seismic constraints on a remnant mesozoic forearc basin in the northeastern south china sea abstract the northeastern of the south china sea scs has undergone tectonic inversion from a mesozoic convergent continental margin to a cenozoic divergent continental margin the remnant subduction related structures such as volcanic arc and forearc basin likely preserve key clues to reveal the dynamic mechanism of this tectonic inversion we applied ocean bottom seismometers obs data combining with multi channel seismic data gravity data drilling data and petrologic observations to reveal geophysical and structural features of the dongsha basin a mesozoic basin in the northeastern scs the mesozoic strata are characterized by large thickness 1 8 km relatively high p wave velocities 3 5 5 5 km s high densities 2 55 2 65 g cm3 alternating folds and thrust faults which are significantly different from the features of cenozoic strata the dongsha basin is flanked by a mesozoic granitic arc to the northwest and an inferred palaeo pacific subduction zone to the southeast indicating that it is geographically located in the mesozoic forearc region considering the similar mesozoic strata detected in the southern margin of the scs and the contemporaneous accretionary complex outcrops observed in the palawan continental terrane we propose that the mesozoic strata in these two regions originated from a common mesozoic forearc basin which was broken by the forearc rifting and subsequently separated by the spreading of cenozoic scs accordingly a continuous palaeo pacific subduction system is reconstructed including the arc forearc accretionary wedge tectonic sequences reviewing the mechanisms of forearc rifting globally and the retreat of volcanic arcs onshore south china we speculate that rollback of the palaeo pacific subduction slab might be one of the causes for the forearc rifting and the initial extension of the scs

curie point depths in northeast china and their geothermal implications for the songliao basin abstract this work integrates processing and interpretation of the combined geomagnetic anomaly model of ngdc 720 v3 1 to better constrain thermal structures of the northeast china lithosphere which shows intensive extension and active volcanism during the mesozoic and cenozoic numerical tests demonstrate that magnetic anomalies calculated at 20 km altitude can be used for accurate curie depth estimation averaged curie depths using a 3d fractal exponent of 3 0 vary from 13 2 0 to 33 4 1 km with a mean of 22 2 4 km in northeast china with temperature dependent thermal conductivity models constrained by incorporated surface heat flow and curie depths in different geological blocks the thermal lithospheric thicknesses are estimated mostly between 45 and 90 km based on 1d steady state thermal conduction equation beneath the songliao basin the moho temperatures are abnormally high ranging from ca 700 to 1000 c and the mantle contributes about 65 of the total surface heat flow the shallow curie points and thermal lithosphere asthenosphere boundary together with high moho temperatures and mantle heat flow support abnormally high geotherms in the songliao basin originated from hot upper mantle upwelling triggered likely by lithospheric delamination

global vector and scalar magsat magnetic anomaly maps empirical and analytical techniques for modeling ionospheric fields in magsat data have been developed that facilitate ionospheric field removal from uncorrected anomalies to obtain better estimates of regional lithospheric anomalies this task has been accomplished for equatorial x z and b component and polar z and b component measurements the techniques for modeling ionospheric fields have been integrated into a processing sequence that incorporates some of the important data processing techniques developed during the last decade data processing techniques include retention of common signal in a correlation analysis of adjacent passes analysis of field differences between dawn and dusk data at points where their orbits cross and retention of common signal in a covariant spherical harmonic analysis procedure results suggest that implementation of the above processing scheme leads to the mapping of the most robust magnetic anomalies of the lithosphere vector components as well as scalar

late mesozoic gravity sliding and oxfordian hydrocarbon reservoir potential of the northern yucatan margin abstract the stratigraphy and structural evolution of the western florida margin is much better understood than its northern yucatan conjugate because previous hydrocarbon exploration has been more extensive on the florida margin in the deep water northeastern gom the post salt section records late jurassic cretaceous gravity sliding of rafted blocks along a basinward dipping layer of callovian salt this study uses approximately 31 000 line km of 2d seismic data to describe a coeval area of widespread gravity sliding along the less studied northern yucatan margin we define three structural domains based on their distinctive salt structures and associated deformation 1 a region in the northeastern study area consists of relatively undeformed late jurassic cretaceous section underlain by 0 300 m thick salt 2 areas in the central and southwestern study area contain late mesozoic gravity slides defined by 300 600 m thick salt rollers and normal faults rooted onto a 1 4 basinward dipping salt detachment and 3 the distal margin of the western and central study area exhibits large salt diapirs up to 6 km tall that deform quaternary stratigraphic units in the central yucatan study area containing late mesozoic gravity slides a stratigraphic unit equivalent to the productive oxfordian norphlet sandstone of the deep water northeastern gom is identified based on its similar seismic character reconstructing the extent of this norphlet equivalent unit along the yucatan margin to its location during oxfordian deposition places it adjacent to an extensive area of norphlet sandstone mapped in the deep water northeastern gom by previous authors our reconstruction suggests a 48 000 km2 fairway of potential aeolian late jurassic reservoirs on the yucatan and campeche margins that includes productive reservoirs in the southeastern bay of campeche which previous authors correlated with the norphlet formation

