geodetic constraints on postseismic deformation following the 1990 ms 7 8 luzon earthquake and implications for luzon tectonics and philippine sea plate motion 1 we have obtained three epochs of high quality dual frequency gps data at 15 stations in the luzon philippines region since the july 16 1990 ms 7 8 earthquake from which we generate horizontal velocity fields at the earth s surface for the 1993 1996 and 1996 1998 periods we use these velocities plus 1996 1998 gps velocity results reported elsewhere from 13 additional stations to show that present day deformation in luzon is dominated by strike slip motion along the philippine fault system the measured strike slip rate across central luzon is faster than the expected long term slip rate on the fault which we attribute to postseismic deformation following the 1990 earthquake the 1993 1996 and 1996 1998 deformation patterns cannot be statistically distinguished within the 10 uncertainties of the data if the observed deformation is interpreted using an elastic half space model with uniform slip below a subsurface locking depth we find a fault slip rate of 40 mm yr and a locking depth of 15 km this is too fast to be a steady interseismic rate as it disagrees with what is known from paleoseismic and other evidence about earthquake recurrence and long term slip rate on the philippine fault system in central and northern luzon also a 15 km locking depth for steady interseismic slip is probably too shallow compared to the depth of rupture in the 1990 earthquake if the deformation is instead interpreted with two types of viscoelastic model we find that the observed velocities can be fit well for a range of values of lower lithosphere viscosity and long term slip rate using a two dimensional 2 d viscoelastic coupling model the minimum allowable lower lithosphere viscosity is 0 5 1019 pa s with an associated long term slip rate of 15 22 mm yr faster long term slip rates also fit well with correspondingly higher values of viscosity with a possibly more realistic model that includes 3 d viscoelastic effects we find preferred long term velocities in the range 20 35 mm yr and viscosities in the range 2 6 1019 pa s our preferred viscosities correspond to maxwell times on the order of 7 20 of the typical earthquake recurrence interval and are several times higher than values found for a number of californian strike slip earthquakes other implications of our 1993 1998 surface velocities are that the philippine sea eurasia ph eu euler vector of seno et al 1993 is a better description of the ph eu convergence rate than a euler vector recently estimated from gps velocities in the northern ph and that the great majority of the normal component of ph eu plate convergence is taking place west of luzon presumably largely at the manila trench we also find that luzon is rotating counterclockwise relative to ph at 1 2 myr only 25 of the rate recently suggested by other workers on the basis of limited gps data

relative paleointensity in sediments a pseudo thellier approach we present a method for normalizing sedimentary records for estimating relative paleointensity of the geomagnetic field similar to that successfully used to obtain absolute paleointensity from thermally blocked remanences it has the advantages that it is more effective in removing unwanted viscous remanence thereby improving agreement among various records and that it allows the estimation of the uncertainty in the relative paleointensity calculated

a new branch of the philippine fault system as observed from aeromagnetic and seismic data abstract the philippine fault is one of the major strike slip faults of the world extending over 1200 km from the lingayen la union region of northwestern luzon to the davao gulf south of mindanao nevertheless its total slip history of displacement and its exact location have been uncertain and even controversial significant new insight has been provided by aeromagnetic and seismic surveys made as part of a basin evaluation project of the philippines bureau of energy development funded by the world bank this paper outlines evidence from these surveys for a previously unrecognized major branch of the philippine fault system and tentatively discusses its paleogeographic implications we have interpreted aeromagnetic data covering most of the length of the fault and seismic profiles crossing it at various locations the philippine fault is well displayed in the aeromagnetic data surprisingly the data also show a major previously unrecognized branch of the philippine fault here named the sibuyan sea branch this branch is almost entirely offshore or is covered by quaternary or recent volcanic rocks it leaves the known philippine fault northeast of masbate island passes along the northeast edge of the sibuyan sea southwest of the bondoc peninsula passes northeast of marinduque island apparently passes through the taal volcano and then offshore west of the batangas peninsula the sibuyan sea branch is 350 km long and is presently active it is more obvious on seismic and aeromagnetic data than the northern part of the known philippine fault and thus may have accommodated more motion we have made a preliminary evaluation of the total slip on the sibuyan sea branch by attempting to restore long wavelength geophysical and geologic features including forearc basins ophiolite belts volcanic belts and regional bouguer free air and magnetic anomalies which suggests the possibility of 200 300 km of slip since about the early or middle miocene in this reconstruction the central luzon basin and luzon volcanic arc are continuous with the forearc basin and volcanic belt on panay as a continuous arc and forearc extending from northern ilocos to southern panay after collision with the palawan mindoro continental block continued motion along the manila trench north of mindoro has ruptured the central philippines along the sibuyan sea branch of the philippine fault thus the sibuyan sea branch could account for most of the post early to middle miocene relative motion between luzon and the visayas

rock magnetic properties and relative paleointensity stack between 13 and 24 kyr bp calibrated ages from sediment cores lake moreno patagonia argentina we conducted a detailed study of natural remanence and rock magnetic properties on three sediment cores from lake moreno patagonia argentina based on these measurements we constructed a relative paleointensity stack for the period 13 24 kyr bp calibrated ages rock magnetic properties of the sediment cores showed uniform magnetic mineralogy and grain size suggesting that they were suitable for relative paleointensity studies the remanent magnetisation at 20 mt nrm20 mt was normalised using the anhysteretic remanent magnetisation at 20 mt arm20 mt the saturation of the isothermal remanent magnetisation at 20 mt sirm20 mt and the low field magnetic susceptibility k spectral analysis showed that the normalised records were not affected by local environmental conditions the recorded pseudothellier paleointensity was compared with records obtained from conventional normalising methods comparing the paleointensity curves with others obtained previously in other lakes in the area has allowed us to reach reliable conclusions about centennial scale features 2008 elsevier b v all rights reserved

