effective medium theories for fluid saturated materials with aligned cracks there is general agreement between different theories giving expressions for the overall properties of materials with dry aligned cracks if the number density of cracks is small there is also very fair agreement for fluid filled isolated cracks however there are considerable differences between two separate theories for fluid filled cracks with equant porosity comparison with recently published experimental data on synthetic sandstones gives a good fit with theory for dry samples however although the crack number density in the laboratory sample is such that first order theory is unlikely to apply expressions correct to second order in the number density provide a worse fit it also appears that the ratio of wavelength to crack size is not sufficiently great for any detailed comparison with effective medium theories which are valid only when this ratio is large the data show dispersion effects for dry cracks and scattering neither of which will occur at sufficiently long wavelengths data from the water saturated samples indicate that the effect of equant porosity is significant although the two theories differ strongly as to just how significant once again and in spite of the reservations mentioned above a reasonable fit between theory and observation can be shown

geochemistry of tertiary igneous rocks of northern luzon philippines evidence for a back arc setting for alkalic porphyry copper gold deposits and a case for slab roll back oligocene to early miocene volcanic rocks are preserved in the central cordillera range and cagayan valley of northern luzon philippines basaltic and andesitic rocks of the pugo formation in the baguio district of the central cordillera were intruded by the 27 to 20 ma calc alkaline central cordillera diorite complex in the southern cagayan valley the subalkaline to alkaline late oligocene mamparang formation overlies the cretaceous caraballo formation and has been intruded by the didipio igneous complex the cordon syenite complex and the palali batholith the didipio complex comprises an early suite of diorites which were intruded by the strongly mineralized stocks of the dinkidi cu au porphyry deposit whole rock geochemical data for intrusive and extrusive rocks of the baguio district range from low k calc alkaline to shoshonitic basalts to dacites with rare earth element ree and high field strength element hfse characteristics of suprasubduction zone magmas and are all interpreted to have been sourced from the same parent melt samples from didipio display higher alkali contents but similar trace element characteristics new age dates for the didipio area range from 25 7 to 24 8 ma the potassic magmas of the cagayan valley are interpreted to have formed in a back arc coeval to the mainarc sequence that is preserved in the baguio miocene rocks this contradicts earlier models which invoke an early miocene arc reversal in the northern luzon archipelago with the switch from early westward subduction to later eastward subduction attributed to a variety of causes the lack of a single compelling trigger for arc reversal combined with the coeval emplacement of arc magmas in the west and back arc magmas in the east in northern luzon is best interpreted as the result of eastward subduction since the late oligocene the presence of 20 ma adakitic magmas in the baguio district may indicate that flattening of the downgoing slab resulted in a hiatus in magmatism and termination of back arc rifting

thermochronology of the south cyclades shear zone ios greece effects of ductile shear in the argon partial retention zone micas and potassium feldspars from the south cyclades shear zone ios cyclades greece yield varied and complex 40ar 39ar apparent age spectra a correlation exists between 40ar 39ar apparent ages and the relative timing of different episodes of recrystallization and grain growth as indicated by fabric and microstructural analysis the 40ar 39ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth and partial to complete resetting of argon systematics in potassium bearing minerals during hercynian m0 alpine m1 and late oligocene early miocene m2 metamorphism deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals modeling suggests the oligo miocene thermal events were of insufficient magnitude and or duration to completely reset the isotopic systematics in these samples our data lead to the concept of the argon partial retention and resetting zone prz defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium bearing minerals within the prz some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained tectonic exhumation of the prz involves movement on crustal scale ductile shear zones accompanied by strongly partitioned deformation and localized recrystallization recrystallization leads to resetting of argon systematics and thus will result in heterogeneous 40ar 39ar age distributions within these ductile shear zones e g in the south cyclades shear zone

experimental calibration of the aluminum in hornblende geobarometer with application to long valley caldera california volcanic rocks a new geobarometer based on the al content of igneous hornblendes in equilibrium with melt fluid biotite quartz sanidine plagioclase sphene and magnetite or ilmenite has been calibrated experimentally the calibration was performed by equilibrating the required phase assemblage over the pressure range 2 8 kbar at 740 780c and then analyzing euhedral hornblendes in equilibrium with glass melt experiments were performed on natural samples of both volcanic and plutonic rocks earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks the experimental calibration differs from the empirical calibrations especially above 5 kbar and shows that the al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought the geobarometers uncertainty is dramatically reduced the derived equation is p plus minus 0 5 kbar 3 minus 3 46 plus minus 0 24 4 23 plus minus 0 13 al sup t the geobarometer is applied to post bishop tuff volcanic rocks from long valley caldera california and reveals that most rhyodacites in this complex erupted from depths of about 6 km these eruptions occurred over 500 000 yr suggesting that the rhyodacitic magma reservoir beneath longmore valley had reached a steady p depth t state less

magmatic epidote and its petrologic significance epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the mesozoic accreted terranes between northern california and southeastern alaska textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase the observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure plutonic rocks bearing magmatic epidote must have formed under moderately high pressures corresponding to lower crustal depths under fairly oxidizing conditions

fission track analysis reveals character of collisional tectonics in new zealand during the late cenozoic the pacific plate has been obliquely converging with the australia plate in south island new zealand this has resulted in westward obduction of continental crust of the pacific plate and consequently a major mountain range the southern alps has formed east of the alpine fault and within the leading edge of the pacific plate we have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the southern alps starting at the alpine fault to better understand the uplift age and structure of the continental collision the results show the following 1 uplift was underway at 7 ma in the south at haast pass and by 5 ma in the north at arthur s pass 2 profiles of the total amount of late cenozoic uplift show an exponential increase toward the alpine fault adjacent to the alpine fault there has been 14 5 to 21 5 km of uplift in the vicinity of haast pass 14 5 to 21 km near mt cook and 12 5 to 15 5 km in the arthur s pass transect adjacent to the alpine fault in the mount cook region the average uplift rate from 5 5 ma to 1 ma was 2 3 to 2 5 mm yr between 1 ma and the present it has averaged 6 2 mm yr the strongly asymmetric uplift pattern is responsible for the exposure of a 13 25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the alpine fault known as the alpine schists 3 late cenozoic uplift of torlesse greywackes and alpine schists was preceeded during the jurassic early cretaceous by 5 8 km of uplift the otago schist belt to the south was uplifted much more during this interval and in addition western parts of the otago schists were uplifted 5 6 km during the late cretaceous 4 the fission track data do not support the notion of significant late cenozoic shear heating about the alpine fault as previously suggested from spatial variations in k ar ages of the otago and alpine schists

