the metamorphic sole of new caledonia ophiolite 40ar 39ar u pb and geochemical evidence for subduction inception at a spreading ridge amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of new caledonia termed peridotite nappe recrystallized in the high temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the eocene metamorphic complex amphibolites mostly display the geochemical features of morb with a slight nb depletion and thus are similar to the youngest late paleocene eocene babb components of the allochthonous poya terrane thermochronological data from hornblende 40ar 39ar zircon and sphene u pb suggest that these mafic rocks recrystallized at 56ma using various geothermobarometers provides a rough estimate of peak recrystallization conditions of 0 5gpa at 800 950c the thermal gradient inferred from the metamorphic assemblage 60 ckm 1 geometrical relationships and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction obduction system and recrystallized when they subducted below young and hot oceanic lithosphere they were detached from the down going plate and finally thrust onto unmetamorphosed poya terrane basalts this and the occurrence of slab melts at 53ma suggest that subduction inception occurred at or near to the spreading ridge of the south loyalty basin at 56ma 2012 american geophysical union all rights reserved

multiple episodes of continental subduction during india asia convergence insight from seismic tomography and tectonic reconstruction abstract high wavespeed tomographic anomalies shallower than 1100 km beneath the india asia collision zone are interpreted as continental slabs subducted during collision combining anomaly positions with paleogeographic reconstructions of india we constrain the spatio temporal evolution of multiple episodes of continental subduction likely related to these anomalies this study highlights the different evolution at lithospheric scales of the western and eastern parts of the collision zone the evolution of the western part is characterized by two episodes of steep subduction of the northern margin of india the first episode involving an area with a lateral extent as large as 1500 km started at about 40 30 and ended by a slab break off process at 15 ma the second episode consists on subduction beneath the hindu kush mountains since 8 ma to the east of the collision zone no anomaly related to steep subduction along the northern edge of india is found we interpret two tomographic anomalies beneath the eastern border of the indian plate beneath burma and beneath the andaman sea as the result of two successive episodes of southeastward extrusion followed by subduction we suggest that both extruded portions were initially located along the northern margin of india and that they slid around the eastern syntaxis then southward along the eastern boundary of indian plate both portions subducted along the eastern border of india south of the eastern syntaxis we provide a rough estimate of the amount of indian lithosphere consumed during these subduction and extrusion episodes by comparing this amount with the total amount of indian lithosphere at the onset of collision we conclude that these processes accommodated most of the india asia convergence during collision

a fossil ocean continent transition of the mesozoic tethys preserved in the schistes lustres nappe of northern corsica abstract the analysis of field relationships between allochthonous crystalline slivers ultramafic units and associated sedimentary cover in the high pressure schistes lustres units of northern corsica allows reconstruction of the pre orogenic architecture of a fossil mesozoic ocean continent transition oct of the tethys basin the studied area encompassing the serra di pigno and col de teghime areas cap corse is composed of an assemblage of prealpine basement sheets made up of serpentinized mantle rocks permian gabbros and gneisses locally associated with paleozoic metabasites and micaschists the sedimentary cover includes numerous intervals of calc schists marbles quartzites micaschists and dolomitic metabreccias yielding a great variety of lithostratigraphical successions the metasedimentary sequences do not show affinities with the typical cover series of the prepiemontais margin domains this implies an origin from a more internal domain oceanward for the studied units in addition the sedimentary cover of the ultramafic basement units do not display the characters of the classical alpine ophiolites but bear lithological features showing affinities with both continental and oceanic environments we assume that the high variability of the sedimentary successions as well as the presence of a basement composed of continental and upper mantle rocks is consistent with an origin at an oct the acidic basement slivers are regarded as former extensional allochthons of continental crust abandoned during breakup on a newly exhumed seafloor of peridotite composition such extensional allochthons will then logically constitute the most internal continental slices buried at great depth during alpine subduction this work shows that a lot has to be expected from a revision of the schistes lustres lithostratigraphy in the light of the new concepts of the evolution of the oct domains

the crustal architecture of myanmar imaged through zircon u pb lu hf and o isotopes tectonic and metallogenic implications abstract the tethys margin in central and eastern asia is comprised of continental terranes separated by suture zones some of which remain cryptic determining the crustal architecture and therefore the geological history of the eastern tethyan margin remains challenging sited in the heart of this region myanmar is a highly prospective but poorly explored minerals jurisdiction a better understanding of myanmar s mineralization can only be realized through a better understanding of its tectonic history itself reflected in at least four major magmatic belts the eastern and the main range provinces are associated with the late permian to early triassic closure of palaeo tethys the mogok mandalay mergui belt and wuntho popa arc are a response to the eocene closure of neo tethys however magmatic ages outside these two orogenic events are also recorded we present new zircon u pb lu hf and o isotope data from magmatic rocks across myanmar which we append to the existing dataset to isotopically characterize myanmar s magmatic belts eastern province permian i type magmatism has evolved ehf 10 9 to 6 4 whilst main range province triassic s type magmatism also records evolved ehf 13 5 to 8 8 the mogok mandalay mergui belt is here divided into the tin province and the mogok metamorphic belt the tin province hosts ca 77 50 ma magmatism with evolved ehf 1 2 to 15 2 and 18o of 5 6 8 3 the mogok metamorphic belt exhibits a more complex magmatic and metamorphic history and granitoids record jurassic late cretaceous and eocene to miocene phases of magmatism all of which exhibit evolved ehf values between 4 6 and 17 6 and 18o between 6 3 and 9 2 from the tagaung myitkyina belt we report a magmatic age of 172 ma and ehf of 18 1 to 10 8 to accommodate the geological evidence we propose a tectonic model for myanmar involving a greater sibumasu where the documented zircon isotopic variations reflect compositional variations in magmatic source and invoke the role of a tengchong block the baoshan block and greater sibumasu were likely assembled on or before the triassic a former andean margin and suture which may lie across the northern shan plateau and reflected in isotopic differences between the northern and southern parts of the mogok metamorphic belt this contiguous sibumasu baoshan block then sutured onto the indochina margin in the late triassic we propose that a tengchong block within myanmar provides for a southerly termination of the meso tethys suture immediately north of the mogok area a discrete tengchong block may explain a discontinuous arc of late triassic to jurassic i type magmatism in central myanmar representing an andean type margin sited above a subducting meso tethys on the margin of sibumasu the tengchong block sutured onto greater sibumasu before the late cretaceous after which subduction of neo tethys drove the magmatism of the wuntho popa arc and ultimately that of the tin province the metallogenic character of granite belts in myanmar reflects the crustal architecture of the region which is remarkable for its prolific endowment of granite hosted sn w mineralization in two quite distinct granite belts related to sequential indosinian and himalayan orogenesis

ultra high pressure metamorphism and orogenesis an alpine perspective article i nfo the pressure temperature time deformation evolution of the high to ultra high pressure units of the western alps has been progressively refined in the last 40 years leading to several paradigm shifts in the understanding of orogenesis this set of information combined with cretaceous tertiary plate kinematic reconstructions and mesozoic palaeogeography indicates that the western alps represent the amalgamation of a cretaceous and an eocene orogen which developed at the expense of the adriatic and european rifted margins respectively in the cretaceous nne directed drift of the adriatic plate parallel to the inherited jurassic structural trends led to the development of a highly oblique subduction boundary in this setting the canavese zone and parts of the sesia zone derived from the hyper extended adriatic margin underwent deformation and metamorphism at conditions ranging from sub greenschist to eclogite facies a lower eocene switch to nnw directed motion perpendicular to the rift related jurassic structural trends culminated in the collision between the proto alps and the brianconnais block at ca 44 ma as a result the brianconnais domain and parts of its hyper extended margin preserved in the u hp piemonte units were accreted to the orogen continued convergence was subsequently accommodated by subduction of the more external valaisan basin and of the thinned european margin the arrival of thick european continental crust at the subduction zone at ca 35 30 ma marked the onset of the final continental collision to a first order the orogen grew through the progressive episodic accretion of units located towards north westerly positions accretion of subducting units at the front of the orogen was coeval with kinematic reworking of tectonic contacts in the hangingwall units locally resulting in renewed deformation metamorphism

early cretaceous origin of the woyla arc sumatra indonesia on the australian plate abstract key to understanding the plate kinematic evolution of the neotethys oceanic domain that existed between the gondwana derived indian and australian continents in the south and eurasia in the north is the reconstruction of oceanic plates that are now entirely lost to subduction relics of these oceanic plates exist in the form of ophiolites and island arcs accreted to the orogen that stretches from tibet and the himalayas to se asia that formed the southern margin of sundaland the intra oceanic woyla arc thrusted over western sundaland the eurasian core of se asia in the mid cretaceous the woyla arc was previously interpreted to have formed above a west dipping subduction zone in the early cretaceous synchronous with east dipping subduction below sundaland the oceanic ngalau plate between the woyla arc and sundaland was lost to subduction we present paleomagnetic results from lower cretaceous limestones and volcaniclastic rocks of the woyla arc middle jurassic radiolarian cherts of the intervening ngalau plate and upper jurassic lower cretaceous detrital sediments of the sundaland margin our results suggest that the woyla arc was formed around equatorial latitudes and only underwent an eastward longitudinal motion relative to sundaland this is consistent with a scenario where the woyla arc was formed on the edge of the australian plate we propose a reconstruction where the ngalau plate formed a triangular oceanic basin between the n s trending woyla arc and the nw se trending sundaland margin to account for the absence of accreted arc rocks in the himalayas as consequence of this triangular geometry accretion of the woyla arc to the western sundaland margin was diachronous accommodated by a southward migrating triple junction continuing convergence of the australia relative to eurasia was accommodated by subduction polarity reversal behind the woyla arc possibly recorded by cretaceous ophiolites in the indo burman ranges and the andaman nicobar islands

forearc hyperextension dismembered the south tibetan ophiolites suprasubduction zone ophiolites are relics of oceanic upper plate forearcs and are typically preserved as discontinuous belts with discrete massifs along suture zones ophiolites usually contain an incomplete condensed section compared to average modern oceanic lithosphere the incompleteness and discontinuity of ophiolites are frequently attributed to dismemberment but tectonic causes remain poorly constrained here we show new paleomagnetic and field geological evidence for the preservation of extensional detachment faults that thinned and dismembered the south tibetan ophiolite belt during the early cretaceous similar to those documented in modern slow and ultraslow spreading ridges these detachments exhumed lithospheric mantle and subophiolitic melange to the seafloor which became unconformably covered by asia derived forearc strata we call this mechanism forearc hyperextension whereby widespread detachment faults accommodate upper plate extension above a subduction zone we propose that hyperextension is the key mechanism responsible for dismemberment of the south tibetan ophiolitic belt shortly after its magmatic accretion

the birth of the xigaze forearc basin in southern tibet abstract the stratigraphic succession of a forearc basin provides crucial information on the history of a convergent plate margin in particular it helps to establish the origin of the underlying ophiolites and to unravel the earliest evolutionary stage of arc trench systems which remain poorly understood the xigaze forearc basin in southern tibet is one of the best examples of a fossil forearc basin this study illustrates detailed stratigraphic and high precision sims u pb zircon geochronological and hf isotopic data from the chongdui formation representing the very base of the xigaze forearc basin succession and reconstructs when and how the basin was formed the chongdui formation includes tuffaceous chert and siliceous mudrocks deposited directly on top of pillow basalts of the xigaze ophiolite and conformably overlain by volcaniclastic turbidites tuff layers are interbedded throughout the unit and their u pb zircon ages range from 119 to 113 ma in the lower member and from 113 to 110 ma in the upper member broadly consistent with the established radiolarian biostratigraphy u pb ages and hf isotope signatures of zircons contained in both tuff layers and turbiditic sandstones indicate clear affinity with magmatic rocks of the lhasa terrane direct depositional and chronostratigraphic relationship with the underlying oceanic crust dated between 131 and 124 ma proves that the xigaze ophiolite is the basement of the xigaze forearc basin after an initial prolonged stage of starved siliceous sedimentation influx of terrigenous detritus began at 113 110 ma reflecting the onset of topographic growth and erosion of the lhasa terrane in response to intense magmatic activity formation of the ophiolitic basement during the early stage of subduction and the subsequent topographic growth of the arc source induced by subduction related magmatism are thus two critical factors for the birth of the xigaze forearc basin similar stratigraphies were identified in the great valley and luzon central valley forearc basins suggesting that the initial geodynamic evolution of the xigaze forearc basin may be common to many other forearc basins worldwide

