the bagdad reflection sequence as tabular mafic intrusions evidence from seismic modeling of mapped exposures two dimensional seismic modeling of exposed rocks implicates subhorizontal tabular mafic intrusions as the likely cause of the unusually prominent reflections that constitute the bagdad reflection sequence brs these basement reflections from depths of 3 15 km are observed on cocorp and other data to extend for some 100 km across the basin and range colorado plateau transition zone in west central arizona the nearby buck mountains fortuitously expose an upended upper crustal block which is used in this study as the basis for a 12 x 6 km seismic model ray tracing through this model produces synthetic reflections from the numerous proterozoic diabase intrusions occurring in the buck mountains these synthetic reflections approximate the scale geometry and character of reflections observed on the cocorp data indicating that the intrusions in the buck mountains furnish a viable geologic analogy for the brs several lines of evidence argue that the major competing hypothesis that tertiary detachment surfaces cause the brs is less satisfactory although we cannot rule out the possibility that the brs is related to hypothetical tertiary intrusions of similar distribution and structure geologic evidence more directly implies that these reflectors are proterozoic in age if so the bagdad reflections serve as strain markers and can be used to constrain subsequent deformation in the arizona transition zone their relatively continuous nature thus argues against widespread disruption of the upper crust in the transition zone during tertiary extension moreover identification of these reflectors as subhorizontal mafic intrusions may have profound consequences for interpretation of similar reflections observed elsewhere by bolstering the contention that mafic intrusions are an important cause of basement reflections in the continental crust

atoll reef island formation and response to sea level change west island cocos keeling islands abstract reef islands around the margin of coral atolls generally comprise unconsolidated holocene sands and gravels overlying a reef flat or cemented conglomerate platform such islands have accreted within the last 3000 4000 years since sea level has reached a level close to present and the reef flat and conglomerate platform have formed island morphology consists of an oceanward ridge a less distinct lagoonward ridge and low lying central depression several alternative models of how such reef islands might have developed are examined in relation to chronology and sediment provenance particularly in the context of the cocos keeling islands where this issue has been debated since darwin visited the atoll which of these models appears most appropriate for an elongate reef island on the atoll margin is assessed using conventional radiocarbon dating of coral shingle and accelerator mass spectrometry ams radiocarbon dating of individual sand grains from pits across west island the dating results suggest that both coral clasts and individual grains of various components are generally reliable and replicable indicators of the chronology of island accumulation implying rapid transport of skeletal material after death of the contributing organisms across the reef flat zone and relatively little reworking the central part of west island appears to have formed first with oceanward accretion up until about 2000 years bp gradual oceanward accretion with lesser lagoonward extension has continued beyond 2000 years bp at the northern and southern ends of the island and a sequence of lagoonward recurving spits has formed adjacent to the inter island passage at the southern end of the island radiocarbon dating of fossil microatolls indicates a gradual fall of sea level over this period which appears to have had little effect on the pattern of island accretion

geophysical geological transect and tectonic evolution of the swiss italian alps a complete alpine cross section integrates numerous seismic reflection and refraction profiles across and along strike with published and new field data the deepest parts of the profile are constrained by geophysical data only while structural features at intermediate levels are largely depicted according to the results of three dimensional models making use of seismic and field geological data the geometry of the highest structural levels is constrained by classical along strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units because the transect is placed close to the western erosional margin of the austroalpine nappes of the eastern alps it contains all the major tectonic units of the alps a model for the tectonic evolution along the transect is proposed in the form of scaled and area balanced profile sketches shortening within the austroalpine nappes is testimony of a separate cretaceous age orogenic event west directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the meliata hallstatt ocean further to the east considerable amounts of oceanic and continental crustal material were subducted during tertiary orogeny which involved some 500 km of n s convergence between europe and apulia consequently only a very small percentage of this crustal material is preserved within the nappes depicted in the transect postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments both detachments nucleating at the interface between lower and upper crust large scale wedging of the adriatic or apulian lower crust into a gap opening between the subduced european lower crust and the pile of thin upper crustal flakes alpine nappes indicates a relatively strong lower crust and detachment between upper and lower crust

reconstructing mid late holocene cyclone variability in the central pacific using sedimentary records from tahaa french polynesia abstract we lack an understanding of the geographic and temporal controls on south pacific cyclone activity overwash records from backbarrier salt marshes and coastal ponds have been used to reconstruct tropical cyclone strikes in the north atlantic basin however these specific backbarrier environments are scarce in the south pacific with cyclone records limited primarily to the period of modern observation this instrumental record suggests a correlation with the el nino southern oscillation enso but longer records are necessary to test this relationship over geologic timescales and explore other potential climate drivers of tropical cyclone variability deep lagoons behind coral reefs are widespread in the pacific and provide an alternative setting for developing long term sedimentary reconstructions of tropical cyclone occurrence coarse grained event deposits within the sediments of a back reef lagoon surrounding tahaa reveal a 5000 year record of cyclone occurrences timing of recent high energy deposits matches well with observed tropical cyclone strikes and indicates coarse deposits are storm derived longer records show tropical cyclone activity was higher from 5000 to 3800 and 2900 to 500 yrs bp comparison to records from the north pacific out of phase and north atlantic in phase suggests a coordinated pattern of storm activity across tropical cyclone basins over the mid late holocene the changes in tropical cyclone activity we observe in the south pacific and across other basins may be related to enso as well as precession driven changes in ocean atmosphere thermal gradients

comparison of vibroseis and explosive source methods for deep crustal seismic reflection profiling in the basin and range province direct comparison of low fold high energy explosive and high fold lower energy vibroseis methods for acquiring deep crustal seismic reflection data in the basin and range province suggests that the high fold common midpoint cmp method there does not provide the best possible image of lower crustal structure during the recent acquisition of a vibroseis profile in the basin and range province we fired single deep shot holes to obtain a coincident single fold explosive section within the upper crust upper 3 s the explosive source and vibroseis records are nearly equivalent for record times below 3 s however comparison of the explosive source gathers and the coincident final 60 fold vibroseis section demonstrates that low fold explosive profiling provides a higher quality image of the midcrust to lower crust 3 10 s the higher record quality of the explosive sources results primarily from the larger seismic energy levels produced by the explosives making them less sensitive to common noise sources whereas deeper than 4 5 s the vibroseis energy levels on individual source efforts fall to that of ambient noise levels the explosions provide signal generated energy exceeding ambient noise levels down to 18 19 s although individual reflections can be correlated on explosion and vibroseis shot gathers reflection events on the 60 fold vibroseis stack do not correlate to those on the single fold explosion profile suggesting that the high fold cmp method in our study did not maintain the integrity of the weak lower crustal reflected arrivals reasons why the high fold cmp method apparently failed include complex even time varying statics nonhyperbolic moveout at long offsets and the difficulty in resolving stacking velocities with data having low signal to noise ratios reflections on the explosion section are longer and imply a greater degree of layering than one would infer from the lower energy vibroseis section

active submarine tectonism and formation of the gulf of saros northeast aegean sea inferred from multi channel seismic reflection data abstract active submarine tectonism and the formation of the gulf of saros northeast aegean sea were investigated by means of multi channel seismic reflection data this area is a key in understanding the development of major structures resulting from the interacting regimes of the western anatolia aegean sea and the north anatolian fault zone naf the major feature forming the gulf of saros is the ganos fault which is one of the main segments of the northern strand of the naf in this study a total of 159 km reflection seismic data were collected processed and interpreted in seven lines in order to map active submarine faults which were used to propose a tectonic model for opening of the gulf of saros interpretation of the seismic sections and their correlation with the geological morphological and earthquake data indicate that there are two main fault systems in the gulf i strike slip faults with normal components which bound the saros trough in the north and south and ii normal faults located within the trough we propose that the saros trough was first formed as a negative flower structure in response to the southward bending of the naf zone possibly in plio quaternary then it evolved as a dilated negative flower structure as the aegean extension intensified in late pliocene quaternary

