lithofacies microfacies and depositional environments of upper cretaceous oceanic red beds chuangde formation in southern tibet the gyangze basin located in southern tibet is one of the representative areas for cretaceous oceanic red beds corbs we investigated and inter correlated several new corb outcrops in the western and southeastern part of the gyangze area in addition to the previously described chuangde section the corbs in the gyangze basin mainly consist of shales thin bedded marls re sedimented limestones and clasts supported conglomerates breccias planktonic foraminifers in the carbonate rocks within corbs belong to the globotruncana ventricosa and globotruncanita calcarata zones indicative of middle campanian age the facies assemblages in the different sections indicate that the depositional environments of corbs range from outer base of slope apron to basin zones within carbonates we observed microfacies types characteristic of basin open deep shelf margin or toe of slope environments gravity transport resulted from the steepening of the sea floor in the gyangze basin owing to subduction during the campanian relatively oxic bottom water conditions prevailed below the toe of slope environment in the gyangze basin during corb deposition however the presence of gray clasts within a multicoloured matrix suggests less oxic conditions at shallower water depths within the basin c 2010 elsevier b v all rights reserved

deformation enhanced fluid distribution in feldspar aggregates and implications for ductile shear zones experiments at 900 c and 1 4 gpa show that the distribution of aqueous fluid in fine grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation the isolated pore distribution is rapidly regained during annealing following deformation the deformation enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength it also increases the bulk transport rate through the aggregate by more than an order of magnitude a change in fluid distribution is not observed in fine grained quartz aggregates deformed at similar conditions if deformation enhanced fluid distribution occurs in naturally deformed feldspathic rocks it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones

franciscan complex olistostrome at crescent city northern california an olistostrome and bounding turbidites are exposed within the late mesozoic franciscan complex along the crescent city california coastline facies grade in character from mutti ricci lucchi 1978 mixed facies b c and d to f the olistostrome to mixed a b and e progressing upwards within the franciscan stratigraphic section the facies f unit outcrop is up to 600 m thick and extends 12 km along strike it consists of oblate to tabular blocks up to 200 m in maximum dimension of greenstone tonalite radiolarian chert limestone phyllite and greywacke dispersed in a scaly argillite matrix the olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites by the presence of abundant recycled sedimentary clasts within the unit and by the presence of sedimentary breccias and associated dismembered slump folded turbidites both within the olistostrome and among subjacent turbidites sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected late mesozoic sierran klamath magmatic are franciscan rocks record an early pervasive layer parallel flattening strain in such features as extensional faults necking and pinch and swell structures several scales of extensional faulting account for the juxtaposition of turbidites of different facies and or with varying degrees of stratal disruption the formation of sandstone lozenges and the formation of scaly foliation in the olistostrome matrix the latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay these were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome the presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix and of sandstone dykes that intrude fractures and associated drag folded turbidite beds indicate that franciscan sediments were not lithified during their early deformation these sediments were deposited in either a trench or trench slope basin and were first deformed either by gravity collapse of the trench slope cover or less likely by vertical loading beneath the toe of the accretionary wedge they later were folded during internal shortening of the growing franciscan accretionary prism

stratigraphic and sedimentological constraints on the age and tectonic evolution of the neotethyan ophiolites along the yarlung tsangpo suture zone tibet

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

eocene development of the northerly active continental margin of the southern neotethys in the kyrenia range north cyprus we focus on an active continental margin related to northwards subduction during the eocene in which sedimentary melange olistostromes forms a key component maastrichtian early eocene deep marine carbonates and volcanic rocks pass gradationally upwards into a thick succession 800 m of gravity deposits exposed in several thrust sheets the lowest levels are mainly siliciclastic turbidites and debris flow deposits interbedded marls contain middle eocene planktonic benthic foraminifera and calcareous nannofossils sandstones include abundant ophiolite derived grains the higher levels are chaotic debris flow deposits that include exotic blocks of late palaeozoic mesozoic neritic limestone and dismembered ophiolite related rocks a thinner sequence 200 m in one area contains abundant redeposited paleogene pelagic limestone and basalt chemical analysis of basaltic clasts shows that some are subduction influenced basaltic clasts from unconformably overlying alluvial conglomerates late eocene oligocene indicate derivation from a supra subduction zone ophiolite including boninites taking account of regional comparisons the sedimentary melange is interpreted to have formed within a flexurally controlled foredeep floored by continental crust gravity flows including large limestone blocks multiple debris flows and turbidites were emplaced followed by southwards thrust imbrication the emplacement was possibly triggered by the final closure of an oceanic basin to the north alanya ocean further convergence between the african and eurasian plates was accommodated by northwards subduction beneath the kyrenia active continental margin subduction zone rollback may have triggered collapse of the active continental margin non marine to shallow marine alluvial fans prograded southwards during late eocene oligocene time marking the base of a renewed depositional cycle that lasted until latest miocene time

