jurassic subsidence history in the hebrides n w scotland abstract geohistory analyses have been applied to the generally well dated triassic jurassic successions of three areas in the hebrides basin north west scotland and the importance of making corrections for decompaction palaeobathymetry and eustatic changes of sea level are assessed the thermo tectonic subsidence patterns in the skye areas north suggest a three phase history of crustal stretching though only the first phase is well developed in the mull area south episodes of relatively rapid subsidence are 1 1 late triassic c210 ma until earliest toarcian 186 ma 2 2 latest toarcian 180 ma until late bathonian c161 ma 3 3 early oxfordian 154 ma until at least early kimmeridgian incomplete these are separated by toarcian 186 180 ma and latest bathonian callovian c161 154 ma episodes of either little subsidence or of uplift but at slower rates than eustatic rise of sea level the intervals between phases at 30 ma and 26 ma are remarkably similar but the wider significance of this remains to be established

interactive interpretation of a submarine fan offshore ireland a case history during the eocene a series of submarine fans developed in the porcupine basin off the southwest coast of ireland interactive interpretation of one the clontarf fan shows the successive development of four sequences seismic facies analysis of these sequences suggests a progressive evolution from a more distal to proximal environment during deposition of the fan seismic facies mapping reveals an extensive series of sand prone distributary systems in the youngest fan sequence amplitude anomalies suggest that this and other fans in the basin are favorable stratigraphic and or structural traps for hydrocarbons

provenance and tectonic setting of upper triassic turbidites in the eastern tethyan himalaya implications for early stage evolution of the neo tethys abstract upper triassic turbidites in the eastern tethyan himalaya preserve archives for early stage evolution of the neo tethys however their provenance and tectonic setting remain controversial here we report petrographic detrital zircon u pb and trace element data of newly discovered sandstones interbedded with layered basalts and whole rock geochemical data of basalts from the yumen melange within upper triassic turbidites and the same types of published data from associated terranes in the eastern gondwana are compiled for comparison the interbedded sandstones are dominated by quartz lithics fragments and plagioclase grains resembling those of the langjiexue group all samples display similar ranges of detrital zircon u pb ages with significant age populations at 280 210 ma 650 450 ma and 1200 800 ma comparable to those of the langjiexue group and coeval turbidites from western myanmar and partly from central tethyan himalaya implying they should share identical provenance trace elements of 300 200 ma aged zircons indicate they were derived from a continental magmatic arc geochemical data from interbedded basalts display typical within plate basalt affinity synthesized with regional geological setting the above observations suggest upper triassic turbidites from eastern and part of central tethyan himalaya and western myanmar were deposited in a marine rift basin of indian passive continental margin with their primary provenance from lhasa however other contemporaneous turbidites from part of central tethyan himalaya and australia had distinct primary sources from indian continent and west papua respectively indicating they may have separate deposystems

jurassic deep water fans in the neo tethys ocean the kashafrud formation of the kopet dagh basin iran abstract the middle jurassic kashafrud formation of northeast iran was deposited in the kopet dagh basin linked to the neo tethys ocean the 2 km thick succession commences with fluvio marine boulder conglomerates that occupy bedrock valleys but the bulk of the formation comprises a deep water succession of interbedded sandstone and mudstone packages with trace fossils and sparse ammonites neogene collisional events deformed the strata to create the kopet dagh mountains the deep water succession represents channel lobe systems and associated levees and overbank areas on slope to basin floor fans northward paleoflow suggests that rivers drained uplands along the paleo tethys suture zone forming fan deltas on a narrow shelf with associated sediment transport to deeper water coarse sands extrabasinal pebbles and plant fragments are present throughout the formation and a thick boulder conglomerate unit lies within the turbidite succession these observations suggest proximity to terrestrial sources throughout deposition sandstone packages are mainly less than 10 m thick maximum 84 m and numerous lensoid packages of thick bedded sandstone tens of meters thick and up to 200 m wide represent submarine channel fills thick bedded lenses and sheets show a relatively narrow range of paleoflow direction and represent major submarine conduits whereas the more varied paleoflow direction of thin bedded packages suggests that many were deposited on levees the presence in the kashafrud formation of potential source rocks in the form of organic rich mudstones and abundant plant fragments suggests that the formation may have contributed to hydrocarbon fields within overlying formations sandstone sheets and channel fills could form local reservoirs

