ava analysis reveals in situ sediment diagenesis at the costa rican decollement complete sediment subduction at the costa rica subduction zone makes this convergent margin an ideal place to investigate the effects of tectonic deformation in situ we present a seismic reflection study along a line located 3 km landward of the middle american trench and oriented parallel to the strike of the decollement the ocean bottom hydrophone obh seismic data include large offsets and incidence angles at the reflectors we derive the p and s waves velocity distribution below the decollement using a p wave analysis of amplitude with reflection angle the investigation shows that there are unexpected large lateral velocity variations at a scale of only a few 100 m the shear wave velocity in the uppermost subducted sediment varies between 300 and 700 m s while the variation of the compressional wave velocity is in a range of 1700 to 2000 m s the variation of the vp vs ratio between 2 8 and 5 2 can only be explained by variations of the pore fluid pressure the modelled velocities correspond to a normalised pore fluid pressure ratio in the range between 0 02 and 0 93 the most reasonable explanation for these observations is the localised presence of fluids which are released during diagenesis by smectite to illite transformation during this process which takes place in three discrete steps the interlayer water of the smectite is added to the pore fluid and the permeability of the sediment is decreased both effects lead to the formation of small overpressured cells

stress induced smectite dehydration ramifications for patterns of freshening and fluid expulsion in the n barbados accretionary wedge abstract presented is new experimental evidence that smectites can partially dehydrate from 18 5 a to 15 4 a hydrates when they are subjected to effective mineral framework stresses above 1 3 0 3 mpa we show here that this process can substantially freshen saline pore fluids and impact our understanding of the hydrogeologic processes in convergent margins where smectites are abundant the general anomalous freshening of pore fluids in the n barbados accretionary wedge was previously proposed to relate to the widespread lateral expulsion of fluids derived from mineral dehydration reactions at great depth in contrast our analysis suggests that a significant part of the widespread anomalous freshening relates to stress induced smectite dehydration that occurred during the ocean drilling program odp pore water extraction process clear evidence for long distance lateral flow appears to be limited to localized chloride and isotopic anomalies developed in a 20 m thick locally mineralized region at the top of decollement horizon at site 948 our reinterpretation is consistent with the fluid flow in the decollement zone being 1 highly transient on time scales of several hundreds of years and 2 channeled laterally into heterogeneous finger like flow paths that are apparently not intersected and or sampled by drilling in most regions at the toe of the wedge the incipient stress induced dehydration of smectite may also be buffering in situ fluid pressures in the wedge maintaining the effective stresses at 1 3 mpa

fold axis parallel extension in an arcuate fold and thrust belt the case of the helvetic nappes abstract the helvetic nappes of western switzerland are discussed as an example of an arcuate foreland fold and thrust belt in which active fold axis parallel stretching occurred fold axis parallel extension is recorded by 1 1 incremental strain data from pressure shadow fibres the significance of pressure shadow fibres for the determination of the deformation history of a region is discussed pressure shadows are used to quantify the amount of and to describe the distribution of fold axis parallel extension occurring in the helvetic nappes 2 2 the extension directions of conjugate systems of en echelon veins it is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern the relative age of the conjugate en echelon vein systems in the helvetic deformation history can be assessed the geometry of the conjugate sets relative to the local anisotropy plane is described and the significance of the preferred orientation of their extension directions is discussed 3 3 fold axis parallel sections a comparison of the regional distribution of the fold axis parallel strain with the shape of the helvetic nappes in fold axis parallel sections shows that the fold axis parallel strain cannot be related to the footwall topography of the nappes it is concluded that the fold axis parallel extension in the helvetic nappes was induced by a change of direction of overthrust shear this change occurred late in the deformation history and was superposed on the already formed nappes the changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting alpine nappe pile relative to the european plate a rotation which lead to the arcuate shape of the western alps

the brahmaputra tale of tectonics and erosion early miocene river capture in the eastern himalaya the himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion within the broader orogen fluvial drainages partly reflect surface uplift different climatic zones and a response to crustal deformation in the eastern himalaya the unusual drainage configuration of the yarlung tsangpo brahmaputra river has been interpreted either as antecedent drainage distorted by the india asia collision and as such applied as a passive strain marker of lateral extrusion latest neogene tectonically induced river capture or glacial damming induced river diversion events here we apply a multi technique approach to the neogene paleo brahmaputra deposits of the surma basin bengal basin bangladesh to test the long debated occurrence and timing of river capture of the yarlung tsangpo by the brahmaputra river we provide u pb detrital zircon and rutile isotopic sr nd and hf and petrographic evidence consistent with river capture of the yarlung tsangpo by the brahmaputra river in the early miocene we document influx of cretaceous paleogene zircons in early miocene sediments of the paleo brahmaputra river that we interpret as first influx of material from the asian plate transhimalayan arc indicative of yarlung tsangpo contribution prior to capture the predominantly precambrian paleozoic zircons indicate that only the indian plate was drained contemporaneous with transhimalayan influx reflecting the river capture we record arrival of detrital material affected by cenozoic metamorphism as indicated by rutiles and zircons with cenozoic u pb ages and an increase in metamorphic grade of detritus as recorded by petrography we interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen whole rock sr nd isotopic data from the same samples provide further support to this interpretation river capture may have been caused by a change in relative base level due to uplift of the tibetan plateau assuming such river capture occurred via the siang river in the early miocene we refute the tectonic aneurysm model of tectonic erosion coupling between river capture and rapid exhumation of the eastern syntaxis since a time interval of at least 10 ma between these two events is now demonstrated this work is also the first to highlight u pb dating on detrital rutile as a powerful approach in provenance studies in the himalaya in combination with zircon u pb chronology

thermal evolution of the gangdese batholith southern tibet a history of episodic unroofing the gangdese batholith southern tibet was part of an andean type arc at the southern margin of asia prior to the collision of india and asia at approximately 50 to 40 ma fission track and 40ar 39ar analyses of 28 rocks from 10 gangdese granitoid plutons along an 250 km length of the batholith in the lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks these data suggest a range of ages for these plutons of 94 to 42 ma with the majority being of tertiary age the postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities we conclude that most of these discontinuities and all of them after 40 ma reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time in addition to the initial cooling of hot magma against cold country rock all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling dropping many tens of degrees in a few million years conversely these plutons all experienced intervals during which they cooled very slowly or not at all these slow cooling intervals lasted from 5 to 50 million years our data indicate that since the collision between india and asia began response to continued convergence has been quite variable in even this relatively small area the data reported here are consistent with a recently proposed model of oligo miocene crustal shortening along the gangdese thrust system in this area

sr and nd isotopes in river sediments from the ganga basin sediment provenance and spatial variability in physical erosion 1 sr and nd concentrations and isotope compositions in sediment of the ganga river from gangotri to rajmahal and its tributaries have been measured to determine provenance and the spatial variability in physical erosion among the ganga subbasins sr and nd in silicates range from 37 to 138 and from 10 to 36 g g with 87sr 86sr and nd of 0 7474 0 8428 and 25 5 to 15 5 respectively the results suggest that 65 of ganga mainstream sediments are derived from the higher himalayan crystallines highlighting intense physical erosion in this region the 87sr 86sr values of sediments in the gangetic plain show nearly identical trends during two seasons with a sharp and significant decrease at barauni downstream of gandak confluence this brings out the major impact of the sediment contribution from the gandak to the ganga mainstream model calculation suggests that about half of the ganga sediment at rajmahal is sourced from the gandak the erosion rates in the himalayan subbasins of the ganga range between 0 5 and 6 mm a where a is years with the gandak having the highest erosion rate high relief and intense precipitation over the headwater basins of the gandak appear to drive the rapid and focused erosion of this basin the results of this study and those in literature suggest that the eastern syntaxis brahmaputra the western syntaxis indus and the gandak have much higher physical erosion rates than the other himalayan basins focused erosion in the hot spots of these river basins contributes significantly to the global riverine sediment budget and influence regional tectonics

erosional history of the himalayan and burman ranges during the last two glacial interglacial cycles abstract the results of a clay mineralogy study combined with major element geochemistry strontium neodymium and oxygen isotopes and 14c ams stratigraphy are reported for deep sea gravity cores located in the bay of bengal md77 180 and the andaman sea md77 169 end 0 and 87sr 86sr from holocene and last glacial maximum lgm sediments of fifteen other cores have also been investigated to identify sediment sources and to estimate oceanic sedimentary transport the data show the contribution of three sources 1 ganges brahmaputra rivers 2 irrawaddy river and 3 sediment derived from the western part of the indo burman ranges the dispersion of the detrital material issuing from these sources has been constrained by the geographic distribution of nd and sr isotopic ratios the lgm sediments are characterized by a significant increase of 87sr 86sr small changes in end 0 and a general decrease of smectite illite chlorite which together imply a decrease in weathering intensity the increased 87sr 86sr are attributed to a decrease in chemical weathering which should release preferentially 87sr during weathering processes in the andaman sea sediments smectite illite chlorite and kaolinite quartz ratios combined with a chemical index of alteration cia indicate that the weathering intensity of the irrawaddy river basin is mainly controlled by the summer monsoon rainfall intensity the wet summer monsoons increase vegetation cover in the plains and favor soil development by the production of pedogenic clays smectite and kaolinite

