the modi taung nankwe gold district slate belt central myanmar mesothermal veins in a mesozoic orogen abstract the recently discovered quartz gold veins comprising the modi taung nankwe gold district in central myanmar are largely hosted by mudstones of the late palaeozoic slate belt mesozoic rocks of the paunglaung mawchi zone separate the slate belt from the shan scarp and shan plateau to the east at modi taung 5 km of exploration adits indicate that the veins within steeply dipping oblique reverse slip shear zones are displaced by movements along the shears intruded by late jurassic calc alkaline dykes and offset on conjugate cross faults drill intercepts show that mineralization extends vertically for more than 500 m coarse visible gold book and ribbon texture and stylolitic laminations and trace metal values are consistent with mineralization at mesothermal depths of several kilometres in the brittle ductile regime our field observations suggest that mineralization took place in the early jurassic following collision of myanmar on the passive western margin of a greater shan thai continental block with an oceanic arc on the overriding plate to the west collision generated intra continental thrusting in and east of the paunglaung mawchi zone metamorphism of plateau rocks thrust west beneath the slate belt to form the mogok metamorphics and ascent of mineralizing fluids expelled from dehydrating underthrust rocks reversal in orogenic polarity initiated late jurassic eastward subduction of oceanic crust beneath myanmar and generation of a magmatic arc with dykes cutting the quartz gold veins renewed east directed thrusting translated the slate belt over arc clastics and probably resulted in exhumation of the mogok metamorphics and offsets of the veins on conjugate faults young k ar rb 87 sr 86 and ar 40 ar 39 ages on the mogok metamorphic belt imply renewed tertiary uplift and possibly intrusion of mid tertiary or palaeogene granites

late cretaceous early palaeogene tectonic development of se asia abstract the late cretaceous early palaeogene history of the continental core of se asia sundaland marks the time prior to collision of india with asia when se asia from the tethys in the west to the palaeo pacific in the east lay in the upper plate of subduction zones in myanmar and sumatra subduction was interrupted in the aptian albian by a phase of arc accretion woyla and mawgyi arcs and in java eastern borneo and western sulawesi by collision of continental fragments rifted from northern australia subsequent resumption of subduction in the myanmar thailand sector explains 1 early creation of oceanic crust in the andaman sea in a supra subduction zone setting 95 ma 2 the belt of granite plutons of late cretaceous early palaeogene age starting 88 ma in western thailand and central myanmar and 3 amphibolite grade metamorphism between 70 and 80 ma seen in gneissic outcrops in western and central thailand and 4 accretionary prism development in the western belt of myanmar until glancing collision with the ne corner of greater india promoted ophiolite obduction deformation and exhumation of marine sediments in the early palaeogene the ranong strike slip fault and other less well documented faults were episodically active during the late cretaceous palaeogene time n to nw directed subduction of the palaeo pacific ocean below southern china vietnam and borneo created a major magmatic arc associated with rift basins metamorphic core complexes and strike slip deformation which continued into the late cretaceous the origin and timing of termination of subduction has recently been explained by collision of a large luconia continental fragment either during the late cretaceous or palaeogene evidence for such a collision is absent from the south china sea well and seismic reflection record and here collision is discounted instead relocation of the subducting margin further west possibly in response of back arc extension which created the proto south china sea is preferred lying between the two subduction related arcs the khorat basin is of predominantly late jurassic early cretaceous age but stratigraphic and apatite fission track data also indicates deposition of 1 2 km of late cretaceous sediments the synformal basin geometry probably arose due to the dynamic topography created by converging tethyan and palaeo pacific subduction zones the aptian albian slowing of basin subsidence and onset of evaporite deposition coincides with collision of the mawgyi and woyla island arcs extensive palaeogene deformation and exhumation 3 km in places affected all margins of the khorat plateau deformation includes folds of the phu phan uplift and strike slip faults thrusts and folds on the southern and eastern margins south of the khorat plateau outcrop and seismic reflection data from the ton le sap basin cambodia and the gulf of thailand indicate syn depositional fault controlled subsidence was important during cretaceous deposition the hot thickened crust developed during the late cretaceous palaeogene events follows the weak indosinian crustal scale inthanon and sukhothai zones which persistently guided the location of later structures including cenozoic extensional and post rift basins and influenced the widespread occurrence of low angle normal faults metamorphic core complexes and eastern gulf of thailand super deep post rift basins

upper crust response to geodynamic processes beneath isparta angle sw turkey revealed by cmt solutions of earthquakes abstract the isparta angle is an important area of sw anatolia where extensions in all directions n s ne sw nw se and e w meet these extensions were determined by normal faulting structures as well as by shallow earthquakes all extensions except the e w one were attributed to the deviatoric stresses in relation to slab forces and or extrusion of anatolia the moment tensor inversion of 40 shallow earthquakes which occurred in the inner part of the isparta angle give focal mechanisms mostly indicating normal faulting inversion of all focal mechanisms of the earthquakes obtained from the moment tensor inversion yields normal faulting characterized by an approximately e w n268 e 3 axis the calculated stress ratio r is 0 6944 indicating a triaxial stress state commonly accepted geodynamic models for the eastern mediterranean region do not include plate boundary forces acting in the east or west direction our hypothesis is that the cause of the e w extension is the combined forces of gravitational potential energy and the hot asthenosphere upwelling through a tear fault in the subducted african plate between the hellenic and cyprus arcs beneath the isparta angle

the role of strike slip faults in the displacement of the palaeotethys suture zone in southeast thailand abstract six n s tectono stratigraphic belts can be recognized in southeast thailand each with a distinct stratigraphy and igneous history the westernmost belt i is thought to be part of the sibumasu block and the easternmost v to include part of the indochina block between those belt iii comprises acid volcanic rocks volcaniclastics and carboniferous to triassic sedimentary rocks with distinctive faunas interpreted to be a volcanic arc belt iv comprises triassic rocks of back arc basin origin belt v is interpreted as a permian accretionary complex on the western flank of indochina and vi is an unconformable cover of jurassic cretaceous red beds correlating with the khorat group a history of the triassic indosinian orogeny is proposed leading to the cratonization of southeast thailand by the end triassic the apparent absence in southeast thailand of rocks from the devonian triassic palaeotethys ocean the inthanon zone of northern thailand is suggested to have resulted from a phase of post indosinian oblique to longitudinal sinistral strike slip faulting that phase is thought to have included displacement on the mae yuam fault which bounds the inthanon zone in northern thailand then followed a phase of transverse sinistral strike slip faulting which offset the tectono stratigraphic belts and faults of the earlier phase

zircon u pb dating and the petrological and geochemical constraints on lincang granite in western yunnan china implications for the closure of the paleo tethys ocean abstract lincang granite is a batholith located in the sanjiang region and is an important research subject for understanding subduction and collision during the paleo tethyan period it is widely exposed in the lincang terrane and extends south into burma based on various petrological and geochemical investigations performed from south to north across the lincang granite a new set of data which includes zircon chronological and hf isotopic data is presented to discuss the origin of the lincang granite and its tectonic significance the lincang granite is a peraluminous high k calc alkaline body with sub parallel ree patterns and a strong negative eu anomaly this anomaly is characteristic of a post collision peraluminous s type granitic batholith the 200 230 ma formation age of the lincang granite was determined using la icp ms zircon u pb dating thus it has been confirmed that the granite formed during the late triassic period and the formation process lasted for approximately 30 ma geochemical and isotopic compositions indicate that the primary magma of lincang granite most likely originated from a crustal source and possibly underwent an assimilation fractionation crystallization afc process during its emplacement the lincang granite formed during the continental collision between the baoshan gengma terrane and the lanping simao terrane after the northeast subduction of the paleo tethyan oceanic plate therefore the late triassic lincang granite is important evidence for the closure of the paleo tethyan ocean

