a 5 my chronology of carbonate platform margin aggradation southwestern little bahama bank bahamas abstract results of integrated age dating are reported from two core borings 60 m and 61 m on the southwestern margin of little bahama bank lbb on the outer margin core gb 1 seven chronostratigraphic units some unconformity bounded are correlated with of the inner margin core gb 2 about 11 km away to the north the southern margin of lbb shows a general shallowing upward succession that records three distinct intervals of differing sediment accumulation this outer margin core progresses from a relatively deep water 30 m margin at 5 6 ma to shallower reef associated sands about 5 ma and reefal conditions 5 3 ma followed by peloidal sands 3 2 ma and subaerial exposure around 2 1 ma and finally intermittent quaternary nonskeletal sedimentation 1 ma to holocene the facies indicate a change from a windward open type margin 30 m water depth to a windward protected type margin reef island this change in the platform margin may have been in response to reef accretion along the margin the long lived reefal or near reef facies in the outer margin core suggests that this margin has been predominantly aggradational since at least the earliest pliocene in contrast to the rapidly prograding margins on other bahamian platforms during the same time period however the general facies succession from transgressive highstand early pliocene to lowstand mid pliocene early pleistocene to intermittent highstand mid late pleistocene is consistent with dated successions from other platforms the three sediment accumulation intervals and their respective facies likely representing third order scale sea level fluctuations are also consistent with changes in both the proposed eustatic curve and the oxygen isotope curve

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

geochemical and sr nd isotopic characteristics of granitic rocks from northern vietnam abstract five major felsic igneous suites from northern vietnam with ages from mid proterozoic to early cenozoic were studied representative granitic rocks from the posen complex mid proterozoic and the dienbien complex late permian to early triassic show geochemical characteristics similar to those of calc alkaline to high k calc alkaline i type granites however the former located in the south china block has significantly higher initial nd isotopic ratios end t 0 7 to 1 5 and older nd isotopic model ages tdm 1 7 ga than the latter end t 4 7 to 9 7 tdm 1 3 1 5 ga which were emplaced south of the song ma suture and thus in the indochina block the generation of both complexes may be attributed to subduction related processes that occurred in two distinct crustal provenances with different degrees of mantle inputs on the other hand jurassic to cretaceous granitic rocks from the phusaphin complex contemporaneous rhyolites from the tule basin and late paleogene granitic rocks from the yeyensun complex all exposed in the south china block between the ailao shan red river shear zone and the song ma suture display geochemical features similar to those of a type granites with intermediate end t values 0 6 to 2 8 and younger tdm ages 0 6 1 1 ga these magmas are suggested to have been generated as a consequence of intraplate extension in the western part of the south china block yunnan and to have been transported to their present position by mid tertiary continental extrusion along the ailao shan red river shear zone related to the india asia collision overall the isotopic and model age data reported in this study indicate that in northern vietnam the most important crust formation episode took place in the proterozoic likewise repeated mantle inputs have played a role in the petrogenesis of phanerozoic granitic rocks