depth to curie temperature or bottom of the magnetic sources in the volcanic zone of la reunion hot spot we present an innovative study to generalize curie point depth cpd determinations at the scale of oceanic volcanic islands an approach which has previously focused largely on continental areas in order to determine the validity of this technique in oceanic environments we first tested the approach on sets of sea floor spreading anomalies assuming that magnetic anomalies are concentrated within the oceanic crust and uppermost mantle the curie depth should deepen as oceanic lithosphere increases in age and thickness away from spreading centers the calculated depths to the magnetic bottom are in agreement with this general pattern on the basis of this test we then applied the method to la reunion island and surrounding oceanic lithosphere the calculated extent of magnetic sources lies at depths between 10 and 30 km and exhibits a complex topography presumably caused by a combination of various magmatic and tectonic lithospheric structures these calculations indicate that magnetic sources extend well below the crust mantle interface at this location to the first order the bottom of the magnetic surface shallows beneath reunion and mauritius islands due to the thermal effect of the hot spot and deepens away from la reunion edifice on the scale of the mascarene basin several discontinuities in the cpd correlate well with major fracture zones

repeat ridge jumps and microcontinent separation insights from ne arabian sea abstract microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume ridge interactions india separated from seychelles at 64 ma by asymmetric sea floor spreading initially when the spreading centre in the mascarene basin jumped towards the indian sub continent between magnetic chrons c29 and c28 the subsequent tectonics is difficult to comprehend since laxmi ridge another microcontinent formed during the later phase most of the studies considered the laxmi ridge as a sliver others considered it to be oceanic crust high resolution deep 25 km seismic data reveals that i the ridge possesses 15 km deep sea ward dipping reflector sdr packages ii normal faulted rift valleys devoid of syn rift sedimentary packages and iii axial magma chambers 5 7 km beneath the ridge top additionally from 2d forward gravity models we deduce that the ridge most possibly comprises of high density oceanic crust we conclude the laxmi ridge to be indeed composed of oceanic crust and a fossil spreading centre we thus identified the ridge jumps and their relation to the seychelles microcontinent separation previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating spreading rate at the ridge and plate ages for repeated ridge jumps the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating we find that the medium spreading rates and high magmatic heating due to the reunion plume and young plates favoured rapid and repeated ridge jumps towards the plume

reply to a s dyke s discussion abstract in our opinion the amino acid data are consistent with the till nonglacial stratigraphy we reject dyke s proposal that the plotting of data in 0 04 increments is appropriate as an unwarranted interpretation that errors are cumulative we also see no grounds for accepting his alternative interpretation that the groups of amino acid ratios reflect various transport read temperature histories of a single population of bell sea shells it is our opinion that the relative sequence of marine incursions in hudson bay is reliable and we repeat that the evidence favors one or more deglacial events we stress that the ages of the units between the tyrrell sea and bell sea end membrers are interpolated and that the chronology of events is currently based on the assumption that the bell sea represents marine incursion at the onset of marine isotope stage 5 dyke has raised a number of points which have concerned us since we started our joint research on the aminostratigraphy of the hudson bay lowlands the answer to many questions will come not from the amino acid results per se but from detailed litho and biostratigraphic logging of the thousands of kilometers exposed along the large rivers that drain into james bay and southwestern hudson bay this work is presently going on let us say in conclusion that analysis of a further 63 shells and shell fragments resulted in a virtually identical frequency distribution to that discussed in our paper we are currently evaluating the stratigraphic integrity of these results field expeditions by the geological survey of canada in 1982 and 1983 into the hudson bay lowlands were specifically designed to log new sections and make additional shell collections we hope to report on these new data in due course

sks splitting beneath transportable array stations in eastern north america and the signature of past lithospheric deformation seismic anisotropy in the upper mantle beneath continental interiors is generally complicated with contributions from both the lithosphere and the asthenosphere previous studies of sks splitting beneath the eastern united states have yielded evidence for complex and laterally variable anisotropy but until the recent arrival of the usarray transportable array ta the station coverage has been sparse here we present sks splitting measurements at ta stations in eastern north america and compare the measured fast directions with indicators such as absolute plate motion surface geology and magnetic lineations we find few correlations between fast directions and absolute plate motion except in the northeastern u s and southern canada where some stations exhibit variations in apparent splitting with backazimuth that would suggest multiple layers of anisotropy a region of the southeastern u s is dominated by null sks arrivals over a range of backazimuths consistent with previous work we document a pattern of fast directions parallel to the appalachian mountain chain suggesting a contribution from lithospheric deformation associated with appalachian orogenesis overall our measurements suggest that upper mantle anisotropy beneath the eastern united states is complex with likely contributions from both asthenospheric and lithospheric anisotropy in many regions

compatibility of high altitude aeromagnetic and satellite altitude magnetic anomalies over canada results from equivalent source distributions derived jointly from high altitude average 4 km aeromagnetic and magsat derived average 400 km magnetic anomalies over canada indicate that long wavelength components 500 2500 km in these fields are extremely compatible with one another with a correlation coefficient of 0 95 the jointly estimated anomaly field at the earth s surface can be used as a long wavelength adjustment surface for regional near surface magnetic anomaly compilations and in assessing the performance of other downward continuation techniques because near surface anomalies are not available over all regions of the world we compare the jointly estimated anomaly field to the results of two different downward continuation techniques the evaluation of anomalies at the earth s surface from spherical harmonic coefficients derived from satellite altitude data and the use of downward continuation methods based on harmonic splines numerical and visual comparisons of these downward conti

