neotectonics of the thakkhola graben and implications for recent activity on the south tibetan fault system in the central nepal himalaya the thakkhola graben is one of many north trending rifts that define the neogene structural pattern of the southern tibetan plateau lying at the southern margin of the plateau and extending to the crest of the himalaya the graben provides an opportunity to evaluate the kinematic relationships between east west extensional strain in southern tibet and north south extensional strain in the himalaya neotectonic and structural mapping of the dangardzong fault along the western margin of the graben reveals a southward decreasing component of normal slip coupled with a southward increasing component of right lateral slip that affects pleistocene basin fill sediments we present 14c ages for river terraces in the thakkhola graben that provide a ca 17 2 ka minimum age on the latest stage of dangardzong fault movement near the southern termination of the graben the dangardzong fault apparently offsets the annapurna detachment an early miocene strand of the east striking south tibetan fault system however the dangardzong fault itself terminates against a young i e younger than ca 17 2 ka strand of the south tibetan fault system the dhumpu detachment structural relationships among the dangardzong annapurna and dhumpu faults suggest that the dangardzong structure is a tear fault in the south tibetan allochthon that accommodates differential amounts and rates of displacement along the south tibetan fault system thus although the south tibetan fault system first developed as part of the structural architecture of the himalaya in miocene time at least some strands have been active as recently as the pleistocene in a regional context the south tibetan fault system serves to accommodate the strain gradient between extension in tibet and shortening in the himalaya

density inhomogeneities isostasy and flexural rigidity of the lithosphere in the transcaspian region the 3 d lithospheric density model for the southeastern part of the caspian sea and the transcaspian area practically coinciding with the territory of the turkmen republic has been constructed based on geophysical data and in accordance with the principle of isostasy from the model selected the anomalous density of the subcrustal layer between the moho discontinuity and the 100 km depth level is found to be 100 kg m3 under the tien shan 50 kg m3 under the kopet dag mountain area 80 kg m3 under the central region of the south caspian basin 50 kg m3 under the eastern part of the basin known as the west turkmenian depression and 45 kg m3 under the murgab depression significant disturbances of the local isostasy are determined both in the northern and central areas of the south caspian basin and also in the area of the kara bogaz swell of the turan platform and for the kopet dag foredeep indicating a high level of stresses in the lithosphere the shape of the turan plate determined by the seismic profiling is accounted for by elastic deformation resulting from the forces acting on the southern edge of the plate in the area of the turan plate kopet dag collision the elastic thickness of the turan plate is estimated as 25 5 km the results obtained seem to confirm the idea that the decomposition of the turan plate has taken place in the zone of the plates interaction and the decomposed material is situated under the kopet dag ridge we propose that the kara bogaz swell is supported by the mantle material upwelling whereas the subsidence of the adjacent part of the south caspian basin may be due to the downgoing mantle flow i e a small convection cell is suggested in that area

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

three dimensional gravity and magnetic modeling of crustal indentation and wedging in the western pyrenees cantabrian mountains 1 the complex crustal structure of the boundary zone between the iberian and european plates from the western pyrenees to the cantabrian mountains is probed by three dimensional 3 d gravity and magnetic modeling constrained by deep seismic profiles the 3 d modeling results support the presence of a continuous iberian crustal root and suggest that the geometry of the orogenic belt that formed in latest cretaceous tertiary times along the northern margin of iberia was conditioned by oblique structures separating areas characterized by different tectonic styles in the western pyrenees and the basque cantabrian basin a thick mesozoic basin presently incorporated to the pyrenean cantabrian belt the relatively narrow thinning of the crust inherited from the mesozoic rifting stage conditioned a structural style in which portions of the southward indenting european lower crust are interpreted to be back thrusted toward the north and uplifted to shallow depths promoting the appearance of significant potential field anomalies in the basque cantabrian basin the strongest aeromagnetic anomaly of the whole iberian mainland is superimposed on the eastern part of a well defined positive gravity anomaly similar in amplitude and wavelength to those located along the north pyrenean zone these observations suggest that the eastern part of the dense lower crustal causative body is strongly magnetized and may correspond to a gabbroic cumulate originated in the axis of the ancient mesozoic rift to the west tertiary compression affected the north iberian cantabrian passive continental margin whose geometry and inherited structures conditioned the formation of a double crustal delamination and the uplift of the cantabrian mountains

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

landscape dynamics and human environment interactions in the northern foothills of cho oyu and mount everest southern tibet during the late pleistocene and holocene abstract here we present an integrated earth surface process and paleoenvironmental study from the tingri graben and the archaeological site of su re located on the southern rim of the tibetan plateau spanning the past ca 30 ka the study area is characterized by cold climate earth surface processes and aridity due to its altitude and location in the rain shadow of the mount everest cho oyu massif and is thus sensitive to climatic and anthropogenic perturbations in this highly dynamic geomorphic environment paired cosmogenic nuclide results from boulders on a massive hummocky moraine in the southern tingri graben reveal complex exposure histories that limit our capability of directly dating the corresponding glacial advance and shed a note of caution on previously published single nuclide based exposure ages along the northern himalaya based on geomorphic considerations however the moraine clearly represents the local last glacial maximum and likely coincided with a 344 109 m drepression of discontinuous permafrost zone relative to today during the global last glacial maximum glgm this greatly intensified permafrost and periglacial hillslope processes and led to fluvial aggradation of the valley floors of 12 m we observe formation of a thick 50 cm pedo complex starting at ca 6 7 ka before present bp and erosional truncation at ca 3 9 ka bp widespread landscape instability and erosion characterize the region subsequent to 3 9 ka and intensifies in the 15th century ad several lines of geo archaeological evidence including the presence of pottery sherds sling shot projectiles and hammer stones within the sedimentary record indicate human presence at su re since ca 3 9 ka bp our data suggest that in the su re tingri area climatic conditions were warm and moist enough to allow vegetation expansion and soil formation only from ca 6 7 3 9 ka followed by weakening of the indian summer monsoon ism strength between ca 4 2 and 3 9 ka which is a prominent climatic event in the wider asian monsoon region and reflected in the investigation area by the 3 9 ka erosional boundary merging our holocene landscape reconstruction with the geoarchaeological evidence we speculate that the combined effect of little ice age lia cooling and an anthropogenic overuse of the landscape led to climatically induced landscape degradation and ultimately to an anthropogenically triggered ecological collapse in the 15th century such a scenario is in line with regional historical data on declining monastery construction and migration of the ethnic group of the sherpas from an earth surface dynamics perspective we find that transient landscape processes on the southern rim of the tibetan plateau are strongly linked to millennial scale changes in the ism intensity and duration we identify three types of unidirectional non linear ism landscape interactions given that the tibetan plateau is the largest high altitude landmass on our planet and our limited understanding of several of the key earth surface processes on the plateau we pinpoint the need for more long term quaternary scale empirical data particularly on permafrost and periglacial processes and human environment interactions