quaternary sediments in the dien bien phu fault zone nw vietnam a record of young tectonic processes in the light of osl sar dating results abstract the dien bien phu fault zone dbp orientated nne to n is one of the most seismically active zones in indochina in nw vietnam this zone is 160 km long and 6 10 km wide cutting sedimentary and metamorphic rocks of the late proterozoic palaeozoic and mesozoic age as well as palaeozoic and late triassic granitoids along the dbp relatively small narrow pull apart basins occur the three largest of which chan nua lai chau and dien bien phu have been studied in detail all of them are bounded by sinistral and sinistral normal faults responsible for offset and deflected drainage presence of numerous shutter ridges and displaced terraces and alluvial fans the normal component of motion is testified to by well preserved triangular facets on fault scarps highly elevated straths in river watergaps overhanging tributary valleys as well as high and uneven river bed gradients our observations indicate a minimum recent sinistral offset ranging from 6 8 to 150 m for holocene valleys to 1 2 9 75 km for middle late pleistocene valleys in different fault segments the thickness of quaternary sediments varies from 5 25 m in the lai chau area to some 130 m in the dien bien phu basin in the lai chau basin the middle terrace 23 m alluvia of nam na river at muong te bridge have been optically stimulated luminescence single aliquot regenerative dose technique osl sar dated at 23 40 to 13 ka these sediments were normal faulted by some 11 m after 13 ka and mantled by vari coloured slope loams 8 12 m thick containing colluvial wedges composed of angular debris these wedges were probably formed due to at least three palaeoseismic events postdating 6 ka in the dien bien phu basin in turn alluvium of the upper holocene terraces has been osl sar dated to 6 5 7 and 1 7 1 0 ka whereas the younger sub recent terrace sediments give ages of 0 5 0 2 ka displaced terraces and alluvial fans allow us to suppose that the sinistral and sinistral normal faults bounding narrow pull apart basins in the southern portion of the dbp fault reveal minimum rates of left lateral strike slip ranging from 0 6 to 2 mm year in holocene and 0 5 3 8 mm year in pleistocene times whereas rates of holocene uplift tend to attain 1 mm year north of lai chau and 0 4 0 6 mm year west of dien bien phu more precise estimations however are difficult to obtain due to poor age control of the displaced drainage rates of quaternary strike slip are comparable with those of the red river fault the sense of movement being however opposite taking into account the presence of two phases of late cenozoic strike slip of contrasting sense of motion as well as the geometry of the two fault zones we hypothesize that the red river and dien bien phu faults are conjugate faults capable of generating relatively strong earthquakes in the future

ar ar ages in phlogopites from marble hosted ruby deposits in northern vietnam evidence for cenozoic ruby formation abstract ruby growth in phlogopite bearing marbles has been indirectly dated using the 40 ar 39 ar laser stepwise heating technique on purified syngenetic phlogopite and other micas from ruby deposits in yen bai luc yen and quy chau mining districts in northern vietnam the principal results indicate the following 1 across the red river shear zone the phlogopites from the yen bai deposits yielded miocene cooling ages between 23 2 and 24 4 ma identical to those previously published using the same dating method on magmatic and metamorphic rocks from the day nui con voi range 2 luc yen ruby deposits in the lo gam zone on the eastern flank of the red river shear zone yielded oligocene cooling 40 ar 39 ar mica ages between 30 8 and 34 0 ma regarding the age of ruby crystallisation itself the most plausible hypothesis is that all rubies in both zones formed during the period 40 to 35 ma diachronism of cooling in adjacent zones leads to the conclusion that around 35 ma the ductile deformation in the lo gam zone ended and the ruby bearing marbles cooled rapidly while the high temperature deformation remained in the red river shear zone resulting in cooling through blocking temperature some 15 ma later 3 the quy chau ruby deposit is restricted to the quy chau shear zone that bounds the eastern part of the oligocene miocene bu khang dome phlogopite and biotite samples reveal miocene cooling ages between 21 and 22 5 ma which are minimum ages for ruby formation these ages could be linked with the end of the extension of the bu khang dome vietnamese ruby formation is linked to cenozoic tectonics resulting from continental collision between the asian and eurasian plates as for other marble hosted ruby deposits in central and southeast asia

north directed triassic nappes in northeastern vietnam east bac bo abstract a careful re examination of the previous tectonic descriptions and the acquisition of new structural and kinematic data have been undertaken in northeastern vietnam the structure of the area consists of a system of slightly metamorphosed but ductilely deformed nappes including recumbent folds formed during the triassic prior to the unconformable deposition of the upper triassic terrigenous sediments these results confirm the previous interpretation of preyunnanaises nappes represented by middle upper paleozoic foliated limestone resting through a flat mylonitic contact over an intermediate song mien unit deprat 1915 the latter made essentially of deformed lower triassic sedimentary and volcanic formations which previously filled the song hien rift bourret 1922a bourret 1922b in the external part of the belt the triassic strata conformably overlie slightly deformed and unmetamorphosed upper paleozoic rocks forming an autochthonous domain alternatively the same triassic succession can be seen resting tectonically through decollement zones directly over the middle paleozoic marbles to form a distinct but subsidiary allochthonous unit our data show that the song chay orthogneiss and its lower paleozoic sedimentary country rocks belong to the main nappe they also demonstrate that this nappe was transported to the n ne a newly discovered ophiolitic melange of supposed oceanic nature and triassic age lying along the song chay fault north of the nui con voi could hypothetically represent the rooted zone of the nappe a tertiary overprinting event in particular accommodated by strike slip movements likely accounts for the present orocline of ne vietnam