jurassic evolution of the gulf of mexico salt basin we present a new hypothesis for the jurassic plate tectonic evolution of the gulf of mexico basin and discuss how this evolution influenced jurassic salt tectonics four interpretations some based on new data constrain the hypothesis first the limit of normal oceanic crust coincides with a landward dipping basement ramp near the seaward end of the salt basin which has been mapped on seismic data second the deep salt in the deep water gulf of mexico can be separated into provinces on the basis of position with respect to this ramp third paleodepths in the postsalt sequence indicate that salt filled the gulf of mexico salt basin to near sea level fourth seismic data show that postsalt sediments in the central louann and the yucatan salt basins exhibit large magnitudes of late jurassic salt detached extension not balanced by equivalent salt detached shortening in our hypothesis callovian salt was deposited in preexisting crustal depressions on hyperextended continental and transitional crust after salt deposition ended rifting continued for another 7 to 12 m y before sea floor spreading began during this phase of postsalt crustal stretching the salt and its overburden were extended by 100 to 250 km 62 155 mi depending on location sea floor spreading divided the northern gulf of mexico into two segments separated by the northwest trending brazos transform the eastern segment opened from east to west leaving the walker ridge salient in the center of the basin as the final area to break apart in some areas salt flowed seaward onto new oceanic crust first concordantly over the basement as a parautochthonous province then climbing up over stratigraphically younger strata as an allochthonous province

asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern south atlantic abstract the southern south atlantic has often been considered a classic example of continental break up in the presence of a starting mantle plume evidence for a mantle plume includes the parana etendeka continental flood basalts which are associated with the rio grande rise and walvis ridge and the wide spread presence of seaward dipping reflectors and high velocity lower crustal bodies along the conjugate margins observations from seaward dipping reflector distributions suggested that lithospheric segmentation played a major role in the pattern of volcanism during break up in this region and consequent numerical modelling was used to test this we tested this hypothesis ourselves by measuring the thickness of the earliest oceanic crust generated this was done through the use of 37 measurements of initial oceanic crustal thickness from wide angle and multichannel seismic profiles collected along the conjugate margins these measurements show that at 450 km south of the parana etendeka flood basalts the oceanic crust is thicker than the global average at 11 7 km farther south the oceanic crust thins reaching 6 1 km at a distance of 2300 km along strike overall the along strike trend of oceanic crustal thickness is linear with a regression coefficient of 0 7 and little indication of segmentation from numerical models representing extension of the lithosphere we find that observed melt volumes are matched with the presence of a hot layer if we assume this region of hot mantle has a thickness of 100 km its excess temperature relative to the asthenosphere has to decrease from 200 to 50 c north to south this decrease in temperature also seen in published thermobarometry results suggests that temperature was the primary control of volcanism during the opening of the southern south atlantic

satellite magnetic anomalies of the antarctic wilkes land impact basin inferred from regional gravity and terrain data abstract the grace gravity and subglacial terrain data of wilkes land are consistent with the presence of a mascon produced by giant impact perhaps at the end of the permian in contrast to the relatively extensive ice probing radar coverage aeromagnetic data coverage is limited across the basin however magsat orsted and champ satellite magnetic observations reveal the thinned crust of the impact site to be associated with the largest satellite altitude crustal magnetic anomaly of antarctica the underlying region of enhanced magnetization is consistent with the grace gravity and bedmap terrain data and extends into south central australia in a reconstructed gondwana the strongly magnetized crust can reflect the impact s thermal enhancement of lower crustal viscous remanent magnetization as well as the production of positively magnetized melt rocks within the fractured crust

reconstruction of the east africa and antarctica continental margins the early jurassic separation of antarctica from africa plays an important role in our understanding of the dispersal of gondwana and pangea previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent ocean boundary cob and determining the amount and distribution of extended continental crust this study focuses on the evolution of the african margin adjacent to the mozambique basin and the conjugate antarctic margin near the riiser larsen sea satellite derived gravity data have been used to trace the orientations and landward limits of fracture zones a 3 d gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust crustal thicknesses together with fracture zone terminations reveal cobs that are significantly closer to the african and antarctic coasts than previously recognized correlation of fracture zone azimuths and identified cobs suggests antarctica began drifting away from africa at approximately 171 ma in a roughly sse direction an areal balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins both margins reveal a trend of increasing extension from east to west our results suggest africa underwent extension of 60 120 km while antarctic crust was stretched by 105 180 km various models tested to determine the direction of extension during rifting suggest that antarctica moved away from africa in a wnw ese direction during the period between 184 and 171 ma prior to the onset of seafloor spreading

first scalar magnetic anomaly map from champ satellite data indicates weak lithospheric field 1 satellite magnetic anomaly maps derived by different techniques from magsat pogo data vary by more than a factor of 2 in the deduced strength of the lithospheric magnetic field here we present a first anomaly map from new champ scalar magnetic field data after subtracting a recent orsted main and external field model we remove remaining unmodeled large scale external contributions from 120 track segments by subtracting a best fitting uniform field in order to preserve n s trending features the data are not filtered along track direct integration of the spherically gridded data yields the final degree 14 65 spherical harmonic expansion of the total intensity anomaly at the mean satellite altitude of 438 km apart from enhanced long wavelength features and a smoother general appearance our initial map is strikingly similar to one of the lower strength magsat pogo maps

