magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

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

transcurrent movements in the burma andaman sea region the drag produced by the northward movement of the indian plate has caused the indoburman ranges an accretionary prism to decouple from the burma plate along the kabaw fault and the burma plate itself has decoupled from the eastern highlands of burma along the sagaing fault the indoburman ranges have probably been dragged north slightly ahead of the burma plate although at times the two plates may have locked together to move as one unit it is suggested that the double arc formed by the indoburman ranges and the andaman nicobar sumatra islands was formerly one large single arc northward translation of this larger single arcuate structure has caused the tail end of the arc i e the andaman nicobar sumatra section to rotate into the burma plate causing a bending of the arc to form the present double arc and compression within the burma plate to form the pegu yoma folding northward movement of the burma plate has also resulted in offset by over 400 km of the irrawaddy river from its former connection through the channel of what is now the chindwin river

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

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

multiple sources for sea rafted loisels pumice new zealand sea rafted loisels pumice is one of the few stratigraphic markers used to correlate late holocene coastal deposits in new zealand along with underlying sea rafted products of the local taupo eruption of ca 1800 yr b p these events have been used to bracket the first arrival of humans at new zealand loisels pumice is dacitic to rhyolitic sio263 78 wt in composition but individual clasts are homogeneous sio2range 1 wt characteristics include very low k2o 0 5 1 75 wt and rb 25 ppm and a mineralogy dominated by calcic and mafic xenocrysts similar features are shared by pumices of the tonga kermadec arc suggesting a common tholeiitic oceanic source interclast diversity of loisels pumice suggests that it is the product of several eruptive events from different volcanoes the differences in glass and mineral compositions found at various sites can be explained if the deposits are from different events a multisource origin can also partially explain the discrepancy in reported14c ages ca 1500 600 yr b p from different localities therefore the value of loisels pumice as a stratigraphic marker is questionable and it does not constrain the arrival of humans the predominant westward drift of historic tonga kermadec arc pumices and prevailing ocean currents suggest a long anticlockwise semicircular transport route into the tasman sea before sea rafted pumice arrival in new zealand the diversity of the pumices indicates that silicic eruptions frequently occur from the predominantly basic oceanic volcanoes

post permian events in the zangbo suture zone tibet the mesozoic geology of the zangbo zone in tibet is closely comparable to that of the zone9s continuation through western burma deposition of triassic turbidites and locally jurassic sediments on and immediately oceanward of the passive asian margin was followed by imbrication above subducting ocean floor in the early cretaceous the subducting slab became detached from the downgoing lithosphere and accreted to asia in the late lower cretaceous emplacement of the detached slab by southward thrusting over its triassic flysch cover to form the zangbo and western burma ophiolite took place respectively in or after the cenomanian and before the albian subsidence and deposition of albian limestones and mudstones in burma and of the xigatse flysch in a fore arc basin in tibet were followed in both regions by pre campanian uplift and campanian to early maastrichtian marine deposition maastrichtian olistostromes resulted from underthrusting and tectonic burial of poorly consolidated campanian sediments their extrusion together with blocks of adjacent older rock and downslope flow of the extruded melange collision of india with asia in the eocene was not associated with ophiolite emplacement and in the zangbo region resulted chiefly in northward directed thrusting of the triassic flysch and ophiolite over the xigatse flysch with consequent narrowing of the fore arc basin

the four plutonic belts of the transhimalaya himalaya a chemical mineralogical isotopic and chronological synthesis along a tibet nepal section