u pb zircon geochronology of major lithologic units in the eastern himalaya implications for the origin and assembly of himalayan rocks models for the origin and deformation of himalayan rocks are dependent upon geometric and age relationships between major units we present field mapping and u pb dating of igneous and detrital zircons that establish the lithostratigraphic architecture of the eastern himalaya revealing that 1 the south tibet detachment along the bhutan china border is a top to the north ductile shear zone 2 late triassic and early cretaceous sedimentary samples from the northern indian margin show a similar age range of detrital zircons from ca 3500 ma to ca 200 ma but the late triassic rocks are distinguished by a significant age cluster between ca 280 and ca 220 ma and a well defined age peak at ca 570 ma 3 an augen gneiss in the south tibet detachment shear zone in southeast tibet has a cambrian ordovician crystallization age 4 main central thrust hanging wall paragneiss and footwall quartzites from the far western arunachal himalaya share similar provenance and late proterozoic maximum depositional ages and 5 main central thrust footwall metagraywacke from the central western arunachal himalaya has a paleoproterozoic maximum depositional age indicated by a single prominent age peak of ca 1780 ma recent work in the eastern himalaya demonstrates that in the early middle miocene the himalayan crystalline core here was emplaced southward between two subhorizontal shear zones that merge to the south a proposed subsequent middle miocene brittle low angle normal fault accomplishing exhumation of these rocks along the range crest can be precluded because new and existing mapping demonstrates only a ductile shear zone here the ca 280 220 ma detrital zircons of the late triassic strata are derived from an arc developed along the northern margin of the lhasa terrane detritus from this arc was deposited on the northern margin of india during india lhasa rifting along strike heterogeneity in main central thrust footwall chronostratigraphy is indicated by detrital zircon age spectrum differences from central western to far western arunachal nonetheless the late proterozoic rocks in the main central thrust hanging wall and footwall in far western arunachal can be correlated to each other and to previously analyzed rocks in the south tibet detachment hanging wall to the west and in the indian craton to the south these findings are synthesized in a reconstruction showing late triassic india lhasa rifting and cenozoic eastern himalayan construction via in situ thrusting of basement and cover sequences along the north indian margin

provenance of late triassic sediments in central lhasa terrane tibet and its implication abstract in southern tibet late triassic sequences are especially important to understanding the assembly of the lhasa terrane prior to indo asian collision we report new data relevant to the provenance of a late triassic clastic sequence from the mailonggang formation in the central lhasa terrane tibet petrographic studies and detrital heavy mineral assemblages indicate a proximal orogenic provenance including volcanic sedimentary and some ultramafic and metamorphic rocks in situ detrital zircon hf and u pb isotope data are consistent with derivation of these rocks from nearby triassic magmatic rocks and basement that comprise part of the newly recognized late permian triassic sumdo cuoqen orogenic belt the new data suggests correlation with the upper triassic langjiexue group which lies on the opposing southern side of indus yarlung ophiolite sediments from both the mailonggang formation and langjiexue group are interpreted to represent formerly contiguous parts of a sequence deposited on the southern flanks of the sumdo cuoqen belt

the yarlung zangbo paleo ophiolite southern tibet implications for the dynamic evolution of the yarlung zangbo suture zone the yarlung zangbo suture zone a major geological structure in tibet is well known as the locus of tectonic emplacement of the tethyan ophiolites current models propose that most of the east tethyan oceanic lithosphere was subducted within a single subduction zone active during the middle or late cretaceous which was completed during the paleogene collision between india and asia the early cretaceous sedimentary giabulin formation in southern tibet includes conglomeratic members that contain ultramafic and mafic plutonic pebbles as well as radiolarian chert clasts that record the erosion of oceanic lithosphere involved in a subduction event which occurred earlier than previously believed geochemical analyses mineral chemistry stratigraphic chronology and sedimentary analysis including source provenance suggest that the pebbly conglomerate was formed through erosion of an unknown ophiolitic source that was geochemically distinguishable from the xigaze ophiolites within the yarlung zangbo suture zone southern tibet we infer the existence of an older ophiolitic source termed the yarlung zangbo paleo ophiolite that was dismembered and eroded during an earlier subduction stage not taken into account in current models q 2000 elsevier science ltd all rights reserved

the clementine bistatic radar experiment during the clementine 1 mission a bistatic radar experiment measured the magnitude and polarization of the radar echo versus bistatic angle beta for selected lunar areas observations of the lunar south pole yield a same sense polarization enhancement around beta 0 analysis shows that the observed enhancement is localized to the permanently shadowed regions of the lunar south pole radar observations of periodically solar illuminated lunar surfaces including the north pole yielded no such enhancement a probable explanation for these differences is the presence of low loss volume scatterers such as water ice in the permanently shadowed region at the south pole