late palaeozoic to mesozoic kinematic history of the talas ferghana strike slip fault kyrgyz west tianshan as revealed by 40ar 39ar dating of syn kinematic white mica abstract the nw trending talas ferghana fault tff in kyrgyzstan central asia is one of the largest intracontinental strike slip faults in the world it extends over a distance of more than 2000 km from southern tourghai to western tarim and exhibits a maximum dextral offset of 200 km during the late palaeozoic to present the history of the fault provides important insights for the understanding of the evolution of southern central asia but remains poorly constrained due to lack of reliable geochronological data we present new ar ar ages and structural data from the kyrgyz west tianshan that elucidate the kinematic history of the tff in the palaeozoic and mesozoic 40ar 39ar ages on mylonitic white micas document a deformational history consisting of several episodes a late carboniferous age of 312 4 ma point to initiation of top to the south and dextral transpressional deformation during a metamorphic overprint in precambrian and palaeozoic rocks along the northern compartment of the tff the main phase of dextral motion along the entire fault occurred in the permian as suggested by minimum ages of 260 290 ma obtained at two different locations in the nw and central parts of the tff partial isotopic resetting occurred between 240 and 210 ma and younger ages of the ar ar mineral ages and structural data argue for a major phase of dextral shearing affecting the entire region from the west tianshan to mongolia in the late permian and leading to formation of almost equally spaced major nw trending dextral strike slip faults the uniform character of this deformation indicates that the process of amalgamation in this part of the caob ended prior to the late permian and central asia evolved as a single coherent continental block since that time

application of the rfφ technique to ellipitical markers deformed by pressure solution abstract the applicability of the r f technique of finite strain analysis to elliptical markers whose shape is modified by pressure solution is investigated variably oriented elliptical markers shortened by removing material from one or both sides in open and closed systems are compared to markers shortened homogeneously by the same amount for the models examined it was found that the r f technique correctly predicts the principal directions the initial marker axial ratio and the axial ratio of a circle deformed in a like manner interpretation of strain ratios in terms of percent shortening requires knowing how much volume was lost equations relating the strain ratio to percent shortening are given for both constant volume and volume loss deformation

exhumation during crustal folding in the namche barwa syntaxis geological observation in the eastern end of the himalaya shows that the asia india suture is folded metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating establish cooling exhumation of rocks from c 30 km depth within the last 4 myr we argue that exhumation is caused by 10 mm yr 1 erosion coeval with crustal scale folding

underthrust sediments fluid migration paths and mud volcanoes associated with the accretionary wedge off costa rica middle america trench styles of deformation and tectonic responses resulting from the convergence of oceanic and continental plates are strongly dependent on fluids in the sediments we estimate the volume of fluids and sediment underthrust beneath the toe of an accretionary wedge and describe evidence of fluid migration observed in seismic data in the middle america trench near costa rica reduction in normal incident travel time of the underthrust oceanic plate section arcward of the deformation front may be related to fluid expulsion within 4 km of the deformation front where overburden is 800 m more than half of the total water in the pore spaces may have been expelled the rate of fluid transfer from the underthrust section is highest in the zone 0 4 km arcward of the deformation front the clarity of the reflection associated with the decollement is exceptional and shows a phase reversal relative to the seafloor this high amplitude phase reversed reflection ends about 4 km arcward of the deformation front suggesting a rapid reduction in the density and or velocity contrast between the dewatering underthrust section and overlying offscraped sediments at this position fluid migration within the wedge produces mud volcanoes or ridges in the midslope region having relief of about 50 m and extent of perhaps 500 m 1000 m a reflection traced beneath a seafloor mud volcano crosses through the accretionary wedge slope cover boundary and may mark a fluid conduit locally high amplitude reflections associated with the base of slope cover are coincident with closure of structure and indicate local traps to upward organized or diffusive fluid flow some fluids appear to migrate along pathways marked by reflections which extend deep within the accretionary wedge but not always extending into the underthrust section near the trench where fluid expulsion rates are highest water may exit to the seafloor seaward of the downslope pinch out of the slope section

tracing the distribution of erosion in the brahmaputra watershed from isotopic compositions of stream sediments bank sediments and suspended loads of the brahmaputra river and its important tributaries were collected from the himalayan front to bangladesh along with most of the important tributaries chemical and isotopic compositions of the sediments are used to trace sediment provenance and to understand erosion patterns in the basin overall isotopic compositions range from 0 7053 to 0 8250 for sr and nd from 20 5 to 6 9 this large range derives from the variable proportions of sediments from himalayan formations with high sr isotopic ratios and low nd and transhimalayan plutonic belt with lower sr isotopic ratios and higher nd the latter are exposed to erosion in the tsangpo and in the eastern tributary drainages overall erosion of the himalayan rocks is dominant representing ca 70 of the detrital influx compositions of the brahmaputra main channel are rather stable between 0 7177 and 0 7284 for sr and between 14 4 and 12 5 for nd throughout its course in the plain from the siang tsangpo at the foot of the himalayan range down to the delta this stability despite the input of large himalayan rivers suggests that the siang tsangpo river represents the major source of sediment to the whole brahmaputra geochemical budget implies that erosion of the namche barwa zone represents about 45 of the total flux at its outflow before confluence with the ganga from only 20 of the mountain area higher erosion rates in the eastern syntaxis compared to the other himalayan ranges is related to the rapid exhumation rates of this region possibly triggered by higher precipitation over the far eastern himalaya and the high incision potential of the tsangpo river due to its very high water discharge

estimation of viscosity contrast and finite strain from deformed elliptical inclusions a method of strain analysis is described which takes into account a possible competence difference between the strain markers and the rest of the rock using data consisting of the sectional shapes and orientations of groups of inclusions from conglomerate like rocks limits are placed on the possible effective viscosity contrast between the inclusions and the rock as a whole as well as on the bulk finite strain suffered by the rock these results are calculated using a computer program based on the theory for the deformation of elliptical cylinders the method can be considered an extension of the rf method but provides more information and involves less restrictive assumptions

strain and tectonic styles related to variscan transpression and transtension in the moravo silesian culmian basin bohemian massif czechoslovakia abstract progressive variscan development of the n s to nne ssw trending dextral wrench zone in the moravo silesian culmian basin on the eastern border of the bohemian massif resulted in the westphalian asturian transpression followed by transtension of the area the kinematics of the structures are indicated by three phase folding n s to nne ssw and e w stretching of rocks and eastward oriented thrust tectonics the estimated displacement based on strain analysis of conglomerates and folds can reach several tens of kilometers and important stretching up to 160 is due to the perpendicular extension transtension in the most ductile part of the zone

acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection shallow water flows and over pressured zones are a major hazard in deepwater drilling projects their detection prior to drilling would save millions of dollars in lost drilling costs i have investigated the sensitivity of seismic methods for this purpose using p wave information alone can be ambiguous because a drop in p wave velocity vp can be caused both by overpressure and by presence of gas the ratio of p wave velocity to s wave velocity vp vs which increases with overpressure and decreases with gas saturation can help differentiate between the two cases since p wave velocity in a suspension is slightly below that of the suspending fluid and vs 0 vp vs and poisson s ratio must increase exponentially as a load bearing sediment approaches a state of suspension on the other hand presence of gas will also decrease vp but vs will remain unaffected and vp vs will decrease analyses of ultrasonic p and s wave velocities in sands show that the vp vs ratio especially at low effective pressures

experimental folding and boudinage under pure constrictional conditions abstract constrictional folds are characterized by true fold axis parallel extension if the rock volume does not vary during deformation studies of such folds in experiments using plasticine layers of different apparent viscosity and power law exponent clearly indicate that fold axis parallel stretch may be accompanied by plastic elongation as well as boudinage of the competent layer characteristic aspects of the experimentally folded competent layers are 1 coeval development of folds and boudins 2 layer thickness not changing during deformation 3 layer parallel shortening in sections perpendicular to the fold stretching axis 4 enlargement of the initial thickness of the competent layer results in increasing fold wavelength and decreasing number of boudins the ratio of dominant wavelength to layer thickness of the constrictional folds can be described mathematically approximately by the equation developed for plane strain folding of power law materials

growth of the namche barwa syntaxis and associated evolution of the tsangpo gorge constraints from structural and thermochronological data abstract the eastern syntaxis of the himalaya namche barwa is dominated by a north plunging antiform which began to decompress grow at approximately 4 ma new fission track analyses on both apatite and zircon combined with previous geochronological ages indicate that the namche barwa dome also extended laterally while growing vertically zircon fission track ages range from 17 6 to 0 2 ma and have a strong relationship to the main faults of the region including the tertiary tsangpo suture with the younger ages inside the fault bounds towards the syntaxis core on the indian plate and the older ages away from the fault apatite ages reveal that the dome has grown laterally and now impinges over the older faulted margin onto the asian plate the dome is traversed by the tsangpo which has followed the trace of the suture for over 1300 km from its source to the entrance of the dome near dania as the tsangpo crosses the dome it departs from the suture but rejoins it some 60 km northeastwards we construe that the suture has been displaced by the growing antiform and as a consequence the antecedent river has been dragged in a left lateral sense along the exhuming north plunging dome restoring the suture to its position prior to 4 ma reveals a path of the tsangpo eastwards across the present southwestern position of the namche barwa indentation this geometric reconstrunction implies that the tsangpo and the brahmaputra were always one and the same river in addition the tsangpo was tectonically forced into juxtaposition with a tributary of the jiali parlung which it probably then captured the capture was due to tectonic forcing in the last 4 ma rather than headward retreat of the paleo brahmaputra as has been previously suggested

climatic control of sediment transport from the himalayas to the proximal ne bengal fan during the last glacial interglacial cycle clay mineralogy siliciclastic grain size major elements 87sr 86sr and end analyses of deep sea sediments cored in the north eastern bay of bengal are used to reconstruct evolution of detrital sources and sediment transport to the proximal part of the bengal deep sea fan during the last climatic cycle end values 13 3 to 9 7 and 87sr 86sr ratios 0 721 0 733 indicate a mixture of sediments originating from the ganges brahmaputra rivers and the indo burman ranges interglacial marine isotopic stages mis 5 and 1 are associated with a higher contribution of sediments from the ganges brahmaputra river system than is the case for glacial mis 6 4 3 and 2 siliciclasitic grain size combined with si al and si fe ratios indicate coarser glacial sediments with numerous turbidite layers glacial turbidite layers display similar clay mineralogical compositions to hemipelagic sediments only few of turbidite layers mis 6 4 and 2 are slightly unradiogenic end 13 3 suggesting a higher contribution of ganges brahmaputra river sediments independently of changes in the sedimentary sources the smectite illite chlorite ratio of cores located on the ne bengal fan indicates higher inputs of primary minerals illite and chlorite from the highlands of the river basins relief during glacial mis 6 4 3 and 2 and an increased contribution of pedogenic minerals smectite and kaolinite during interglacial mis 5 and 1 maximum smectite illite chlorite ratios during the warm sub stages of mis 5 suggest an intensification of summer monsoon rainfall associated with higher rates of physical erosion of the indo gangetic flood plain and or dominant summer hydrological conditions transporting a higher proportion of sediments deriving from the ganges brahmaputra rivers to the ne bengal fan in addition a higher production of smectite in soils of the indo gangetic flood plain during periods of intensification of monsoon rainfall cannot be excluded