mid cenozoic fluvio deltaic to marine environments of the salin sub basin central myanmar abstract the salin sub basin is one of several basins within the wide elongate north south trending central basin in myanmar a field based campaign targeted the deposits of the reportedly oligocene shwezetaw padaung and okhmintaung formations which previously have been described only from spatially restricted and incomplete core records and limited field studies a minimum of 2 5 km thickness of these formations in the sub basin records a high sedimentation rate and long term subsidence which broadly kept pace with sediment input facies analysis based on sedimentary logs and panel diagram interpretation interprets these formations as deposits of large scale broadly southward flowing fluvial systems that interacted with deltaic and shallow marine environments in the southwestern parts of the basin a switch from south to southwest flowing rivers midway down the basin suggests these systems fed sediment to offshore basins in the west the overall environment was somewhat similar to the modern day irrawaddy river and delta despite the recorded high sedimentation rates and the coeval subsidence the depositional environment does not show much variation the main control is overall relative sea level rise associated with a northeastward marine transgression into the basin with contemporaneous smaller scale cycles of sea level rise and fall in terms of basin development this considerable thickness of clastic sediment indicates significant emergent sources palaeoenvironment reconstructions suggest that the south flowing fluvial systems sourced sediment from the northern margin of the basin but there were also some channels joining the main channel from the western margin of the basin and fewer channels from the northeast

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

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

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

geochronological geochemical and sr nd hf isotopic constraints on the petrogenesis of late cretaceous a type granites from the sibumasu block southern myanmar se asia abstract the late cretaceous to paleogene granitoids occur widespread in the sibumasu block within myanmar se asia which show a close association with tin tungsten mineralization however the precise timing petrogenesis and tectonic significance of these granitoids are poorly constrained so far in this study we present a detailed study on geochronology elemental and sr nd hf isotopic geochemistry for the hermyingyi and taungphila granites in southern myanmar with the aim of determining their petrogenesis and tectonic implications la icp ms u pb dating of zircon grains from the two granites yield ages of 69 70 ma indicating a late cretaceous magmatic event these granitic rocks are weakly peraluminous and belong to the high k calc alkaline series they are both characterized by high sio2 k2o na2o feot feot mgo and ga al ratios and low al2o3 cao mgo p2o5 and tio2 contents enriched in rb th u and y but depleted in ba sr p and eu suggesting an a type granite affinity moreover they display prominent tetrad ree patterns and non charac trace element behavior which are common in late magmatic differentiates with strong hydrothermal interaction or deuteric alteration the granites belong to a2 type and probably formed at a high temperature and anhydrous condition they have zircon ehf t values from 12 4 to 10 0 and whole rock end t values from 11 3 to 10 6 with paleoproterozoic tdm2 ages 1741 1922 ma for both hf and nd isotopes geochemical and isotopic data suggest that these a type granites were derived from partial melting of the paleoproterozoic continental crust dominated by metaigneous rocks with tonalitic to granodioritic compositions without significant input of mantle derived magma and followed by subsequent fractional crystallization by integrating all available data for the regional tectonic evolution in se asia and adjacent regions we attribute the formation of the late cretaceous a type granites to a back arc extension in the hinterland behind the subduction zone which is induced by the rollback of the flat neo tethyan subducting slab around ca 70 ma

west directed thrusting south of the eastern himalayan syntaxis indicates clockwise crustal flow at the indenter corner during the india asia collision abstract whether continental deformation is accommodated by microplate motion or continuum flow is a central issue regarding the nature of cenozoic deformation surrounding the eastern himalayan syntaxis the microplate model predicts southeastward extrusion of rigid blocks along widely spaced strike slip faults whereas the crustal flow model requires clockwise crustal rotation along closely spaced semi circular right slip faults around the eastern himalayan syntaxis although global positioning system gps data support the crustal flow model the surface velocity field provides no information on the evolution of the india asia orogenic system at million year scales in this work we present the results of systematic geologic mapping across the northernmost segment of the indo burma ranges located directly southeast of the eastern himalayan syntaxis early research inferred the area to have experienced either right slip faulting accommodating northward indentation of india or thrusting due to the eastward continuation of the himalayan orogen in the cenozoic our mapping supports the presence of dip slip thrust faults rather than strike slip faults specifically the northern indo burma ranges exposes south to west directed ductile thrust shear zones in the hinterland and brittle fault zones in the foreland the trends of ductile stretching lineations within thrust shear zones and thrust sheets rotate clockwise from the northeast direction in the northern part of the study area to the east direction in the southern part of the study area this clockwise deflection pattern of lineations around the eastern himalayan syntaxis mirrors the clockwise crustal rotation pattern as suggested by the crustal flow model and contemporary gps velocity field however our finding is inconsistent with discrete strike slip deformation in the area and the microplate model

