direct determination of the timing of sea level change during termination ii an outcrop within the last interglacial terrace on barbados contains corals that grew during the penultimate deglaciation or termination ii we used combined 230th and231pa dating to determine that they grew 135 8 0 8 thousand years ago indicating that sea level was 18 3 meters below present sea level at the time this suggests that sea level had risen to within 20 of its peak last interglacial value by 136 thousand years ago in conflict with milankovitch theory predictions orbital forcing may have played a role in the deglaciation as may have isostatic adjustments due to large ice sheets other corals in the same outcrop grew during oxygen isotope 18o substage 6e indicating that sea level was 38 5 meters below present sea level about 168 0 thousand years ago when compared to the 18o signal in the benthic v19 30 v19 28 record at that time the coral data extend to the previous glacial cycle the conclusion that deep water temperatures were colder during glacial periods

seismic images of continental lithosphere seismic reflection profiles in usa and west germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere the british institutions reflection profiling syndicate birps found advantage in working offshore and has obtained over 12 000 km of profiles around britain during the past decade giving the densest regional coverage anywhere reflections are observed from strata in sedimentary basins from low angle faults that can be followed from surface to mid crust from a laterally variable reflective zone in the lower crust from the moho and from fault like features in the upper mantle there is evidence around britain that faults bounding sedimentary basins are older structures which have been reactivated it is argued that lower crustal reflectivity around britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation there is also evidence of reflections preserved from older orogenic features so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated most reflections in the lower crust arise from rocks that have been highly strained evidence of lateral variability in the reflective character of the moho suggests that the moho has been actively involved in the structural evolution of the lithosphere upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere

mt and reflection an essential combination summary at many localities in the world there have been coincident comprehensive electromagnetic em studies and seismic reflection profiles conducted unfortunately over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information this paper is an attempt to convince the reader that a collocated magnetotelluric mt study should in almost every case be made wherever a seismic reflection survey is undertaken examples are shown from six studies in which the em results aided the geological tectonic interpretations of the seismic sections also difficulties with the mt technique are discussed and the interpretations of conducting zones within the lower crust are examined finally a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust

contrasting styles of rifting models and examples from the eastern canadian margin we present the results of a dynamical model of lithospheric rifting and rupture which show that a wide range of crustal thinning patterns across rifted passive margins can be produced by varying the steady state geotherm lithospheric composition dry versus wet materials and strain rate the basic mechanism of continental rupture is assumed to be passive rifting and necking we use a numerical thermomechanical model of lithosphere extension based on a finite element approach when plasticity is significant i e at lower temperatures or for harder materials deformation is unstable and thinning takes place abruptly over a narrow area conversely a progressive thinning across the margin is observed when creep is dominant i e in warm or ductile conditions cooling and associated hardening of the thinned area can occur during extension and cause the locus of extension to migrate laterally in these circumstances rupture is likely to take place asymmetrically along one edge of the thinned area producing a narrow margin and a very wide conjugate the eastern margins of canada and their conjugates across the north atlantic provide examples which cover this range of theoretical profiles the crustal thinning patterns inferred from deep seismic data and the duration of rifting compare well with model results we discuss also the constraints that these geodynamical models provide on such current issues as the seismic reflectivity of the lower crust or the location of the ocean continent boundary in wide areas supposedly underlain by 5 km thin continental crust

on the dynamics of extensional basin geological and geophysical data from the north china bohai basin and basin and range province were examined and compared they are similar to each other in many respects surficial geological structures are characterized by a series of half grabens with their one flank constituted by normal fault those extensional structures usually extend to a depth of 6 8 km therefore the stress condition in the upper 8 km can be written as sigma 2 sigma x sigma y where x y denote the directions of maximum compression and maximum tension on the horizontal plane while z signifies the vertical direction

origin of deep crystal reflections seismic profiling across high grade metamorphic terranes in canada abstract in an attempt to better understand the origin of deep crustal reflections lithoprobe has sponsored or co sponsored seismic reflection surveys across tracts of high grade metamorphic rock in the archean superior craton the proterozoic grenville orogen and the phanerozoic cordilleran orogen common to these three diverse terranes are near surface zones of prominent seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology at some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks stratified assemblages sills etc whereas in other regions it was generated by extreme attenuation stretching and ductile flow of weakly layered or irregularly organized rocks it seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust with reflections originating at lithological boundaries and zones of mylonite

crustal evolution of the rhinegraben area 1 exploring the lower crust in the rhinegraben rift by unified geophysical experiments abstract unified geophysical investigations of the lithosphere in the rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process the new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust 1 1 the lower crust as the layer beneath the conrad discontinuity with a p wave velocity of about 6 5 km s or greater refraction seismics 2 2 the laminated band of reflections as seen in the near vertical reflection seismic experiments in many parts of the continents 3 3 the ductile part of the crust below the brittle ductile transition devoid of earthquakes in seismically active regions 4 4 the electrical conductivity of the lower crust indicative of dry or wet conditions or still unknown conduction phenomena in the rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process in which the crust of the rhinegraben rift system has been subjected to three different natural dynamic processes 1 uplift by 2 to 3 km with subsequent erosion of the rhinegraben shoulders black forest and vosges mountains caused decompression possibly leading to the formation of a low velocity high electrical conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the black forest 2 the brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust which became about 5 to 7 km thinner below the graben than below the shoulders 3 the deepest hypocentres in the black forest dinkelberg area if projected onto the neighbouring reflection profile would be located 7 to 8 km within the laminated lower crust beneath the southern black forest indicating a discrepancy between the top of the lower crust as defined by the brittle ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band the rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations

interpretation of the reflections in the siljan ring area based on results from the gravberg 1 borehole abstract during 1985 about 80 km of surface multichannel seismic reflection data were collected across the meteorite impact in the siljan ring area in central sweden the area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 ma thereby forming a circular ring about 40 km in diameter dolerite intrusions ranging in age from 850 to 1700 million years are also present the seismic data revealed several high amplitude laterally continuous sub horizontal reflections in the northern part of the structure the high amplitude reflections and a possible intermediate low velocity zone were contributing factors in choosing the site for the gravberg 1 deep earth gas test well drilling and vertical seismic profiling vsp found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre impact area extent of at least 800 km 2 studies of amplitude and frequency versus offset afvo show the observations are compatible with a model of simple granite dolerite granite layering

crustal structure of the goban spur continental margin northeast atlantic from deep seismic reflection profiling deep reflection data across the uk continental margin are used to test theories concerning the origin of a pervasive reflective fabric in the lower continental crust seen on many deep reflection profiles worldwide the origin of the reflections is at present little understood but clearly represents an important crustal process the western approaches margin wam line was designed to determine the behaviour of the lower crustal reflections in crust which has undergone significant extension the wam profile across the continental shelf is characterized by bright continuous lower crustal reflections some of which reach 30 km in length and have minimum reflection coefficients of 0 1 deep water multiples in the area of the continental slope partially obscure reflections from the lower crust but detailed analyses show that reflectors are present in the lower crust on the continental slope adjacent to the continent ocean boundary the lower crust on the wam profile is well imaged and free of noise and multiples this very thin crust exhibits the same reflectivity pattern as is observed on the continental shelf i e the upper half of the crust is blank and the lower half contains sub horizontal reflections the lower crustal reflections adjacent to the continent ocean boundary are less continuous than the reflections seen on the shelf and are interpreted as being the remnants after extension of the lower crustal reflectors observed on the continental shelf the lower crustal reflectors are thus interpreted to pre date the extensional episode that led to the opening of the atlantic the preservation of thinned sections of both transparent upper crust and reflective lower crust adjacent to the continent ocean boundary suggests that extension by pure shear was the mechanism by which the crust was thinned in the goban spur area