the origin and p t evolution of peridotites and serpentinites of ne new caledonia prograde interaction between continental margin and the mantle wedge the pouebo and diahot terranes of ne new caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c 40 ma metamorphism this eocene high p event has been linked with the sw directed obduction of the new caledonian ophiolite an extensive ultramafic nappe that dominates outcrop in the south of the island in the north ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with but separated from the eclogites and blueschists by foliated talc chlorite serpentine carbonate bearing rocks the base of the largest and best preserved ultramafic body at yambe is marked by a distinctive 2 m thick layer of high p mylonite that preserves evidence for early blueschist facies conditions s1 as inclusions in eclogite facies minerals textural evidence preserved in olivine bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high p terrane as serpentinite tectonites sourced from hydrated mantle formerly in the hangingwall of the eocene subduction zone serpentinite emplacement accompanied burial of the ne new caledonian margin at t 500 c and p 16 kbar the ultramafic fragments were buried to depths of 50 60 km in the subduction zone where olivine was stable and coarse grained garnet omphacite rich assemblages developed in low strain domains within enclosing mylonites host metabasic and metasedimentary rocks from the structurally highest portions of the high p belt have a prograde record identical to that of the ultramafic tectonites the early emplacement and similar p t history of host rocks and ultramafic masses suggest that ne new caledonia preserves a fossil slab mantle wedge boundary reactivated during exhumation

is spreading prolonged episodic or incipient in the andaman sea evidence from deepwater sedimentation abstract the central andaman basin cab is generally accepted to be a site of continuous sea floor spreading since the early pliocene 4 0 ma the adjacent alcock and sewell rises and part of the east andaman basin have been interpreted as probable miocene oceanic crust published seismic lines across the eastern half of the spreading centre show that 100 s m thickness of sediment are present right up to the central trough the central trough margins are faulted uplifted and tilted away from the central trough the youngest sediment is ponded and onlaps the tilted central trough margin while older faulted sediment lies within the trough such a configuration is incompatible with continuous spreading instead either spreading in the central basin was episodic probably comprising a late miocene early pliocene phase of spreading followed by extension accommodated in the alcock and sewell rise area by faulting and dike intrusion and then a recent quaternary return to spreading in the central trough or the central trough marks an incipient spreading centre in hyper thinned continental or possibly island arc crust to resolve these possibilities regional satellite gravity data was inverted to determine crustal type and thickness the results indicate the cab is oceanic crust however the adjacent regions of the alcock and sewell rises and the east andaman basin are extended continental crust these regions were able to undergo extension before seafloor spreading and when seafloor spreading ceased unpublished seismic reflection data across the east andaman basin supports the presence of continental crust under the basin that thins drastically westwards towards the spreading centre episodic seafloor spreading fits with gps data onshore that indicate the differential motion of india with respect to se asia is accommodated on widely distributed structures that lie between the trench and the sagaing fault

locked and loading megathrust linked to active subduction beneath the indo burman ranges it is unclear whether subduction is still active beneath the indo burman mountain range analyses of gps measurements from this region reveal a locked megathrust fault implying that subduction is active and could generate a large earthquake

mesozoic cenozoic geological evolution of the himalayan tibetan orogen and working tectonic hypotheses the himalayan tibetan orogen culminated during the cenozoic india asia collision but its geological framework and initial growth were fundamentally the result of multiple previous ocean closure and intercontinental suturing events as such the himalayan tibetan orogen provides an ideal laboratory to investigate geological signatures of the suturing process in general and how the earth9s highest and largest orogenic feature formed in specific this paper synthesizes the triassic through cenozoic geology of the central himalayan tibetan orogen and presents our tectonic interpretations in a time series of schematic lithosphere scale cross sections and paleogeographic maps we suggest that north dipping subducting slabs beneath asian continental terranes associated with closure of the paleo meso and neo tethys oceans experienced phases of southward trench retreat prior to intercontinental suturing these trench retreat events created ophiolites in forearc extensional settings and or a backarc oceanic basins between rifted segments of upper plate continental margin arcs this process may have occurred at least three times along the southern asian margin during northward subduction of neo tethys oceanic lithosphere from 174 to 156 ma 132 to 120 ma and 90 to 70 ma at most other times the tibetan terranes underwent cordilleran style or collisional contractional deformation geological records indicate that most of northern and central tibet the hoh xil and qiangtang terranes respectively were uplifted above sea level by jurassic time and southern tibet the lhasa terrane north of its forearc region has been above sea level since 100 ma stratigraphic evidence indicates that the northern himalayan margin of india collided with an asian affinity subduction complex forearc arc system beginning at 60 ma both the himalaya composed of indian crust and tibet show continuous geological records of orogenesis since 60 ma as no evidence exists in the rock record for a younger suture the simplest interpretation of the geology is that india asia collision initiated at 60 ma plate circuit paleomagnetic and structural reconstructions however suggest that the southern margin of asia was too far north of india to have collided with it at that time seismic tomographic images are also suggestive of a second more southerly neo tethyan oceanic slab in the lower mantle where the northernmost margin of india may have been located at 60 ma the geology of tibet and the india asia suture zone permits an alternative collision scenario in which the continental margin arc along southern asia the gangdese arc was split by extension beginning at 90 ma and along with its forearc to the south the xigaze forearc rifted southward and opened a backarc ocean basin the rifted arc collided with india at 60 ma whereas the hypothetical backarc ocean basin may not have been consumed until 45 ma a compilation of igneous age data from tibet shows that the most recent phase of gangdese arc magmatism in the southern lhasa terrane initiated at 70 ma peaked at 51 ma and terminated at 38 ma cenozoic potassic adakitic magmatism initiated at 45 ma within a 200 km wide elliptical area within the northern qiangtang terrane after which it swept westward and southward with time across central tibet until 26 ma at 26 to 23 ma potassic adakitic magmatism swept southward across the lhasa terrane a narrow 20 km width orogen parallel basin developed at low elevation along the axis of the india asia suture zone the kailas basin and greater himalayan sequence rocks began extruding southward between the south tibetan detachment and main central thrust the kailas basin was then uplifted to 4 km elevation by 20 ma after which parts of the india asia suture zone and gangdese arc experienced 6 km of exhumation between 20 and 16 ma between 16 and 12 ma slip along the south tibetan detachment terminated and east west extension initiated in the northern himalaya and tibet potassic adakitic magmatism in the lhasa terrane shows a northward younging trend in the age of its termination beginning at 20 to 18 ma until volcanism ended at 8 ma we interpret the post 45 ma geological evolution in the context of the subduction dynamics of indian continental lithosphere and its interplay with delamination of asian mantle lithosphere

eocene arc continent collision in new caledonia and implications for regional southwest pacific tectonic evolution new caledonia preserves evidence that constrains models for the tectonic evolution of the southwest pacific region onland geology reflects four main tectonic phases 1 early mesozoic development of subduction related terranes and their accretion to the gondwana margin 2 cretaceous passive margin development and sea floor spreading during the gondwana breakup 3 foundering of an oceanic basin and the eocene arrival of thinned gondwana margin crust at a southwest facing subduction zone resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast and 4 detachment faulting during extensional collapse resulting in unroofing of metamorphic core complexes the last phase explains supposedly anomalous metamorphic gradients in the northeast of the island

a palaeo tibet myanmar connection reconstructing the late eocene drainage system of central myanmar using a multi proxy approach strain resulting from the collision of india with asia has caused fundamental changes to asian drainage patterns but the timing and nature of these changes are poorly understood one frequently proposed hypothesis involves the connection of the palaeo tsangpo drainage to a precursor to the irrawaddy river of central myanmar in the palaeogene to test this hypothesis we studied the provenance of palaeogene fluvio clastic sedimentary rocks that crop out in central myanmar namely the late middle eocene early oligocene pondaung and yaw formations isotopic analysis on bulk rock and petrographic data indicate a primary magmatic arc source and a secondary source composed of recycled metamorphosed basement material although the exact location of both sources is hardly distinguishable because burmese and tibetan provinces share common lithological features the presence of low grade metamorphic fragments the heterogeneity in sr nd isotopic values of bulk sediments and westward directed palaeoflow orientations indicate a proximal source area located on the eastern asian margin central myanmar was the locus of westward prograding deltas opening into the indian ocean supplied by the unroofing of an andean type cordillera that extended along the burmese margin we found no evidence to support a palaeo tsangpo irrawaddy river at least during the late eocene supplementary material data locations and isotopic and petrographic results are available at www geolsoc org uk sup18655

recognizing remnants of magma poor rifted margins in high pressure orogenic belts the alpine case study magma poor rifted margins are being increasingly recognized in present day atlantic type systems however findings of fossil areas floored by exhumed mantle or hyper extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre existing rifted margins this discrepancy may be due to the common reactivation of lithological contacts during subduction orogeny potentially masking pre orogenic relationships and most importantly to the frequent lack of a preorogenic layer cake architecture hindering retro deformation of multiply deformed tectonic units this study outlines a methodology to detect sections of magma poor hyper extended rifted margins without a layer cake architecture in multiply deformed metamorphosed terrains this approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of alpine orogens in the latter domains continental basement and hydrated peridotites were exhumed at the basin floor during jurassic rifting along long offset detachment systems extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre rift sediments fromthe hanging wall blocks lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn to post rift meta sediments and or serpentinized subcontinental mantle are widespread within sections of alpine type orogenic belts that underwent high to ultra high pressure metamorphism however similar associations may arise from a variety of processes other than rift related lithospheric thinning in magma poor environments including subduction melange dynamics or deposition of sedimentary melanges along convergent divergent margins the partial preservation of rift related lithostratigraphic associations may still be assessed despite the lack of biostratigraphic evidence by 1 the consistency of the lithostratigraphic architecture over large areas despite pervasive alpine deformation which rules out chaotic mixing during subduction exhumation 2 the presence of clasts of basement rocks in the neighboringmeta sediments indicating the original proximity of the different lithologies 3 evidence of brittle deformation in continental basement and ultramafic rocks pre dating alpine metamorphism indicating that theywere juxtaposed by fault activity prior to the deposition of post rift sediments and 4 the similar alpine tectono metamorphic evolution of ophiolites continental basement and meta sediments a re assessment of basement cover relationships in the north western alps following this approach combined with published studies on exhumedmantle domains sampled in the rest of thewestern alps indicates that several tectono metamorphic units fromthemost deformed metamorphosed part of the belt between the canavese line and the penninic front sample hyper extended lithosphere related to the jurassic opening of the western tethys relative plate motion during cretaceous tertiary basin inversion was largely accommodated at the transition between areas floored by hyper extended crust or hydrated subcontinental mantle and domains consisting of thicker continental crust as a result distal margins were preferentially subducted whereas the proximal domains and the brianconnais paleo high underwent relatively minor deformation and metamorphism the high pressure alpine tectono metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present day distal margins the recognition of preserved pre alpine relationships between continental basement post rift sediments and or serpentinized ultramafic rocks calls for a re assessment of the relative role of subduction and rifting dynamics in establishing the present day orogen architecture