peculiar geometry of northern luzon philippines implications for regional tectonics of new gravity and paleomagnetic data 1 the northern termination of the philippine archipelago is remarkably abrupt the distinctive almost rectangular shape of northern luzon is a consequence of the almost simultaneous disappearance near 18 n of the central cordillera the much narrower sierra madre and the broad floodplain of the cagayan river the floodplain is underlain by a deep sedimentary basin that is closed off just inland of the coast by an ene wsw structural high known as the sicalao ridge gravity surveys show that the ridge has steep flanks that are almost certainly defined by faulting in the same region there is also a dramatic change in the luzon arc which is the volcanic expression of subduction of the south china sea the volcanic centers in the central cordillera the north luzon segment of the arc are all now inactive but further north eruptions are frequent on the small eastern islands of the bashi segment the differences between the north luzon and bashi segments suggest that they had very different geological histories until the late neogene but the evolution of northern luzon is so poorly understood that it is not even known on which plate it originated new paleomagnetic data and the existence of the sicalao ridge provide important constraints on reconstructions and when combined with geological data favor the possibility that in the paleogene the central cordillera and sierra madre were combined as parts of an arc at the southern margin of the philippine sea the sicalao ridge can be interpreted as a rift margin feature created during the detachment of luzon from continental sundaland

gassmann s fluid substitution and shear modulus variability in carbonates at laboratory seismic and ultrasonic frequencies carbonates have become important targets for rock property research in recent years because they represent many of themajoroilandgasreservoirsintheworld someareundergoing enhanced oil recovery most laboratory studies to understand fluid and pressure effects on reservoir rocks have been performed on sandstones but applying relations developed for sandstones to carbonates is problematic at best we measure in the laboratory nine carbonate samples from the same reservoir at seismic 3 3000 hz and ultrasonic 0 8 mhz frequencies samples are measured dry humidified and saturated with liquid butane and brine our carbonate samples showed typical changes in moduli as a function of porosity andfluid saturation however we explore the applicability of gassmann s theory on limestone and dolomite rocks in the context of shear and bulk modulus dispersion andgassmann stheoryassumptions forourcarbonatesetat high differential pressures and seismic frequencies the bulk modulus of rocks with high aspect ratio pores and dolomite mineralogy is predicted by gassmann s relation we also explore in detail some of the assumptions of gassmann s relation especiallyrock framesensitivitytofluidsaturation our carbonate samples show rock shear modulus change from dry to brine saturation conditions and we investigate several rock fluid mechanisms responsible for this change to our knowledge these are the first controlled laboratory experimentsoncarbonatesintheseismicfrequencyrange

brunswick subduction complex in the canadian appalachians record of the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick subduction complex in the new brunswick part of the canadian appalachians records the late ordovician to late silurian collision between laurentia and the gander margin of avalon the brunswick complex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the quebec reentrant of the laurentian margin this part of the margin experienced less underthrusting and exhumation and overprinting by orogen parallel faulting than the adjacent promontories where collision started earlier the early southeast to east vergent thrust related structures represent a progressive d1 deformation that formed in response to northwestward subduction of the previously extended gander margin and subsequent tectonic unroofing of the subduction complex the original shallow northwestward dipping envelope to s1 was deformed in the late silurian by d2 upright folds and associated shear zones into a steep belt during terminal collision the d2 structures probably formed in response to sinistral transpression together d1 and d2 indicate that convergence was oblique and sinistral most rocks incorporated in the subduction complex formed in the tetagouche back arc basin that evolved from rifting of an arenig magmatic arc built on the gander margin into a wide marginal basin subduction was initiated in the late ordovician 455 ma in the back arc basin following collision of the middle ordovician popelogan arc with laurentia in the caradoc shortly after its opening in the late arenig 473 ma the closure history of the iapetus ocean involved more than one subduction zone and arc continent collision and rivals the southwestern pacific ocean in its complexities

thermodynamic calculation of emplacement pressures for batholithic rocks california implications for the aluminum in hornblende barometer the aluminum in hornblende ah barometer has been widely used to estimate the emplacement pressure p of granitic rocks application of the barometer has remained controversial because the controls on the aluminum content of hornblende imposed by temperature t magma composition and fluid composition remain incompletely understood in this paper the p t conditions of equilibration for 19 rocks from the mesozoic batholiths of california are estimated by simultaneous evaluation of 1 the p sensitive reaction tremolite phlogopite 2 anorthite 2 albite 2 pargasite 6 quartz k feldspar and 2 previously published amphibole plagioclase thermometer equilibria p estimates for the batholiths range from 1 2 to 7 4 kbar and are consistent with independent geologic evidence the highest p samples 5 1 7 4 kbar are located near the san andreas fault furthermore it appears that regional p gradients determined by previous ah barometry studies are valid but that the absolute values of the p estimates will need to be revised somewhat 1 5 kbar i suggest that the method of this study provides a viable thermodynamics based alternative to conventional ah barometry that requires no a priori assumptions about t or bulk composition

new paleomagnetic constraints on the cenozoic tectonic evolution of the north arm of sulawesi indonesia abstract extensive paleomagnetic sampling 34 sites was carried out in cenozoic formations of the north arm of sulawesi between 120 and 124 e consistent results obtained between 120 and 122 e suggest the existence of a unique post miocene clockwise rotation of about 20 25 of the western part of the north arm this rotation probably occurred during the northward drift of the north arm along the palu matano sinistral transcurrent fault between 122 5 and 124 e both clockwise and counterclockwise rotations with various amplitudes from 6 to 85 are encountered and these very likely correspond to individual microblock rotations due to pervasive deformation along shear zones this result is consistent with the complex fault system of the gorontalo and kotamobagu shear zones that have been recognized on radar imagery and by spot image analyses