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

the structural setting of the song ma region vietnam and the indochina south china plate boundary problem abstract the song ma song da region of northern vietnam contains the song ma anticlinorium a polydeformed polymetamorphosed early palaeozoic island arc forearc terrane accreted to the south china plate in siluro devonian times the song ma anticlinorium is not an indosinian subduction zone and nor is the post triassic post cretaceous song ma fault the song ma fault is one of many northwest trending post cretaceous high angle reverse oblique slip faults and thrusts responsible for shortening and strike slip transposition of northern vietnam folds penecontemporaneous with this faulting include the song ma anticlinorium which is formed probably of thrust ramp folds the faulting and penecontemporaneous folding developed during oligocene sinistral strike slip on the song hong fault and was reactivated during pliocene quaternary times we conclude that the present plate boundary is a broad deformation zone between da nang and the song hong and that parts of the south china plate extend well to the south of the song hong fault

evidence for mesozoic shear along the western kunlun and altyn tagh fault northern tibet china the strike slip faults of north tibet accommodate part of the cenozoic convergence between india and asia along the karakax valley south of yecheng and near the xidatan trough south of golmud the active traces of the altyn tagh and kunlun faults follow narrow belts of metamorphic rocks the deformation recorded in those mylonites is sinistral strike slip rb sr and 40ar 39ar ages of deformation from syntectonic fabrics formed at 350 400 c 120 ma argon loss suggests that deformation was associated to a 250 300 c thermal pulse that lasted 5 20 ma after the onset of movement unroofing occurred much later around 25 ma when sudden cooling suggests a component of thrusting or more likely normal faulting the cretaceous shear may be related to collision between the qiantang and the lhasa blocks the karakax and xidatan shear zones may have formed a unique continuous boundary in the cretaceous which was later reused by the tertiary strike slip faults leading to potentially calculable offsets along the altyn tagh fault

emplacement and cooling of the dien bien phu granitic complex implications for the tectonic evolution of the dien bien phu fault truong son belt nw vietnam the dien bien phu dbp fault strikes nearly perpendicular to the general tectonic framework separating northwest vietnam into two compartments the dbp and muong lay granites are exposed respectively east and west of the fault the u pb age of 277 2 ma obtained for the muong lay granite is similar to the age of the chieng khuong complex exposed within the song ma structure east of the dbp fault these two magmatic intrusions are considered as belonging to a same tectonic structure related to the subduction of the song ma ocean under the indochina block during the indosinian orogeny the dbp fault with a total post permian dextral offset of 30 40 km shifts the song ma suture it could then be connected to the ailao shan shuanggou suture to the north the undeformed dbp granite gave a u pb age of 230 1 ma sealing the indosinian deformation on the fault the ladinian carnian activity of the fault is confirmed by the 40ar 39ar biotite age of 228 4 ma obtained from the paragneiss forming the fault zone a geodynamic model is proposed to describe the various tectonic phases recorded by on the dbp fault it associates field data stratigraphic observations and geochronology analyses u pb ar ar fission tracks following the late triassic exhumation of the granite and the end of the indosinian orogeny marked by the norian unconformity the granite was buried under jurassic and lower cretaceous sediments a strong cooling event during the early late cretaceous 100 ma is associated with the reactivation of the dbp fault and the accommodation of a southward thrusting of the song da terrane onto the truong son belt this newly recognised tectonic phase could be associated to the subduction of the paleo pacific plate under the indochina block no data are available yet to constrain the tertiary evolution of the dbp system however the tertiary deformation is likely similar to the cretaceous one