seismic investigation of a bottom simulating reflector and quantification of gas hydrate in the black sea a bottom simulating reflector bsr which marks the base of the gas hydrate stability zone has been detected for the first time in seismic data of the black sea the survey area is in the northwestern black sea at 44 45 n and 31 5 32 5 e in this paper seismic wide angle ocean bottom hydrophone obh and ocean bottom seismometer obs data are investigated with the goal to quantify the gas hydrate and free gas saturation in the sediment an image of the subsurface is computed from wide angle data by using kirchhoff depth migration the image shows the bsr at 205 270 m depth below the seafloor and six to eight discrete layer boundaries between the seafloor and the bsr the top of the hydrate layer and the bottom of the gas layer cannot be identified by seismic reflection signals an analysis of traveltimes and reflection amplitudes leads to 1 d p wave velocity depth and density depth models an average s wave velocity of 160 m s 1 between the seafloor and the bsr is determined from the traveltime of the p to s converted wave the normal incidence pp reflection coefficient at the bsr is 0 11 where the p wave velocity decreases from 1840 to 1475 m s 1 velocities and density are used to compute the porosity and the system bulk modulus as a function of depth the gassmann equation for porous media is used to derive explicit formulae for the gas hydrate and free gas saturation which depend on porosity and on the bulk moduli of the dry and saturated sediment a gas hydrate saturation depth profile is obtained which shows that there is 38 10 per cent hydrate in the pore space at the bsr depth where the porosity is 57 per cent obs 24 this value is derived for the case that the gas hydrate does not cement the sediment grains a model that is supported by the low s wave velocities there is 0 9 or 0 1 per cent free gas in the sediment below the bsr depending on the model for the gas distribution in the sediment the free gas layer may be more than 100 m thick as a result of a zone of enhanced reflectivity which can be identified in the subsurface image

surficial evidence of fluid expulsion from the costa rica accretionary prism the nature and distribution of authigenic carbonates chemosynthetic bacterial mats and unique macrobenthic chemosynthetic communities of bivalves and tube worms are important for evaluating and reconstructing present and past fluid venting of accretionary complexes this paper describes the authigenic carbonates chemosynthetic fauna and fluid venting observed at the four tectonic regions of the costa rica accretionary wedge in february 1994 during an alvin diving program of 20 submersible dives we found no surficial evidence of highly focused fluid venting at the toe of the prism outermost 3 km as implied by the absence of authigenic carbonates and chemosynthetic fauna the absence of vent communities on the lower 3 km of the prism and the relatively elevated heat flow with respect to the adjacent incoming cocos plate langseth and silver this issue suggests diffuse rather than focused flow through the toe of the prism twelve active and relict vent sites marked by small clusters of live vesicomyid clams are localized at the bases and tops of out of sequence thrusts implicating fracture permeability as the fluid conduit in the lower slope region but upslope from the toe vast authigenic carbonates and seven active and relict vent sites marked by large dense clusters of chemosynthetic organisms predominate the largest mud diapir in the mid slope region fluid expulsion appears to be more restricted on the upper slope with only 2 small but dense vents marked by chemosynthetic fauna observed at one wall of one canyon

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

combined interpretation of marine controlled source electromagnetic and reflection seismic data in the german north sea a case study this paper focuses on the combined analysis and interpretation of controlled source electromagnetic csem and multichannel reflection seismic mcs data along one profile in the german north sea with the goal to reduce ambiguities in interpretation the shallow water environment of the north sea is characterized by a complex geological development which includes rifting several ice age cycles a propagating shelf margin mass transport deposits and salt dome formation seismic and electromagnetic methods are sensitive to different physical properties of the seabed and therefore complement each other we analyse the mcs data with a migration velocity tomography and an amplitude variation with offset analysis and discuss seismic velocities and densities for true amplitude recovery the amplitude distortions are calibrated with in situ logging data the csem data are analysed in 2 d for which for the first time data were included that were acquired while the instrument was towed on the seafloor in addition to the stationary sites the csem inversions are constrained by seismic horizons the joint interpretation focuses on two seismic reflectors one can be interpreted as an unconformity marking a lithological change from fresh water bearing glacial deposits to compacted sediments below and the other one as a layer of fine grained deposits potentially capping patchy shallow gas occurrences this exemplary case study shows how the combination of both methods can benefit by interpreting complex geology and eliminating ambiguous explanations

tectonic signatures in centimeter scale velocity porosity relationships of costa rica convergent margin sediments 1 the costa rica convergent margin investigated during ocean drilling program leg 170 offers a unique opportunity to compare the tectonic effects of rapid subduction on incoming oceanic sediments to their laterally equivalent underthrust counterparts and to terrestrially derived wedge materials elevated pressure laboratory measurements of ultrasonic compressional and shear wave velocity and porosity are used to examine the importance of tectonic lithologic and diagenetic controls on physical and elastic properties of sediments in these three key tectonic domains depositional and stress path histories of the three domains for example can be distinguished by 1 trends of in situ velocity porosity and derived measurements correspondence and 2 the mechanical response of representative materials to isotropic consolidation a compressional wave velocity porosity model critical for the application of seismic imaging to margin wide physical property and mass balance estimates is developed from the laboratory measurements and shown to be consistent with the information derived from lwd bulk density and migrated seismic reflection data this consistency of the velocity porosity model over the large range of both frequency and length measurement scales is a key result supporting the assertion that core and borehole physical property measurements can be extrapolated to larger domains finally dewatering and overpressure effects critical factors in subduction zone and fault process dynamics and increasingly common multiphase converted wave imaging targets are discussed in terms of laboratory estimated in situ compressional and shear wave velocity relationships

plio pleistocene exhumation of the eastern himalayan syntaxis and its domal pop up the eastern termination of the himalayan orogen forms a structural syntaxis that is characterised by young from 10 to 7 km in elevation in the core of the syntaxis very high erosion rates documented in sediment downstream of the gorge at the foot of the himalaya contribute 50 of total detritus to the sediment load of the brahmaputra the initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback or to the geometry of the indian plate indentor with the corner being thrust beneath the asian plate resulting in buckling which accommodates shortening both processes may be important the northern third of the syntaxis corresponds to a steep domal pop up structure bounded by the india asia suture on three sides and a thrust zone to the south within the dome greater himalaya rocks equilibrated at 800 c and 25 30 km depth during the miocene with these conditions potentially persisting into the latest miocene and possibly the pliocene with modest decompression prior to 4 ma this domal pop up corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers in this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion we then discuss its cause with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis we use zircon and rutile u pb white mica ar ar and zircon fission track dating methods to extract age data from bedrock brahmaputra modern sediments including an extensive compilation of modern detrital chronometry from the eastern himalaya and neogene palaeo brahmaputra deposits of the surma basin bangladesh numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of 800 c to 250 c our new data include u pb bedrock rutile ages as young as 1 4 ma from the namche barwa massif and 0 4 ma from the river downstream of the syntaxis combined with existing data our new data and heat flow modelling show that i the detrital age signature of the modern syntaxis is unique within the eastern himalayan region ii the rocks within the domal pop up were 575 75 c only 1 2 myr ago iii the neogene surma basin does not record evidence of the rise and erosion of the domal pop up until latest pliocene pleistocene time iv pleistocene exhumation of the north easternmost part of the syntaxis took place at rates of at least 4 km myr with bedrock erosion of 12 21 km during the last 3 ma v the inception of rapid syntaxial exhumation may have started as early as 7 ma or as late as 3 ma and vi the yarlung tsangpo is antecedent and subsequently distorted by the developing antiform together our data and modelling demonstrate that the domal pop up with its exceptional erosion and topographic relief is likely a pleistocene feature that overprinted earlier structural and metamorphic events typical of himalayan evolution

the problem of strain marker centers and the fry method abstract the fry method and its variants will underestimate finite strain when applied to strain markers deformed heterogeneously at marker scale despite homogeneous behavior at a population scale this difference results from the post deformation centers not coinciding with the original pre deformation centers the amount of error may rapidly approach 50 for small strains in ideal cases but is likely to be in the range of 20 30

neural network interpretation of lwd data odp leg 170 confirms complete sediment subduction at the costa rica convergent margin abstract the internal structure of a convergent plate boundary was the focus of odp leg 170 in 1996 at the subduction zone off costa rica although the structure of the subduction zone is rather well known from seismic surveys prior to drilling of odp leg 170 it was a matter of discussion whether it is accretionary or non accretionary with a neural network approach we confirm the evidence gained during drilling of leg 170 that at least presently no lower plate sediments are transferred to the upper plate by accretion to supplement lithological information logging while drilling geophysical data have been included in this study and were interpreted in terms of lithology using a genetically trained artificial neural network

fluid pressure in the frontal thrust of the oregon accretionary prism experimental constraints seismic reflection profiles of many accretionary prisms including the oregon prism exhibit high amplitude reversed polarity reflections from the decollement and other thrusts it has been suggested that these reflectors image fault zones with enhanced fluid content due to dilation by very high fluid pressure we present measurements of velocity as a function of effective stress on samples of the oregon frontal thrust that show that velocity decreases by up to 15 as fluid pressure rises from hydrostatic to lithostatic conditions under constant confining stress in fault zone and wall rock alike synthetic seismic modeling shows that the frontal thrust reflector at this location is the result of a thin low velocity zone in the plane of the fault 100 300 m s slower than the walls combining models with experimental results we conclude that fluid pressure of 86 to 98 of lithostatic stress reduces velocity enough to generate the reflections