fracture parameter estimation from well log data we evaluated a method of deriving seismic fracture parameters from vertical well log data with the assumption that the fractured medium is transversely isotropic with a horizontal axis of symmetry hti one approximation we used is that the observed vertical p wave velocity is the same as the background isotropic p wave velocity of the hti medium another assumption was that the fractures and cracks are noninteractive and penny shaped using these approximations we generated the fracture compliance matrix for each layer fracture parameters were then derived by constructing the hti stiffness matrix for those layers we tested our method using vertical well log data from a tight sand reservoir in colorado usa thomsen style parameters were derived and gas filled fractures were identified on this log the identified gas filled fractures were compared to the production log data the fracture density was also obtained at the well location within the depth of interest we also found some problems and limitations caused by approximating vertical p wave velocity the same as the background isotropic p wave velocity

estimation of fracture parameters from reflection seismic data part iii fractured models with monoclinic symmetry existing geophysical and geological data indicate that orthorhombic media with a horizontal symmetry plane should be rather common for naturally fractured reservoirs here we consider two orthorhombic models one that contains parallel vertical fractures embedded in a transversely isotropic background with a vertical symmetry axis vti medium and the other formed by two orthogonal sets of rotationally invariant vertical fractures in a purely isotropic host rock using the linear slip theory we obtain simple analytic expressions for the anisotropic coefficients of effective orthorhombic media under the assumptions of weak anisotropy of the background medium for the first model and small compliances of the fractures all effective anisotropic parameters reduce to the sum of the background values and the parameters associated with each fracture set for the model with a single fracture system this result allows us to eliminate the influence of the vti background by evaluating the differences between t

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

estimation of modified fluid factor and dry fracture weaknesses using azimuthal elastic impedance abstractwe consider the problem of fluid identification and fracture detection in unconventional reservoir tight gas sand and shale gas characterization we begin with a simplification of the stiffness parameters and the derivation of a linearized reflection coefficient and azimuthal elastic impedance ei the accuracy of the simplification is confirmed in application to gas bearing fractured rocks with low porosity and small fracture density we have developed a modified fluid factor that is more sensitive to fluid type and less influenced by porosity a two step inversion workflow is evaluated based on the derived linearized reflection coefficient and azimuthal ei including 1 a damped least squares inversion for azimuthal ei constrained by an initial model and 2 a bayesian markov chain monte carlo inversion for the modified fluid factor and dry fracture weaknesses stability and accuracy are examined with synthetic data from which we conclude that the modified fluid factor and dry fracture we

elastic wave velocity characterization of gas hydrate bearing fractured reservoirs in a permafrost area of the qilian mountain northwest china abstract there are two types of gas hydrate bearing reservoirs in the permafrost area of qilian mountain most of the gas hydrates occur mainly in the fractured mudstone reservoirs and rarely in the pores of the sandstone reservoirs in this study for the acoustic velocity characterization of the fractured gas hydrate reservoirs of the qilian mountain permafrost area some mudstone core samples were collected for physical rock experiments such as the acoustic experiment and the porosity and permeability experiment an acoustic velocity numerical simulation of gas hydrate reservoirs was performed according to the biot theory and the differential effective medium theory with the conditions of multiple gas hydrate occurrence models including the suspension model the semi cementation model and the cementation model and considering both infinite and penny shaped cracks fracture porosity was added to the core samples that only contain matrix porosity with fracture porosity ranging from 0 01 to 5 the variation laws between acoustic velocity with fractured porosity and hydrate saturation are obtained 1 in the case of an infinite crack if the fractured porosity is 0 01 1 the p wave velocity decreases rapidly in the case of the three occurrence models if the fractured porosity is higher than 1 the acoustic velocity decreases gradually if the crack shape is a penny shaped crack the p wave velocity decreases almost linearly with increasing fracture porosity 2 if the hydrate occurrence model is the suspension model the p wave velocity increases slightly with increasing hydrate saturation if the occurrence model is the semi cementation model or the cementation model when the gas hydrate saturation of the infinite crack ranges from 0 to 80 the acoustic velocity increases approximately linearly whereas when the gas hydrate saturation ranges from 80 to 100 the velocity increases rapidly if the crack is a penny shaped crack the velocity increases almost linearly with increasing gas hydrate saturation from 0 to 100 3 it is found that the fractured gas hydrate reservoirs of the qilian mountain permafrost area contain both penny shaped and infinite cracks of which the infinite crack is the main crack shape the gas hydrate occurrence in the qilian mountain permafrost area mainly follows the suspension model this has significance for the seismic exploration and log evaluation of gas hydrate bearing fractured reservoirs in the permafrost area of the qilian mountain in studying the acoustic velocity characterization the crack shapes and occurrence models of gas hydrate reservoirs in the study area

onland signatures of the palawan microcontinental block and philippine mobile belt collision and crustal growth process a review abstract the collision of the palawan microcontinental block with the philippine mobile belt had significantly influenced the geological evolution of the philippines multiple collisions involving several fragments through space and time resulted into the collage of terranes of varying origin exposed in this part of central philippines cusping of the overriding plate volcanic arc gap ophiolite emplacement incipient back arc rifting island rotation and tilting raised coastal terraces metamorphism intrusion of igneous rocks and steepened subducted slab as seen in focal mechanism solutions are some of the manifestations of this collision a late early miocene to early middle miocene age 20 16 ma is proposed for the major collision between the palawan indenter and the philippine mobile belt the collision boundary is located from the northern part of mindoro through the central mountain range swinging east of sibuyan island in the romblon island group and finally threading along the buruanga peninsula and eastern side of the antique ophiolite complex before exiting and connecting with the negros trench the collision through accretion and crustal thickening has contributed to the crustal growth of the philippine archipelago

fluid substitution effects on seismic anisotropy we derive equations for hti and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media the derivations are based on the anisotropic gassmann equation and linear slip theory we assess the influence of fluid substitution gas brine and oil on elastic moduli velocities anisotropy and azimuthal amplitude variations we find that in the direction normal to fractures p wave moduli increase as much as 56 and p wave velocity increases up to 19 for gas to brine substitution for the direction parallel to fractures p wave velocity remains almost constant when porosity is low 5 but can increase up to 4 if porosity is high 25 since p waves in two different directions have different sensitivities to fluids and fractures the thomsen s parameters defined for hti and orthorhombic symmetries and are sensitive to fluid types and fractures we also found that is sensitive to porosity for liquid saturation but insensitive to porosity for the case of gas saturation gassmann assumes and as has been observed that shear modulus does not depend on fluids and we observe no changes in shear wave splitting for different fluids the azimuthal amplitude variation is dependent on fluid types fractures and porosity we observe up to 12 increase in azimuthal amplitude variation for low porosity gas sands after brine saturation and 6 decrease for high porosity gas sands we find that the percentage changes in gas to oil substitution are about half that of the gas to brine case the equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy

early paleozoic paleogeography and accretionary history of the newfoundland appalachians new paleomagnetic results were obtained from several units in the central mobile belt of the north central newfoundland appalachians combined with recently published data the results place important limits on interpretations of early paleozoic paleogeography and evolution of the appalachian caledonian orogen the ordovician appalachian margin of north america located at low paleolatitudes and facing roughly south was separated from the southern margin of iapetus avalon armorica by 3500 km primary directions in pillowed volcanic rocks of the moreton9s harbour group that are part of an early ordovician island arc show that these rocks were formed near the north american margin at a paleolatitude of 12 s early to middle ordovician volcanic rocks of the roberts arm chanceport and summerford groups on the other hand were extruded at paleolatitudes of 30 40 s unconformably overlying volcanics of the silurian botwood group were extruded subaerially at a paleolatitude of 24 s combined with tectonostratigraphic data these results suggest the following tectonic history in early ordovician time northerly subduction of the main iapetus basin was accompanied by the formation of a volcanic arc and a back arc basin near the north american margin middle ordovician emplacement of ophiolites on the north american margin reflects the closure of this back arc basin iapetus i and continued northerly subduction in silurian time closed the main basin iapetus ii

north luzon and the philippine sea plate motion model insights following paleomagnetic structural and age dating investigations results of one of the most comprehensive paleomagnetic and supporting geological programs ever carried out in offshore se asia on north luzon northern philippines are reported six new results based on 66 sites are reported from a total collection of 243 individual sites declinations in the data subset are sometimes scattered likely reflecting combinations of major plate and local rotations in both clockwise and counterclockwise directions and thus have a somewhat limited value for tectonic modeling the inclination data are however much more valuable and can be best explained if north luzon traveled as part of the philippine sea plate for most of its history a scenario which is compatible with the known geology of the eastern philippines and broader region in the proposed model for all of its eocene pliocene history north luzon is placed on the western edge of the philippine sea plate effectively always just to the west of the site where the benham plateau formed 40 ma the paleomagnetic data indicate a substantial northward migration of the area since the start of the neogene with an earlier interval stretching back to at least the mid early cretaceous when this part of the plate occupied equatorial latitudes post 15 ma motion of the plate has involved the indentation of the palawan microcontinental block into the western side of the philippine archipelago deformations induced by this process offer the most likely explanation for the scattered declinations observed in north luzon and areas a short distance to the south

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

evidence for early cretaceous oceanic crust trapped in the philippine sea plate abstract the huatung basin is a small oceanic basin located east of taiwan previous age estimates from magnetic lineation studies indicated an eocene age for the basin and formation from the central basin spreading center of the west philippine basin during the last phase of spreading in middle eocene new ar ar ages obtained on gabbros dredged on oceanic basement highs of the huatung basin are early cretaceous these old ages are consistent with early cretaceous ages determined on radiolarian assemblages from lanyu island luzon arc we have performed magnetic anomalies modeling for an early cretaceous oceanic crust our results are in good agreement with new ar ar ages determinations the best fit is indeed obtained with an opening of the huatung basin during the early cretaceous from 131 to 119 ma with a half spreading rate varying between 25 and 30 mm yr the spreading center appears to be located south of the actual basin the abnormal depth 5500 m instead of 5900 m and thickness 12 km instead of 6 km of the crust beneath the basin indicate that there was probably an excess supply of magma during its formation we propose that the basin is a fragment of the former proto south china sea or possibly the new guinea basin that has been trapped by the philippine sea plate

tectono stratigraphic terranes in cape breton island nova scotia implications for the configuration of the northern appalachian orogen cape breton island is divided into four terranes on the basis of contrasts in pre carboniferous geology the blair river complex in the north is an exposure of north american grenvillian basement analogous to the humber zone basement in newfoundland ordovician to devonian metavolcanic metasedimentary gneissic and granitic rocks of the aspy terrane are correlative with parts of the gander terrane of newfoundland and new brunswick the bras d9or terrane characterized by low pressure gneisses a carbonate clastic platform sequence and late precambrian early cambrian plutons may be correlative with units previously included in the gander terrane in southern newfoundland and the avalon terrane in southern new brunswick the mira terrane in southeastern cape breton island including late precambrian to early cambrian volcanic and sedimentary sequences and fossiliferous cambrian ordovician units is clearly part of the avalon terrane therefore with the exception of the dunnage terrane which is not represented in cape breton island the terranes of newfoundland continue through cape breton island they are offset to the northwest to the mainland part of the appalachian orogen in new brunswick

subduction zone retreat and recent tectonics of the south island of new zealand the central south island of new zealand is a zone of recent oblique continent continent convergence between the australian and pacific plates convergence is localized on the alpine fault and its broader expression has created the southern alps we propose that many of the strike variations in orogenic style of this zone e g physical extent and style of crustal shortening argon release patterns and fission track observations can be explained by differential amounts of subduction zone retreat beneath the orogen the observed patterns suggest greater eastward retreat of the subducting pacific plate in the south than in the north to support this hypothesis we compare geodynamic models having varying amounts of subduction zone retreat with geological and geophysical observations incorporating subduction zone retreat not only creates tectonic variations similar to those observed but also provides a geodynamically consistent explanation for the size shape and location of the broad negative bouguer gravity anomaly associated with the orogen in addition these geodynamic models suggest that subduction zone retreat may result in local topographic lows which may serve as preferential locations for the formation of intermontane basins and associated lakes possibly explaining the chain of south island lakes our preferred interpretation of the geologic history of the central south island convergence has an initial phase of pacific plate lower crust and mantle subduction underthrusting with no retreat followed by a second phase of subduction zone retreat in the southern part of the zone this retreat may be a consequence of oblique convergence which creates interference beneath fiordland between the south east dipping australian plate and the underthrust west dipping pacific plate to the north and forces retreat