a one million year long sequence of marine terraces on sumba island indonesia abstract an exceptional sequence of raised coral reef terraces is visible at cape laundi on the north coast of the island of sumba in the savu sea in which at least eleven terraces wider than 100 m six of them even being over half a kilometer wide are geomorphologically distinguishable between an ancient patch reef now at 475 m and the present sea level esr date estimations of almost unrecrystallized corals most of them in situ make possible a tentative identification of the terraces corresponding to isotope stages 15 ca 600 ka ago and 9 ca 330 ka ago when the uplift trend deduced from the present altitude of dated terraces 0 49 0 01 mm yr is extrapolated to the whole raised section most geomorphological features appear to correspond to interglacial stages of the astronomically calibrated oxygen isotope record up to stage 27 ca 0 99 ma beyond the matuyama jaramillo geomagnetic boundary the upper terraces would in this case be twice as old as previously estimated ca 0 5 ma main terraces seem to correspond to high sea levels at certain interglacial stages and substages but in some cases the action of low sea levels at glacial stages cannot be ruled out for terraces between 30 and 70 m in altitude th u and esr dates reveal that aragonitic corals at similar elevations may differ in age as much as 100 ka this can be explained by taking into account a polycyclic origin for several terraces reconstructions available of changes in the eustatic sea level during the last 250 kyr and the fact that coral reef development may have been scattered at some periods only without totally covering subjacent older formations the sumba sequence of marine terraces at cape laundi is among the longest and most complete for the past million years reported in the literature its relatively good state of preservation will probably give impetus to a number of new studies on the chemical physical isotopic morphological palaeoclimatic pedological ecological and diagenetic changes which have occurred during the past one million years

relations between sill intrusions and bedding parallel extensional shear zones in the mid continent rift system of the lake superior region abstract the mid continent rift system is a continuous u shaped feature extending from kansas north through the lake superior region and then south through michigan the logan sills on the northwest side of the lake were intruded during the rifting stage 1109 ma deformation features in the country rock adjacent to the logan sills indicate two episodes of shear strain the first consists of minor thrusts and mesoscopic shear bands both suggesting top to the south southwest movement parallel to the bedding the second rotates the previously formed shear bands in the opposite sense top to the north northeast and is localized at the basal contact between the sills and the country rock we attempt to explain the relations observed with a tectonic model in which sill intrusions take advantage of bedding parallel shear zones that are the surface expression of a south dipping detachment plane the detachment plane steps up in the layered proterozoic sedimentary sequence and allows extension during rifting in this scenario the first episode of shear strain might be related to bedding parallel extension and the second to shear stress induced by the intrusion of magma along the detachment from the south the polarity of both events is supported by small scale kinematic indicators such as veins fiber growths and foliation

geodynamics of rifting and implications for hydrocarbon habitat abstract rifting activity is probably mainly governed by frictional forces exerted on the base of the lithosphere by the convecting sub lithospheric upper mantle combined with deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system and by far filed stresses controlling the interaction of lithospheric plates moving relative to each other mantle plumes rising from the deep mantle do not appear to be a major driving force of rifting therefore it is questioned whether a distinction between active and passive rifts is still justified lithospheric extension triggers by decompression partial melting of the lower lithosphere and upper asthenosphere magmatic processes contribute materially to thinning of the subcrustal lithosphere and probably also of the lower crust this could explain the frequently observed discrepancy between upper crustal extension by faulting and lower crustal attenuation as defined by refraction and deep reflection seismic data the duration of the rifting stage of palaeorifts and passive margins is highly variable depending on plate interaction crustal separation can be achieved after as little as 7 ma and as much as 280 ma of rifting activity the availability of crustal discontinuities does not appear to play a significant role in the time required for crustal separation to be achieved the structural style of rifts is controlled by the physical state of the lithosphere the mode and amount of its extension and the lithological composition of pre and syn rift sediments subsidence of rift basins is controlled by the balance between isostatic adjustment of the crust to lithospheric stretching and uplift induced by mechanical and thermal attenuation of the subcrustal lithosphere post rift basin subsidence is mainly governed by the decay of rift induced thermal anomalies densification of the lower crust and intra plate stresses provide overprinting effects in view of this it is unlikely that quantitative analyses of post rift basin subsidence give a direct measure of lithospheric extension tectonically active rifts palaeorifts and passive margin basins contain major hydrocarbon provinces their hydrocarbon charge can rely exclusively on pre rift syn rift or post rift sedimentary sequences or a combination thereof maturation of source rocks can be achieved during the syn and or post rift stage of basin evolution during rifting conductive and convective heat transfer accounts for elevated geothermal gradients these play an important role in the maturation of pre and syn rift source rocks thermal gradients decrease asymptotically during the post rift stage

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

seismic images and evolution of the iberian crust in the pyrenees abstract crustal images of the pyrenean range based on coincident wide angle and vertical deep seismic profiles are presented in this study wide angle experiments in the thickened iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust in contrast reversed sounding on a line across the pyrenean range shows wide angle reflectivity its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust this observation is consistent with a new image of vertical incidence reflectivity obtained by coherency weighted migration of the coincident ecors traverse this profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust the reflections dip towards the axis of the range the upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple continuous three layered model previously used for geodynamical reconstructions the reflective zones cannot be brought back to the horizontal image of a layered lower crust of proposed extensional origin by plate unbending to the pre alpine stage the dipping reflections may result from compressional deformation as the south pyrenean deep crustal domain has undergone both hercynian and alpine orogenic deformation if part of the present reflectivity pattern was due to alpine remobilization the corresponding lower crustal deformation would have spread out thereby accomodating convergence by thickening even outside the axial zone where the upper crustal deformation appears to be concentrated

late quaternary coseismic uplift events on the huon peninsula papua new guinea deduced from coral terrace data up to six regressive terraces occur on the holocene raised reef tract and up to 15 occur on late pleistocene raised reef tracts along 40 km of coastline at huon peninsula papua new guinea we suggest that the regressive terraces represent repeated episodic uplift caused by great earthquakes ages of holocene coseismic uplift events are established by radiocarbon dating the late pleistocene events are bracketed by u series ages of the raised reef tracts on which they occur the mean recurrence interval of great earthquakes that caused the uplift events is 970 to 1165 years in the holocene and probably the same in the late pleistocene the interval ranged from about 200 to 1900 years the uplift rate increases parallel to the coast from northwest to southeast and the amplitude of coseismic uplifts generally increases similarly although some events produced uplift with little shore parallel tilting the mean amplitude of coseismic uplifts throughout the study area is 3 m in for both holocene and late pleistocene sequences large late quaternary landslides are numerous and some probably were triggered by the great earthquakes that caused coseismic uplift there appears to be no continuum between historical large earthquakes at huon peninsula of magnitudes 7 that produced no or only minor uplift and the great earthquakes represented by meter scale coseismic uplifts and very large landslides two tectonic subregions are recognized which were uplifted together by some holocene events but not by others there is no surface trace of holocene faulting between the subregions and a buried fault is thought to separate them