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

wide angle images of the mediterranean ridge backstop structure the mediterranean ridge is an accretionary wedge consisting of a large pile of tectonically deformed sediments in the past the mediterranean ridge has been interpreted as a prism related to hellenic subduction however geophysical evidence pasiphae cruise 1988 has indicated the presence of a high velocity body southward of the hellenic trench which was subsequently interpreted as an extended continental backstop during the recent imerse project coincident wide angle and normal incidence seismic data were collected over the ridge along the ne sw medriff corridor explora cruise 1994 the objectives of the imerse project were to acquire the seismic observations necessary for a high resolution structural model of the western mediterranean ridge including the structure of the newly interpreted continental backstop here we present the results of recent re modelling of two ship expanding spread seismic refraction data esp 8 and 9 recorded during the pasiphae cruise the modelling of these data has allowed an approximate constraint on the position of the juncture between the mediterranean accretionary complex and its buttress also presented are processing and modelling results of the recently acquired imerse wide angle data travel time modelling of these data has allowed the construction of six multi azimuth velocity depth models these models show the existence of high velocity material at depth p wave velocity 5 km s inferred to be extended backstop and constrain its southward extent all models show the presence of a messinian evaporite layer near the surface the evaporite layer is observed to vary in thickness from 0 to 2 0 km with the existence of local thickening and thinning due to depositional factors tectonism and salt dissolution in addition combined analysis of the wide angle velocity models with coincident seismic reflection and swath bathymetry data indicates that the geometry of the continental backstop varies laterally as a consequence of transform faulting possibly related to the southward rotation of the peloponnesus thrust sheet and backstop emplacement

nitrogen isotopes in thermal fluids of a forearc region jalisco block mexico evidence for heavy nitrogen from continental crust 1 the jalisco block jb is a geologically and tectonically complex part of northwestern mexico characterized by active subduction type volcanism rifting and old stable structures thermal springs and groups of springs are widely distributed over jb bubbling gas from seven thermal springs located within different tectonic environments of the jb was analyzed for he 20ne and n2 concentrations and 15n ratios all gases are n2 dominant 84 with the exception of one sample rio purificacion which has a significant ch4 content about 50 all collected gas samples are relatively high in he up to 1500 ppm vol and with 3he 4he values ranging from 0 6 to 4 5 ra all measured nitrogen isotope ratios are heavier than air with 15n values ranging from 0 5 to 5 0 the relative n2 excess with respect to air saturated water computed on the basis of n2 and 20ne contents indicates the contribution of a nonatmospheric n2 source all the samples show a good correlation between 15n and the relative excess of n2 with 15n 5 3 for the maximum n2 excess of 100 due to a presumed lack of seafloor sediment involved in the subduction process such a 15n positive value seems to reflect the addition to the fluids of a heavy nitrogen originating from metamorphism processes of rocks occurring within the overlying continental crust

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

carbon dioxide earth degassing and seismogenesis in central and southern italy 1 we present the first regional map of co2 earth degassing from a large area most of central and south italy derived from the carbon of deep provenance dissolved in the main springs of the region the investigation shows that a globally significant amount of deeply derived co2 10 of the estimated global co2 emitted from subaerial volcanoes is released by two large areas located in western italy the anomalous flux of co2 suddenly disappears in the apennine in correspondence to a narrow band where most of seismicity concentrates here at depth the gas accumulates in crustal traps generating co2 overpressurized reservoirs which induce seismicity

progression from south directed extrusion to orogen parallel extension in the southern margin of the tibetan plateau mount everest region tibet abstract in the south tibetan himalaya major detachment systems exhumed midcrustal rocks from different structural levels that are exposed in the ama drime and mount everest massifs the south tibetan detachment system stds accommodated exhumation of the greater himalayan series ghs until the middle miocene field and laboratory based structural data presented here indicate that early melt present deformation in the footwall of the stds accommodated large scale flow of a low viscosity middle crust decompression related anatexis and emplacement of leucogranites in the structurally highest positions of the ghs 16 18 ma mark the final stages of south directed extrusion within a relatively narrow solid state mylonite zone that progressed into a brittle detachment estimates of mean kinematic vorticity and deformation temperatures record a progression in deformation conditions from ultra mylonitic leucogranite 400 500 c 51 67 pure shear to mylonitic marble 300 c 41 55 pure shear to mode

an integrated gravity model for europe s crust and upper mantle we present an integrated gravity model of the european lithosphere based on an analysis of a number of new data sets leading to a much higher resolution than provided by previous models first of all a recent crustal model eucrust 07 is used to quantify the crustal contribution to the observed gravity field and to identify the effect of mantle heterogeneity the new gravity field model is based on a combination of satellite champ and grace and terrestrial data we also use these data sets to estimate residual mantle gravity anomalies and residual topography reflecting the effect of mantle density variations induced by temperature and compositional heterogeneity the separation of these effects is vital for a proper assessment of mantle structure and evolution in addition we utilize a new tomographic model for p and s velocity anomalies beneath europe which is a priori corrected for crustal structure using eucrust 07 the seismic velocity anomalies were subsequently converted into temperature anomalies using a mineral physics approach we estimate the effect of temperature variations on the gravity field and subtract it from the total mantle field the residual fields point to an important role of compositional density anomalies in the upper mantle a number of key features of the compositional density distribution so far invisible in seismic tomography data are detected for the first time

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

the measurement of δ15n in crustal rocks by static vacuum mass spectrometry application to the origin of the ammonium in the cornubian batholith southwest england the release of ammoniacal nitrogen from muscovite biotite orthoclase microcline and a whole rock granite by vacuum pyrolysis has been investigated under the heating regimes employed it was found that micas lose most of their nitrogen below the temperature at which the powders begin to sinter ca 1100 c the peak release of n occurring in either the 900 c or 1000 c heating step it is possible that the loss of ammonium is linked to the loss of h2o and perhaps f for the feldspars there was insignificant release of n below the melting point of the minerals about 1150 c suggesting that diffusion of ammonium or ammonia is the principal control for both micas and feldspars there was significant kinetic fractionation of the nitrogen isotopes during stepped heating from these results a technique has been devised for the routine determination of 15n and ammonium concentration in whole rock granites by static vacuum mass spectrometry the ammonium concentrations and 15n values have been determined in whole rock samples from the cornubian batholith southwest england the ammonium concentrations were in the range 8 to 187 ppm and the 15n values were in the range 5 1 to 10 2 most of the granite samples had 15n values between 8 4 and 10 2 and are within the range that have been obtained from amphibolite facies metasediments these granites are also typified by high 87sr86srinit 0 7130 to 0 7166 and high normative corundum 2 31 to 2 55 taken together the data suggest that the ammonium present within these granites is of biological origin and that it was incorporated via the anatexis of metasediments which originally contained organic material the samples were collected over a distance of 120 km and the limited range in 15n suggests either 1 1 the 15n values within the metasedimentary protolith were rather homogeneous or 2 2 the nitrogen isotopes were homogenised during the incorporation of ammonium into the granite magmas two of the granites had lower 15n values of 5 1 and 7 0 these granites also have lower ammonium concentrations 8 and 15 ppm lower 87sr86srinit 0 7094 0 7095 and lower normative corundum 1 44 and 1 63 the preferred explanation is that they were derived from or have had a contribution from a second crustal source containing sedimentary or metasedimentary material with a lower proportion of pelitic to other constituents

helium and carbon isotopic compositions of hot spring gases in the tibetan plateau abstract the tibetan plateau is one of the most active geothermal areas in the himalayan geothermal belt however the heat source of the geothermal manifestation is unknown because of a lack of quaternary volcanoes in this area except in western yunnan and northern tibet in order to unravel the heat source and the origin of volatile components in hot spring gases in the southern tibetan plateau we have measured helium and carbon isotope ratios of hot spring gases from yangbajain district and some northern sites the observed helium compositions in the hot spring gases are explained by the mixing of an air component and underground components whose mean end member 3 he 4 he ratio is 0 12 ratm for yangbajain and 0 22 ratm for the northern sites respectively despite the lack of quaternary volcanism such 3 he 4 he ratios are an order of magnitude higher than that of stable continental crust 0 02 ratm these results may imply the existence of intrusive magmas that contain mantle helium in the thick tibetan crust the mantle helium with high 3 he 4 he ratio may have been diluted to the observed values by radiogenic helium produced in the crust the observed high co2 3 he ratios of the samples 1010 1012 relative to that of morb 2 109 indicate that most carbon dioxide in the samples is derived from crustal materials such as organic matter and marine carbonate minerals regional differences are observed not only in the mean end member 3 he 4 he ratios of the underground components but also in the 13c co2 values of the hot spring gases between yangbajain 9 1 to 6 3 and the northern sites 4 3 to 1 0 these features suggest heterogeneity among basement rocks the lower 3 he 4 he ratio in yangbajain may be due to a larger addition of radiogenic 4 he from basement rocks that are enriched with uranium and thorium the higher 13 c co2 value of the hot spring gases from the northern sites may be due to the addition of carbon dioxide from underlying sedimentary rocks containing more marine carbonate minerals