rb sr biotite and whole rock data from the kapuskasing uplift and their bearing on the cooling and exhumation history rb sr isotopic ratios were measured on biotite and whole rocks from a suite of 21 granitoid rocks from the southern kapuskasing uplift results of 18 of the whole rock analyses fall on an isochron with an age of 2 677 0 057 ga and sri 0 70080 biotite model ages range from 2 50 to 1 93 ga in a general spatial relationship of decreasing age with depth in the structural section as calibrated with the igneous hornblende barometer for tonalitic rocks and garnet pyroxene barometers for granulites near the michipicoten belt in the west biotite ages of 2 ga reflect disturbances also recorded by paleomagnetic and whole rock rb sr systems to the east in a 70 km long northwest southeast transect toward the ivanhoe lake fault zone ages decrease from 2 50 to 1 95 ga in a southern transect ages are in the range 2 30 2 38 ga to within 15 km of the fault reflecting consistent erosion levels corresponding to 0 53 mpa but decrease to 1 93 ga to the east in concert with paleopressures in the 0 7 1 0 gpa ra

approximate fluid substitution for vertical velocities in weakly anisotropic vti rocks despite the prevalence of elastic anisotropy in rocks most models used routinely for wave propagation imaging and rock physics assume an isotropic earth the main reason for using isotropic approximations is that we seldom measure enough parameters to characterize the stiffness tensor of a rock completely fluid substitution is an important example because of an incomplete knowledge of the stiffness tensor often we choose isotropic equations over their anisotropic form assuming weak anisotropy we derive an approximate form of the anisotropic fluid substitution equation for seismic waves propagating along the symmetry axis of a transversely isotropic medium the approximation takes the form of the usual isotropic calculation with a simple first order correction proportional to the thomsen anisotropic parameter thus requiring only three elastic constants because can be either greater than or less than zero in a vti medium we can explain why isotropic fluid substitution sometimes overpredicts

the west philippine basin an eocene to early oligocene back arc basin opened between two opposed subduction zones 1 based on geological and geophysical data collected from the west philippine basin and its boundaries we propose a comprehensive cenozoic history of the basin our model shows that it is a back arc basin that developed between two opposed subduction zones rifting started around 55 ma and spreading ended at 33 30 ma the initial spreading axis was parallel to the paleo philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin reaching the former one at an r r r triple junction spreading occurred mainly from this second axis with a quasi continuous counter clockwise rotation of the spreading direction the gagua and palau kyushu ridges acted as transform margins accommodating the opening arc volcanism occurred along the palau kyushu ridge eastern margin during the whole opening of the basin whereas the paleo philippine arc decreased its activity between 43 and 36 ma the western margin underwent a compressive event in late eocene early oligocene time leading to the rising of the gagua ridge and to a short subduction episode along eastern luzon in the western part of the basin the spreading system was highly disorganized due to the presence of a mantle plume overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed shortly after the end of the spreading a late stage of amagmatic extension occurred between 30 and 26 ma in the central part of the basin being responsible for the deep rift valley that cut across the older spreading fabric

elastic wave propagation in media with parallel fractures and aligned cracks a model of parallel slip interfaces simulates the behaviour of a fracture system composed of large closely spaced aligned joints the model admits any fracture system anisotropy triclinic the most general monoclinic orthorhombic or transversely isotropic and this is specified by the form of the 3 3 fracture system compliance matrix the fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy to compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 3 matrices when the fractures are assumed on average to have rotational symmetry transversely isotropic fracture system behaviour and the background is assumed isotropic the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters in addition to the two isotropic parameters of the isotropic background dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints i e there are two additional parameters due to the presence of the microcracks which play roles in the stress strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints thus knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation as the aligned microcracks which were assumed to be ellipsoidal with very small aspect ratio are allowed to become non fiat i e have a growing aspect ratio the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks the divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0 3

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

analysis of fluid substitution in a porous and fractured medium to improve quantitative interpretation of seismic data we analyze the effect of fluid substitution in a porous and fractured medium on elastic properties and reflection coefficients this analysis uses closed form expressions suitable for fluid substitution in transversely isotropic media with a horizontal symmetry axis hti for the hti medium the effect of changing porosity and water saturation on 1 p wave moduli 2 horizontal and vertical velocities 3 anisotropic parameters and 4 reflection coefficients are examined the effects of fracture density on these four parameters are also studied for the model used in this study a 35 increase in porosity lowers the value of p wave moduli by maximum of 45 consistent with the reduction in p wave moduli p wave velocities also decrease by maximum of 17 with a similar increment in porosity the reduction is always larger for the horizontal p wave modulus than for the vertical one and is nearly independent of fracture density the magnitude of the anisotropic parameters of the fractured medium also changes with increased porosity depending on the changes in the value of p wave moduli the reflection coefficients at an interface of the fractured medium with an isotropic medium change in accordance with the above observations and lead to an increase in anisotropic amplitude variation with offset avo gradient with porosity additionally we observe a maximum increase in p wave modulus and velocity by 30 and 8 respectively with a 100 increase in water saturation water saturation also changes the anisotropic parameters and reflection coefficients increase in water saturation considerably increases the magnitude of the anisotropic avo gradient irrespective of fracture density from this study we conclude that porosity and water saturation have a significant impact on the four studied parameters and the impacts are seismically detectable

elastic anisotropy due to aligned cracks in porous rock all theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way for example to the case of stiff pore fluids or of the absence of equant porosity or of a moderately high frequency band because of these restrictions previous theory is not suitable for application to the upper crust where the pore fluid is brine kf k820 the equant porosity is often substantial p 0 1 and the frequency band is sonic to seismic this work removes these particular restrictions recognizing in the process an important mechanism of dispersion a notable feature of these more general expressions is their insensitivity at low frequency to the aspect ratio of the cracks only the crack density is critical an important conclusion of this more general model is that many insights previously achieved concerning the shear wave splitting due to vertical aligned saturated cracks are sustained however conclusions on crack orientation or crack aspect ratio which were derived from p wave data or from shear wave critical angles may need to be reconsidered further the non linear coupling between pores and cracks due to pressure equalization effects means that the linear schoenberg muir calculus may not be applied to such systems the theory receives strong support from recent data by rathore et al on artificial samples with controlled crack geometry

