provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

deep structure of the northern kerguelen plateau and hotspot related activity summary seismic refraction profiles were carried out in 1983 and 1987 throughout the kerguelen isles southern indian ocean terres australes antarctiques franqaises taaf and thereafter at sea on the kerguelen heard plateau during the md66 keobs cruise in 1991 these profiles substantiate the existence of oceanic type crust beneath the kerguelen heard plateau stretching from 46 s to 55 s including the archipelago seismic velocities within both structures are in the range of those encountered in standard oceanic crust however the kerguelen isles and the kerguelen heard plateau differ strikingly in their velocity depth structure unlike the kerguelen isles the thickening of the crust below the kerguelen heard plateau is caused by a 17 km thick layer 3 velocities of 7 4 km s 1 or so within the transition to mantle zone below the kerguelen isles are ascribed to the lower crust intruded and or underplated by upper mantle material the crust mantle boundary below the kerguelen heard plateau is abrupt and devoid of any underplated material the difference in structure between the northern edge of the kerguelen plateau including the archipelago and the kerguelen heard plateau may be related to variability of the time dependent hotspot activity the kerguelen heard plateau was emplaced during the cretaceous time 110 ma when the volcanic output rate of the kerguelen plateau and the ninetyeast ridge was high as well as high potential temperature the northernmost kerguelen plateau and the archipelago were emplaced during tertiary time 40 45 ma as the volcanic output rate reduced furthermore intraplate volcanism continued in the kerguelen archipelago for at least 40 ma the isostatic compensation of the kerguelen isles and the kerguelen heard plateau is achieved by low density mantle material as shown by refraction and geoid studies the velocity depth structure below the kerguelen isles is similar to that found below intraplate oceanic islands such as hawaii despite the differences in age the crust below iceland 0ma and the kerguelen plateau 100 120 ma are strikingly akin the similarity between the kerguelen heard plateau and iceland then strongly supports a similar origin for both structures the kerguelen heard plateau being a fossil equivalent of present day iceland crustal thickening beneath the kerguelen heard plateau which may result from an iceland type setting i e an active spreading centre over a hotspot is mostly produced by thickening of layer 3 layer 2 representing 25 per cent only of the thickness of the igneous crust the kerguelen isles despite the initial volcanism near the active southeast indian ridge behave as a midplate volcanic island and are definitely not representative of the whole kerguelen plateau structure

the neogene to recent rallier du baty nested ring complex kerguelen archipelago taaf indian ocean stratigraphy revisited implications for cauldron subsidence mechanisms 1 ups cnrs fre 2566 orsayterre department des sciences de la terre universite de paris sud f 91405 orsay cedex france bbonin geol u psud fr 2 cnrs umr 6524 magmas et volcans department de geologie petrologie geochimie universite jean monnet f 42023 saint etienne cedex 2 france 3 cnrs ep 125 department des sciences de la terre universite de nice sophia antipolis f 06108 nice cedex 2 france 4 department of geology university college dublin belfield dublin 4 eire 5 gemoc macquarie university sydney australia

are granitic intrusions scale invariant published length l and thickness t data on 135 laccolith and 21 granite intrusions define power law relationships of the form l kt a typical of systems exhibiting scale invariant fractal behaviour both data sets are characterised by an exponent a 1 0 88 0 1 for laccoliths and 0 80 0 20 for plutons that reflects an inherent preference for scale invariant tabular sheet geometries these power law size relationships can be explained in mechanical terms by the need for an incoming magma sheet to travel laterally some distance before vertical thickening can occur sheet thickness is a function of available magma pressure which for an intrusion fed by a feeder dyke is proportional to the vertical magma transport distance