grustal shortening in the alpine orogen results from deep seismic reflection profiling in the eastern swiss alps line nfp 20 east the deep crustal seismic line nfp 20 east crosses almost the entire swiss alps despite the complex geometry of the well exposed nappe structure and the considerable axial plunge of some of the units the vibroseis survey yielded coherent reflections from several individually identifiable nappe contacts in the northern part of the survey the vibroseis data closely match the internal structure of the helvetic nappes and the underlying autochthonous parautochthonous mesozoic sediments on the northern flank of the aar massif an external basement uplift these mesozoic sediments seem to rise from a depth of approximately 7 8 km below sea level to the surface in a series of steps which is interpreted to represent crustal shortening achieved by a combination of folding and thrusting in the southern part of the survey it was possible to image a number of thin slivers of mesozoic carbonates pinched between slabs of penninic basement nappes as well as nappe contacts between lithologically contrasting units in addition it seems that the insubric fault zone which marks the contact between the penninic zone and the southern alps and which outcrops about 30 km to the south of the survey shows up as steeply north dipping reflections the lower european crust in the northern part of the survey is relatively transparent as opposed to the adriatic lower crust whose reflective nature may stem from shear zones associated with mesozoic crustal stretching the base of both the european and adriatic crust coincides with a 1 s thick band of laterally discontinuous reflections this reflection moho drops to greater depths going from the north toward the center of the alpine chain where it disappears with a steep southerly dip the moho reappears as a reflection band farther south this moho gap is situated above the lithospheric root and may be caused by perturbations related to subduction of lower crustal material the crustal scale structure obtained from the vibroseis data may be interpreted as a continent continent collision with wedge shaped indentation of a piece of adriatic crust into the european crust and vertical escape of the material in the core of the orogen along steeply dipping faults

insights on the upper mantle beneath the eastern alps analyses of ps and sp receiver functions from datasets collected by permanent and temporary seismic stations image a seismic discontinuity due to a negative velocity contrast across the entire eastern alps the receiver functions show the presence of the discontinuity within the upper mantle with a resolution of tens of kilometers laterally it is deeper 100 130 km below the central portion of the eastern alps and shallower 70 80 km towards the pannonian basin and in the central alps comparison with previous studies renders it likely that the observed discontinuity coincides with the lithosphere asthenosphere boundary lab east of 15 e longitude while it could be associated with a low velocity zone west of 15 e

crustal structure and tectonics of the northern part of the southern granulite terrane india deep seismic reflection studies investigating the exposed archean lower continental crust of the southern granulite terrane india yield important constraints on the nature and evolution of the deep crust including the formation and exhumation of granulites seismic reflection images along the kuppam bhavani profile reveal a band of reflections that dip southward from 10 5 to 15 0 s two way time twt across a distance of 50 km the bottom of these reflections beneath the dharwar craton is interpreted as the moho further south another reflection band dipping northward is observed these bands of reflectivity constitute a divergent reflection fabric that converges at the moho boundary observed at the mettur shear zone reflection fabrics that intersect at a steep angle are interpreted as a collisional signature due to the convergence of crustal blocks which we infer resulted in crustal thickening and the formation of granulites anomalous gravity and magnetic signatures are also observed across the mettur shear zone the gravity model derived from the bouguer gravity data corroborates seismic results the tectonic regime and seismic reflection profiles are combined in a 3 d representation that illustrates our evidence for paleo subduction at a collision zone the structural dissimilarities and geophysical anomalies suggest that the mettur shear zone is a suture between the dharwar craton in the north and another crustal block in the south this study contributes significantly to our understanding of the operation of archean plate tectonics here inferred to involve collision and subduction furthermore it provides an important link between the gondwanaland and global granulite evolution occurring throughout the late archean

the development of polygonal fault systems by syneresis of colloidal sediments abstract polygonal fault systems occur in numerous sedimentary basins worldwide are generally located on passive margins in onlap fill units and tend to comprise the finest grained sediments in this geological setting these fault systems have been most thoroughly described in the central north sea basin and the detailed structure shows a significant correlation with lithological variations both vertically and laterally extension measured in stacked decoupled tiers of polygonal faults correlates positively with both clay fraction and smectite content lateral facies variations are also observed and indicate that time equivalent sequences upslope from the smectite rich polygonally faulted sediments are coarse grained clay poor and undeformed this leads us to believe that the structure and geometry of the fault system are controlled by the colloidal nature of the sediments and that the volumetric contraction measured on seismic sections can be accounted for by syneresis of colloidal smectitic gels during early compaction syneresis results from the spontaneous contraction of a sedimentary gel without evaporation of the constituent pore fluid this process occurs due to the domination of interparticle attractive forces in marine clays dependent on environment and is governed by the change of gel permeability and viscosity with progressive compaction the process of syneresis can account for a number of structural features observed within the fault systems such as tiers of faults the location of maximum fault throw and growth components at upper fault tips as such this paper represents the first attempt to correlate microscale properties of clay rich sediments to their macroscale seismic character

deep seismic reflection profiling in the swiss alps explosion seismology results for line nfp 20 east in september 1986 a 120 km long seismic line was recorded through the swiss alps the line traverses major units involved in thin and thick skinned alpine tectonics our preliminary interpretation indicates that 1 lithologic boundaries such as basement cover contacts although severely deformed during the alpine orogeny can be identified on the seismic sections 2 the top of the aar massif an external basement massif extends deep into the alpine orogen 3 the upper and lower parts of the crust are more or less transparent but are separated by a swarm of reflectors at mid crustal level 4 these mid crustal reflectors might be related to trapped fluids from alpine metamorphism and 5 the moho appears as a bright reflection that steepens from the north toward the south and terminates abruptly near the center of the alpine chain perhaps because of alpine deformation

preliminary results from a geophysical study across a modern continent continent collisional plate boundary the southern alps new zealand abstract the southern alps of south island new zealand is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement a joint us nz geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three dimensional model of the deformation the measurements undertaken include active source and passive seismology magnetotelluric and electrical studies and petrophysics preliminary models for the active source seismic measurements across south island confirm in general terms a thickened crust under the southern alps a high velocity lower crustal layer and a major crustal discontinuity associated with the alpine fault the anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements shear wave splitting and resistivity the mid crust under the southern alps coincides with a major electrical conductivity high which possibly corresponds to fluid in the crust the top lies at about 15 km close to the base of shallow seismicity east of the alpine fault offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to south island these data are showing complex structure particularly off western and southeastern south island the complexity in structure high quality data and consistency in results from several techniques indicates that the south island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular and the continental lithosphere in general

results of nfp 20 seismic reflection profiling along the alpine section of the european geotraverse egt summary recent deep crustal seismic reflection profiling across the central alps of eastern and southern switzerland has provided a detailed image of this continental collision zone as part of the swiss national science project nfp 20 a series of reflection profiles was recorded during two phases along n s transects crossing the alpine edifice the eastern and southern profiles coincide with the alpine segment of the european geotraverse egt and in part with the earlier swiss geotraverse these data together with other geophysical and extensive geologic information form a unique and comprehensive volume of crustal information for this region of the alps on a larger scale the new reflection data also add to a growing set of profiles crossing the alpine fold belt which includes an additional traverse carried out by nfp 20 across the western swiss alps and the ecors crop profile across the western alps the initial results of the reflection data across the central alps outline a crustal framework involving northward indentation of the adriatic hinterland into the subducting european foreland this occurs beneath the collapsed oceanic basins of the penninic allochthon which is defined by its highly reflective crystalline nappes the present crustal thickness within the alpine collision zone has doubled to about 60 km by both vertical and horizontal displacements along an inferred complex detachment system controlled by the adriatic wedge the south plunging european foreland implies that portions of the thickened crust have been subducted into the upper mantle this general framework of flake tectonics is laterally consistent with the ecors crop results from the western alps however the western alps show a higher degree of crustal imbrication which may be related to the presence of the rigid ivrea body and an overall greater amount of crustal shortening compared to the central alps