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

variations of the geomagnetic field during the holocene relative paleointensity and inclination record from the west pacific odp hole 1202b abstract we conducted detailed rock magnetic investigations on 36 m of drill core collected during ocean drilling program leg 195 at hole 1202b 24 48 24 n 122 30 00 e in the southern okinawa trough with the goal of extracting a reliable paleointensity signal with centennial resolution an age depth model was established from a chronology obtained by accelerator mass spectromety 14c dating the sedimentary section spans almost the entire holocene 0 9 4 kyr and exhibits sedimentation rates close to 400 cm kyr the magnetic properties are dominated by stable pseudo single domain magnetite high field hysteresis data and the grain size sensitive ratio of anhysteretic remanent magnetization arm to low field magnetic susceptibility indicate a narrow range of grain sizes and concentrations magnetic parameters vary by a factor of 4 thereby fulfilling the usual criteria for a relative paleointensity study the relative geomagnetic paleointensity was obtained by normalizing the intensity of natural remanent magnetization nrm by the arm and the low field magnetic susceptibility both normalizations yield nearly identical results r 0 89 spectral analysis indicates that the record is not significantly affected by local environmental conditions comparison of this west pacific paleointensity curve with other curves suggests a geomagnetic origin for the signal millennial scale features of our record correlate to variations of the archeomagnetic dipole moment this suggest that the sediments at hole1202b recorded changes of the geomagnetic field over the studied time interval

evidence for multiple paleomagnetic intensity lows between 30 and 50 ka bp from a western equatorial pacific sedimentary sequence abstract a paleomagnetic study was carried out on the radiocarbon dated md97 2134 core located in the western equatorial pacific southern papua new guinea margin rock magnetic investigations revealed changes of the magnetic mineralogy along the hemi pelagic sedimentary sequence but the reconstruction of past direction and relative paleointensity variations of the geomagnetic field remained feasible four successive paleointensity drops are recorded between 30 and 50 ka bp the largest one is associated with an abrupt swing of declination and inclination interpreted as a smoothed signature of the laschamp excursion 41 ka bp the succession of four events of weak intensity between 30 and 50 ka bp introduces a complex behaviour of the geomagnetic field in the time interval spanning over the laschamp and the mono lake excursions

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

paleomagnetism of the zambales ophiolite luzon northern philippines abstract paleomagnetic results are reported from 52 sites in the zambales ophiolite the sites provide vertical sampling from the overlying sediments in the north and south to layered gabbros and dunites in the acoje and coto mines in the west they extend over an area of approximately 100 by 50 km a late miocene pliocene intrusion in the tarlac region gives a direction marginally distinguishable from the present field indicating a small clockwise rotation this is consistent with other indications of clockwise rotation since late miocene in luzon early and mid miocene sediments from barlo in the north and tarlac in the south indicate a period of rapid counterclockwise rotation during the miocene again consistent with other results from luzon eocene oligocene sediments overlying the ophiolite in the north and south indicate further counterclockwise rotation with northerly translation the interpretation of the directions in the ophiolite itself is more problematical directions similar to those in the eocene oligocene and in mid miocene sediments probably reflect secondary magnetizations and magnetization associated with the unroofing of the ophiolite we interpret the most strongly counterclockwise rotated magnetizations represented by a 44 ma sill in the barlo area and of the 46 ma dykes at coto as the primary direction of the ophiolite the counterclockwise rotation and northward translation interpreted from our results is consistent with the holloway model of the origin of the philippines however this model neglects the role of the philippine sea plate in the tectonic history of the philippines the counterclockwise rotation is also consistent with rotation of luzon in the left lateral shear between the philippine sea plate and asia as suggested by sarewitz and karig our data do not distinguish between counterclockwise rotation of a single block or of a number of smaller blocks the data suggest post late miocene clockwise rotation for luzon when it was coupled to the philippine sea plate our data are not consistent with a philippine sea plate origin for zambales