40ar 39ar geochronology of the charnockites and granulites of the kan nack complex kon tum massif vietnam abstract the truong son belt forms the eastern rim of the indochina block in southeast asia the age of the metamorphism mainly along nw se mylonitic shear zones that affects this belt has been formerly determined at about 240 250 ma this age corresponds to the indosinian tectonometamorphic episode the kon tum massif situated to the south of this belt comprises high temperature rocks the kan nack complex including charnockites and granulites the main charnockitic outcrops restricted to the song ba valley establish the intrusive nature of these magmatic rocks within granulite facies material basic charnockitic rocks are mainly quartz enderbites to norites and hornblende pyroxene granulite facies rocks the 40 ar 39 ar age of intrusion cooling of charnockitic magmas is determined from primary magmatic biotites at about 245 ma in the east of the kan nack complex some granulite facies rocks exhibit relicts of primary granulite facies parageneses whereas others show evidence of overprinting by a retrogressive low grade metamorphism ar ar dating confirm this evolution giving ages of 400 ma for primary relict granulite facies phases and 260 270 ma from the most retrogressed samples establishing the youngest limit for the granulite facies metamorphism granulites intruded by charnockites in the song ba valley yield ages of about 250 ma equivalent to the ages of the charnockites and have evidently been completely reset by these high temperature intrusions therefore the kan nack complex of the kon tum massif is not an independent unit with respect to the indosinian orogen but represents the deep crustal part of this belt

indosinian nw trending shear zones within the truong son belt vietnam 40ar39ar triassic ages and cretaceous to cenozoic overprints abstract several nw trending ductile and partly mylonitic shear zones cross the truong son belt of central vietnam along the song ma and song ca valleys and north of the kontum block from da nang to aluoi and khe sanh high grade metamorphic rocks of amphibolite facies showing a retrograde evolution and consisting of ortho and paragneisses metavolcanics amphibolites marbles and quartzites are exposed along these structures they display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near horizontal to gently plunging mineral stretching lineation indicating a strike slip tectonic regime of deformation along the southernmost fault zone at least various and consistent kinematic indicators including s c structures asymmetric tails of porphyroclasts prove that the strain and metamorphism have been generated by a phase of dominantly non coaxial deformation with a dextral sense of shear 40ar 39ar dating applied on the high grade metamorphic rocks minerals occurring along these zones provide plateau cooling ages closely around 245 ma establishing that this event took place in the lowermost triassic as an early phase of the indosinian orogeny the existence of indosinian movements in vietnam as they have been defined by previous authors during the early century is now accurately confirmed and this is the first insight in the occurrence of ductile strike slip tectonics of indosinian age along nw se fault zones well expressed in central vietnam is a cretaceous thermal and deformational overprint marked by epimetamorphism which took place between 90 and 120 ma as attested by low temperature degassing ages this cretaceous event is not found further north in the song ma zone where younger ages appear as a result of the influence of cenozoic shear movements along the red river fault which displays 20 30 ma ages on the northern flank of song ca in the bu khang phu hoat core complex comparable oligocene lower miocene ages 20 35 ma yielded by biotite and phlogopite reflect a strong overprinting and attest to a rapid uplift of the basement

crustal contamination of late neogene basalts in the dien bien phu basin nw vietnam some insights from petrological and geochronological studies early pliocene zanclean basalts in the dien bien phu pull apart basin in nw vietnam associated with the presently sinistral dien bien phu fault zone have been dated by the k ar method at 4 4 4 9 and 5 4 5 2 ma rapid migration of basaltic magma to the surface in the dien bien phu fault zone may be due to pliocene transtension of the crust in this region resulting from asthenospheric upwelling induced by lateral displacement of the mantle the basalts are moderately phyric 10 and consist of olivine hyalosiderite plagioclase bytownite labradorite and orthopyroxene bytownite labradorite phenocrysts and a fine grained crystalline matrix olivine hortonolite plagioclase labradorite clinopyroxene pigeonite and augite k feldspar the presence of fe rich olivine and orthopyroxene phenocrysts indicates that the basalts are sio2 saturated oversaturated olivine tholeiites which formed under water undersaturated conditions the dien bien phu basalts contain both mantle derived pyroxenites dunites gabbros and crustal sillimanite mullite mg fe spinel wallrock xenoliths indicative of crustal contamination during the ascent of the basaltic magma the basalts show selective enrichment in some mobile elements k rb sr and th a feature considered to be a result of metasomatism these rocks classified on the basis of their normative composition as quartz tholeiites could represent primary olivine tholeiites basalts in which the geochemical signatures were modified by the processes of contamination 2006 elsevier ltd all rights reserved