reflection polarity of the midcrustal surrency bright spot beneath southeastern georgia testing the fluid hypothesis a small reflection seismic experiment with an explosive source was conducted in southeastern georgia to determine the polarity of an unusually strong midcrustal reflector the surrency bright spot sbs which was found at a depth of approximately 16 km during earlier cocorp profiling in the region in addition to being very bright the sbs is notable for being unusually flat and horizontal for about half of its 4 km length as these characteristics are similar to those of fluid caused reflections at shallow depths it has been suggested that the sbs may be caused by in situ midcrustal fluids if caused by fluid enclosed in fracture porosity in solid rock the reflection would be expected to exhibit a negative polarity from the top of the porous zone the vibroseis data however gave ambiguous results with regard to polarity due to the limited bandwidth and inherent uncertainties about the phase of the source signal the new experiment consisted of four dynamite shots each recorded at three receiver stations by seismic group recorders sgr borrowed from amoco production company comparison of the dynamite records with geophone polarity tests indicate that the sbs is characterized by a positive reflection coefficient at its top this result itself does not negate the fluid hypothesis a fluid fluid interface could cause the positive reflection as well as the flat spot nature of the reflector waveform modeling shows that the sbs is caused by two or more thinly spaced reflectors a relatively high impedance layer about 120 m thick provided a good match to the observed dynamite data but requires a lower boundary having a slightly smaller reflection coefficient than the upper boundary 0 7 versus 1 0 a 150 m thick porous zone model also provided a relatively good fit to the observed dynamite data but in this model the polarity test requires that the initial sbs reflection of positive polarity be caused by a fluid fluid interface within the porous zone and that the top of the porous zone be relatively non reflective though these observations do not preclude the fluid hypothesis they certainly make the high impedance model the simpler of the two alternatives presented here the additional constraints imposed by the modeling also suggest that the sbs is more complex than these simple two interface models in either of these cases wavelet tuning contributes in part to the unusually large amplitude of the sbs reflection

observation and tentative interpretation of a double bsr on the nankai slope seismic data collected during the french japanese kaiko tokai cruise of r v l atalante on the upper slope of the eastern nankai margin reveal the simultaneous presence at two distinct depths below the seafloor of two bottom simulating reflector bsr type reflectors the upper bsr is traced as a continuous reflector over about 10 km as water depth decreases from 850 m to 550 m its depth below seafloor decreases from 200 m to 40 m the lower bsr is traced at 50 100 m below the upper one the two bsrs end abruptly near the summit of the daichii tenryu knoll into an area where the 3 5 khz record suggests active gas expulsion through the seabed the observed depth of the upper bsr fits the predicted one for the base of the methane gas hydrate stability zone as estimated from present temperature and pressure conditions at the seafloor and in the slope sediments thus we interpret the upper bsr as an active methane hydrate bsr we further suggest that the lower bsr is a residual hydrate related bsr this could have followed a recent migration of the base of the methane hydrate stability zone from the lower bsr to the upper one as possible causes for this migration we discuss sea bottom warming and tectonic uplift the bsr migration could have occurred as a response to a 1 2 c sea bottom warming or with an equivalent effect an event of fast uplift of the seafloor by about 90 m we do not discard other interpretations of the lower bsr such as an active hydrate related bsr formed from a mixture of gases

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

holocene sea level changes in french polynesia abstract new data on holocene sea levels have been obtained in 28 atolls and 10 high islands in the society tuamotu gambier and austral islands in an area of the south pacific as wide as western europe sea level indicators are often very accurate 0 1 m and include exposed corals abandoned algal ridges and reef frameworks in growth position emerged tidal notches and skeletal reef conglomerates in which the position of the former low water level at the time of cementation has been determined by petrological analysis over 110 new samples have been dated by radiocarbon the overall pattern is that of a msl standstill at 0 8 1 0 m between 5000 and 1250 yr b p a set of radiocarbon dated samples covering the period 4500 1250 yr b p almost continuously is evidence that lasting msl drops could not ever have occurred below 0 7 m throughout this time the highest sea level peak about 1 0 m seems to have been reached between 2000 and 1500 yr b p since 1500 yr b p msl has been dropping gradually from about 1 0 m to the present position which has been reached only recently some islands show however a slightly different pattern the holocene emergence is smaller than average in moorea and tahiti and larger in rurutu makatea anaa and possibly hereheretue some data suggest also the possibility of a slight increase in the holocene emergence towards the south possible geodynamic isostatic oceanic and climatic causes of the sea level variations observed are discussed

neogene sedimentation history of the congo fan abstract high resolution seismic reflection data were interpreted to develop a model for the neogene sedimentation in the congo fan area identification of two reflectors representing marine isotope stages 6 and 12 allows a distinction between the early and the late quaternary a single sediment source system during the late paleogene early neogene gradually changes into a two sediment source system in the late neogene since the late quaternary the congo river has been the prevailing sediment source the evolving benguela current further modified the system and led to a high biogenic productivity evidence for gas in the sediments can be found in the whole area of investigation additionally indications for a tectonic control of the congo canyon have been found in the distribution of sedimentary units and unit thicknesses

geological significance of seismic reflections in collision belts geological problems raised by the interpretation of deep seismic reflections in collision belts result from the structural complexity of the crust and the lack of surface control and well documeted references some of these problems are illustrated by examples coming from various tectonic settings in france and adjacent areas and tentative interpretations are proposed

deep seismic probing of the continents and their margins abstract application of deep seismic methods to studies of the crust and lithospheric mantle receives considerable interest and the methods are constantly refined and new methods are developed which allows the extension of studies to new subjects and regions deep seismic methods are applied to a long range of geoscientific subjects in research on the formation and development of crust and lithosphere in continental and oceanic environments from being an experimental discipline some 30 40 years ago a series of methods can now be applied almost routinely to research on the lithosphere however in many applications the methods are used up to their limits at the present technological state therefore development of methods has high priority in the seismic community this volume provides an overview of recent development of deep seismic techniques and their application to the imaging and probing of the continents and their margins

gravity interpretation of a unified 2 d acoustic image of the central alpine collision zone summary four individual deep seismic reflection profiles across the eastern and southern swiss alps have been combined along the trend of the bouguer gravity anomalies onto the european geotraverse the profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide angle profiles running parallel to the strike of the alpine arc the contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles the resulting acoustic image depicts the subduction of parts of the lower crust of the european plate beneath the adriatic promontory of the african plate at a low angle of 15 based on the interpretation of the seismic data orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the european plate and wedging of the european and adriatic middle and lower crusts the latter being ill constrained by the seismic evidence alone the south vergent thrusting of the southern alps can be accounted for by the observed downbending of the adriatic moho and the lower crust in conjunction with the inferred wedging at mid crustal levels using the geometric constraints provided by seismic data gravity modelling of the alpine lithosphere asthenosphere system relative to stable central european platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth such a gently dipping subduction zone is at odds with prominent models on alpine geodynamics which favour a near vertical orientation of the subducting lithosphere verschluckung in agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen both seismic and gravimetric evidence therefore suggest that late orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper middle and lower parts of the crust the amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab

late holocene sea levels and coral reef development in vahitahi atoll eastern tuamotu islands pacific ocean abstract preliminary quantitative data are provided on the latest stage of development and morphological evolution of the closed atoll vahitahi in the eastern tuamotus from before 4300 to at least 2400 yrs b p sea level was at least 0 7 m higher than at present during this time the development of a flat reef pavement now slightly emerged gradually separated the lagoon from the ocean at about 1200 yrs b p the water level of the lagoon already closed was still slightly above the present level the response of the reef was a rapid upward accretion until about 3000 yrs b p then an oceanward development at rates averaging from 0 04 to 0 13 m yr 1

velocity structure of a bottom simulating reflector offshore peru results from full waveform inversion abstract much of our knowledge of the worldwide distribution of submarine gas hydrates comes from seismic observations of bottom simulating reflectors bsrs full waveform inversion has proven to be a reliable technique for studying the fine structure of bsrs using the compressional wave velocity we applied a non linear full waveform inversion technique to a bsr at a location offshore peru we first determined the large scale features of seismic velocity variations using a statistical inversion technique to maximise coherent energy along travel time curves these velocities were used for a starting velocity model for the full waveform inversion which yielded a detailed velocity depth model in the vicinity of the bsr we found that the data are best fit by a model in which the bsr consists of a thin low velocity layer the compressional wave velocity drops from 2 15 km s down to an average of 1 70 km s in an 18 m thick interval with a minimum velocity of 1 62 km s in a 6 m interval the resulting compressional wave velocity was used to estimate gas content in the sediments our results suggest that the low velocity layer is a 6 18 m thick zone containing a few percent of free gas in the pore space the presence of the bsr coincides with a region of vertical uplift therefore we suggest that gas at this bsr is formed by a dissociation of hydrates at the base of the hydrate stability zone due to uplift and subsequently a decrease in pressure