zircon u pb and hf isotopic constraints on the onset time of india asia collision the time of initial collision between india and asia has been extremely controversial despite the fact that it is vital to constraining the orogenesis and subsequent evolution of the himalayas and tibetan plateau here we report u pb and hf isotope analysis of detrital zircons from two principal foreland basins that is the sangdanlin and gyangze basins respectively in the western and central parts of southern tibet our data suggest that asian derived clastic sediments started contributing to sedimentation on the indian continental margin earlier than generally thought at 60 ma in both basins near the yarlung zangbo suture zone in southern tibet no evidence for the existence of an intra oceanic arc within the neotethys is observed we conclude that 60 ma can be used to constrain the onset time of the india asia collision at least in central tibet after this initial collision closure of the neotethys propagated both westward and eastward with the final closure occurring at 50 and 45 ma in northwestern and eastern himalayas respectively our conclusion differs significantly from the previous view that the india asia collision may have started in northwestern himalaya and propagated eastward with diachronous suturing

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

the yarlung suture melange lopu range southern tibet provenance of sandstone blocks and transition from oceanic subduction to continental collision abstract with the aim of better understanding the history of ocean closure and suturing between india and asia we conducted a geologic investigation of a siliciclastic matrix tectonic melange within the western yarlung suture zone of southern tibet lopu range region 50 km northwest of saga the siliciclastic matrix melange includes abundant blocks of ocean plate stratigraphy and sparse blocks of sandstone metapelite and metabasite blocks in the melange exhibit lower greenschist facies mineral assemblages indicating that they were not deeply subducted we obtained detrital zircon u pb geochronologic and sandstone petrographic data from sandstone blocks in the melange and sandstone beds from tethyan himalayan strata exposed to the south of the suture the sandstones from both units are all similar in u pb detrital zircon age spectra and petrography to the nearby tethyan cretaceous paleocene sangdanlin section which records the earliest appearance at 59 ma of arc affinity strata deposited conformably on indian affinity strata two paleocene sandstones one of which is a schistose block incorporated in the siliciclastic matrix melange yielded indistinguishable maximum depositional ages of 59 ma mesozoic asian affinity sandstone blocks previously documented in the siliciclastic matrix melange 200 500 km along strike to the east are notably absent in the lopu range region we documented a gradational transition in structural style from the block in matrix melange in the northeast to the south vergent tethyan thrust belt in the southwest blocks of tethyan himalayan strata increase in size and the volumetric proportion of matrix decreases from northeast to southwest we conclude that no arc affinity sandstone blocks were incorporated into the subduction complex until india asia collision at 59 ma when the xigaze forearc basin became overfilled and tethyan himalayan strata entered the trench as collision progressed there was a gradual transition in structural style from block in matrix melange formation to imbricate style thrust belt formation

on induction effects of geomagnetic daily variations from equatorial electrojet and solar quiet sources at low and middle latitudes 1 we investigate the spatiotemporal behavior of the magnetic vertical component z of the daily ionospheric current systems the equatorial electrojet eej and solar quiet sq variations considering induction in the mantle and oceans the inducing eej and sq current systems are provided by the comprehensive model of sabaka et al 2004 the three dimensional 3 d conductivity model of the earth includes oceans of laterally variable conductance and a spherical conductor 1 d underneath our model studies demonstrate that induction effects in z due to the eej are negligible everywhere inland for all local times at champ altitude 400 km the magnetic signal induced by eej above the oceans does not exceed 2 5 of the external field during local noon this in particular means that considering the induction effects is not necessary when modeling the eej current strength from inland surface magnetic measurements and or satellite data as expected induction in the oceans strongly affects the sq field the model studies show that the anomalous induction effect defined as the difference between results obtained with 1 d and 3 d conductivity models of sq is substantial at champ altitude comprising 50 of the total field it is therefore necessary to consider induction in the oceans when modeling sq variations for both ground based and satellite data finally we demonstrate that the anomalous behavior of the daily variations in z at south indian sites namely a large positive prenoon peak can be explained by 3 d induction of the sq variations with no contribution from the eej

formation of taconic melanges and broken formations in the hamburg klippe central appalachian orogenic belt eastern pennsylvania abstract the hamburg klippe of the central appalachian orogenic belt exposed in eastern pennsylvania displays a complex record of poly phase melange and broken formation development in a convergent margin setting it includes an imbricate stack of tectonic slices which consist of upper cambrian to upper ordovician deep water and continental slope sedimentary rocks emplaced by gravity sliding onto the laurentian passive margin during deposition of the upper ordovician martinsburg formation based on their internal structure and stratigraphy the block matrix ratios and relations and the inferred tectonic settings of origin we have differentiated the following melanges and broken formations whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision i sedimentary broken formations without exotic blocks formed by in situ and local down slope remobilization during the early stages of closure of the octoraro sea basin in the early ordovician ii two types of sedimentary melanges with exotic blocks formed at the front of the advancing accretionary wedge during the middle ordovician subduction of the laurentian continental margin beneath a microcontinent magmatic arc tectonic assembly iii layer parallel extensional broken formation and a diapiric melange formed in the outer trench and at the toe of the accretionary wedge respectively during the early late ordovician iv precursory olistostromes formed during the late ordovician collisional episodes as the hamburg klippe was emplaced in the martinsburg formation on the downgoing laurentian continental margin and v contractional deformation related broken formations formed at the base of main thrust faults overprinting the previously formed melanges and broken formations this sequential development of different melange types in the central appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences the structural level of melange formation within the accretionary wedge and the kinematics of deformational processes