lower crustal xenoliths from queensland australia evidence for deep crustal assimilation and fractionation of continental basalts abstract a suite of mafic granulite facies xenoliths from north queensland possesses petrographic and geochemical features of basaltic cumulates crystallized at lower crustal pressures negative correlations between incompatible trace elements and mg and positive correlations between compatible trace elements and mg suggest the xenoliths are genetically related and crystallized from a continuously evolving melt zr hf y hree ti and v do not correlate with mg but show excellent negative correlations with al2o3 content reflecting the proportion of cumulate plagioclase to clinopyroxene these chemical trends also suggest the trace element concentrations have not been affected by subsolidus recrystallization the xenoliths have a large range in sr and nd isotopic compositions 87 sr 86 sr 0 70239 to 0 71467 nd 9 5 to 6 1 which cannot be produced by crystal fractionation alone and excellent correlations between isotope ratios and mg suggest the variable isotope compositions are not due to mantle source heterogeneities these mafic xenoliths are proposed to be cumulate products from a melt undergoing simultaneous assimilation and fractional crystallization afc the data illustrate that only a few percent afc in lower crustal environments can dramatically change the sr and nd isotopic composition of a basaltic melt and suggest the use of caution when inferring mantle source isotopic compositions from continental basalts additionally the nd isotopic data plot on a positive trend on an sm nd isochron diagram with an age of 570 ma however if these xenoliths formed by afc the positive trend reflects mixing between two isotopic end members and has no age significance the correlations between sr and nd isotopic compositions with mg degrade as the isotopic ratios are back calculated to earlier times suggesting the xenoliths are relatively young the xenoliths may be related to the cenozoic igneous activity which occurs throughout eastern australia

petrology and geochemistry of granulite xenoliths from central hoggar algeria implications for the lower crust granulitic xenoliths enclosed in neogene alkali basalts from the atakor and adrar n addjer volcanic massifs in central hoggar southern algeria include mafic metaigneous and felsic metasedimentary rocks the main mineral phases of metaigneous xenoliths are plagioclase and pyroxenes whereas quartz feldspars sillimanite garnet and sapphirine occur in metasedimentary granulites the metaigneous xenoliths represent a magmatic sequence of cumulitic and non cumulitic rocks which resembles gabbroic bodies associated with anorthosites and has calc alkaline affinities similar types and distributions of granulitic xenoliths are widespread and are probably characteristic of the lower crust in general suggesting its homogeneity on a large scale but heterogencity on a local scale

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

the evolution of the layered lower crust and the moho through geological time in western europe contribution of deep seismic reflection profiles deep seismic reflection images from a set of profiles shot in western europe have been reviewed and compared and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the moho the disappearance of variscan mountain roots is related to the set up of a new moho at a typical 30 km depth and the creation of seismic layering in the lower crust deep seismic profiles suggest that these processes resulted at least in part from magmatic intrusion partial crustal melting and metamorphism of deep crustal rocks into eclogite on the other hand the layered lower crust is greatly attenuated beneath cretaceous basins and tertiary rifts in relation to prominent moho upwellings the unusual amplitude of the moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the tertiary rifts suggest that the moho and part of the layering are comparatively young features related to interactions between crust and mantle beneath triassic jurassic basins the layered lower crust was not affected by the subsidence of the basement with the whole crustal thinning being entirely concentrated in the upper crust this indicates that the layered lower crust and the moho were formed or restored during or after the main rifting phase seismic data reveal constraints on the processes that affect the crust mantle transition and seem to restore the moho to its typical depth after any mechanical deformation of the lithosphere

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

coralgal composition of drowned carbonate platforms in the huon gulf papua new guinea implications for lowstand reef development and drowning abstract collision between the south bismarck plate and the northern edge of the australian plate has produced an actively subsiding foreland basin in the western huon gulf a series of drowned carbonate platforms and pinnacles are preserved on this margin due to a combination of this rapid subsidence and eustatic sea level changes over the last 450 ka we analyzed sedimentary and coralgal data from the platforms to better understand lowstand reef development and drowning in the huon gulf the recovered limestones are divided into five main sedimentary facies coral reef coralline foraminiferal nodule coralline foraminiferal crust halimeda and planktonic foraminiferal limestones based on a comparison with modern analogues in the indo pacific and elsewhere we identified coral reef deep fore reef slope deeper fore reef slope and pelagic hemipelagic paleoenvironmental settings an analysis of facies relationships and their paleoenvironmental meanings revealed lowstand corals reefs preserved at the top of the platforms that grew within 10 m of sea level two different coral assemblages were identified within this facies 1 a shallow high energy reef community characteristic of windward margins and limited to the deep platforms 1947 2121 2393 m and 2 another shallow community but indicative of more moderate lower energy reef conditions and limited to the middle 1113 1217 1612 m and shallow platforms 823 m the change from high to lower energy reef growth conditions suggests that oceanographic climatic conditions in the huon gulf have changed substantially through time primarily through the closure of the gulf as a result of tectonic rotation and uplift of the huon peninsula over the last 450 ka despite major environmental perturbations i e relative sea level and temperature changes the platforms and the shallow water coral reefs exposed at the top have been able to re establish themselves time and time again over the last 450 ka we also identified two different incipient drowning scenarios influenced by the rate of relative sea level rise more rapid drowning in the middle and deep platforms produced a thin veneer of coralline foraminiferal nodule and halimeda limestones over the shallow coral reef material while the slower drowning experienced by the shallowest platforms allowed thick coralline foraminiferal crust limestones to develop we recognize three main stages of platform development 1 initiation and growth characterized by shallow coral reef growth as the platforms grew close to sea level during the lowstands 2 incipient drowning marked by a shift to coralline foraminiferal nodule crust and halimeda limestones as the platforms began to drown during rapid eustatic sea level rise and continued subsidence and 3 the complete drowning of the platforms characterized by platform turn off increased bioerosion fe mn precipitation and pelagic hemipelagic sedimentation as the platform surfaces finally drop below the photic zone

motion of the south bismarck plate papua new guinea the absolute motion of the south bismarck plate was first estimated by tregoning et al 1998 from three site velocities estimated from global positioning system gps observations we report an improved estimate of the euler vector for this plate using site velocities derived from new gps data which include the velocity of a site located 25 km from the pole of rotation the gps velocities of madang witu jacquinot bay and finschafen can be modelled to within 3 mm yr using a single pole of rotation located at 6 75 s 147 98 e with a clockwise rotation rate of 8 11 my the known tectonic features and available geophysical data surrounding the south bismarck plate can also be explained by a rotation of the south bismarck plate about this pole