a comparison of methods used in measuring finite strains from ellipsoidal objects abstract the available methods for measuring strains in two and three dimensions from passive ellipsoidal objects are compared in an attempt to determine the most useful and precise procedure for the structural geologist a comparison of data collection techniques showed that the use of thin orthogonally cut slabs or mylar overlays used with a monocomparater provides the most reproducible data lisle s theta curve and shimamoto and ikeda s algebraic methods provided the most precise and probably most accurate two dimensional data while miller and oertel s procedure and possibly dunnet s phase 5 program gave the best three dimensional results an examination of errors encountered in strain analyses suggests that all of the available methods give accurate orientations of the finite strain ellipsoid however the magnitudes of strain ratios show large variations that are dependent on the sample size and procedure used shimamoto and ikeda s method again proved to be the most precise giving reproducible results with as few as ten elliptical objects the samples used in the above comparison are part of a larger analysis of strains occurring in southeastern maine structural elements observed in four selected areas of avalonian belt rocks and the strain data collected suggest that the region has undergone at least three non coaxial deformations with d1 d2 d3

timing and mechanism of the rise of the shillong plateau in the himalayan foreland island arcs provide insights into the origin and recycling of continental crust but questions remain concerning source metasomatism the depth of differentiation the potential role of amphibole fractionation and the time scales involved basaltic andesites on batan island philippines contain 150 ma peridotite fragments encased in hornblendite and gabbroic rinds produced via melt rock reaction the peridotites have some of the lightest 7li and 57fe values yet measured in mantle rocks these values are too fractionated to have been created solely by equilibrium partial melting and thus require a combination of melt depletion and slab fluid addition and may be derived from diffusion modified melt channel wall rocks stable isotope signals are easily modified by diffusive equilibration between peridotite and host magma so the preservation of light 7li and 57fe here suggests magma ascent rates of 10 km yr 1 we show that melt wall rock reactions at 25 30 km depth led to the crystallization of amphibole plagioclase followed by gabbroic fractionation at 7 km depth the former provides a location and mechanism for the cryptic amphibole fractionation observed in these and perhaps many other arc lavas and may obviate the requirement for delamination of cumulates

structural and strain analyses of the middle part of the talassian alatau ridge middle asia kirgizstan abstract the middle part of the talassian alatau ridge is composed of upper precambrian lower paleozoic greenschists the usunakhmatian sheet which were overthrust on unmetamorphosed rocks of nearly the same age the talassian and kumishtagian sheets three phases of deformation can be recognized in the usunakhmatian sheet the most intensive of which was the late silurian early devonian d 2 deformation in the usunakhmatian sheet the axis of maximum extension is subhorizontal and parallel to f 2 fold axes r xz varies from 2 4 to 7 5 and k from 0 03 to 1 75 flattening is widespread but the strain of many samples is nearly plane strain in the talassian sheet r xz varies from less than 1 8 to 4 4 and k from 0 to 0 35 r xz and k values increase near large faults r xz values determined by quartz and chert grains were found to be linearly related a three stage strain history is related to d 2 deformation the earliest was simple shear parallel to the axial surfaces of folds following this the fold and thrust system was transformed by pure shear with an orogen parallel axis of extension some minor increments of strain are locally represented by pressure shadows around pyrite crystals

deformed conglomerates and textural zones in the otago schists south island new zealand abstract the prehnite pumpellyite to greenschist facies rocks of the classic low grade otago schist terrane are subdivided into a number of textural zones based on the development of slaty cleavage and the appearance of segregation lamination in metagreywackes in this study conglomerates from fifteen localities have been studied in order to investigate the relationship between ductile strain and the sequence of textural zones strain measurements on lithic pebbles show a clear correlation between strain and textural zone ranging from around 40 shortening in tz iia with the development of slaty cleavage to more than 70 in tz iiia corresponding to the appearence of segregation lamination the much lower and more prolate strains of the quartzose pebbles compared to the corresponding lithic clasts may be explained by differential strain partitioning between clast and matrix microstructural evidence including well developed strain shadows and solution seams indicates that solution transfer was the dominant deformation mechanism the development of quartz segregation laminae in tz iii is due to three related processes the development of mica rich solution seams demarcating quartzo feldspathic microlithons the development of amalgamated and recrystallised strain shadow overgrowths on clastic grains and the formation rotation and disruption of early formed veins the large increase in strain from tz ii to tz iii is responsible both for increased segregation and for the rotation and flattening of segregation features into the foliation thus producing a segregation lamination large strains developed by solution transfer during the low grade metamorphism of greywackes will lead inevitably to segregated schists

the problem of shearing along axial plane foliations abstract the structural significance of axial plane foliations cannot be understood unless we make a distinction between rotation of the material plane foliation and rotation of the geometrically defined x y plane of the strain ellipsoid if the foliation rotates as a material plane at any stage of deformation then its final orientation will be different from that of the x y plane it is suggested that reorientation of foliation takes place by some combination of the formation of foliation e g recrystallization along the x y plane and passive rotation of the material plane foliation in the same continuous deformation the deviation between the foliation and the x y plane is then much less than 5 degrees however because of this deviation a considerable amount of shear strain may develop along the foliation the analysis thus explains how a foliation can be approximately parallel to the x y plane and yet be a plane of shearing

paleogene evolution of the burmese forearc basin and implications for the history of india asia convergence the geological history of the burmese subduction margin where india obliquely subducts below indochina remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader tibetan himalayan orogen various scenarios for the evolution of the orogen have been proposed including a collision of india with myanmar in the paleogene a significant extrusion of myanmar and indochina from the india asia collision zone or very little change in paleogeography and subduction regime since the india asia collision this article examines the history of the burmese forearc basin with a particular focus on eocene oligocene times to reconstruct the evolution of the burmese margin during the early stages of the india asia collision we report on sedimentological geochemical petrographical and geochronological data from the chindwin basin the northern part of the burmese forearc and integrate these results with previous data from other basins in central myanmar our results show that the burmese margin acted as a regular andean type subduction margin until the late middle eocene with a forearc basin open to the trench and fed by the denudation of the andean volcanic arc to the east we show that the modern tectonic configuration of central myanmar was formed 39 37 million years ago when the burmese margin shifted from an andean type margin to a hyper oblique margin the forearc basin was quickly partitioned into individual pull apart basins bounded to the west by a quickly emerged accretionary prism and to the east by synchronously exhumed basement rocks including coeval high grade metamorphics we interpret this shift as resulting from the onset of strike slip deformation on the subduction margin leading to the formation of a paleo sliver plate with a paleo fault system in the accretionary prism pull apart basins in the forearc and another paleo fault system in the backarc this evolution implies that hyper oblique convergence below the burmese margin is at least twice older than previously thought our results reject any india asia convergence scenario involving an early paleogene collision of india with myanmar in contrast our results validate conservative geodynamic models arguing for a close to modern precollisional paleogeography for the indochina peninsula and indicate that any postcollisional rotation of indochina if it occurred at all must have been achieved by the late middle eocene

understanding himalayan erosion and the significance of the nicobar fan a holistic view of the bengal nicobar fan system requires sampling the full sedimentary section of the nicobar fan which was achieved for the first time by international ocean discovery program iodp expedition 362 west of north sumatra we identified a distinct rise in sediment accumulation rate sar beginning 9 5 ma and reaching 250 350 m myr in the 9 5 2 ma interval which equal or far exceed rates on the bengal fan at similar latitudes this marked rise in sar and a constant himalayan derived provenance necessitates a major restructuring of sediment routing in the bengal nicobar submarine fan this coincides with the inversion of the eastern himalayan shillong plateau and encroachment of the west propagating indo burmese wedge which reduced continental accommodation space and increased sediment supply directly to the fan our results challenge a commonly held view that changes in sediment flux seen in the bengal nicobar submarine fan were caused by discrete tectonic or climatic events acting on the himalayan tibetan plateau instead an interplay of tectonic and climatic processes caused the fan system to develop by punctuated changes rather than gradual progradation

tectonics exhumation and drainage evolution of the eastern himalaya since 13 ma from detrital geochemistry and thermochronology kameng river section arunachal pradesh the exhumation history of the central himalaya is well documented but lateral variations in exhumation remain poorly constrained in this study we identify sediment source areas and examine the late neogene exhumation history of the eastern himalaya from the synorogenic sedimentary record of its foreland basin we present nd and hf isotopic data as well as apatite and zircon fission track analyses from the miocene pliocene siwalik group along the recently dated kameng river section in arunachal pradesh northeastern india our isotopic data show that siwalik group sediments deposited between 13 7 and 2 6 ma in arunachal pradesh were mainly derived from higher himalayan source rocks in contrast sediments deposited between ca 7 and 3 ma have far less negative end and ehf values that require involvement of the gangdese batholith and yarlung suture zone source areas via the brahmaputra river system consequently these sediments should also record incision of the namche barwa massif by this river source area exhumation rates of himalayan derived sediments determined from detrital zircon fission track data were on the order of 1 8 km m y in the fastest exhuming areas these rates are very similar to those calculated for the central himalaya and have been relatively constant since ca 13 ma our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to shillong plateau uplift during the miocene as reported elsewhere the zircon fission track data further suggest that exhumation of the namche barwa massif between 7 and 3 ma was much slower than the 10 km m y rate recorded in the recent past detrital apatite fission track data indicate deformation of the siwaliks due to forward propagation of the frontal thrust since around 1 ma