an 84 kyr paleomagnetic record from the sediments of lake baikal siberia we have conducted a paleomagnetic study of sediment cores obtained from the selenga prodelta region of lake baikal russia this record which spans approximately the last 84 kyr contributes to a better understanding of the nature of geomagnetic field behavior in siberia and is a useful correlation and dating tool we demonstrate that the lake baikal sediments are recording variations in the geomagnetic field the directional record displays secular variation behavior with a geomagnetic excursion at 20 ka and additional excursions appearing as large amplitude secular variation at 41 61 and 67 ka smoothing of the geomagnetic excursion behavior occurs in lake baikal sediments owing to the intermediate sedimentation rate 13 cm kyr 1 the lake baikal relative paleointensity record correlates to absolute paleointensity data for the last 10 kyr and to relative paleointensity records from the mediterranean sea and indian ocean for the last 84 kyr this correlation suggests a strong global i e dipole component to these records and further supports the reliability of sediments as recorders of relative geomagnetic paleointensity we show that a relative geomagnetic intensity stratigraphy has a potential resolution of 7 kyr by correlating continental and marine records the geomagnetic intensity stratigraphy helps constrain the age of the difficult to date lake baikal sediments

environmental magnetism of antarctic late pleistocene sediments and interhemispheric correlation of climatic events abstract recent developments in paleomagnetism and environmental magnetism provide new tools for the detailed correlation of climatically induced magnetic mineralogy changes in sedimentary sequences studies of these changes contribute to the reconstruction of climate history for the glacial interglacial cycles of the late pleistocene and to the delineation of the range of natural variability for global climate during the past hundred thousands years here we show that sharp coercivity minima observed in fine grained sediments from the continental rise of the western antarctic peninsula correlate to the major rapid cooling events of the northern atlantic heinrich layers we interpret such an environmental magnetic signal in terms of variations in deep sea diagenetic processes of sulfide formation which reflect changes in the input of detrital organic matter controlled by sea ice extent with the inherent uncertainties in age controls the sedimentary paleoclimatic markers of the two hemispheres are almost contemporaneous but interhemispheric time lags or leads of the order of 1 2 kyr such as those recently reported from the greenland and antarctic ice cores are also compatible with the data

a comparison of different techniques for relative paleointensity parallel relative paleointensity records that are supposed to display the same geomagnetic variations often show discrepancies we compared various techniques of normalization with different parameters involving alternating fields a f and thermal demagnetization of sediment samples spanning the same period from the indian and the pacific oceans the results are basically unchanged using any technique there is also no causal relationship between changes in rock magnetic properties and mismatch between the two records thus discrepancies between parallel records do not depend on methods for relative paleointensity and any technique can be used with confidence provided that the characteristic component has been properly isolated and the sediments are magnetically homogeneous complications are more likely due to subtle variations in the sediment remanent magnetization acquisition these results point to the importance of sedimentological lithology and physical properties factors in controling the process of magnetization

rock magnetic properties and relative paleointensity stack for the last 300 ka based on a stratigraphic network from the subtropical and subantarctic south atlantic abstract we present a detailed study of natural remanence and rock magnetic properties of eight sediment cores from a south north profile across the subtropical front stf in the south atlantic which previously have been combined into a stratigraphic network based on these measurements we construct a first relative paleointensity rpi stack for the central south atlantic sas 300 covering the last 300 ka the degree of down core homogeneity of magnetic mineral concentration as well as magnetic mineral content and grain sizes vary between all cores and are quantified by high resolution rock magnetic measurements in the cores north of and close to the stf the magnetic remanence is mainly carried by magnetite the cores in the south also contain a high coercive mineral fraction based on a linear model of the sediment matrix we propose to apply principal component analysis pca of natural remanent magnetization nrm anhysteretic and isothermal remanent magnetization arm and irm to determine a homogeneity interval where remanences are only affected by a single environmental signal this reduces lithologic and climatic influence upon the relative paleointensity record for our data set pca indicates that all remanences in the 30 mt to 80 mt demagnetization interval are dominated by a single environmental influence single core rpi estimates are then obtained by normalizing the nrm fraction in this homogeneity interval with respect to either irm arm in the same interval or by by direct averaging we obtain the stack sas 300 which is compared to other paleointensity series such as sint 800 or the rpi records from odp sites 1089 subantarctic south atlantic and 983 gardar drift the detailed documentation of environmental influences combined with the presentation of seven different recordings of the same geomagnetic signal allows to connect southern and northern hemisphere rpi data also with respect to possible common environmental distortions

a geophysical study of the manila trench luzon philippines 1 crustal structure gravity and regional tectonic evolution the manila trench subduction zone is an active convergent plate margin between the south china sea and the northern philippines the trench trends northerly and is associated with a volcanic arc an east dipping benioff zone beneath luzon and a well developed fore arc basin system the luzon trough fore arc basins lie landward of the manila trench and contain up to 4 5 km of cenozoic sediments offscraping and accretion of turbidite sediments deposited in the manila trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system the uplifted zambales ophiolite and its offshore extension form the landward side of the fore arc basin multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope the major structural decollement at the manila trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the south china sea basin have presumably been subducted north of lingayan gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the luzon central valley and from the southern cordillera central based on regional geological and tectonic observations we infer that subduction probably began at the manila trench in late oligocene to middle miocene time the long term average convergence rate at the manila trench is estimated to be between 10 and 20 mm yr and may be slowing in the north owing to the collision of taiwan with eurasia