u pb dating of high temperature metamorphic episodes in the kon tum massif vietnam abstract in vietnam recent studies demonstrated that the kon tum massif was part of the indosinian mountain system together with the truong son belt one sector of the kon tum massif song ba valley is a granulite facies terrain for which ar ar and u pb ages cluster around 245 250 ma in the area of song bien river also metamorphosed under granulite facies conditions ar ar ages of 410 ma are displayed by biotite in this study u pb dating of monazite and zircon has been used to understand the reason for these two groups of ages taking into account the metamorphic mineral assemblages related to granulite facies metamorphism in the two districts for granulites of song ba valley ages of 245 250 ma are confirmed from monazites in the song bien district ages of 465 470 ma displayed by monazites are interpreted to correspond to the metamorphic climax zircons from one of these samples record the age of anatexis that has affected the granulitic rocks the younger age recorded in the song ba valley is considered to reflect high temperature related to charnockitic intrusions emplaced near the end of the indosinian orogeny overprinting the earlier formed granulites and resetting the u pb and ar ar ages for the first time two ht events are dated in the kon tum massif one occurring during the ordovician and the other during the permo triassic

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

the song ma anticlinorium northern vietnam the structure of an allochthonous terrane containing an early palaeozoic island arc sequence abstract the song ma anticlinorium displays evidence for four deformations the first two accompanied by high grade to greenschist facies metamorphism and the third by production of syn low grade metamorphic sheath folds the fourth occurred during post cretaceous folding and thrusting which produced a thrust fold complex across northern vietnam whilst the song ma anticlinorium may contain a metamorphosed island arc volcano sedimentary sequence this palaeozoic sequence cannot represent the active plate boundary in triassic cenozoic or recent times the song ma anticlinorium is not an indosinian subduction zone but is an allochthonous terrane which accreted to the south china plate in siluro devonian times

geology and geochemistry of the shuanggou ophiolite ailao shan ophiolitic belt yunnan province sw china evidence for a slow spreading oceanic basin origin abstract the early carboniferous shuanggou ophiolite lies in the middle segment of the ailao shan orogenic belt between the south china block to the north and the indochina block to the south the ophiolite consists of meta peridotite gabbro diabase and basalt capped by radiolarian bearing siliceous rocks no layered gabbros or sheeted dikes have been observed the meta peridotite underwent low degrees of partial melting consistent with the low magma budget of this oceanic lithosphere whole rock rare earth element analyses of gabbro indicate a geochemical affinity with normal mid ocean ridge basalts consistent with the crystallization order of plagioclase followed by clinopyroxene recognized in the gabbros the ophiolite is believed to have formed in a small slow spreading oceanic basin collision of the indochina block with the south china block in the late paleozoic was responsible for the closure of the oceanic basin and emplacement of the ophiolite in the ailao shan orogenic belt

la icpms zircons u pb dating of permo triassic and cretaceous magmatisms in northern vietnam geodynamical implications abstract in northeastern vietnam the major tectonic episode responsible for nappes emplacement is triassic these allochtonous structures are intruded by granitic melts two post tectonic massifs showing no sign of deformation have been dated by the la icpms zircon u pb techniques dating reveals a multiphase history with zircon cores showing evidence of proterozoic magmatism the emplacement of the phia bioc granite intrusive in allochtonous units is 248 245 ma an age which assesses a younger limit for the major nappes tectonic this tectonic could be synchronous of the tectonometamorphic strike slip faulting events 250 245 ma defined in the truong son belt as the indosinian orogen the phia bioc intrusion is probably linked with the intra plate magmatism of the emeishan large igneous province or with magmatism associated with the paleotethys closure the age of the phia oac granite intrusion in displaced units is much younger at 87 3 1 2 ma this granite is probably linked to the magmatic activity produced during the paleo pacific plate subduction under the se asia continental plate during the mesozoic although the cenozoic red river fault system is close to these two plutons this last thermotectonic episode has not been strong enough to disturb the u pb system zircons rims do not show any tertiary magmatic or metamorphic overprint