deep seismic reflection constraints on paleogene crustal extension in the south central intermontane belt british columbia following growth of the canadian cordillera during the mesozoic the southern cordillera was subject to extension during the paleocene and eocene that correlated with widespread volcanic activity in south central british columbia including across much of the nechako chilcotin plateau in 2008 geoscience bc acquired 330 km of deep vibroseis reflection profiles on the plateau mostly over the stikinia arc terrane but also over its eastern contact with the oceanic cache creek terrane all seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the moho which is defined by the downward termination of reflectivity at 11 12 s in the uppermost crust extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth extension in the deeper upper and middle crust which was partly controlled by antiforms likely related to earlier shortening was accommodated on a network of anastomosing shear zones

coral record of equatorial sea surface temperatures during the penultimate deglaciation at huon peninsula uplifted coral terraces at huon peninsula papua new guinea preserve a record of sea level sea surface temperature and salinity from the penultimate deglaciation remnants have been found of a shallow water reef that formed during a pause similar to the younger dryas in the penultimate deglaciation at 130 000 2000 years ago when sea level was 60 to 80 meters lower than it is today porites coral which grew during this period has oxygen isotopic values and strontium calcium ratios that indicate that sea surface temperatures were much cooler 22 2 c than either last interglacial or present day tropical temperatures 29 1 c these observations provide further evidence for a major cooling of the equatorial western pacific followed by an extremely rapid rise in sea level during the latter stages of termination ii

evidence for shear zones in the lower crust offshore britain lower crustal reflections seen on british institutions reflection profiling syndicate birps deep seismic reflection data recorded offshore the united kingdom are commonly short convex up and crosscutting consequently the lower crust should not be described as layered as much of the reflecting structure is not fully resolved and may be quite complex moreover the lower crustal reflective zone lcrz is not a uniformly reflective layer but contains reflective bands that wrap around nonreflective transparent zones because transparent zones probably correspond to a lack of a reflecting fabric understanding why these features are unreflective may help constrain the cause of the reflectivity of the lower crust although transparent zones may have a variety of explanations e g large shear pods or frozen magma bodies the geometry of reflective bands and transparent zones is perhaps best explained by the geometry of shear zones and low strain lozenges predicted both by combining theoretical lithospheric strength profiles with slip line field theory and by drawing analogies with structures developed in highly tectonised high grade metamorphic terrains this interpretation implies that lower crustal reflections may be from shear zones and also has implications for the way the lower crust deforms in particular extension may be accommodated by the relative movement of low strain lozenges along intervening zones of simple shear the lower crust may be stretched during extension but it must also transfer deformation between the upper crust and the mantle the observation on birps data of mantle reflections interpreted as mantle shear zones implies that deformation in the mantle is localised however the manue reflections are never colinear with faults in the upper crust requiring at least locally a component of sub horizontal bulk strain within the lower crust which may greatly increase the amount of shearing therein and hence perhaps may increase the reflectivity of the lcrz

lower crustal reflectivity from waveform inversion summary deep seismic reflection profiles show that the lower part of the continental crust often contains many strong subhorizontal reflections based on forward modelling these reflections are interpreted as a sequence of alternating high and low velocity layers a waveform inversion provides an optimum model by minimizing the misfit between observed data and synthetic seismograms however it is very difficult to invert for the whole data set partly because of the large computation time needed in modelling and partly because the high amplitude reflected energy in the upper crust dominates the misfit function to overcome these difficulties we propose a three step strategy for inverting the deeper part of seismic reflection data over a narrow aperture in which the velocity structure can be assumed to be 1 d first both source wavelet and recorded wavefield are propagated in the plane wave domain downwards to a particular depth for a given 1 d medium because the source receiver distances are short for deep reflections it is assumed that these reflections are at normal incidence it is also assumed that the near surface velocity is known or has been estimated using other inversion schemes the downward propagation algorithm takes account of nearsurface effects such as multiple reflections and attenuation second starting from a known background velocity model a non linear iterative waveform inversion is applied to the propagated data set the method is based on minimizing the difference sample by sample between observed and calculated wavefields in a least squares sense the forward modelling which consists of calculating the data from the model uses a reflectivity method to compute the synthetic seismogram in the plane wave domain the model perturbation is calculated with the help of the conjugate gradients method the non linear inversion gives the short wavelengths of impedance contrast which is a function of density and velocity in the lower crust third an interpretational step is tried in order to recover the medium wavelengths of velocity the algorithm is tested on synthetic data and then applied to a part of the wam data set south west of england a good fit to the real data with the optimum model predicts that the velocity contrast of the layers in the lower crust varies from 0 2 km s to 0 5 km s and the thickness from at least 0 075 km to 1 km

gps and seismological constraints on active tectonics and arc continent collision in papua new guinea implications for mechanics of microplate rotations in a plate boundary zone 1 the island of new guinea is located within the deforming zone between the pacific and australian plates that converge obliquely at 110 mm yr new guinea has been fragmented into a complex array of microplates some of which rotate rapidly about nearby vertical axes we present velocities from a network of 38 global positioning system gps sites spanning much of the nation of papua new guinea png the gps derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in png we simultaneously invert gps velocities earthquake slip vectors on faults and transform orientations in the woodlark basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region the data are best explained by six distinct tectonic blocks the australian pacific south bismarck north bismarck and woodlark plates and a previously unrecognized new guinea highlands block significant portions of the ramu markham fault appear to be locked which has implications for seismic hazard determination in the markham valley region we also propose that rapid clockwise rotation of the south bismarck plate is controlled by edge forces initiated by the collision between the finisterre arc and the new guinea highlands

layered crust mantle transition zone below a large crustal intrusion in the norwegian danish basin abstract the crust and uppermost mantle below the large positive gravity anomaly in the central part of the norwegian danish basin the silkeborg gravity high sgh is investigated using controlled source seismic data a more than 80 km long and 20 km thick intrusion is interpreted the seismic velocity within the intrusion varies from 6 8 km s at the top 12 km depth to 7 2 7 7 km s at the moho level 30 31 km depth the characteristics of the moho reflectivity vary along the profile below the central part of the intrusion the moho reflectivity is very weak due to a small velocity contrast between the lowermost crust 7 7 km s and the uppermost mantle 7 9 8 0 km s the seismic data show a ringing moho below the western part of the intrusion the coda trailing the main pmp reflection is about 1 0 s long and is composed of 4 5 wavelets we demonstrate that this feature may be explained by a layered transition zone between 30 and 35 km depth where high velocity layers of mantle affinity 7 9 8 05 km s alternate with layers of typical lower crustal velocity 6 7 7 3 km s the characteristics of this layering which causes the pmp coda are modelled by matching synthetic seismograms to the observed data synthetic seismograms calculated using the reflectivity method for 1d models constrain the nominal layer thickness to 100 500 m waveform modelling in 2d constrains the layered internal structure of the transition zone and the length of the heterogeneous zone along the profile we interpret the layered crust mantle transition zone as solidified melts of mantle affinity which were intruded into the lowermost crust the layered zone is proposed to be associated with the extensional tectonism that occurred in the area in late carboniferous early permian which also caused the large crustal intrusion the subsequent cooling may have initiated the formation of the danish basin