seismic images of crust and upper mantle beneath tibet evidence for eurasian plate subduction seismic data from central tibet have been combined to image the subsurface structure and understand the evolution of the collision of india and eurasia the 410 and 660 kilometer mantle discontinuities are sharply defined implying a lack of a subducting slab beneath the plateau the discontinuities appear slightly deeper beneath northern tibet implying that the average temperature of the mantle above the transition zone is about 300 c hotter in the north than in the south there is a prominent south dipping converter in the uppermost mantle beneath northern tibet that might represent the top of the eurasian mantle lithosphere underthrusting the northern margin of the plateau

p wave tomography of the mantle under the alpine mediterranean area 1 we study the upper mantle p wave velocity structure below the euro mediterranean area down to 1000 km depth by seismic travel time tomography we invert summary residuals constructed with both regional and teleseismic first arrival data reported by the international seismological centre isc 1964 1995 introducing some alternative strategies in the travel time tomographic approach and a new scheme to correct teleseismic data for global mantle structure our high resolution model pm0 5 is parameterized with three dimensional 3 d linear splines on a grid of nodes with 0 5 spacing in both horizontal directions and 50 km vertical spacing we obtain about 26 root mean square rms reduction of residuals by inversion in addition to roughly 31 reduction after summary rays formation and selection sensitivity analyses are performed through several test inversions to explore the resolution characteristics of the model at different spatial scales the distribution of large scale fast anomalies suggests that two different stages of a convection process presently coexist in very close regions the mantle dynamics of western central europe is dominated by blockage of subducted slabs at the 660 km discontinuity and ponding of seismically fast material in the transition zone contrarily in the eastern mediterranean fast velocity material sinks into the lower mantle suggesting that the flow of the cold downwelling here is not blocked by the 660 km discontinuity on a smaller scale the existence of tears in the subducted slab lithospheric detachment all along both margins of the adriatic plate as proposed by some authors is not supported by our tomographic images

oceanic upper mantle structure from experimental scaling of vs and density at different depths summary isotropic shear wave velocities from the ekstrom dziewonski s20 global tomographic model and residual crust free gravity are used to determine density velocity relations for various mantle layers by inversion the residual gravity field was obtained by subtracting the crustal density inhomogeneities from the observed surface gravity the residual field is dominated by upper mantle signatures as the effect of dynamic topo graphy is also reduced from the gravity field before hand the gravity seismic inversion is almost independent of the viscosity distribution within the earth there exists a reliable correlation between residual gravity and shear wave velocities in the upper mantle under oceans but not under continents this result supports the idea that lateral density variations in the oceanic upper mantle are mostly driven by the temperature distribution while under the continents temperature and composition induced density anomalies are superimposed the inversion is therefore restricted to the oceanic area and was done by a least squares adjustment in the spatial domain the density velocity ratios obtained for the subcrustal layer down to 75 km and for the deep part of the oceanic upper mantle below 225 km are very close to the ones predicted from mineral physics analyses however for the oceanic layer from 75 to 225 km depth a significantly lower density velocity ratio is obtained this may be explained by a compositional layering dry partially depleted peridotite residuum underlain by damp fertile peridotite and separated by the g discontinuity at a depth of about 70 km for the mantle transition zone and for the lower mantle the results obtained are less reliable but relative minima in the density velocity ratio are found for the layers around the phase transition discontinuities and for the depth interval from about 900 to 1100 km applying the adjusted velocity to density relations to the seismic tomography model a mantle density distribution under the oceans is found which explains about 85 per cent of the longest wave crust free gravity field degrees 1 5 and 70 per cent of the overall field up to degree 20 which was the limit of this investigation

seismic evidence of alpine crustal thickening and wedging from the western pyrenees to the cantabrian mountains north iberia 1 the alpine collision between the iberian and european plates resulted in a complex crustal structure beneath the northern iberian peninsula as revealed from a new set of seismic refraction wide angle reflection profiles the study area is characterized by two major e w ranges the cantabrian mountains and the pyrenees which are relayed to the south by the iberian chain important variations in crustal thickness and velocity distribution are found in a 560 km long e w transect in contrast to the typical 30 to 32 km thick european variscan crust of the western end of the profile a continuous alpine crustal root is evidenced from the cantabrian mountains to the central pyrenees with a moho depth of 46 48 km locally rising to 40 km depth in between beneath the basque cantabrian basin another outstanding feature is the inferred presence of high velocities of 6 40 6 75 km s at midcrustal depths which can be associated with portions of a lower crustal wedge from the northern european domain indenting the southern iberian crust during the alpine stage of compression this indentation produces the delamination of the iberian crust with northward underthrusting of its lower half and the consequent crustal thickening the indenting wedge has a discontinuous presence along the longitudinal section as it was controlled and or affected by n s to ne sw structures further evidence of the northward subduction of the iberian plate is provided by another profile toward the aquitaine basin while a n s profile across the iberian chain reveals an alpine midcrustal thickening beneath this belt with moho depths of 42 km

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

3d gravity modeling of the triassic salt diapirs of the cubeta alavesa northern spain abstract up to now subsurface information permitted the delineation of the top of the triassic salt all throughout the cantabro navarro domain although little was known on the location and geometry of its base and thus on the estimation of the total salt thickness a 3d gravity inversion scheme combined with a 3d analytic method has been conducted to map out the geometry of the main salt structures of the basin the gravity modeling results have been constrained by well log information and available geological and reflection seismic data the combined 3d scheme integrated with available geological and geophysical data has allowed us to obtain the geometry of the main diapirs that characterize the central and marginal regions of the basin from our interpretation the salinas de anana diapir has almost vertical flanks and can be divided into two different parts one of them forming a lateral overhang of the main body the salinas de oro diapir has near vertical flanks and a main axis in the n s direction also the anomaly is rather more extensive than the outcrop of the diapir which implies an important expansion of non outcropping salt in this area like the hoz sobron diapir the salinas de ollo diapir is long and narrow stretching in the nw se direction which includes three important highs plus an intense zone of salt migration the estella and alloz diapirs crop out individually in spite of being connected at depth also two non outcropping salt domes have been detected to the south of atauri that like the estella diapir are related to the thrust front we point out the gravity signature of the murguia diapir which shows an intense gravity high probably due to the presence of high density rocks in the cap rock or more probably due to the existence of triassic volcanites of ophitic texture pinched off into the diapir

origin of carbon in fumarolic gas from island arcs abstract we have measured 3 he 4 he ratios 13c values and co 2 3 he ratios of fumaroles and hot spring gases in three volcanic areas in japan the 3 he 4 he ratios 13c values and co 2 3 he ratios vary from 02 05 ratm 9 7 to 2 7 and from 7 74 109 to 1 18 1011 respectively based on these data and those in the literature we present the first quantitative estimates of the upper mantle sediment and limestone including slab components in co2 of volcanic and hydrothermal fluids from subduction zones taking compiled c isotopic data of limestone and organic sediment assuming that the co 2 3 he ratio and 13c value of mid ocean ridge basalt morb are representative of the upper mantle component and that later isotopic and elemental fractionations are minimal it is possible to deconvole each fraction of c on the basis of simple mixing equations up to 20 of the carbon in high temperature volcanic gases in subduction zones is derived from a morb type source while the major fraction of the gases is attributable to co2 produced by decarbonation of subducted marine limestone and slab carbonate

dissected hydrologic system at the grand canyon interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine geochemical study of water and gas discharging from the deeply incised aquifer system at the grand canyon arizona provides a paradigm for understanding complex groundwater mixing phenomena and quaternary travertines deposited from cool springs provide a paleohydrologic record of this mixing geochemical data show that springs have marked compositional variability those associated with active travertine accumulations deeply derived endogenic waters are more saline richer in co2 and elevated in 87sr 86sr relative to springs derived dominantly from surface recharge of plateau aquifers epigenic waters endogenic waters and associated travertine are preferentially located along basement penetrating faults we propose a model whereby deeply derived fluids are conveyed upward via both magmatism and seismicity our model is supported by 1 gas analyses from spring waters with high he ar and he n2 and 3he 4he ratios indicating the presence of mantle derived he 2 large volumes of travertine and co2 rich gases in springs recording high co2 fluxes and 3 87sr 86sr in these springs that indicate circulation of waters through precambrian basement geochemical trends are explained by mixing of epigenic waters of the colorado plateau aquifers with different endogenic end member waters in different tectonic subprovinces endogenic waters are volumetrically minor but have significant effects on water chemistry they are an important and largely unrecognized component of the hydrogeochemistry and neotectonics of the southwestern united states