a fault zone conductor beneath a compressional inversion zone northeastern honshu japan 1 a resistivity section based on magnetotelluric data was obtained for the shonai plain fault in northeastern honshu japan faults in this area were created as normal faults during the opening of the japan sea in the miocene but are now reactivated as high angle reverse faults under compressional tectonics geological interpretations of the resistivity section support the proposed fault reactivation an estimated east dipping conductor along a deep part of the shonai plain fault system probably represents a fluid rich zone around the fault zone the high 3he 4he ratio near the fault indicates transportation of mantle fluid through this fluid rich zone these evidences may reflect the development of pronounced fracture permeability after fault rupturing as in the fault valve hypothesis

upper oligocene lower miocene gangrinboche conglomerate in the xigaze area southern tibet implications for himalayan uplift and paleo yarlung zangbo initiation abstract the gangrinboche conglomerate exposed along the yarlung zangbo suture zone records a crucial stage of the himalayan orogeny the type section of these strata in the xigaze area southern tibet including the qiuwu and dazhuka formations was studied by integrated stratigraphic sedimentologic petrographic and geochemical techniques palynological data and detrital zircon u pb ages indicate that the qiuwu formation was deposited during the latest oligocene to the earliest miocene most probably 23 ma while the overlying dazhuka formation was deposited during 23 18 ma the qiuwu formation was deposited in a deltaic setting and detritus was entirely derived from the gangdese magmatic arc in the north the dazhuka formation in contrast was deposited in mainly braided river environments and contains clasts derived from both the gangdese arc in the north and the himalayan orogen in the south clasts derived from the south first occur at the base of the dazhuka formation and increase in abunda

sedimentary history of the tethyan basin in the tibetan himalayas after an epicontinental phase the sedimentary rocks in the tibetan himalayas document a complete wilson cycle of the neo tethyan tethys ill evolution between the gondwana supercontinent and its northward drifting margin lhasa block from the late permian to the eocene during the triassic rift stage the basin was filled with a huge clastic dominated sediment wedge with up to 5 000 m of flysch in the northern zone widespread deltaic clastics and shallow water carbonates of late norian to earliest jurassic age in the southern zone mark in conjunction with decreasing tectonic subsidence the transition to the drift stage some 4 500 m of jurassic and early cretaceous shallow water carbonates and siliciclastics accumulated on the tethyan indian passive margin deepening upward sequences with condensed beds at their tops alternate with repeated progradational packages of shelf sediments extensive abyssal sediments with basaltic volcanics in the northern deep water zone reflect continued ocean spreading and thermal subsidence paleomagnetic data gained separately for the northern indian plate and the lhasa block indicate that the neo tethys reached its maximum width about 110 ma ago with a spreading rate of 4 8 cm year before it commenced to close again during the remnant basin stage in the late cretaceous and paleogene a shallowing upward megasequence capped by a carbonate platform developed in the southern inner shelf realm in the northern slope basin plain zone turbidites and chaotic sediments derived from both the acretionary wedge and the steepening slope of the passive margin accumulated the depositional center of the remnant basin shifted southward as a result of flexural subsidence and southward overthrusting the sediments from the triassic to the paleogene are tentatively subdivided into five mega sequences which are controlled mainly by regional tectonics climatic influence e g carbonate deposition due to northward plate motion is partially subdued by terrigenous input and or increased water depth during the oligocene and miocene crustal shortening led to rapid uplift and the deposition of fluvial molasse in limited basins

