lower crust and underlying mantle and slow cooling after the peak temperature we report 21 new amphibole biotite and muscovite 40ar 39ar ages that complement previously published data these data suggest slow cooling 3 5 c myr over several tens of million years after the peak temperature 800 c at 600 ma followed by faster cooling 10 c myr after the final amalgamation of west gondwana we estimate that 30 myr was required to heat the middle crust to the peak temperature and that anatectic and plutonic bodies remained in the magmatic state for 40 myr this protracted thermal evolution likely had major effects on the rheology of the middle crust and on the tectonic evolution of this orogen for example the correlation of u pb zircon crystallization ages and 40ar 39ar biotite cooling ages in the anatectic core of the orogen denotes a diachronic thermal evolution likely related to 3d deformation involving successive upwelling of anatectic components within a thrust unit crosscutting the pre existing fabric channel flow like this study also highlights that classical structural analysis techniques relying on changes in pressure or temperature conditions to identify the succession of deformation phases are not efficient at deciphering the tectonic evolution of hot slowly cooling orogenic belts where the temperature varies slowly over tens of million years allowing diachronic episodes of deformation to occur under nearly similar pressure and temperature conditions

magmatic expression of tectonic transition from oceanic subduction to continental collision insights from the middle triassic rhyolites of the north qiangtang block abstract the tectonic transition from subduction to collision is a fundamental process during orogenesis yet the magmatic expression of this transition and related deep geodynamic processes remain unclear this study focuses on a newly identified volcanic belt within the moyun zaduo sulu area of the north qiangtang block and presents new zircon u pb data that indicate that this belt formed during the middle triassic 247 241 ma a time characterized by a regional transition from subduction to collisional tectonism the volcanic belt is located to the south of a permian to early triassic arc and is dominated by high k calc alkaline and peraluminous rhyolites these rhyolites have low mg nb ta and eu values contain low contents of sr have high rb sr and whole rock end t values and show positive zircon ehf t values all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material the migration of middle triassic volcanism in this region was most likely caused by rollback of the subducting longmucuo shuanghu tethyan oceanic slab combining our new data with previously published results of numerical modeling of subduction collisional processes and regional data from north central tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision this combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction the transition from subduction to collision and collisional tectonism

carboniferous to early triassic magmatism and accretion in alxa nw china implications for accretionary orogenesis of the southern altaids the alxa occupies a critical position between the tianshan beishan to the west and the solonker suture to the east and is significant for understanding the accretionary orogenesis of the southern altaids to unravel the tectonic history of the alxa we undertook an integrated study of field geology geochemistry and geochronology of magmatic rocks and an accretionary complex six granites and one rhyolite from the zhusileng hangwula arc zha show peraluminous high k calc alkaline composition and one gabbro is tholeiitic they show incompatible element rich patterns and negative nb ta anomalies on the primitive mantle normalized trace element spider diagrams la icp ms zircon u pb dating reveals 348 329 and 267 250 ma magmatic events these data indicate that the zha was a japan type island arc from late paleozoic to early triassic the engger us melange comprises n morb type pillow basalt tuff chert siliceous mudstone limestone and turbidite which are tectonically juxtaposed by block in matrix structure this melange is interpreted as a carboniferous permian accretionary complex these new data combined with previous studies confirm that the paleo asian ocean subducted bidirectionally in the alxa in carboniferous early triassic a consistent divergent subduction system existed from alxa to solonker before the terminal closure of the paleo asian ocean in the mid late triassic supplementary material https doi org 10 6084 m9 figshare c 4962230