multiple sources of the upper triassic flysch in the eastern himalaya orogen tibet china implications to palaeogeography and palaeotectonic evolution abstract the upper triassic flysch langjiexue group tentatively named the shannan terrane of the eastern himalaya orogen has been tectonically assigned either to the tethys himalaya or the yarlung zangbo suture zone yzsz in this work geochronology of detrital zircon u pb isotope shows that the shannan terrane is characterized by the population of 260 200 ma peak ca 240 ma strongly supporting the view of no affinity to the tethys himalaya the detrital zircons dated as 400 290 ma display relatively positive ehf t values of 5 0 to 15 0 with t dm c ages of 2 6 1 3 ga for the shannan terrane whereas highly negative are of 20 0 to 5 0 for the lhasa terrane indicating that the two terranes have different devonian carboniferous sources numerous cr spinels found in the shannan terrane but not in the lhasa terrane exhibit contents in cr 2 o 3 and cr of mainly 44 100 and 48 95 in tio 2 of 0 01 1 0 and in al 2 o 3 of 5 257 respectively denoting several parent lithologies these differences suggested that the shannan terrane has multiple sources not only from the lhasa terrane but also from oceanic island arc seamount and mid ocean ridge areas as well as likely from greater india and australia considering the early cretaceous diabase dykes within the upper triassic flysch defined to the comei bunbury large igneous province we propose that the langjiexue group could have been deposited on the ocean between india and australia and would have not stopped till the lhasa terrane was separated from australia during the terminal triassic according to the cenozoic deformation and metamorphic history and palaeogeography of the langjiexue group we postulate that the shannan terrane could have been loaded onto the greater india during the middle early cretaceous and subsequently drifted northward to the collision zone of india and asia implying that it does not represent an accretionary prism within the yzsz

late cretaceous and cenozoic intra plate compressional deformations in the alpine foreland a geodynamic model abstract the mesozoic grabens and wrench induced troughs of western and central europe developed in response to triassic early cretaceous intra plate tensional stresses which affected the arctic north atlantic and the tethys borderlands during the rifting phase preceding the opening of the respective oceans this resulted in the fragmentation of the continental crust of the alpine foreland during the late cretaceous and tertiary alpine orogeny collision related compressive stresses exerted on the foreland induced the reactivation of these fracture systems causing the inversion of mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present alpine deformation front the structural style of these inverted basins and basement uplifts is indicative of compressional and transpressional deformations the total amount of crustal shortening associated with these deformations is unlikely to exceed a few tens of kilometres these displacements require a coupling between the foreland and the orogen at the alpine a subduction zones and within the foreland a decoupling at intra crustal levels between the crust and mantle and or at deeper lithospheric levels the example of the alpine foreland indicates that compressive stresses inducing major deformations can be transmitted from a collisional plate boundary through continental lithosphere over distances of up to 1300 km circumstantial evidence suggests that collision related compressive stresses not large enough to give rise to significant deformations can be transmitted through continental lithosphere over even greater distances

composition and sediment dispersal pattern of the upper triassic flysch in the eastern himalayas china significance to provenance and basin analysis the paleogeography and basin type of upper triassic flysch langjiexue group in the eastern himalayan orogen are disputed in order to shed new light on the flysch s origin we applied different sedimentological methods assemblages of heavy minerals and clastic components of sandstones were utilized to determine the primary depositional composition heavy mineral indices s m ratios thickness of sandstone siltstone s versus slate mudrock m and paleocurrent data were combined to reveal the sediment dispersal pattern and the location of the source areas in the analyzed sandstones heavy minerals such as zircon rutile tourmaline apatite and anatase are most common and zircon is predominant most over 60 ztr values range from 60 to 98 and systematically increase southward as a provenance sensitive parameter ruzi values vary in large magnitude and are significantly higher in both the east and west 20 than in the center the majority of s m ratios decrease from north to south suggesting an overall decrease in grain size to the south paleocurrent directions vary between 120 and 270 main vector 205 and 185 after 20 counterclockwise correction displaying a radial curved pattern variable heavy mineral assemblages indicate different sources and the sandstones fall in the recycled and mixed arc orogeny fields of dickinson triplots together supporting the view of multiple sources results of the ztr values s m ratios and paleocurrent directions illustrate a dispersal pattern corresponding to a submarine fan system the provenance and submarine fan dispersal pattern along with the basin configuration deep basin with oceanic affinities suggest that the langjiexue group accumulated in a remnant basin between lhasa greater india and australia where the sediments dispersed into the basin toward the developing orogen suture zone and not away from the orogen challenging the provenance direction for the traditional remnant basin model