crustal thickness variation from a continental to an island arc terrane clues from the gravity signatures of the central philippines abstract offshore and ground gravity data were utilized to estimate crustal thickness across the central philippines where a transition from continental to island arc terrane occurs significant differences in gravity anomalies were observed between the palawan microcontinental block pcb and the philippine mobile belt pmb two major terranes that came together through arc continent collision islands of the pcb mindoro tablas romblon sibuyan and western panay made up of an assortment of continent derived sedimentary and igneous rocks and slivers of ophiolitic bodies register lower bouguer anomalies compared to that displayed by masbate island in the pmb the calculated crustal thickness of this region exhibits a complex moho topography of non uniform depth across the collision zone with the thickest parts 32 km corresponding with ophiolitic units emplaced consequent to arc continent collision on the other hand relatively thinner crust 21 km within the collision zone coincides with areas surmised to have undergone attenuation following intra arc rifting the same characteristics are observed offshore of western mindoro and within the marinduque basin areas known to have experienced crustal thinning following regional tectonic rearrangements that triggered riftings and intra basin openings

a new insight on the geometry of subducting slabs in northern luzon philippines abstract we used hypocentral and focal mechanism data in order to characterize the tectonic configuration of northern luzon and propose a model for describing the geometry of the subducted slab of the eurasian plate beneath the northern segment of the manila trench we took into consideration some of the observed bathymetric features i e the bend in the trench line of the manila trench at 20 n lat and the collision and subduction of an extinct mid oceanic ridge at 16 17 n lat and the intraplate deformation pattern in the north luzon ridge region in depicting a model that is consistent with most of the observed features and phenomena in the study area an earthquake catalog covering the period from 1619 to 1997 was used in studying the distribution of strain energy released while earthquakes covering the instrumental years 1963 1997 were used to analyze vertical distributions of earthquakes in sections we refined the location of some of the historical earthquakes using macroseismic data the inclusion of historical records despite their inaccurate locations could prevent misidentification of some regions as aseismic zones or gaps in seismicity especially along structures with long return periods a focal mechanism database that covers the period from 1963 to 1997 was used to decipher intraplate deformation pattern and subduction process of the slab the focal mechanism database consists of first motion solutions from previous authors and cmt solutions from harvard university seismicity strain energy release and focal mechanism maps and cross sections were drawn in order to have a three dimensional visualization of the geometry and stress regimes of seismogenic zones a new model of the subducted slab of the eurasian plate beneath the manila trench is proposed the model suggests the collision and subsequent partial subduction of a buoyant plateau at around 20 n lat to explain the sharp bend in the trench line the complicated deformation pattern on the overriding plate fronting the bend and the shallow dip of the subducted slab beneath this zone a tear in the slab is also inferred to be present as evidenced by the observed gap in strain energy release and the abrupt change in dip from shallow to steep south of 18 n lat the gap in seismicity and strain energy release 65 300 km depth at around 17 n lat may be used to infer the trajectory and location of the subducted extinct mid oceanic ridge mor the aseismic behavior is probably caused by the subducted ridge which is still hot and is deforming plastically this is supported by the heatflow data which shows high values along the extinct mor the subducted part of the mor may serve as the weakest zone where this tear could be localized the tear may also explain the cause of the abrupt termination of the eastern chain of volcanoes south of 18 n lat the above model which is a refinement of the model introduced by yang et al tectonophysics 258 1996 85 is consistent with the observed seismicity and deformation pattern observed bathymetric features spatial distribution and geochemical character of volcanism in northern luzon

paleomagnetic results from luzon and the central philippines samples were collected from 86 paleomagnetic sites from the islands of luzon marinduque mindoro panay negros cebu and mindanao in the philippine arc the sampling sites range in age from pleistocene to jurassic characteristic directions of magnetization of the samples were determined by the use of vector plots curie temperature determinations thin section studies and hysteresis studies showed that remanence of these samples is carried by fine grained pseudo single domain magnetite positive fold tests from miocene data from panay jurassic data from mindoro and cretaceous data from cebu suggest that the magnetization of these regions was acquired prior to folding rotations reported below are measured with respect to the axial goecentric dipole field the plio pleistocene data set shows no resolvable rotation for the 22 sites this data set suggests that the various terranes that make up the philippine arc have behaved as a single unit during the past 5 m y or that deformation has been below the limits of resolution the inclination data from the plio pleistocene sites have anomalously shallow inclination and are consistent with other plio pleistocene data from vietnam taiwan and the marianas these data support earlier suggestions for a late neogene offset dipole effect the late miocene sites fall into two separate groups ten sites from western luzon show evidence for around 20 of clockwise rotation in contrast to this late miocene samples from the bicol region negros marinduque and mindanao are not rotated the cause of the postlate miocene clockwise rotation of luzon is unknown but a pliocene collision of the north luzon arc with taiwan is suggested early neogene results also separate into two different populations the population from marinduque shows evidence for a large counterclockwise rotation the second early neogene population comes from panay cebu and mindanao and clearly shows evidence for a clockwise rotation the validity of this rotation is further supported by a fold test and a reversal test these early neogene data sets are consistent with a middle to late miocene collision of the palawan continental terrane and the central philippine arc data from six dikes of possible oligocene age from the zambales ophiolite are highly discordant from the present field being rotated approximately 60 clockwise the directions from these dikes are similar to a direction reported earlier from late oligocene sediments also from the zambales region these two data sets support the interpretation that the eocene direction from zambales is recording a large clockwise rotation of the region data from the mesozoic sites are from two regions data from the cretaceous pandan formation of cebu are discordant with data from the upper jurassic from mindoro the presence of a fold test from each region and a reversal test from mindoro supports the interpretation that each of these data sets is reliable the vgp of mindoro is displaced southward from the late jurassic vgp of south china suggesting a post jurassic southward migration of mindoro