a cocorp deep reflection profile across the buried reelfoot rift south central united states abstract a cocorp deep reflection survey across the northern mississippi embayment reveals essential features of the late precambrian early paleozoic reelfoot rift stratified reflections delineating the sedimentary fill of the rift basin are of variable thickness the base ranging from 2 0 to a maximum of 3 5 s twt 4 to 8 km depth in contrast the base of phanerozoic the sedimentary section immediately east of the rift is at 1 4 s 2 8 km depth correlation with a basement penetrating well indicates that the base of the upper cretaceous holocene mississippi embayment supergroup base of the cambrian ordovician knox group and the lower upper cambrian bonneterre formation are basin wide seismic stratigraphic markers clastic sedimentary strata of the lamotte formation which immediately overlie precambrian basement show clear angular discordance with the overlying subhorizontal bonneterre formation indicating that the base of the bonneterre marks the transition from active extension to passive subsidence within the reelfoot rift palinspastic restoration of the basement suggests relatively modest extension associated with rift formation 17 the blytheville arch an axial antiformal feature as well as a number of lesser structures indicative of multiple episodes of fault reactivation are also evident on the cocorp profile the crystalline basement beneath the reelfoot rift is relatively devoid of reflections down to 5 0 s 12 15 km depth beyond which it passes gradationally into a complexly reflective diffractive middle and lower crust the lower crust however does not exhibit the distinctly laminated character commonly observed in mesozoic cenozoic extensional provinces the crust mantle transition beneath the rift is characterized by a gradational downward cessation of crustal reflectivity at 12 13 s 40 km depth followed by an unreflective zone which in turn is followed by a distinct horizontal reflection at 14 5 s the latter is not observed on existing profiles outside the rift at present it is not possible to determine strictly whether the deep horizon represents moho or alternatively a discrete feature sill within the uppermost mantle however its spatial association with the reelfoot rift suggests that the two are genetically related regional relations together with the existence of dipping fabric in the deep crust beneath the rift suggest the possibility that the reelfoot rift formed along the southern extension of the circa 1 ga grenville front

crustal structure and reflectivity of the swiss alps from three dimensional seismic modeling 2 penninic nappes surface geological mapping laboratory measurements of rock properties and seismic reflection data are integrated through three dimensional seismic modeling to determine the likely cause of upper crustal reflections and to elucidate the deep structure of the penninic alps in eastern switzerland results indicate that the principal upper crustal reflections recorded on the south end of swiss seismic line nfp20 east can be explained by the subsurface geometry of stacked basement nappes in addition modeling results provide improvements to structural maps based solely on surface trends and suggest the presence of previously unrecognized rock units in the subsurface construction of the initial model is based upon extrapolation of plunging surface structures velocities and densities are established by laboratory measurements of corresponding rock units iterative modification produces a best fit model that refines the definition of the subsurface geometry of major structures we conclude that most reflections from the upper 20 km can be ascribed to the presence of sedimentary cover rocks especially carbonates and ophiolites juxtaposed against crystalline basement nappes thus in this area reflections appear to be principally due to first order lithologic contrasts this study also demonstrates not only the importance of three dimensional effects sideswipe in interpreting seismic data but also that these effects can be considered quantitatively through three dimensional modeling

the cover basement contact beneath the rawil axial depression western alps true amplitude seismic processing petrophysical properties and modelling abstract since 1986 several near vertical seismic reflection profiles have been recorded in switzerland in order to map the deep geologic structure of the alps one objective of this endeavour has been to determine the geometries of the autochthonous basement and of the external crystalline massifs important elements for understanding the geodynamics of the alpine orogeny the pnr 20 seismic line w1 located in the rawil depression of the western swiss alps provides important information on this subject it extends northward from the penninic front across the helvetic nappes to the prealps the crystalline massifs do not outcrop along this profile thus the interpretation of near basement reflections has to be constrained by down dip projections of surface geology true amplitude processing rock physical property studies and modelling 3 d seismic modelling has been used to evaluate the seismic response of two alternative down dip projection models to constrain the interpretation in the southern part of the profile true amplitude processing has provided information on the strength of the reflections density and velocity measurements on core samples collected up dip from the region of the seismic line have been used to evaluate reflection coefficients of typical lithologic boundaries in the region the cover basement contact itself is not a source of strong reflections but strong reflections arise from within the overlaying metasedimentary cover sequence allowing the geometry of the top of the basement to be determined on the basis of near basement reflections the front of the external crystalline massifs is shown to extend beneath the prealps about 6 km north of the expected position a 2 d model whose seismic response shows reflection patterns very similar to the observed is proposed

amplitude versus offset modeling of the bottom simulating reflection associated with submarine gas hydrates abstract a bottom simulating seismic reflection bsr that parallels the sea floor occurs worldwide on seismic profiles from outer continental margins the bsr coincides with the base of the gas hydrate stability field and is commonly used as indicator of natural submarine gas hydrates despite the widespread assumption that the bsr marks the base of gas hydrate bearing sediments the occurrence and importance of low velocity free gas in the sediments beneath the bsr has long been a subject of debate this paper investigates the relative abundance of hydrate and free gas associated with the bsr by modeling the reflection coefficient or amplitude variation with offset avo of the bsr at two separate sites offshore oregon and the beaufort sea the models are based on multichannel seismic profiles seismic velocity data from both sites and downhole log data from oregon odp site 892 avo studies of the bsr can determine whether free gas exists beneath the bsr if the saturation of gas hydrate above the bsr is less than approximately 30 of the pore volume gas hydrate saturation above the bsr can be roughly estimated from avo studies but the saturation of free gas beneath the bsr cannot be constrained from the seismic data alone the avo analyses at the two study locations indicate that the high amplitude bsr results primarily from free gas beneath the bsr hydrate concentrations above the bsr are calculated to be less than 10 of the pore volume for both locations studied

late holocene sea level changes in the northwest tuamotu islands french polynesia abstract field surveys of several sea level indicators exposed in situ reef framework conglomerates coral colonies and tridacna shells in a growth position sea corrosion notches carried out on six atolls from the nw tuamotus mataiva rangiroa arutua kaukura apataki and takapoto and data from four subsurface boreholes drilled through mataiva show that during the late holocene mean sea level msl reached a maximum elevation at approximately 0 9 m it remained above the present msl from between 6000 and 5500 yr b p until at least 1200 yr b p human settlements on the atolls were extremely unlikely and probably impossible throughout this time the area investigated seems to have been tectonically quite stable during the late holocene a local curve of msl variations may be representative of the regional eustatic pattern

quaternary atoll development new insights from the two dimensional stratigraphic forward modelling of mururoa island central pacific ocean knowledge about the quaternary evolution of mid ocean atolls comes mainly from drilling and field observations carried out on a number of pacific carbonate islands however little is known about the early to mid pleistocene atoll development history especially at margin and foreslope settings using previous field and subsurface data from mururoa atoll and a process based modelling software dionisos a two dimensional forward stratigraphic model of atoll development is proposed for the past 1 8 million years myr observational data from vertical to inclined coring seismic and bathymetric surveys indicate that from approximately 0 45 to 0 40 million years before present ma carbonate deposition at mururoa atoll resulted in a series of mostly prograding reef units the model is first constrained at the base by the shape and topography of the pre quaternary basement a number of sensitivity tests were performed to define the respective influence of variant parameters the best fit development scenario that accounts for the overall geometry and stratigraphic architecture of the quaternary sediment packages is obtained by using the sea level curve by miller et al 2005 uniform subsidence rate of 105 m myr 1 and carbonate production rates gradually increasing from 0 50 to 8 mm yr 1 between 1 80 ma and the present additional controlling parameters include subaerial erosion at a constant rate of 0 25 m kyr wave energy and sediment transport processes the stratigraphic forward model predicts a succession of three distinct types of carbonate systems that have developed since the earliest pleistocene toe of slope systems from 1 80 ma to about 0 80 ma open platform systems from 0 80 ma to 0 50 ma and framework reef systems from about 0 50 ma to the present the development of these different systems is most likely to be controlled by climate and changes in sea level cycles during the low amplitude 41 kyr cycle periods of the earliest pleistocene ambient conditions were not conducive to framework reef growth shallow water carbonate sedimentation was dominantly gravity driven operating along the platform foreslopes only during the mid pleistocene climate transition narrow open platform units have developed at the upper parts of the pre quaternary basement flanks with the onset of the high amplitude 100 kyr sea level modes and climate restoration reef frameworks started to be generated these models from mururoa agree with a number of previous studies suggesting that most of the true framework reefs were not initiated prior to 0 50 ma mururoa atoll is demonstrated to be a robust analogue for providing more realistic interpretations of the development history of pacific atolls further modelling with three dimensional dionisos could generate better predictions by taking into account hydrodynamic and transport parameters more accurately