3 d magnetotelluric imaging of the phayao fault zone northern thailand evidence for saline fluid in the source region of the 2014 chiang rai earthquake abstract seismicity in thailand had been relatively low for decades prior to the mw 6 5 earthquake of 5 may 2014 which came as a surprise and was followed by thousands of aftershocks most of the epicenters were located in the transition region between the mae lao segment mls and the pan segment ps of the phayao fault zone pfz we conducted a 3 d magnetotelluric mt survey 31 sites to image the deep pfz structure the shallow 3 d resistivity structure matches very well with the surface geology while the deeper structures disclose many interesting resistive and conductive anomalies however the most interesting feature of this study is the large conductive anomaly ml located at a depth of 4 km to the mid crust beneath the mls near the seismogenic zone our current hypothesis is that the ml conductor has a highly interconnected aqueous fluid content and also plays crucial role in the earthquake sequence of the 5 may 2014 event as our previous seismic waveform study revealed that the mls has a relatively high fault plane instability the fluid within the fractured fault would further reduce the fault strength the accumulated pre existing tectonic stress from the north can therefore overcome the maximum frictional strength of the mls and hence cause it to slip and produce the main shock with the local structural heterogeneities and fluid in the fractured fault zones the aftershocks then occurred on both the ps and mls this is in contrast to the mae chan fault zone mcfz in the north which many scientists expected to generate a larger magnitude earthquake than any other faults since instrumental record it has only generated a few mw 4 earthquakes some of our mt stations were located within the mcfz however there is no deep conductor as the conductor lies beneath the mls a lack of interconnected fluid within the deep fault beneath the mcfz might be one of the reasons for the lower seismic activity from the mcfz other geophysical methods such as seismic tomography are necessary in order to confirm the presence of the fluid beneath the mls and also the lack of a deep conductor beneath the mcfz

magnetotelluric imaging of the creeping segment of the san andreas fault near hollister 1 fluids may play a significant role in the earthquake rupture process since fluids can dramatically change the electrical resistivity of crustal rocks electromagnetic exploration methods can image in situ fault zone fluids magnetotelluric mt data has been collected on segments of the san andreas fault saf in central california exhibiting a range of seismic behavior from locked to creeping this paper describes mt data from the creeping segment of the saf near hollister california inversion models image a zone of enhanced conductivity beneath the surface trace of the saf that has similar geometry and greater depth extent than the fault zone conductor at parkfield comparison with geologic and seismic tomographic studies show this zone is bounded by the san andreas and calaveras faults and coincident with a zone of low seismic p wave velocities combined with previous mt studies this indicates a correlation between seismicity and the presence of fault zone fluids

the indus yarlung zangbo ophiolites from nanga parbat to namche barwa syntaxes southern tibet first synthesis of petrology geochemistry and geochronology with incidences on geodynamic reconstructions of neo tethys the purpose of this first synthesis is to summarize findings on the yarlung zangbo suture zone yzsz ophiolites and discuss still remaining problems the yzsz studied for almost 30 years and is the youngest of the sutures recognized on tibet plateau it is now acknowledged that the yzsz is a complex assemblage of sedimentary metamorphic and igneous rocks created during and shortly after the collision between india and eurasia the ages of the various lithologies span a time interval from the jurassic to the middle miocene with some permian and devonian exotic blocks from melange zone the yzsz is characterized by ophiolitic complexes and ophiolitic melange the ophiolites are of two types non dismembered and dismembered sections the non disturbed sections although tectonically reworked are observed along the segment from dazhuqu to jiding in xigaze area and spontang ophiolite the dismembered sequences are found in various locations such as nidar kiogar jungbwa saga sangsang xigugabu and luobusa the incomplete stratigraphic log could be connected to intraoceanic or orogenetic origins the ophiolites are distributed into two groups of ages the luobusa zedang and kiogar sequences being jurassic lower cretaceous whereas all other sequences are of lower cretaceous age compilation of geochronological data suggest that some ophiolite sequences might have evolved for over more than 70 my from their initial genesis to obduction which occurred around 70 90 my ago ophiolites differ in terms of petrological and geochemical aspects however they were all generated in suprasubduction zone and more specifically in arc few fore arc and back arc settings synthesis of more than 700 geochemical analyses show variable mixing of components from n morb type to iat cab and to oib end members the jurassic ophiolites show the maximum of arc component while the lower cretaceous ones show little to strong mixing in addition most ophiolites were created in short lived 30 my basins and generated close to the eurasiatic continental margin we propose that ladakh tibet ophiolites were generated in a suprasubduction context similar to mariana arc interarc and back arc or tonga lau system the variable arc signature of these ophiolites is directly related to their initial position within the suprasubduction system

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

mega debris flow deposits from the oligo miocene pindos foreland basin western mainland greece implications for transport mechanisms in ancient deep marine basins the turbidite dominated oligo miocene pindos foreland basin of western mainland greece contains two thick 60 72 m matrix supported conglomerates the conglomerates are ungraded and contain three clast types 1 polymict rounded extrabasinal clasts long axes 3 50 cm 2 tightly folded intrabasinal clasts long axes 1 10 m and 3 tabular largely undeformed intrabasinal blocks long axes 18 300 m clasts are isolated within a slit dominated matrix these chaotic matrix supported conglomerates are interpreted as mega debris flow deposits during transport extrabasinal clasts were supported by a combination of matrix cohesion and clast dispersive pressure folded intrabasinal clasts were supported by a combination of buoyancy archimedes principle and clast dispersive pressure the large tabular clasts were transported by gravity sliding gliding within the flow on films at high pore fluid pressure these different clast support mechanisms were active simultaneously within the pindos mega debris flow deposits as a result the deposits have no systemic vertical stratigraphy in contrast to many described large scale mass flow deposits the mega debris flow deposits are significantly thicker than most described ancient siliciclastic debris flow deposits and provide an ancient analogue for the thick recent siliciclastic debris flow deposits on continental margins