the changning menglian suture zone a segment of the major cathaysian gondwana divide in southeast asia abstract in southwest china the major cathaysian gondwana divide the palaeo tethyan suture is very well delineated by a narrow north south zone of oceanic siliceous sedimentary rocks and dismembered ophiolite complexes including probable remains of reef capped oceanic islands the location of this zone the changning menglian suture is not well appreciated in the english language literature probably because of the failure to recognise the approximately 600 400 km simao terrane which lies to the east of the suture many analyses mistakenly include the simao element in the sibumasu terrane thus placing a terrane of distinct cathaysian affinities on the gondwana side of the major palaeogeographic divide a measure of the size and time span of palaeo tethys is contained in the changning menglian suture zone sedimentary rocks deep marine cherts are dated between early devonian and middle permian and limestones interpreted as caps to seamounts extend the age range of the ocean to late permian these data agree with recent views of cathaysian elements rifting from gondwana in the silurian or devonian followed by a major rifting event on the north gondwana margin early in the permian subduction created an active continental margin on the western edge of the simao terrane throughout much of the triassic and the closure of this branch of palaeo tethys is marked by the cessation of this activity in the early late triassic

discussion on the role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia journal vol 163 2006 1025 1036 p h leloup p tapponnier r lacassin write in his recent paper searle 2006 acknowledges that the 1000 km long ailao shan red river shear zone is a large miocene left lateral shear zone but speculates that left lateral slip started after 21 ma and claims that the total finite offset remains unknown from this he concludes that continental extrusion was only a relatively minor tectonic factor during the india asia collision as long argued by other workers e g england houseman 1986 cobbold davy 1988 dewey et al 1989 houseman england 1993 we summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts timing of left lateral shear along the ailao shan red river shear zone the ailao shan red river shear zone is composed mostly of high grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left lateral shear parallel to the belt e g tapponnier et al 1986 1990 leloup et al 1993 1995 2001 the crystallization of the granitoids has been dated between 22 and 35 ma fig 1d f for example by scharer et al 1990 1994 and zhang scharer 1999 leading those workers to propose that left lateral shear started at least c 35 ma ago in contrast searle 2006 claims that all the deformed granitoids found within the shear zone predate left lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation rather than a lower limit thus suggesting a maximum age of 21 ma for this deformation a clear understanding of the p t history and in situ deformation history of the intrusions as well as of their relationships with surrounding paragneisses is fundamental for interpreting correctly the geochronological

oligocene miocene bu khang extensional gneiss dome in vietnam geodynamic implications a large oligocene miocene extensional gneiss dome in vietnam has implications for the kinematics of the ailao shan red river shear zone and rifting of the south china sea a major northeast dipping extensional shear zone separates the bu khang dome from less metamorphosed units above shearing deformation occurred during the exhumation of high pressure rocks 40 ar 39 ar ages on micas reveal oligocene miocene ages from 36 to 21 ma after a high pressure episode of probable oligocene age the area was subjected to northeast southwest extension during the opening of the south china sea a kinematic model involving left lateral shear and block rotation within a north south dextral shear zone integrates our observations in the bu khang dome with south china sea rifting