crustal structure in the carpatho pannonian region insights from three dimensional gravity modelling and their geodynamic significance a three dimensional gravity modelling of the carpatho pannonian region was carried out to get a better image of the moho boundary and the most prominent intra crustal density heterogeneities at first only the major density boundaries were considered the bottom of the tertiary basin fill the moho discontinuity and the lithosphere to asthenosphere boundary density contrasts were represented by relative densities the improved density model shows a transitional unit of high density at the base of the crust along the teisseyre tornquist zone in the western carpathians an extensive relatively low density unit was inferred in mid crustal levels the border zone between the southern carpathians and the transylvanian basin is characterized by a sharp step like contact of the two crustal units the moho configuration reveals important information on the tectonic evolution of the region zones of continental collision are represented by thick moho roots eastern alps eastern carpathians transpressional orogenic segments however are different in the western carpathians the moho is a flat surface in the dinarides a medium moho root is observed the southern carpathians are characterized by a thick crustal root the differences are explained with the presence or absence of subductible oceanic crust along the carpathians during the extrusion of pannonian blocks

convective hydrothermal c02 emission from high heat flow regions abstract in addition to volatiles released from volcanoes the flux of co2 to the atmosphere from other sources e g metamorphism and subsurface magmatism represents an important aspect of the global carbon cycle we have obtained a direct estimate of the present day atmospheric co2 flux from convective hydrothermal systems within subaerial seismically active high heat flow regions geothermal systems of the salton trough california u s a and the taupo volcanic zone new zealand provide benchmarks for quantifying convective hydrothermal co2 fluxes from such regions co2 fluxes from the salton trough 109 mol yr 1 and the taupo volcanic zone 8 109 mol yr 1 were computed using data on convective heat flow and the temperatures and co2 concentrations of reservoir fluids the similarity in specific co2 flux 106 mol km 2 yr 1 from these two disparate geologic tectonic settings implies that this flux may be used as a baseline to compute convective hydrothermal co2 emission from other areas of high heat flow if this specific flux is integrated over high heat flow areas of the circum pacific and tethyan belts the total global co2 flux could equal or exceed 1012 mol yr 1 adding this flux to a present day volcanic co2 flux of 4 1012 mol yr 1 the total present day earth degassing flux could balance the amount of co2 consumed by chemical weathering 7 1012 mol yr 1

a deep seismic crustal transect from the ne iberian peninsula to the western mediterranean the crustal structure of the transition from the ne iberian peninsula to the western mediterranean is characterized by large scale variations related to its successive compressional and extensional cenozoic tectonics the spanish esci spanish acronym for seismic studies of the iberian crust program recorded 450 km of deep vertical reflection profiles on land and at sea to complete a first crustal transect from the pyrenees to the south balearic basin highly energetic north dipping reflections imaged by combining the etude continentale et oceanique par reflexion et refraction sismique ecors and esci land profiles mark a crustal thickening toward the pyrenees the reflective band becomes horizontal beneath the catalan coastal ranges and the moho is found at 11s no evidence of crustal thinning affecting the iberian mainland is observed the transition to the mediterranean basin is constrained by complementary onshore offshore recordings forward modeling combined with multichannel processing of the wide angle data set shows that a strong thinning of the continental crust by a factor of 2 over a distance of 50 km affects both the catalan and balearic flanks of the valencia trough this thinning affects mainly the deep crust characterized by low velocities of 6 4 6 5 km s suggesting a removal of part of the lower crust during the rifting processes velocities of 7 8 7 9 km s are interpreted in the uppermost mantle with no evidence of underplating the moho appears as a marked discontinuity throughout the transect the nw part of the balearic promontory is characterized by a thick and reflective crust from 12 to 30 km depth an intensely fracturated evaporitic sequence is seen for 100 km along the south balearic basin the seismic images of the continent ocean transition are disturbed by these prominent sediments however the deep reflectivity suggests a crustal thinning moho at 6 7 s to be related to the middle miocene present extensional tectonics

nature of the upper crust beneath central tibet abstract a wide angle reflection refraction seismic line obtained through sino french joint research near the bangung nujiang suture in tibet 1 yielded crustal velocity estimates for both p and s waves avp vs ratio of 1 65 was derived fo waves propagating in the upper 50 c km near the surface a shallow low velocity zone is expected to form in a granitic upper crust for both p and s waves a two dimensional ray tracing study indicates that the observed first p and s waves 1 can be interpreted as diving waves above a low velocity zone where a positive gradient exists the low velocity zone commences at a depth of 10 km and extends possibly to 20 km or deeper

aqueous and isotope geochemistry of mineral springs along the southern margin of the tibetan plateau implications for fluid sources and regional degassing of co2 springs issuing from different faults and shear zones along the crest of the himalayas tap three different levels of crust beneath the tibetan plateau from structurally highest to lowest these are the tingri graben the south tibetan detachment system stds and the ama drime massif adm the aqueous chemistry reflects water rock interactions along faults and is consistent with mapped rock types major ion chemistry and calculated temperatures indicate that spring waters have circulated to greater depths along the n s trending faults that bound the tingri graben and ama drime detachment add compared to the stds suggesting that these structures penetrate to greater depths springs have excess co2 n2 he and ch4 compared to meteoric water values implying addition from crustal sources the 3he 4he ratios range from 0 018 to 0 063 ra and are consistent with a crustal source for he the 13c values of dissolved inorganic carbon dic and co2 gas range from 5 5 to 3 8 and 13 1 to 0 3 versus peedee belemnite respectively sources of carbon are evaluated by calculating isotopic trajectories associated with near surface effervescence of co2 positive 13c values of the tingri graben and stds springs are consistent with decarbonation of marine carbonates as the source of co2 negative values for the add springs overlap with mantle values but are best explained by metamorphic devolatilization of reduced sedimentary carbon the 15n values of n2 range from 2 2 to 2 1 versus air and are explained by mixtures of air derived nitrogen metamorphic devolatilization of sedimentary nitrogen and nitrogen from near surface biogenic processes co2 flux is estimated by scaling from individual springs 105 mol a 1 per spring to extensional structures across the southern limit of the tibetan plateau and likely contributes between 108 and 1011 mol a 1 up to 10 to the global carbon budget

evidence for present day leucogranite pluton growth in tibet several geophysical surveys carried out in southern tibet have revealed the occurrences of bright spots of high electrical conductivity at 15 20 km depth corresponding to zones of seismic attenuation and correlated to high heat flow at the surface such zones have been variably interpreted as intruding magmas partially melted middle crust or concentrations of free fluids here we show that experimentally derived electrical conductivities of hydrous granite melts under pressure temperature h2o conditions appropriate for crust derived magmas are in perfect agreement with those inferred from magnetotelluric data strongly suggesting that the tibetan bright spots image present day pluton assembly in the middle crust in a manner analogous to what happened during the miocene to form the leucogranite plutons now exposed southward in the high himalaya range

was the himalayan orogen a climatically significant coupled source and sink for atmospheric co2 during the cenozoic the hypothesis that the himalayan orogen was a climatically significant coupled source and sink for atmospheric co2 during the cenozoic is evaluated in light of the timing duration and co2 fluxes associated with himalayan metamorphism and chemical weathering we suggest that diachronous eohimalayan metamorphism occurred over a 20 m y time span middle eocene to early oligocene with total metamorphic co2 production of 4 10 1018 mol because this is much greater than the amount of carbon stored in the atmosphere and oceans and because uplift and accelerated erosion began at least 5 m y after the peak of metamorphism we conclude that it is implausible that co2 produced by metamorphism in the himalayan orogen was consumed millions of years later by erosion enhanced weathering in this orogen assuming a global climate silicate weathering feedback we estimate that metamorphic co2 degassing from the himalayan orogen would have produced a warming of 0 5 c and enhanced weathering in this orogen would have produced a cooling of 0 2 c thus direct climate effects of this degassing and weathering were likely to have been minor