evolution of the western approaches trough abstract the western approaches trough corresponds to a major mesozoic rift which became partly inverted during the late oligocene and miocene sediments contained in this basin range in age from permian to neogene and are some 7000 m thick the rifting stage of the western approaches trough spans permian to aptian times there is only minor evidence for volcanic activity important wrench deformations preceded the accumulation of wealden clastics in the rapidly subsiding southwest channel basin and in the brittany trough early aptian wrench deformations coincided with crustal separation in the bay of biscay during the albian to early oligocene the western approaches trough was tectonically inactive and subsided regionally late oligocene to miocene compressional deformations induced the upwarping of a major anticlinorium having a structural relief of some 3000 m

a crustal model for a northeast southwest profile through ireland abstract the irish caledonian suture seismic project icssp consisting of a 250 km seismic refraction line was carried out in conjunction with and as an extension of the caledonian suture seismic project cssp of the university of durham 51 seismic recording sites at 5 km intervals were deployed on the northwest flank of the suture zone in ireland recordings were made from three shot points at the southwest of the line and from 31 shot points of the durham programme in the north irish sea a crustal model is presented which is based exclusively on a travel time interpretation the p g velocities recorded in eastern ireland rise quite rapidly with depth to 6 3 6 5 km s but in the western part of the profile lower velocities between 6 0 and 6 3 km s were recorded prominent energy in later arrivals can be correlated by two reflected phases from a mid crustal boundary and the moho a low velocity zone is present at the northeastern end of the line and a mid crustal layer with an average velocity of about 6 4 km s is separated from the lower crust mean velocity 6 8 km s by a well established mid crustal boundary zone of 1 2 km thickness at about 20 km depth the moho around 32 km at its deepest shallows towards the sea at both ends of the line it is a sharp discontinuity under western ireland but a transition zone of 3 4 km thickness in the east an interpretation is given which takes into account 1 a the geologic and tectonic history of ireland and 2 b data derived from the gravity and magnetic fields

western extension of the iberian european plate boundary during the early cenozoic pyrenean convergence a new model abstract a dense seismic net work allows of drawing the western prolongation of the northern boundary of the iberian plate this early cenozoic deformed boundary is disrupted by two prominent nw se transform faults a kinematic reconstruction with a nw late cretaceous lower eocene motion of the iberian plate is proposed the azores biscay rise is interpreted as a short lived subduction zone whereas king s trough could be a transform fault

structural framework and tectono stratigraphic evolution of the porcupine seabight basin offshore western ireland abstract the porcupine seabight basin is an intra cratonic rift basin which evolved through recurrent episodes of lithospheric stretching structure contour and isopach maps presented here have been constructed following the interpretation of approx 10 000 km of seismic data together with all the available well data this integrated database has allowed the tectono stratigraphic history of the basin to be traced in detail from its earliest inception the basin was initially formed during the carboniferous when up to 3 7 km of autuno carboniferous sediments were deposited on a caledonian and hercynian structured basement late permian and early mesozoic sediments at least 1 km thick are preserved in contiguous grabens and their presence is also inferred within localized sub basins late liassic crustal doming heralded the onset of the final rifting episode and in the north promoted severe erosion and a rapid discharge of clastic sediments rapid subsidence block faulting and magmatism accompanied the deposition of up to 3 5 km of middle upper jurassic sediments in asymmetrical sub basins the transition from stretching to passive subsidence across the jurassic cretaceous boundary in combination with localized inversion abrupt eustatic and environmental changes generated a basin wide unconformity the rift drift transition in the adjacent proto atlantic rift system is recorded in the porcupine seabight basin by voluminous volcanism and a late lower cretaceous unconformity cretaceous sediments are strongly transgressive and up to 6 km thick these were superseded during the early palaeogene by several regressive transgressive cycles before the arrival of an upper slope fauna in the mid middle eocene cenozoic sediments are up to 4 km thick and record the interplay between 1 the location of the basin pn the pyrenean alpine foreland superimposed on the periphery of the icelandic plume and 2 its development as a marginal atlantic shelf region the former is thought to be responsible for eo oligocene faulting subsidence and igneous activity in the north east part of the basin and the latter for unconformities and a change in the style of sedimentation principally due to oceanic circulation changes