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

structural and tectonic development of the indo burma ranges abstract the indo burma ranges form an enigmatic mountain belt with fragments of evidence for an early accretionary history jurassic jade belt hp lt metamorphism early cretaceous ophiolites highly deformed triassic turbidites pane chaung formation pcf kanpetlet schists it remains uncertain whether this early history involved collision of a microcontinent mt victoria land mvl unconformably sealed by aptian cenomanian limestones or can be explained entirely as an accretionary type ophiolite on the western margin of the west burma terrane wbt complex deformation in the deepwater triassic jurassic late cretaceous and paleogene deepwater sequences is replaced in the late eocene early oligocene by molasse deposition these events mark closure of the neo tethys ocean between india and the ibr wbt and the onset of major dextral translation 2000 km 40 ma recent between the coupled india ibr wbt region and sundaland in the late miocene recent major transpressional deformation affected the ibr and central basin of the wbt the late deformation events sedimentary depocentres and impinging thick crustal regions of the eastern himalayas and shillong plateau have all affected the overall shape wedge taper of the modern ibr with the wedge and retro wedge behaving anomalously compared with typical accretionary prisms all tectonic models proposed for the ibr wbt have weaknesses or ambiguities and there is considerable scope for future research to resolve the many outstanding tectonic metamorphic structural and sedimentary issues these are important tasks because the ibr is a key region for understanding the development of northern gondwana the himalayan orogeny and se asia as well as providing insights into the complex development of highly oblique collisional margins

zircon u pb ages in myanmar magmatic metamorphic events and the closure of a neo tethys ocean abstract previous isotopic age studies have concluded that the main metamorphism in the mogok metamorphic belt mmb in myanmar was tertiary and caused by collision with or underthrusting by india we present here results of zircon u pb age determinations on 18 samples from myanmar largely on intrusive rocks from the north trending 550 km long shan scarps segment of the sickle shaped mmb the oldest determination is 491 ma on orthogneiss north of mandalay interpreted as a protolith age and similar to the stratigraphic age of the bawdwin volcanics augen gneiss at kyaukse has a 114 ma protolith age our zircon u pb ages on undeformed intrusive rocks in the mmb range from cretaceous to miocene the oldest 128 ma on a diorite near yebokson implies an earliest cretaceous minimum age for regional metamorphism here younger ages on undeformed intrusions include 91 ma on diorite at mokpalin 72 ma on the weakly foliated garnet bearing nattaung granite 44 and 48 ma respectively on the mec and sedo granites and 17 20 ma on granite dykes for the 128 ma diorite sr and nd isotopes indicative of a continental crust component and mapped continuity of metamorphic host rocks suggest correlation with mid early cretaceous plutons in the eastern transhimalayas zircon u pb ages for the mec sedo and other granites within the mmb and slate belt help to define a late cretaceous to early eocene age range for mostly ilmenite series peraluminous granites within the western granite belt and more restricted western tin belt of myanmar and southern thailand in the wuntho popa arc west of the dextral sagaing fault our new u pb zircon age of 105 ma on diorite at salingyi supports previous k ar ages 106 93 ma for pre tertiary intrusions in the cretaceous this arc may have occupied the andaman sea and continued northwards through the mokpalin diorites and west of the mmb to the 103 80 ma i type plutons in the gandise batholith of southern tibet we interpret the mmb and slate belt as part of a western myanmar block separated in the jurassic from the shan plateau to the east by the southwestward continuation of the bangong nujiang luxi neo tethys i ocean we relate most intrusive and metamorphic events in and near the mmb to westward subduction of this ocean and end jurassic collision of the plateau with the overriding western myanmar slate belt block to orogenic polarity reversal and early cretaceous westward translation of the plateau sequence over the suture zone and to eastward subduction of the ancestral indian ocean or neo tethys ii beneath myanmar with generation of the wuntho popa arc beginning before the late cretaceous the mmb includes meta plateau rocks near mandalay but meta slate belt rocks to the north and south within the mmb previous monazite xenotime thorite ages of eocene oligocene and oligocene early miocene cooling ages on micas with no deformation in eocene granites suggest a high temperature thermal event related to intrusion of these and other granites during a previously proposed regional extension our data imply that the main fabric forming metamorphic event in the mmb pre dated the india asia collision

la icp ms apatite fission track dating a practical zeta based approach abstract the la icp ms method is becoming increasingly popular for uranium determinations in fission track dating of apatite zircon or titanite this is because the approach has several advantages over the classical external detector method edm including faster sample throughput simultaneous acquisition of additional data such as u pb age information and trace element abundances while removing the need for neutron irradiation two different approaches are used to determine u contents in la icp ms fission track dating an absolute dating approach or a zeta based determination analogous to the classical edm absolute age dating by la icp ms potentially suffers from small but systematic deviations in apatite u contents which in turn propagate through to minor systematic deviations in the accuracy of absolute fission track age determinations a zeta based approach typically requires time consuming counting of large numbers of zeta standard grains usually durango apatite so as to yield a precise zeta factor for every la icp ms session containing unknowns the modification of the zeta based approach proposed here has two major advantages firstly it employs just one large primary la icp ms session to determine a precise primary zeta factor on a large number of counted durango primary zeta grains during subsequent secondary la icp ms sessions with unknowns no further fission track counting of the primary zeta standard is required this is because we reanalyse a subset of the primary zeta grains to calculate a session specific zeta fractionation factor which is related to variations in the instrumental operating conditions primarily plasma tuning between primary and secondary la icp ms sessions this enables us to reuse the primary zeta factor and thus avail of its precision derived from the large spontaneous track count the second advantage is that reusing the primary zeta grains by applying a session specific zeta fractionation factor allows us to verify that background and drift corrections applied during the secondary la icp ms session were fully appropriate this method has been successfully tested by dating samples of known apatite fission track age by comparing edm and la icp ms data from the same sample and by participating in a round robin test between international fission track laboratories where blind fission track dating of two unknown samples was undertaken our la icp ms apatite fission track dating approach is also easily modifiable for fission track dating of zircon or titanite if suitable age standards are employed

geochronology geochemistry and sr nd hf isotopic compositions of late cretaceous eocene granites in southern myanmar petrogenetic tectonic and metallogenic implications abstract the geochronology petrogenesis and tectonic setting of granites associated with tin tungsten mineralization in southern myanmar remain unclear this work presents whole rock geochemical sr nd isotopic and zircon u pb and hf isotopic data for felsic intrusions in the sibusima terrane zircon u pb dating indicates formation in the late cretaceous early eocene 84 48 ma and suggests that these intrusions likely represent a southward extension of the coeval magmatic belt in the tengchong terrane primarily classified as high k calc alkaline the granites show strong enrichments in large ion lithophile elements e g cs rb and k depletions in nb ta p and ti and negative ba and sr anomalies on primitive mantle normalized diagrams p2o5 cao al2o3 mgo tio2 contents and nb ta ratios decrease with increasing sio2 contents possibly consistent with evolution trends in fractionated i type granitic magmas in addition these intrusions display a wide range of negative end t 14 6 5 5 and zircon ehf t values 22 7 5 7 suggesting a dominant sibusimacrustal source with a minor mantle contribution notably the eocene and a few paleocene granites show more contribution of mantle material based on higher end t 8 5 5 5 and zircon ehf t 11 0 3 2 values large variations in zircon hf isotopic compositions within an individual eocene granitic intrusion e g 15 2 5 7 and corresponding biotite rich enclave 17 8 0 2 likely indicate magma mixing considering the spatial temporal distribution of magmatism in the west burma and sibusima terranes the studied late cretaceous eocene felsic magmas likely formed in a continental arc setting during normal subduction 100 60 ma and subsequent roll back of the neo tethyan oceanic lithosphere 60 50 ma additionally most of granites have the elevated sn with increasing rb sr ratios and decreasing tio2 contents and nb ta ratios suggesting sn enrichment is primarily controlled by magmatic fractionation meanwhile sn and li depletions in some granites deviate from the magmatic evolution trend likely as a result of fluid exsolution the exsolved sn rich fluids could have made a genetic contribution to hydrothermal sn mineralization in southern myanmar

late miocene coesite eclogite exhumed in the woodlark rift late miocene pliocene eclogites were exhumed in the woodlark rift of eastern papua new guinea an actively extending region west of the woodlark basin seafl oor spreading center we report the discovery of coesite in late miocene eclogite from the lower plate of one of the d entrecasteaux islands metamorphic core complexes within the woodlark rift zircon crys tallization temperatures 650 675 c and 238 u 206 pb age ca 8 ma and rutile thermometry 695 743 c combined with garnet pyroxene thermometry 600 760 c and garnet pyroxene phengite barometry 18 27 kbar indicate that the coesite eclogite was exhumed from mantle depths 90 km to the earth s surface at plate tectonic rates cm yr 1 this late miocene coesite eclogite is the youngest exhumed ultrahigh pressure uhp rock on earth and its preservation ahead of the westward propagating seafl oor spreading center forces reevaluation of models for uhp exhumation as well as the geologic and tectonic evolution of the woodlark rift

polyphase deformation in a fore arc back arc basin salin subbasin myanmar burma the salin subbasin of myanmar part of the larger central basin is a fore arc back arc basin couplet situated between an oblique subduction zone to the west and a major right lateral strike slip fault to the east surface and subsurface expressions of folds and faults suggest that the basin experienced north northwest directed extensional deformation in the miocene followed by east northeast directed pliocene pleistocene transpressional deformation resulting in a variety of structural styles including thrust faults oblique reverse faults strike slip faults and normal faults fault propagation folds cored by west dipping thrust faults in the basin center are located above steps in the top of basement surface most likely fault controlled unconformities at the base of and within the pliocene pleistocene synorogenic section indicate that the thrust faults were active during the pliocene pleistocene the southeastern region contains broad north northwest trending uplifts and east northeast striking normal faults associated with thickened miocene synorogenic deposits pliocene pleistocene sedimentary rocks lie unconformably above the miocene section and are folded above the uplifts reflecting pliocene pleistocene compression in the southern part of the basin miocene sedimentary rocks thicken dramatically over the 20 n uplift and the yedwet uplift suggesting that the north northwest striking faults that bound them represent pliocene pleistocene inversion of miocene normal faults during the miocene the burma plate acted as a fore arc sliver coupled with the india plate subducting obliquely underneath it and moved northward relative to asia along the sagaing fault normal faulting and local basin formation took place at this time when the northern part of the burma plate collided with eastern asia in the pliocene transpressional deformation predominated creating thrust and reverse faults positive flower structures and inverted basins

india and sunda plates motion and deformation along their boundary in myanmar determined by gps using a regional gps data set including 190 stations in asia from nepal to eastern indonesia and spanning 11 years we update the present day relative motion between the indian and sundaland plates and discuss the deformation taking place between them in myanmar revisiting measurements acquired on the main boundary thrust in nepal it appears that points in southern nepal exhibit negligible deformation with respect to mainland india including these points using a longer time span than previous studies and making an accurate geodetic mapping in the newest reference frame allows us to refine the present day indian motion our results confirm that the current motion of india is slower than predicted by the nuvel 1a model and in addition our india eurasia motion is significantly 5 mm yr slower than previous geodetic determinations this new indian motion combined with a refined determination of the sundaland motion gives way to a relative india sunda angular velocity of 20 2 n 26 1 e 0 370 myr in itrf2000 predicting a relative motion of 35 mm yr oriented n10 at the latitude of myanmar there the sagaing fault accommodates only 18 mm yr of right lateral strike slip only half of the shear component of motion we present two models addressing how and where the remaining deformation may occur a first model of distributed deformation implies convergence on the arakan subduction the northern continuation of the now famous sumatra andaman trench and wrench faulting in the arakan wedge the second model uses localized deformation where deformation observed west of the sagaing fault is entirely due to elastic loading on a faster and oblique arakan subduction 23 mm yr this latter model predicts that a major earthquake of mw 8 5 may occur every century on this segment of the subduction