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

chlorine isotopes of thermal springs in arc volcanoes for tracing shallow magmatic activity the evaluation of the status of shallow magma body i e from the final intrusion stage to quiescence and back to activity one of the key parameters that trigger and sustain volcanic eruptions has been challenging in modern volcanology among volatile tracers chlorine cl uniquely exsolves at shallow depths and is highly hydrophilic consequently cl enrichment in volcanic gases and thermal springs has been proposed as a sign for shallow magmatic activities however such enrichment could also result from numerous other processes e g water evaporation dissolution of old chloride mineral deposits seawater contamination that are unrelated to magmatic activity here based on stable isotope compositions of chloride and dissolved inorganic carbon as well as previous published 3he 4he data obtained in thermal springs from two recently erupted volcanoes la soufriere in guadeloupe and montagne pelee in martinique in the lesser antilles arc we show that the magmatic cl efficiently trapped in thermal springs displays negative 37cl values 0 65 0 65 consistent with a slab derived origin but distinct from the isotope compositions of chloride in surface reservoirs e g seawater local meteoric waters rivers and cold springs displaying common 37cl values of around 0 using this 37cl difference as an index of magmatic cl we further examined thermal spring samples including a 30 year archive from two thermal springs in guadeloupe covering samples from its last eruption in 1976 1977 to 2008 and an island wide sampling event in martinique in 2008 to trace the evolution of magmatic cl in the volcanic hydrothermal systems over time the results show that magmatic cl can be rapidly flushed out of the hydrothermal systems within 30 to 80 years after the eruption much quicker than other volatile tracers such as co2 and noble gases which can exsolve at greater depths and constantly migrate to the surface because arc volcanoes often have well developed hydrothermal systems where magmatic cl is easily transferred to the surface following its exsolution from shallow magma body we suggest that 37cl has great potential to be a unique proxy to monitor the cessation and revival of infrequent arc volcanoes particularly at centennial time scales

travertine deposits from along the south tibetan fault system near nyalam tibet a newly investigated travertine deposit in southern tibet provides a window into holocene hydrological geomorphic and climatic processes near the boundary of the tibetan plateau and high himalaya travertine deposited as a result of the degassing of co 2 rich groundwater as it emerges on the earth s surface is in many cases formed along the trace of major crustal scale faults in primarily extensional tectonic regimes a travertine platform measuring roughly 1 km by 0 5 km exists near the town of nyalam in southern tibet along a major himalayan down to the n normal fault the south tibetan fault system a wide variety of travertine depositional textures and features are recorded in the platform on a series of terraces active travertine deposition was observed from spring mounds and seeps along the base of the platform at the modern river level palaeotemperatures of spring water calculated from 18 o of the travertine range from 9 to 25 c which closely matches the temperatures recorded from modern springs in the area a complex geomorphological landscape records interaction between growing alluvial fans travertine accumulation and a rapidly down cutting river with associated fluvial terraces river incision was contemporaneous with travertine deposition as indicated by cemented fluvial river gravel layers interbedded with travertine high 87 sr 86 sr ratios in the travertine mean of 0 7168 indicate subsurface fluid interaction with radiogenic crystalline rocks of the underlying greater himalaya uranium series ages of the travertine platform range from 5400 a 950 a to 11 600 a 1000 a and indicate a younging progression from higher terraces near the valley wall to lower terraces at present day river level travertine that overlies a river gravel terrace 18 m above river level formed at 11 600 a this date yields a local incision rate of 1 6 mm a 1 consistent with estimated fluvial incision rates in the high himalaya the range of our u series ages coincides with an interval of higher precipitation associated with greater intensity of the indian monsoon which led to elevated spring discharge and carbonate precipitation in this part of the high himalayas

three dimensional flexural modelling of the ebro basin ne iberia summary the ebro basin the southern foreland basin of the pyrenees has undergone a complex evolution in which apart from the pyrenees the iberian range and the catalan coastal ranges have played an important role both as sediment sources and as basin confining structures the deflected basement underlying the ebro basin dips north suggesting a lithospheric scale control on the structure of this basin this is compatible with the results of subsidence analyses which show that the study area is not in a local mode of isostatic compensation in order better to understand the mechanisms that led to the present configuration of the ebro basin and particularly the relevance of the various kinds of un loading e g surrounding fold and thrust belts basin topography subsurface loads we carried out a 3 d kinematic modelling study that accounts for the flexural state of the lithosphere subjected to various loads applied at its lateral boundaries and the sedimentary fill of the basin we also included the effect of neogene extensional tectonics along the eastern basin margin which is related to the opening of the valencia trough we show the suitability of the 3 d lithospheric scale flexural modelling approach to the study of ne iberia modelling results point to a relatively strong lithosphere in this area with values of effective elastic thickness ranging from 10 to 35 km in the ebro basin increasing towards the pyrenees we also find that the topographic tectonic load itself is insufficient to explain the observed basement deflection thus an extra subsurface load beneath the pyrenees corresponding to the underthrusted iberian lithosphere is required the effect of lithospheric stretching in the valencia trough on the ebro basin is appreciable only in its eastern part where the lithosphere was uplifted this had considerable repercussions on the sedimentary and erosional regime of the ebro basin we have analysed the link between the stretching related tectonically uplifted areas and the erosional patterns observed onshore northeast iberia

insight into the structure of the upper rhine graben and its basement from a new compilation of bouguer gravity abstract a compilation of gravity data from the upper rhine graben urg is presented that includes all the main data sources from its german and french parts this data is used to show that the urg consists of at least two arc shaped and asymmetric rift units that tectonically are the basic building blocks of the graben in this sense the urg does not differ from other continental rifts such as the african rifts this division should replace the now classical geomorphologic division of the urg into three segments based on their different trends moreover the gravity suggests that the faults in the central and southern segments are continuous and have the same trend appearing to respond as a single kinematic unit changes in the gravity field in the graben are shown to reflect not only the structure of the graben but also the highly variable composition of the basement in this respect the urg is quite different from some other tertiary continental rifts where possible changes in the composition of the basement are mostly masked in the gravity field by the effect of the overlying low density sediments this characteristic is used to study the extent of some of the main basement units that underlie the graben

precambrian ammonium biogeochemistry a study of the moine metasediments scotland abstract the content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of precambrian age whole rock samples and mica separates from the moine succession proterozoic of scotland were analysed by a combination of capacitance manometry and static vacuum mass spectrometry prior to the analyses the presence of the ammonium ion was confirmed by obtaining infrared ir absorption spectra from the micas principally biotites whole rock ammonium contents are high for amphibolite facies metasediments 140 422 ppm the ammonium ion was not detected in the ir absorption spectra of biotites from one schist however this sample contains neither garnet nor white mica for most of the samples the 15n values are very positive between 12 4 and 16 6 with one sample having a lower value of 8 4 the major variations between samples can be interpreted in terms of original sedimentary features for the ammonium bearing samples muscovites have concentrations between 365 and 633 ppm biotites between 845 and 1739 ppm for individual samples nh4 mu nh4 bi varies between 0 31 and 0 43 a much narrower range than has been reported previously there are no significant or consistent differences in 15n between co existing biotite and white mica nor between the 15n values of the mineral separates and their respective whole rock values the latter suggests minimal selective partitioning of isotopes between micas and plagioclase feldspar the only other potential host for ammonium in the rocks assuming that the ammonium has been derived from organic matter originally present at the time of deposition and by comparison with previous studies it can be inferred that the original 15n value of the moine sediments was somewhere between 5 and 10 similar to recent sediments this suggests that the nitrogen cycle in the proterozoic shelf sea from which the moine sediments were deposited was broadly similar to the present day overall the study underlines how under developed the n system is compared to other stable isotope systems notably c and s and we use the opportunity to highlight some areas that could be developed in the future however one aspect is clear future ammonium studies of metamorphic rocks will need to be performed at high resolution visible scale of heterogeneity with strong mineralogical petrological and field controls