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

thermo mechanical approach to validation of deep crustal and lithospheric structures inferred from multidisciplinary data application to the western and northern alps current multidisciplinary studies confront the difficult problem of validation and verification of internal compatibility of geophysical and geological data of different nature quality and origin our geodynamic approach exploits the natural interdependence between the data related to different physical properties of geological structures density rheology temperature stress using the latter as input for a thermo mechanical finite element model we can verify whether the inferred properties and geometries i are mechanically stable ii require rheologically consistent stresses strains realistic thermal conditions etc and 3 will evolve in a geologically realistic way in the future this mechanically balanced approach is tested on the alpine orogen ecors and nfp20 profiles in the framework of the geofrance 3d programme the results suggest a number of corrections and adjustments that can be made to existing seismically and gravity predicted geometries of structures such as the ivrea body and those of the depleted subducted crust

large gas hydrate accumulations on the eastern nankai trough inferred from new high resolution 2 d seismic data previous studies have revealed the presence of a widespread bottom simulating reflector bsr on the eastern nankai slope as well as the occurrence of enigmatic high amplitude reflections that extend well above the bsr new high resolution 2 d seismic data were collected on the eastern nankai slope during the french japanese sfj cruise in year 2000 and ava analyses of the enigmatic reflectors are conducted at the studied location these analyses suggest that high amplitude anomalies above the bsr delineate the top of gas hydrate rich sediments several tens of the sediment porosity would be filled with gas hydrates between the bsr and a sharp boundary 30 to 60 m above to account for these observations we propose that an invasion by free gas of the present day gas hydrate stability zone occurred in the past several mechanisms for this intrusion are discussed

basement cover relationships in the tambo nappe central alps switzerland geometry structure and kinematics in the pennine zone of the central alps the tambo nappe forms a thin crystalline sliver embedded in the mesozoic cover four tertiary alpine deformations are observed the d1 ductile deformation is linked to the progressive eocene stacking of the nappes towards the northwest during d1 basement deformation is governed by imbricate tectonics whereas cover is thin skinned and intensively folded these different structures reflect the original strong rheological contrast between basement and cover during the heterogeneous and ductile d2 deformation the behaviour of the basement and cover became similar the strong vertical d2 shortening associated with a top to the east shear led to the folding of the prior se dipping structures developing se vergent folds with axes close to the e w l2 stretching lineation d2 corresponds to post collisional crustal thinning following d2 crustal thickening the d3 and d4 deformations occurred under retrograde conditions and can be correlated with uplift and late dextral movement on the insubric line respectively

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

crustal structure beneath the eastern swiss alps derived from seismic refraction data abstract the eastern part of the swiss central alps is densely covered by a network of seismic refraction and wide angle reflection profiles recorded mostly parallel to the tectonic strike of the alps and along the newly acquired n s oriented european geotraverse egt to obtain a well constrained crustal transect along the egt an initial model was constructed from information based mainly on the coincident reflection seismic profile of the swiss national research program nfp20 and short range observations of the egt data for the shallow structure and on the along strike wide angle profiles for the deep structure the refinement of the initial model by 2 d raytracing during the subsequent interpretation of the egt data leads to a detailed p wave velocity distribution of the crustal cross section beneath the central swiss alps and its adjacent areas in general the distinctly layered crustal structure below the alpine foreland thickens considerably as the alps are approached reaching a maximum thickness of nearly 60 km below the insubric line the upper and middle crust has velocities between 6 0 and 6 2 km s except for the area below the southern part of the molasse basin and the helvetic nappes a distinct lower crust with a relatively low velocity of 6 5 6 6 km s is found below the penninic nappes the lower crust thickens remarkably merging probably with the high velocity zone of 6 6 km s at a depth of about 21 km which has been interpreted as the top of the indenting lower crust of the adriatic promontory of the african plate a clear vertical offset between the smoothly south dipping european and the more rapidly rising adriatic crust mantle boundaries is found the complex structures of the upper crust beneath the alps caused by the nappe tectonics can only be partly resolved by the refraction seismic data a south dipping high velocity zone within the penninic nappe pile and a reflector beneath the northern front of the aar massif can possibly be interpreted as incomplete images of the shallow heterogeneous 3 d structure

impacts of eustasy and hydro isostasy on the evolution and landforms of pacific atolls consideration of eustatic and hydro isostatic effects on late quaternary sea levels in the tropical pacific ocean indicates that the configuration of modern atolls with emergent annular reef flats is a transient morphology not developed until post mid holocene time annular atoll reefs perched atop carbonate platforms which cap buried volcanic edifices are underlain by 8 28 m of holocene limestone disconformably overlying a substratum of last interglacial or older limestone comparable thicknesses 9 23 m of holocene sediment are present beneath atoll lagoons that are uniformly 85 m deep during glacio eustatic drawdowns in global sea level by 120 125 m carbonate platforms of modern atoll provinces rose abruptly from the sea as clusters of subaerial limestone plateaus flanked by steep cliffs modern analogues are provided by emergent atolls uplifted on the flexural arches of trench forebulges slow subsidence coupled with karstic erosion of emergent atolls during the last glaciation lowered the surfaces of last interglacial reef edifices by the amounts needed to provide accommodation space for holocene reef growth during the postglacial eustatic rise in sea level modern atoll reef caps began to grow after 9 ka when rising holocene sea level overtopped degraded remnants of interglacial reefs but remained submerged until carbonate buildups approached sea level in mid holocene time 6 4 ka classic atoll morphology with circlets of multiple islets dotting annular reefs formed in combination with late holocene hydro isostatic drawdown in tropical pacific sea level in response to equatorial ocean siphoning a facet of global isostatic adjustment gia to deglaciation early holocene eustatic rise in sea level and late holocene hydro isostatic decline in sea level combined to produce a regionally variable mid holocene highstand in tropical pacific sea level that stood 1 0 2 6 m above modern sea level cemented mid holocene paleoreef flats now stranded well above sea level serve as resistant foundations for non migratory pinned islets that were not present along atoll rims until after the local crossover date when ambient high tide level first fell below mid holocene low tide level existing atoll landforms have a time depth generally 1 2 ka

thrust tectonics deep structure and crustal subduction in the alps and himalayas the structural restoration of collision orogenic belts onto crustal templates provides important insights into the tectonic evolution deep structure and amounts of plate convergence after the initial contact between two continental masses balanced cross sections have been constructed parallel to the local displacement directions across the western alps and western himalayas and demonstrate very large amounts of crustal shortening above intra crustal detachments to achieve a balance substantial volumes of lower crust must have been subducted beneath the two tectonic hinterlands a model of eclogite metamorphism is invoked to facilitate this subduction and to explain the varying isostatic responses of the alpine and himalayan hinterlands patterns of eclogite metamorphism are controlled by the geometry of thrust profiles on a lithospheric scale the development of crust mantle detachments being of crucial importance such a profile is proposed for the himalayas suggesting that relatively small volumes of the footwall crust succumbed to eclogite metamorphism in the alps however a steeper thrust profile apparently developed emplacing mantle onto crust causing wholescale eclogite metamorphism in the underlying franco swiss crust the resultant density increase would provide a mechanism of isostatic collapse and flexural subsidence in the po plain region