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

triassic radiolarian faunas from the mae sariang area northern thailand and their paleogeographic significane abstract early to late triassic spathian to carnian radiolarians were obtained from the bedded chert sequence of the mae sariang group distributed in northern thailand based on the similarity of radiolarian fauna and petrographical characteristics it is inferred that the fine grained siliceous and calcareous sediments of the mae sariang group are equivalent to those belonging to the eastern marginal facies of the sibumasu block moreover the occurrence of an early carnian radiolarian assemblage from bedded chert shows that the closure of the paleotethys ocean between the sibumasu and indochina blocks in northern thailand occurred after the early carnian

a middle permian middle triassic accretionary complex and a late triassic foredeep basin forerunners of an indosinian late triassic thrust complex in the thailand malaysia border area abstract the semanggol formation of nw peninsular malaysia is a middle permian late triassic sequence of predominantly radiolarian chert sandstone and mudstone including turbidites and conglomerate the belt of country occupied by this unit extends into thailand where various names including na thawi formation have been applied to its correlatives fossil evidence particularly radiolarian has established its age but also revealed that it is tectonically complex with numerous out of sequence slices interpreted here to be caused by thrusting the model proposed here involves initially in the middle permian accumulation in the oceanward part of an accretionary complex as palaeotethys began subducting beneath indochina east malaya this regime it is proposed continued until about the end of the middle triassic when sibumasu collided with indochina east malaya bringing an end to subduction but as crustal shortening continued into the late triassic a foredeep basin formed in front of the now inactive subduction zone and accretionary complex and the youngest part of the semanggol formation was deposited during this final stage the whole package of rocks comprising those in the accretionary complex and those deposited in the foredeep basin underwent lateral compression resulting in a thrust complex the semanggol formation and its thailand correlatives occupy part of a n s belt of imbricately thrust deeper water sediments which include slope deposited carboniferous and lower permian beds that belt is interpreted as a series of thrust slices juxtaposing rocks of different ages referred to here as the songkhla semanggol terrane its western boundary is a n s line of inferred thrusting which coincides with a major westward facies change to platform carbonates of middle permian to late triassic age called here the rattaphum kodiang tectonic line

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 origin of melanges cautionary tales from indonesia abstract the origin of block in matrix melanges has been the subject of intense speculation by structural and tectonic geologists working in accretionary complexes since their first recognition in the early twentieth century because of their enigmatic nature a number of important international meetings and a large number of publications have been devoted to the problem of the origin of melanges as melanges show the effects of the disruption of lithological units to form separate blocks and also apparently show the effects shearing in the scaly fabric of the matrix a tectonic origin has often been preferred then it was suggested that the disruption to form the blocks in melanges could also occur in a sedimentary environment due to the collapse of submarine fault scarps to form olistostromes upon which deformation could be superimposed tectonically subsequently it has proposed that some melanges have originated by overpressured clays rising buoyantly towards the surface incorporating blocks of the overlying rocks in mud or shale diapirs and mud volcanoes two well known examples of melanges from the banda and sunda arcs are described to which tectonic and sedimentary origins were confidently ascribed which proved on subsequent examination to have been formed due to mud diapirism in a dynamically active environment as the result of tectonism only indirectly evidence from the australian continental shelf to the south of sumba shows that large quantities of diapiric melange were generated before the diapirs were incorporated in the accretionary complex comparable diapirs can be recognised in timor accreted at an earlier stage evidence from both timor and nias shows that diapiric melange can be generated well after the initial accretion process was completed the problem is why when diapirism is so abundantly found in present convergent margins is it so rarely reported from older orogenic belts many occurrences of melanges throughout the world to which tectonic and or sedimentary and origins have been ascribed may in future investigations prove to have had a diapiric origin it is emphasised that although the examples of diapiric melange described here may contain ophiolitic blocks they were developed in shelf or continental margin environments and do not contain blocks of high grade metamorphic rocks in a serpentinous matrix such melanges originate diapirically during subduction in a mantle environment as previous authors have suggested

east flank of the sibumasu block in nw thailand and myanmar and its possible northward continuation into yunnan a review and suggested tectono stratigraphic interpretation abstract the east flank of the sibumasu block was a passive continental margin and in nw thailand is marked by the absence of the autochthonous middle permian triassic platform carbonates which are widespread across the rest of sibumasu further west instead the carbonates are represented by hemipelagic cherts mudstones and sandstones including turbidites during the northward drift of sibumasu following its early permian rifting from gondwana an accretionary complex was present where palaeotethyan pelagic rocks as old as devonian were subducted beneath the sukhothai volcanic arc at the time of sibumasu s collision with the sukhothai arc beginning in the middle triassic the accretionary complex was thrust westwards across the east flank of sibumasu it is suggested that in the late triassic the thrust pile which had been the accretionary complex underwent erosion and was the source of terrigenous clastic rocks deposited further west in a foredeep basin the boundary of sibumasu s east flank with the permo triassic carbonate platform further west is the arcuate mae ping nam teng fault system notwithstanding later cenozoic strike slip displacement those faults as well as the mae yuam fault are interpreted to have had an earlier history of westward directed indosinian thrusting northwards in myanmar and yunnan the sibumasu permo triassic carbonate shelf continues as the shan plateau and baoshan block the east flank is represented by the changning menglian belt and the palaeotethys cryptic suture in thailand possibly joins with the lancangjiang suture