comment on the species and age determination published by li et al discovery of radiolaria from upper cretaceous oceanic red beds in daba kangmar and its paleogeographic implication palaeogeography palaeoclimatology palaeoecology 312 2011 127 137 abstract li et al 2011a discovery of radiolaria from upper cretaceous oceanic red beds in daba kangmar and its paleogeographic implication palaeogeography palaeoclimatology palaeoecology 312 127 137 reported three radiolarian assemblages from cretaceous oceanic red beds corbs in chuangde formation which they assigned to the late cretaceous with only very few exceptions all species have been erroneously determined and consequently the age of the samples was wrongly interpreted none of these radiolarians suggest a late cretaceous age late santonian to early maastrichtian as proposed in the publication on the contrary all radiolarians are indicators of an early cretaceous age early barremian to earliest albian

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

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

the himalaya bengal fan denudation accumulation system during the past 20 ma mass balances for both denudation in the himalayas and sediment accumulation in the subhimalayan basins including the bengal deep sea fan but excluding the indus fan yield 7 1 106 km3 and 7 4 106 km3 s 20 rock of 2 75 g cm3 density respectively for the past 20 million years coarsening and increased sediment accumulation rates in the foreland basin and in the bengal foredeep indicate accentuated tectonic activity and unroofing in the himalayas since that time the sediment volume includes 1 106 km3 of neogene bengal fan sediment that was lost via the nicobar fan to the sunda accretionary wedge in addition the indian peninsular rivers contributed about c 0 6 106 km3 of solid load to the basins average denudation during the past 20 m y as derived from geothermobarometric data and restored cross sections occurred most rapidly along the high himalayan crystalline chain vertical unroofing c 1000 m m y northward lateral retreat of southern himalayan slope exposed to monsoonal rain

various types of olistostromes in a closing ocean basin tethyan himalaya cretaceous tibet abstract the cretaceous rocks of the central tethyan himalayas display different types of deep sea olistostromes and chaotic deposits which are accompanied by turbidites the deposits were incorporated into an accretionary prism and more or less deformed according to the composition and provenance of their clasts we distinguish four types of olistostromes 1 p type derived from the passive indian continental margin 2 po type derived from both the passive indian margin and ocean floor pelagic sediments and oceanic crust 3 poa type with clasts from the same sources as the po type but also containing material from the active margin along the lhasa block and 4 a and oa type mainly derived from the active margin accretionary prism and magmatic arc the occurrence of these different types of deep sea chaotic deposits in space and time is closely related to the evolution of the neo tethyan basin from a wide open ocean basin to a narrowing remnant and trench basin prior to collision whereas pure p type olistostromes formed when the basin was still wide predominating a type deposits characterize a late stage of basin evolution prior to collision mixed type olistostromes record an intermediate stage of basin evolution and require specific syn sedimentary tectonic activity

geochemistry of sedimentary rocks from melange and flysch units south of the yarlung zangbo suture zone southern tibet abstract the mesozoic yamdrock melange and the triassic flysch lie immediately south of the yarlung zangbo suture zone yzsz ophiolitic belt in southern tibet the whole rock chemical signature of sandstones mostly greywackes red shales and black shales of the yamdrock melange and flysch units do not present significant variations between the different rock types and geological units and is concordant with a continental passive margin source despite fairly important chemical weathering the geochemical signature was not affected by significant sedimentary recycling nor significant heavy mineral accumulation ree patterns show an enrichment in lree typical of shales and indicate that the neo tethyan turbidites mostly derive from a felsic old upper continental crust source slight enrichments in ti and sc indicate a mafic contribution that could originate from mafic blocks of enriched intraplate geochemical affinity found in the sedimentary units sedimentary rocks from both units were thus derived from the old indian continental crust including granitic and intraplate mafic blocks but were deposited in different tectonic settings in the neo tethys ocean the homogeneity of the yzsz data which cover the whole neo tethyan basin and correlation with himalayan data support the formation of all himalayan lithotectonic units along a continuous passive margin of northern india