exhumation of the schistes lustres complex in situ laser probe 40ar 39ar constraints and implications for the western alps the schistes lustres sl suture zone occupies a key position in the alpine chain between the high pressure hp brianconnais domain and the ultrahigh pressure uhp dora maira massif and reached subduction depths ranging from c 40 65 km cottian alps in order to constrain the timing of hp metamorphism and subsequent exhumation several phengite generations were differentiated on the basis of habit texture paragenesis and chemistry as belonging to the first or second exhumation episode respectively d2 or d3 or to earlier stages of the tectono metamorphic evolution ten carefully selected samples showing d2 d3 d2 d3 or earlier mostly peak temperature phengite population s were subjected to laser probe 40ar 39ar analysis the data support the results of the petrostructural study with two distinct age groups crystallization ages for d2 and d3 phengite at 51 45 and 38 35 ma respectively the data also reveal a coherent age cluster at 62 55 ma for peak temperature phengite associated with chloritoid which were preserved in low strain domains the age of the d3 event in the sl complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs exhumation rates of the order of 1 2 mm yr 1 are obtained for the sl complex which are compatible with velocities documented for accretionary wedge settings similarly cooling velocities are only moderate c 5 c myr 1 which is at variance with recent estimates in the nearby uhp massifs

evolution from an oblique subduction back arc mobile belt to a highly oblique collisional margin the cenozoic tectonic development of thailand and eastern myanmar

high pressure veins in eclogite from new caledonia and their significance for fluid migration in subduction zones abstract this paper describes the petrology and geochemistry of high pressure veins and associated metasomatic selvages that are hosted by eclogite in the pouebo eclogite melange of northern new caledonia isotopic and geochemical evidence indicates that the mafic host rock represents seafloor altered fractionated morb that underwent eclogite facies metamorphism in a subduction zone within the host rock are cm thick garnet quartz phengite veins that are enveloped by garnet poor omphacite rich selvages or bleach zones petrography thermometry oxygen isotope characteristics and mass balance calculations are used to show that the veins largely formed by fluid mediated mass transfer from the bleach zones during prograde metamorphism minerals in the veins are free of inclusions but the vein garnets preserve complex chemical zoning features that are not present in the host rock garnets vein garnets have mn and hree zoning patterns that are indicative of progressive garnet growth during prograde metamorphism whereas mg and ca contents reveal prominent sector zoning and fine scale intergrowth features we propose that the veins formed over a prolonged period during subduction by local circulation of fluid that was sourced from prograde dehydration of minerals in the host rock fluid circulation may have been driven by episodic microcracking sealing around garnet porphyroblasts which led to significant mass transfer and progressive vein growth mass balance calculations and phengite trace element compositions also require the additional of pelite derived components to the veins these components were probably introduced into the veins at conditions close to peak metamorphism via a relatively small external fluid flux this model for vein formation is consistent with previous studies that suggest fluid flow in deeply subducted oceanic crust is highly restricted in many cases the delay of fluid migration after hydrous mineral breakdown may provide an important source of water for arc magmas as free fluid is transported to deeper levels in subduction zones

geochemical evolution and tectonic significance of boninites and tholeiites from the koh ophiolite new caledonia the central chain ophiolites in new caledonia are fragments of a supra subduction zone ssz ophiolite now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick middle triassic to upper jurassic volcaniclastic sequence most of the fragments were formed by a single tholeiitic magmatic episode but one of these the koh ophiolite was formed by two tholeiitic magmatic episodes separated by boninites the first event in the koh ophiolite formed cumulate gabbros dolerites plagiogranites and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements lree and abnormally low tio2 0 5 at mg 60 shortly after their eruption these tholeiitic lavas were overlain by a high ca boninitic unit with a basal section of boninite pillows flows and breccias and an upper section of boninitic dacites and tuffs the last magmatic phase involved eruption of evolved tholeiitic basalts as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite this second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts babb erupted during the early rifting history of modern back arc basins the boninitic volcanics belong to a high ca series with slightly lower sio2 al2o3 and tio2 compared to those from modern island arc systems and they lack the positive zr spike relative to adjacent rare earth elements ree in normalised element variation patterns these boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40 60 extension of the initial basin crust and eruption of the upper tholeiites the dominance of babb like tholeiites throughout the central chain ophiolites in new caledonia the restricted occurrence of boninites and the stratigraphy and chemistry of the koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation we suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre the occurrence of a thick blanket of calc alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the koh boninites were not associated with the initiation of subduction a close modern analogy for the koh ophiolite exists on the hunter ridge protoisland arc between southernmost vanuatu new hebrides island arc and the fijian islands there high ca boninites lacking positive zr spikes occur together with low ti tholeiites and more typical babb tholeiites where the southern spreading centre of the north fiji basin is propagating into the protoarc crust of the hunter ridge

apatite fission track analysis apatite fission track aft analysis has been widely used during the past 30 years to constrain the low temperature thermal histories of many igneous metamorphic and sedimentary rocks in a wide range of geological settings applicable geological settings include orogenic belts rifted margins faults sedimentary basins cratons and mineral deposits the types of geologic problems that can be addressed include the timing and rates of tectonic events sedimentary basin evolution the timing of hydrocarbon generation and ore mineralization the absolute age of volcanic deposits the effects of major climatic changes on the near surface geothermal gradient and long term landscape evolution early work by naeser 1967 and wagner 1968 1969 first established the basic procedures that enabled the fission track dating method to be applied routinely to these geologic problems fleischer et al 1975 summarized the early studies of the broader discipline of nuclear track detection in different solid state materials more recent comprehensive overviews of fission track applications have been provided by naeser and mcculloh 1989 wagner and van den haute 1992 gallagher et al 1998 van den haute and de corte 1998 dumitru 2000 and gleadow et al 2002 etched natural fission tracks in several apatite grains are shown in figure 1 successful aft analysis is limited by the following 1 the availability of apatite from which useful aft data can be obtained often due to a lack of apatite of sufficient grain size and quality within available rock types or alternatively the lack of available rock samples due to minimal outcrop exposure or other reasons 2 economic considerations in terms of the time and money required to obtain sufficient aft data 3 the inherent limitations of aft data to resolve geological thermal history information often related to limited numbers of accumulated spontaneous fission tracks due to

tectonic setting origin and obduction history of the spontang ophiolite ladakh himalaya nw india abstract the spontang ophiolite forms the highest tectonic thrust slice above the mesozoic early tertiary continental margin of the north indian plate in the ladakh zanskar himalaya detailed field mapping combined with geochemical analysis has defined two major units a full ophiolite sequence spontang ophiolite overlain by an upper unit consisting of 500 m thick basalt andesite volcanic and volcano sedimentary rocks of island arc affinity spong arc the spontang ophiolite comprises a harzburgitic mantle sequence gabbroic and ultramafic cumulates isotropic gabbros and highly tectonized sheeted dikes feeding pillow lavas with a few uncommon highly fractionated plagiogranites a separate lherzolitic peridotite unit affected by possible transform related shearing was thrust over the harzburgites in the west possibly during the early stages of subduction initiation beneath the ophiolite whole rock geochemistry and u pb geochronology show that the ophiolite formed at a normal mid ocean ridge sp

ion microprobe zircon u pb age and geochemistry of the myanmar jadeitite combined geochemistry and geochronology of the myanmar jadeitite were determined bulk rock trace element compositions display u shaped ree patterns with pronounced positive eu anomalies the total ree abundances are very low less than half chondritic and the high field strength elements and some large ion lithophile elements are moderately enriched these features indicate a metasomatic origin there are three groups of zircons with different interior characteristics cathodoluminescence mineral inclusions chemical compositions and sensitive high resolution ion microprobe u pb ages group i zircons with a mean age of 163 2 3 3 ma mostly have distinct oscillatory zoning highest u and th contents and na free mg rich mineral inclusions and thus indicate an igneous formation of oceanic crust or hydrothermal serpentinization and or rodingitization event in the middle jurassic group ii zircons with a mean age of 146 5 3 4 ma have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions suggesting that formation of the myanmar jadeitites as well as subduction of the eastern indian oceanic plate occurred in the late jurassic group iii zircons have an age of 122 2 4 8 ma which represents a later unknown thermal event discovery of the middle jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern indo burman range

neo tethyan magmatism and metallogeny in myanmar an andean analogue abstract plate margin tectonism magmatic and hydrothermal processes and the genesis of mineral deposits are intricately linked in myanmar two near parallel magmatic belts that together contain a significant proportion of that country s mineral wealth have contrasting metallogenic endowments the mogok mandalay mergui belt hosts crustal melt s type granites with significant tin tungsten mineralization while the wuntho popa arc comprises i type granites and granodiorites with copper gold mineralization the spatial juxtaposition of the two belts and their distinct but consistent metallogenic endowment bears strong similarities to the metallogenic belts of the south american cordillera recent u pb age dating has shown the potential for the two belts to be near contemporary from the late cretaceous to eocene here we investigate whether an andean type setting during subduction of neo tethys could explain the observed magmatism and mineralization within these two belts

fossil oceanic core complexes recognized in the blueschist metaophiolites of western alps and corsica tethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks by contrast tothe complete mantle crustal sections of semail type ophiolite sheets tethyan ophiolites are characterized by asmaller amount of mafic rocks gabbros and basalts by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle derived peridotites and associatedgabbroic intrusions therefore they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean continent transition at distal passive margins in the very first models offormation of the tethyan ophiolites in the years 1980 the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present dayoceans in particular at that time little work had focused on the distribution origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics here we reconsider the geology of sometypical metaophiolites from the western alps and corsica and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the tethyan ophiolites in general revisitedexamples include a set of representative metaophiolites from the blueschists units of the western alps queyrasregion and from alpine corsica golo valley field relationships between the ophiolitic basement andthe metasedimentary metavolcanic oceanic cover are described outlining a typical character of the tethyanophiolite lithological associations jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle gabbo basement in a way strictly similar to what is observed in the non metamorphicappennine ophiolites or chenaillet massif this confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction this juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of oceanic core complexes occs at the axis of slow spreading ridges this fundamental plate tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries in addition fieldrelationships between the discontinuous basaltic formations and the ultramafic mafic basement indicate that thistectonic stage is followed or not by a volcanic stage we discuss this issue in the light of available field constraints

collisional emplacement history of the naga andaman ophiolites and the position of the eastern indian suture abstract dismembered late mesozoic ophiolites occur in two parallel belts along the eastern margin of the indian plate the eastern belt closely following the magmatic arc of the central burma basin coincides with a zone of high gravity it is considered to mark a zone of steeply dipping mafic ultramafic rocks and continental metamorphic rocks which are the locus of two closely juxtaposed sutures in contrast the western belt which follows the eastern margin of the indo burma range and the andaman outer island arc broadly follows a zone of negative gravity anomalies here the ophiolites occur mainly as rootless subhorizontal bodies overlying eocene oligocene flyschoid sediments two sets of ophiolites that were accreted during the early cretaceous and mid eocene are juxtaposed in this belt these are inferred to be westward propagated nappes from the eastern belt emplaced during the late oligocene collision between the burmese and indo burma andaman microcontinents ophiolite occurrences in the andaman islands belong to the western belt and are generally interpreted as upthrust oceanic crust accreted due to prolonged subduction activity to the west of the island arc this phase of subduction began only in the late miocene and thus could not have produced the ophiolitic rocks which were accreted in the late early eocene

a 6000 km long neo tethyan arc system with coherent magmatic flare ups and lulls in south asia magmatic arcs typically exhibit non steady state evolution with episodic flare ups and lulls yet the main drivers remain contentious situated in the southwest margin of southeast asia sumatra records a long lived magmatic arc that is still poorly constrained in age and tempo detrital zircon data from sumatra delineate major arc magmatic pulses at ca 212 102 85 52 and 22 11 ma the mid cretaceous to early eocene zircons mostly yield high positive ehf t values indicating magma derivation from juvenile sources and matching well with those of the gangdese batholiths in the southern lhasa terrane these similarities substantiate an extended 6000 km neo tethyan arc system from southern tibet to sumatra that exhibits concurrent magmatic lulls ca 150 105 and 85 65 ma and flare ups ca 105 85 and 65 40 ma the late cretaceous magmatic lull coincided with a period of strong regional deformation and increasingly fast northward drift of india likely attributable to neo tethyan flat slab subduction periodic pulses of neo tethyan arc magmatism most likely correlated with repeated steepening and shallowing of slab dip rather than india eurasia convergence rates