drainage network analysis and tectonics of mindanao southern philippines abstract mindanao southern philippines is the site of an active arc continent collision which began in the late pliocene the principal tectonic features are faults and blind folds and duplexes which are not easily detectable in this island on which 75 of the surface is covered by pleistocene and holocene sediments and volcanics a detailed study of the drainage pattern and its discontinuities in the agusan davao and the cotabato basins when compared with field data radar data and subsurface structures observed on seismic documents enables a correlation of distant seismic lines and features existing on the basin edges the drainage network analysis based on description and interpretation of drainage anomalies and frequency allows us to distinguish 1 subhomogenous drainage areas which correspond to areas bearing consistent geological characteristics and 2 drainage anomalies the results have been checked in the field and compared with the tectonic features in the mountainous areas the extension of these structures could then be traced confidently further towards the flat lowlands we point out the presence of two distinct anomaly directions that intersect in eastern mindanao 1 numerous nnw anomalies in the intra arc basin of agusan davao and both pacific and central cordilleras appear as young features in the morphology and are due to strike slip faults of the arc continent collision some of the faults have proven to be reactivated middle miocene faults related to the initial rifting of the agusan davao basin 2 n s anomaly directions are transpressive ridges due to the collision except one anomaly which corresponds to the trace of the philippine fault zone and connects with the collisional features in contrast the cotabato basin in western mindanao lies between two terranes of different nature in the cotabato basin area the nw and nnw anomaly directions are also present and similarly correspond to strike slip faults and thrust related folds respectively pull apart basins occur in places where sagging is evidenced an additional set of anomalies trends ene and corresponds to the active extensional field below the quaternary sediments some of the en echelon features corresponding to blind flat and ramps structures were identified these observations provide reference examples for the evolution of basins generated within the compressive and strike slip setting of accreted belts

spatial variations of the crustal stress field in the philippine region from inversion of earthquake focal mechanisms and their tectonic implications abstract this paper presents the spatial variations in the crustal stress field for the philippine region based on the stress configuration we divided the stress regime in the area of the philippines into four parts 1 trench perpendicular compressive stress axes 1 with an intermediate plunge were observed along the eastern subduction systems of the philippine mobile belt pmb which suggests that the subduction of the philippine sea plate psp is the primary factor controlling the stress distribution 2 characterized by systematically rotated 1 with very shallow plunge the central western portion of the philippines appears to be largely affected by the collision between the palawan block and the pmb 3 the 1 distribution in the northern part of luzon island north of 14 n is compatible with the plate motion of the psp whereas the extensional axes 3 has a fan shaped stress signature with n s direction at the south and a more spreading direction to the north which implies its north escapement 4 bounded by the scarborough seamount the northern and southern portions of the manila trench reveal a very distinct stress pattern with a trench parallel 3 to the north and a trench perpendicular 3 to the south this observation may infer a potential effect caused by the presence of the oceanic bathymetric highs on the behavior of the subduction process in addition to the identification of the four provinces with different stress regimes we suggest that the philippine fault zone may play an important role to alter the stress state in the region of the philippines

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

paleosecular variations 0 19 000 years recorded by sediments from escondido lake argentina abstract paleomagnetic and sedimentological studies carried out on four cores about 11 m long from the bottom sediments of escondido lake southwestern argentina are described this lake is geologically related to the set of lakes of glacial origin located in western patagonia using several magnetic studies the predominance of pseudo single single domain magnetite as main carrier as well as an increase of concentration and grain size with depth was determined the pairs of magnetic susceptibility and intensity of remanent magnetization profiles show a good correlation with the sedimentological macroscopic descriptions and also with those of shorter cores previously studied the stability of the natural remanent magnetization was investigated by alternating field demagnetization within lake correlation was based on lithology magnetic susceptibility and intensity of remanent magnetization because the patterns of these parameters showed similarity for every core radiocarbon dating suggests higher deposition rates during the early stage of the lacustrine basin late pleistocene the transfer function depth age also allowed constructing a secular variation record from south argentina for the past 19 000 years the declination and inclination logs show some anomalous directions at about 18 000 calibrated years