predicting the geothermal heat flux in greenland a machine learning approach geothermal heat flux ghf is a crucial boundary condition for making accurate predictions of ice sheet mass loss yet it is poorly known in greenland due to inaccessibility of the bedrock here we use a machine learning algorithm on a large collection of relevant geologic features and global ghf measurements and produce a ghf map in greenland which we argue is within 15 accuracy the main features of our predicted ghf map include a large region with high ghf in central north greenland surrounding the northgrip ice core site and hotspots in the jakobshavn isbrae catchment upstream of petermann gletscher and near the terminus of nioghalvfjerdsfjorden glacier our model also captures the trajectory of greenland movement over the icelandic plume by predicting a stripe of elevated ghf in central east greenland finally we show that our model can produce substantially more accurate predictions if additional measurements of ghf in greenland are provided

new insights into the magmatism in the northern margin of the south china sea spatial features and volume of intraplate seamounts the extensive intraplate seamounts are obvious features in the northern south china sea scs however the distribution volume and origin of these seamounts are not well understood which greatly hinders our understanding of magmatism in the scs based on high resolution bathymetric data and 147 seismic profiles and combining gravity and magnetic data we first identify 45 seamounts in the northern margin of the scs and simulate their shape with elliptical cones results show that the total volume of these 45 seamounts above seafloor is estimated at about 1885 3078 km3 and the total volume of intrusive magma above moho is about 0 15 mkm3 which is close to the estimates for classic large igneous provinces across the world these seamounts are mostly located on the continental slope with thin crust approximately 12 18 km which reduces the overlying pressure and shortens the magmatic conduits the dominant azimuth of elliptical major axis in seamounts is consistent with the synrift and synspreading fault strikes ne nee indicating that these pre existing faults provide magmatic conduits for the subsequent postrift intraplate seamounts based on three existing clues i e 1 the intraplate seamounts high velocity layer and hainan mantle plume are contiguous in 3 d space 2 the high velocity layer is thicker beneath the continental shelf but thinner beneath the slope and 3 the basalts dredged from certain seamounts show oib type geochemical features we propose a magmatic upwelling pattern which contains hainan mantle plume to explain the spatial and morphological characteristics of these intraplate seamounts

distribution of the crustal magnetic anomaly and geological structure in xinjiang china based on the high order crustal magnetic field model ngdc 720 v3 we investigate the distribution of crustal magnetic anomaly the decay characteristics of the anomaly and the relationship between the magnetic anomaly and geological structure in xinjiang china topography of the magnetic layer basement is studied through curie isothermal surface using the power spectrum method it is found that south tarim basin junggar basin and turpan hami basin have strong positive magnetic anomaly whereas west kunlun mountain altun mountain tianshan mountain and altai mountain have weak or negative anomaly the magnetic anomaly well reflects the regional tectonic structure i e three alternating mountains intervened by two basins the magnetic anomaly on the ground surface in tarim basin is well corresponding to the mafic dykes the decay of the magnetic anomaly with altitude indicates that xinjiang is a large massif composed of several magnetic blocks with different sizes in different directions the curie surface presents a feature of being shallow under mountains whereas being deep under basins roughly having an anti mirror correspondence with the moho depth

tearing and breaking off of subducted slabs as the result of collision of the panama arc indenter with northwestern south america we present two regional lithospheric cross sections that illustrate eastward and southeastward dipping subducted slabs to depths of 315 km beneath the surface of colombia in northwestern south america these cross sectional interpretations are based on relocated earthquake hypocentral solutions models supported on gravity and magnetic regional data and coda q q c tomography the method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system mapping of earthquake defined benioff zones combined with tomographic imaging reveals the presence of an 240 km long east west striking slab tear named here the caldas tear the proposed caldas tear separates a zone of shallow 20 30 dipping southeastward subduction in the area of colombia adjacent to panama and the caribbean sea which is not associated with subduction related volcanism from an area of steeper 30 40 dipping slab adjacent to the eastern pacific ocean that is associated with an active north south chain of active arc volcanoes we propose that the caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the panama indenter an extinct island arc that began subducting beneath northwestern south america about 12 ma the area south of the panama indenter is miocene oceanic crust of the nazca plate which subducts eastward beneath northwestern south america at normal angles and melts to form a north south trending active volcanic arc in addition to the formation of the caldas tear we propose that impedance of the thicker crustal area of the panama arc indenter over the past 12 ma may have led to down dip break off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate depth earthquakes beneath east central colombia

resolution of direction of oceanic magnetic lineations by the sixth generation lithospheric magnetic field model from champ satellite magnetic measurements the champ satellite continues to provide highly accurate magnetic field measurements from decreasing orbital altitudes 350 km at solar minimum conditions using the latest 4 years 2004 2007 of readings from the champ fluxgate magnetometer including an improved scalar data product we have estimated the lithospheric magnetic field to spherical harmonic degree 120 corresponding to 333 km wavelength resolution the data were found to be sensitive to crustal field variations up to degree 150 down to 266 km wavelength but a clean separation of the lithospheric signal from ionospheric and magnetospheric noise sources was achieved only to degree 120 this new mf6 model is the first satellite based magnetic model to resolve the direction of oceanic magnetic lineations revealing the age structure of oceanic crust

