amphibolites of the ashe and alligator back formations north carolina samples of late proterozoic early paleozoic oceanic crust the amphibolites of the late proterozoic early paleozoic ashe and alligator back formations in western north carolina represent metamorphosed tholeiitic basalts unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the grenville age basement rocks on the basis of their geochemistry the amphibolites are divisible into three compositional groups i ii and iii the protoliths of which appear to have evolved from different parental magmas at a spreading center group ii intermediate ti amphibolite the predominant variety in the study area constitutes a highly fractionated suite zr 40 200 ppm tio2 0 7 2 3 wt with geochemical characteristics similar to those of n type morb formed from a heterogeneous mantle source group iii high ti amphibolite is enriched in high field strength incompatible elements zr nb ti y ree s especially ti tio2 2 8 3 4 wt and is interpreted as t type morb on the basis of its zr nb and y nb ratios group i low ti amphibolite restricted to the ashe formation is characterized by extremely low abundances of incompatible elements including ti tio2 0 45 wt and u shaped ree profiles requiring the mixing of a depleted morb type mantle source with a fluid phase enriched in lree s the juxtaposition of depleted low ti basalt group i amphibolite and morb like basalts group ii and group iii amphibolites is suggestive of a back arc basin setting as has been postulated for many ophiolites but it can also be accounted for by multi stage melting of an upper mantle source adjacent to a mantle plume in a mid oceanic ridge environment in either case the ashe and alligator back amphibolites represent metamorphosed oceanic crust material generated at a spreading center

reverse time migration from irregular surface by flattening surface topography abstract prestack reverse time migration rtm is a powerful tool to provide high resolution structure images of crust and upper mantle in wide angle seismic experiments as a standard migration algorithm assumes that data are recorded on a flat surface rtm meets severe challenge on the issue of complicated topography where many seismic surveys are carried out up till now there are two popular techniques for handling elevation changes i e elevation static correction or time shift and wave equation datuming numerous studies demonstrate that the elevation static correction is inaccurate for non vertically traveling energy and subsequently may migrate reflectors to incorrect positions wave equation datuming is more accurate but more sophisticated and too computationally demanding for routine use here we present an alternative scheme to deal with topography in prestack rtm that can migrate seismic data acquired in areas with irregular topography to the reflector positions directly without datuming or elevation static corrections in this scheme surface topography is flattened by a transformation of coordinates from cartesian to curvilinear such a transformation is equivalent to map a rectangular grid onto a curved one and is implemented by the introduction of boundary conforming grid then we use an explicit finite difference scheme with second order accuracy to discretize the elastic wave equations and perform the rtm in the curvilinear coordinate system moreover a flux corrected transport fct algorithm adapted from hydrodynamics is incorporated in the rtm implementation which can not only overcome problems of numerical dispersion that arise in finite difference algorithms but also offer the opportunity to use a coarse grid to obtain the same accuracy as conventional finite difference methods on a dense grid the fctrtm method presented here has no limitations on surface irregularity complexity of the subsurface structure or velocity function extensive numerical examples demonstrate the feasibility and effectiveness of our method

reverse time migration of offset vertical seismic profiling data using the excitation time imaging condition to apply reverse time migration to prestack finite offset data from variable velocity media the standard time zero imaging condition must be generalized because each point in the image space has a different image time or times this generalization is the excitation time imaging condition in which each point is imaged at the one way traveltime from the source to that point reverse time migration with the excitation time imaging condition consists of three elements 1 computation of the imaging condition 2 extrapolation of the recorder wave field and 3 application of the imaging condition computation of the imaging condition for each point in the image is done by ray tracing from the source point this is equivalent to extrapolation of the source wave field through the medium extrapolation of the recorded wave field is done by an acoustic finite difference algorithm imaging is performed at each step of the finite difference extrapolation by extracting from the propagating wave field the

reverse time migration a prospect of seismic imaging methodology abstract reverse time migration rtm is a seismic imaging method to map the subsurface reflectivity using recorded seismic waveforms the practice in exploration seismology has long established a two fold approach of seismic imaging using velocity modeling building to establish the long wavelength reference velocity models and using seismic migration to map the short wavelength reflectivity structures among various seismic migration methods for different situations rtm is the only method that is capable to use all seismic wave types that can be computed numerically being initiated in early 1980 s rtm seeks an image of the subsurface reflectivity as the best match in an image space between the extrapolation of time reversed waveform data and the prediction based on estimated velocity model and source parameters judging the image quality in the same space of forming the images is more advantageous than the approaches of modeling and inversion which seek the solution in the model space but judge its fitness in data space considering that most seismic migration applications today still use primary reflection as the only signal the capability of rtm to use all computable wave types is unique and helpful reducing the imaging artifacts due to mistaking non primary waves as primary reflections hence we refer to those rtm algorithms using only primary reflections as the first generation rtm methods and the rtm algorithms making a full use of primary reflections multiple reflections and other non primary waveform data as the second generation rtm methods this paper reviews the development history of the rtm along with its major challenges current solutions and future perspectives