paleosecular variations recorded by holocene pleistocene sediments from lake el trebol patagonia argentina paleomagnetic studies carried out on six cores from the bottom sediments from lake el trebol south argentina are described measurements of directions declination and inclination d and i and intensity of natural remanent magnetisation nrm magnetic susceptibility at low and high frequency specific x and volumetric k isothermal remanent magnetisation irm saturation isothermal remanent magnetisation sirm back field and anhysteric remanent magnetisation arm were carried out stability of the nrm was investigated by alternating field demagnetisation rock magnetic studies suggest that the main carriers of magnetisation are ferrimagnetic minerals predominantly pseudo single domain magnetite records of nrm and k show a good correlation with the sedimentological description and also with those of shorter cores previously studied the tephra layers were identified from the lithologic profiles and also from the magnetic susceptibility logs due to their different chronological meaning and their rather bad behaviour as magnetic recorder these layers were removed from the sequence and the gaps that were produced along the profiles by the removal were closed obtaining a shortened depth radiocarbon age estimates from these cores and from earlier studies allow us to construct a paleosecular variation psv record for the past 24 000 years these results show good correlation with the previous one the spectrum obtained from spectral analysis shows major peaks at periodicities of about 1150 1500 2490 3630 5240 and 11 810 years both clockwise and counterclockwise precession of the geomagnetic vector is evident from analysis of the bauer plots with a preponderance of clockwise movement

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

facies and evolution of silurian coral microbialite reef complex maumee ohio usa abstract silurian dolomite that crops out in a quarry at maumee ohio usa forms a thick reef complex roughly 600 m in diameter and about 68 m thick it is composed in ascending stratigraphic order of the lockport dolomite and the greenfield dolomite the lockport dolomite at maumee contains multistory stromatolite thrombolite and coral microbialite reefs such reefs have never been reported previously in this unit in ohio and indiana thrombolite and stromatolite reefs are interpreted to have formed in a subtidal setting possibly within a hypersaline lagoon wave stress affected the coral microbialite reefs by limiting the number of metazoans and promoting the formation of fibrous cement inferred to have been marine precipitates originally in the form of magnesian calcite thrombolites an stromatolites form 20 60 by volume of coral microbialite reefs and they contributed substantially to the syndepositional stabilization of these reefs by serving as binders and encrusters of metazoans especially branching corals such stabilization sharply limited the amount of sediment available for the formation of flank beds next to coral microbialite reefs evolution of the maumee reef complex mrc involved two shallowing upward sequences each of which was terminated by subaerial exposure the older interval of subaerial exposure is represented by an intraformational unconformity within the lockport dolomite the younger unconformity at the top of the lockport dolomite is correlated with the base of the a 1 evaporite in the michigan basin parts or all of patterns shown by coral microbialite reefs and microbialite reefs in the mrc may provide an analogue for better understanding the nature and evolution of pinnacle reefs and patch reefs in the michigan basin region and other reefs elsewhere

the shear wave velocity structure of the crust and uppermost mantle beneath tagaytay philippines inferred from receiver function analysis depths of the partially melted zone and the moho discontinuity are probably at 18 kilometers and 34 kilometers respectively beneath tagaytay seismic station luzon philippines this conclusion is based on the shear wave velocity structure determined in this study which indicates the presence of a low velocity zone and high velocity zone at these depths the shear wave velocity model was derived from the inversion of five receiver functions of teleseismic earthquakes recorded by the broadband seismometer at tagaytay however due to the very limited information to constrain our initial model other models with comparable waveform fit were also obtained but with deeper low velocity zone and shallower moho

south atlantic and north atlantic geomagnetic paleointensity stacks 0 80 ka implications for inter hemispheric correlation recent coring of high deposition rate atlantic sediments has led to the development of north atlantic napis and south atlantic sapis geomagnetic paleointensity stacks the sapis stack comprises five records from the sub antarctic south atlantic 41 471s 6 101e four are from piston cores collected during the ocean drilling program odp leg 177 site survey cruise earth planet sci lett 175 2000 145 the other is odp site 1089 leg 177 j geophys res 2001 submitted for publication all but one 4 pc03 of the sapis cores have oxygen isotope records which are used in conjunction with lithologic and geomagnetic variability to derive an optimized correlation to site 1089 the site 1089 age model is derived by mapping the benthic and planktic isotope data to those from nearby core rc11 83 which has 14 calibrated radiocarbon ages in the 11 41 ka interval below this level the chronology is derived by matching the site 1089 benthic oxygen isotope data to the specmap stack the napis stack comprises six records from a wide area of the north atlantic philos trans r soc ser a 358 2000 1009 correlation between cores was based on marine isotopic stage boundaries augmented by millennial scale features heinrich events and fluctuations in concentration dependent magnetic parameters napis was placed on the gisp2 chronology using the marine to ice core oxygen isotope correlation proposed by voelker et al radiocarbon 40 1998 517 for one of the napis cores in the resulting age model the laschampevent recorded in five of six cores falls in a very narrow o1 kyr age range and coincides with the 10 be and 36 cl peak measured in the ice cores comparison of the two stacks placed on their own independent age models indicates that common millennial scale paleointensity features are preserved although more work is needed to define the true global content of the paleointensity record as well as the precise age of many features it is readily apparent that paleointensity can provide a global correlation tool at a resolution unattainable from isotope data alone r 2002 published by elsevier science ltd