heterogeneous structure around the jemez volcanic field new mexico usa as inferred from the envelope inversion of active experiment seismic data summary we analyse active experiment seismic data obtained by the 1993 jemez tomography experiment jtex programme to elucidate the heterogeneous structure of the jemez volcanic field which is located at the boundary between the colorado plateau and the rio grande rift using a single isotropic scattering assumption we first calculate the envelope green s functions for the upper and lower crust and the uppermost mantle by fitting the theoretical envelopes with the observed three component data we estimate depth dependent features of the scattering coefficients around valles caldera we estimate the ratios of scattering coefficients rather than scattering coefficients themselves because of the uncertainty of the seismic efficiency of the explosive sources and knowledge of absolute site amplification factors the strongest scattering coefficients are observed at a shallow depth beneath the valles caldera this is considered to be related to the complex structure caused by two episodes of caldera formation and the ensuing resurgent uplift in the caldera etc the depth dependent characters of the scattering coefficients for the colorado plateau and the rio grande rift are similar to each other a transparent upper crust and a heterogeneous lower crust small and large scattering coefficients respectively however the scattering coefficients beneath the rio grande rift are several times larger than those beneath the colorado plateau depths of the lower crust and the moho boundary beneath the rio grande rift are shallower than those of the colorado plateau from their geological settings and other geophysical results around the region we infer that the larger scattering coefficients of the rift are associated with rift formation and volcanic activity such as magma ascent from the upper mantle to the crust

dating the source of lower crystal reflectivity using birps deep seismic profiles across the lapetus suture abstract six birps deep seismic reflection profiles around the british isles that cross the trend of the lapetus suture allow a geophysical definition of the suture zone for 900 km along its strike and demonstrate that specific reflective structures have survived since the mid palaeozoic caledonian orogen in an area northeast of england spanning 70 km along caledonian strike and 200 km across strike three profiles show very considerable similarities in the reflectivity patterns of the lower crust when these similarities are mapped the boundaries separating regions of lower crust with the same reflective character run parallel to caledonian strike this strongly suggests that the distribution of reflectivity in the lower crust in this area is directly controlled by caledonian structures geologic models for the area imply that many of the reflectors themselves may also be of caledonian age however though an age for the imposition of patterns of reflectivity can be inferred the age of an individual reflector cannot be determined unless it can be traced to outcrop or unless there is independent evidence as to the physical nature of the reflector

interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the pannonian basin summary the geologic origin of subhorizontal reflections often observed in crustal seismic sections was investigated by establishing metamorphic facies and strength of rocks in depth and correlating these properties to seismic reflection sections from eastern hungary estimation of the depths of metamorphic mineral stability zones utilized the principles developed by fyfe et al and known geothermal data of the area the strength versus depth profile was derived by relating local seismic p wave interval velocities to meissner et al s activation energy the results show that the series of subhorizontal reflections observed in the pannonian basin are a consequence of combined metamorphic and rheologic changes in depths the synthesis of the integrated data set suggests that the retrograde alteration of the pre tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water conducting capacity the subhorizontal reflections of highest energy of the consolidated crust below the percolation threshold originate in the depths of greenschist amphibolite and granulite metamorphic mineral facies which were formed in geothermal and pressure conditions similar to those existing today these results imply the overprint of earlier variscan metamorphic sequences of the crust by more recent retrograde metamorphic processes

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

on the relationships between the bushveld complex and its felsic roof rocks part 1 petrogenesis of rooiberg and related felsites a major question concerning the bushveld complex is the relationship between the layered mafic rocks and the overlying rooiberg group felsites and related granophyres here we assemble bulk rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks the data indicate that the rooiberg group consists of distinctive magnesian and ferroan lavas the former dominates the basalts to rhyolites of the basal dullstroom formation while nearly all the dacites to rhyolites of the overlying damwal kwaggasnek and schrikkloof formations are ferroan the ferroan rocks also include the stavoren granophyre which exists regionally as a several hundred meter thick concordant sheet between the bushveld complex and rooiberg lavas the compositions of the magnesian lavas are similar to calc alkaline granitoids found in convergent margins suggesting that the lavas could have originated in a mantle affected by previous archean subduction events that are recorded by xenoliths and inclusions in diamonds from most kaapvaal kimberlites in contrast the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings the hypothesis that these rocks are fractional crystallization products of bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations based on compositional characteristics and geologic relations the stavoren granophyre is the most likely candidate for the residual liquid that escaped from the top of the bushveld complex whether the bulk of the bushveld province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper hidden crustal magma reservoir remains unclear

the effects of three dimensional structure on two dimensional images of crustal seismic sections and on the interpretation of shear zone morphology summary crustal scale seismic images provide information on the geometry of subsurface structure in this paper we examine shear zones as they provide geometrical constraints on the evolution of the crust and as they provide pathways for the migration of mineral rich fluids from the lower crust however they typically appear in seismic images of the deep crust as laterally continuous bands of discontinuous reflections with individual reflections often having high amplitudes geological mapping of exposed shear zones show them to have a complex 3 d structure yet crustal scale seismic reflection surveys use single or at the most only a few profiles and therefore only create 2 d images of these structures the processing and imaging of the multifold common midpoint cmp data assumes that the seismic energy comes entirely from within the plane of the section in this paper we use full waveform 3 d synthetic data to consider the effects that 3 d topography on a reflector has on reflection character on a 2 d profile we base our synthetics on an observed shear zone and test models with both a single layer and anastomizing layers we show that topography on the reflector out of the plane of the section may cause spurious events both above and below the expected target depth we derive the basic understanding using a simple isotropic homogeneous model however we then demonstrate that this is a robust phenomenon and is endemic on all 2 d sections even if the overburden is not homogeneous we demonstrate that we obtain similar results with a velocity gradient or for a more extreme case with a 2 km deep basin filled with low velocity sediment for crustal scale seismic profiles in particular the effect is pervasive as neither stacking nor migration can discriminate against out of plane energy and the 2 d stack represents the 3 d seismic response of a broad swath centred on the profile however we conclude that using the modelled data it is possible to identify qualitatively where there is significant contamination from out of plane topography and show examples from a shear zone in the archaean yilgarn block in western australia

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

timing and duration of the last interglacial evidence for a restricted interval of widespread coral reef growth abstract we report new mass spectrometric u series ages for eight last interglacial fossil reefs along the continental margin of western australia corals were selected in growth position from localities that are characterized by apparently low levels of diagenesis and relative tectonic stability so that the fossil reefs provide critical information on last interglacial sea levels without requiring corrections for tectonic movements in addition we have improved the constraint on the timing of onset of reef growth by recovering drill core coral from the base of the reefs uranium and thorium isotopes were measured with high levels of precision leading to improvements in age resolution and allowing samples which have undergone diagenetic exchange of uranium and thorium to be more easily identified and discarded these data supplement our previous results for rottnest island and leander point leading to more than seventy mass spectrometric u series ages from which constraints can be placed on the timing duration and character of the last interglacial sea level highstand reliable ages show that reef growth started contemporaneously at 128 1 ka along the entire western australian coastline while relative sea levels were at least 3 m above the present level because western australia is located far from the former penultimate glacial maximum ice sheets and are not significantly effected by glacial unloading these data constrain the timing of onset of the last interglacial period to 128 1 ka assuming reef growth started soon after sea level approached interglacial levels a unique regressive reef sequence at mangrove bay constrains the timing of termination of the last interglacial period to 116 1 ka the major episode of reef building however both globally and locally along the western australian coast is restricted to a very narrow interval occurring from 128 ka and 121 ka suggesting that global ocean surface temperatures were warm and or sea levels were stable enough to allow prolific reef growth only during the earlier part of the last interglacial