the origin of the loncopue trough in the retroarc of the southern central andes from field geophysical and geochemical data abstract this work analyzes through field geophysical geochemical and geochronological data a particular setting of the southern andes in which an extensional system is flanking the mountain front this setting is represented by the loncopue trough whose origin is discussed for this part of the andes this work reconstructs 3 dimensionally the structure of this basin and its evolution through time since jurassic times as a series of isolated depocenters to late cretaceous eocene times with the construction of a positive relief that subsequently was relaxed through two extensional stages during the late oligocene early miocene and the pliocene quaternary respectively the last episode of extension is characterized by an initial stage with a series of caldera collapses in the latest pliocene early quaternary this passed to a stage of development of a broad basaltic plateau in pre and inter glacial times circumscribed with new radiometric data to the pleistocene quaternary products in northern and central loncopue trough have chemical relations that are intermediate between the arc front and the southern loncopue trough volcanic rocks studied in previous works thus retroarc eruptions would be part of an extended arc configuration that goes from typical arc series in the northern part of the trough to within plate series in the south low elastic thicknesses computed from gravity data in this work correlate with the area of retroarc volcanic activity magnetic data have allowed determining the curie isotherm showing two areas of relatively abnormal heat flow one along the loncopue trough itself and the other in the foreland zone this scenario is discussed through three main hypotheses an occurrence linked to a slab steepening after a shallow subduction in the area co seismic crustal stretching linked to giant earthquakes in the subduction zone and finally slab tearing associated with asthenospheric upwelling

mapping p wave azimuthal anisotropy in the crust and upper mantle beneath the united states abstract much progress has been made on revealing seismic structure and mantle dynamics beneath the united states us with the earthscope usarray project seismic anisotropy revealed by shear wave splitting studies provides important constraints on constructing geodynamic models with regard to the seismic images but the shear wave splitting observations have poor vertical resolution and so their interpretations are often not unique in this work we used a large number of arrival time data from local and distant earthquakes recorded by the usarray to determine the first p wave azimuthal anisotropy tomography of the crust and upper mantle beneath the us our results show that fast velocity directions fvds in the lithosphere under the tectonically active areas correlate well with the surface tectonic features suggesting that the p wave anisotropy mainly reflects the present deformation a circular pattern of the fvds centered in the great basin is revealed which is well consistent with the specific circular shear wave splitting observations there suggesting that the anisotropy occurs in the crust and uppermost mantle in contrast beneath the stable cratonic region the fvds revealed by this study differ from the shear wave splitting observations but consistent with the features of gravity and magnetic anomalies indicating that the p wave fvds mainly reflect the fossil anisotropy in the lithosphere whereas the s wave splitting observations mainly reflect the significant anisotropy in the asthenosphere the present results shed new light on the seismic anisotropy in the crust and upper mantle and provide new constraints on constructing geodynamic models beneath the us

thermal structure of the panama basin by analysis of seismic attenuation abstract using recordings of earthquakes on oceanic bottom seismographs and onshore stations on the coastal margins of colombia panama and ecuador we estimate attenuation parameters in the upper lithosphere of the panama basin the tomographic images of the derived coda q values are correlated with estimates of curie point depth and measured and theoretical heat flow our study reveals three tectonic domains where magmatic hydrothermal activity or lateral variations of the lithologic composition in the upper lithosphere can account for the modeled thermal structure and the anelasticity we find that the costa rica ridge and the panama fracture zone are significant tectonic features probably related to thermal anomalies detected in the study area we interpret a large and deep intrinsic attenuation anomaly as related to the heat source at the costa rica ridge and show how interactions with regional fault systems cause contrasting attenuation anomalies

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

new global vector magnetic anomaly maps derived from magsat data new global maps of the vector magnetic anomaly field of earth are derived from magsat data improvement on earlier published vector and scalar maps is achieved with the following procedure 1 the noise level of vector measurements associated with the discontinuities of the in flight sensor attitude determination is reduced by selecting the data 2 a monthly model of the main field is computed to account for secular variation using only quiet day data 3 the time dependence of the ring current effect is taken into account using hourly values of the dst index based on ground obseivations 4 sectorial indices based on ground observations are introduced to select profiles corresponding to low magnetic activity these indices provide a better discrimination of quiet data than the global kp index usually used for that purpose 5 a time and longitude dependent dynamic model of the equatorial electrojet is derived and used to correct magnetic measurements at low latitudes anomaly maps of x and z components are obtained using selected measurements within the reduced altitude range 350 425 km these maps are compared with earlier magsat anomaly maps and their respective resolutions are discussed these maps evidence large anomalies of lithospheric origin within the oceanic domain in particular the z map displays a characteristic pattern of long wavelength anomalies in these areas it is first shown that some of these anomalies particularly in the north atlantic and indian oceans may be correlated with the age of the lithosphere then a significant similarity between magnetic and geoid anomalies above ocean basins is evidenced this suggests that long wavelength oceanic magnetic anomalies may also be attributed to variations of the thickness of the magnetized layer associated with ocean floor topography