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

phanerozoic plate boundaries in mainland se asia the himalayas and tibet recently available geological data on tibet yunnan and burma facilitate recognition of phanerozoic subduction systems and two continental fragments of gondwanaland which extend through mainland se asia a cambrian subduction system and 5 mesozoic early caenozoic collision belts are identified in the region indochina eastern thailand and central tibet accreted to china in the early triassic western se asia and s tibet separated from gondwanaland in the permian or early triassic and collided with asia in the late triassic and the western burma island arc system collided with asia in the jurassic upper triassic flysch and schist in the eastern indoburman ranges were accreted to western burma in the jurassic early cretaceous during eastward subduction and formation of a back arc thrust belt following collision of greater india with arc complexes to the n in the cretaceous and with tibet in the early eocene western burma moved northwards about 450 km along the sagaing namyin ridge thrust transform fault formation of the 3 major belts of tin granites and the porphyry copper deposits in the region took place in specific tectonic settings within the subduction systems and collision belts

indo burma range a belt of accreted microcontinents ophiolites and mesozoic paleogene flyschoid sediments this study provides an insight into the lithotectonic evolution of the n s trending indo burma range ibr constituting the southern flank of the himalayan syntaxis paleogene flyschoid sediments disang barail that represent a shallow marine to deltaic environment mainly comprise the west central sector of ibr possibly resting upon a continental base on the east these sequences are tectonically flanked by the eocene olistostromal facies of the disang which developed through accretion of trench sediments during the subduction the shelf and trench facies sequences of the disang underwent overthrusting from the east giving rise to two ophiolite suites naga hills lower ophiolite nhlo andvictoria hills upper ophiolite vhuo but with different accretion history the ophiolite and ophiolite cover rock package were subsequently overthrusted by the proterozoic metamorphic sequence originated from the burmese continent the nhlo suite of late jurassic to early eocene age is unconformably overlain by mid eocene shallow marine ophiolite derived clastics on the south the vhuo of mesozoic age is structurally underlain by continental metamorphic rocks the entire package in victoria hills is unconformably overlain by shallow marine late albian sediments both the ophiolite suites and the sandwiched continental metamorphic rocks are thrust westward over the paleogene shelf sediments these dismembered ophiolites and continental metamorphic rocks suggest thin skinned tectonic detachment processes in ibr as reflected from the presence of klippe of continental metamorphic rocks over the nhlo and the flyschoid disang floor sediments and half windows exposing the disang beneath the nhlo

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

the tectonic and metallogenic framework of myanmar a tethyan mineral system myanmar is perhaps one of the world s most prospective but least explored minerals jurisdictions containing important known deposits of tin tungsten copper gold zinc lead nickel silver jade and gemstones a scarcity of recent geological mapping available in published form coupled with an unfavourable political climate has resulted in the fact that although characterized by several world class deposits the nation s mineral resource sector is underdeveloped as well as representing a potential new search space for a range of commodities many of myanmar s known existing mineral deposits remain highly prospective myanmar lies at a crucial geologic juncture immediately south of the eastern himalayan syntaxis however it remains geologically enigmatic its mesozoic recent geological history is dominated by several orogenic events representing the closing of the tethys ocean we present new zircon u pb age data related to several styles of mineralization within myanmar we outline a tectonic model for myanmar from the late cretaceous onwards and document nine major mineralization styles representing a range of commodities found within the country we propose a metallogenetic model that places the genesis of many of these metallotects within the framework of the subduction and suturing of neo tethys and the subsequent himalayan orogeny temporal overlap of favourable conditions for the formation of particular deposit types during orogenic progression permits the genesis of differing metallotects during the same orogenic event we suggest the evolution of these favourable conditions and resulting genesis of much of myanmar s mineral deposits represents a single evolving mineral system the subduction and suturing of neo tethys

modelling the effect of arbitrary p t t histories on argon diffusion in minerals using the macargon program for the apple macintosh argon diffusion in mineral grains has been numerically modelled using p t t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the earth s crust for long durations the macargon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion it can be run on an apple macintosh computer with arbitrary p t t histories used as input finite difference equations are used in the calculation of 40ar concentration profiles across individual diffusion domains the associated macspectrometer generates model spectra after a p t t history has been specified the form of model 40ar39ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40ar39ar apparent age spectra particularly in cases involving metamorphic rocks where complex p t t histories might apply although modelled spectra cannot be directly compared to experimentally determined 40ar39ar age spectra especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion they do provide insight into theoretically expected age spectra for samples that have experienced complex p t t histories

two parallel magmatic belts with contrasting isotopic characteristics from southern tibet to myanmar zircon u pb and hf isotopic constraints before the india asia collision neotethyan subduction gave rise to an andean type convergent margin on the southern margin of asia to investigate the spatial and temporal distribution of the subduction related magmatism we undertook a combined determination of zircon u pb ages and hf isotopes of mesozoic to paleogene intrusive and volcanic rocks from southern tibet to myanmar to characterize the two parallel magmatic belts that have previously been considered separately one belt extends from the gangdese batholith in the southern lhasa sub terrane to the lohit batholith the sodon pluton and the popa loimye arc in the west burma block and the other from the central lhasa plutonic belt to the bomi chayu batholith the dianxi batholith and the shan scarps batholith in central myanmar the gangdese belt as the main arc component consists typically of i type granitoids that contain magmatic zircons showing positive ehf t values in contrast the central lhasa plutonic belt belt is dominated by s type granites in which most zircons show negative ehf t values suggesting the involvement of older continental crust in their petrogenesis the distinct geochemical characteristics indicate not only distinct tectonic settings of their genesis but also the diverse nature of the crust forming the exotic continental ribbons amalgamated to asia supplementary material details of sample locations and analytical results are available at https doi org 10 6084 m9 figshare c 4311485

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

tethyan suturing in southeast asia zircon u pb and hf o isotopic constraints from myanmar ophiolites ophiolites that crop out in southeast asia represent the relics of the tethys ocean which existed between the continents of gondwana and laurasia during much of the mesozoic two ophiolite belts in myanmar i e the eastern belt and the western belt have been conventionally regarded as parts of a single suture connecting with the yarlung tsangpo suture in the tibetan plateau and displaced by the dextral sagaing fault here we present for the first time a combined analysis of zircon secondary ion mass spectrometry u pb ages and hf o isotopes of two myanmar ophiolites the kalaymyo ophiolite from the western belt and the myitkyina ophiolite from the eastern belt our results show that the kalaymyo ophiolite has an early cretaceous age ca 127 ma coeval with neo tethyan ophiolites along the yarlung tsangpo suture in contrast the myitkyina ophiolite was formed during the middle jurassic ca 173 ma and thus the eastern belt is the southern continuation of the meso tethyan bangong nujiang suture in the tibetan plateau consequently we argue that the two myanmar ophiolite belts belong to two different sutures of the meso tethys and neo tethys and that the boundary between the sibumasu and west burma blocks is a jurassic suture rather than a transcurrent shear zone

should sibumasu be renamed sibuma the case for a discrete gondwana derived block embracing western myanmar upper peninsular thailand and ne sumatra the luxi nujiang suture which extends from yunnan into myanmar is shown to continue into thailand and the malacca strait it forms the eastern boundary of a discrete block here called the irrawaddy block which was formerly considered to be part of gondwana derived sibumasu it has a distinct palaeozoic succession with thick lower permian glacially influenced diamictite bearing marine mass flow intervals deposited at the gondwana margin the rest of sibumasu east of the suture also has local thin lower permian diamictite units however they have been interpreted as ice rafted deposits laid down at a greater distance from the gondwana margin in ne sumatra a diamictite bearing carboniferous lower permian succession has also been described as of mass flow origin and it is suggested that it is also part of the irrawaddy block it requires that the bounding suture is aligned se beneath the malacca strait in the late cretaceous palaeogene the dextral india australia oceanic transform propagated onshore as a strike slip fault bounding on the east the north going india plate it disrupted the irrawaddy block and for much of its length coincided with the line of the earlier suture insofar as the acronym sibumasu was coined to include sumatra consideration should be given to renaming this block sibuma

structures uplift and magmatism of the western myanmar arc constraints to mid cretaceous paleogene tectonic evolution of the western myanmar continental margin abstract knowledge of trans himalayan tectono magmatic evolution is critical to understanding the complex pre collisional history of southern eurasia active continental margin it has been proposed that magmatic rocks of the trans himalayan batholith extending from southern tibet to southeast asia are now exposed as the western myanmar arc and central granite belt in myanmar yet origin emplacement and relationships of the two juxtaposed belts remain poorly constrained in this study 2d seismic and drilling data for the western myanmar arc zircon u pb age and hf isotope and whole rock geochemical data for magmatic rocks from the arc have been applied our seismic profiles borehole stratigraphic sequences and zircon u pb data show that a typical arc basin system was well developed along the western myanmar continental margin the magmatic arc has experienced at least three igneous events in the mid cretaceous 110 90 ma latest cretaceous early paleocene 69 64 5 ma and eocene 53 38 ma as well as three associated uplift processes in the late cretaceous eocene and late oligocene whole rock geochemical characteristics and zircons showing variable but predominately positive e hf t values suggest a significant juvenile mantle source involving a proportion of ancient subducted sediments and juvenile crustal materials for these typical arc related magmatic rocks the identification of mid cretaceous to paleogene magmatic rocks having positive e hf t values from the western myanmar arc 1 indicates that the magmatism can be correlated with the gangdese arc within the lhasa terrane of the southern tibetan plateau 2 provides evidence for the proximal derived model that paleogene sediments in the central myanmar basin were from the western myanmar arc but were not delivered by the paleo yarlung tsangpo irrawaddy river system from the gangdese arc and 3 enables a model of eastward subduction of the neo tethyan indian oceanic crust to reflect onset of the magmatism at the mid cretaceous and a long existed back arc extension in western myanmar