3d cartographic modeling of the alpine arc we built a 3d cartography of the alpine arc a highly non cylindrical mountain belt using the 3d geomodeller of the brgm french geological survey the model allows to handle the large scale 3d structure of seventeen major crustal units of the belt from the lower crust to the sedimentary cover nappes and two main discontinuities the insubric line and the crustal penninic front it provides a unique document to better understand their structural relationships and to produce new sections the study area comprises the western alpine arc from the jura to the northwest up to the bergell granite intrusion and the lepontine dome to the east and is limited to the south by the ligurian basin the model is limited vertically 10 km above sea level at the top and the moho interface at the bottom we discarded the structural relationships between the alps sensus stricto and the surrounding geodynamic systems such as the rhine graben or the connection with the apennines the 3d model is based on the global integration of various data such as the dem of the alps the moho isobaths the simplified geological and tectonic maps of the belt the crustal cross sections ecors crop and nfp 20 and complementary cross sections specifically built to precise local complexities the database has first been integrated in a gis project to prepare their implementation in the geomodeller by homogenizing the different spatial referencing systems the global model is finally interpolated from all these data using the potential field method the final document is a new tri dimensional cartography that would be used as input for further alpine studies

the 3d crustal structure of the alps of eastern switzerland and western austria interpreted from a network of deep seismic profiles abstract a 3d crustal model of the alps of eastern switzerland and western austria was constructed a total of twenty two dimensional deep seismic reflection profiles recorded between 1986 and 1991 were two dimensionally interpreted based on depth migrated line drawings and using correlation with surface geology refraction derived velocity depth models and correlation with intersecting or neighboring profiles the network of crustal cross sections was then unified into 3d surfaces this analysis was restricted to four crustal discontinuities 1 the top of the european basement first european basement unit including the foreland encountered going downward from the topographic surface 2 the top of the lower crust 3 the top of the european moho transition zone and 4 the european moho simulation of the seismic profiles using 3d normal incidence and shot ray tracing enabled the quantification and correction of distortions caused by out of plane reflected energy the inferred 3d crustal configuration is likely the result of the latest phase of alpine shortening during the late tertiary it depicts a smooth sse dipping european moho overlain by a concordant lower crust in the external alpine domain and a presumably wedge shaped body of lower crust in the internal alpine domain in contrast to the lower crustal discontinuities the top of the european basement reveals a highly structured 3d geometry dominated by thrust tectonics the pronounced structural disharmony with respect to the lower crust implies an intracrustal decoupling mechanism during the formation of the external basement uplifts with the zone of decoupling possibly located near the top of the lower crust comparison of the top basement topography with the mapped surface geology indicates a close relation between the basement geometry and the shape of the austroalpine basal thrust delineating the prattigau halfwindow in the west and the engadine window in the east

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

late pleistocene and holocene sea level change in the australian region and mantle rheology summary spatial and temporal variations in sea level are produced by the melting of the late pleistocene ice and by the earth s response to the redistribution in surface loads by examining different parts of the sea level curves of the past 20 000 yr from geographically widely distributed regions it becomes possible to constrain models of the melting history of the ice sheets and of the earth s rheology observations from sites away from the former arctic ice sheets such as the australian and south pacific region are particularly important for constraining the total meltwater volumes added into the oceans in the past 20 000 yr and the rates at which this occurred these observations indicate that the antarctic ice sheets provided a significant contribution to the sea level rise at a rate that was approximately synchronous with the melting of the laurentide ice sheet except for the interval 9000 6000 yr ago when it may have lagged behind minor melting of the antarctic ice sheet appears to have continued throughout the late holocene differential observations of the late holocene sea level change recorded at sites in the same region are particularly useful for estimating parameters describing the earth s non elastic response to surface loading the effective parameters used here are a lithospheric thickness an upper mantle viscosity and a lower mantle viscosity describing the response below 670 km depth with the observations used here it is not possible to separate the lithospheric thickness h from the upper mantle viscosity and the viscosity results are based on the assumption that 50 h 100 km neither do these observations provide a resolution of the depth dependence of viscosity in the upper mantle and the resulting estimates are effective parameters only differential sea levels along continental margins and along the shores of large gulfs and bays constrain the effective upper mantle viscosity to be about 1 2 1020 pa s 1 while differential values from islands of different sizes are suggestive of a somewhat lower value the lower mantle taken to be below 670 km depth viscosity is about two orders of magnitude greater than this the estimated ice and rheological models explain many of the holocene sea level observations throughout the australian and southern pacific region the tilting of continental margins as exemplified by observations of variable amplitudes of the holocene high stands and the variable times at which sea levels first reached their present level along the north queensland coast and great barrier reef is well represented by these models differential holocene sea levels observed along the narrow spencers gulf of south australia are also well explained by the models and in neither case is it necessary to invoke tectonic motions predicted late holocene sea levels at small to medium sized islands are characterized by small amplitude high stands that reached their maximum values about 4000 2000 yr ago consistent with observations from the society cook and tuamotu islands

sediments with gas hydrates internal structure from seismic avo we interpret amplitude variation with offset avo data from a bottom simulating reflector bsr offshore florida by using rock physics based synthetic seismic models a previously conducted velocity and avo analysis of the in situ seismic data showed that the bsr separates hydrate bearing sediments from sediments containing free methane the amplitudes at the bsr are increasingly negative with increasing offset this behavior was explained by p wave velocity above the bsr being larger than that below the bsr and s wave velocity above the bsr being smaller than that below the bsr we use these avo and velocity results to infer the internal structure of the hydrated sediment to do so we examine two micromechanical models that correspond to the two extreme cases of hydrate deposition in the pore space 1 the hydrate cements grain contacts and strongly reinforces the sediment and 2 the hydrate is located away from grain contacts and does not affect the stiffness of the sediment frame only the second model can qualitatively reproduce the observed avo response thus inferred internal structure of the hydrate bearing sediment means that 1 the sediment above the bsr is uncemented and thereby mechanically weak and 2 its permeability is very low because the hydrate clogs large pore space conduits the latter explains why free gas is trapped underneath the bsr the seismic data also indicate the absence of strong reflections at the top of the hydrate layer this fact suggests that the high concentration of hydrates in the sediment just above the bsr gradually decreases with decreasing depth this effect is consistent with the fact that the low permeability hydrated sediments above the bsr prevent free methane from migrating upwards

paleoshoreline record of relative holocene sea levels on pacific islands abstract understanding the history of relative holocene sea levels on pacific islands is important for constraining fundamental geodynamic theories interpreting the environments of early human occupation sites and forecasting future environmental conditions on the islands an observational paleoshoreline record is provided by emergent paleoshoreline indicators formed at higher relative sea levels hence standing at higher elevations than modern counterparts emergent paleoshoreline notches in limestone seacliffs record paleo high tide levels and emergent paleoreef flats record paleo low tide levels whereas emergent paleobeachrock locally records paleo intertidal levels both paleonotches and paleoreefs occur along the coasts of high standing islands exposing volcanic bedrock and uplifted reef complexes but low lying coralline atolls lack sufficient relief to preserve paleonotches controls on relative holocene sea level include global eustatic and regional hydro isostatic changes in ambient sea level relative to island landmasses and shifts in the elevations of islands relative to sea level caused by thermal subsidence of the oceanic lithosphere or thermally rejuvenated loci of hotspot volcanism by flexure of the lithosphere under the load of growing volcanic edifices hawaii samoa society islands by arching of the lithosphere over trench forebulges loyalty islands niue bellona rennell and by tectonism within forearc belts between active volcanic chains and trenches mariana islands tonga vanuatu the dominant pattern of relative sea level change where not overprinted by local tectonism or lithospheric flexure was a uniform early holocene rise in eustatic sea level followed by a regionally variable late holocene hydro isostatic drawdown in sea level the resultant was a mid holocene highstand in relative sea level that affected the development of shoreline morphology throughout the tropical pacific ocean the earliest human migrations into intra oceanic island groups of both the northwestern and southwestern pacific ocean followed close on the heels of the mid holocene sea level highstand and took advantage of newly attractive coastal environments engendered by sea level drawdown the effects of the mid holocene highstand were modified to varying degrees in different island groups by geodynamic uplift or subsidence