variations in late cenozoic recent strike slip and oblique extensional geometries within indochina the influence of pre existing fabrics abstract from yunnan to northern thailand late cenozoic recent faults strike predominantly nne ssw n s to nne ssw and ne sw to ene wsw associated sedimentary basins are aligned ne sw to n s the regional fault patterns are commonly interpreted as strike slip dominated deformation throughout the area releasing bend and en echelon stepping patterns on faults bounding sedimentary basins indicate sinistral displacement on ne sw to ene wsw trending faults yet in the escape tectonics model left lateral displacement on the ne sw to ene wsw faults is thought to have occurred late in the miocene whilst earlier motion was dextral however in yunnan the ne sw shmax direction required for dextral motion on the n s sagaing nanting and gaoligong fault zones is consistent with sinistral motion on ene wsw striking faults which is still their sense of motion today in northern thailand the dextral sinistral switch model during the miocene is not tenable because the fang basin is of late oligocene pliocene age and requires similar age sinistral motion on the ene wsw mae chan fault in order to have opened in an alternative model northern thailand is interpreted to have evolved predominantly by oblique extension the golden triangle area marks a transition from transtensional deformation in the north to oblique extension in the south the activation of pre existing fabrics strongly affects both strike slip and extensional faults and has given rise to the similar extensional and strike slip fault patterns multiple episodes of basin inversion in northern thailand during the miocene require short term changes in stress pattern to produce the inferred changes in stress pattern it is suggested that stresses radiating out from the himalayan syntaxis exert a strong influence but were not the only important forces acting on the region

crustal heterogeneity in the 2000 western tottori earthquake region effect of fluids from slab dehydration abstract in order to understand the generating mechanism of the western tottori earthquake m 7 3 that occurred on 6 october 2000 in southwest japan we determined high resolution 3d p and s wave velocity and poisson s ratio structures in the epicentral area we used 17 542 p and 13 831 s wave high quality arrival times from 721 tottori aftershocks and other local microearthquakes recorded by the dense high sensitivity seismic network hi net installed recently on the japan islands significant variations of up to 4 for seismic velocity and 9 for poisson s ratio are revealed in the aftershock area areas with large coseismic slips and high aftershock activity are associated with high p wave velocity v p high poisson s ratio and high electrical conductivity which may represent strong and competent parts of the fault zone containing fluids the western tottori mainshock hypocenter is located in a boundary zone where both velocity and poisson s ratio change drastically both p and s wave velocities are slower in the lower crust under the mainshock hypocenter and low frequency microearthquakes were detected within the slow anomalies around the moho discontinuity before and after the occurrence of the 2000 western tottori earthquake these results and other related evidence suggest that the strong crustal heterogeneity in the western tottori source area is associated with fluids and arc magma under nearby the daisen volcano which may have influenced the nucleation and rupture process of the western tottori earthquake the arc magma and fluids are thought to result from the dehydration reactions of the subducting philippine sea slab in this region these results suggest that the nucleation of shallow earthquakes could be controlled by a deep structure and regime and so it is important to study the deeper crust and upper mantle structure and processes for clarifying the generating mechanism of large crustal earthquakes

conglomerates record the tectonic evolution of the yarlung tsangpo suture zone in southern tibet the histories of individual conglomeratic units along the yarlung tsangpo river suture zone in southern tibet reflect significant phases in the mesozoic to cenozoic tectonic evolution of this area several temporally distinct conglomerate units are recognized along the suture and their detailed examination permits analysis of the collision between india and asia upper jurassic to lower cretaceous conglomerates crop out within the sangri group along the southern lhasa terrane they are dominated by limestone and andesitic volcanic cobbles derived entirely from the lhasa terrane these rocks have experienced amphibolite facies metamorphism and exhibit a strong penetrative regional foliation thick successions of the palaeocene liuqu conglomerate crop out within the suture from xigaze to lhaze they contain detritus sourced from intra oceanic terranes associated with the suture zone as well as clasts of indian affinity while lhasa and xigaze terrane derived material is notably absent these conglomerates record an early suture zone event prior to india asia collision uppermost oligocene to lower miocene gangrinboche facies conglomerates crop out on the southern edge of the lhasa terrane along the length of the suture several correlative units within this facies exhibit broadly similar stratigraphic histories a basal depositional contact upon an eroded lhasa terrane surface is ubiquitous with initial clast derivation from the north up section the first arrival of coarse grained suture zone and india derived clasts is abrupt these southerly derived clasts predominate by the top of most sections an a really restricted succession of gently dipping late neogene ultramafic breccias unconformably overlies folded liuqu conglomerate near quanggong other neogene sediments are extensive west of mount kailas deposition of coarse clastic sediments is presently continuing along the length of the yarlung tsangpo discrimination and detailed investigation of each of these units will improve our understanding of the evolution of the india asia collision