u th pb geochronology of zircon and monazite from syenite and pincinite xenoliths in pliocene alkali basalts of the intra carpathian back arc basin abstract phreatomagmatic eruptions in the lucenec basin in slovakia ejected fragments of igneous rocks of unknown age and ranging in composition from gabbro to syenite and orthopyroxene granodiorite pincinite two kinds of syenites with different fe3 contents composed of anorthoclase and interstitial metaluminous trachyte to alkali rhyolite glass represent the final products of alkali basalt differentiation at depths of 10 and 15 18 km pincinite composed of plagioclase quartz orthopyroxene ilmenite and interstitial peraluminous dacite glass is a quenched supersolidus anatectic melt formed at the depth of 17 20 km by high temperature dehydration melting of biotite bearing gneissic protolith zircon from pincinite and syenite and monazite from syenite were dated using excimer laser ablation icp ms the obtained ages cluster at 5 8 0 1 and 5 4 0 1 ma thus corroborating the genetic link between the two rock types and constraining the residence time of at least 300 100 ka of the parental basalt in the deep crustal magma storage zones the inferred ages overlap the upper miocene pliocene boundary and are considerably younger than spatially associated basaltic lava flows dated using the k ar method consequently the fluviolacustrine poltar formation overlying the maar structure must have sedimented during the pliocene and not during the upper miocene pontian as previously assumed

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

a precise 40ar 39ar reference age for the durango apatite u th he and fission track dating standard the distinctive durango apatite is widely distributed as a reference material for mineralogy geochemistry and thermochronology the apatite occurs within the cerro de mercado open pit iron mine located in the northern portion of durango city mexico it is largely confined to vugs and open fractures within the lower portion of the ore deposit apatite and iron ore formed in situ and concurrently along with emplacement of small felsic intrusions along the southern margin of the chupaderos caldera complex this activity occurred between eruptions of two major ignimbrites from the caldera complex four single crystal sanidine anorthoclase 40 ar 39 ar ages from these ignimbrites and from a conformable older unit have been used to compute a reference age of 31 44 0 18 ma 2 for the apatite itself four additional multi crystal measurements are in general agreement with this age also in agreement are a set of 24 u th sm he ages recently determined directly on the apatite which have a mean of 31 02 1 01 ma 1 with a standard error s e of 0 22 ma

improved measurement of fission track annealing in apatite using c axis projection apatite fission track length data are used extensively for thermal history inversion however several studies have documented instances of poor reproducibility of length data we address this problem by using c axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by barbarand et al 2003a 2003b a new simplification reduces the c axis projection model from six to four fitted parameters normalizing for track angle using c axis projection improves every aspect of length measurement reproducibility examined it accelerates convergence of mean length in single analyses increases consistency among replicate measurements by a single analyst enhances consistency of measurements of the same mounts by different analysts and improves the match between analyses conducted with and without cf irradiation c axis projection is also shown to enhance the thermal sensitivity of length data based on these results we assert that c axis projection is a good means of compensating for observer bias although it does not overcome differences caused by experimental error

last glacial cycle hydrological change at lake tyrrell southeast australia abstract lake tyrrell is the largest playa in the murray basin of southeast australia optical dating of transverse dune lunette sediments extends the lake s radiocarbon chronology to the last interglacial period the highest lake level was attained 131 000 10 000 yr ago forming lake chillingollah a megalake that persisted until around 77 000 4000 yr ago pedogenesis of its sandy lunette continued until buried by a silty clay lunette deflated from the lake floor 27 000 2000 yr ago the dated soil stratigraphic units correlate with the upper tyrrell beds and contain evidence that humans visited the lakeshore before 27 000 yr ago the lake chillingollah megalake was synchronous with very high lake levels in monsoon dominated australia yet it was not influenced by tropical monsoon systems it was filled instead by increased winter rainfall from westerly low pressure fronts greater effective precipitation across australia is evident the result of a weakened subtropical high pressure zone

jurassic dextral movement along the dien bien phu fault nw vietnam constraints from 40ar 39ar geochronology abstractthe nne ssw trending dien bien phu fault which dextrally displaces the nw se trending song ma suture in northwestern vietnam is widely considered one of the most seismically active faults in indochina in order to better understand the fault s activity this study reports new 40ar 39ar geochronological data for mica schists and mylonites from several areas along the dien bien phu fault showing 40ar 39ar ages of 194 212 ma for mica schists and 158 198 ma for mylonites these ages suggest that after the collision of the south china and indochina blocks dextral shearing deformation initiated along the dien bien phu fault in the jurassic significantly earlier than previously thought in light of the relevant tectonic events in indochina the successive suturing collision of the indochina simao and sibumasu blocks may have been responsible for the initiation of dextral shearing along the dien bien phu fault which in turn resulted in offset of the song ma suture