ductile extensional shear zones in the lower crust of a passive margin we describe and interpret an unpublished set of ion geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the uruguayan volcanic rifted margin we distinguish two main groups of reflectors in the lowermost continental crust a first group at depths ranging from 32 km below the continent to 16 km in the continent ocean transition comprises reflectors continuous over tens of kilometers peculiarly visible near the mantle crust boundary a second group of reflectors dipping toward the ese oceanward is widely distributed in the lower crust these reflectors are slightly curved and tend to merge and become sub parallel with the first group of reflectors together they draw the pattern of thick shallow dipping top to the continent shear zones affecting the lower continental crust such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn tectonic volcanic formations sdr a major portion of the continental crust behaved in a ductile manner and recorded a component of top to the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle

stochastic modeling of the reflective lower crust petrophysical and geological evidence from the ivera zone northern italy crustal scale seismic wave equation modeling demonstrates that wavelength scale velocity fluctuations can dominate the seismic response over a wide range of offsets and that a wide range of velocity distributions qualitatively can explain deep seismic data this range can be narrowed by studying the detailed petrophysical and structural properties of middle and lower crustal exposures the ivrea zone in the southern alps is a well studied sliver of extended lower continental crust brought into its present upright position as a result of late alpine lithospheric shortening the high degree of structural and petrophysical complexity and in particular the limited exposure of the ivrea zone favor a statistical rather than deterministic approach to develop a two dimensional crustal scale seismic model for this purpose we digitized two 1 25 000 geological maps from the central ivrea zone on a 60 m grid we find that the ivrea zone is characterized by a bimodal velocity distribution and a band limited self affine structure having a fractal dimension of 2 7 a characteristic horizontal scale of 500 to 1000 m and an aspect ratio around 4 0 assuming statistical stationarity a realization of this stochastic model was used to develop synthetic seismograms typical of ivrea type lower continental crust our results indicate that 1 ivrea type lower crust can explain qualitatively commonly observed lower crustal reflectivity 2 relatively minor changes in bulk composition and or metamorphic grade may lead to dramatic changes in reflectivity which cannot be explained by current generic models on the origin of lower crustal reflections and 3 travel time interpretation of the synthetic wide angle data may lead to erroneous large scale velocity structures

moho depth determination from waveforms of microearthquakes in the west bohemia vogtland swarm area 1 the west bohemia vogtland area is known for its increased geodynamic activity with reoccurrence of intraplate earthquake swarms previous geophysical studies namely active and passive seismic investigations revealed a high velocity lower crust in this area with increased reflectivity to refine this result and retrieve a more detailed structure of the deep crust and the moho discontinuity we analyzed waveforms of local microearthquakes that occurred in this area during the 2008 swarm the waveforms of earthquakes were grouped into clusters with similar focal mechanisms and the clusters were processed separately we developed a new multiazimuthal approach in data processing to increase resolution of moho phases in the waveforms we applied the waveform cross correlation of the p and s waves and rotated aligned and stacked the seismograms to extract the moho sms pmp and pms reflected converted phases these phases were inverted for laterally varying moho depth by ray tracing and a grid search inversion algorithm the model retrieved was verified using modeling of full waveforms computed by the discrete wave number method the multiazimuthal approach reveals details in the velocity structure of the crust mantle transition at each station instead of a single interface with a sharp velocity contrast the inversion indicates a reflective zone at moho depths with one or two strongly reflective interfaces which is in agreement with the zone interpreted by previous investigations the thickness of the zone varies from 2 to 4 km within the depth range of 27 31 5 km and is delimited by reflections from its top and bottom boundaries sometimes with strong reflectors within the zone the average vp vs ratio determined from the moho reflections and conversions is 1 73

long listening multichannel seismic profiles in the valencia trough valsis 2 and the gulf of lions ecors a comparison abstract the long listening multichannel seismic mcs profiles recorded during the valsis and ecors experiments were analysed in order to understand the rifting processes of a young passive margin the seismic lines were interpreted within a geological framework which is considerably improved by the results from industrial wells the disturbed upper crust off catalonia is tentatively connected with a variscan front the origin of lamellae in the lower crust beneath the valencia shelf is discussed a variscan origin is rejected and a relation to a jurassic extensional event is proposed a companion basin to the onshore maestrazgo basin is located off shore of valencia and these two basins are probably connected by the left lateral strike slip castellon fault this fault is parallel to the north pyrenean fault which is the trace of the rotation of spain relative to europe the castellon fault may be connected with the asturian fault which is located on the atlantic side of spain a contour map down to the moho has revealed some new deep features and the results are compared with the previous refraction results the sedimentary column of the deep north balearic provencal basin is much thicker than that of the eastern part of the valencia trough and this diversity in the sedimentary infilling is probably related to a difference in the subsidence processes a comparison of the valencia trough to the gulf of lions shows that the thinning of the continental crust looks similar in both regions but the crust is thinner beneath the valencia continental shelf than that below the gulf of lions in both regions the moho rises possibly at the expense of the continental crust by a process as yet unexplained petrological or and thermal variation

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

rapid fluctuations in sea level recorded at huon peninsula during the penultimate deglaciation about 140 000 years ago the breakup of large continental ice sheets initiated the last interglacial period sea level rose and peaked around 135 000 years ago about 14 meters below present levels a record of last interglacial sea levels between 116 000 years to 136 000 years ago is preserved at reef vii of the uplifted coral terraces of huon peninsula in papua new guinea however corals from a cave situated about 90 meters below the crest of reef vii are 130 000 2000 years old and appear to have grown in conditions that were 6 c cooler than those at present these observations imply a drop in sea level of 60 to 80 meters after 130 000 years sea level began rising again in response to the major insolation maximum at 126 000 to 128 000 years ago the early about 140 000 years ago start of the penultimate deglaciation well before the peak in insolation is consistent with the devils hole chronology

growth of the maritime continent and its possible contribution to recurring ice ages the areal extent of the maritime continent the islands of indonesia and surrounding region has grown larger by 60 since 5 ma we argue that this growth might have altered global climate in two ways that would have contributed to making recurring ice ages possible first because rainfall over the islands of the maritime continent not only is heavier than that over the adjacent ocean but also correlates with the strength of the walker circulation the growth of the maritime continent since 5 ma may have contributed to the cooling of the eastern tropical pacific since that time scaling relationships between the strength of the walker circulation and rainfall over the islands of the maritime continent and between sea surface temperature sst of the eastern tropical pacific and the strength of easterly wind stress suggest that the increase in areal extent of islands would lead to a drop in that sst of 0 75 c although only a fraction of the 3 4 c decrease in ssts between the eastern and western tropical pacific the growth of the maritime continent may have strengthened the walker circulation increased the east west temperature gradient across the pacific and thereby enabled ice sheets to wax and wane over canada since 3 ma second because the weathering of basaltic rock under warm moist conditions extracts co2 from the atmosphere more rapidly than weathering of other rock or of basalt under cooler or drier conditions the increase in weathering due to increasing area of basalt in the maritime continent may have drawn down enough co2 from the atmosphere to affect global temperatures simple calculations suggest that increased weathering of basalt might have lowered global temperatures by 0 25 c possibly important for the overall cooling