the permian fusulinoidean faunas of the sibumasu and baoshan blocks their implications for the paleogeographic and paleoclimatologic reconstruction of the cimmerian continent abstract permian fusulinoidean faunas occur in mainly four stratigraphic levels in the baoshan block of west yunnan and the sibumasu block of southeast asia which constituted part of the eastern cimmerian continent the oldest fauna from the upper part of the dingjiazhai formation in the baoshan block consists of pseudofusulina eoparafusulina and a new boultoniid genus and is assignable to the yakhtashian artinskian the second one which occurs in the basal part of the ratburi limestone and its equivalent strata in the sibumasu block is represented by monodiexodina and is probably referable to the bolorian kungurian the third fauna composed of eopolydiexodina rugososchwagerina yangchienia chusenella jinzhangia and several other genera is dated to the murgabian wordian and occurs in the lower part of the shazipo and daaozi formations in the baoshan block and the main part of the ratburi limestone in the sibumasu block the youngest fauna of probably dzhulfian wuchiapingian age is found in the upper part of the ratburi limestone and contains nanlingella reichelina codonofusiella and a few staffellid genera a smaller foraminiferal genus shanita found from the upper part of the ratburi limestone and the upper part of the shazipo formation is also an important element of the foraminiferal assemblage near the midian dzhulfian capitanian wuchiapingian boundary in the baoshan and sibumasu blocks in the eastern cimmerian continent low generic diversity throughout the permian and the paucity of tethys characterizing neoschwagerinid and verbeekinid genera during middle permian time are two remarkable features of the permian fusulinoidean faunas in the cimmerian continent the generic diversity of permian fusulinoidean faunas in space and time gradually increases from the early permian to late middle permian as well as from the eastern cimmerian areas to western ones the temporal increase of the generic diversity can be explained by the northward drift of the cimmerian continent during permian time in contrast the lower generic diversity of the eastern cimmerian permian fusulinoidean faunas against western ones is possibly due to an oblique arrangement of the continent to paleolatitude thus the western cimmerian continent was more proximal to the tropical tethyan domain than its eastern part in addition the middle permian cimmerian paleobiogeographic region is likely to be subdivided into two subregions the western tethyan cimmerian and the eastern gondwanan cimmerian based on the distribution pattern of verbeekinid and neoschwagerinid fusulinoideans and overall generic diversity the scarce occurrence or total absence of these essentially tethys indicating fusulinoideans in the baoshan and sibumasu blocks suggests that the eastern cimmerian continent was still far from the equatoro tropical cathaysian domain and was probably in a warm temperate or subtropical zone until the end of the permian the eastern cimmerian areas finally migrated into a tropical zone by the late triassic judging from well developed carnian sponge coral buildups in the chaiburi formation in the sibumasu block

detrital zircon record of paleozoic and mesozoic meta sedimentary strata in the eastern part of the baoshan block implications of their provenance and the tectonic evolution of the southeastern margin of the tibetan plateau abstract the baoshan block is one of the important members in the southeastern margin of the tibetan plateau cambrian to jurassic meta sedimentary strata and their metamorphic counterparts were well preserved in the eastern part of the baoshan block zircon u pb dating trace elements and hf isotope data for both the cambrian to jurassic meta sedimentary sequences and the lincang granite were used to deduce the provenance of sediments and crustal affinity with eastern gondwanaland the lincang granite outcropped in the fengqing area is typical s type with crystallization age of 230 ma and narrow range of zircon e hf t values from 15 5 to 10 detrital zircons from paleozoic strata range in age from archean to early paleozoic with age peaks at 2 5 0 95 and 0 6 ga triassic age peak 230 ma was also detected in the jurassic strata detrital zircon e hf t values exhibit a wide range from negative to positive for each of the four major age groups showing the host magma of zircons from these groups have diverse sources the baoshan block should be along the indian margin as the qiangtang tengchong and simao indochina blocks in early paleozoic based on the provenance analyses sediment inputs eroded from both the lincang granite and the coeval igneous rocks in the baoshan and gongshan blocks were the dominant contributions to the jurassic strata after the amalgation of the baoshan and simao indochina blocks tectonic processes linking dispersion of the baoshan sibumasu terrane from the gondwana supercontinent to the collision with the simao indochina block and the deposition of the cambrian to jurassic sedimentary sequences were reconstructed

ocean plate stratigraphy in east and southeast asia abstract ancient accretionary wedges have been recognised by the presence of glaucophane schist radiolarian chert and melange recent techniques for the reconstruction of disrupted fragments of such wedges by means of radiolarian biostratigraphy provide a more comprehensive history of ocean plate subduction and successive accretion of ocean floor materials from the oceanic plate through offscraping and underplating reconstructed ocean floor sequences found in ancient accretionary complexes in japan comprise from oldest to youngest pillow basalt limestone radiolarian chert siliceous shale and shale and sandstone similar lithologies also occur in the melange complexes of the philippines indonesia thailand and other regions this succession is called ocean plate stratigraphy ops and it represents the following sequence of processes birth of the oceanic plate at the oceanic ridge formation of volcanic islands near the ridge covered by calcareous reefs sedimentation of calcilutite on the flanks of the volcanic islands where radiolarian chert is also deposited deposition of radiolarian skeletons on the oceanic plate in a pelagic setting and sedimentary mixing of radiolarian remains and detrital grains to form siliceous shale in a hemipelagic setting and sedimentation of coarse grained sandstone and shale at or near the trench of the convergent margin radiolarian biostratigraphy of detrital sedimentary rocks provides information on the time and duration of ocean plate subduction the ages of detrital sediments becomes younger oceanward as younger packages of ops are scraped off the downgoing plate ops reconstructed from ancient accretionary complexes give us the age of subduction and accretion direction of subduction and ancient tectonic environments and is an important key to understanding the paleoenvironment and history of the paleo oceans now represented only in suture zones and orogenic belts

geochronological elemental and sr nd hf o isotopic constraints on the petrogenesis of the triassic post collisional granitic rocks in nw thailand and its paleotethyan implications abstract new u pb geochronological petrologic elemental and sr nd hf o isotopic data for the granites from the inthanon and sukhothai zones in nw thailand in conjunction with correlations with sw china are presented to constrain the age and position of the paleotethys ocean in this region and the associated assembly of southeast asia the geochronological data show that the granitic rocks in the inthanon and sukhothai zones herein named group 1 and group 2 granites respectively yield similar crystallization ages of 230 200 ma group 1 samples are characterized by monzogranite and granite with i and s type geochemical affinity and group 2 samples by i type monzogranite and granodiorite they have generally similar chondrite normalized ree and pm normalized multi element patterns but distinct sr nd hf o isotopic compositions group 1 samples have slightly higher initial 87sr 86sr ratios 0 7111 0 7293 but lower end t values 11 1 to 14 1 than those of group 2 samples 87sr 86sr i 0 7073 0 7278 and end t 8 3 to 11 0 group 1 samples show the lower ehf t values 5 4 to 18 2 older tdm 1 62 2 40 ga and higher 18o values 7 95 to 9 94 than those of group 2 samples ehf t of 11 1 to 4 80 tdm of 0 96 1 95 ga and 18o of 4 95 to 7 98 for the triassic crystallization zircons these geochemical signatures are similar to the kwangsian and indosinian granites in the south china and indochina blocks but distinct from those of the gangdese i type granite and sibumasu paleozoic granite our data suggest that group 1 samples mainly originated from the early paleozoic supracrustal rocks containing metapelite and metavolcanic components which had previously experienced the surface weathering group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component synthesis of our data with available regional observations indicates that the inthanon zone represents the main suture zone of the eastern paleotethyan ocean in nw thailand and links with the changning menglian suture zone in sw yunnan sw china in nw thailand a switch from the eastward subduction of the paleotethyan oceanic plate to the collision of the sibumasu with indochina blocks occurred at 237 ma and syn and post collisional time being at 237 230 ma and 200 230 ma respectively the late triassic granites in the inthanon and sukhothai zones are representative of the post collisional magmatic products