gondwana to asia plate tectonics paleogeography and the biological connectivity of the indian sub continent from the middle jurassic through latest eocene 166 35 ma abstract using the most up to the date information available we present a considerably revised plate tectonic and paleogeographic model for the indian ocean bordering continents from gondwana s middle jurassic break up through to india s collision with asia in the middle cenozoic the landmass framework is then used to explore the sometimes complex and occasionally counter intuitive patterns that have been observed in the fossil and extant biological records of india madagascar africa and eastern eurasia as well those of the more distal continents although the paleogeographic model confirms the traditional view that india became progressively more isolated from the major landmasses during the cretaceous and paleocene it is likely that at various times minor physiographic features principally ocean islands provided causeways and or stepping stone trails along which land animals could have migrated to from the sub continent aside from a likely link albeit broken by several marine gaps to africa for much of this time it is notable that the present day recent biota of madagascar indicates that the ancestors of five land mammal orders plus bats crossed the 400 km wide mozambique channel at different times in the cenozoic it is possible that the kerguelen plateau connected india and australia antarctica in the mid cretaceous approximately 115 90 ma later the seychelles mascarene plateau and nearby elevated sea floor areas could have allowed faunas to pass between southern india and madagascar in the late cretaceous from around 85 65 ma with an early cenozoic extension to this path forming as a result of the reunion hot spot trace islands growing on the ocean floor to the ssw of india the modelling also suggests that india s northward passage towards asia with eventual collision at 35 ma involved the ne corner of the sub continent making a glancing contact with sumatra followed by burma from 57 ma late paleocene onwards a scenario which is compatible with the fossil record indicating that india asia faunal exchanges began occurring at about this time finally we contend that a number of biologically based direct terrestrial migration routes that have been proposed for last 15 m y of the cretaceous asia to india antarctica to madagascar and or india can probably be dismissed because the marine barriers likely varying from 1000 up to 2500 km were simply too wide

provenance analysis of upper cretaceous strata in the tethys himalaya southern tibet implications for timing of india asia collision abstract the provenance of upper cretaceous strata in the tethys himalaya provides critical constraints on the closure time of the neo tethys ocean and the initial india asia collision this paper reports detailed petrographic studies in situ detrital zircon u pb ages and lu hf isotopic analyses whole rock nd isotopes and cr spinel electronic microprobe data from upper cretaceous clastic sedimentary rocks of the tethys himalaya near gyangze southern tibet the berriasian coniacian jiabula formation consists of black mudstone chert and minor quartz arenite and is dominated by detrital zircons with archean cambrian u pb ages which were most likely derived from the indian continent overlying conformably is the santonian maastrichtian chuangde formation which consists of red shale limestone and chert the chuangde formation is in turn overlain by the late maastrichtian late paleocene zongzhuo formation which is composed of black mudstone and lithic sandstone enclosing various olistoliths of sandstone limestone and chert the rilang conglomerate is a lens which is located within the upper part of the zongzhuo formation and consists of an upward thinning and fining succession of volcanic conglomerate sandstone and black mudstone the zongzhuo formation and the rilang conglomerate record an abrupt influx of cretaceous zircon grains with juvenile hf isotopic compositions arc related cr spinels and positive end 0 sediments suggesting an arc and suture zone provenance the change in provenance of upper cretaceous strata from the southern indian continent to a northern arc and suture zone is attributed to either 1 initial collision between the indian plate and lhasa terrane or 2 initial collision between the indian plate and an intra oceanic arc we prefer option 1 that the initiation of india asia collision occurred during maastrichtian 70 65 ma

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

new constraints on the india asia collision the lower miocene gangrinboche conglomerates yarlung tsangpo suture zone se tibet abstract lower miocene conglomerates crop out along the length of the yarlung tsangpo suture zone on the southern margin of the lhasa terrane these conglomerates known by various local names are correlated herein as the gangrinboche conglomerates all units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the lhasa terrane is ubiquitous at most localities the tops of sections are either removed by erosion or truncated by north directed thrusts these conglomeratic molasse units developed in response to the india asia collision and record aspects of its development in all units initial clast derivation was from the lhasa terrane on the northern margin of the yarlung tsangpo suture zone up section the first appearance of clasts derived from terranes within the suture zone and the northern margin of india all of which lie to the south of any outcrops of gangrinboche conglomerates is observed although these units were previously thought to be eocene analysis of fossil and structural constraints indicates early miocene deposition as development of the gangrinboche conglomerates records a significant phase in the evolution of the india asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models

crustal and uppermost mantle structure and seismotectonics of north china craton abstract we determined a 3 d p wave anisotropic tomography of the crust and uppermost mantle beneath north china craton ncc using 107 976 p wave arrival times from 16 073 local earthquakes recorded by 380 seismic stations our results show significant lateral heterogeneities beneath ncc the lower crust and uppermost mantle beneath the north china basin show widespread low velocity anomalies which may reflect high temperature materials caused by the late mesozoic basaltic magmatism in the ncc low velocity anomalies also exist beneath the trans north china orogen which may reflect asthenospheric upwelling since late mesozoic large crustal earthquakes generally occurred in high velocity zones in the upper to middle crust while low velocity and high conductivity anomalies that may represent fluid filled fractured rock matrices exist in the lower crust to the uppermost mantle under the source zones of the large earthquakes the crustal fluids may lead to the weakening of the seismogenic layer in the upper and middle crust and hence cause the large crustal earthquakes the nw se p wave fast velocity directions seem to be dominant in the uppermost mantle under the central parts of eastern ncc suggesting that these mantle minerals were possibly regenerated but keep the original fossil anisotropy formed before the new lithospheric mantle was produced during the mesozoic to cenozoic