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

bimodal magmatism produced by delamination geochemical evidence from late palaeozoic volcanic rocks from the yili block western tianshan northwestern china

magma flow paths and strain patterns in magma chambers growing by floor subsidence a model based on magnetic fabric study of shallow level plutons in the stiavnica volcano plutonic complex western carpathians the miocene stiavnica volcano plutonic complex western carpathians exposes two nearly coeval intra caldera plutons their roof basement of a stratovolcano and associated volcanic rocks the complex thus provides insights into mechanisms of magma chamber growth beneath large volcanoes as inferred from the anisotropy of magnetic susceptibility ams these plutons were emplaced through significantly different processes the diorite as a discordant stock with steep fabric and the granodiorite as a tabular bell jar pluton in detail we interpret that the latter was assembled in two stages first an upper layer intruded as a thin sill along a major subhorizontal basement cover detachment the subhorizontal magnetic fabric and strongly oblate ams ellipsoid in this layer record intrusive strain where the magma flow paths were subparallel to the pluton roof second in the lower layer of the pluton magnetic foliations dip moderately to the nw and wnw to vertical and are associated with down dip to subhorizontal lineations and prolate to weakly oblate shapes of the ams ellipsoids such a fabric pattern is compatible with piecemeal floor subsidence where magma flowed along multiple subsiding fault bounded blocks based on this case example we develop a conceptual model for magma flow paths and strain patterns for four main modes of floor subsidence 1 piston cauldron subsidence is characterized by convergent flow and radial principal stretching above the magma chamber floor 2 the piecemeal floor subsidence leads to steep to inclined magma flow paths in conduits along fault bounded blocks 3 asymmetric trapdoor subsidence produces first divergent flow paths near the conduit sides changing into convergent paths in the narrower space near the kinematic hinge and 4 symmetric cantilever funnel subsidence will lead to divergent flow from a central feeder and thus circumferential principal stretching of the magma if the growing pluton develops a two layer structure all the flow paths and associated strains are affected by the flat lying pluton roof and will convert into horizontal flattening as the roof is approached

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

mode of emplacement of the slaufrudalur pluton southeast iceland inferred from three dimensional gps mapping and model building abstract the slaufrudalur pluton is a granitic pluton in the tertiary lava pile of southeast iceland excellent exposures of its roof and walls made it possible to map the shape of the pluton with high resolution gps based on the gps mapping and field observations we reconstructed the three dimensional shape of the slaufrudalur pluton with the aim to test which implications on the mechanisms of emplacement can be derived from this approach the reconstructed pluton shape is characterised by steep walls and a flat roof at map scale this shape and the internal compositional layering indicate that the pluton was probably emplaced by cauldron subsidence along subvertical faults that are parallel with the strike of the regional fissure swarms at the roof contact the pluton exploited the original layering of the flood basalts at outcrop scale however the roof was modified by magmatic stoping which resulted in a step like pattern on a scale that cannot be resolved in the three dimensional model hence the reconstruction of the three dimensional shape of the slaufrudalur pluton combined with structural field studies provided valuable information about the mechanism of its emplacement on pluton scale for a comprehensive understanding of all mechanisms involved in the emplacement of the slaufrudalur pluton detailed structural field studies remain essential

evolution of the himalaya abstract the compression and attendant deformation of a thick and vast sedimentary prism formed since early riphean times on the northern continental margin of the indian craton gave rise to the himalaya mountains as a result of convergence and collision of the indian and asian plates the oceanic trench sediments tectonically implanted with sea floor material and intimately associated with calc al kaline volcanics in the narrow sindhu tsangpo belt extending from kohistan through dras leh darchen mansarovar to shigatse and beyond represent the subduction island arc complex which developed south of the dynamic southern margin of the asian continent and was welded to the colliding indian plate during the late eocene to oligocene period this complex is fringed to the north by a wide zone of andean type granitic bodies the evolution of the himalayan orogen is closely connected with the development of the present day andaman nicobar indonesia island arc subduction system in the southeast and the makran ranges oman trench in the southwest the evolution of the himalaya was accomplished in four major phases of tectonic upheaval during the late cretaceous to palaeocene karakoram phase late eocene to oligocene malla johar phase middle miocene to pontian sirmurian phase and late pliocene to middle pleistocene siwalik phase while the karakoram phase marks the convergence of continents and the malla johar phase represents the collision and subduction it was during the sirmurian upheaval that the main tectonic features developed and the himalaya acquired its distinctive structural complexion