collision propagation in papua new guinea and the solomon sea the mountain system in papua new guinea png is growing by accretion of australian margin strata to the front of the papuan fold and thrust belt in the south and by accretion of exotic terranes along its ne margin presently the finisterre terrane is being accreted along the ne margin of png and the collision point is migrating eastward within the solomon sea east of the collision point the new britain trench marks the tectonic front between the solomon sea and south bismarck sea plates whereas to the west the tectonic front lies in the southern part of the papuan fold and thrust belt present day sedimentary environments within the western solomon sea are inferred from interpretation of seismic reflection data similar facies are observed accreted to the front of the finisterre terrane in the markham valley the frontal leron formation is largely fluvial and shallow marine and the erap complex contains elements of turbidites and pelagic deposits there appears to be a smaller volume of coarse proximal marine strata accreted to the front of the finisterre terrane than is expected from interpretation of reflection profiles in the western solomon sea this difference may be attributed to a combination of submarine erosion in the markham canyon partial incorporation of these deposits into the erap complex and a difference in depositional rates and products carried to the solomon sea in the past relative to what is observed today based on plate motions alternative collision geometries and the youngest dates available for the erap complex we determine a rate of progression of the collision point of about 212 km m y during the past 1 m y

orientation of nucleating faults in anisotropic media insights from three dimensional deformation experiments abstract physical experiments have been undertaken to investigate the influence of planar rheological anisotropy on the orientation of fault structures in three dimensional flow associated with layer normal shortening the experimental results indicate that plane strain is a special case characterised by the lowest angle between the faults and the principal extension direction for any combination of rheology and flow type during progressive pure shear a model is proposed wherein the role of the anisotropy appears to be two fold first it determines that shear bands operate internally by a mechanism involving rotating slip surfaces during fault nucleation the initial orientation of the faults is determined by the optimisation of the driving processes in the propagating shear bands as a partial function of the angle they make with the external anisotropy second it induces directionally dependent variation in the resistance to flow which manifests itself differently according to the symmetry of the strain ellipsoid in three dimensions x y 1 z in flattening flow it is proposed that anisotropy induced geometrical hardening leads to an increase in the angles made by the shear bands with the principal extension directions of the deformation in constriction the same effect is seen in xz sections but in yz sections it is suggested that the wide range of fault orientations observed is due to anisotropy induced geometrical softening it is possible that the low angle of dip of crustal scale extensional detachment faults may reflect anisotropic behaviour of the lower continental crust

evidence of holocene uplift in east new britain papua new guinea 1 we present the first published evidence of uplift along the eastern coast of new britain papua new guinea based upon 230th dating of uplifted holocene coral reefs although uplift rates of the nearby huon peninsula are well constrained 2 3 m ka little is known about vertical motion along the eastern margin of new britain based upon relative sea level curves for papua new guinea present sample elevations and estimates of paleowater depth of four in situ holocene corals we calculate an average uplift rate of 1 6 0 4 m ka for the southeast coast of new britain we suggest underplating of subducted sediment as a possible mechanism for uplift of the reefs

coral indicators of past sea level change a global repository of u series dated benchmarks fossil corals provide valuable data for reconstructing past sea levels as they are often well preserved in the fossil record and can be dated with u series methods here we present a global and internally consistent database of usingle bondth dated fossil coral sea level indicators including full consideration of all known associated uncertainties both vertical and chronological we include carefully determined taxon specific depth distributions rather than blanket depth uncertainty terms as used in most previous work this is based on a synthesis of extensive modern ecological information on depth ranges these ranges are found to be spatially variable between ocean basins between regions and on sub regional scales because depth itself is not limiting instead depth distributions arise from complex physical chemical and biological interactions with coral reef growth distribution and composition one of the main causes for recognition of the greater depth variability of coral taxa has been the routine inclusion of deep diving and rov surveys in coral ecological studies over the past few decades which has broken through the shallow water bias of early surveys by adding frequent observations on deeper occurrences although more are needed it is also clear from our assessment that coral habitat depth distributions must be determined on the species level to reduce uncertainties in reconstructions of past sea levels and that application to sea level studies then requires these studies also to identify fossil corals to the species level samples identified only to the genus level give rise to wide uncertainties in habitat depth and hence sea level our database contains extensive metadata to assist evaluations of dating quality as well as geomorphic and stratigraphic metadata we demonstrate with examples how such metadata can help to evaluate sea level reconstructions for example by identifying outlier points one example discusses the last interglacial lig where we use the available data with their uncertainties to assess probabilistically the time at which local sea levels exceed that of the present which yields a mean age of 124 6 ka with 95 probability bounds at 118 5 and 129 5 ka we conclude with identification of key outstanding issues relating to i current incomplete understanding of tectonic setting including the current lack of independent verification of uplift subsidence rates and reliance of somewhat unsatisfactory and circular use of the elevation of last interglacial deposits ii the depth distributions of coral taxa and iii the complete documentation of stratigraphic geomorphological and other contextual information with suggestions for strategies to address these issues

pleistocene sea levels and tectonic uplift based on dating of corals from sumba island indonesia we collected and dated fossil corals from the uplifted reef tracts of sumba island mass spectrometry tims and ams was used to obtain reliable and precise radiochronological results it is possible to identify outcrops corresponding to major sea level high stands isotope stage 1 starting at 7 1 kyr bp 5a c a 86 kyr bp 5c c a 105 kyr bp 5e between 119 and 132 kyr bp and 9 c a 305 kyr bp the radiochronological data are thus broadly consistent with the timing of sea level fluctuations predicted by the astronomical theory of paleoclimates comparison with similar data from other terrace sequences and from benthic 18o records indicates that the average tectonic uplift rate at cape laundi sumba is on the order of 0 2 0 5m kyr the lowest values 0 3m kyr lead to the build up of polycyclic topographic terraces in which it is possible to find corals grown during different interstadial or even interglacial times