origin and assembly of the tethyside orogenic collage at the expense of gondwana land abstract the tethysides are a superorogenic complex flanking the eurasian continent to the south and consisting of the cimmerides and alpides products of palaeo and neo tethys respectively we here review their evolution mainly on the basis of new maps showing the distribution of sutures magmatic rocks certain palaeobiogeographically and palaeoclimatologically significant taxa and facies and fragments of pan african 900 450 ma orogenic system forming the basement of many tethyside blocks these are supplemented by palaeomagnetic data reported in the literature a fundamental tenet of this paper is that major sutures which contain ophiolite fragments represent tectonic sections between continental blocks where oceanic crust has been subducted palaeo tethys came into existence largely in late carboniferous time coevally it began to be consumed by both internal and peripheral subduction zones which continued into the permian some of these had been inherited from pre tethyan times in the later permian rifting subparallel with the northern margin of gondwana land began between the zagros and malaysia separating a cimmerian continent from n gondwana land and thus heralding the opening of neo tethys and other smaller oceans that were back arc basins of palaeo tethys this rifting possibly also extended farther west into crete and mainland greece however the north china block yangtze block huanan block the eastern moity of the qangtang block north tibet and annamia all originally pieces of the end proterozoic early palaeozoic gondwana land had already separated from it in pre late carboniferous times possibly during the devonian all of these blocks and the cimmerian continent were characterized by cathaysian floral elements in late palaeozoic time palaeomagnetic and palaeontological data showing the original gondwana land affinity of these continental blocks are supplemented by correlating late proterozoic early palaeozoic pan african sutures orogenic belts and sedimentary basin fragments across tethyside sutures late permian foraminiferal provinces are related to this palaeogeographical interpretation by triassic times most cimmeride subduction zones were already in existence the cimmerian continent accelerated its separation from gondwana land and locally in the late permian began disintegrating internally along the waser rushan pshart banggong co nu jiang mandalay ocean by late triassic time all of the chinese blocks except lhasa and annamia had collided with each other and with laurasia the resulting enormous orogenic collage had a soft cushion between itself and laurasia in the form of the enormous accretionary complex of the songpan ganzi this connection enabled laurasian land vertebrates to reach south east asia by late triassic time in late triassic to middle jurassic times most major cimmeride collisions were completed widespread aridity in central asia occurred in late jurassic time probably in the rain shadow of the newly formed cimmeride mountain wall neo tethyan subduction systems formed along the s margin of the cimmerides or within neo tethyan oceanic lithosphere during the jurassic most if not all were north or east dipping they continued the northerly migration of the tethyside blocks evolution of the tethysides influenced the distribution of marine and terrestrial organisms and affected sea level changes and patterns of atmospheric circulation during much of the mesozoic and cainozoic it is likely to have reflected the surface expression of a persistent trend in the large scale convective circulation in the mantle that continuously transported material northward into the tethyan domain