evolution of the boundary between the philippine sea plate and australia palaeomagnetic evidence from eastern indonesia abstract the boundary between the philippine sea and australian plates is the left lateral sorong fault system of eastern indonesia until recently modelling this boundary for the period before about 5 ma was difficult the tertiary motion of the philippine sea plate was uncertain and palaeomagnetic data from areas adjacent to the fault were lacking recent geological and palaeomagnetic studies of the area north of the sorong fault have elucidated the tertiary motion history of the philippine sea plate providing a reference for examining movements within the fault system we report new palaeomagnetic data from within the sorong fault zone from the islands of taliabu and obi taliabu is part of the sula platform and is considered to be derived from australia pelagic limestones from the upper cretaceous tanamu formation of taliabu yielded a direction of d 329 1 i 34 9 implying counter clockwise rotation and a formation latitude of 19 5 s sula and misool are postulated to be part of a single microcontinent which had a different late cretaceous mid tertiary movement history from australia the sula platform was transported to its present position by movement along the sorong fault system in the late miocene obi includes rocks of philippine sea and australian origin all the new sites are in rocks of philippine sea plate origin since the early neogene the philippine sea plate which includes all islands north of the sorong fault has rotated 40 clockwise and moved 10 15 northwards philippine sea plate rocks within the sorong fault zone record similar latitude shifts but different rotations in north obi the upper oligocene anggai river formation and the middle miocene woi formation record 60 and 30 counter clockwise rotations respectively the sense of rotation is consistent with motion within a left lateral fault system with the philippine sea and australian plates providing the shear couple in contrast the woi formation in southeast obi records 15 20 clockwise rotation this area is separated from the zone of counter clockwise movement in north obi by a strand of the sorong fault arc volcaniclastic rocks from the upper cretaceous leleobasso formation of northwest obi have a primary magnetisation with a mean direction of d 357 1 i 21 9 these rocks formed at 11 n or 11 s depending on the interpreted rotation history and indicate a pacific rather than indian ocean origin a volcanic arc at the southern edge of the philippine sea plate collided with eastern new guinea at 25 ma the philippine sea australia plate boundary then changed from subduction to strike slip as the philippine sea plate began its neogene rotation initiating the sorong fault system we suggest that many of the arc fragments in the new guinea orogenic belt originated in the southern philippine sea plate arc which has subsequently been dismembered by strike slip faulting

structural evolution and tectonic significance of the eastern highlands shear zone in cape breton island the canadian appalachians the eastern highlands shear zone in cape breton island of the canadian appalachians is characterized by an amphibolite facies deformation zone over 5 km wide overprinted by a greenschist facies mylonite zone about 1 km wide deformation zones in both metamorphic grades dip steeply to the southeast with movement direction pitching steeply to the southwest and shear sense indicators indicate the same sense of shear that is an east over west dip slip movement with minor sinistral strike slip component deformation in both conditions is constrained to the late silurian to early devonian mainly late silurian it is suggested that the greenschist facies deformation represents the last stage of a single episode of deformation that occurred initially under amphibolite facies conditions the west vergent shearing along the shear zone is antithetic to the westward subduction that led to the silurian continent continent collision which is interpreted by tectonic wedging in this part of the canadian appalachians

p and s wave anisotropy of a synthetic sandstone with controlled crack geometryl1 a method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix a synthetic sandstone was manufactured from sand cemented with an epoxy glue the cracks of known geometry were introduced into the material in the manufacturing stage by emplacing thin metallic discs in the sand epoxy matrix these discs were chemically leached out of the consolidated porous sandstone acoustic anisotropy and shear wave splitting were observed in the synthetic sandstones for the dry sample the observed angular dependence of the p and s wave velocities at 100 khz compares well qualitatively with the theoretical models of hudson and of thomsen quantitatively however the experimental data fits hudson s model better for the case of a saturated sample the experimental results are in excellent agreement with thomsen s model hudson s model on the other hand predicts a different angular dependence for p waves this demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process the results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry

effects of progressive mylonitization on ar retention in biotites from the santa rosa mylonite zone california and thermochronologic implications the santa rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the peninsular ranges batholith a wide variation in k ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation induced reduction in grain size we suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes previous workers have shown that deformation in the santa rosa mylonite zone involved the formation of mylonites and an imbricate series of low angle faults which divide the area into structural units field work petrographic studies and tem analysis of deformation mechanisms in biotite show that the granodiorite mylonite the lowermost structural unit formed below the granodiorite solidus the granodiorite mylonite varies from protomylonite to ultramylonite with regions of high strain distributed heterogeneously within the zone previously reported biotite and hornblende k ar dates from the granodiorite protolith below 82 89 ma and the asbestos mountain granodiorite above 61 68 ma the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units other workers fission track dates on sphene zircon and apatite from the granodiorite mylonite and the asbestos mountain granodiorite suggest thermal homogenization and rapid cooling to 100 c by ca 60 ma within and adjacent to the mylonite zone k ar dates on 5 samples of biotite from variably deformed granodiorite range from 62 76 ma dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction 40ar 39ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum 40ar 39ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39ar recoil loss k analysis of samples allowing k ar dates to be calculated is therefore recommended as an adjunct to 40ar 39ar step heating analysis in rocks that have experienced similar deformation during mylonitization biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0 05 m locally within porphyroclasts as large as 1 mm because biotite compositions are relatively uniform fe fe mg mn ti alvi 0 47 0 52 and show no systematic variation with grain size compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in ar retention ar closure temperatures calculated with appropriate diffusion parameters for the observed grain sizes are in the range 220 280 c and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the asbestos mountain granodiorite above the mylonite zone changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330 c 80 ma above the closure temperature for thermally activated diffusion of argon in biotite and continued to a minimum of ca 220 260 c 62 ma