amplitude effects due to multiscale impedance contrasts and multiple scattering implications for ivrea type continental lower crust summary studying seismic wave propagation in non deterministic media is interesting as some parts of the earth e g in the continental lower crust may be amenable to a stochastic description in this paper we study the acoustic response of a 1 d binary stochastic fractal medium and separate the single from the multiple scattered wavefield we show that for our models with scales that partly overlap the seismic bandwidth amplitude changes occur due to both the presence of multiple scattering and the multiscale nature of the impedance contrasts we study these changes as functions of sc a measure of the nominal bimodal reflection coefficient ka a measure of the relative length scales of the medium compared to the wavelength of the probing waves and c the scatterer density as our models are binary two lithologies the results should be interpreted as an upper bound for the general class of acoustic 1 d stochastic fractal model responses we present a measure to quantify the interference of single and multiple scattered wavefields and use this measure to study the importance of the three aforementioned dependences since stochastic fractal media have been proposed for ivrea type lower crust we relate our results to such a model and find that amplitudes may increase by up to 45t15 per cent the contribution of multiple scattering to this increase ranges from 1t 1t o 20t15 per cent the main contribution being due to the multiscale nature of the impedance contrasts we claim that the parameters used to model ivrea type lower crust sc 5 per cent c 5r10 x2 1 jkaj24 and m 9 6r10 x1 are robust although we recognize that the scatterer density c is determined by the vertical sampling dz in our models the heuristic argument that extra geometrical spreading in two three dimensions will further diminish the importance of multiple scattering effects combined with our results makes it tempting to claim that multiple scattering is not important for 2 3 d generalizations of our 1 d models however in the light of indirect evidence for the importance of multiple scattering in ivrea type lower crust reported by other authors we argue that elasticity p s conversions may be important therefore we believe that it is necessary to study the 2 d case with a similar direct approach to that presented in this paper before a satisfactory conclusion can be drawn about the validity of the standard processing of deep seismic reflection data and the importance of multiple scattering

rapid sea level fall and deep ocean temperature change since the last interglacial period we have dated huon peninsula papua new guinea and barbados corals that formed at times since the last interglacial period applying both 230 th and 231 pa dating techniques as a test of age accuracy we show that marine isotope stage mis 5e ended prior to 113 1 8 0 7 kyr when sea level was 319 m during mis 5b sea level was 357 m at 92 6 8 0 5 kyr having dropped about 40 m in approximately 10 kyr during the mis 5c 5b transition sea level then rose more than 40 m during the mis 5b 5a transition also in about 10 kyr mis 5a lasted until at least 76 2 8 0 4 kyr at a level of 324 m at that time combined with earlier data that places mis 4 sea level at 381 m at 70 8 kyr our late mis 5a data indicate that sea level fell almost 60 m in less than 6 kyr 10 6 m kyr during the mis 5 4 transition the magnitude of the drop is half that of the glacial interglacial amplitude and approximatelyequivalent to the volume of the present dayantarctic ice sheet during this interval the minimum average rate of net continental ice accumulation was 18 cm yr likely facilitated by efficient moisture transport from lower latitudes at three specific times 60 6 8 0 3 50 8 8 0 3 and 36 8 0 2 kyr during mis 3 sea level was between 385 and 374 m sea level then dropped to 3107 m at 23 7 8 0 1 kyr early in mis 2 before dropping further to last glacial maximum lgm values and then rising to present values during the last deglaciation times of rapid sea level drop correspond to times of high winter insolation at low northern latitudes and high winter latitudinal gradients in northern hemisphere insolation supporting the idea that these factors mayhave resulted in high water vapor pressure in moisture sources and efficient moisture transport to high latitude glaciers therebycontributing to glacial buildup we combined our sea level results with deep sea n 18 o records as a means of estimating the temperature and ice volume components in the marine n 18 o record this analysis confirms large deep ocean temperature shifts following mis 5e and during termination i deep ocean temperatures changed bymuch smaller amounts between mis 5c and 2 maximum temperature shift in the deep pacific is about 2 whereas the shift at a site in the atlantic is 4 under glacial conditions temperatures at both sites are near the freezing point the shift in the atlantic is likelycaused bya combination of changing proportions of northern and southern source waters as well as changing temperature at the sites where these deep waters form

tectonic controls on facies transitions in an oblique collision the western solomon sea papua new guinea the western solomon sea is the site of a closing ocean basin and an incipient arc continent collision between the bismarck arc and the australian continental margin in papua new guinea migrated seismic reflection profiles and hawaii mr1 sidescan sonar data indicate that sedimentation within the solomon sea basin is controlled by topographic gradients generated by flexure of the solomon sea plate turbidites delivered to the basin by the submarine markham canyon extend farther eastward down the axis of the deeper new britain trench north side of the solomon sea than they do in the shallower trobriand trough south side of the solomon sea the stratigraphic record of the foredeep in the zone of arc continent collision is controlled by the steep topography of the australian continental margin a long 1 5 3 m y period of deep marine turbidite deposition is followed by a short 50 100 k y period of shallow marine deposition and a long 0 5 1 m y period of fluvial deposition comparisons between the foredeep record of taiwan and the papua new guinea collision indicate that the steep topography of the australian continental margin exerts significant control over the evolution of the foredeep and the taiwan foredeep is more controlled by the dynamic link between the flexural properties of the lithosphere and the orogenic load

dipping seismic reflectors electrically conductive zones and trapped water in the crust over a subducting plate the lithoprobe program across the northern cascadia subduction zone at vancouver island has obtained geophysical data that suggest a dipping zone of trapped free pore water in the overlying continental crust land and marine multichannel seismic reflection profiles show several bands of reflectors the primary one at a depth of about 30 km is about 5 km thick dips inland and appears to coincide with an electrically conductive layer defined by magnetotelluric data the downgoing oceanic crust is imaged as a thin reflector dipping at about 15 and 10 km deeper than the primary reflector a location that is consistent with seismicity and seismic refraction data both the reflectors and the conductor have been modeled by near horizontal lamellae with 1 4 saline water porosity detailed geothermal measurements defining heat flow that decreases inland indicate that the dipping bands are nearly isothermal at a temperature of 400 500 c it is suggested that free water generated by dehydration of the downgoing oceanic plate migrates upward until reduced temperature results in hydration reactions and mineral precipitation that forms an impermeable barrier low vertical permeability is required to keep the water from separating to the top of the layer this can be reconciled with the low electrical resistivity if pore interconnection is through thin grain boundary tubes as predicted by theoretical equilibrium pore geometry although very small effective grain size is necessary an abrupt transition from high permeability to very low permeability is predicted at a several percent porosity in agreement with the modeled porosity such pore geometry with low permeability may be restricted to temperatures above about 400 c i e in greenschist to amphibolite facies conditions where grains will deform to equilibrium geometry and cracks are annealed the region above the reflective and conductive band is estimated to be within blueschist conditions consistent with the inferred high seismic velocity from refraction data and high density from gravity data

reflection refraction seismics in the gulf of corinth hints at deep structure and control of the deep marine basin abstract the gulf of corinth is a natural laboratory for the study of seismicity and crustal deformation during continental extension seismic profiling along its axis provides a 24 fold normal incidence seismic reflection profile and wide angle reflection refraction profiles recorded by sea bottom seismometers obs and land seismometers at wide angle incidence the land receivers document the moho at 40 km depth under the western end of the gulf north of aigion rising to 32 km depth under the northern coast in the east of the gulf both refraction and normal incidence reflection sections image the basement under the deep marine basin that has formed by recent extension the depth to the base of the sedimentary basin beneath the gulf constrained by both methods is no more than 2 7 km with 1 km of water underlain by no more than 1 7 km of sediment less than what was expected from past modeling of uplift of the south coast in the east of the gulf unlike the flat sea bottom the basement and sedimentary interfaces show topography along this axial line several deeps are identified as depocenters which suggest that this axial line is not a strike line to the basin it appears instead to be controlled by several faults oblique to the s60 e overall trend of the south coast of the gulf their more easterly strikes being consistent with the instantaneous direction of extension measured by earthquake slip vectors and by gps

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