nature of accretion related to paleo tethys subduction recorded in northern thailand constraints from melange kinematics and illite crystallinity abstract we reconstructed the accretion process related to paleo tethys subduction recorded in northern thailand based on melange and thrust structures and metamorphic temperatures derived from illite crystallinity data melange formation was characterized by hydrofracturing and cataclastic deformation with mud injection under semi lithified conditions followed by shear deformation and pressure solution illite crystallinity data suggest metamorphic temperatures below 250 c during melange formation the combined structural and metamorphic data indicate that during melange formation the accretionary complex related to paleo tethys subduction developed at shallow levels within an accretionary prism asymmetric shear fabrics in melange indicate top to south shear after correction for rotation associated with collision between the indian and eurasian continents the trend of the paleo tethys subduction zone is estimated to have been n80 e we conclude that the paleo tethys was subducted northward beneath the indochina block from the permian to triassic

p t t constraints on the development of the doi inthanon metamorphic core complex domain and implications for the evolution of the western gneiss belt northern thailand abstract the western gneiss belt in northern thailand is exposed within two overlapping cenozoic structural domains the extensional doi inthanon metamorphic core complex domain located west of the chiang mai basin and the mae ping strike slip fault domain located west of the tak batholith new p t estimates and u pb and 40ar 39ar age determinations from the doi inthanon domain show that the gneiss there records a complex multi stage history that can be represented by a clockwise p t t path u pb zircon and titanite dating of mylonitic calc silicate gneiss from the mae wang area of the complex indicates that the paragneissic sequence experienced high grade medium pressure metamorphism m1 in the late triassic early jurassic ca 210 ma in good agreement with previously determined zircon ages from the underlying core orthogneiss exposed on doi inthanon late cretaceous monazite ages of 84 and 72 ma reported previously from the core orthogneiss are attributed to a thermal overprint m2 to upper amphibolite facies in the sillimanite field u pb zircon and monazite dating of granitic mylonite from the doi suthep area of the complex provides an upper age limit of 40 ma late eocene for the early stage s of development of the actual core complex by initially ductile low angle extensional shearing under lower amphibolite facies conditions m3 accompanied by near isothermal diapiric rise and decompression melting 40ar 39ar laserprobe dating of muscovite from both doi suthep and doi inthanon provided miocene ages of ca 26 15 ma representing cooling through the ca 350 c isotherm and marking late stage development of the core complex by detachment faulting of the cover rocks and isostatic uplift of the sheared core zone and mantling gneisses in the footwall similarities in the thermochronology of high grade gneisses exposed in the core complex and shear zone domains in the western gneiss belt of northern thailand and also in northern vietnam laos yunnan and central myanmar suggest a complex regional response to indentation of southeast asia by india

cenozoic deformation and exhumation of the kampot fold belt and implications for south indochina tectonics latest mesozoic to earliest cenozoic deformation affected se asia s sundaland core the deformation event bridges the mesozoic se asian fusion with the cenozoic era of rifting translation basin formation and the creation of modern se asian oceans southern cambodia and vietnam are central to this shift but geological investigations of the region are in their infancy based on apatite and zircon fission track analyses afta and zfta stratigraphic and structural observations seismic data thermal maturity and igneous rock dating the geological evolution of southern cambodia and vietnam is investigated diverse depositional styles igneous activity structural deformation and subsurface unconformities testify to a highly variable phanerozoic tectonic setting major latest cretaceous to paleocene thrusting and uplift affected the kampot fold belt and surrounding regions and the associated up to 11 km exhumation probably exceeds earlier denudation events since at least permian time the present relief of the bokor mountains rising high above the kampot fold belt represents an artifact after differential erosion and only 2 5 4 5 km of erosion affected this area the latest cretaceous to paleocene orogenesis affected much of greater indochina probably owing to plate collision along eastern sundaland or a combination of collisions along both east and west sundaland afta and zfta data document protracted cooling of cretaceous granites and locally elevated thermal gradients persisting a few tens of million years after their emplacement the thermal gradient had stabilized by early miocene time and miocene cooling probably reflects a renewed denudation pulse driven by either regional tectonism or climate enhanced erosion

the granites of the southeast asian tin belt four granite provinces have been delineated each with its own distinctive pattern of mineralization 1 the main range province endogenous greisen bordered vein swarms of cassiterite and wolframite 2 the eastern province magnetite cassiterite skarns base metal sulphides with antimony in thailand 3 the western peninsular thailand burma province endogenous greisen bordered vein swarms and pegmatites of cassiterite and wolframite 4 the north thailand migmatitic province endogenous vein and skarn replacement scheelite and fluorite deposits with some tin and local antimony in all provinces but particularly in the main range granitoids designated as two phase variants have been recognized where xenocrysts and xenoliths of coarse primary texture granite are enclosed in and corroded by an invasive equigranular quartzo feldspathic matrix these rocks form an essential part of the granite sequence in all provinces and have probably resulted from the infiltration and disruption of the host granite by late stage magmatic fluids whole rock geochemistry from peninsular malaysia shows that the granites from the main range and eastern provinces comprise two contrasted suites which correspond approximately to the i and s type categories advocated by chappell white 1974 in addition it is shown that individual plutons within batholiths in the two provinces have distinctive geochemical parameters variation diagrams of plutons having the intrusive sequence primary texture granite two phase granite microgranite show linear trends with increasing sio 2 na 2 o rb w sn and u and decreasing sr ba th and all other major elements

petrochemistry u pb zircon age and palaeotectonic setting of the lampang volcanic belt northern thailand two ne trending belts of mainly subaerial dacitic to rhyolitic flows and tuffs occur in the area between the towns of lampang and denchai in northern thailand in the western belt doi ton the rocks have been pervasively altered to quartz keratophyre rocks in the eastern belt doi luang are generally less altered mobile chemical components such as na 2 o and k 2 o show wide variation particularly in samples from the doi ton belt however low zr tio 2 ratios and low nb and ta contents support an origin at a convergent plate margin a positive epsilon nd value of 4 9 for rhyolite from the doi luang belt supports derivation from a primitive crustal source a rhyolite sample from the doi luang belt yielded a u pb zircon age of 240 1 ma early mid triassic the doi ton and doi luang belts are part of the lampang volcanic belt which can be traced to the north into the lincang jinghong volcanic belt in southern china comparison with published petrological data from the lincang jinghong belt shows strong similarity including the widespread development of keratophyric mineralogy and chemistry the lampang lincang jinghong belt formed at an early mid triassic convergent plate margin and is similar in age and tectonic setting to the more mafic phetchabun volcanic belt on the east side of the nan river suture zone these data constrain the timing of final amalgamation between the indochina and shan thai terranes to mid triassic or younger