suppressing unwanted internal reflections in prestack reverse time migration prestack reverse time migration a wave equation technique using two way propagation correctly handles multiarrivals and enables imaging of overturned reflections however image artifacts occur when backscattered waves cross correlate these artifacts are particularly strong where high velocity contrasts occur a method for removing unwanted internal reflections during propagation of both the source and receiver wavefields is presented this method applies a directional damping term to the wave equation in areas of the velocity model where unwanted reflections occur tests on synthetic data show good suppression of image artifacts

plutonism in three orogenic pulses eastern blue ridge province southern appalachians department of geological sciences university of north carolina mitchell hall chapel hill nc 27599 3315

evidence for the younger dryas climatic event in southeast asia abstract this article reviews evidence for the younger dryas climatic event from the limited data available from mainland and island southeast asia and from deep sea cores from the region data from highland indonesia and regional deep sea cores suggest a climatic reversal at the time of the younger dryas may be more widely detectable with finer resolution pollen analysis and additional radiocarbon dating but it should be remembered that dates of this age cannot be calibrated locations are suggested in the region where more sites and finer resolution analysis is needed before definite statements on the younger dryas can be made at present the best evidence for its occurrence comes from one sumatran pollen diagram but even that is very tentative

investigating the history of east asian monsoon and climate during the last glacial interglacial period 0 140 000 years mineralogy and geochemistry of odp sites 1143 and 1144 south china sea abstract monsoon climate is an important component of the global climatic system a comprehensive understanding of its variability over glacial interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate continental weathering and productivity a detailed multiproxy study including bulk and clay mineralogy grain size analysis phosphorus geochemistry sedex extraction organic matter characterization and nitrogen stable isotopes was carried out on samples from odp sites 1143 and 1144 leg 184 south china sea covering the past 140 000 years we tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial interglacial cycle when sea level variations linked to the growth and melting of ice caps interact with monsoon variability during interglacial periods of high sea level summer monsoon was strong and humid and warm climate characterized the adjacent continent and islands clay minerals bear signals of chemical weathering during these intervals high calcite and reactive phosphorus mass accumulation rates mars indicate high productivity especially in the southern region of the basin during glacial intervals strong winter monsoon provided enhanced detrital input from the continent as indicated by high detrital mar glacial low sea level resulted in erosion of sediments from the exposed sunda shelf to the south and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas enhanced supply of nutrients from the continent both by river and eolian input maintained high primary productivity reduced circulation during these periods possibly induced active remobilization of nutrients such as phosphorus from the sediments intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in china and marine climatic records in the north atlantic confirming a teleconnection between low and high latitude climate variability

late pleistocene and holocene development of the interior peat accumulating basin of tropical tasek bera peninsular malaysia the stratigraphy of holocene peat deposits from an interior lowland basin of tropical peninsular malaysia provides insight into the hydrological ecological and climatic controls on tropical peatland development interpretations about vegetation and climate dynamics including storm intensity over the past 5000 years are integrated with regional and global climatic and eustatic changes to serve as a proxy for past ecology and climate tasek bera basin tbb is a lowland dendritic basin in tropical peninsular malaysia accumulation of organic matter occurred in local lakes during the last glacial maximum lgm but widespread peat deposition did not start until 5300 bp when climatic changes led to the evolution of a wetland system peat accumulation progressively expanded with terrestrialisation of channels and subbasins to paludification of the riparian part of the lowland forest zone lithofacies can be distinguished despite the highly diverse peat and organic rich sediment composition as a result of the climatic and hydrologic changes over the past 5300 years peat accumulation rates ranging from 0 7 to 2 5 mm year are highest in pandanaceae environments and lowest in high ash swamp forests and environments dominated by cyperaceae transects of peat cores across the mire system show the stratigraphic complexities of the peat is the result of hydrological variations and thus plant successional changes ash yield shows that periods of high mineral matter input into the drainage system occurred since the mid holocene indicating rapid and cyclic changes in frequency and magnitude of runoff events and by inference storms stratigraphic analysis of peat and ash yield are tools that provide a proxy for development of holocene hydrologic events in topogenous peat deposits