tectonic structure of the convergent pacific margin offshore costa rica from multichannel seismic reflection data the middle america trench between the cocos ridge and a well studied corridor off the nicoya peninsula has a more varied morphology and structure than previously reported the morphological positive features on the lower plate significantly affect the upper plate structure the cocos ridge has uplifted the margin opposite the osa peninsula northwest of cocos ridge numerous seamounts on the oceanic crust sculptured the margin as they subducted a seamount and a huge slump in the trench axis that currently block lateral sediment transport affect the sediment currently accreted and subducted the greater portion of the trench sediment is subducted beneath a lower slope accretionary mass beneath the middle and upper slope is a margin wedge consisting of a high velocity rock with few internal reflections its upper surface has a nondirectional random relief commonly 500 m high in the middle slope area overlying this surface is a low velocity cover of slope sediment which shows little transgressive stratigraphy and can be traced landward into an inferred eocene section beneath the shelf the shelf basement is composed of nicoya complex ophiolite with the same acoustic velocity similar structure and no apparent dividing geologic boundary with the margin wedge we favor a seaward continuation of the nicoya complex to the middle slope and emphasize the evidence for a non steady state tertiary tectonic history

drainage evolution and exhumation history of the eastern himalaya insights from the nicobar fan northeastern indian ocean abstract the eastern himalayan syntaxis where the yarlung tsangpo sharply bends is one of the areas experiencing most rapid exhumation on earth the rapid exhumation is often regarded as the result of capture of the yarlung tsangpo by the brahmaputra river however both the timing of integration of the yarlung tsangpo brahmaputra river and initiation of the rapid syntaxial exhumation are debated as the ultimate sedimentary trap of the yarlung tsangpo brahmaputra river the nicobar fan is a window to look into the drainage evolution and exhumation history of the eastern himalaya international ocean discovery program expedition 362 drilled the nicobar fan for the first time recovering fan sediments dating back to the early miocene 19 ma we apply trace elements and sr nd isotopes to investigate the provenance of the sediments in the nicobar fan with the aim of constraining the timing of integration of the yarlung tsangpo brahmaputra river and initiation of the rapid syntaxial exhumation the geochemical and sr nd isotope compositions indicate an eastern himalayan source dominated by the greater himalaya with significant gangdese arc contribution and primarily carried by the brahmaputra river flux of gangdese arc material appears to have been continuous from the base of the nicobar fan suggesting that the yarlung tsangpo brahmaputra river has been established at least since 19 ma synchronously with the sharp rise in sedimentation rate the abrupt change of geochemical and isotope compositions at 9 2 ma indicates an increase in erosion of the greater himalaya as the result of initiation of rapid exhumation in the broad syntaxial region the proportion of greater himalayan material increased again at 3 5 1 7 ma consistent with a younger pulse of rapid exhumation focused in the core of the syntaxis since 3 5 ma our results show that initiation of the rapid syntaxial exhumation postdated integration of the yarlung tsangpo brahmaputra river by at least 10 m y therefore tectonic uplift rather than river capture could be responsible for the initiation of the rapid syntaxial exhumation

orogen parallel extension and oblique tectonics the relation between stretching lineations and relative plate motions many collisional orogens exhibit two distinct deformation phases the first and relatively early phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds second phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall kinematic modeling based on modern oblique convergent boundaries and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion but that the correspondence is not simple in particular the orientation of stretching lineations is a function of finite strain kinematics of deformation and angle of subduction and convergence and it will consistently underestimate the angle of oblique convergence second phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence examples are drawn from the south central canadian rockies the western alps the variscides of ireland and france and the himalaya

deformational and strain patterns of an intracontinental collision ductile shear zone an example from the higher garhwal himalaya the higher himalayan metamorphic belt of garhwal appears to have evolved in a major 15 20 km wide ne dipping ductile shear zone of the overthrust type due to intracontinental crustal shortening during cenozoic continental collision within the indian plate out of the four major distinct deformation phases the most widespread d2 deformation phase is marked by a prominent s2 foliation axial planar to the reclined and recumbent f2 folds a coaxial ne plunging l2 stretching lineation and syntectonic growth of index metamorphic minerals these deformational structures are developed during the sw directed d2 ductile shear deformation irrespective of the orientation of the s2 foliation and thrust zones regional strain patterns in quartzite of the lesser himalayan garhwal group beneath the main central thrust mct and the central crystalline zone using rf data on quartz clasts and aggregates reveal that the maximum values of finite strain es are attained within narrow mylonitic shear zones within the broad ductile shear zone during its progressive translation towards higher structural levels in late d2 and d3 deformation phases the shortening was accommodated by development of thrust sheets with the mct and jutogh thrust surfaces coinciding with zones of mylonite and maximum strain at higher levels ductile shear zones became compressional brittle ductile zones which resulted in the development of sw verging f3a and f3b folds during the southward migration of the thrust sheets in the lesser himalaya

evolution of the nankai trough decollement from the trench into the seismogenic zone inferences from three dimensional seismic reflection imaging we mapped the amplitude of the nankai trough subduction thrust seismic reflection from the trench into the seismogenic zone with three dimensional seismic reflection data the decollement thrust forms within the lithologically homogeneous lower shikoku basin facies along an initially nonreflective interface the reflection develops from a porosity contrast between accreted and underthrust sedimentary material because of accretionary wedge consolidation and rapid loading and delayed consolidation of the underthrust section a decollement amplitude map shows a significant decline from high amplitudes at the trench to barely detectable levels 25 30 km landward three other observations coincide with the amplitude decline 1 the decollement initially steps down to deeper stratigraphic levels 2 the wedge taper increases dramatically and 3 the thrust becomes seismogenic the amplitude decline and the coincident decollement and accretionary wedge tectonic and seismogenic behavior are attributed to the loss of fluids and potentially loss of excess fluid pressures downdip along the subduction thrust

mineralogy and geochemistry of bay of bengal deep sea fan sediments odp leg 116 evidence for an indian subcontinent contribution to distal fan sedimentation

strains and structures associated with a terrane bounding stretching fault the melones fault zone central sierra nevada california abstract an examination of the structures total strains and rock types across the melones fault zone indicates that in the central sierra nevada this fault separates and locally imbricates two distinct terranes east of the fault a widespread early jurassic cleavage displays west northwest to northwest strikes and is associated with moderate to high strain intensities these structures form in jura triassic metasedimentary rocks in part derived from a continental source a weak northwest striking cleavage and low strain intensities occur west of the fault in lower jurassic volcanic rocks and upper jurassic slates and sandstones the fault zone itself consists of steep east dipping ductile brittle shear zones or stretching faults along which the early jurassic structures are transposed parallel to the jura cretaceous and fault related structures large flattening type strains are common in the fault zone and may be the result of heterogeneous strains developed when the shear zones bend around resistant blocks or by volume losses total strains in the eastern hanging wall reflect the superposition of fault related on older strains therefore stretching lineations in this hanging wall do not reflect directions of fault motion stretching lineations in the footwall and fault zone kinematic indicators in the fault zone and a small eastward jump in metamorphic grade suggest an east over west or reverse sense of movement no evidence for significant strike slip or oblique slip motion was found removal of footwall strains indicates that the jura cretaceous shortening across the fault was responsible for both the transposition of older structures east of the fault and the rotation of the fault to its present steep dips thus the melones fault is a large ductile brittle shear zone along which the eastern hanging wall merced river terrane moved to the southwest over the western footwall eastern foothills terrane

including strain data in balanced cross sections abstract almost all published balanced cross sections include cleaved and or strained rocks and may therefore violate the requirement that bed lengths and formation areas are preserved between undeformed and deformed states knowledge of distortions presence and type of cleavage and overprinting relations in poly deformed segments are necessary to define area changes related to volume loss strike elongation and poly deformation work in the southern and central appalachians has shown variation in structural complexity and changing deformation styles both along and across the thrust belt this necessitates separate treatment of individual sheets during balancing regions of little or no interstratal slip pinning points should be chosen for each thrust sheet and each fold strain in the sheet in external sheets where deformation intensity increases towards thrusts pin points should be back limb inflection points in intermediate and internal sheets where cleavage is approximately coeval with folding pin points should be at fold hinges where cleavage and strain are heterogeneously developed throughout a stratigraphic sequence a line length balance using massive relatively uncleaved beds e g knox group southern appalachians can be used as a basis for restoring the section unstraining of deformed layers given strain data and cleavage trajectories for individual thrust sheets provides line lengths and formation areas which can be fitted to the key bed of the undeformed section cleavages which partially or fully postdate folding indicate continued shortening in the thrust wedge and modify earlier pinning points in upper sheets these cleavages and poly deformed segments are difficult to restore as strain data are generally scarce in the metamorphosed internal thrust sheets

structural development during flysch basin collapse the fencemaker allochthon east range nevada abstract during mesozoic deformation in north central nevada a thick succession of triassic siliciclastic and hemipelagic carbonate strata that were deposited in a deep water basin shortened against and overrode penecontemporaneous shallow water carbonates and interbedded siliciclastics at the eastern margin of the basin initial rheologic contrasts developed between lithologic units during shallow pretectonic burial localized displacements within fine grained poorly cemented units and along lithologic boundaries changes in theology during low pressure and low temperature deformation led to more brittle conditions possibly due to fluid expulsion and subsequent displacements truncated bedding structures in triassic strata suggest that the w facing slope of the shelf to basin transition localized thrusting collapse of the basinal strata against strata of the basin margin and formation of a thrust at the shelf to basin transition demonstrate the ability of stratigraphic declivities and associated rheologic contrasts to buttress deformation and localize thrusting

fluid flow paths in the middle america trench and costa rica margin the hydrology of the subducting plate and its dewatering behavior through the shallow subduction zone is linked to the structure and deformation of the forearc prism the nature of the seismogenic zone the composition of seawater for selected elements and the composition of the residual slab subducted to depths of magma generation at the volcanic arc two locally independent systems of fluid flow govern the transport of heat and chemistry through the costa rica subduction complex a dominantly nonaccretionary subduction zone one fluid system is the margin wedge decollement and underthrust sediment section fluid sources include local sediment compaction and mineral dehydration at depth a second flow system occurs in basement beneath the sedimentary sequence on the incoming plate this region is characterized by extremely low conductive heat flow and the sediment overlying basement has pore water geochemistry similar to that of seawater flow nearly parallel to the trench could be directed by permeability associated with faults and driven by a combination of differential heating and earthquake strain cycling