methane hydrate and free gas on the blake ridge from vertical seismic profiling seismic velocities measured in three drill holes through a gas hydrate deposit on the blake ridge offshore south carolina indicate that substantial free gas exists to at least 250 meters beneath the bottom simulating reflection bsr both methane hydrate and free gas exist even where a clear bsr is absent the low reflectance or blanking above the bsr is caused by lithologic homogeneity of the sediments rather than by hydrate cementation the average methane hydrate saturation above the bsr is relatively low 5 to 7 percent of porosity which suggests that earlier global estimates of methane in hydrates may be too high by as much as a factor of 3

seismic image of the deep crust at the eastern margin of the alps austria indications for crustal extension in a convergent orogen abstract a 39 km long deep seismic reflection profile recorded during two field campaigns in 1996 and 2002 provides a first detailed image of the deep crust at the eastern margin of the eastern alps austria the ese wnw trending low fold seismic line crosses austroalpine basement units and extends approximately from 20 km west of the penninic window group of rechnitz to 60 km sse of the alpine thrust front the explosive source seismic data reveals a transparent shallow crust down to 5 km depth a complexly reflective upper crust and a highly reflective lowermost crust the upper crust is dominated by three prominent west dipping packages of high amplitude subparallel reflections the upper two of these prominent packages commence at the eastern end of the profile at about 5 and 10 km depth and are interpreted as low angle normal shear zones related to the miocene exhumation of the rechnitz metamorphic core complex in the western portion of the upper crust east dipping and less significant reflections prevail the lowermost package of these reflections is suggested to represent the overall top of the european crystalline basement along the western portion of the line the lower crust is characterised by a 6 8 km thick band of high amplitude reflection lamellae typically observed in extensional provinces the moho can be clearly defined at the base of this band at approximately 32 5 km depth due to insufficient signal penetration outstanding reflections are missing in the central and eastern portion of the lower crust we speculate that the result of accompanying gravity measurements and lower crustal sporadic reflections can be interpreted as an indication for a shallower moho in the east preferable at about 30 5 km depth the high reflectivity of the lowermost part of the lower crust and prominent reflection packages in the upper crust the latter interpreted to represent broad extensional mylonite zones emphasises the latest extensional processes in accordance with eastward extrusion

seismic constraints on the nature of lower crustal reflectors beneath the extending southern transition zone of the colorado plateau arizona events maintain a constant amplitude with offset which is most consistent with a mafic layering model one exception is a high amplitude 10 db above background event near the base of lower crustal reflectivity which abruptly decreases in amplitude at increasing offsets the event has a pronounced s wave response which along with its negative avo trend is a possible indication of the presence of fluids in the lower crust the arizona transition zone is an active but weakly extended province which causes us to discard models of lower crustal layering resulting from shearing because of the high degree of strain required to create such layers instead we favor horizontal basaltic intrusions as the primary origin of high impedance reflectors based on 1 the fact that most xenoliths in eruptive basalts of the transition zone are of mafic igneous composition 2 indications that a pulse of magmatic activity crossed the transition zone in the late tertiary period and 3 the high regional heat flow observed in the transition zone the apparent presence of fluids near the base of the reflective zone may indicate a partially molten intrusion we present a mechanism by which magma can be trapped and be induced to intrude horizontally at rheologic contrasts in extending crust

2 d seismic modeling of the variscan foreland thrust and fold belt crust in nw spain from escin 1 deep seismic reflection data abstract the foreland thrust and fold belt cantabrian zone of the variscan belt in nw spain and the transition to the hinterland westasturian leonese zone was the location of a seismic experiment in 1991 the seismic reflection profile escin 1 is 140 km long and runs in an e w direction the interpretation and seismic modeling of the main reflective interfaces in the profile were made integrating available geological and geophysical data including surface geology deep seismic reflection data from escin 1 transmission velocities from a borehole refraction and laboratory data the geological and velocity model of the crust was tested obtaining synthetic seismograms and can be correlated with surface geology the velocity model images the main crustal structures interpreted from escin 1 the basal detachment of the foreland thrust and fold belt dips gently from 12 km in the easternmost part of the profile to 16 km in the transition to the hinterland to the west a new interpretation is proposed for the structure above the basal detachment in the eastern end of the profile where the basal detachment at a depth of 12 km is duplicated at 6 5 km by a n dipping alpine thrust that also duplicates part of the basement in the western part two deep reflective bands dip westward and the most conspicuous one can be followed from 16 27 km depth the two bands previously interpreted as crustal ramps join a reflective lower crust that is located between 25 and 29 km and fades westwards a reflective moho interpreted at the base of the reflective lower crust dips and fades in the same direction from 30 34 km

late quaternary barrier and fringing reef development of bora bora society islands south pacific first subsurface data from the darwin type barrier reef system the universally known subsidence theory of darwin based on bora bora as a model was developed without information from the subsurface to evaluate the influence of environmental factors on reef development two traverses with three cores each on the barrier and the fringing reefs of bora bora were drilled and 34 uranium series dates obtained and subsequently analysed sea level rise and to a lesser degree subsidence were crucial for holocene reef development in that they have created accommodation space and controlled reef architecture antecedent topography played a role as well because the holocene barrier reef is located on a pleistocene barrier reef forming a topographic high the pedestal of the fringing reef was pleistocene soil and basalt barrier and fringing reefs developed contemporaneously during the holocene the occurrence of five coralgal assemblages indicates an upcore increase in wave energy age depth plots suggest that barrier and fringing reefs have prograded during the holocene the holocene fringing reef is up to 20 m thick and comprises coralgal and microbial reef sections and abundant unconsolidated sediment fringing reef growth started 8780 50 yr bp accretion rates average 5 65 m kyr 1 the barrier reef consists of 30 m thick holocene coralgal and microbial successions holocene barrier reef growth began 10 030 50 yr bp and accretion rates average 6 15 m kyr 1 the underlying pleistocene reef formed 116 900 1100 yr bp i e during marine isotope stage 5e based on pleistocene age depth and coralgal palaeobathymetry the subsidence rate of bora bora was estimated to be 0 05 to 0 14 m kyr 1 in addition to subsidence reef development on shorter timescales like in the late pleistocene and holocene has been driven by glacioeustatic sea level changes causing alternations of periods of flooding and subaerial exposure comparisons with other oceanic barrier reef systems in tahiti and mayotte exhibit more differences than similarities

trapping and migration of methane associated with the gas hydrate stability zone at the blake ridge diapir new insights from seismic data abstract the blake ridge diapir is the southernmost of a line of salt diapirs along the carolina trough diapirs cause faulting of the superjacent sediments creating pathways for migration of fluids and gas to the seafloor we analyzed reflection seismic data from the blake ridge diapir which is located in a region with known abundant gas hydrate occurrence a striking feature in these data is a significant shallowing of the base of gas hydrate stability bghs over the center of the diapir the seafloor is warped up by about 100 m above the diapir from about 2300 m to about 2200 m the bghs as indicated by a bottom simulating reflection bsr is about 4 5 s off the flanks of the diapir rising to about 4 15 s at the center above the diapir a fault system appears to rise vertically from the bghs to about 0 05 s below the seafloor 40 50 m it then diverges into several steeply dipping faults that breach the seafloor and cover an area 700 m in diameter other secondary faults diverge from the main fault or emerge directly from the bghs near the crest of the diapir gas and other fluids may migrate upward through the faults we performed complex trace analysis to compare the reflection strength and instantaneous frequency along individual reflections a low frequency anomaly over the center of the diapir indicates high seismic attenuation this is interpreted to be caused by migration of fluids probably methane along fault pathways the migration of gas i e probably mainly methane through the gas hydrate stability zone is not yet understood we speculate that pore fluids in the faults may be too warm and too salty to form gas hydrate even at depths where gas hydrate is stable away from the diapir alternatively gas hydrates may seal the fault walls such that water supply is too low to transform all the gas into gas hydrates the shallowing of the bsr may reflect increased heatflow above the diapir either caused by the high thermal conductivity of the underlying salt or by advective heat transport along with fluids high pore water salinity shifts the gas hydrate stability to lower temperatures and may also play a significant role in bsr shallowing we therefore investigated the possible effect of pore water salinity on shallowing of the bsr we found that bsr shallowing may theoretically be entirely caused by increased salinity over the diapir although geologically this would not be reasonable this observation demonstrates the potential importance of pore water salinity for lateral variations of bsr depths in particular above salt structures