crustal volumes of the continents and of oceanic and continental submarine plateaus global topographic data and the assumption of airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus the calculated crustal volumes are 7182 x lo6 km3 for the continents 242 x lo6 km3 for continental submarine plateaus and 369 x lo6 km3 for oceanic plateaus the falkland plateau and the lord howe rise are the two largest continental submarine plateaus with volumes of 48 x lo6 km3 and 47 x lo6 km3 respectively total continental crustal volume is 7581 x lo6 km3 including the volume of continental sediments on the ocean floor 160 x lo6 km3 in good agreement with previous estimates continental submarine plateaus on the seafloor comprise 3 2 of the total continental crustal volume the largest oceanic plateaus in order of decreasing size are the ontong java plateau the kerguelen plateau the caribbean the chagos laccadive ridge the ninetyeast ridge and the mid pacific mountains together they comprise 54 of the total anomalous crustal volume in oceanic plateaus an upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3 7 km3 yr a value that assumes accretion of all oceanic plateaus with a total volume of 4 9 of the continental crustal volume on a 100 myr time scale even if a substantial fraction of the crustal volume in oceanic plateaus is subducted accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate subduction of continental submarine plateaus with the oceanic lithosphere on a 100 myr time scale gives an upper bound to the continental crustal subtraction rate of 2 4 km3 yr much larger than recent estimates of crustal subtraction by subduction of seafloor sediments effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus a potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents entrapment of the continental fragments in the seafloor and subduction of the fragments with the oceanic lithosphere this process may be occurring in the mediterranean for corsica and sardinia

tectonic setting origin and obduction history of the spontang ophiolite ladakh himalaya nw india abstract the spontang ophiolite forms the highest tectonic thrust slice above the mesozoic early tertiary continental margin of the north indian plate in the ladakh zanskar himalaya detailed field mapping combined with geochemical analysis has defined two major units a full ophiolite sequence spontang ophiolite overlain by an upper unit consisting of 500 m thick basalt andesite volcanic and volcano sedimentary rocks of island arc affinity spong arc the spontang ophiolite comprises a harzburgitic mantle sequence gabbroic and ultramafic cumulates isotropic gabbros and highly tectonized sheeted dikes feeding pillow lavas with a few uncommon highly fractionated plagiogranites a separate lherzolitic peridotite unit affected by possible transform related shearing was thrust over the harzburgites in the west possibly during the early stages of subduction initiation beneath the ophiolite whole rock geochemistry and u pb geochronology show that the ophiolite formed at a normal mid ocean ridge sp

structure profonde du mont ross d apres la refraction sismique iles kerguelen ocean indien austral mont ross is the main volcanic feature of the kerguelen archipelago terres australes et antarctiques francaises this newly formed volcano buildup over 2 ma provides us with an outstanding model

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

geophysical images and a crustal model of intrusive structures beneath the messum ring complex namibia the early cretaceous messum complex in namibia is an excellent example of the type of composite mafic and alkaline igneous ring complexes associated with continental rifts worldwide this study integrates reflection seismic gravity and aeromagnetic data from a profile across messum and reveals a heavily intruded root zone extending vertically to moho depths 40 km the crustal section to either side of the messum complex is characterized by a seismically transparent upper crust to 20 km depth and a laminar reflective lower crust below the crust below the messum complex is seismically transparent at all depths but it shows a combination of high seismic reflection strength and weak coherency relative to the unintruded crust and p velocities about 0 2 0 3 km s higher at all depths the seismic attributes are best explained by a network of mafic intrusions and this is supported by steep sided positive magnetic and gravity anomalies centered on the complex modelling indicates that about 30 gabbroic material in the crustal column can explain the velocity and potential field data an increase in reflective strength below 20 km suggests the degree of intrusion is greater in the lower crust but the difference is not resolved in the velocity or gravity models the new geometric and compositional constraints on the crust below the messum complex contradict the hypothesis that messum was the sole eruptive center for an estimated 8600 km3 of crustally derived rhyodacitic magmas that now form part of the regional parana etendeka volcanic sequences to our knowledge this is the first study to provide a seismic image of the whole crust below a typical igneous ring complex and our conclusion that the cross sectional area of the complex is the same in the lower as in the upper crust could not have been predicted from surface studies the data resolution is insufficient to define intrusion geometry in detail but the cylindrical 3 d shape throughout the crust and the lack of major breaks in the vertical distribution of intrusions indicate depth independent controls on the nature of the intruded zone the suggestion is that the magma transport and emplacement took place in a brittle fashion at all levels of the crust in response to conditions of high fluid magma pressure and a high magma flux