heterogeneous hydrofracture development and accretionary fault dynamics ocean drilling program observations at many accretionary wedges indicate that fluid flow in low permeability sediments is focused along faults rich in clay minerals recent ring shear experiments show that shear failure generally reduces permeability in muddy faults to values well below the surrounding sediments and that very little increase occurs in the fault permeability even when continued failure proceeds under steadily reduced effective loads it is thus unlikely that observed focused flow occurs through the matrix of muddy faults and we suggest that focused flow can only be explained by the development of open fracture systems the lithostatic fluid pressures necessary for hydrofracturing in weak sediments cannot however exist throughout the imbricate thrust system and basal decollement zone because a zero basal shear stress condition would result in regional instability and accretionary wedge collapse we propose that the regional hydrofracture networks must propagate heterogeneously leaving relatively strong asperity regions at sublithostatic fluid pressures to balance forces in the wedge in three dimensions

negative polarity seismic reflections along faults of the oregon accretionary prism indicators of overpressuring either thrusting of higher over lower impedance sediment or reduction of impedance locally in the fault zone can produce negative polarity reflections along the protothrusts and the frontal thrust of the oregon accretionary prism the vertical displacement of protothrusts averages only 27 m is uncorrelated to the occurrence of polarity reversals and is insufficient to generate the impedance contrasts associated with the negative polarity reflections the negative polarity reflections from the protothrusts are probably due to localized impedance reduction in the fault zone waveform modeling of these negative polarity reflections indicates they develop due to a single interface with negative impedance contrast this seismic signal could be produced by a fault zone with a sharp upper boundary with low impedance fault material and a transition down section to sediment of higher impedance an impedance inversion due to the 1 km throw on the frontal thrust could create the observed negative polarity reflections interval velocities do not decrease across the frontal thrust but core scale velocity measurements indicate a velocity decrease in the frontal thrust therefore the negative polarity reflections are apparently produced by a localized decrease in impedance waveform modeling of frontal thrust reflections suggests the low impedance interval is about 20 m thick with discrete upper and lower boundaries moderately dipping faults are an efficient path for fluid expulsion across the turbidites with low equivalent vertical permeability fluid migration from the over pressured section up the faults can locally reduce the velocity sufficiently to cause the negative polarity reflections

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

thrust related strain gradients and thrusting mechanisms in a chevron folded sequence southeastern australia an e directed imbricate fan thrust system within a chevron folded monotonous quartz rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust sheets upper parts of thrust sheets show low to moderate strains xz strains 30 1 tight inclined chevron folds ila 30 with predominantly w dipping axial surfaces intense phyllitic cleavage with a strong down dip mineral stretching lineation and long curved tapering quartz pressure shadows on pyrite such differences in fold geometry the intensity of the cleavage and the geometry of quartz fibres in pressure shadows require thrust sheet emplacement to be synchronous with the internal deformation of sheets thrust sheets show a change from an approximately constant volume plane strain coaxial deformation in their upper parts to non coaxial high strain deformation with incremental i between those expected for pure shear and simple shear i e 0 i 5 45 in their basal parts observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust sheet volumes both during the initiation and propagation of subsurface detachment faults and with their emplacement to shallower crustal levels strain softening related to chevron folding and more intense cleavage development within a 500 m basal zone has controlled the emplacement of these turbidite dominated thrust sheets transport and internal deformation of the thrust sheets did not involve gravitationally induced spreading but required either a push from the rear or some form of underthrusting

regional structural and strain analyses of terranes in the western metamorphic belt central sierra nevada california abstract a comparison of total strains in 225 samples from the western metamorphic belt sierra nevada shows that strain types and intensities strongly depend on rock type greywackes and clast supported conglomerates are least strained whereas more intense strains occur in lapilli tuffs pebbly mudstones and slates large belts of volcanic rock record plane strain whereas nearby slate and greywacke sequences record flattening type strain suggesting that volume losses of 45 65 occurred in slates and 0 26 in greywackes lapilli tuffs record less than 11 volume loss the differences in volume loss and strain from one rock type to another require that discontinuities in total strain occur at most lithologic contacts independently of the lithologic control regional structural patterns and strain fields support the delineation of three terranes foothills merced river and northern sierra and two distinct domains western and eastern foothills terranes at these latitudes within the western metamorphic belt these terranes each bordered by large ductile shear zones differ in average strain intensity and orientation but not in strain type in the foothills terrane strain intensities decrease from moderate values in the western foothills terrane to low values in the eastern foothills terrane strain intensities in the study area are highest in the melones and bear mountains fault zones and in terranes east of the melones fault structures associated with regional deformation in each terrane do not extend far across terrane boundaries multiple generations of regionally developed structures are therefore rare in this part of the western metamorphic belt outside of fault zones we see one regionally developed cleavage in the foothills terrane a single but older cleavage in the merced river terrane and two still older cleavages in the northern sierra terrane however younger deformations may have reactivated and rotated older structures in adjacent terranes the timing and kinematics of the deformation in the foothills terrane suggest that faulting and rigid body rotations preceded pervasive shortening continued fault movement and rotation of faults to steep dips if orientations of ductile structures reflect plate motions structures in the merced river terrane suggest jurassic triassic orthogonal nne convergence whereas structures in the foothills terrane support late jurassic orthogonal ne convergence changing to oblique ene convergence about 150 ma ago

valemount strain zone a dextral oblique slip thrust system linking the rocky mountain and omineca belts of the southeastern canadian cordillera the valemount strain zone vsz a narrow zone of high orogen parallel op strain in pebble conglomerate of the late proterozoic miette group is the footwall expression of a thrust fault of the western edge of the rocky mountain belt marking the eastern limit of a wide zone of op fabrics distributed through the omineca crystalline and western rocky mountain belts of the southeastern canadian cordillera kinematic indicators from the vsz and the adjacent bear foot thrust zone show that both thrust and dextral displacement are associated with folding and thrust motion in the rocky mountains thereby linking the southern rocky mountain belt to the omineca belt by an oblique slip thrust regime that is tectonically unrelated to the southern rocky mountain trench transverse shortening of thrust sheets and subsequent distribution of op shear are invoked to explain the parallelism of stretching lineations and fold axes strain and kinematic data and the thrust belt geometry of the vsz suggest that op lineations are a product of a large amount of transverse shortening during slightly oblique a type subduction thus op lineations are not representative of relative plate motions between north america and accreted terranes but probably are a function of footwall buttressing of thrustmore sheets a mechanism that may be widely applicable to the internal zones of collisional orogens less

10be imaging of sediment accretion and subduction along the northeast japan and costa rica convergent margins sediment accretion and subduction at convergent margins play an important role in the nature of hazardous interplate seismicity the seismogenic zone and the subduction recycling of volatiles and continentally derived materials to the earth s mantle identifying and quantifying sediment accretion essential for a complete mass balance across the margin can be difficult seismic images do not define the processes by which a prism was built and cored sediments may show disturbed magnetostratigraphy and sparse biostratigraphy this contribution reports the first use of cosmogenic 10be depth profiles to define the origin and structural evolution of forearc sedimentary prisms biostratigraphy and 10be model ages generally are in good agreement for sediments drilled at deep sea drilling project site 434 in the japan forearc and support an origin by imbricate thrusting for the upper section forearc sediments from ocean drilling program site 1040 in costa rica lack good fossil or paleomagnetic age control above the decollement low and homogeneous 10be concentrations show that the prism sediments are older than 3 4 ma and that the prism is either a paleoaccretionary prism or it formed largely from slump deposits of apron sediments low 10be in costa rican lavas and the absence of frontal accretion imply deeper sediment underplating or subduction erosion

seismically inferred dilatancy distribution northern barbados ridge decollement implications for fluid migration and fault strength a 5 x 25 km three dimensional seismic survey of the lower part of the northern barbados ridge accretionary prism creates a three dimensional image of a major active decollement fault the fault is usually a compound negative polarity reflection modeled as a low velocity high porosity zone less than 14 m thick this thickness is significantly less than that defined by drilling of a 40 m zone of deformation at ocean drilling program odp site 671b located within the surveyed area we infer that the seismically defined fault is a thin high porosity zone and is thus an undercompacted high fluid pressure dilatant section if these inferences are correct then map view variations in seismic reflection waveform and amplitude illustrate complex patterns of fault zone fluid content and fluid migration paths the amplitude map suggests kilometre wide channels of locally high porosity and thus focused fluid flow these paths are only subparallel to the expected minimum head as inferred from the shape of the overlying sediment wedge other factors must modify fluid concentrations and ultimately migration several areas of positive polarity fault reflections define square kilometre sized regions inferred to be lower porosity sections producing strong asperities in an otherwise weak fault one coincident with site 671b may explain the success of drilling through the fault here all other holes drilled in the area were within the negative polarity regions and were unsuccessful in penetrating through the entire fault zone possibly because of instability associated with high fluid pressures and a weak fault odp leg 156 planned for 1994 will test inferences related to fault permeability and fluid pressures

petrography and whole rock geochemistry of the tertiary sylhet succession northeastern bengal basin bangladesh provenance and source area weathering abstract detrital modes and the geochemical compositions of sandstones and mudrocks from the paleocene to plio pleistocene jaintia barail surma tipam and dupitila groups of the sylhet basin in bangladesh have been examined to identify their provenance and source area weathering conditions barail and surma group sandstones are quartz rich feldspar poor and are rich in sedimentary and metamorphic lithic fragments indicating a recycled orogen source geochemically the succession is divisible into three parts paleocene late eocene jaintia group mudrocks form separate trends and have very high chemical index of alteration cia values suggesting derivation from a stable cratonic source the second part the late eocene to miocene barail and surma groups forms single trends on variation diagrams maximum cia ratios decrease from the barail to the surma group suggesting rapid influx of detritus from a common felsic source undergoing active uplift compatible with weathering and erosion proxies from the bengal and indus fans average bulk geochemical and ree characteristics of these two groups are identical as are a cn k trends these features and comparison with potential himalayan sources support derivation of both the barail and surma groups from that orogen and hence supply of himalayan detritus to the sylhet basin as early as the late eocene rather than from the neogene the third part the non marine tipam late miocene pliocene and dupitila pliocene pleistocene groups forms slightly differing geochemical trends and has higher maximum cia values ree fractionation between sandstones and mudrocks is more marked but ree patterns and parameters remain unchanged suggesting that the overall source remained unchanged higher cia and greater ree fractionation in the tipam and dupitila groups may be a product of their fluvial deposition and more intense weathering on the floodplain during intensification of the south asian monsoon from 4 ma onwards

ability of the fry method to characterize pressure solution deformation abstract the applicability of the fry method to marker distributions deformed by pressure solution is examined four types of pressure solution deformation were modeled they differ by the spacing of the dissolution surfaces relative to the distance between marker centers the amount of volume loss and whether or not markers in contact with dissolution surfaces are dissolved completely the results show that the fry method correctly characterizes each type of deformation but without additional information such as the amount of volume loss it can be used only where the deformation is constant volume and where dissolution surfaces are more closely spaced than marker centers