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

structural insights into the early stages of exhumation along an orogen scale detachment the south tibetan detachment system dzakaa chu section eastern himalaya abstract structural transects through the south tibetan detachment system stds in the dzakaa chu valley tibet reveal a 1000 m thick low angle

mantle volatiles in spring gases in the basin and range province on the west of beijing china constraints from helium and carbon isotopes abstract the mantle degassing observed at the earth surface demonstrates both a provenance of fluids in the mantle and a pathway to the surface quantities of this process are discovered on the plate boundaries where there are plenty of active volcanoes and active faults releasing plenty of mantle volatiles however in intraplate tectonic settings without obvious mantle plume the work for mantle degassing observed in spring gasses seems comparatively limited we selected the basin and range province on the west of beijing an area in the inner part of north china craton to discuss the mantle degassing based on the helium and carbon isotopes of spring gasses and the previous works on seismic tomography and fault slip rate the spring gas helium and carbon co2 isotopes indicate the mixture of crustal and mantle materials the helium ratios reported as rc ra air corrected 3he 4he ratio ra 1 4 10 6 ra is the air ratio vary in the range of 0 33 2 08 the calculated mantle helium contributes 4 26 of helium in spring gasses and the remaining is generated in the crust by radiogenic decay of u th series with tiny air mixture co2 acquires analytical 13cv pdb values in the range from 20 3 to 10 2 affected by carbonate precipitation the unaffected values are calculated to be 8 5 5 1 by temperature dependent isotope fractionation indicating the mixture of mantle and crustal limestone materials the mantle volatiles are possibly generated in the upwelling asthenosphere in that the 3he 4he ratio corresponds well with the negative anomaly of p wave velocity at the depth of 70 km imaged by seismic tomography the 3he 4he ratio also correlates with time averaged fault slip rate suggesting higher slip rate renders more permeable mantle vent these consequences help to construct a conceptual model for intraplate mantle degassing that the mantle volatiles generate in the upwelling asthenosphere and uprise through faults and fractures whose permeabilities are controlled by slip rates

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

orogen parallel extension and exhumation enhanced by denudation in the trans himalayan arun river gorge ama drime massif tibet nepal focused denudation and mid crustal flow are coupled in many active tectonic settings including the himalaya where exhumation of mid crustal rocks accommodated by thrust faults and low angle detachment systems during crustal shortening is well documented new structural and u th he apatite data from the mount everest region demonstrate that the trans himalayan ama drime massif has been exhumed at a minimum rate of 1 mm yr between 1 5 and 3 0 ma during orogen parallel extension the ama drime massif offsets the south tibetan detachment system and therefore the south tibetan detachment system is no longer capable of accommodating south directed mid crustal flow or coupling it with focused denudation previous investigations interpreted the nne ssw striking shear zone on the west side of the ama drime massif as the main central thrust zone however our data show that the ama drime massif is bounded on either side by 100 300 m thick normal sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset quaternary deposits and record active orogen parallel extension when combined with existing data these results suggest that the ama drime massif was exhumed during orogen parallel extension that was enhanced by or potentially coupled with denudation in the trans himalayan arun river gorge this model provides important insights into the mechanisms that exhumed trans himalayan antiformal structures during orogen parallel extension along the southern margin of the tibetan plateau

travertines depositional morphology and the bacterially constructed constituents abstract investigation of travertine accumulations throughout central italy and the west central u s has shown that the carbonate is precipitated in response to both inorganic and organic processes individual deposits range up to 85 m thick and hundreds of square kilometers in areal extent all of the carbonate is low magnesian calcite water chemistry temperature and morphology of the accumulation greatly influence the constituents comprising these deposits harsh environmental situations favor inorganic deposits while increasingly more moderate conditions result in the formation of a greater abundance of organically precipitated material morphological variations of travertine deposits recognized include 1 waterfall or cascade 2 lake fill 3 sloping mound fan or cone 4 terraced ound and 5 fissure ridge gross morphology internal stratification and constituents comprising these deposits vary systematically depending upon the type of accumulation and chemistry of the waters bacterially precipitated calcite forms a large percentage of the carbonate in many travertine accumulations exceeding 90 of the framework grains comprising some of the lake fill deposits bacteria are among the first taxa to inhabit and reproduce in harsh spring environments and produce a variety of fascinating constituents the bacteria are primarily rods generally 0 2 m in diameter and less than 1 0 m in length the rods readily decay resulting in calcite crystals loaded with micropores the basic building block of bacterially constructed travertine is a clump of bacteria averaging 20 m in diameter enclosed in a single crystal of calcite aggregates of these crystals produce a variety of deposits including 1 crudely laminated carbonate mud 2 finely laminat d layers of mud 3 intraclasts 4 foam rock and 5 shrubs the shrubs are most striking commonly forming layers 1 3 cm thick but also producing bacterial pisoids at some locales bacterial stromatolites composed of layers of shrubs alternating with finely laminated layers of bacterial mud comprise essentially the entire deposit the shrub layers are the result of flourishing summer growth of bacteria and furthermore show remarkable daily laminae 0 1 0 5 mm thick the importance of bacteria in the formation of travertine and their universally recognized abundance in modern sediments provides an impetus for a reexamination of the role of bacteria in the origin of other types of ancient deposits

strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab everest massif tibet this paper presents quantitative data on strain deformation temperatures and vorticity of flow at the top of the greater himalayan slab the data were collected from the tibetan side of the everest massif where two low angle normal faults bound the upper surface of the greater himalayan slab the earlier and structurally lower lhotse detachment and the later and structurally higher qomolangma detachment greenschist to sillimanite grade quartz rich metasedimentary rocks exposed in the rongbuk to north col region of the everest massif are characterized by cross girdle quartz c axis fabrics indicating approximate plane strain conditions fabric opening angles progressively increase with depth beneath the overlying lhotse detachment and indicate progressively rising deformation temperatures of 525 625 50 c at depths of 300 600 m beneath the detachment deformation temperatures of c 450 c are indicated by fabric opening angles in epidote amphibolite facies mylonites located closer to the overlying detachment a top down to the north normal shear sense is indicated by the asymmetry of microstructures and c axis fabrics but the degree of asymmetry is low at distances greater than 400 m beneath the detachment and sillimanite grains are drawn into adjacent conjugate shear bands but still appear pristine indicating that deformation occurred at close to peak metamorphic temperatures these quenched fabrics and microstructures indicate rapid exhumation in agreement with previous isotopic dating studies mean kinematic vorticity numbers w m were independently calculated by three analytical methods calculated w m values range between 0 67 and 0 98 and indicate that although a simple shear component is generally dominant particularly in greenschist facies mylonites located between the lhotse and overlying qomolangma detachments there is also a major component of pure shear in samples located at 400 600 m beneath the lhotse detachment pure and simple shear make equal contributions at w k 0 71 our integrated strain and vorticity data indicate a shortening of 10 30 perpendicular to the upper surface of the greater himalayan slab and confirm that the upper surface of the slab is a stretching fault with estimated down dip stretches of 10 40 assuming plane strain deformation measured parallel to the flow plane transport direction

nitrogen isotope compositions of metasedimentary rocks in the catalina schist california implications for metamorphic devolatilization history abstract in the catalina schist subduction zone metamorphic complex california metasedimentary rocks show a decrease in n concentration and an increase in 15nair with increasing metamorphic grade lowest grade lawsonite albite rocks contain 632 185 ppm n with 15 n 1 9 0 6 whereas highgrade amphibolite equivalents contain 138 76 ppm n with 15 n 4 3 0 8 loss of n accompanied devolatilization reactions that evolved h2o rich c o h s n fluids through consumption of chlorite and phengitic white mica and production of mineral assemblages containing muscovite biotite garnet and kyanite whole rock n concentrations of up to 200 ppm in veins and pegmatites produced during high p t metamorphism reflect the redistribution of n during devolatilization and partial melting of the metasedimentary rocks bulk fluid rock n isotope fractionations 15n 15nfluid 15nrock of 1 5 1 were calculated with the rayleigh distillation equation taking into account variability in rock composition by comparison of samples with similar k2o concentrations these fractionations are similar to but slightly lower than published calculated fractionations for n2 nh4 exchange at the temperature range of 350 600 c over which most of the devolatilization occurred in the catalina schist approximately 3 4 to 2 25 the n systematics appear to be explained by n2 nh4 exchange and a devolatilization process intermediate in behavior to batch volatilization and rayleigh distillation the observed shifts in n concentration and 15n cannot be explained by nh3 nh4 exchange at these temperatures using the equilibrium models the distillation devolatilization process implicated in this study may govern the behavior of other trace elements partitioned into hydrous fluids during devolatilization e g b cs u similarity of the calculated fluid 15n 1 5 to 5 5 with compositions of natural gases inferred to be derived from metasedimentary sources indicates the possibility of using n as a tracer of large scale volatile transport