triassic marine biogeography constrains the palaeogeographic reconstruction of tibet and adjacent areas abstract palaeobiogeographic studies can provide valuable constraints on palaeogeographic reconstructions the triassic marine bivalves from tibet and adjacent areas including the tectonic units of qamdu qiangtang lhasa himalaya north china songpan ganzi south china simao and indochina and from three relevant and representative triassic biogeographic regions i e the western tethys japan and new zealand are rigorously analysed at the generic level analyses of diversity changes of the six clades and of the total bivalve fauna against latitude in the studied areas reveal that the diversity is highest along the northern margin and gradually declines southward to the southern margin of the tethyan ocean during the triassic a dynamic triassic palaeobiogeographic pattern is presented applying cluster analysis and analysis of taxonomic diversity gradients and endemism to four time intervals i e the early triassic epoch the anisian carnian and the norian rhaetian three provinces i e the north eastern tethyan ne south eastern tethyan se and the himalayan hi province are recognised from the early triassic to the carnian the hi joined the se and a new province the arctic pacific ap formed in the norian rhaetian in the study area the dynamic palaeobiogeographic pattern is used in the palaeogeographic reconstruction of the study area such as position and timing of collision processes of the palaeo tethys and meso tethys during the triassic

the embryonic himalayan foreland basin revealed in the eastern yarlung zangbo suture zone southeastern tibet abstract understanding of early foreland basin development is key to reconstruct the timing and nature of early collisional processes though the nature of the early himalayan foreland basin system is well established in the central himalaya range it lacks analysis in the eastern yarlung zangbo suture zone in this article we investigate the region s lithological and structural evolution and analyze the geochemistry and geochronology of detrital zircons and bulk rocks in the jiacha jindong region southeastern tibet we show that 1 the previously named cretaceous langxian melange comprises two separate stratigraphic units the newly revised early eocene 56 ma langxian unit and the triassic langjiexue group 2 the langxian unit overthrusts the oligocene to miocene dazhuka formation and is overlain and or underlain by the highly deformed langjiexue group 3 shale normalized ree y patterns of marbles with light ree depletion la enrichment and positive y anomaly plus sr isotope ratios 0 707481 0 707897 together demonstrate the langxian unit was originally deposited in a marine environment 4 1270 concordant detrital zircons from 20 rock samples indicate a bilateral provenance from both the gangdese arc and or recycled forearc strata in the north and the langjiexue group in the south the evidence illustrates that the langxian unit records the early india asia collision and its protoliths could have been deposited in a distal foredeep basin to the embryonic eastern himalaya orogeny the simplest explanation is that it reflects an underfilled marine basin with sources from the early mountain range to the north and a region of forebulge erosion to the south

sandstone provenance and tectonic evolution of the xiukang melange from neotethyan subduction to india asia collision yarlung zangbo suture south tibet abstract the xiukang melange of the yarlung zangbo suture zone in south tibet documents low efficiency of accretion along the southern active margin of asia during cretaceous neotethyan subduction followed by final development during the early paleogene stages of the india asia collision here we present integrated petrologic u pb detrital zircon geochronology and hf isotope data on different types of sandstone blocks in the xiukang melange three groups of sandstone blocks with different provenance and depositional setting are distinguished by their petrographic geochronological and isotopic fingerprints blocks of turbiditic quartzarenite originally sourced from the indian continent were deposited in pre cretaceous time on the northernmost edge of the indian passive margin and eventually involved into the melange at the early stage of the india asia collision two distinct groups of volcaniclastic sandstone blocks were derived from the central lhasa block and gangdese magmatic arc one group was deposited in the trench and or on the trench slope of the asian margin during the early late cretaceous and the other group in a syn collisional basin just after the onset of the india asia collision in the early eocene the largely erosional character of the asian active margin in the late cretaceous is indicated by the scarcity of off scraped trench fill deposits and the relatively small subduction complex developed during limited episodes of accretion the xiukang melange was finally structured in the late paleocene eocene when sandstone blocks of both indian and asian origin were progressively incorporated tectonically in the suture zone of the nascent himalayan orogen

major cretaceous volcanic province in southern rockall trough abstract a group of three large curvilinear ridges called the barra volcanic ridge system has been mapped in the acoustic basement of southern rockall trough typically each ridge is about 2 km high and 20 km wide at its base a crudely layered acoustic character moderate density but high strength of magnetisation point to a volcanic sedimentary ash composition for the ridges seismic continuity with the acoustic basement of the rest of rockall trough suggests that the trough basement is of similar composition an age for the ridges of lower cretaceous is indicated by well ties and consideration of regional geology volumetrically the ridges are on the scale of hot spot features such as the wyville thomson ridge