remnants of a cretaceous intra oceanic subduction system within the yarlung zangbo suture southern tibet extensive field investigations along the yarlung zangbo suture zone in southern tibet reveal the presence of now fragmented remnants of a south facing intra oceanic subduction system this system developed within tethys during the cretaceous the associated arc forearc ophiolite and subduction complex were emplaced onto the leading edge of india at the end of the cretaceous rapid sedimentation in oblique slip basins and disruption of water saturated sediments into melange was widespread and concomitant with ophiolite emplacement we describe the tectonic entities that developed during this previously unrecognized phase of tethys tibet evolution and present a new model for the evolution of this portion of tibet

shape and intrusion style of the coastal batholith peru abstract the coastal batholith of peru extends over 1600 km parallel to the coast along the andean trend gravity profiles on three traverses across the batholith indicate the geometry is essentially that of a flat slab with average thickness from 2 0 3 2 km and a thick root 4 10 km wide to the west granitic material does not extend to depths greater than 3 km below sea level datum this study supports recent gravity work which indicates plutons are commonly thin 5 km or less in thickness detailed mapping in the lima segment of the coastal batholith reveals thin plutons where space was made dominantly by downward displacement via floor depression however early roof uplift also created some space stoping occurs but is not a major space maker floor depression may be modelled by cantilever or piston mechanisms and although the strong marginal deformation with mylonites tuffisites microbreccia faults and shear zones suggests the piston model best describes the mechanism of emplacement of much of the coastal batholith some space was probably made by a cantilever mechanism in brief space making processes involved early roof uplift and regional doming then floor depression mainly by piston and probably subsidiary cantilever mechanisms and finally local stoping producing the cut out rectilinear nature of the batholith the coastal batholith formed on shallow partial melting of hydrous basaltic marginal basin rocks between 5 and 10 km floor depression occurred as the crustal column foundered into an actively deflating layer of partial melt this is an efficient space making process and is limited here to shallow levels of the upper crust only the melts ascended to within 2 or 3 km of the surface up dyke like conduits then spread horizontally to form tabular plutons

bainang terrane yarlung tsangpo suture southern tibet xizang china a record of intra neotethyan subduction accretion processes preserved on the roof of the world the bainang terrane an intra oceanic island arc subduction complex into which tethyan oceanic rocks were accreted during the cretaceous is preserved within the yarlung tsangpo suture zone of tibet the lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central tethyan domain from late triassic to mid cretaceous time these rocks are preserved within a south verging imbricate thrust stack of thin 1 km thick northward younging tectonic slices five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts the northern tract which accumulated on the north side of neotethys was probably separated from its southern counterpart by a mid ocean ridge detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract oceanic plate stratigraphy of the northern tract records its northward travel and mid cretaceous late aptian approach towards a south facing intra oceanic subduction zone rocks in the southern tract developed closer to the indian subcontinent and experienced thermotectonic subsidence and mid jurassic basic alkaline intraplate magmatism they were probably accreted late in the cretaceous variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion the overall tectonostratigraphy of the terrane reflects its development in a remote intra oceanic setting