the volcaniclastic series from the luang prabang basin laos a witness of a triassic magmatic arc abstract the paleogeographic evolution of south east asia sea during the early mesozoic is still poorly understood and a number of models have recently been put forward to account for the geodynamic evolution of sea the luang prabang basin north laos located in the core of a paleogeographic jigsaw in sea recorded a long lasting volcanism that spanned for 35 my from the earliest triassic up to late triassic as evidenced by combined stratigraphic and geochronological u pb zircon analyses performed on both volcanic and volcaniclastic series the volcanic rocks are arc tholeiites and calk alkaline andesites to dacites the volcaniclastic rocks contain in part volcaniclasts produced contemporaneously with sedimentation both the volcanic and volcaniclastic series display geochemical features characteristic of a subduction related volcanism therefore the luang prabang basin documents a magmatic arc in a good agreement with the recent recognition of neighboring ophiolitic rocks in the luang prabang area following a passive margin setting that prevailed from the late carboniferous to the late permian an active margin then initiated along the western margin of the indochina block this active magmatic arc developed as the result of an east dipping subduction below the indochina block during most of the triassic at least from ca 250 to 215 ma subsequently this oceanic subduction episode must have been followed by a continental collision of the indochina block with the eastern simao block at a period that remains to be defined

the phuket terrane a late palaeozoic rift at the margin of sibumasu abstract it is widely accepted that sibumasu rifted from gondwana in the late palaeozoic but the rifts themselves have not previously been documented in southeast asia this paper identifies the pre middle permian kaeng krachan group of upper peninsular thailand as the infill of one such rift which is given the name phuket terrane indirect evidence suggests the rift infill is several kilometres thick and glacially influenced diamictites are conspicuous in the succession there are significant similarities with the 3 km thick pre middle permian rift infill of the carnarvon basin of western australia east of the khlong marui fault belt the succession is thinner and diamictites are a minor component a tectono stratigraphic model is proposed involving gondwana glaciers dropping their load at the present western margin of the phuket terrane from where it was re sedimented in the rapidly subsiding marine rift basin it is suggested that the khlong marui fault formed part of the eastern boundary of the rift system the three pagodas fault belt similarly juxtaposes different pre middle permian successions rifting ceased in the early permian and a passive margin formed as the mesotethys ocean widened the upper part of the kaeng krachan group and the overlying ratburi limestone representing the post rift sequence

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

the bentong raub suture zone abstract it is proposed that the bentong raub suture zone represents a segment of the main devonian to middle triassic palaeo tethys ocean and forms the boundary between the gondwana derived sibumasu and indochina terranes palaeo tethyan oceanic ribbon bedded cherts preserved in the suture zone range in age from middle devonian to middle permian and melange includes chert and limestone clasts that range in age from lower carboniferous to lower permian this indicates that the palaeo tethys opened in the devonian when indochina and other chinese blocks separated from gondwana and closed in the late triassic peninsular malaysia segment the suture zone is the result of northwards subduction of the palaeo tethys ocean beneath indochina in the late palaeozoic and the triassic collision of the sibumasu terrane with and the underthrusting of indochina tectonostratigraphic palaeobiogeographic and palaeomagnetic data indicate that the sibumasu terrane separated from gondwana in the late sakmarian and then drifted rapidly northwards during the permian triassic during the permian subduction phase the east malaya volcano plutonic arc with i type granitoids and intermediate to acidic volcanism was developed on the margin of indochina the main structural discontinuity in peninsular malaysia occurs between palaeozoic and triassic rocks and orogenic deformation appears to have been initiated in the upper permian to lower triassic when sibumasu began to collide with indochina during the early to middle triassic a type subduction and crustal thickening generated the main range syn to post orogenic granites which were emplaced in the late triassic early jurassic a foredeep basin developed on the depressed margin of sibumasu in front of the uplifted accretionary complex in which the semanggol formation rocks accumulated the suture zone is covered by a latest triassic jurassic and cretaceous mainly continental red bed overlap sequence

the lancang river zone of southwestern yunnan china a questionable location for the active continental margin of paleotethys abstract in an attempt to constrain a late paleozoic tectono metamorphic event along the lancang river zone fourteen samples were processed for k ar dating on fine mineral fractions and detrital muscovites from this zone in southwestern yunnan china the samples include mica schists mylonites and gneisses from the proterozoic lancang group and phyllites from the western part of the simao basin in addition one ar ar analysis was performed on separated phengites from a blueschist of the central part of the lancang group the results reveal a considerable spread of ages the tectonic evolution of the zone is constrained by the new data which accentuate two temporally separate but spatially overlapping events i a late carboniferous high p low t metamorphism related to an east vergent late paleozoic thrust belt inverting a devonian to carboniferous marginal basin of the yangtze platform and ii an upper permian and triassic low p high t belt caused by a post orogenic stage of rifting with distinct petrological and geochemical similarities of the igneous rocks to the emeishan large igneous province these results imply that no active continental margin accounts for the subduction of the paleotethys main branch proposed to be recorded either along the lancang river or the changning menglian belt

late devonian sedimentary record of the paleotethys ocean the mae sariang section northwestern thailand abstract an 11 m thick condensed sequence of late devonian limestones in northwestern thailand exhibits faunal associations and sedimentological microfacies data which are indicative of a pelagic facies setting the entire long ranging section is completely free from clastic input similar successions are known worldwide in a few sections only the mae sariang section is characterised by low sedimentation rates as recognised by a number of hardgrounds neptunian dikes and fe mn crusts the succession comprises a number of pelagic faunal elements e g conodonts cephalopods and pelagic ostracodes the fauna records rare macrofossils and the faunal diversity is low the very condensed section ranges from the late rhenana to praesulcata conodont biozones but contains some global events as undoubtedly shown by biostratigraphical and carbon isotope results including major kellwasser and hangenberg biotic crises in terms of plate tectonics this important succession most probably belongs to the inthanon zone comprising remnants of the paleotethys ocean

combined effects of eurasia sunda oblique convergence and east tibetan crustal flow on the active tectonics of burma it is widely accepted that deformation of the india sunda plate is the result of partitioned hyper oblique convergence presently sub meridian dextral strike slip faulting accommodates this india sunda motion in a buffer zone the burma platelet this wide dextral strike slip shear zone is complicated by the side effect of the tibet plateau collapse that can be described in term of crustal flow and gravity tectonics the loss of potential energy related to this plateau collapse affects most of the burmese platelet particularly in its northernmost part interaction of these two distinct geodynamic processes is recorded in the gps based regional strain field the analysis of seismic focal mechanism but also from direct geologic observations both onshore and offshore myanmar and bangladesh we propose the apparent e w shortening component of this so called partitioned hyper oblique subduction is only the effect of regional gravitational forces related to the tibet plateau collapse whereas the ns strike slip faulting accommodates the india sunda motion