the lhasa terrane record of a microcontinent and its histories of drift and growth article i nfo the lhasa terrane in southern tibet has long been accepted as the last geological block accreted to eurasia before its collision with the northward drifting indian continent in the cenozoic but its lithospheric architecture drift and growth histories and the nature of its northern suture with eurasia via the qiangtang terrane remain enigmatic using zircon in situ u pb and lu hf isotopic and bulk rock geochemical data of mesozoic early tertiary magmatic rocks sampled along four north south traverses across the lhasa terrane we show that the lhasa terrane has ancient basement rocks of proterozoic and archean ages up to 2870 ma in its centre with younger and juvenile crust phanerozoic accreted towards its both northern and southern edges this finding proves that the central lhasa subterrane was once a microcontinent this continent has survived from its long journey across the paleo tethyan ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the qiangtang terrane to the north and with the indian continent to the south zircon hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks e g 50 90 0 70 and 30 100 to the mesozoic magmatism in the southern central and northern lhasa subterranes respectively we suggest that much of the mesozoic magmatism in the lhasa terrane may be associated with the southward bangong nujiang tethyan seafloor subduction beneath the lhasa terrane which likely began in the middle permian or earlier and ceased in the late early cretaceous and that the significant changes of zircon ehf t at 113 and 52 ma record tectonomagmatic activities as a result of slab break off and related mantle melting events following the qiangtang lhasa amalgamation and india lhasa amalgamation respectively these results manifest the efficacy of zircons as a chronometer u pb dating and a geochemical tracer hf isotopes in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics

tectonostratigraphy and provenance of an accretionary complex within the yarlung zangpo suture zone southern tibet insights into subduction accretion processes in the neo tethys abstract accretionary complexes record critical information about the history of subduction and accretion along the southern margin of asia prior to the india asia collision this paper presents detailed field mapping petrographic and detrital zircon u pb data from an accretionary complex within the yalung zangpo suture zone southern tibet from structurally higher to lower levels the accretionary complex consists of a serpentinite melange the chert matrix tangga melange the mud matrix pomunong melange and the coherent rongmawa formation the tangga melange consists of late triassic early cretaceous abyssal chert and siliceous shale with blocks of chert and mafic to ultra mafic rocks the tangga melange was accreted beneath ophiolitic rocks during the aptian the pomunong melange consists of a late jurassic early cretaceous hemipelagic siliceous shale and chert matrix with blocks from early permian seamounts and from late cretaceous trench fill sandstones it was accreted beneath the tangga melange after the aptian but prior to 71 ma structurally beneath these melanges to the south the uppermost cretaceous rongmawa formation consists of turbiditic sandstone pelagic chert and siliceous shale and records a transition in depositional setting from lower abyssal plain to upper trench detrital zircons from sandstone blocks of the pomunong melange and the coherent rongmawa formation display similar u pb age spectra and are dominated by peaks at 71 231 ma 481 693 ma and 701 1372 ma these age peaks overlap with igneous crystallization ages and detrital zircon ages from sedimentary strata in the lhasa terrane our data indicate that the serpentinite melange tangga melange pomunong melange and rongmawa formation comprise a southward younging accretionary complex that developed during the northward subduction of neo tethyan oceanic lithosphere beneath the south margin of the lhasa terrane and that all exposed ophiolitic and accretionary complex assemblages were accreted to the asian margin prior to the india asia continental collision

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

kinematic evolution of the eastern tethyan himalaya constraints from magnetic fabric and structural properties of the triassic flysch in se tibet anisotropy of magnetic susceptibility ams combined with structural analysis are used in this work with the aim to characterize the tectonic evolution of the triassic flysch within the eastern tethyan himalaya thrust belt in se tibet the attitude of the magnetic foliation and lineation are concordant with the planar and linear structures of tectonic origin defined by the preferred orientation of the iron bearing silicates two different tectonic domains can be defined a the southern domain is controlled by the eohimalayan tectonic foliation s1 recorded in the magnetic foliation which trends east west and dips to the north b the northern domain is dominated by the neohimalayan magnetic foliation with wnw ese strike and dips to the south opposite to the vergence of the main structures a slightly prolate magnetic ellipsoid has been found in between the two domains recording the intersection of s1 and the subtle development of the s2 tectonic foliation hinterland propagation of the deformation lead to the great counter backthrust generation pointed out by the ssw steeply plunging magnetic lineation furthermore different orientations of magnetic foliation may indicate an early miocene c 208 clockwise vertical

origin of the xigaze ophiolite yarlung zangbo suture zone southern tibet abstract the xigaze ophiolite tibet displays unusual lithological petrological textural and structural characteristics there are no large masses of cumulate gabbros but dolerite intrusives throughout the whole ophiolite sequence some of which were intruded into already serpentinized peridotites only minor residual harzburgites and dunites in dominantly iherzolitic peridotites equilibrated at low temperatures and pressures and relatively low temperature deformation structures in the uppermost peridotites these features suggest a very low heat flow at the spreading center where the xigaze ophiolite was formed in good agreement with a discontinuous and slowly accreting spreading center origin however this ophiolite does not represent a typical mid oceanic ridge ophiolite rather it was formed in a small basin located at the southern margin of eurasia hence within a preexisting oceanic lithosphere the opening of the nearly n s xigaze paleo ridge resulted from the w e drift of africa relative to eurasia from 180 to 110 ma the n s emplacement onto the continent of the xigaze ophiolite formed 120 to 110 ma ago can be correlated to changes in direction of motion of the african and indian plates a primary intra oceanic thrusting event probably occurring at 110 or 85 ma and the final obduction near 50 ma during the india eurasia collision