measuring the seismic properties of tibetan bright spots evidence for free aqueous fluids in the tibetan middle crust seismic bright spots are commonly interpreted to mark fluid concentrations but their nature melt or aqueous is usually inferred only from circumstantial evidence of the geologic setting a band of bright spot reflections has been imaged by project indepth international deep profiling of tibet and the himalayas at about 15 km depth along 150 km of the northern yadong gulu rift southern tibet we use indepth three component wide angle seismic data to measure seismic velocities at the bright spot reflector and theoretical rock physics bounds to constrain the nature of the fluids merging of data from multiple bright spots allows us to use a one dimensional approximation travel time modeling yields average p and s velocities for the upper crust above the bright spots of 5 3 0 2 and 3 2 0 2 km s 1 respectively reflection amplitude variation with offset avo modeling constrains the p and s velocities of the bright spots to 3 0 0 8 and 1 6 0 8 km s 1 respectively multiple modeling procedures suggest these velocities are not model dependent our results imply that of the order of 10 volume of free aqueous fluids in the tibetan middle crust produces the observed bright spot reflections the presence of relatively large quantities of free aqueous fluids presumably mostly saline supercritical h2o does not preclude the presence of melt but does constrain the maximum temperature at the bright spots to the wet granite solidus about 650 c and thus the maximum surface heat flow to 110 mw m 2 the observed bright spots can alternatively be explained as a result of transient flow of aqueous fluids through a lower temperature and lower heat flow southern tibetan crust

the hatton basin and continental margin crustal structure from wide angle seismic and gravity data results from a wide angle seismic and gravity study between the rockall bank and the iceland basin in the north atlantic are presented crustal and sedimentary structures are resolved in the hatton basin and across the hatton continental margin hcm east of magnetic anomaly 24 the structure of the oceanic crust west of the anomaly is also determined gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide angle seismic data are poor a two layer sedimentary sequence is present both in the hatton basin and across the continental margin the lower layer with p wave velocity of about 4 km s is interpreted as pre eocene synrift sediments and is up to 3 5 km thick a younger and thinner 1 2 5 km postrift sequence with a velocity of about 2 km s defines a strong velocity contrast which suggests an erosional unconformity surface the sedimentary structure is distinctly different from that in the rockall trough where a third intermediate layer vp 3 km s occurs the three layer crust characterized by two intracrustal reflections pip1 and pip2 varies from 30 km thick under the rockall bank to about 15 km below the hatton basin where it is stretched by a factor of 2 relative to onshore ireland the crust is thinnest below the hatton bank where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit below the hcm a region of thick lower crust with anomalously high velocity vp 7 2 km s is resolved by the seismic and gravity data it is connected west of anomaly 24 to a region of oceanic crust which is thicker than in the iceland basin these relationships between the thick lower crust below the hcm and the oceanic crust in the iceland basin are interpreted as evidence for magmatic underplating consistent with previous models for the hcm the inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 early eocene time

geodynamic evidence for a chemically depleted continental tectosphere the tectosphere namely the portions of earth s mantle lying below cratons has a thermochemical structure that differs from average suboceanic mantle the tectosphere is thought to be depleted in its basaltic components and to have an intrinsic buoyancy that balances the mass increase associated with its colder temperature relative to suboceanic mantle inversions of a large set of geodynamic data related to mantle convection using tomography based mantle flow models indicate that the tectosphere is chemically depleted and relatively cold to 250 kilometers depth below earth s surface the approximate equilibrium between thermal and chemical buoyancy contributes to cratonic stability over geological time

deep structure of the voring margin the transition from a continental shield to a young oceanic lithosphere the present day lithospheric structure across the norwegian margin in the voring region is presented the voring margin is characterized by the presence of large volume of magmatic underplating thick mesozoic basins and prominent crustal thinning results from recent deep seismic experiments in the voring basin and voring marginal high and 3 d gravity modelling have been implemented in a regional 2 d lithospheric transect that runs from the norwegian caledonian belt to the oceanic domain crossing the transition between the precambrian fennoscandian baltic shield and the cenozoic northern north atlantic oceanic lithosphere the modelling approach integrates elevation gravity geoid and heat flow data under the assumptions of thermal steady state and local isostasy the results confirm and refine the major trends of the crustal geometry which is characterized by a moho depth varying from about 45 km beneath the caledonian thrusts to 30 km beneath the trondelag platform to 20 14 km beneath the voring basin and to 13 km in the oceanic domain the lithosphere thins from the norwegian caledonian belt 190 km thick to the oceanic domain 60 km in a stepwise manner reflecting the progressive seawards migration of lithospheric deformation since the paleozoic our lithospheric thickness results are different from those predicted by post rift lithospheric cooling models in the oceanic domain observed residual bathymetry 600 m is interpreted as the remaining effect of a deep seated thermal perturbation during the upper cretaceous which would also produce a heat flow anomaly of 15 mw m 2 alternatively residual bathymetry can be interpreted in the absence of thermal anomalies as produced by depletion of the sublithospheric mantle extending down to 130 230 km depth depending on the density contrast 10 30 kg m 3

automatic detection location and quantification of earthquakes application to tsunami warning we describe a fully automated seismic event detection and location system providing for real time estimates of the epicentral parameters of both local and distant earthquakes the system uses 12 telemetered short period stations with a regional aperture of 350 km as well as two 3 component broad band stations detection and location of teleseismic events is achieved independently and concurrently on the short period and long period channels the long period data is then used to obtain an estimate of the seismic momentm0 of the earthquake through the mantle magnitudemm as introduced byokal andtalandier 1989 in turn this estimate ofm0 is used to infer the expected tsunami amplitude at papeete within 15 minutes of the recording of rayleigh waves the performance of the method is discussed in terms of the accuracy of the epicentral parameters and seismic moment obtained in real time as compared to the values later published by the reporting agencies our estimates are usually within 3 degrees of the reported epicenter and the standard deviation on the seismic moment only 0 19 unit of magnitude for a population of 154 teleseismic events

paleoatmospheric consequences of co2 released during early cenozoic regional metamorphism in the tethyan orogen the eocene was the warmest epoch of the cenozoic with published estimates of eocene atmospheric co2 content ranging from two to six times the current value calculations of co2 consumption by silicate weathering show that co2 fluxes to the atmosphere of ca 1018 myr could account for inferred paleocene eocene atmospheric co2 contents and the resulting greenhouse effect would have contributed to eocene warmth extensive portions of the tethyan orogen underwent regional metamorphism culminating in the eocene prograde metamorphism may have been contemporaneous with the late paleocene global warming we calculated the amount of metamorphic co2 produced at depth in the himalayan orogen with data on the timing of the india asia collison duration of prograde regional metamorphism and proportions and bulk compositions of metamorphic co2 source rocks if co2 was generated at a constant rate over a 10 myr period of prograde metamorphism in the himalayan orogen we estimate that ca 1018 1019 moles myr of metamorphic co2 were produced at depth significant expulsion of metamorphic co2 to the atmosphere may have occurred by focused fluid flow along shear zones such as the extensive main central thrust in the himalayan orogen an additional ca 1018 myr could have been contributed by eocene regional metamorphism in the mediterranean tethys i e from the alps to turkey the extensive metamorphism associated with the india asia collision and the closing of tethys may have contributed to co2 greenhouse warming in the early to mid cenozoic