metamorphism and tectonic evolution of the lancang paired metamorphic belts south western china the lancang metamorphic terrane consists of an eastern low p t belt and a western high p t belt divided by a n s trending fault protoliths of both units are mid late proterozoic basement and its cover the low p t belt includes the permian lincang batholith related amphibolite facies rocks of the damenglong and chongshan groups and permo triassic volcanic and volcaniclastic rocks most whole rock rb sr isochron and u pb zircon ages of the lincang batholith are in the range 290 279 and 254 212 ma respectively metamorphism of the low p t belt reaches upper amphibolite with local granulite facies 735 c at 5 kbar subsequently retrogressed at 450 500 c during post triassic time the high p t rocks grade from west to east from blueschist through transitional blueschist greenschist to epidote amphibolite facies estimated p t conditions follow the high p intermediate facies series up to about 550 600 c at which oligoclase is stable the 40ar 39ar plateau age of sodic amphibole in blueschist is 279 ma the paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south east asia since the latest precambrian tectonic juxtaposition of paired belts with contrasting p t conditions perhaps during collision of the baoshan block with south east asia suggests that an intervening oceanic zone existed that has been removed the baoshan block is a microcontinent rifted from the northern periphery of gondwana successive collision and amalgamation of microcontinents from either gondwana or the panthalassan ocean resulted in rapid southward continental growth of c 500 km during the last 200 ma hence the lancang region in south east asia represents a suture zone between two contrasting microcontinents

permo carboniferous sequences of gondwana affinity in southwest china and their paleogeographic implications abstract in southwest china permo carboniferous depositional sequences of gondwana affinity were first discovered in the himalayan region by the expedition team of acadimia sinica in 1975 by 1983 gondwana affinity permo carboniferous sequences had been identified also in the lhasa block the south qiangtang block in tibet and the tengchong and baoshan blocks in yunnan these sequences are characterised by the presence of successions which are interpreted as glacio marine deposits and by the presence of gondwana affinity biota the distribution of gondwana affinity permo carboniferous indicates that basins whose remnants are now represented by the yarlung zangbo suture and the bangong nujiang suture did not exist at that time and the vast area south of the lungmu shuanghu jinshajiang zone in tibet and west of the changning menglian belt in yunnan formed a part of the northern margin of gondwana

correlation of triassic stratigraphy between the simao and lampang phrae basins implications for the tectonopaleogeography of southeast asia abstract based on our extensive fieldwork in southwestern yunnan and northern thailand followed by detailed stratigraphic and paleontological studies we propose that the triassic simao basin in yunnan can be correlated with the triassic lampang phrae basin in thailand strata equivalent to those in the southern lancangjiang sub basin have not been identified in northern thailand we consider that during the triassic the simao and the lampang phrae basins belonged to the same tectonopaleogeographic unit the orogenic belt to the east of this unit includes the nan uttaradit and ailaoshan sutures the shan thai block in northern thailand can be divided from east to west into the sukhothai the inthanon and the shan terranes according to tectonopaleogeographic correlation our results support the idea that the sukhothai terrane including the lampang phrae basin belongs to the cathaysian domain and not to gondwana domain and that the geosuture corresponding to the changning menglian suture in yunnan must lie to the west of the sukhothai terrane in thailand

closure of the east paleotethyan ocean and amalgamation of the eastern cimmerian and southeast asia continental fragments abstract the phanerozoic record of southeast asia preserves the history of opening and subsequent consumption of the paleotethyan ocean and the assembly of continental fragments into asia as part of the broader scale reconstruction of pangea however uncertainty remains as to which of the many suture zones in southeast asia represents the relict of the main ocean when final ocean closure occurred and the assembly history of the eastern cimmerian and southeast asia continental fragments our overview of the geological features of the suture zones the bounding continental fragments and their magmatic metamorphic and sedimentary records resolves many of these key issues the sedimentary biogeographical structural lithological geochemical and geochronological data from the changning menglian inthanon and bentong raub suture zones argue for their linkage with the longmu co shuanghu suture zone in central tibet and together constitute the main east paleotethyan ocean relict the eastward subduction of the ocean resulted in the development of a series of magmatic arc back arc basin and continental fragments in southeast asia including from west to east the lincang sukhothai east malaya arc the jinghong nan sa kaeo back arc basin the simao west indochina fragment the luang prabang loei back arc basin the south indochina fragment the wusu and truong son back arc basins the north indochina fragment the jinshajiang ailaoshan song ma branch back arc basin and the south china block assembly of these fragments resulted in indosinian high temperature and high pressure metamorphism and related tectonothermal event available data indicate a switch from subduction of the main east paleotethyan ocean to the collision of the sibumasu with simao indochina blocks at 237 ma with subsequent syn and post collisional events at 237 230 ma and 230 200 ma respectively along the changning menglian inthanon and bentong raub suture zones the timing of initial syn and post collision events along the jinshajiang ailaoshan song ma suture zone with its record of back arc basin closure is at 247 ma 247 237 ma and 237 200 ma generally 10 ma older than that along the changning menglian inthanon and bentong raub suture zones our synthesis of all available data enables establishment of a comprehensive geodynamic model for the east paleotethyan evolution this model links the spatial temporal pattern across southeast asia into a series of tectonic events including ocean back arc basin opening subduction closure subsequent assemblage and orogenic collapse along with associated igneous metamorphic and sedimentary activities

some stratigraphic and sedimentologic constraints on the features of the changning menglian tethys yunnan china abstract the interpretation of the lower carboniferous basalt middle carboniferous to middle permian carbonate succession in the central zone of the changning menglian belt in western yunnan china as a seamount is one of the major arguments supporting a large ocean model of the changning menglian tethys our field investigations and laboratory work lead to the conclusion that the basalt carbonate succession of the changning menglian belt is not genetically related to seamount it is a normal lithological succession that developed on continental crust and it is underlain and overlain by marine carbonate and or clastic deposits

an alternative plate tectonic model for the palaeozoic early mesozoic palaeotethyan evolution of southeast asia northern thailand burma abstract an alternative model for the geodynamic evolution of southeast asia is proposed and inserted in a modern plate tectonic model the reconstruction methodology is based on dynamic plate boundaries constrained by data such as spreading rates and subduction velocities in this way it differs from classical continental drift models proposed so far the different interpretations about the location of the palaeotethys suture in thailand are revised the tertiary mae yuam fault is seen as the emplacement of the suture east of the suture we identify an indochina derived terrane for which we keep the name shan thai formerly used to identify the cimmerian block present in southeast asia now called sibumasu this nomenclatural choice was made on the basis of the geographic location of the terrane eastern shan states in burma and central thailand and in order not to introduce new confusing terminology the closure of the eastern palaeotethys is related to a southward subduction of the ocean that triggered the eastern neotethys to open as a back arc due to the presence of late carboniferous early permian arc magmatism in mergui burma and in the lhasa block south tibet and to the absence of arc magmatism of the same age east of the suture in order to explain the presence of carboniferous early permian and permo triassic volcanic arcs in cambodia upper triassic magmatism in eastern vietnam and lower permian middle permian arc volcanites in western sumatra we introduce the orang laut terranes concept these terranes were detached from indochina and south china during back arc opening of the poko song ma system due to the westward subduction of the palaeopacific this also explains the location of the cathaysian west sumatra block to the west of the cimmerian sibumasu block