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

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

early eocene sedimentary recycling in the kailas area southwestern tibet implications for the initial india asia collision abstract syncollisional sedimentary rocks in the himalayan orogen record important information about the early history of the india asia collision this article presents a combined stratigraphic sedimentologic micropaleontologic and provenance data for the early eocene clastic strata dajin formation in the kailas area southwestern tibet the dajin formation comprises ungraded and normally graded matrix and clast supported conglomerates ungraded and cross stratified sandstones and massive to poorly laminated mudstones deposition of the rocks is characterized by subaerial and subaqueous debris flows waning gravity flows and suspension fallout the larger benthic foraminifera and the youngest ages of the detrital zircons constrain the depositional age to the earliest eocene 56 54 ma the dajin formation contains abundant sandstone and mudstone clasts indicating significant sedimentary recycling in the source area u pb ages of detrital zircons are mostly clustered at 120 54 ma and have positive e hf t values suggesting a gangdese affinity clasts of the dajin formation are interpreted as having recycled from the forearc strata that were originally derived from the gangdese magmatic arc highly immature texture and recycled provenance lead us to propose that deposition of the dajin formation was a result of the development of fold thrusts in the gangdese forearc our new results and published data from the coeval strata in the himalayan orogen indicate that an underfilled foreland basin was initiated soon after the initial india asia collision and the dajin formation records the wedge top depozone of the basin system

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

provenance of the upper cretaceous eocene deep water sandstones in sangdanlin southern tibet constraints on the timing of initial india asia collision abstractthe first arrival of sedimentary material from asia onto the indian continental margin provides a minimum constraint on the timing of initial india asia collision a combination of petrology detrital cr spinel geochemistry and zircon u pb dating of the upper cretaceous to eocene deep water succession at the sangdanlin section southern tibet provides evidence for major provenance change within the strata the upper cretaceous paleocene denggang formation quartzarenites contain zircons with dominant proterozoic ordovician u pb ages with an additional age peak of early cretaceous which we interpret to be derived from the northern indian margin by contrast the lithic sandstones of the early to middle eocene sangdanlin and zheya formations are dominated by zircons younger than 200 ma showing one major peak at 80 125 ma and two subdominant peaks at 54 70 and 180 196 ma comparable to those from the gangdese magmatic arc cr spinels in the sangdanlin and zheya formations are abundant and char

the casanova complex of the northern apennines a melange formed on a distal passive continental margin abstract the cretaceous palaeocene casanova complex occurs in two thrust sheets of the eugeosynclinal ligurids of the northern apennines it is a sedimentary melange with ophiolitic and quartzose turbidites or limestone shale olistostrome submarine debris flows as matrix exotic blocks of ophiolite and granite serpentinite breccias and lenticular ophiolitic breccias and olistostromes contribute to the melange character of the complex deformational structures include soft sediment slump folds indicating a sw dipping palaeoslope and boudins a gradational slumped top to the melange small scale faults in chert blocks and deformation associated with the emplacement of the exotic slide blocks the blocks were shed as rotational slides from submarine fault scarps and are surrounded by haloes of debris created by submarine weathering the stacking pattern of the blocks with the originally stratigraphically highest ophiolite lithologies lowest in the pile of blocks is explained by a diverticulation model with progressively deeper erosion mechanical analysis shows that the blocks were stable when partly exposed resting on a soft sediment substratum criteria which distinguish the casanova complex from a tectonic melange and which may be of value in other melanges are discussed previous interpretations of the complex as a precursor olistostrome to northeastward nappe emplacement the bracco ridge model are rejected the melange is believed to have formed on ocean crust as a result of turbidite and debris flow sedimentation soft sediment deformation block faulting gravity sliding and submarine erosion at the distal edge of a uniformly sw dipping continental margin

evidence for middle triassic to miocene dual subduction zones beneath the shan thai terrane western thailand from magnetotelluric data abstract kanchanaburi province western thailand is in the middle of shan thai terrane paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the shan thai terrane during the late triassic there was a westward subduction in the east where the lampang chiang rai block subducted beneath the shan thai terrane in addition in the early tertiary the western burma terrane subducted underneath the shan thai in the west resulting in an eastward subduction a pioneer survey of this region was therefore conducted using magnetotelluric mt techniques with the aim of finding deep structures associated with these ancient subduction zones 39 mt stations were deployed to cover most of the province resistivity structures are obtained from the 3 d inversion the near surface resistivity structures correspond well with the surface geology the mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3 porosity in which the remnant dehydrated fluid was accumulated during the subduction in addition the westward and eastward subduction zones generated mafic ultramafic intrusions which appear as two conductive zones on the east and west respectively these two conductors and the conductive lower crust support the hypotheses of the middle triassic to miocene dual subduction zones