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

role of crustal thickening and extensional collapse in the tectonic evolution of the sevier laramide orogeny western united states the effects of crustal thickening and extensional collapse of continental crust combined with variations in plate convergence forces provide a coherent explanation for evolution of the cretaceous early tertiary sevier laramide orogeny crustal thickening of the sevier laramide hinterland region was triggered by the combination of compressive plate convergence forces and subduction induced conductive heating of the crust eastward progradation of the locus of sevier laramide deformation through time reflects the progressive widening of the region of crustal thickening the region of maximum crustal thickening migrated outward until about 80 to 75 ma when it encountered excessively strong lithosphere of the colorado plateau the locus of deformation was then transmitted laterally across both a hinterland that had attained maximum crustal thickness and the rigid colorado plateau into the laramide rocky mountain foreland an episode of vigorous extensional collapse of orogenically thickened crust affected the southern cordillera between about 75 and 35 ma early tertiary waning of plate convergence rates and continued subduction induced conductive heating of thickened crust allowed extensional collapse along the southern cordillera region to drive the colorado plateau block northward the result was the late laramide phase of rocky mountain foreland deformation because of this compressive deformation in the central and southern laramide rocky mountain region continued through eocene time rather than ceasing at 56 ma as it did in the northern cordillera

the exhumation of the indo burman ranges myanmar abstract the indo burman ranges ibr are a mountain range comprised of mesozoic cenozoic rocks which run the length of western myanmar extending into india and bangladesh to the west lies the indian ocean and to the east lies the central myanmar basin cmb along which the irrawaddy river flows the ibr are considered to be an accretionary prism developed at the juncture of the indian and sunda plates and a number of hypotheses have been proposed for their evolution however in order for these hypotheses to be evaluated the timing of ibr evolution needs to be determined we undertook a two pronged approach to determining the timing of uplift of the ibr 1 we present the first low temperature thermochronological age elevation profiles of the ibr using zft aft and zhe techniques our data show a major period of exhumation occurred around the time of the oligo miocene boundary we tentatively suggest subject to further verification an additional period of exhumation at or before the late eocene 2 we carried out a detailed multi technique provenance study of the sedimentary rocks of the ibr and arakan coastal region to their west and compared data to coeval rocks of the cmb we determined that during eocene times rocks of the cmb and ibr were derived from similar local provenance that of the myanmar arc to the east therefore at this time there was an open connection from arc to ocean by contrast by miocene times provenance diverged rocks of the cmb were deposited by a through flowing irrawaddy river with detritus derived from its upland source region of the mogok metamorphic belt and cretaceous paleogene granites to the north such a provenance is not recorded in coeval rocks of the ibr indicating that the ibr had uplifted by this time providing a barrier to transport of material to the west to the previously published list of viable proposals to explain the exhumation of the range we add a new suggestion the period of exhumation around the time of the oligo miocene boundary could have been governed by a change to wedge dynamics instigated by a major increase in the thickness of the incoming bengal fan sediment pile

ichnology of the laisong formation late eocene early oligocene of the naga hills indo burma range ibr paleoenvironmental significance abstract the late eocene early oligocene laisong formation basal barail group from the naga hills indo burma range has yielded a low diversity of trace fossils i e chondrites gyrolithes ophiomorpha palaeophycus planolites and thalassinoides three ichnofabrics namely ophiomorpha palaeophycus thalassinoides and chondrites planolites have been identified the ophiomorpha palaeophycus ichnofabric grades upsection into chondrites planolites ichnofabric in a fine to medium grained sandstone unit through a succession of alternating thinly bedded sandstone and mudstone containing thalassinoides ichnofabric the ichnofabric distribution is primarily controlled by the change in depositional energy levels from low to high a low diversity trace fossils and lack of graphoglyptids in the laisong formation of naga hills supports the presence of a proximal part of the hyperpycnal lobes of delta fed marine coarse grained turbidite system