late paleozoic faunal climatic and geographic changes in the baoshan block as a gondwana derived continental fragment in southwest china abstract the carboniferous and permian of the baoshan block consist of three major depositional sequences a lower carboniferous carbonate sequence a lower permian siliciclastic sequence and a middle permian carbonate sequence these three sequences were interrupted by two major regressive events first the namurian uplift ranging in age from serpukovian to gzhelian and second the post sakmarian regression occurring probably at artinskian time in the baoshan block although the precise time interval of the latter event is still unclear the baoshan block is characterized by warm water highly diverse and abundant faunas during the early carboniferous by cold water and low diversity faunas during the early permian and by possibly warm water but low diversity faunas during the middle permian the sweetognathus bucaramangus conodont fauna constrains the upper boundary of the diamictite bearing siliciclastic deposits dingjiazhai formation to the sakmarian to early artinskian as well as the eruption of the rifting basalts woniusi formation to at least the post early artinskian paleozoogeographically affiliation of the faunas in the baoshan block changed from eurasian in the early carboniferous to peri gondwanan in the early permian and to marginal cathaysian cimmerian in the middle permian cimmerian blocks have more or less comparable geohistory to one another in the carboniferous and permian during the middle permian the eastern cimmerian blocks such as sibumasu s s baoshan and tengchong are not far from the palaeoequator but apparently more distant than the western cimmerian blocks based on the presence or absence of some index taxa such as the fusulinaceans eopolydiexodina and neoschwagerina and the corals thomasiphyllum and wentzellophyllum persicum

basaltic activity preserved in an upper permian radiolarian chert from the paleo tethys in the inthanon zone northern thailand basaltic sandstone composed mainly of clay to sand sized fragments of glass and basaltic rock has been discovered in a pelagic chert sequence in northern thailand the sandstone conformably underlies and is partly intercalated with the upper permian wuchiapingian early changhsingian radiolarian bearing chert geological age dating based on radiolarians petrographic examination of the bedded chert with the basaltic sandstone and geochemical studies were conducted to investigate the origin and sedimentary mode of the lithologic sequence the bedded chert overlying the basaltic sandstone yielded late permian radiolarians and was correlatable with the wuchiapingian follicucullus charveti albaillella yamakitai assemblage zone as well as the early changhsingian neoalbaillella ornithoformis assemblage zone lithofacies and microfacies of the bedded chert indicated deposition in a pelagic realm of the paleo tethys although the tectonic setting was difficult to estimate based on geochemical features because of element mobility and alteration the basaltic fragments likely originated as oceanic basalt such as oceanic island basalt microfacies and sedimentary structures of the chert and sandstone indicated that the studied sequence is composed of intermittent high sedimentation rates mainly gravity currents of basaltic fragments in a background setting of very slowly accumulating radiolarian chert the sedimentary mode of the sequence the rare terrigenous influx and the presence of fusulinoidean or other foraminiferal tests indicate that the basaltic rocks fragment were originally sourced from an oceanic topographic high such as a seamount or an oceanic plateau and that the volcanic activity occurred in the chert accumulated oceanic basin of the paleo tethys in the wuchiapingian

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

discussion of tectonic models for cenozoic strike slip fault affected continental margins of mainland se asia abstract understanding the roles of cenozoic strike slip faults in se asia observed in outcrop onshore with their offshore continuation has produced a variety of structural models particularly pull apart vs oblique extension escape tectonics vs slab pull driven extension to explain their relationships to sedimentary basins key problems with interpreting the offshore significance of major strike slip faults are 1 reconciling conflicting palaeomagnetic data 2 discriminating extensional and oblique extensional fault geometries from strike slip geometries on 2d seismic reflection data and 3 estimating strike slip displacements from seismic reflection data focus on basic strike slip fault geometries such as restraining vs releasing bends and strongly splaying geometries approach the gulfs of thailand and tonkin suggest major strike slip faults probably do not extend far offshore splays covering areas 10 000 s km2 in extent are characteristic of the southern portions of the sagaing mae ping three pagodas and ailao shan red river faults and are indicative of major faults dying out the areas of the fault tips associated with faults of potentially 100 km displacement scale appropriately with global examples of strike slip faults on log log displacement vs tip area plots the fault geometries in the song hong yinggehai basin are inappropriate for a sinistral pull apart geometry and instead the southern fault strands of the ailao shan red river fault are interpreted to die out within the nw part of the song hong yinggehai basin hence the fault zone does not transfer displacement onto the south china seas spreading centre the strike slip faults are replaced by more extensional oblique extensional fault systems offshore to the south the sagaing fault is also superimposed on an older paleogene early miocene oblique extensional rift system the sagaing fault geometry is complex and one branch of the offshore fault zone transfers displacement onto the pliocene recent andaman spreading centre and links with the west andaman and related faults to form a very large pull apart basin

nd isotopic composition of late devonian seawater in western thailand geotectonic implications for the origin of the sibumasu terrane abstract conodonts collected from sections near the small towns of thong pha phum and mae sariang in the westernmost part of thailand are used to reconstruct the neodymium nd isotopic composition of seawater during the late devonian the study provides the first devonian seawater signatures recognized within the australian shelf of northeastern gondwana and the adjacent paleotethys ocean at thong pha phum site the seawater was characterized by very low e nd values from 13 1 to 18 2 and very high sm nd ratios between 0 36 and 0 66 in contrast the seawater at the mae sariang site was characterized by much more radiogenic signatures e nd values from 8 7 to 11 1 and uniform low sm nd ratios between 0 20 and 0 23 extremely low e nd values recognized at thong pha phum attest to a passive margin continental setting and a paleogeographic position very close to a continental area where paleoproterozoic and neoarchean rocks were eroded thus the isotopic data provide strong evidence that during late devonian time the sibumasu terrane was situated in the proximity to the archean cratons of western australia presumably adjacent to the carnarvon intracratonic basin moreover sibumasu may not have been situated in an outboard position on the shelf as previously suggested but could have been directly attached to the australian continental crust by contrast low and uniform sm nd ratios of seawater at mae sariang resemble those of the variscan and the present day oceanic seawaters therefore a pelagic setting within the paleotethys ocean is postulated for the mae sariang succession this conclusion is also constrained by minor temporal changes in e nd values and suggests that the paleozoic of mae sariang is not part of the sibumasu terrane but belongs to the inthanon zone