continental crust under the southern porcupine seabight west of ireland abstract two new seismic refraction wide angle reflection profiles demonstrate that the crust beneath the southern porcupine seabight out to water depths in excess of 4000 m is of continental type they also reveal the rifted margin of the porcupine basin on its eastern side crustal thickness under the seabight inclusive of sediments which are up to 6 km thick decreases from 23 km in the east to about 10 km at a sharp continent ocean transition in the west

graben hydrocarbon occurrences and structural style major hydrocarbon occurrences types of traps and structural styles have been synthesized from the sirte basin the suez and viking grabens and from other normal faulted regions hydrocarbons occur in a stacked succession of one or more basins pregraben graben fill and interior sag preservation of pregraben accumulations depends on late initiation of crustal arching and limitation of uplift to the graben shoulders furthermore a stable pregraben tectonic environment is required to ensure internal continuity of subsequent fault block closures hydrocarbon traps in the pregraben and graben fill sediments are primarily dependent on the multidirectional orientation of normal faults tilting of fault blocks and in many cases flexing or erosion parallel with block edges the fault pattern is dominated by longitudinal faults subparallel with the graben axis and secondarily by oblique faults the direction and degree of block rotation are influenced by fault profile planar or listric degree of extension fault pattern doglegs junctions terminations downwarping of the subsequent sag basin and isostatic adjustments between large blocks fold closures result from the upward termination of faults into forced folds which may be accentuated by drag during renewed faulting passive drape and differential compaction extend he folds to shallower depths because they form along block edges the flexures have trends that mimic the multidirectional fault patterns truncation traps are usually developed from the erosional retreat of fault scarps and they can also mimic the fault patterns where the sedimentary succession is complete an interior sag basin is superimposed across the graben its lower part is structured mostly by drape and differential compaction across the underlying graben fault blocks and fault block topography the upper part is usually an unsegmented syncline and hydrocarbon traps at this level include depositional buildups reefs turbidite mounds and salt diapirs or other structures caused by superimposed deformation unrelated to graben tectonics

deposystem architectures and lithofacies of a submarine fan dominated deep sea succession in an orogen a case study from the upper triassic langjiexue group of southern tibet abstract over thirty stratigraphic sections of the himalaya orogen upper triassic langjiexue group in southern tibet china were studied to interpret the environments and lithofacies the facies associations channel a lobe b levee interchannel c and basin plain d with nine facies a1 3 b1 3 and c1 3 were distinguished they form six architectural elements channel interchannel overbank levee crevasse splay outer fan lobe fan fringe and basin plain taking into account the facies analysis sub deposystem correlation paleocurrent dispersal pattern and restoration of primary stratal width the langjiexue group displays the architecture of a coalescing submarine fan dominated deep sea deposystem measuring about 400 500 km 600 700 km in size or even more one of the largest pre cenozoic submarine fans ever reported subdivisionally four fans lacking inner fans could have coalesced laterally within the submarine fan deposystem and at least six submarine fan developments were vertically succeeded by mid to outer fan deposits with progradational to retrogradational successions according to the range of 30 70 of sandstone content the fan deposystem is mud and sand rich suggesting a medium far over 400 600 km transport of sediment from the source area

late triassic paleogeographic reconstruction along the neo tethyan ocean margins southern tibet abstract sandstone petrographic and u pb detrital zircon analyses of upper triassic sedimentary rocks from the northern margin of india tethyan himalaya sequence and southern margin of eurasia lhasa terrane provide new constraints on the mesozoic paleogeography of neo tethyan ocean basins the upper triassic nieru formation of the tethyan himalaya sequence ths near lazi city 29 n 87 5 e is dominated by indian affinity precambrian detrital zircons which are typical of the majority of the ths however the upper triassic langjiexue formation of the ths exposed to the east at 90 93 e longitude includes significant populations of permian to early jurassic 291 184 ma detrital zircons for which there is no known indian source in addition the upper triassic nieru formation near kangma town 28 5 n 90 e located 200 km to the southeast of lazi city yielded detrital zircon age spectra that are similar to those of langjiexue formation based on detrital zircon age spectra comparisons we propose that both the langjiexue and nieru formations were derived from continental crustal fragments that were adjacent to the northwestern margin of australia furthermore we suggest that these ths units and age equivalent strata in northwest australia west sulawesi timor and west papua comprised a late triassic submarine fan along the northern australian shelf the upper triassic mailonggang formation in the southern lhasa terrane 35 km northeast of lhasa city 30 n 91 5 e is dominated by permian detrital zircons which were likely derived from proximal lhasa terrane sources the mailonggang formation differs from all age equivalent strata in the tethyan himalaya therefore we interpret that it was separated from greater india by the neo tethyan ocean