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

rock relationships in the mogok metamorphic belt tatkon to mandalay central myanmar abstract the mogok metamorphic belt mmb over 1450 km long and up to 40 km wide consists of regionally metamorphosed rocks including kyanite and sillimanite schists and granites lying along the western margin of the shan plateau in central myanmar and continuing northwards to the eastern himalayan syntaxis exposures in quarries allow correlation of palaeozoic meta sedimentary early mesozoic meta igneous and late mesozoic intrusive rocks within a 230 km long northerly trending segment of the mmb from tatkon to kyanigan north of mandalay and with the mogok gemstone district 100 km to the northeast relationships among the metamorphic and intrusive rocks with sparse published radiometric age controls indicate at least two metamorphic events one before and one after the intrusion of late jurassic to early cretaceous calc alkaline rocks these relationships can be explained by either of two possible tectonic histories one constrained by correlation of mid permian limestones across myanmar requires early permian and early jurassic regional metamorphic events prior to an early tertiary metamorphism in the western part of but within a shan thai western myanmar block the second not compatible with a single laterally continuous permian limestone requires pre upper jurassic regional metamorphism and orogenic gold mineralization in the mergui group and western myanmar early cretaceous collision of an east facing mergui western myanmar island arc with the shan plateau and early tertiary metamorphism in the mmb related to reversal in tectonic polarity following the arc plateau collision

the valaisan controversy revisited multi stage folding of a mesozoic hyper extended margin in the petit st bernard pass area western alps abstract the valaisan units in the alps sample remnants of a mesozoic basin located along the distal margin of the european plate the extent and timing of crustal thinning and the presence absence of rift related magmatism in this basin have been investigated in the newly defined punta rossa unit at the petit st bernard pass western alps the punta rossa unit consists of laterally discontinuous slivers of paleozoic basement rare meta pillow lavas and abundant metasedimentary breccias these rock types are located along the multiply folded interface between serpentinized ultramafics and radiolaria bearing garnet chloritoid micaschists fault breccias pre dating alpine metamorphism are common at the top of the ultramafics and throughout the continental basement slivers but they are absent in all other lithologies these observations suggest that subcontinental mantle and paleozoic basement were juxtaposed by brittle faulting before the overlying sediments were deposited rift related thinning was probably accompanied by minor mafic magmatism as indicated by pillow lavas directly in contact with garnet chloritoid micaschists the punta rossa unit preserves evidence of a multi stage alpine evolution post high pressure isoclinal folding fctd is associated with a pervasive axial planar cleavage sctd defined by chloritoid and white mica following re heating to 400 c sctd was statically overgrown by garnet and chloritoid prior to large scale recumbent folding at greenschist facies conditions frec interference between fctd and frec is responsible for the regional occurrence of basement rocks resting upon mesozoic metasediments following frec shear zones with top to the south kinematics dissected the tectonic pile prior to the formation of upright folds with nne ssw trending fold axes therefore the punta rossa unit preserves evidence of complete crustal excision in the valaisan basin with exhumation of ultramafics and minor mafic magmatism multi stage deformation and a laterally discontinuous pre alpine architecture typical of hyper extended rifted margins are responsible for the complex outcrop pattern observed in the field

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

a tectonic model reconciling evidence for the collisions between india eurasia and intra oceanic arcs of the central eastern tethys abstract despite several decades of investigations inferences on the timing and nature of collisions along the mesozoic cenozoic eurasian margin remain controversial we assimilate geological and geophysical evidence into a plate tectonic model for the india eurasia collision that includes continuously closing topological plate polygons constructed from a time dependent network of evolving plate boundaries with synthetic plates constructed for now subducted ocean floor including back arc basins that formed on the southern eurasian margin our model is regionally constrained and self consistent incorporating geophysical data from abyssal plains offshore west australia and east antarctica including jurassic age data from offshore northwest australia limiting much of northern greater india to a 1000 km long indenter originally reaching to the wallaby zenith fracture zone southern eurasia and southeast asia are riddled with dismembered oceanic arcs indicating long lived tethyan intra oceanic subduction this intra oceanic subduction system was well established from cretaceous time in the india eurasia convergence zone in the neotethys which was consumed during greater india s northward trajectory towards eurasia from the early cretaceous fragments of obducted oceanic crust within the yarlung tsangpo suture zone between india and eurasia predominantly date to the late jurassic or mid cretaceous barremian aptian the various ophiolites along strike and a hiatus in subduction related magmatism during the tithonian aptian suggest that there was at least one generation of intra oceanic arc formation whose plate boundary configuration remains uncertain paleomagnetic and magmatic studies suggest that the intra oceanic arc was at equatorial latitudes during the early cretaceous before subduction resumed further north beneath the eurasian margin lhasa terrane with another hiatus in subduction related magmatism along southern lhasa during 80 65 ma possibly as the back arc spreading centre approached the active andean style margin in our model greater india collided with the tethyan intra oceanic arc in paleocene eocene time finally closing the tethyan seaway from mid late eocene time which is consistent with the age of the youngest marine deposits found between india and eurasia geological evidence from the collision zone indicates an age of initial arc continent collision by 52 ma followed by the soft initial continent continent collision between india and eurasia by 44 2 ma this timing is supported by marine geophysical data where the spreading centres in the indian ocean record a drastic decline in seafloor spreading rates and changes in spreading directions first at 52 ma followed by another reorganisation at 43 ma the abandonment of spreading in the wharton basin and the onset of extrusion tectonics in asia by 36 ma are likely indicators of hard complete continent collision and highlight the multi stage collisional history of this margin our continuously evolving network of mid ocean ridge and subduction zone geometries and divergence convergence vectors through time provide a basis for future refinements to assimilate new data and or test alternative tectonic scenarios

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

timing of syenite charnockite magmatism and ruby and sapphire metamorphism in the mogok valley region myanmar we thank the oxford burma aung san suu kyi trust for funding research and fieldwork visits to myanmarfor ms ng and lr geochronology was funded by ucsb and nsf grants ear 1348003and ear 1551054

multiple alternating forearc and backarc ward migration of magmatism in the indo myanmar orogenic belt since the jurassic documentation of the orogenic architecture of eastern neotethys in se asia abstract we integrated our new data with a review of published work on the indo myanmar orogenic belt imob of the eastern neotethys in myanmar western yunnan china and naga hills india in order to better understand subduction accretion processes of the neotethys ocean in se asia and interactive relationships between the gondwana and laurasia supercontinents from east to west the imob is divided into three geographic units the shan thai plateau myanmar central basin and indo burma range the mogok metamorphic and slate belt is within the shan thai plateau the shan thai plateau and eastern myanmar central basin contain paleozoic sedimentary successions low grade to amphibolitic facies metamorphic rocks and the luxi ultramafic and myitkyina mogok ultramafic diorite belts unmetamorphosed and metamorphosed sediments have similar detrital zircon age peaks of 1000 ma and 700 ma or 500 ma and they are separated by the luxi ultramafic belt whose end 0 and 87sr 86sr i values are similar to surrounding granite diorite and migmatite and the os 4 8 8 8 is indicative of old sub continental lithospheric mantle rather than an oceanic suture zone these data and relations suggest that these regions belong to the cimmerian continental sliver derived from the gondwana supercontinent the myitkyina mogok ultramafic diorite belt located along the eastern margin of the myanmar central basin that was associated with the continental sliver contains peridotite andesite hornblende gabbro diorite granodiorite and plagiogranite all with arc geochemical signatures and ages of 177 166 ma the source region of some of these igneous rocks contains a component of em1 which is close to low 87sr 86sr i ratio of a migmatite in the cimmerian continental sliver as shown by sr nd isotopes values suggesting this belt developed as a jurassic continental margin arc the mogok metamorphic and slate belt contains granodiorite diorite tonalite granite and migmatite extending from the gongshan tengchong and luxi areas in western yunnan china to mandalay kalaw padatchaung and moulmein in myanmar these rocks have arc geochemical character and yield ages of 185 170 ma 128 112 ma 91 ma and 75 45 ma suggesting development of four episodes of arc magmatism along the central axis of the myanmar central basin there are magmatic arc related 106 94 ma diorite granodiorite and tonalite as well as eocene and neogene present 38 0 ma andesite dacite trachyte and rhyolite accretion took place along the central axis of the myanmar central basin towards the indo burma range and westward to the present trench axis on the margin of the indian ocean as shown by westward younging of accretionary complexes and trench slope basins a jurassic accretionary complex located in the hpakan area in the north of the myanmar central basin contains 189 147 ma high pressure metamorphic rocks such as jadeitite garnet mica schist and glaucophane schist in the eastern belt of the indo burma range early cretaceous 119 115 ma accretionary complexes consist of upper plate triassic unmetamorphosed and low grade metamorphic sediments and lower plate amphibolites high pressure metamorphic rocks and jurassic early cretaceous oceanic plate stratigraphy containing morb and oib type basalts and thin bedded radiolarian cherts late early cretaceous 100 ma and late cretaceous to middle eocene 75 45 ma trench slope basin deposits containing abundant fossil plants and shallow water sedimentary structures cover most of the indo burma range where sediments rest with an angular unconformity on older accretionary complexes the presence of earlier magmatic arcs accretionary complexes and trench slope basins suggests that the present trench on the margin of the indian ocean has evolved by continuous eastward subduction since the jurassic the magmatic arc axis migrated eastward landward during some time periods jurassic to early cretaceous and late cretaceous to middle eocene and westward seaward migration during others early cretaceous to late early cretaceous and middle eocene to present during continuous east dipping subduction

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

structure and kinematics of the indo burmese wedge recent and fast growth of the outer wedge 1 the northern sunda subduction zone offshore burma and the associated indo burmese wedge mark the active eastern boundary of the burma platelet jammed between the india plate and the sunda plate the aim of this paper is to provide a structural and kinematics analysis of the indo burmese wedge based on seismic reflection geodetic and geological field data we found out that the indo burmese wedge is the place of diffuse strain partitioning right lateral shearing in the innermost part and e w shortening in the outermost part in the outer wedge thick skinned deformation overprints thin skinned deformation it can be explained by the required preservation of the critical taper after the fast westward propagation of the outer wedge above a very efficient clayey decollement layer the thick skinned deformation is here characterized by major right lateral strike slip faults the kaladan fault and the chittagong coastal fault and yet a typical internal deformation of the indo burmese wedge we suggest that the westward migration of internal right lateral shear deformation allows preservation of the strain partitioning ratios between internal n s trending right lateral shearing and external e w shortening some seismic lines show that the outer wedge deformation is not older than 2 ma its propagation could have been enhanced by the large amount of sediments filling up the sylhet flexural basin formed after the pliocene tectonic uplift of the shillong plateau

order from chaos a field based estimate on bulk rheology of tectonic melanges formed in subduction zones abstract many tectonic hp lt melanges exhibit block in matrix structures in contrast to the pervasively deformed matrix the blocks of various lithologies and pt histories usually appear undeformed if we accept that an hp lt melange reflects persistent high strain deformation in a deep 30 km subduction channel the record of these rock associations can provide insight into stress state rheology and flow patterns along a plate interface the bulk rheology of a block in matrix structure is controlled by a the dominant deformation mechanism of the matrix b the volume fraction of blocks engulfed in that matrix and c the geometry of the blocks the microstructures of the matrix indicate distributed deformation by dissolution precipitation creep dpc while crystal plastic deformation is subordinate dpc is grain size sensitive controlled by the type of interfaces and requires an aqueous pore fluid at quasi lithostatic pressure the rheology of a rock body undergoing deformation by dpc is believed to be newtonian viscous matrix viscosities are estimated to be on the order of 10 19 pa s or less here we explore the effect of rigid blocks embedded in a weak matrix on the bulk rheology using stereology we determine the block size distributions as well as the total block volume fractions for block in matrix structures from several regions the volume fractions are found to range from 2 to 70 with typical values between 5 and 50 using published mixing laws we model the subduction channel as a suspension of rigid spheres with a linear viscous matrix and predict that for the typical values of 5 to 50 the viscosity increase is less than one order of magnitude consequently the influence of blocks on the bulk viscosity of a subduction channel is minor instead bulk rheology is primarily controlled by the deformation behaviour of the matrix material