late permian and early to middle triassic radiolarians from the hat yai area southern peninsular thailand implications for the tectonic setting of the eastern margin of the sibumasu continental block and closure timing of the paleo tethys abstract the fine clastic and siliceous sedimentary successions distributed in the hat yai area southeastern peninsular thailand were examined using radiolarian biostratigraphy lithology and stratigraphy the studied succession was essentially divided into two kinds of fine grained sedimentary rock units the lower shale unit and the upper chert unit the lower shale unit and the upper chert unit yielded late middle to early late permian and early to middle triassic radiolarians respectively we clarified that the sedimentary rocks distributed in the study area are not entirely carboniferous but contain some sediments from the permian and triassic this age determination suggests that the clastic chert succession distributed in the study area should be distinguished from the carboniferous yaha formation and correlated with the permian to triassic semanggol formation lithological change with increasing siliceous composition was observed in the uppermost part of the lower shale unit and the geological age based on the radiolarians indicates a nearly conformable stratigraphic relationship for the lower shale unit and the upper chert unit the stratigraphy and lithology of the permian to triassic succession in the study area together with geological correlation around southernmost thailand and northern malaysia suggest that the triassic chert should be interpreted as continental slope sediments overlying permian clastic and or calcareous facies rather than typical pelagic deep water sediments formed on an abyssal plain the depositional environment of the chert in this area was likely restricted to the vicinity of a continental slope considering with the wide distribution of triassic platform carbonates over southeastern peninsular thailand the continental margin of the sibumasu along the paleo tethys was represented by a stable passive margin during the middle early late triassic time on the basis of the sedimentary setting and stratigraphy in the permian and triassic it is suggested that the closure of the paleo tethys between the sibumasu and indochina continental blocks took place at least after the middle triassic in southeastern peninsular thailand

structural styles and deformation fabrics of accretionary complexes intricately deformed stratally disrupted and mixed rocks produced at subduction zones have been considered structurally intractable however marine geophysical studies deep sea drilling analyses of subaerially exposed complexes and theoretical advances have recently produced breakthroughs in elucidating the structural enigmas of these accretionary complexes the penrose conference structural styles and deformational fabrics of accretionary complexes assembled specialists from the above and related research areas and addressed the research frontiers of this dynamic topic this conference in eureka california in may 1984 drew participants from great britain france canada japan and new zealand

preliminary geochemical study of volcanic rocks in the pang mayao area phrao chiang mai northern thailand tectonic setting of formation abstract the least altered permian mafic volcanic rocks from the pang mayao area phrao district chiang mai province part of chiang rai chiang mai volcanic belt have been analyzed and are found to be mid ocean ridge and ocean island basalts the mid ocean ridge basalts occur as lava flows or dike rocks they are equigranular fine to medium grained and consist largely of plagioclase clinopyroxene and olivine these basalt samples are tholeiitic and have compositions very similar to t morb from the region where the du toit fracture zone intersects the southwest indian ridge the ocean island basalt occurs as pillow breccia and lava flows or dike rocks they are slightly to moderately porphyritic with phenocrysts microphenocrysts of clinopyroxene olivine plagioclase and or fe ti oxide the groundmass is very fine grained and made up largely of felty plagioclase laths with subordinate clinopyroxene these basalt samples are alkalic and chemically analogous to those from haleakala volcano maui hawaiian chain these mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back arc basin

zircon and monazite upb dating of the doi inthanon core complex northern thailand implications for extension within the indosinian orogen abstract the doi inthanon metamorphic core complex occupies a prominent central position within the northwestern thai gneiss belt which has been inferred to be as old as precambrian and as young as permo triassic u pb dating of zircon from core orthogneisses of the complex suggests that the orthogneisses were derived from granitic protoliths with essentially the same late triassic early jurassic age as the mae cham pluton 205 3 4 ma to the west of the doi inthanon complex monazite ages from the orthogneisses indicate that low pressure and high temperature metamorphism and migmatitization occurred in the late cretaceous 84 72 ma limited published radiometric age data from elsewhere in the migmatitic gneiss belt suggest that widespread minor plutonism accompanied this thermal event a younger u pb data of 26 8 0 5 ma for zircon and monazite from the mae klang granite which intruded mylonitic gneisses on the eastern side of the doi inthanon complex indicates that there was also significant plutonism in the late oligocene these u pb ages constrain the development of the complex and hence of the migmatitic gneiss belt as a whole to an interval between late cretaceous and early miocene ductile shearing uplift and tectonic denudation via detachment faulting chronologically overlap the initiation of extensional basin development to the east and southeast of the gneiss belt and imply a genetic connection thus oligocene early miocene thermal rifting from the gulf of thailand northward into mainland thailand may have been the direct product of crustal extension rather than the transtensional product of regional strike slip faulting

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

nested strike slip duplexes and other evidence for late cretaceous palaeogene transpressional tectonics before and during india eurasia collision in thailand myanmar and malaysia the late cretaceous palaeogene structural and tectonic events in the northern region of sundaland represent a poorly known but potentially important orogenic development in se asia extensive late cretaceous s type granites extending from malaysia through thailand to myanmar have long been used to infer an episode of crustal thickening an inference supported by the presence of late cretaceous eocene ophiolites in myanmar however structural evidence for a well developed associated fold and thrust belt has been lacking apatite and zircon fission track studies of thailand indicate a broad regional uplift from about 80 ma to 40 ma suggesting an episode of crustal thickening but the uplift amount is modest and not that expected for a major fold and thrust belt left lateral motion on major nw se trending strike slip fault zones mae ping and three pagodas faults in myanmar and thailand has been attributed to himalayan tibetan escape tectonics this paper demonstrates that the fault zones are considerably more complicated than just nw se trends they are a network of branching faults with important north south trends as well as nw se trends they form large scale nested strike slip duplex structures and affect a region over 1000 km long and up to 250 km wide this diffuse branching network of strike slip fault trends may represent a substantial late cretaceous palaeogene transpressional belt himalayan escape tectonics represent later increments of deformation with both oligocene sinistral and late oligocene recent dextral reactivation

permian of myanmar abstract permo triassic carbonates with minor basal clastics cover extensive tracts of the shan plateau in the eastern part of myanmar these carbonates are grouped into the permian thitsipin limestone formation and the late permian early triassic nwabangyi dolomite formation the thitsipin formation occurs as isolated masses in many places faulted against older rocks it is now known that there is a regional unconformity between the thitsipin limestone formation and the underlying silici carbonates of devonian age in shan state and between the thitsipin limestone formation and carboniferous meta siliciclastics in kayah state mon state kayin state and tanintharyi division some of the thitsipin limestone is dolomitized and passes transitionally upward into the nwabangyi dolomite formation permian shelf carbonates occur within two different sequences the silici carbonate sequence of the shan thai block and the meta siliciclastic sequence of the mergui terrane suggesting that the permian limestones were deposited after the emplacement of the mergui terrane onto the southern margin of the shan thai block this event occurred when the shan thai block was in the southern hemisphere