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

finite strain patterns of nevadan deformation western klamath mountains california finite strain analyses of galice formation metagraywackes from the footwall block of the orleans fault western klamath mountains california show a dramatic increase in strain ratios structurally upward approaching the orleans fault r xz strain values range from less than 3 at a distance of 2 km from the fault to 6 5 adjacent to the fault strain ellipse orientations are relatively consistent x directions are commonly northwest southeast and subhorizontal and trend subparallel to the trace of the orleans fault finite strain ellipse orientations and the clockwise rotational strain recorded by microscopic kinematic indicators are both compatible with the interpretation of dextral oblique convergence on the orleans fault during the nevadan orogeny

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

sr nd os evidence for a stable erosion regime in the himalaya during the past 12 myr modern erosion of the himalaya the world s largest mountain range transfers huge dissolved and particulate loads to the ocean it plays an important role in the long term global carbon cycle mostly through enhanced organic carbon burial in the bengal fan to understand the role of past himalayan erosion the influence of changing climate and tectonic on erosion must be determined here we use a 12 myr sedimentary record from the distal bengal fan deep sea drilling project site 218 to reconstruct the mio pliocene history of himalayan erosion we use carbon stable isotopes 13c of bulk organic matter as paleo environmental proxy and stratigraphic tool multi isotopic sr nd and os data are used as proxies for the source of the sediments deposited in the bengal fan over time 13c values of bulk organic matter shift dramatically towards less depleted values revealing the widespread late miocene ca 7 4 ma expansion of c4 plants in the basin sr nd and os isotopic compositions indicate a rather stable erosion pattern in the himalaya range during the past 12 myr this supports the existence of a strong connection between the southern tibetan plateau and the bengal fan the tectonic evolution of the himalaya range and southern tibet seems to have been unable to produce large re organisation of the drainage system moreover our data do not suggest a rapid change of the altitude of the southern tibetan plateau during the past 12 myr variations in sr and nd isotopic compositions around the late miocene expansion of c4 plants are suggestive of a relative increase in the erosion of high himalaya crystalline rock i e a simultaneous reduction of both transhimalayan batholiths and lesser himalaya relative contributions this could be related to an increase in aridity as suggested by the ecological and sedimentological changes at that time a reversed trend in sr and nd isotopic compositions is observed at the plio pleistocene transition that is likely related to higher precipitation and the development of glaciers in the himalaya these almost synchronous moderate changes in erosion pattern and climate changes during the late miocene and at the plio pleistocene transition support the notion of a dominant control of climate on himalayan erosion during this time period however stable erosion regime during the pleistocene is suggestive of a limited influence of the glacier development on himalayan erosion

crack propagation sequence and pore fluid conditions during fault bend folding in the appalachian valley and ridge central pennsylvania bedding parallel bpv strike sv and cross fold cfv veins represent a sequence of polyphase fracturing during the development of a fault bend fold in lower paleozoic carbonate beds in the appalachian valley and ridge province pennsylvania brecciation and layer parallel shearing played important roles in the development of the earliest vein set bpv which propagated prior to folding fluid inclusions in calcite bpv are highly saline brines 23 4 wt naci trapped at conditions close to lithostatic p 180 mpa t 267 c while passing through the lower kink plane of a fault bend fold strike joints propagated in dolomitic beds located on the extensional side of neutral surfaces stylolitization of strike joint surfaces accompanied slip of the hanging wall up a ramp renewed extension upon passing through a second kink plane led to propagation of antitaxial sv along the stylolitized joints by the crack seal process slightly less saline fluids 22 4 wt naci were trapped in sv at fluid pressures 144 mpa and temperatures 217 c with the concurrent formation of lateral ramps the carbonates moved to the upper flat and were subjected to strike parallel extension as manifested by the propagation of antitaxial cfv due to further mixing of fresh waters the salinity of fluids forming cfv decreased 20 5 wt nacl with trapping conditions at p 116 mpa and t 179 c the fluid evolution path from bpv to cfv through sv shows a modest decrease in salinity with a sharp decrease in possible trapping pressures

seismic evidence for fluids in fault zones on top of the subducting cocos plate beneath costa rica summary in the 2005 ticocava explosion seismology study in costa rica we observed crustal turning waves with a dominant frequency of 10 hz on a linear array of short period seismometers from the pacific ocean to the caribbean sea on one of the shot records from shot 21 in the backarc of the cordillera central we also observed two seismic phases with an unusually high dominant frequency 20 hz these two phases were recorded in the forearc region of central costa rica and arrived 7 s apart and 30 40 s after the detonation of shot 21 we considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active source seismic experiment such high frequency phases following shot 21 were not recorded after shots 22 23 and 24 all in the backarc of costa rica which might suggest that they were produced by some other source however earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude in addition we would have expected to see more arrivals from such an earthquake on other seismic stations in central costa rica we therefore investigate whether the high frequency arrivals may be the result of a deep seismic reflection from the subducting cocos plate the timing of these phases is consistent with a shear wave from shot 21 that was reflected as a compressional s p and a shear s s wave at the top of the subducting cocos slab between 35 and 55 km depth the shift in dominant frequency from 10 hz in the downgoing seismic wave to 20 hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency the spectral amplitude characteristics of the s p and s s phases from shot 21 are consistent with a very high v p v s ratio of 6 in 5m thick slab parallel layers this result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near lithostatic conditions the overpressured shear zone probably takes up fluids from the downgoing slab and it may control the lower limit of the seismogenic zone

extreme localized exhumation at syntaxes initiated by subduction geometry some of the highest and most localized rates of lithospheric deformation in the world are observed at the transition between adjacent plate boundary subduction segments the initiating perturbation of this deformation has long been attributed to vigorous erosional processes as observed at nanga parbat and namche barwa in the himalaya and at mount st elias in alaska however an erosion dominated mechanism ignores the 3 d geometry of curved subducting plates here we present an alternative explanation for rapid exhumation at these locations based on the 3 d thermomechanical evolution of collisions between plates with nonplanar geometries comparison of model predictions with existing data reproduces the defining characteristics of these mountains and offers an explanation for their spatial correlation with arc termini these results demonstrate a bottom up tectonic rather than top down erosional initiation of feedbacks between erosion and tectonic deformation hence the importance of 3 d subduction geometry

factors affecting finite strain estimation in low grade low strain clastic rocks the computer strain analysis methods sape mrl and dtnnm have permitted the characterization of finite strain in two different regions with contrasting geodynamic scenarios 1 the talas ala tau tien shan kyrgyzs republic and 2 the somiedo nappe and narcea antiform cantabrian to west asturian leonese zone boundary variscan belt nw of iberia the performed analyses have revealed low strain values and the regional strain trend in both studied areas this study also investigates the relationship between lithology grain size and percentage of matrix and strain estimates the two methodologies used the results show that these methods are comparable and the absence of significant finite strain lithological control in rocks deformed under low metamorphic and low strain conditions

pencil structure and strain in weakly deformed mudstone and siltstone abstract weakly deformed mudstone and siltstone middle ordovician knobs formation of the appalachian valley and ridge province south western virginia u s a show strain dependent transitions between bedding fissility pencil structure and cleavage pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain z ranges between 9 and 26 shortening assuming a plane strain and constant volume where strains are less bedding fissility dominates pencil fragments are defined by intersecting fracture sets subparallel to the pre existing bedding fissility and cleavage their long axes are both parallel to the bedding cleavage intersection and to the inferred y axis of the tectonic strain ellipsoid pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post tectonic stress relaxation pencils show variations in size and shape depending on lithology grain size and composition degree of initial clay preferred orientation degree of cleavage development type of cleavage total bulk strain and degree of strain homogeneity their shape l w however is a direct measure of total y z strain since strain determinations from chlorite pressure fringes on framboidal pyrite within the pencil fragments give y z 0 913 0 019 l w pencil structure is therefore a potential strain marker in weakly deformed rocks