eucrust 07 a new reference model for the european crust 1 we present a new digital model eucrust 07 for the crust of western and central europe and surroundings 35n 71n 25w 35e available results of seismic reflection refraction and receiver functions studies are assembled in an integrated model at a uniform grid 15 0 15 0 the model consists of three layers sediments and two layers of the crystalline crust besides depth to the boundaries we provide average p wave velocities in the upper and lower parts of the crystalline crust the new model demonstrates large differences in the moho depth compared to previous compilations over 10 km in some specific areas e g the baltic shield furthermore the velocity structure of the crust is much more heterogeneous than in previous maps eucrust 07 offers a starting point for numerical modeling of deeper structures by allowing correction for crustal effects beforehand and to resolve trade off with mantle heterogeneities citation tesauro m m k kaban and s a p l cloetingh 2008 eucrust 07 a new reference model for the european crust geophys res lett 35 l05313 doi 10 1029 2007gl032244

mantle fluids in the karakoram fault helium isotope evidence abstract the karakoram fault kkf is the 1000 km long strike slip fault separating the western himalaya from the tibetan plateau from geologic and geodetic data the kkf is argued either to be a lithospheric scale fault with hundreds of km of offset at several cm a or to be almost inactive with cumulative offset of only a few tens of kilometers and to be just the upper crustal localization of distributed deformation at depth here we show 3he 4he ratios in geothermal springs along a 500 km segment of the kkf are 3 100 times the normal ratio in continental crust providing unequivocal evidence that a component of these hydrologic systems is derived from tectonically active mantle mantle enrichment is absent along the indus yarlung suture zone isz just 35 km southwest of the kkf suggesting that the mantle fluids flow only within the kkf within the last few ma the kkf must have accessed tectonically active tibetan mantle northeast of the mantle suture which we therefore locate vertically beneath the kkf very close to the surface trace of the isz hence in southwestern tibet indian crust may not now be underthrusting substantially north of the isz even though miocene underthrusting may have placed indian crust north of the isz in the lower half of the tibetan plateau crust this is in significant contrast to central and eastern tibet where underthrust indian material not only forms the lower half of the tibetan crust but is also currently underthrusting for 200 km north of the isz our new constraint on kkf penetration to the mantle allows an improved description of the continuously evolving karakoram fault as a tectonically significant yet perhaps geologically ephemeral lithospheric structure

southern limit of mantle derived geothermal helium emissions in tibet implications for lithospheric structure the isotopic composition of helium emitted from geothermal springs in the southern tibetan plateau reported as rc ra rc air corrected sample 3 he 4 he ra air 3 he 4 he ranges from 0 013 to 0 38 and defines two principal domains in southernmost central tibet helium isotope ratios are typical of radiogenic helium production in the crust rc ra 6 0 05 crustal helium domain further north there is a resolvable 3 he anomaly consistent with a mantle contribution r ra s 0 1 mantle helium domain the highest values of 0 27 0 38 ra occur at the southern end of the karakoram fault the boundary between the two domains lies 50 100 km north of the indus zangpo suture zone there seems to be no association between the 3 he anomaly and zones of active normal faulting and litho tectonic crustal units such as the ultramafic rocks of the indus zangpo suture zone and the gangdese intrusive belt although scavenging of mantle derived helium stored in large ultrabasic and basic intrusions in the crust cannot be ruled out entirely we argue that the 3 he anomaly most plausibly reflects degassing of volatiles from young quaternary mantlederived melts intruded into the crust as such it defines the southern limit of recent mantle melting and mantle melt extraction beneath the tibetan plateau the southern limit of the 3 he anomaly coincides with the junction between the indian and asian plates in the region where the indian lithospheric slab steepens and is subducted beneath tibet as suggested by seismic studies recent mantle melting and melt extraction is confined to the asian mantle but the southern limit of the melt zone may have migrated northwards during the last 10 ma as the indian lithosphere has progressively underthrust the himalayas and tibet fl 2000 elsevier science b v all rights reserved

nitrogen isotope tracers of high temperature fluid rock interactions case study of the catalina schist california abstract nitrogen isotope data for metasomatized rocks veins and pegmatites in the catalina schist subduction zone metamorphic complex allow futher characterization of complex high p t metasomatic proceses and evaluation of the scales of isotopic equilibration and fluid transport during subduction zone metamorphism throughout the catalina schist n resides predominantly as nh4 in white mica which occurs in nearly all bulk compositions i e metasedimentary metamafic and to a lesser extent metaultramafic melange at all grades within each metamorphic unit of the catalina schist ranging in grade from lawsonite albite to amphibolite facies 15n values of mica in metasomatized metamafic and metaultramafic rocks are consistent with the metasomatic addition of n from nearby devolatilizing metasedimentary rocks into the initially n poor mafic and ultramafic rocks within each unit uniformity of mica 15n in metasomatized rocks relative to the 15n of metasedimentary rocks in the same unit implies mixing of n from nearby heterogeneous metasedimentary sources perhaps producing fluids with unifrom 15n at up to the kilometer scale however the trend in 15n of metasedimentary sources with increasing metamorphic grade is inconsistent with larger scale up temperature transfer of fluid in this case n2 bearing in the catalina schist paleosubduction zone such flow at scales of up to tens of kilometers has been inferred through previous oxygen isotope study nitrogen isotope compositions are instead believed to have been controlled at a more local scale than the o isotope systematics due to the more rock dominated fluid rock mass balance for n the 15n of muscovite in leucosomes and pegmatites in amphibolite grade metasedimentary exposures matches that of muscovite in metasedimentary hosts implying minimal n isotope fractionation during migmatization processes and possible transfer of metasedimentary n isotope signatures in silicate melts these results illustrate the potential of the n isotope system to yield valuable information regarding fluid rock interactions in the crust and mantle the data for the catalina schist demonstrate the ability of n isotopes to trace the transfer of sediment derived c o h s n fluids and silicate melts and show the expected benefit of the n isotope system in having a differing fluid rock mass balance relative to the more commonly used stable isotope systems that can yield unique constraints in quantitative models of crustal fluid processes

spatial variations in gas and stable isotope compositions of thermal fluids around lake van implications for crust mantle dynamics in eastern turkey we investigate the helium 3he 4he and carbon 13c isotope compositions and relative abundance ratios co2 3he of gas samples together with the stable isotope compositions of dissolved carbon and sulfur and the oxygen and hydrogen isotopic compositions of the associated water phase from a number of geothermal fields located around lake van in eastern anatolia turkey the mantle derived helium component which is likely transferred to the crust beneath eastern turkey by recent magmatism is found to constitute up to 96 e g nemrut caldera of the total he content in fluids as regards the spatial distribution of he samples collected from areas of pliocene quaternary volcanics are characterized by a wide and generally higher range of r ra ratios 0 93 to 7 76 ra compared to those of non volcanic regions 0 85 to 1 0 ra co2 3he ratios vary over a wide range 2 4 105 3 8 1013 but are mostly higher than that of the nominal upper mantle 2 109 oxygen hydrogen isotope values of the waters are conformable with the global meteoric water line and indicate a local meteoric origin sulfate in waters is most probably derived from dissolution of marine carbonates and terrestrial evaporite units temperatures calculated by so4 h2o isotope geothermometry lie between 40 and 199 c and are in poor agreement with reservoir temperatures estimated from silica geothermometers discordant temperatures may be due to either the relatively slow rate of isotopic equilibrium between water and sulfate or mixing of geothermal water with sulfate bearing shallow waters which may modify the 18o value the 13c co2 values of gas samples are consistently lower than those of their water counterparts consistent with loss of co2 from waters by degassing mixing between mantle and various crustal c sources appears to be the main control on the c isotope composition the principal origin of co2 in all samples is crustal lithologies mainly limestone 85 to 98 of the total carbon inventory thus the crustal carbon flux is at least 10 times that from the mantle there is a broad correlation between high 3he 4he values and thinner crust in the western part of the lake van region where several historically active volcanoes are located this observation indicates that localized volcanic and magmatic activity exerts the primary control on the balance between mantle and crustally derived volatiles in the region