revised tectonic evolution of the eastern indian ocean published plate tectonic models for the australian antarctic plate pair imply geologically improbable scenarios at either or both ends of the cretaceous rift and spreading system controversy also exists around the location of and motion at the plate boundary extending west of australia antarctica through the kerguelen plateau region we present a plate tectonic model of relative motions among india australia and antarctica from the onset of continental rifting to the establishment of rapid seafloor spreading at 43 ma the model conforms to a wide range of geological geophysical evidence and reconstructs the formation of both the western kerguelen region and the eastern tasman region the incorporation of spatio temporally continuous plate boundaries reveals the presence of a plate boundary beneath the contiguous central kerguelen plateau and broken ridge for 65 million years to investigate the relationship between the plate boundary system and the kerguelen plume we test three alternative absolute reference frames using a fixed hotspot reference frame the indian ocean mid ocean ridge system remains within 500 km of the kerguelen plume while the proximity of the plate boundaries and the plume is more variable with a moving hotspot reference frame proximity between the plume plate boundaries and the central kerguelen plateau broken ridge for 65myrs suggests that these specific features were not formed by a single short lived 5 10 myr pulse of magmatic activity but rather by a 25 myr period of relatively high magma flux followed by 40 myr period of lower volume magmatic activity an interpretation not excluded by the relatively sparse dredge and drill ages

exploding xenoliths and the absence of elephants graveyards in granite batholiths abstract biotite monzogranite near the eastern margin of the late devonian south mountain batholith at portuguese cove nova scotia surrounds a large 20 m 25 m anisotropic decimetre scale interbedded psammite and semipelite xenolith of the metamorphic regional greenschist thermal cordierite hornfels country rocks of the cambro ordovician meguma group the xenolith has two distinct types of surfaces against the monzogranite i one rounded diffuse and apparently old surface that had prolonged contact with the granite magma and ii two straight sharp and apparently new surfaces that had more recently come into contact with the granite magma more than 375 thin sheets width n 224 0 1 0 5 cm n 66 0 5 1 0 cm n 51 1 0 5 0 cm n 30 5 0 10 0 cm n 3 10 cm n 2 ranging in composition from monzogranite to pegmatite to coarse quartz sericite cut the xenolith parallel to bedding producing a cumulative volume expansion of 14 in the xenolith aspect ratios in some of the thin sheets exceed 1000 1 several sheets cut the xenolith perpendicular to bedding producing a volume expansion of

underplating process from melange formation to duplexing example from the cretaceous shimanto belt kii peninsula southwest japan accretionary complexes are considered to form through off scraping and underplating which results in the lateral and vertical growth of an accretionary prism accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process although the origin of melange is considered controversial in order to better understand the origin of melange and its role in accretion in particular during underplating we have studied the relationship between the formation of melange and duplexing in the miyama assemblage here the melange is composed dominantly of shale basalt chert and sandstone this study stresses the following features 1 the structure of the miyama assemblage appears to be a composite suite of landward and seaward dipping duplexes 2 the melange fabric displays a systematic asymmetry which may have resulted through shearing along the decollement and 3 facing of horses within duplexes changes from north to south coinciding with a change in the sense of shear inferred from the melange fabric which suggests that it formed prior to duplexing

the meta igneous granulite xenoliths from kerguelen archipelago evidence of a continent nucleation in an oceanic setting xenoliths of meta igneous origin occur as one of the two main types of ultramafic and mafic xenoliths entrained by alkaline lavas of the kerguelen islands these are designated type ii xenoliths and are subdivided into three mineralogical groups subtype iia and iic xenoliths are interpreted as crystallisation products of basaltic melts that were emplaced near the crust mantle boundary during the early tholeiitic transitional magmatic activity of the kerguelen islands younger magmatism became more alkaline and subtype iib xenoliths were formed as high pressure alkaline cumulates related to the last alkaline volcanic stage subsequently the plagioclase bearing type ii rocks have been re equilibrated under granulite facies conditions this addition of mafic material around the crust mantle boundary is consistent with seismic evidence for crustal thickening to 14 20 km calculated compressional seismic velocities vp for the basic granulites are consistent with the range of observed vp in the low velocity region underlying the oceanic crust such growth in the thickness of the oceanic crust may be caused by intrusion of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the oceanic setting this study suggests that basic granulites can account for the observed seismic characteristics of oceanic plateaux and can be important components of kerguelen oceanic lithosphere where there has been large scale magma production moreover we speculate that the kerguelen islands and perhaps the surrounding plateau represent a continental nucleation process