crustal magnetic anomaly and curie surface beneath tarim basin china and its adjacent area based on the geomagnetic field model ngdc 720 v3 national geophysical data center the distribution of crustal magnetic anomalies and the curie surface beneath tarim basin china and its adjacent area were investigated the results show that the spatial distribution of the magnetic anomalies in tarim basin coincides with the regional tectonic structure i e a basin sandwiched between mountain ranges shallow parts of the curie surface are located in uplifted zones of the basin and correspond well to high values of heat flow in contrast deep parts of the curie surface having low values of heat flow are distributed along the depression zones of the basin the curie point depth is consistent with the measured heat flow value with a correlation coefficient of 0 65

parallelism between the maximum exhumation belt and the moho ramp along the eastern tibetan plateau margin coincidence or consequence abstract knowledge of how strain is partitioned and accommodated in crustal shortening provides fundamental constraints on evaluating tectonic models of orogenic plateaus a number of tectonic models have been proposed for the central longmen shan along the eastern tibetan plateau margin including mid crustal channel flow upper crustal shortening and whole crust shearing however it remains controversial whether these models work in the min shan northern longmen shan area and if so how the models function in three dimensions one straightforward test is to evaluate the timing and rate of uplift on the principle crustal faults here we present 36 fission track dates from this region our data show that the southeastern min shan has experienced the most rapid exhumation in this area since the late miocene due to slip along the huya thrust and coeval northwestward tilting of the min shan fault block further we identify a maximum exhumation belt meb along min shan and longmen shan that is characterized by the greatest amount of exhumation in the late cenozoic and the formation of 5000 m peaks along the eastern tibetan plateau margin interestingly the meb overlies the moho ramp between the tibetan plateau and the sichuan basin the parallelism between the meb and the moho ramp leads us to a crustal scale tectonic model in which the tibetan upper crust undergoes shortening and thrusting and the lower crust undergoes pure shear thickening moreover a 15 angle exists between the trend of the meb and that of the longmen shan implying that the longmenshan thrust belt is a reactivated structure which explains the along strike variations of fault kinematics and activity we conclude that the eastern tibetan crust is partially decoupled and future studies need to quantify strain partitioning in both the upper and the lower crust during continental orogenesis

the geomagnetic power spectrum summary combining champ satellite magnetic measurements with aeromagnetic and marine magnetic data the global geomagnetic field has now been modelled to spherical harmonic degree 720 an important tool in field modelling is the geomagnetic power spectrum it allows the comparison of field models estimated from different data sets and can be used to identify noise levels and systematic errors a correctly defined geomagnetic power spectrum is flat white for an uncorrelated field such as the earth s crustal magnetic field at long wavelengths it can be inferred from global spherical harmonic models as well as from regional grids marine and aeromagnetic grids usually represent the anomaly of the total intensity of the magnetic field appropriate corrections have to be applied in estimating the geomagnetic power spectrum from such data the comparison of global and regional spectra using a consistently defined azimuthally averaged geomagnetic power spectrum facilitates quality control in field modelling and should provide new insights in magnetic anomaly interpretation

on the trade off between seismic anisotropy and heterogeneity numerical simulations and application to northeast japan p wave tomography has been recently used to study 3 d azimuthal and radial anisotropy of subduction zones and continental regions however the fundamental issue about the trade off between the isotropic and anisotropic structures is still unclear in this study we investigate this issue systematically with comprehensive synthetic tests our results indicate that good ray coverage in the azimuth for azimuthal anisotropy and incidence for radial anisotropy is required for determining reliable anisotropic models the isotropic and anisotropic structures are strongly coupled and smearing effects are significant when the rays used in the inversion are limited in a small range of azimuth or incidence we therefore plot ray azimuth and ray incidence ellipses at every grid nodes and propose to use the normalized length of the short axis i e the ratio of the short axis and long axis lengths for estimating the ray coverage quantitatively applying our novel approach to a large number of high quality arrival time data of local shallow and intermediate depth earthquakes we obtained new tomographic images of 3 d p wave azimuthal and radial anisotropy in northeast japan both the azimuthal and radial anisotropy results are determined reliably for the shallow parts of the study region whereas the smearing effects are significant in the deeper part of the mantle wedge and the subducting slab our results show dominant trench normal and vertical fast anisotropy in the mantle wedge while trench parallel and horizontal fast anisotropy in the subducting slab which indicates different dynamics in different domains of the subduction zone

emag2 a 2 arc min resolution earth magnetic anomaly grid compiled from satellite airborne and marine magnetic measurements 1 a global earth magnetic anomaly grid emag2 has been compiled from satellite ship and airborne magnetic measurements emag2 is a significant update of our previous candidate grid for the world digital magnetic anomaly map the resolution has been improved from 3 arc min to 2 arc min and the altitude has been reduced from 5 km to 4 km above the geoid additional grid and track line data have been included both over land and the oceans wherever available the original shipborne and airborne data were used instead of precompiled oceanic magnetic grids interpolation between sparse track lines in the oceans was improved by directional gridding and extrapolation based on an oceanic crustal age model the longest wavelengths 330 km were replaced with the latest champ satellite magnetic field model mf6 emag2 is available at http geomag org models emag2 and for permanent archive at http earthref org cgi bin er cgi s erda cgi n 970