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

tectonic and metamorphic architecture of the hp belt of new caledonia abstract new caledonia includes a well exposed example of high pressure orogenic belt formed as a result of cenozoic plate tectonic reorganizations at the australia pacific plate boundary the metamorphic belt is relatively well accessible and has deserved extensive attention mainly for its metamorphic and petrological features the architecture of this belt however still lacks general consensus the aim of this paper is i to review the nature and origin of the main tectonic units of the belts and ii to revisit the available structural models describing its architecture and geodynamic evolution for that we integrate the available field petrological geochemical and geochronological data together with new results from large scale field survey and focused ar ar geochronological data all together these data allow proposing a new model for the tectonic and metamorphic architecture of the belt also comprising a newly established tectonostratigraphic unit this architecture resembles that characterizing alpine type metamorphic belts formed in non pacific settings and clearly differs from cordilleran type belts found in eastern pacific regions moreover the collected data also allow refining our understanding of the oceanic and transitional oceanic lithosphere involved in the eocene subduction and plausible analogies with hyper extended margins known in present day ocean continent transition zones

amount of asian lithospheric mantle subducted during the india asia collision abstract body wave seismic tomography is a successful technique for mapping lithospheric material sinking into the mantle focusing on the india asia collision zone we postulate the existence of several asian continental slabs based on seismic global tomography we observe a lower mantle positive anomaly between 1100 and 900 km depths that we interpret as the signature of a past subduction process of asian lithosphere based on the anomaly position relative to positive anomalies related to indian continental slab we propose that this anomaly provides evidence for south dipping subduction of north tibet lithospheric mantle occurring along 3000 km parallel to the southern asian margin and beginning soon after the 45 ma break off that detached the tethys oceanic slab from the indian continent we estimate the maximum length of the slab related to the anomaly to be 400 km adding 200 km of presently asian subducting slab beneath central tibet the amount of asian lithospheric mantle absorbed by continental subduction during the collision is at most 600 km using global seismic tomography to resolve the geometry of asian continent at the onset of collision we estimate that the convergence absorbed by asia during the indentation process is 1300 km we conclude that asian continental subduction could accommodate at most 45 of the asian convergence the rest of the convergence could have been accommodated by a combination of extrusion and shallow subduction underthrusting processes continental subduction is therefore a major lithospheric process involved in intraplate tectonics of a supercontinent like eurasia

tectonic evolution of the sibumasu indochina terrane collision zone in thailand and malaysia constraints from new u pb zircon chronology of se asian tin granitoids three principal granite provinces are defined across se asia as follows 1 the western thailand myanmar burma province consists of hornblende biotite i type granodiorite granites and felsic biotite k feldspar garnet tourmaline granites associated with abundant tin mineralization in greisen type veins new ion microprobe u pb dating results from phuket island show zircon core ages of 212 2 and 214 2 ma and a thermal overprint with rims of 81 2 1 2 and 85 75 ma 2 the north thailand west malaya main range province has mainly s type biotite granites and abundant tin mineralization resulting from crustal thickening following collision of the sibumasu plate with indochina during the mid triassic biotite granites around kuala lumpur contain extremely u rich zircons up to 38000 ppm that yield ages of 215 7 and 210 7 ma 3 the east malaya province consists of dominantly permian triassic i type hornblende biotite granites but with subordinate s type plutons and a type syenite gabbros biotite k feldspar granites from tioman island off the east coast of malaysia also yield a zircon age of 80 1 ma showing cretaceous magmatism in common with province 1 geological and u pb geochronological data suggest that two east dipping in present day coordinates subduction zones are required during the triassic one along the bentong raub palaeo tethyan suture and the other west of the phuket burma province 1 belt supplementary material a full description of u pb analytical methods used and data tables are available at www geolsoc org uk sup18523

anomalously fast convergence of india and eurasia caused by double subduction prior to collision with eurasia the indian plate rapidly accelerated northwards numerical simulations show that the combined pull of two slabs in two parallel north dipping subduction systems could have caused this pulse of rapid movement

holocene eruptions of mt popa myanmar volcanological evidence of the ongoing subduction of indian plate along arakan trench in the western part of myanmar the geodetic and seismic data indicate the ongoing highly oblique subduction of indian plate under eurasian plate volcanoes of burma arc however did not produce eruptions in the recorded history and are considered extinct such perception questions the ongoing subduction as well as keeping local officials unaware of possibility of future volcanic eruptions in the country we have investigated the youngest lava flows and pyroclasts of mt popa which is the best preserved polygenetic volcanic edifice in myanmar the ar ar dating of the youngest products of the volcano provided very young radiometric ages which were unable to be measured accurately the radiocarbon dating of paleosols intercalated with the most recent ash layers of the volcano has shown that mt popa produced several eruptions in the beginning of holocene the youngest eruption occurred 8000 bp and included the 1 3 km3 gravitational collapse of the volcanic cone with deposition of the 11 km long debris avalanche immediately followed by emplacement of 0 1 km3 of pyroclastic flow of calc alkaline basaltic andesite composition the collapse direction as well as complex morphology of the resulted crater were prearranged by geometry of the listric fault which was formed during the pre collapse asymmetric gravitational spreading of the volcanic cone the recently erupted products are geochemically similar to products of young monogenetic volcanoes of monywa area located 150 km north mt popa and both display patterns consistent with magma generation at an active subduction system low magma production rate of mt popa 3 10 5 km3 year averaged for the 1 ma of the volcano history is in agreement with the highly oblique angle and slow rate of the subduction the fact of the holocene eruptions of calc alkaline composition in burma arc represents volcanological evidence of the ongoing subduction in this part of the collision zone between india and asia

exhumation of west sundaland a record of the path of india abstract the indian plate commenced its northward migration towards eurasia in the early cretaceous the lateral effect of this migration on the western edge of the sunda plate in southeast asia still remains equivocal in order to assess this effect we evaluate several key sectors characterized by deep crustal exhumation along a n s transect from the southern malay peninsula to the east himalayan syntaxis the evaluation is aided by a structural analysis of vertical movements and basin development five major metamorphic domes with similar geodynamic evolution maximum p t burial conditions and exhumation are studied exhumation of these domes migrated with time between late cretaceous in the stong complex north malaysia in the south and late miocene in the gaoligong shear zone south china in the north as documented by published work and our new fission track data presented herein deformation is characterized by a n s oriented extension that followed the more regional e w oriented plate tectonic shortening creating local core complexes and syn kinematic magmatism in the footwall of crustal scale detachments which displays a consistent temporal northward migration the n s extension was associated with the onset of hanging wall deposition in the sedimentary basins of western sundaland e g malay sumatra and thai basins during continuous exhumation of the footwall to upper brittle levels our multifaceted analysis of structural and thermochronological data shows a similar succession of tectonic thermal and sedimentary events in west sundaland that was driven by the gradual northward migration of india starting from cretaceous times we infer that the principal mechanism was driven by the subduction of an excess topography of greater india rifted continental margin during its underplating resulting in uplift thermal anomalies extensional exhumation and associated subsidence

using 4 c to characterize lithologies and fluids in clastic reservoirs multicomponent seismic has been around for a long time without gaining widespread acceptance the potential benefits of having both compressional p wave and shear s wave information have never been disputed but the cost of acquiring and processing the large data volumes has been high and the quality of the s wave data has sometimes not met expectations however recent technological developments in the area of sources sensors telemetry and data processing are making multicomponent data increasingly popular as a way to provide better quality data at lower cost although shear information can be accessed through the avo response of the p waves the pure shear ss mode or the converted shear wave ps mode provide direct and more accurate information about the shear properties of the subsurface getting this improved information about the elastic properties of the subsurface is indeed one of the main reasons for acquiring multicomponent data there are two major ways of acquiring multicomponent seismic data land multicomponent seismic is typically acquired using dedicated vertically and horizontally polarized traction sources to generate downgoing p and s wave modes the seismic can then be recorded as three component 3 c or nine component 9 c data sets the latter involves three consecutive 3 c recordings of direct and converted modes by activating each dedicated source mode in turn i e vertical followed by in line and cross line horizontal components marine multicomponent data known as 4 c is acquired using the traditional air gun sources to generate a pressure wave pp but the receivers are on the seafloor to record the 3 d vector field the fourth component is a hydrophone recording the pressure field just like ordinary seismic streamers by placing three component geophones on the seafloor converted shear waves known as the ps mode can also be recorded a propagating wave front will in addition to

discovery of early cretaceous rocks in new caledonia new geochemical and u pb zircon age constraints on the transition from subduction to marginal breakup in the southwest pacific abstract new u pb dating of detrital zircon and geochemical features of permian mesozoic arc derived volcanic rocks and volcaniclastic turbidites graywackes when compared with those of the volcanic rocks associated with unconformable late cretaceous shallow water sediments reveal that subduction in new caledonia once thought to be extinct in the late jurassic ca 150 ma was still active at least from ca 130 to 95 ma the accumulation of volcanic arc derived sediments during the late early cretaceous suggests that as in new zealand active margin activity went on for a short time in spite of the assumed subduction jamming by the hikurangi plateau at ca 100 ma meanwhile the rift related magmatic activity that preceded the marginal breakup migrated eastward from ca 130 ma 130 95 ma in eastern australia to 110 ma 110 82 ma in new zealand and finally to ca 89 ma 89 83 ma in new caledonia and generated large volumes of silicic magma in contrast marginal basins opened synchronously at

tectonics of the western margin of the shan plateau central myanmar implication for the india indochina oblique convergence since the oligocene abstract we present new data and interpretations on the neogene tectonics of the shan scarp area central myanmar and its relationship with the india indochina oblique convergence we describe ductile and brittle fabrics associated with the major features in this area the mogok metamorphic belt mmb the shan scarp and the sagaing fault from these data we identify a succession of two tectonic regimes first a dominant nnw sse trending extension marked by ductile stretching that characterizes the mmb and associated n70e brittle normal faults later from middle or upper miocene to the present these structures were cross cut by brittle right lateral faults among which the most important are the n20w transpressive shan scarp fault zone and the n s sagaing fault to explain this transition from a dominant transtensive to a transpressive stress regime that occurred during miocene we place our data within a larger geodynamic context we suggest that like the intraplate deformation in the indian ocean the end of spreading in the south china sea the opening of the andaman basin or the end of subduction within the indo burma range the change in the tectonic regime in central myanmar could be in response to a major miocene regional plate kinematic reorganization

the bengal depositional system from rift to orogeny abstract the bengal depositional system is defined as the surface depositional environments and the underlying sediment accumulation extending from the alluvial lacustrine and paludal sediments of the lower ganges and brahmaputra rivers across the bengal delta the bangladesh continental shelf and slope to and including the bengal fan together it is one of the greatest sediment accumulations in the modern world and is comparable in volume to the great sediment accumulations of the geological past the history of formation started with the mesozoic breakup of eastern gondwanaland the northward drift of india its collision with the southern margin of asia rotation and bending of the western sunda arc and the penetration of the indian continental mass into southern asia during this history the regional tectonics evolved and sources and provenance of the sediments changed with the ultimate uplift of the tibetan plateau and the himalayas

diachronous cooling along the mogok metamorphic belt shan scarp myanmar the trace of the northward migration of the indian syntaxis abstract we present new observations and new mineral ages eight by the 40 ar 39 ar and nine by the 40 k 40 ar methods from high grade metamorphic and foliated intrusive rocks collected along the mogok metamorphic belt of central myanmar that complete preliminary results we previously published all 30 ages presented here confirm the oligocene to middle miocene age of the mogok metamorphism moreover the ages are young from the gulf of martaban in the south to the china myanmar border in the north showing a clear diachronism these results confirm a northward migration of the tectonic event responsible for the cooling of our samples at a velocity similar to the india relative to indochina velocity at the same period in addition intensive field work along a 800 km long section of the mogok metamorphic belt has revealed the numerous ductile structures that resulted from a strong nnw sse trending extension from the analogy between the present day local extrusion around the eastern indian syntaxis and the extension observed along the mogok metamorphic belt and from the diachronism we document we suggest that the central part of myanmar has recorded the northward passage of the eastern indian syntaxis with respect to indochina from oligocene to middle miocene