the late oligocene early miocene himalayan belt constraints deduced from isotopic compositions of early miocene turbidites in the bengal fan abstract we present a mineralogical and isotopic study of early miocene turbidites from the bengal fan which are the oldest sediments recovered during odp leg 116 clay fractions are dominated by illite and chlorite and have low 18o 11 16 the clays are dominantly derived from a metamorphic precursor by physical erosion and have undergone only minor low t water rock exchange and chemical weathering sr and nd isotope analysis of silt and clay fractions and coarse muscovite biotite and feldspar fractions are remarkably similar to those of the presently exposed high himalaya crystalline hhc sequence the data show that a close analogue of the hhc was already subaerially exposed to active erosion during early miocene rb sr data on the biotite separates has been used to evaluate the time elapsed between cooling through the closure temperature for sr in biotite in the hhc ca 325 c and final sedimentation in the bengal fan despite the large uncertainties in the initial 87 sr 86 sr values of sedimentary mixtures our estimates of the elapsed time between cooling and deposition are short 4 14 ma assuming a transport time scale 1 ma deduced exhumation rates are in the same range of modern values 1 3 mm yr since ca 30 ma given that a the rocks exposed at the surface b the style of weathering and c rates of exhumation were similar to the present we infer that the factors controlling erosion of the early miocene himalayan belt were also similar to the present the data suggest that the early miocene himalayan belt had significant relief and induced orographic effects on precipitation similar to the modern mountain range it appears that the thrusting along the main central thrust mct or similar structures has uplifted metamorphic basement analogous to the present hhc at sufficiently high rate to maintain steep relief throughout miocene time despite high average erosion rates

provenance of the tertiary sedimentary rocks of the indo burman ranges burma myanmar burman arc or himalayan derived the indo burman ranges in western myanmar extend along the sunda arc subduction zone and may be divided into a western portion of neogene sedimentary rocks and an eastern portion of palaeogene sedimentary rocks separated by the kaladan fault both himalayan and burman sources have been proposed for these sediments our thermochronological analyses on detrital grains isotopic analyses on bulk rock and petrographic and heavy mineral data indicate that the palaeogene indo burman ranges contain a significant component of arc derived material interpreted as derived from the burmese portion of the mesozoic tertiary arc to the east and older crustal component is also identifiable which may have been sourced from the himalaya or the burmese margin by contrast the neogene indo burman ranges show dominant derivation from the himalaya a minor arc derived component may have been sourced from the trans himalaya or recycled from the arc derived paleogene indo burman ranges

two types of ductile deformation regional metamorphic belt abstract the geometry of plastic strains of regional metamorphic rocks is divided into two types one is the uniaxial extension strain prolate and the other the biaxial shortening strain oblate from the plane strain simple shear to the uniaxial shortening geometry the uniaxial extension geometry is found in deformation of metamorphic rocks of the sambagawa ryoke kamuikotan and franciscan belts while the biaxial type is seen in deformation of the shimanto the alpine the appalachian and the caledonian metamorphic rocks the classification of the metamorphic belts by means of strain geometry and strain path is independent of the metamorphic facies series the difference in strain geometry of metamorphic belts is responsible mainly for the tectonic stress regime in metamorphism it is suggested that the biaxial type belongs to the collision type orogeny and the uniaxial type to the cordilleran type orogeny

the structure and the phyllosilicates chemistry crystallinity and texture of talas ala tau tien shan kyrgyz republic comparison with more recent subduction complexes abstract geological mapping and structural analysis of the talas ala tau tien shan kyrgyz republic have revealed a complex structure composed of folds with axial plane cleavage and thrust faults verging towards the ne the main structures of the range correspond to minor tertiary and carboniferous permian deformation superimposed on the main deformation event that took place during the baikalian orogeny the pervasive axial plane cleavage diminishes in penetrativity from the hinterland to the foreland in both the uzunakhmat and karagoin sheets the main thrusts developed phyllonitic shear related rocks on the hangingwall immediately above the thrust planes a crystal chemical study of the phyllosilicates growth during the baikalian deformation event along a cross section revealed changes in the crystallinity composition and lattice parameters of them the phyllosilicates present in the talas ala tau rocks were crystallized in very low grade metamorphic conditions that is below 300 c as indicated by their kubler index ki which decreases from sw towards the ne detailed tem study of the phyllosilicates reveals a clear textural difference at the lattice level between samples with higher or lower ki parameters there is also a clear difference in crystal chemical parameters ki and b and composition between the phyllosilicates growth in relation to the axial plane cleavage and the ones belonging to the thrust related phyllonites the first ones are more affected by the ferrimuscovitic vector than the phyllosilicates of phyllonites closer to the theoretical phengitic component huge ranges of values of phengitic content of micas at sample level are interpreted as the result of a decompression path from at least 8 kbar we propose a subduction geodynamic environment for the regional deformation and the origin of the phyllosilicates as they are similar to those obtained in more recent accretionary complexes

geophysical evidence for dewatering and deformation processes in the odp leg 170 area offshore costa rica abstract we use a combination of borehole data from ocean drilling program odp leg 170 and multichannel seismic reflection mcs data to quantify thickness changes in underthrust sediments away from the boreholes sediments thrust beneath the upper plate at convergent margins may be more rapidly loaded than in any other environment depending on the porosity and permeability of the available fluid pathways these sediments can compact and dewater very rapidly as observed in this area offshore the nicoya peninsula costa rica rapid thinning and dewatering was previously interpreted in this area from mcs data but the lack of velocity data in this deep water environment caused ambiguity in the estimates of thickness change we employ a non linear inversion technique using detailed density data primarily logs and some laboratory measurements and coincident mcs data to create 1d synthetic seismograms and detailed velocity functions at three odp drill sites because only a small part of one hole was logged with a sonic tool and the laboratory measurements significantly underestimate in situ velocities these results provide the most accurate estimate of the velocity profiles we used these velocity functions to depth migrate seven mcs lines in the vicinity of the trench and lower slope spanning a distance of 9 km along strike analysis of the depth migrated images shows that there is significant variation along strike in how the underthrust section compacts which appears to be related to the distribution of normal faults on the cocos plate we interpret that preferentially rapid dewatering in the upper part of the underthrust section may lead to deformation below the original decollement and detachments at deeper stratigraphic levels

structure inferred mechanical properties and implications for fluid transport in the decollement zone costa rica convergent margin faults in a variety of tectonic settings can act as both conduits for and barriers to fluid flow sometimes simultaneously documenting the interaction between hydrologic and tectonic processes in active faults in situ is the key to understanding their mechanical behavior and large scale fluid transport properties we present observations of the plate boundary decollement zone at the middle america trench off costa rica showing that it is structurally divisible into an upper brittle fracture dominated domain overlying a lower ductile domain pore water geochemical evidence shows that along fault flow is occurring specifically in the upper brittle domain but is hydrologically isolated from fluids in the underlying footwall sediments we propose a model for the mechanics of these contrasting domains in which differing stress paths coexist in the upper and lower parts of the decollement zone the data suggest a mechanically controlled permeability anisotropy at a scale of several meters to 10 m across the decollement zone this documentation of separate yet simultaneously active mechanical and hydrologic subregimes within a decollement provides a relatively simple explanation for enhanced along fault permeability coexisting with reduced cross fault permeability without requiring matrix scale permeability anisotropy

pressure and porosity influences on vp vs ratio in unconsolidated sands elevated pore pressures commonly encountered in the shallow unconsolidated section of the sedimentary column present a significant hazard during the drilling and completion of offshore wells the porosity of the ocean floor sediments is high and a cover of low permeability clay can prevent the underlying sediments from draining even at very shallow depths below the seafloor as further deposition loads the sediment the entrapped fluids impede normal compaction by becoming pressurized the lowered effective stress that results from the higher pore fluid pressure produces a proportional drop in the strength of non cohesive sediments resulting in very weak shallow sands this weakness can result in washouts or fracturing in the shallow strata potentially leading to the loss of the well and of neighboring wells

cenozoic structural and metamorphic evolution of the eastern himalayan syntaxis namche barwa combined geological and geochronological investigations of the eastern himalayan syntaxis in the namche barwa region of tibet reveal the first order elements of its cenozoic tectonic evolution the syntaxis is characterized by a northeast plunging antiform and is bounded by two northeast striking strike slip shear zones a left slip shear zone on the western side and a right slip shear zone on the eastern side these strike slip shear zones are linked by east westtrending thrusts and served either as 1 a roof thrust to a large duplex system or 2 transfer faults to a south directed thrust system that accommodated northward indentation of a folded indian plate an east west trending pop up structure in the core of the antiform juxtaposes a granulite bearing complex over sillimanite bearing gneisses of gangdese affinity to the north and of indian affinity to the south previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at v800 c the first at high pressures 14 15 kbar followed by a second event at 8 10 kbar zircons from mafic granulites yield four populations of concordant u pb ion microprobe ages two groups are at v65 ma and v160 ma and likely crystallized during andean type gangdese magmatism prior to the indo asian collision a third cluster at v40 ma exhibits very low th u ratios and is interpreted to have crystallized in the presence of fluids associated with a high pressure granulite facies metamorphic event during the early stages of the indo asian collision subsequent to high pressure metamorphism in the western himalaya syntaxis between v50 and 43 ma a fourth cluster of zircons yields ages between 11 and 25 ma and th u ratios that decrease systematically with decreasing age we interpret the youngest zircon age v11 ma to represent the timing of moderatepressure high grade metamorphism with the older ages and higher th u ratios being a result of mixing with a restitic igneous component this interpretation coupled with a v 8m a 40 ar 39 ar age on hornblende from a metadiorite within the core of the antiform suggests that the namche barwa syntaxis has been characterized by rapid cooling and exhumation since at least late miocene time despite its contrasting structural setting the miocene and younger metamorphism and cooling history in the namche barwa syntaxis are strikingly similar to those of the nanga parbat syntaxis of the western himalaya fl 2001 elsevier science b v all rights reserved

absolute reflection coefficients from deep seismic reflections abstract many deep seismic reflection profiles show bright sub horizontal layered reflections in the lower continental crust accurate estimates are obtained for normal incidence reflection coefficients for the brightest of these reflections by using a marine seismic acquisition system that has been calibrated using deep water sea bottom multiples minimum bounds are placed upon these reflection coefficients by assuming minimum losses due to scattering anelastic attenuation and geometric spreading reflection coefficients of around 0 1 are obtained from the layered lower crust and around 0 15 from the moho it is argued that constructive interference between successive reflections from a layered sequence of reflectors is often unimportant consequently these coefficients can be used to place useful limits on the absolute acoustic impedance contrasts present at depth