engineering characteristics of ankara greywacke under the foundation of the beytepe open air theatre ankara turkey the construction of a large open air theatre having with an audience capacity of 7 000 is planned at the beytepe campus of hacettepe university located near ankara turkey in the heart of anatolia the foundation of the open air theatre is composed of weak and heavily jointed ankara greywacke an estimation of the strength and deformation of the ankara greywacke was required for assessments of short and long term stability accordingly rock mass characterization of the ankara greywacke was investigated in detail by scan line surveys and back analyses of in situ tests performed on the excavated surface of the rock mass the final version of the hoek and brown criterion hoek et al in proceedings of the north american rock mechanics society meeting toronto canada pp 1 6 2002 together with and improvements proposed by sonmez and gokceoglu int j rock mech min sci 43 671 676 2006 were considered together for the back analyses of small slope benches subjected to plate loading test loaded to failure in addition the plate loading test data were used with some well known empirical equations for predicting of deformation modulus of rock masses to calculate ranges of values of the geological strength index gsi the static and pseudostatic slope stabilities of the audience seating structures to be supported by benches excavated into the rock were also analyzed the stability assessments revealed the unlikelihood of large rock mass failures for the short term construction stages or for the long term as constructed conditions under static or pseudo static conditions additionally the analyses showed that there was no need for anchors between the concrete seating structures and greywacke rock mass

nature of ophiolite occurrences along the eastern margin of the indian plate and their tectonic significance upper mesozoic to lower eocene ophiolitic rocks occur in two parallel belts along the eastern margin of the indian plate the eastern belt passes through central burma sumatra and java and coincides with a zone of gravity highs resulting from steeply dipping mafic rocks it denotes the locus of the subduction where these ophiolites were accreted just prior to middle eocene time in contrast the western belt which passes through nagaland manipur western burma and andaman is flanked to the east by a negative gravity anomaly zone in it the ophiolites occur as rootless subhorizontal bodies tectonically overlying eocene oligocene flysch sedimentary rocks they are inferred to be nappes that propagated westward from the eastern belt during late oligocene terminal collision of the indian and eurasian continental blocks ophiolite occurrences in andaman and mentawai islands belonging to the western belt are generally linked with active subduction west of the island arc this subduction began only in late miocene time and thus it could not have produced the ophiolites which had been emplaced on land much earlier

timing of metamorphism of the lansang gneiss and implications for left lateral motion along the mae ping wang chao strike slip fault thailand the mae ping fault mpf western thailand exhibits dominantly left lateral strike slip motion and stretches for 600 km reportedly branching off the right lateral sagaing fault in myanmar and extending southeast towards cambodia previous studies have suggested that the fault assisted the large scale extrusion of sundaland that occurred during the late eocene early oligocene with a geological offset of 120 150 km estimated from displaced high grade gneisses and granites of the chiang mai lincang belt exposures of high grade orthogneiss in the lansang national park part of this belt locally contain strong mylonitic textures and are bounded by strike slip ductile shear zones and brittle faults geochronological analysis of monazite from a sample of sheared biotite k feldspar orthogneiss suggests two episodes of crystallization with core regions documenting th pb ages between c 123 and c 114 ma and rim regions documenting a significantly younger age range between c 45 37 ma these data are interpreted to represent possible magmatic protolith emplacement for the lansang orthogneiss during the early cretaceous with a later episode of metamorphism occurring during the eocene textural relationships provided by in situ analysis suggest that ductile shearing along the mpf occurred during the latter stages of or after this metamorphic event in addition monazite analyzed from an undeformed garnet two mica granite dyke intruding metamorphic units at bhumipol lake outside of the mae ping shear zone produced a th pb age of 66 2 1 6 ma this age is interpreted to date the timing of dyke emplacement implying that the mpf cuts through earlier formed magmatic and high grade metamorphic rocks these new data when combined with regional mapping and earlier geochronological work show that neither metamorphism nor regional cooling was directly related to strike slip motion