north palawan block philippines its relation to asian mainland and role in evolution of south china sea the island of mindoro the northern part of palawan island and the reed bank area southwestern philippines together constitute a continental fragment the north palawan block lying within an island arc oceanic setting the permian to paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the philippine archipelago these rocks also suggest that the north palawan block once occupied a pre drift position contiguous with the south china mainland four prominent pre neogene regional unconformities are recognized both onshore and offshore the china mainland in taiwan and in the north palawan block the synchrony of these unconformities and the facies relation of the unconformity bounded sedimentary units strongly suggest a common pre neogene history for all these areas by contrast an important regional unconformity occurring at the top of the middle miocene throughout the palawan area is absent from the asian mainland extension in the south china sea basin since the mesozoic which has separated the north palawan block from the asian mainland has been approximately uniform from west to east however there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea floor spreading from the foregoing observations and using the most recent magnetic spreading anomaly data for the south china sea a suite of palinspastic reconstructions has been prepared which shows the evolution of the south china sea area from the late triassic to the pliocene the reconstructions illustrate 1 the convergent continental margin setting of the north palawan block during much of the jurassic and cretaceous 2 the late cretaceous inception of the philippine island arc system 3 the subsequent counter clockwise rotation of the arc system from the late eocene onward 4 the paleocene to middle miocene opening of the south china sea and 5 the early to middle miocene collision between the north palawan block and the palawan subduction system

regional stress influence on the opening direction of crater amphitheaters in southeast asian volcanoes abstract holocene volcanoes in the philippines and indonesia were studied to determine the possible relationship between the regional maximum horizontal stress hmax and the opening direction of volcanic amphitheatre craters the study was conducted using publicly available shuttle radar topographic mission srtm images and the world stress map wsm the results corroborate findings in similar research on japanese and indonesian volcanoes indicating that opening of craters occur at an acute angle relative to the hmax direction further investigation of volcanoes in southwest and southeast luzon philippines regions where fault structures and their kinematics are better constrained reveal a promising tectonic stress related rationale for crater opening direction several volcanoes in these regions of the philippines appear to have amphitheatre craters open in the direction related to the fault underlying the volcano these observations are consistent with those derived from analogue models of volcanic cones deformed by basal strike slip faulting these findings can be useful in advancing our understanding of volcanic flank failure and for hazards preparedness against such catastrophic events

cenozoic plate reconstruction of the south china sea region abstract reconstructions of the south china sea region at 60 ma 40 ma 30 ma 20 ma 10 ma and 5 ma are presented we have attempted to place the south china sea basin in a regional tectonic framework the tectonic evolution of the major blocks surrounding the south china sea were analyzed as well as the relative motions of the indian and australian plates we have tried to correct the tectonic models available in this region a 3 d graphics terminal was used to derive rotation poles for the different tectonic blocks and our model was then tested to determine its self consistency when the model conflicted with previous interpretations the input data were evaluated for alternative explanations at least two and possibly three stages of extension can be recognized in this region the earliest one active in the late cretaceous to eocene involved nw se extension the second one active from the late eocene to early miocene involved north south extension the third stage of extension which probably trended nw se can be dated as post oligocene the first extensional event produced the ne sw trending proto south china sea and a series of sedimentary basins along the south china margin following the southeastward extrusion of indochina the proto south china sea was mostly consumed at the palawan trough renewed north south extension in the south china continental margin started the present day south china sea spreading in the oligocene the southeastward extrusion of indochina blocked by sundaland resulted in the nw se trending opening of the south china sea basin in the early miocene collision of the north palawan microcontinental block with the west philippines block stopped the opening of the south china sea at the end of early miocene spreading activity switched to the sulu sea basin in the middle miocene but collision between the sulu ridge and the west philippines at mindanao halted the opening of the sulu sea at the end of the middle miocene in the late miocene greater india continued its northward path and seems to have ripped open the andaman sea in the pliocene subduction along the northern manila trench placed the north luzon arc on a collision path with the east asia continental margin at taiwan our reconstructions along with detailed geological and geophysical information may be used as a predictive tool for basin evolution models and block interactions in this region the development of the south china sea basin the gulf of thailand the malay basin and the central thailand basins are the result of collision induced extensional forces the sulu celebes and sumatra basins were formed as a consequence of prolonged subduction the opening of the pearl river mouth west natuna south china sea sulu and possibly celebes basins were terminated by various plate collisions during the course of plate reorganizations major boundary faults have changed their slip senses during different stages of evolution

tectonic geomorphology and paleoseismology of the surigao segment of the philippine fault in northeastern mindanao island philippines abstract the philippine fault is a major strike slip fault that traverses the entire philippine archipelago for more than 1250 km and has generated at least 10 surface rupturing earthquakes for the past 200 years to better understand its characteristics we have conducted review of historical earthquakes tectonic geomorphic mapping and paleoseismic trenching along the 100 km long surigao segment the northernmost segment of the philippine fault on mindanao island we mapped the surigao fault based on aerial photographs and identification of well defined geomorphic features in the field combining this with historical accounts and paleoseismic trenching we have identified and mapped the surface rupture of the 1879 m w 7 4 surigao earthquake paleoseismic trenching conducted at two sites also led us to identify evidence of at least four surface rupturing earthquakes including the 1879 event during the past 1300 years