curie point depth in the sw caribbean using the radially averaged spectra of magnetic anomalies abstract we have estimated the curie point depth cpd using the average radial power spectrum in a tectonically complex area located in the sw caribbean basin data analyzed came from the world digital magnetic anomaly map and three methods have been used to compare results and evaluate uncertainties centroid spectral peak and forward modeling results show a match along the three methods suggesting that the cpd values in the area ranging between 6 km and 50 km the results share the following characteristics a high values 30 km are in continental regions b there is a trend of maximum cpd values along the sw ne direction starting from the central cordillera in colombia to the maracaibo lake in venezuela c there is a maximum cpd at the sierra nevada de santa marta colombia as well as between costa rica nicaragua and nicaragua honduras borders the lowest cpd values

merging airborne magnetic surveys into continental scale compilations regional compilations of airborne magnetic data are becoming more common as national databases grow grids of the magnetic survey data are joined together to form geological province scale or even continental scale compilations the advantage of these compilations is that large tectonic features and geological provinces can be better mapped and interpreted we take a holistic approach to the joining of survey grids the leveling of the grids into a regional compilation is treated as a single inverse problem we use the weighted least squares method to find the best adjustment for each survey grid such that the data value differences in the grid overlap areas are minimized the method spreads any inconsistencies between grids among all of the grid overlap areas and minimizes the introduction of long wavelength errors into the composite grid this is an improvement on the conventional approach of joining grids sequentially a comparison of leveled data over western australia with diurnally corrected long aero

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

fifth generation lithospheric magnetic field model from champ satellite measurements 1 six years of low orbit champ satellite magnetic measurements have provided an exceptionally high quality data resource for lithospheric magnetic field modeling and interpretation here we describe the fifth generation satellite only magnetic field model mf5 the model extends to spherical harmonic degree 100 as a result of careful data selection extensive corrections filtering and line leveling the model has low noise levels even if evaluated at the earth s surface the model is particularly suited for inferring large scale structure and composition of the lithosphere it is also meant to serve as the long wavelength part of continental and global scale marine and aeromagnetic anomaly maps

testing early cretaceous africa south america fits with new palaeomagnetic data from the etendeka magmatic province namibia abstract we provide an independent test for the proposed pre breakup fits between the south american and african continental margins based on new palaeomagnetic data in the course of modelling the opening of the south atlantic ocean several reconstructions have been proposed which correspond to different estimates of the amount of pre drift extension in the south american and african continental margins and amounts of displacement along intracontinental accommodation zones the parana etendeka large igneous province erupted during a brief time interval between 136 and 131 ma just prior to opening of the south atlantic and has magmatic products on both the african etendeka lavas and south american parana lavas plates here we provide new palaeomagnetic constraints on the etendeka volcanic rocks these yield a declination of d dx 314 4 7 0 and inclination of i ix 42 7 8 3 plat 47 5 plon 88 9 k 26 1 a95 6 3 n 21 when our results are combined with another recently published dataset we obtain an etendeka palaeopole at 49 1 s 87 6 e d dx 315 9 3 8 i ix 43 9 4 4 k 23 4 a95 3 5 n 75 we compare three recent models for south atlantic ocean opening discuss which of the models best fits the palaeomagnetic data and present a revised reconstruction for west gondwana during the early cretaceous

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

sensitivity of ice cemented antarctic soils to greenhouse induced thawing are terrestrial archives at risk abstract we modeled the sensitivity of six ice cemented slope deposits from the western mcmurdo dry valleys antarctica to failure by shallow thaw induced planar sliding the deposits examined have purportedly remained physically stable without morphologic evidence for downslope movement for millions of years could they fail in the near future from greenhouse induced warming to address this question we first prescribed various increases in mean summertime soil surface temperature mssst and modeled numerically the resultant changes in soil thaw depths using a one dimensional heat diffusion equation including the effects of latent heat of fusion second we calculated the minimum thaw depths required to facilitate failure by shallow planar sliding for each deposit for all numerical simulations we maintained present soil moisture conditions and used a mohr coulomb based equation of safety factor third we calculated the rate of subsurface meltwater flow assuming darcy s law our results show that although most deposits contain sufficient subsurface ice to induce sliding upon thawing lateral rates of water flow of as much as 40 m day for some colluvial deposits prohibit the build up of requisite pore pressures for failure on the other hand silty deposits that contain gravimetric water 15 occur on slopes 20 and possess low hydraulic conductivities 30 60 cm day common in the dry valleys region could fail if mssst and by inference mean summertime atmospheric temperatures increase by 4 to 9 c this temperature increase is similar to that predicted to occur from greenhouse induced warming in antarctica over the next century

interpretation of an aeromagnetic survey of the spanish mainland abstract an aeromagnetic survey of the spanish mainland was carried out in 1987 a first interpretation of this survey is given here with particular reference to the structure and tectonic development of the spanish part of the iberian peninsula this involves the integration of the geophysical interpretation where possible with the current understanding of the tectonic framework as determined from previous geological and geophysical investigations of particular interest is the apparent structural symmetry and the possible manner in which this might be associated with the generally acknowledged structural units of spain this gross structural symmetry even seems to extend to the northern and southern coastlines where the magnetic responses are somewhat similar in character from a magnetic point of view the spanish mainland appears to be dominated by a series of sub circular structures about an ese wnw axis centered on the gredos mountains and remaining open in the axial direction into portugal