role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia the 1000 km long nw se striking left lateral ailao shan red river shear zone runs from the southeastern corner of tibet to the gulf of tonkin and the south china sea it has been used as the prime example of a lithospheric scale strike slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of indo china away from the indian plate indentor central to the model of continental extrusion is that such faults cut through the entire lithosphere that shear heating resulted in high grade metamorphism and local anatexis and that the ages of sheared granites along the fault also date the timing of strike slip shearing however structural data from the red river shear zone clearly show that vertical strike slip faulting post dated metamorphism and granite emplacement most granites along the shear zone are mantle related granodiorites or within plate alkali granites formed prior to shearing along the red river shear zone left lateral kinematic indicators are ubiquitous within the red river mylonites but they are always lower temperature fabrics formed after peak sillimanite metamorphism and after granite crystallization it is suggested that left lateral strike slip shearing along the red river shear zone started after 21 ma not at 35 ma as previously thought and the fault was purely a crustal structure none of the geological features used to propose the 500 1000 km offsets are robust and the total finite offset remains unknown

inversion of apatite fission track data for thermal history information apatite fission track length distributions and ages provide an integrated record of the low temperature 20 150 c thermal history of the host rock over geologic time scales the information content of these data is quantified by applying a stochastic optimization technique simulated annealing to determine the range of thermal histories compatible with synthetic apatite fission track data given a mathematical description of fission track annealing the goodness of fit between the observed and predicted data parameters and a slowly decreasing control parameter are treated as analogous to the free energy and temperature respectively of a thermodynamic system on this basis a boltzmann type probability distribution is defined and used to accept or reject random thermal history perturbations a priori information used to constrain the model space may include the temperature range in which the solutions are sought present day temperature the complexity of the thermal history and the maximum duration of the data sensitive thermal history this approach allows determination of the range of thermal histories compatible with an observed confined track length distribution and associated fission track age and it provides an estimate of the probability distribution of temperatures at a specific time results for synthetic data sets for diverse model thermal histories suggest that the resolution of thermal history is generally poor parts of the thermal history to which the data are most sensitive can be resolved to no better than about 10 c for ideal data bounds on acceptable time temperature space are sensitive to the assumed complexity of the thermal history but are relatively insensitive to typical uncertainties associated with the data application of this approach to real data is fundamentally limited by the uncertainty associated with extrapolating annealing models to geologic conditions

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

oligo miocene granitic magmatism in central vietnam and implications for continental deformation in indochina two granitoid intrusions within the bu khang extensional complex in central vietnam have been dated by u pb and rb sr geochronology a monazite u pb age of 26 0 0 2 2 myr was obtained for the bu khang pluton and 23 7 1 6 1 7 myr for monazite allanite and zircon from the dai loc intrusion these ages date crystallization of magmas previously assigned precambrian to devonian rb sr analyses of k feldspar and biotite fractions from the samples yield ages of 19 8 0 6 2 myr and 19 6 0 5 myr respectively the thermal history recorded by the different geochronometers implies an average exhumation rate of 2 mm yr 1 corresponding to 9 km of unroofing magmatism was either i induced passively by lithospheric thinning driven by changes in regional tectonic stresses or ii triggered actively by an ascending plume tertiary exhumation and magmatism documented elsewhere in indochina e g ailao shan red river and wang chao shear zones favours a regional tectonic cause for extension and granitoid magmatism in the bu khang complex on the other hand the presence of an upwelling thermal anomaly since at least 35 ma causing mantle melting below indochina is supported by shear wave velocity variations in the mantle and source geochemistry of both the bu khang plutons and the red river belt intrusions in either case tertiary exhumation of the bu khang complex can account for previously undocumented ne sw directed extension which is required in northern vietnam to account for structural changes related to the opening of the south china sea