on the emplacement of tabular granites granites in both crystalline terranes and continental magmatic arcs tend to be circular to elliptical in map view and vary in width from about 3 to 100 km available gravity and structural data suggests that many of these plutons are tabular in shape with an average thickness of about 3 km ductile structures observed around mesozonal granites indicate that space is created by a combination of lateral and vertical displacements of wall rocks whereas contact relationships of epizonal plutons imply that only vertical displacements are involved during emplacement in both settings magma arrives at the emplacement site via one or more vertical feeder zones and flows laterally with the exception of very high level epizonal plutons structural studies suggest that space for many tabular intrusions must be provided mainly by floor depression lopolith emplacement rather than roof lifting laccolith emplacement an emplacement model for this type of tabular granite is proposed which involves progressive depression of the floor of an initially horizontal chamber as it is filled by one or more vertical conduits a crustal scale balance in the rates of melt extraction magma ascent and pluton filling is required by the model and transfer of material from the source to the pluton is accommodated by broadly distributed deformation of low strain magnitude the process is evaluated with end member cantilever and piston sinking mechanisms the models predict that large 10 100 km wide tabular plutons 3 km thick can be emplaced quickly 100 a to 1 ma with floor depression and related wall rock strain rates similar those expected during tectonic deformation 10 10 to 10 15 s 1 bulk strains in the intervening crustal column rarely exceed a strain ratio of 1 5 which is likely to remain undetected in the geological record unless the required deformation is accommodated on discrete structures such as normal faults or shear zones at the base of the pluton

cosmogenic 3he and 21ne age of the big lost river flood snake river plain idaho the big lost river flood in southeastern idaho occurred 20 500 calibrated yr b p on the basis of dates derived from cosmogenic 3 he and 21 ne measurements of samples from flood deposited boulders and from scour features this date corresponds to a date of 16 900 14 c yr b p and is close in age to several other cataclysmic flood events in western north america it may mark evidence for widespread warming at the end of the pleistocene in western north america the big lost river flood was smaller than some other late pleistocene floods such as the bonnevi1le flood and the missoula floods thus some samples exposed after the flood had significant amounts of cosmogenic 3 he and 21 ne that was acquired before the flood occurred

tethyan subducted slabs under india abstract tomographic imaging of the mantle under tibet india and the adjacent indian ocean reveals several zones of relatively high p wave velocities at various depths under the hindu kush region in northeastern afghanistan and southern tajikistan a regional northward dipping slab is seen in the entire upper 600 km of the mantle and is apparently still attached to the lithosphere of the indian plate under northern pakistan this same slab shows a roll over structure with the deeper portion overturned and dipping southward as can also be seen in the distribution of earthquake hypocenters farther east southeast e g in the vicinity of nepal a well resolved anomaly below 450 km depth is connected to the slab under the hindu kush but seems to be separated from the lithosphere above 350 km these upper mantle anomalies are interpreted as the remnants of delaminated sub continental lithosphere that went down when greater india continued to converge northward with asia after 45 ma the deeper high velocity anomalies under the indian sub continent appear clearly separated from the shallower ones as well as from each other and are inferred to represent remnants of oceanic lithospheric slabs that have sunk into the lower mantle and were subsequently overridden by the indian plate they occur at depths between 1000 and 2300 km and occasionally descend down to the core mantle boundary the anomalies form three parallel wnw ese striking zones we interpret the two southern zones as remnants of oceanic lithosphere that was subducted when the neo tethys ocean closed between india and tibet in the cretaceous and earliest tertiary the northern deep mantle zone under northern afghanistan the himalayas and the lhasa block in southern tibet may represent the last subducted remnant of the paleo tethys ocean which is thought to have closed before the hauterivian stage of the early cretaceous the middle zone continues southeastward as a rather straight high velocity zone towards sumatra where it becomes convex southward and parallel to the subduction zone under the sunda arc comparison of this straight middle zone near india with the shallower upper 600 1000 km northern zone which displays a cusp like shape near the yunnan sw china syntaxis of the eastern himalayas supports the notion that the shallow northern zone represents later subduction than the deeper middle zone the suggestion of a counterclockwise rotation 20 of the indian plate during tertiary indentation of asia is supported by these features the present day latitudes of 5 35 n of the deep slabs under india and adjacent areas correspond to the approximate paleolatitudes of the cretaceous subduction zones the slab remnants in the middle mantle occur therefore near the ancient locations where they started their downward journey which implies that lateral movements in the deeper mantle were not large