the central iberian arc an orocline centered in the iberian massif and some implications for the variscan belt an arcuate structure comparable in size with the ibero armorican arc is delineated by variscan folds and magnetic anomalies in the central iberian zone of the iberian massif called the central iberian arc its sense of curvature is opposite to that of the ibero armorican arc and its core is occupied by the galicia tras os montes zone of nw iberia which includes the rheic suture other zones of the iberian massif are bent by the arc but the ossa morena and south portuguese zones are not involved the arc formed during the late carboniferous at final stages of thermal relaxation and collapse and an origin related with right lateral ductile transpression at the scale of the variscan belt is proposed the central iberian arc explains the width of the central iberian zone clarifies the position of the allochthonous terranes of nw iberia and opens new perspectives for correlations with the rest of the variscan belt in particular with the armorican massif whose central zone represents the continuation of the southwest branch of the arc detached by strike slip tectonics

national geophysical data center candidate for the world digital magnetic anomaly map marine and airborne magnetic anomaly data have been collected for more than half a century providing global coverage of the earth furthermore the german champ satellite is providing increasingly accurate information on large scale magnetic anomalies the world digital magnetic anomaly map project is an international effort to integrate all available near surface and satellite magnetic anomaly data into a global map database teams of researchers were invited to produce candidate maps using a common pool of data sets here we present the national geophysical data center ngdc candidate to produce a homogeneous map the near surface data were first line leveled and then merged by least squares collocation long wavelengths were found to agree surprisingly well with independent satellite information this validates our final processing step of merging the short wavelength part of the near surface data with long wavelength satellite magnetic anomalies

an ellipsoidal harmonic representation of earth s lithospheric magnetic field to degree and order 720 1 while high degree models of the earth s gravity potential have been inferred from measurements for more than a decade corresponding geomagnetic models are difficult to produce the primary challenge lies in the estimation of the magnetic potential which is not completely determined by available field intensity measurements and cannot be computed by direct integration described here is the methodology behind the third generation of the national geophysical data center s degree 720 magnetic model key issues are 1 the ellipsoidal harmonic representation of the magnetic potential 2 the reduction of ambiguities by a suitable penalty function and 3 the use of an iterative method to estimate the model coefficients the ngdc 720 model provides the lithospheric magnetic field vector at any desired location and altitude close to and above the earth s surface anticipated uses are in geological and tectonic studies of the lithosphere as a local correction for magnetic navigation and heading systems and the calibration of ground marine airborne and spaceborne magnetometers the ngdc 720 model is available at http geomag org models ngdc720 html and for long term archive at http earthref org cgi bin er cgi s erda cgi n 989

the satellite along track analysis in planetary magnetism summary many physical characteristics of planets such as their topography magnetic and gravitational fields are routinely detected and measured by spacecrafts at satellite altitudes even if little is known about the measured signal it is possible to separate the large scale components from other contributions by a spectral analysis carried out along the spacecraft orbit this procedure which dates back to the early age of the satellite era is routinely applied with spherical harmonics analysis for the study of large scale planetary magnetic fields particularly those that vary rapidly in time in this paper we review the approximations of this procedure for investigating internal and external magnetic fields of planets we show that the magnetic field analyses along the orbits are limited by a finite frequency resolution that is known to introduce a spectral leakage this leakage may lead to artificial spatio temporal variations of the magnetic field such as an apparent internal field secular variation an asymmetry of the magnetospheric field and a regional distortion of the lithospheric field structures we quantify the errors for the earth s lithospheric field and display its distribution in the spatial and the spectral domains we also discuss these limits for the moon and mars lithospheric magnetic fields our analytical results illustrate the pros and cons of the orbit by orbit analyses and should allow us to avoid the pitfall of geophysical overinterpretation of some artificial magnetic field variations

crustal structure beneath the paleozoic parnaiba basin revealed by airborne gravity and magnetic data brazil abstract the parnaiba basin is a large paleozoic syneclise in northeastern brazil underlain by precambrian crystalline basement which comprises a complex lithostructural and tectonic framework formed during the neoproterozoic eopaleozoic brasiliano pan african orogenic collage a sag basin up to 3 5 km thick and 1000 km long formed after the collage the lithologic composition structure and role in the basin evolution of the underlying basement are the focus of this study airborne gravity and magnetic data were modeled to reveal the general crustal structure underneath the parnaiba basin results indicate that gravity and magnetic signatures delineate the main boundaries and structural trends of three cratonic areas and surrounding neoproterozoic fold belts in the basement triangular shaped basement inliers are geophysically defined in the central region of this continental scale neoproterozoic convergence zone a 3 d gravity inversion constrained by seismological data reveals that basement inliers exhibit a 36 40 5 km deep crustal root with borders defined by a high density and thinner crust forward modeling of gravity and magnetic data indicates that lateral boundaries between crustal units are limited by brasiliano shear zones representing lithospheric sutures of the amazonian and sao francisco cratons tocantins province and parnaiba block in addition coincident residual gravity residual magnetic and pseudo gravity lows indicate two complex systems of eopaleozoic rifts related to the initial phase of the sag deposition which follow basement trends in several directions