a double island arc between taiwan and luzon consequence of ridge subduction analysis of geomorphological geochronological geochemical and geophysical features in the segment of the taiwan luzon arc between taiwan and luzon the bashi segment allows the recognition of a double arc structure the two volcanic chains are separated by 50 km just north of luzon 18 n and converge near 20 n islets in the western chain are older and largely composed of volcanic rocks of miocene to pliocene age they all show low relief lateritic platforms and wave cut terraces and are covered by massive recrystallized limestone in contrast all active volcanoes in this segment of the taiwan luzon arc belong to the eastern chain where most islets are quaternary in age the volcanoes have well developed cone shapes and well preserved deposits of near vent facies magmas of the eastern chain have higher ksi la n la yb n and lowerend than their counterparts at the same latitude in the western chain therefore the magmas erupted in the eastern chain were derived from more enriched mantle sources than the magmas erupted in the western chain moreover the available seismological data seem to suggest an abrupt increase of the dip angle from 30 at 18 n to 80 at 20 n thus the double arc structure is located in the region where the benioff zone suddenly changes in analogy with the lesser antilles arc we propose a geodynamic model in which the double arc in the bashi strait is the tectonic manifestation of the subduction of the aseismic scarborough seamount chain the extinct mid ocean ridge of the south china sea before that ridge reached the manila trench the western chain was the volcanic front when the ridge reached the subduction zone at 5 4 ma its buoyancy temporarily interrupted the subduction thus causing a time gap in magmatic activity furthermore this ridge arc collision was probably also responsible for regional uplift causing extensive sub aerial weathering and erosion as well as massive reef formation in the western chain when subduction started again the dip angle became shallower in response to the extra buoyancy of the downgoing ridge if the depth of magma generation remained constant the shallower dip angle would have naturally led to an eastward shift of the volcanic front thus producing the younger eastern chain moreover we speculate that the abrupt change of the dip angle may have torn the downgoing slab thus allowing more enriched continental lithospheric material to invade the mantle wedge from the northwest thus imprinting a geochemically more enriched signature on the magmas of the eastern chain

40ar 39ar age pattern associated with differential uplift along the eastern highlands shear zone cape breton island canadian appalachians abstract the eastern highlands shear zone in cape breton island is a crustal scale thrust it is characterized by an amphibolite facies deformation zone 5 km wide formed deep in the crust that is overprinted by a greenschist facies mylonite zone 1 km wide that formed at a more shallow level hornblende 40ar 39ar plateau ages on the hanging wall decrease towards the centre of the shear zone in the older zone over 7 8 km from the centre the ages are between 565 and 545 ma in the younger zone within 4 5 km of the centre they are between 425 and 415 ma and in the transitional zone in between they decrease abruptly from 545 to 425 ma pressures of crystallization of plutons in the hanging wall based on the al in hornblende barometer and corresponding to depth of emplacement increase towards the centre of the shear zone and indicate a differential uplift of up to 28 km associated with movement along the shear zone the age pattern is interpreted to have resulted from the differential uplift the pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende ar blocking temperature 500 c until differential uplift occurred the 40ar 39ar ages in the zone 425 415 ma thus date shear zone movement or the last stage of it in contrast rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of 565 555 ma plutons in the area long before shear zone movement the transitional zone corresponds to the ar partial retention zone before differential uplift the 40ar 39ar age pattern thus reflects a neoproterozoic to silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone a similar k ar age pattern has been reported for the alpine fault in new zealand it is suggested that such isotopic age patterns can be used to help constrain the ages kinematics displacements and depth of penetration of shear zones

tertiary arc rifting in northern luzon philippines the north luzon terrane nlt comprising the section of luzon north of the philippine fault is one of the largest arc terranes in the philippine archipelago numerous features suggest that the nlt is a late oligocene to early miocene analogue for the processes in the modern intra arc rift zone at the northern end of the mariana trough first the nlt has bifurcating magmatic arcs sharing similar magmatic histories these include the northern sierra madre nsm and cordillera central cc magmatic arcs which are separated by the cagayan basin but which are linked in the caraballo range to the south the rock record indicates that the nsm cc and caraballo ranges were active arcs in late eocene to late oligocene time second seismic reflection and well data indicate that the cagayan basin formed by extensional faulting in late oligocene to early miocene time third alkalic arc magmatism recognized to be a precursor of intra arc rifting in modern settings occurred at the juncture of the nsm and cc arcs in late oligocene to early miocene time fourth oceanic crust represented by the itogon ophiolite formed at the southwestern end of the cagayan basin in late oligocene to early miocene time major and trace element chemistry show that the itogon sheeted dikes have tholeiitic arc and backarc basin basalt affinities the rock record and geophysical offshore data suggest that the nlt was developing in an island arc system above the subducting west philippine plate in late eocene time rifting occurred in the island arc from late oligocene to early miocene time but did not mature into backarc spreading most likely because of the collision of the benham rise a basaltic rise in the west philippine basin with the nlt the arc rifting in the nlt may be another manifestation of the extensional tectonism that affected most of southeast asia in late oligocene to early miocene time during which the south china and southeast sulu basins formed subsequent to arc rifting the history of the nlt has been linked to the subduction of the south china plate along the manila trench the structural history of the cagayan basin and magmatic history of the southern cc suggest that the subduction in the manila trench at the latitude of the nlt began about 15 ma

40 ar 39 ar mineral ages from the southern cape breton highlands and creignish hills cape breton island canada evidence for a polyphase tectonothermal evolution 40 ar 39 ar ages for hornblende and muscovite from plutons and metamorphic country rocks in the southern cape breton highlands and creignish hills together with previously published u pb rb sr and 40 ar 39 ar ages document a complex tectonothermal history that included 1 episodes of plutonic activity at ca 600 565 560 555 500 490 470 and 410 395 ma all of which cooled through 500 400 c within ca 20 m y and 2 deformation in the late precambrian 550 ma and the late silurian middle devonian 415 380 ma the latter was associated with evelopment of a positive flower like structure during maintenance of medium pressure greenschist amphibolite facies regional metamorphic conditions that may be traced northward along the length of the cape breton highlands it may be correlated with the kingston complex and associated mylonite zone exposed in the avalon composite terrane of southern new brunswick northward it merges with a major high grade poly deformed belt within the gan