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

structure of a hypovolcanic acid complex inferred from magnetic susceptibility anisotropy measurements the western red hills granites skye scotland thulean igneous province the geochemistry and the injection mechanism of hypovolcanic ring dykes have been extensively studied but such is not the case for their internal fabric the tertiary western red hills epigranites of the isle of skye are a classic example of such intrusions using anisotropy of magnetic susceptibility measurements we present the first structural data of their internal magmatic fabric the magnetic foliations equated with the magmatic flow planes have strikes which roughly follow the walls of the different intrusions they dip steeply toward the convex wall of each intrusion the lineations or maximal magnetic susceptibility axes generally have shallow plunges except in the latest granite intrusion these structures appear to be related to the compressional deformation of each intrusion toward the end of its crystallization this shortening would be a consequence of a radial and compressive stress field acting after each injection of magma this radial stress field is interpreted as the effect of high magma pressures originating from the acid magma chamber underlying the ring dyke complex at a shallow depth

open system processes in the genesis of silica oversaturated alkaline rocks of the rallier du baty peninsula kerguelen archipelago indian ocean abstract the rallier du baty peninsula forms the southwestern part of the kerguelen archipelago indian ocean whose magmatic activity is related to the long lived 115 ma kerguelen plume the peninsula is mostly made of alkaline rocks constituting two well defined ring complexes this paper focuses on the northern ring complex which is not yet known recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6 2 to 4 9 ma and two later volcanic systems 40ar 39ar dating of a trachytic ignimbrite linked to the dome carva volcano complex yields an age of 26 3 ka this represents the last major eruptive event on the kerguelen archipelago the volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas the volume of erupted felsic magma is by far the larger and is represented by abundant pyroclastic deposits and lava flows boulders of plutonic rocks are found to the northwest of dome carva and represent intermediate rocks i e monzogabbros and monzonites that are not present at the surface basic rocks are mostly trachybasalts and trachyandesites while true basalts are scarce their mineralogy consists chiefly of plagioclase olivine diopside and oxides sieve textured plagioclase is common as well as corroded olivine and diopside phenocrysts peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks they consist of abundant sanidine augite and magnetite phenocrysts and interstitial quartz aegerinic pyroxenes and na amphiboles ring dykes of quartz poor alkali feldspar syenites display the same mineralogy except hornblende is common and replaces diopside hornblende is particularly abundant in intermediate monzogabbros major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of mgo cao p2o5 tio2 feo ba sr and ni from basic to felsic rocks however the scattering of the data from the basic rocks indicates that other processes have operated the overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control an increase of incompatible elements from trachyte to rhyolite is observed the felsic lavas display an increase of 87sr 86sr i without any significant variations in the nd isotopic composition the genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation mixing between different basic magmas and probable assimilation of ba rich oceanic crust major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution the overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation high 87sr 86sr i of the trachytes is interpreted to reflect interaction with an ocean derived component probably during assimilation of hydrothermally altered oceanic crust boulders of amphibole bearing monzonites and monzogabbros found to the northwest of dome carva are thought to represent intermediate magma composition that formed at depths but did not erupt

comments on the growth of accretionary wedges uplift within accretionary foreland wedges has been explained by the development of duplexes which provides a mechanism for underplating in submarine accretionary wedges with little deformation in the upper part of the wedge we interpret duplex development beneath the costa rica forearc a zone widely considered to be a prime example of nonaccretion because of the apparent absence of an imbricate fan of thrusts at the toe of the wedge large scale structural geometry of accretionary wedges is compatible with low angle trenchward dipping backstops in centrist to reverse angle backstops used commonly in models of return flow within wedges which have been invoked to explain the presence of high p t metamorphic rocks we suggest that emplacement of high p t metamorphic rocks are better explained in a collisional setting where known uplift rates are high than by flow within noncollisional accretionary wedges

plutonic processes in transitional oceanic plateau crust structure age and emplacement of the south rallier du baty laccolith kerguelen islands