large scale structure of the eclogite blueschist belt of new caledonia abstract this paper considers the large scale geometry of the northern part of the eclogite blueschist belt of new caledonia based largely on classical style structural analysis but in consideration of modern concepts regarding crustal extension exhumation of high pressure rocks and uplift shear zones played an important role in the evolution of the region early shear zones and high pressure metamorphism appear to be associated with overthrusting of an ultramafic sheet middle stage shear zones are associated with large scale continental extension during which the high pressure rocks were exhumed the extended crust was subsequently folded during renewed compression producing an orogen scale antiform throughout the high pressure belt that folds all previously formed structures late stage shear zones formed when the orogen was once again thrown into extension relatively youthful normal faults caused late block faulting uplifting a regional peneplain and producing the present geomorphology previously published interpretations of the structure and geometry of the belt have suggested that the high pressure rocks are found in the core of a regionally developed antiform interpreted as a metamorphic core complex our structural mapping shows that allochthonous slices of high pressure rocks are draped over a younger unrelated foliation antiform lower grade retrogressed equivalent rocks are found within the core of the antiform thus the metamorphic core complex model is rejected

magmatic evolution of the western myanmar arc documented by u pb and hf isotopes in detrital zircon abstract the evolution of the magmatic arc formed along the asian margin of neo tethys is a key to understand india asia convergence in contrast to the well studied trans himalayan magmatic rocks in tibet the southeast extension of the arc is still poorly documented in myanmar where it is largely buried by sedimentary rocks this study investigates the magmatic evolution of the western myanmar arc and the potential along strike variation of subduction related magmatism by analyzing u pb and hf isotopes of detrital zircons from the chindwin basin in western myanmar our data indicate that the western myanmar arc wma experienced a long lasting magmatism since the early cretaceous with a main magmatic stage between 110 and 80 ma and a subordinate magmatic stage at 70 40 ma arc derived detrital zircons yield positive e hf t values and young model ages 0 1 1 2 ga indicating magma generation from a juvenile source such geochronologic and geochemical signatures compare well with those of the kohistan ladakh gangdese arc in tibet indicating uniform magmatic arc activity all along the asian margin of neo tethys

paleogene evolution of the burmese forearc basin and implications for the history of india asia convergence the geological history of the burmese subduction margin where india obliquely subducts below indochina remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader tibetan himalayan orogen various scenarios for the evolution of the orogen have been proposed including a collision of india with myanmar in the paleogene a significant extrusion of myanmar and indochina from the india asia collision zone or very little change in paleogeography and subduction regime since the india asia collision this article examines the history of the burmese forearc basin with a particular focus on eocene oligocene times to reconstruct the evolution of the burmese margin during the early stages of the india asia collision we report on sedimentological geochemical petrographical and geochronological data from the chindwin basin the northern part of the burmese forearc and integrate these results with previous data from other basins in central myanmar our results show that the burmese margin acted as a regular andean type subduction margin until the late middle eocene with a forearc basin open to the trench and fed by the denudation of the andean volcanic arc to the east we show that the modern tectonic configuration of central myanmar was formed 39 37 million years ago when the burmese margin shifted from an andean type margin to a hyper oblique margin the forearc basin was quickly partitioned into individual pull apart basins bounded to the west by a quickly emerged accretionary prism and to the east by synchronously exhumed basement rocks including coeval high grade metamorphics we interpret this shift as resulting from the onset of strike slip deformation on the subduction margin leading to the formation of a paleo sliver plate with a paleo fault system in the accretionary prism pull apart basins in the forearc and another paleo fault system in the backarc this evolution implies that hyper oblique convergence below the burmese margin is at least twice older than previously thought our results reject any india asia convergence scenario involving an early paleogene collision of india with myanmar in contrast our results validate conservative geodynamic models arguing for a close to modern precollisional paleogeography for the indochina peninsula and indicate that any postcollisional rotation of indochina if it occurred at all must have been achieved by the late middle eocene

origin and tectonic evolution of upper triassic turbidites in the indo burman ranges west myanmar abstract the pane chaung formation is exposed in the indo burman ranges and has been involved in collision between the indian plate and west burma block however controversies exist over the provenance and paleogeographic reconstruction of the pane chaung formation this study presents new petrographical and detrital zircon u pb ages and hf isotopic data from the pane chaung formation in rakhine yoma and chin hills west myanmar the depositional age of the pane chaung formation is late triassic indicated by the carnian norian halobia fossils and maximum depositional ages between 233 0 2 5 ma and 206 2 1 8 ma upper triassic sandstones contain 290 200 ma detrital zircons ehf t values of 6 to 11 and t dm c of 1 6 to 0 6 ga interpreted to be derived from west papua region the most abundant zircon age population of 750 450 ma is derived from pan african orogenic belts in australia zircons of 1250 900 ma age were derived from the grenvillian orogen in australia archean zircons are interpreted to be derived from the yilgarn and pilbara cratons in western australia detrital zircon ages of the pane chaung formation are distinct from similar aged strata in indochina and sibumasu but comparable to nw australia carnarvon basin and greater india langjiexue formation it is suggested that the pane chaung formation was deposited in a late triassic submarine fan along the northern margin of australia

burma terrane part of the trans tethyan arc during collision with india according to palaeomagnetic data convergence between the indian and asian plates has reshaped large parts of asia changing regional climate and biodiversity yet geodynamic models fundamentally diverge on how convergence was accommodated since the india asia collision here we report paleomagnetic data from the burma terrane at the eastern edge of the collision zone and famous for its cretaceous amber biota to better determine the evolution of the india asia collision the burma terrane was part of a trans tethyan island arc and stood at a near equatorial southern latitude at 95 ma suggesting island endemism for the burmese amber biota the burma terrane underwent significant clockwise rotation between 80 50 ma causing its subduction margin to become hyper oblique subsequently it was translated northward on the indian plate by an exceptional distance of at least 2000 km along a dextral strike slip fault system in the east our reconstructions are only compatible with geodynamic models involving a first collision of india with a near equatorial trans tethyan subduction system at 60 ma followed by a later collision with the asian margin

early cretaceous wedge extrusion in the indo burma range accretionary complex implications for the mesozoic subduction of neotethys in se asia the indo burma range ibr of myanmar the eastern extension of the yarlung tsangpo neotethyan belt of tibet in china contains melanges with serpentinite greenschist facies basalt chert sericite schist silty slate and unmetamorphosed triassic sandstone mudstone and siltstone interbedded with chert in the east and farther north high pressure blueschist and eclogite blocks in the naga hills melange our detailed mapping of the mindat and magwe sections in the middle ibr revealed a major 18 km antiformal isocline in a melange in which greenschist facies rocks in the core decrease in grade eastwards and westwards symmetrically outwards to lower grade sericite schist and silty slate and at the margins to unmetamorphosed sediments and these metamorphic rocks are structurally repeated in small scale imbricated thrust stacks in the mindat section the lower western boundary of the isoclinal melange is a thrust on which the metamorphic rocks have been transported over unmetamorphosed sediments of the triassic pane chaung group and the upper eastern boundary is a normal fault these relations demonstrate that the ibr metamorphic rocks were exhumed by wedge extrusion in a subduction generated accretionary complex along strike to the north in the naga hills is a comparable isoclinal melange in which central eclogite lenses are succeeded outwards by layers of glaucophane schist and glaucophanite and to lower grade greenschist facies sericite schist and slate towards the margins in the natchaung area from west to east unmetamorphosed triassic sediments overlie quartzites sericite schists actinolite schists and meta volcanic amphibolites derived from morb type basalt which are in fault contact with peridotite olivine in the peridotite has undulatory extinction suggesting deformation at 600 700 c similar to the peak temperature of the amphibolite these relations suggest generation in a metamorphic sole the amphibolites have u pb zircon ages of 119 3 ma and 115 ma which are close to the zircon ages of nearby calc alkaline granite and diorite which belong to an active continental margin arc that extends along the western side of the shan thai block the ibr accretionary complex and the active continental margin arc were generated during early cretaceous 115 128 ma subduction of the neotethys ocean

magmatic history of central myanmar and implications for the evolution of the burma terrane the burma terrane is a microplate at the eastern edge of the himalayan tibetan orogen the origin of which remains poorly understood its basement comprises metamorphic and igneous rocks forming the wuntho popa arc wpa and has been correlated with tibetan gondwana or transtethyan rocks yet little is known about the magmatic history of the wpa we report elemental and sr nd isotopic compositions of magmatic rocks crystallization zircon and apatite u pb and exhumation apatite fission track ages from rocks and river sands and structural measurements from the wuntho ranges central myanmar where the wpa is best exposed we show that the wpa in the wuntho ranges is characterized by two magmatic events at 108 90 ma and 46 32 ma magmatism is subduction related for both events characterized by depleted nd and sr isotopic compositions with more enriched values with time apatite fission track data suggest arc exhumation during the 39 22 ma time interval partly overlapping with the last magmatic event structural data indicate nw se striking tilting folding and thrusting that we associate with at least two phases of deformation in the cretaceous and the late paleogene correlating the wpa with tibetan gondwana or transtethyan rocks based on its magmatic history remains ambiguous however models arguing for a transtethyan origin for the wpa are most compatible with our results combined with available burmese geological data

cenozoic structural and metamorphic evolution of the eastern himalayan syntaxis namche barwa combined geological and geochronological investigations of the eastern himalayan syntaxis in the namche barwa region of tibet reveal the first order elements of its cenozoic tectonic evolution the syntaxis is characterized by a northeast plunging antiform and is bounded by two northeast striking strike slip shear zones a left slip shear zone on the western side and a right slip shear zone on the eastern side these strike slip shear zones are linked by east westtrending thrusts and served either as 1 a roof thrust to a large duplex system or 2 transfer faults to a south directed thrust system that accommodated northward indentation of a folded indian plate an east west trending pop up structure in the core of the antiform juxtaposes a granulite bearing complex over sillimanite bearing gneisses of gangdese affinity to the north and of indian affinity to the south previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at v800 c the first at high pressures 14 15 kbar followed by a second event at 8 10 kbar zircons from mafic granulites yield four populations of concordant u pb ion microprobe ages two groups are at v65 ma and v160 ma and likely crystallized during andean type gangdese magmatism prior to the indo asian collision a third cluster at v40 ma exhibits very low th u ratios and is interpreted to have crystallized in the presence of fluids associated with a high pressure granulite facies metamorphic event during the early stages of the indo asian collision subsequent to high pressure metamorphism in the western himalaya syntaxis between v50 and 43 ma a fourth cluster of zircons yields ages between 11 and 25 ma and th u ratios that decrease systematically with decreasing age we interpret the youngest zircon age v11 ma to represent the timing of moderatepressure high grade metamorphism with the older ages and higher th u ratios being a result of mixing with a restitic igneous component this interpretation coupled with a v 8m a 40 ar 39 ar age on hornblende from a metadiorite within the core of the antiform suggests that the namche barwa syntaxis has been characterized by rapid cooling and exhumation since at least late miocene time despite its contrasting structural setting the miocene and younger metamorphism and cooling history in the namche barwa syntaxis are strikingly similar to those of the nanga parbat syntaxis of the western himalaya fl 2001 elsevier science b v all rights reserved