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

azimuthally anisotropic elastic impedance inversion for fluid indicator driven by rock physics the normal to tangential fracture compliance ratio is usually used as a fracture fluid indicator ffi for fluid identification in fractured reservoirs with a new parameterization for fracture weaknesses we define a new ffi based on azimuthally anisotropic elastic impedance ei inversion and fractured anisotropic rock physics models first we derived a new azimuthally anisotropic ei equation with a similar expression for the isotropic and anisotropic ei parts to remove the exponential correction of ei which is attributable to weak anisotropy then we built a fractured anisotropic rock physics model used for the estimation of well log parameters for the normal and tangential fracture weaknesses which builds the initial background low frequency trend of fracture weaknesses finally based on the azimuthally anisotropic ei inversion method with the cauchy sparse and low frequency information regularization we estimated a ffi applied to fluid identification in fractured reservoirs tests on both synthe

east asia plate tectonics since 15 ma constraints from the taiwan region abstract 15 ma ago a major plate reorganization occurred in east asia seafloor spreading ceased in the south china sea japan sea taiwan sea sulu sea and shikoku and parece vela basins simultaneously shear motions also ceased along the taiwan sinzi zone the gagua ridge and the luzon ryukyu transform plate boundary the complex system of thirteen plates suddenly evolved in a simple three plate system eu ph and pa beneath the manila accretionary prism and in the huatung basin we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations these two patterns are rotated by 15 they were formed by seafloor spreading before 15 ma and belonged to the same ocean named the taiwan sea half spreading rate in the taiwan sea was 2 cm year from chron 23 to 20 51 to 43 ma and 1 cm year from chron 20 43 ma to 5b 15 ma five plate kinematic reconstructions spanning from 15 ma to present show implications concerning the geodynamic evolution of east asia amongst them the 1000 km long linear gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the ph sea plate the formation of taiwan was driven by two simple lithospheric motions i the subduction of the ph sea plate beneath eurasia with a relative westward motion of the western end a of the ryukyu subduction zone ii the subduction of eurasia beneath the philippine sea plate with a relative southwestward motion of the northern end b of the manila subduction zone the luzon arc only formed south of b the collision of the luzon arc with eurasia occurred between a and b east of a the luzon arc probably accreted against the ryukyu forearc

benham rise unveiled morphology and structure of an eocene large igneous province in the west philippine basin abstract the benham rise is an oceanic large igneous province at the western margin of the philippine sea it has an ocean island basalt geochemistry a surface area of 0 11 mkm2 and a volume of 0 13 mkm3 a crustal thickness of 15 km is estimated from gravity anomalies volcanism was most active in the early stages of its formation during the lutetian from 47 9 ma to 41 3 ma although volcanic activity extended to 26 ma multibeam bathymetry with almost 100 coverage has revealed the morphology of benham rise for the first time it consists of a main body 310 km by 330 km with the narra loro and molave spurs extending 100 200 km from its eastern side the main body is built on a shield platform from 5200 m to 3800 m sub sea the platform flanks consist of 3 15 km wide terraces with scarps as high as 100 m to 300 m the platform is surmounted by a crest exhibiting caldera morphology at an average depth of 2500 m the crest is named apolaki caldera and may be the world s largest known caldera with a diameter of 150 km features like a breached rim intra caldera benches and a resurgent dome indicate a multi phase volcanic history consisting of both quiet and explosive eruptions the bathymetry and publicly available geological and geophysical data record three main stages in the tectonic history of benham rise shield building caldera formation and post caldera late stage volcanism the platform base is interpreted to have formed over a hotspot beneath the central basin spreading center during the shield building phase between 47 9 ma to 42 5 ma the arrowhead shaped molave spur is interpreted to have formed late in this stage along a ridge that propagated away from the hotspot the caldera formation followed and is interpreted to have occurred before 41 3 41 5 ma when benham rise was still joined with urdaneta plateau the narra spur was formed by late stage hotspot related volcanism as benham rise and urdaneta plateau separated and ended with the eruptions of the vinogradov seamount at 26 ma

paleomagnetism of the samar ophiolite implications for the cretaceous sub equatorial position of the philippine island arc abstract samar island in the eastern part of central philippines is underlain by a complete ophiolite suite the samar ophiolite we present the first geochronological and paleomagnetic data for the samar ophiolite whole rock k ar dating of two basalt samples yielded an age of 100 2 2 7 ma and 97 9 2 8 ma thirteen sites in four localities yielded characteristic remanent magnetization with in situ direction of d 340 i 24 k 15 95 11 and tilt corrected direction of d 342 i 27 k 15 95 11 these values suggest that the ophiolitic basement rocks of samar formed in the late cretaceous at a paleolatitude of 14 s 6 the paleolatitude is several degrees south of the sub equatorial positions calculated for the three other mesozoic ophiolites of the philippine mobile belt pmb whose paleomagnetism had been previously studied the pmb ophiolites in eastern and central philippines share a common age geochemistry and paleolatitude with the halmahera ophiolite suggesting that they originated from a mesozoic supra subduction zone that spanned a few degrees north of the equator to around 15 s

origin and evolution of the west philippine basin a new interpretation abstract combined analyses of the magnetic lineations and seafloor structures of the west philippine basin show that it was formed by spreading from the central basin spreading center in two distinctly different spreading phases before and after subduction was initiated along the palau kyushu trend at 45 ma b p these episodes are recognized by differences in the strike of the magnetic lineations spreading rates density and strike of fracture zones and basement relief from 60 45 ma b p spreading was ne sw relative to present orientations at a half rate of 44 mm a at about 45 ma b p the spreading direction changed to a more n s direction with a reconfiguration of the central basin spreading center into numerous short e w segments offset by closely spaced n s transform faults spreading slowed to a half rate of 18 mm a and ceased at 35 ma b p thus the west philippine basin originated at 45 ma b p by the trapping of normal ocean crust west of the initial subduction along the palau kyushu trend the 45 35 ma b p period represents the dying phase of spreading on the central basin spreading center following isolation of the west philippine basin from the plate driving forces of the pacific

predicting subsurface co2 movement from laboratory to field scale abstractfinding an appropriate model for time lapse seismic monitoring of co2 sequestered carbonate reservoir poses a great challenge because carbonate rocks have varying textures and highly reactive rock fluid system we introduced a frequency dependent model based on eshelby s inclusion and differential effective medium dem theory that can account for heterogeneity in microstructure of rocks and squirt flow we showed that the estimated velocities from the modified dem theory match well with the laboratory measurements ultrasonic of velocities of carbonate rocks saturated with co2 rich water the theory predicts significant decrease in saturated p and s wave velocities in the seismic frequency band as a consequence of porosity and permeability enhancement by the process of chemical dissolution of carbonates with the saturating fluid the study also showed the combined effect of chemical reaction and free co2 saturation on the elastic properties of rock we observed that the velocity dispersion and