late cretaceous provenance change in the jiaolai basin east china implications for paleogeographic evolution of east asia abstract east china experienced crustal extensions during the early cretaceous and the latest cretaceous cenozoic for a long time previous studies have recognized a short lived compressional event between these two extensional regimes during the early late cretaceous but not much attention has been paid to it although it may shed new light on the tectonic evolution of east asia and the mechanism of the north china craton destruction the jiaolai basin located at the margin of east china consists of a thick succession of cretaceous sedimentary rocks and thus may help in further interpreting the mechanism and influence of the short lived compressional event here we apply integrated sedimentological methods to sandstone samples of both the lower cretaceous laiyang group and upper cretaceous wangshi group our findings suggest that there was a provenance change in the jiaolai basin from the early cretaceous granitic and metamorphic rock origin to the late cretaceous basic intermediate volcanic origin combined with the zircon u pb ages we conclude that the provenance change was caused by the uplift of the coastal mountains along the east asian margin from the relatively distal sulu orogenic belt and jiaobei terrane during the early cretaceous to the intra basinal volcanic rocks of the qingshan groupduring the late cretaceous after careful evaluation of the provenance histories orogenic exhumations widespread unconformities and paleoclimate changes that occurred in sedimentary basins both along the east asian margin and in the hinterland we finally estimate the possible range of influence and extent of the short lived compression event during the early late cretaceous

geochemical and geochronological constraints on the origin and emplacement of the yarlung zangbo ophiolites southern tibet

a precise u pb zircon age for the inishtrahull syenitic gneiss county donegal ireland smd zircon fractions from the deformed alkaline igneous complex on inishtrahd county donegal ireland yield near concordant u pb ages of 1779 3 ma interpreted as dating the igneous crystallization of the gneiss9s precursor sm nd depleted mantle model ages range from 1912 ma to 1978 ms the similarity of the model ages and the crystallization age indicates that the protoliths were derived from a depleted mantle source essentially uncontaminated by older crust the isotopic data confirm previous tentative correlations of the inishtrahull rocks with those of islay in southwestern scotland archaean crust is apparently absent from this region consistent with recent models placing a southern limit on the archaean along the line of the great glen fault zone pre caledonian orthogneisses in scotland and ireland south of the great glen fault zone have generally been correlated with the late archaean lewisian complex of northwestern scotland e g wilkinson 1907 mccallien 1930 bowes hopgood 1975 dixon et al 1990 although aftalion max 1987 concluded that the lewisian was absent from the annagh division gneisses of north mayo ireland there are now several lines of evidence to suggest that archaean crust may be absent southeast of the great glen fault zone e g fitches et al 1990 daly 1990 halliday et al 1985 a large area of pre caledonian basement here termed the rhinns complex is thought to be present within the fault bounded colonsay west islay block fig 1 bentley et al 1988 fitches et al 1990 marcantonio et al 1988 which extends from colonsay in

crustal structure restoration and evolution of the greater himalaya in nepal south tibet implications for channel flow and ductile extrusion of the middle crust

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

late permian to early mesozoic rifting and sedimentation offshore nw ireland abstract a late permian triassic and early lias rift succession up to 1 km thick is revealed from offshore nw ireland sediments are preserved in deep narrow grabens from the donegal basin in the north southwestwards through the erris and slyne troughs to a series of newly defined basins collectively forming the north porcupine basin with a further structure the galway graben on the porcupine bank this line of basins superimposed on a caledonoid fractured basement underwent a complex structural evolution but since the late permian they developed primarily as a wrench modified rift system during the latest lias crustal doming west of britain may have produced a regional angular unconformity and created a source for thick middle jurassic alluvial clastics deposited in the main porcupine basin to the south early tertiary transpression of the north porcupine area caused late stage fragmentation of the basin the rift succession consists of continental red beds bounded by two dateable transgressive intervals these intervals provide valuable stratigraphic control for constraining the timing and scale of seaways propagating across the fracturing pangean supercontinent from the late permian to the early jurassic evidence is also presented for an episode of syn rift volcanism temporally unique to western britain and ireland