the petrology and mineralogy of mt popa volcano and the nature of the late cenozoic burma volcanic arc mt popa is situated on a line of widely spaced calc alkaline volcanic centres in the central lowlands of burma latites rhyodacites and ignimbrites are interbedded with strongly folded arenaceous sediments of topmost miocene and pliocene age these are overlain unconformably by a composite cone of basalt and basaltic andesite lavas and pyroclastic deposits of pleistocene or early recent age the two groups of lavas are regarded as a single petrographic suite in which several continuous and discontinuous mineral reaction series are recognized phenocryst phases analysed by microprobe are olivine bronzite hypersthene biotite diopsidic augite salite magnesio hastingsite magnesio hornblende titaniferous magnetite bytownite oligoclase and sanidine whole rock analyses are characterized by relatively high k ba sr rb and zr high mg mg fe ratios of 0 85 to 0 65 in both phenocrysts and whole rock high k rb 767 and low sr 87 sr 86 0 70431 and rb sr 0 0155 ratios suggest that the basalts are relatively primitive mantle derived compositions modified slightly by fractionation with negligible crustal contamination the more siliceous lavas may have been derived by fractional crystallization of such magmas at crustal levels this high k calc alkaline suite is typical of a continental margin orogenic environment analyses of contemporaneous lavas from elsewhere in burma define three n s volcanic lines which become more alkaline from w to e the magmatism is attributed to the waning stages of orogeny as eastward subduction of the indian plate beneath china gave way to strike slip movement associated with the andaman sea spreading centre within this tectonic model magmas were probably generated by hydrous mantle melting above dehydrating oceanic crust of the subduction zone

cretaceous crustal structure and metamorphism in the hinterland of the sevier thrust belt western u s cordillera combined structural and geochronologic studies to the west of the sevier belt at lat 39 degree 40 degree n indicate that peak metamorphism of the upper 15 km of crust occurred during the late cretaceous 70 90 ma overlapping in time with the intrusion of muscovite bearing granites and pegmatites metamorphism was accompanied by penetrative deformation top to the east layer parallel shear both were localized at the currently exposed structural levels resulting in impressive vertical and lateral strain gradients and t sub max gradients of 50 c km inception of thrust faulting in the sevier belt may have predated peak metamorphism and intrusion of granites in the hinterland by 10 15 m y but it continued during and ended simultaneously with the end of metamorphism at about 70 ma the rise of muscovite bearing granites and hot fluids generated by metamorphism and crustal anatexis at greater depth provides and explanation for the observed localized metamorphism and deformation at relatively high structural levels

nyainqentanglha shear zone a late miocene extensional detachment in the southern tibetan plateau a major low angle ductile shear zone containing s c mylonites involves metamorphosed granitic rocks at the southeastern edge of the nyainqentanglha mountain range southern tibet prominent triangular facet geomorphology is developed by valley erosion of the detachment surface defined by the top of the southeast dipping mylonitec shear zone kinematic criteria consistently indicate a top to southeast sense of shear the ductile shearing deformation is inferred from isotopic cooling ages to have occurred during the interval 11 5 ma late miocene we interpret the shear zone as a regional extensional detachment its development indicates when the extensional tectonics in this area started which in turn may mark the time when the maximum sustainable surface elevation and perhaps crustal thickness was reached in southern tibet

late miocene ductile extension and detachment faulting mykonos greece the geometry and style of structural relations at map to microscopic scale on mykonos cyclades aegean sea greece suggests ductile extension and detachment faulting during the late miocene this island is dominated by granitic rocks that have a well developed shallowly dipping mylonitic foliation and a northeast southwest trending mineral elongation lineation kinematic indicators in these rocks exhibit a top to the northeast down dip sense of shear textural relations suggest that conditions during mylonitic deformation were greenschist to amphibolite facies and published k ar geochronology suggests a late miocene age for this deformation event mylonitic rocks lie in the footwall of a low angle fault and fault striae parallel to the elongation lineation suggest kinematic coordination between mylonitic deformation and movement along the fault translation along the fault juxtaposes younger tilted sedimentary rocks of low metamorphic grade upon the once deeper mylonitic rocks the style and geometry of deformational fabrics and map relations on this island are strikingly similar to those observed in metamorphic core complexes of the western united states

granite related tin metallogenic events and key controlling factors in peninsular malaysia southeast asia new insights from cassiterite u pb dating and zircon geochemistry