anaa tuamotu islands central pacific an incipient rising atoll abstract the 4 5 m high raised reefs in anaa atoll certainly date back to the last interglacial age the holocene reefs which have yielded dates between 3200 and 2000 yrs b p are slightly more elevated than in other tuamotu atolls and appear to be rising at a relative rate of about 0 1 mm yr 1 this rising trend of the island is recent and may be the result of thermal rejuvenation effected when the part of the oceanic lithosphere on which the island is situated approached the hot spot of the society islands

anomalously cold temperatures observed at the base of the gas hydrate stability zone on the u s atlantic passive margin in situ measurements to depths of 415 m below sea floor in methane hydrate bearing sediments on the u s atlantic passive margin indicate that temperatures at the bottom simulating reflector bsr are anomalously low by 0 5 2 9 c if the bsr marks the base of gas hydrate stability bghs several hypotheses may explain the occurrence of the bsr at inappropriate pressure temperature p t conditions 1 if the bsr does not mark the bghs then p t conditions need not be sufficient to dissociate gas hydrate at this depth 2 the bsr may lie at nonequilibrium p t conditions due to incomplete readjustment in response to upper pleistocene holocene climate change however the occurrence of the blake ridge bsr at an overly shallow depth cannot be easily explained by realistic combinations of pressure driven deepening sea level rise and temperature driven shoaling bottom water temperature changes 3 the properties of sediments or pore fluids may inhibit the stability of gas hydrate in particular capillary forces arising in the fine grained montmorillonite rich sediments of the blake ridge may lead to shoaling of the bsr in this setting

the deep crust of the southern rhine graben reflectivity and seismicity as images of dynamic processes abstract the rhine graben part of the european cenozoic rift system deserves special attention because of its location in the foreland of the alpine orogen the phanerozoic evolution of the lithosphere in this region is defined by a set of major geodynamic events ranging from the variscan orogeny late orogenic crustal re equilibration to the interference of rifting and alpine orogeny in tertiary times the rhine graben is one of the most detailedly studied continental grabens prospecting for hydrocarbons in its sedimentary fill and intensive pre site studies for the continental deep drilling program ktb on its eastern crystalline flank black forest provided a comprehensive set of geoscience data seismic investigations ranging from a deep seismic network of near vertical reflection to concurrent wide angle refraction experiments were accompanied by seismological and geological surveys therefore a direct observational comparison between the two depth definitions of crustal subdivisions elastic vs strength rheological was possible in large parts of western europe deep seismic sounding dss investigation distinguishes between a crystalline upper crust of comparatively low p wave velocities 5 9 6 0 km s with little and discrete reflectivity and a strongly reflective lower crust with higher p wave velocities about 6 4 6 8 km s both are separated by the conrad discontinuity in contrast strength and rheological considerations define a brittle upper crust and a ductile lower crust the boundary between these two realms is usually referred to as the brittle ductile transition zone which in seismically active regions is imaged by the deepest crustal hypocentres the present brittle ductile transition in the rhine graben region as defined by the deepest hypocentres does not necessarily coincide with the structural separation in an upper and lower crust as seen in deep seismic studies this suggests that the laminated lower crust developed during the crustal re equilibration phase at the end of the variscan orogeny the second part of this paper concentrates on evidence of recent tectonic activities related to rifting processes impacting on the variscan lower crust a remarkable concentration of deep crustal earthquakes reaching to the top of an asymmetrical thinned lower crust is observed at or near the eastern border fault of the southern rhine graben and is possibly induced by the young rifting process we suggest that this asymmetry could be a consequence of the uppermost mantle detaching from the upper crust in collision with the alpine orogen this allows us to propose the distinction between the influence of compression extension continental subduction and intra crustal detachment during graben formation the dynamic interference of rifting and foreland compression shown by structural and seismological observations leads to the notion of small scale collisional escape tectonics associated with an intra continental transform fault at the southern end of the rhine graben the rhine sao ne transform zone

microatolls as sea level indicators on a mid ocean atoll microatolls are discoid corals with flat upper surfaces that develop when upward coral growth is constrained by exposure at low tide they have been widely viewed as important and relatively precise indicators of modern and paleo sea level positions because of their sensitivity to the water air interface though this has rarely been directly established by survey to a precise datum this study involved an accurate survey of 282 microatolls growing in a range of intertidal environments open reef flats interisland passages lagoons across the cocos keeling islands a mid ocean atoll in the indian ocean the upper surface elevation of modern microatolls was found to vary through more than 40 cm across the environments in which these corals occur on cocos representing more than 30 of the spring tide range much of the variation reflects subtle ponding at low water levels across extensive low gradient reef surfaces associated with interisland passages microatolls in open reef flat habitats are typically constrained within the narrowest and most consistent elevation range around the atoll the environment in which individual fossil microatolls occurred when they were alive is significant in reconstructing former sea levels

preliminary laboratory investigations into the attenuation of compressional and shear waves on near surface marine sediments the attenuation of compressional and shear waves q p and q s has been studied by several authors but most of these investigations were performed on deep buried reservoir sandstones in order to distinguish between gas and condensate reservoirs and water saturated sandstones we present a preliminary investigation into the use of seismic wave attenuation as a measure of the geotechnical parameters of the near surface marine sediments a little studied geological setting a 6 9 m long gravity core was taken from the continental slope of the barents sea at a water depth of 2227 m the core was primarily composed of brown to olive grey clayey mud having a high content of foraminifers and being locally bioturbated the values of q p and q s were determined using the rise time method at 19 and 18 different points of the core respectively and they were correlated with geotechnical parameters such as wet bulk density porosity water content shear strength and c p ratio the ratio of shear strength to overburden pressure the calculated correlation coefficients for all correlations ranged from 0 39 to 0 41 suggesting that the attenuation characteristics of seismic waves could not be used to derive geotechnical parameters of marine sediments however with such a small data set it is difficult to determine clearly whether attenuation is primarily a frequency dependent parameter and consequently not related to sediment properties or whether the limited number of data points is the main factor responsible for the low correlation coefficients observed moreover several different methods are available for the computation of the quality factor q and the rise time method may not be the most appropriate means of determining the attenuation on near surface marine sediments

understanding the history of extreme wave events in the tuamotu archipelago of french polynesia from large carbonate boulders on makemo atoll with implications for future threats in the central south pacific abstract numerous large carbonate boulders up to 164 tonnes in mass were investigated on the reef flat and beaches of makemo atoll in the tuamotu archipelago of french polynesia to reveal the past occurrence and to anticipate the future potential threat of extreme wave events possibly generated by tropical cyclones and tsunamis the modern reef edge and emerged mid holocene coastal landforms were identified as sources of boulders mobilized during extreme wave events in the past the minimum flow velocities produced by extreme wave events were estimated to exceed 5 4 15 7 m s at the reef edge on different parts of the atoll comparison of uranium thorium ages of boulder coral fabric with written historical records indicates that two large boulders 77 and 68 tonnes were possibly emplaced on the reef flat by a powerful cyclone in february 1878 although most boulder dates are older than the earliest historical cyclone and tsunami records in french polynesia their ages concur with the following a periods of storminess i e increased cyclone activity compared to today in the central south pacific over the last millennium and b periods of high sea surface temperature sst at the great barrier reef possibly associated with higher than normal ssts pacific wide that facilitated the generation of cyclones affecting the central south pacific ocean none of the boulders on makemo were dated younger than ce1900 implying that the last century has not experienced extreme waves of similar magnitude in the past nevertheless the findings suggest that waves of comparable magnitudes to those that have transported large boulders on makemo may recur in the tuamotus and threaten island coasts across the central south pacific in the future