3d reverse time migration using the acoustic wave equation an experience with the seg eage data set kirchhoff is the most commonly used 3d prestack migration algorithm because of its speed and other economic advantages but it uses a high frequency ray approximation to the wave equation and therefore has difficulties in imaging complex geologic structures where multipathing occurs e g beneath rugose horizons such as faulted salt domes where traveltime calculations become difficult

late quaternary palaeoecology palynology and palaeolimnology of a tropical lowland swamp rawa danau west java indonesia abstract sedimentological limnological and palynological analyses of a sediment core from a lowland site in west java indonesia provide a detailed palaeoenvironmental record for the late glacial and the holocene the record suggests open vegetation under inferred drier climatic conditions for the late glacial however there is no unequivocal evidence for cooler conditions at this time the onset of the holocene coincides with a change to more humid climatic conditions with the development of a fern rich closed forest vegetation type dramatic changes in diatom community composition provide a striking record of habitat change associated with lake shallowing but this process appears to be a result of basin in filling rather than variations in precipitation evaporation balance associated with climatic fluctuations evidence for human impact on the vegetation development is restricted to the last few hundred years

estimated source wavelet incorporated reverse time migration with a virtual source imaging condition manygeophysicistsperformreverse timemigrationusingavarietyofartificialsources to obtain the source wavefield upon processing the seismic data however it is difficult to recover the original phase and amplitude of the source wavelet used for seismic exploration regardless of the source we have therefore used several artificial source wavelets such as ricker or the first derivative gauss wavelets expressed by well known functions there are some differences between these artificial source wavelets and the original source wavelets resulting in imperfect migration images artificial source wavelets tend to distort the exact location of subsurface reflectors and they create noise around the boundary of the stratum to solve this problem we applied the source estimation technique to the reverse time migration algorithm the source estimation technique approximates the source wavelet to the original exploration source wavelet by a deconvolution method this technique is used in full waveform inversion and provides better inversion results as demonstrated by other studies toprovetheeffectofreverse timemigrationwithsourceestimation wetested this algorithm on the sigsbee2a model seg eage 3d salt model and 3d real field land data using the resulting images of these three models we found that the source estimation technique can yield better migration images to suppress the artefacts produced in the migration image we used a wavenumber filter and laplacian filter on 2d and 3d examples respectively furthermore we used the pseudo hessian similar to the source illumination to scale the migration image because the virtual source imaging condition was used for reverse time migration

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

retrograde metamorphism of eclogite in the southern appalachian mountains u s a a case involving seamount subduction this work supports a growing body of evidence that the ashe metamorphic suite ams of the eastern blue ridge province in north carolina has an ensimatic origin and is part of a subduction related accretionary melange marking the taconic suture between the north american craton and the inner piedmont in a palinspastic reconstruction the thrust fault at the base of the ams appears to have intercepted the greatest depths i e highest p metamorphic rocks beneath parts of the ams now exposed adjacent to the grandfather mountain window the greatest volume of mafic rock is found in these same areas we suggest that the nascent subduction related basal thrust fault was deflected downward by an obstacle in the form of an isolated mafic volcanic edifice on the oceanic crust a sea mount pelitic and mafic rocks dominate the ams north of the grandfather mountain window retrograded eclogite occurs in the amphibolite near the base of the ams textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses a eclogite i facies omphacite garnet quartz b eclogite ii facies omphacite garnet epidote quartz c symplectic diopside plagioclase garnet epidote quartz d amphibolite facies diopside plagioclase garnet epidote hornblende quartz e amphibolite facies plagioclase garnet epidote hornblende quartz p t conditions estimated from geothermobarometry applied to relevant mineral compositions are c 720 c and c 16 kbar for b eclogite ii facies c 655 c and c 8 5 kbar for e amphibolite facies

3d acoustic prestack reverse time migration1 a prestack reverse time migration algorithm which operates on common source gathers recorded at the earth s surface from 3d structures is conceived implemented and tested reverse time extrapolation of the recorded wavefield a boundary value problem and computation of the excitation time imaging condition for each point in a 3d volume an initial value problem are both performed using a second order finite difference solution of the full 3d scalar wave equation the algorithm is illustrated by processing synthetic data for a point diffractor an oblique wedge and the french double dome and fault model