normalized remanence in sediments of the western equatorial pacific relative paleointensity of the geomagnetic field piston cores from the ontong java plateau collected during the eurydice expedition have been used to estimate regional geomagnetic paleointensity rock magnetic measurements of the sediments show that 1 the remanence is carried by stable pseudo single domain magnetite 2 the grain sizes and concentrations are relatively uniform with respect to the normalization technique and 3 susceptibility is an adequate parameter for normalizing remanent intensity in order to determine relative variations in the intensity of the paleomagnetic field we therefore have measured the remanent intensity both natural and demagnetized to 15 mt and susceptibility of archive quarters of three cores at centimeter intervals using an automated through bore magnetometer a further test of the normalized intensity records is provided by the coherence function between the normalized remanence and the susceptibility of the cores one of our cores displays high coherence over a broad range of frequencies while for two others coherence is insignificantly different from zero at the 95 level of confidence oxygen isotopic records paleontological datum levels and the brunhes matuyama boundary provide an independent temporal framework for inter core comparison when the two cores which pass our acceptance criteria are placed on a common time scale the coherence between the normalized intensity records is significantly different from zero at well above the 95 level of confidence suggesting some common input signal over the same band of frequencies furthermore there is a great deal of similarity between the records from the ontong java plateau and a record from core rc10 167 taken some 3000 km to the north the distribution of intensities obtained from these sedimentary records has the same shape as the absolute virtual dipole moment data obtained from lava flows 0 5 ma with a slightly smaller standard deviation the inter core agreement within a single region the similarity of records from 3000 km apart and the general correspondence of statistical character of these sediments with lava flow data all argue that the normalized intensity records presented here reflect variations in the intensity of the ancient geomagnetic field spectral analysis of the paleointensity time series shows that there a is significant enhancement of variance concentrated in frequencies around 33 thousand years over the period 0 350 ka this feature is however lacking for the period 350 700 ka

mayon volcano philippines change of monitoring strategy after microgravity and gps measurements from 1992 to 1996 abstract mayon volcano is part of the bicol volcanic chain on the island luzon philippines during this century there were ten activity periods distributed almost regularly because of the density of population about one million people living in the vicinity of the volcano three seismological observatories are in operation measurements of gravity changes started in 1992 just before the eruption of february march 1993 two profiles at the slope were established connected to a regional network around the volcano in order to enable the determination of mass changes between the campaigns the height control was provided by parallel gps measurements in all the network consists of 26 points which were remeasured with three gravimetres at least three times within one campaign during five campaigns within 4 years the differential gps gives no significant changes of the elevation within 4 cm nevertheless the gravity increased significantly by up to 1500 nm s2 equivalent to 150 gal as no significant change of elevation is observed gps no extended shallow magma chamber system below the volcano can be proved this is in accordance with geochemical results indicating a rather undifferentiated magma the youngest lava which is of interest for the eruption dynamics belongs to the medium k basaltic andesite field of the k2o vs sio2 diagram a rather qualitative check of groundwater level changes reveals that these cannot be the sources for the observed gravity changes thus the increase of gravity after the eruption of february 1993 can be explained by a mass redistribution in the volcanic vent from above the level of the gravity points to below practical conclusions of these results lead to changes in the monitoring strategy deformation measurements did not reveal any volcanic activities at least for the eruption of 1993 no significant deformation was observed gravity could be an indicator for long term changes thus repeated gravity measurements gps at selected points could be used in parallel to seismic monitoring to detect slow mass movements prior to changes in seismicity

a rapid method for magnetic granulometry with applications to environmental studies we have developed a rapid method of characterizing the fine scale variation sub micron to hundreds of microns m in the grain size of magnetite in long sediment cores this method is a new rock magnetic tool and can be readily applied to the following disciplines 1 environmental studies 2 hydrology 3 petrology and 4 geomagnetism the method consists of a comparative plot of the depth variation of two magnetic parameters the first parameter low field less than 0 1 millitesla mt or 1 oe susceptibility in a steady field is plotted on the abscissa the second parameter anhysteretic remanent magnetization arm imparted by a smoothly decreasing af of peak value 100 mt 1 000 oe in the presence of a weak steady field 0 01 0 1 mt or 0 1 1 oe is plotted on the ordinate for a sediment column in which the magnetic mineral is magnetite a plot of the two magnetic parameters shows transitions in the average magnetic grain size with depth as clearly discernible changes in the slope of a regression line fit to the individual points arm

a new high resolution geomagnetic relative paleointensity record for the north american holocene a comparison of sedimentary and absolute intensity data a new high resolution paleointensity record for north america has been constructed using holocene sediments from lake pepin minnesota lake pepin sediments yield the same holocene paleosecular variation curve as nearby lake st croix and satisfy all of the criteria recommended for paleointensity studies absolute paleointensity data for north america recorded by holocene volcanic and archeomagnetic samples provide an independent record of geomagnetic field paleointensity against which the relative intensity records from lake pepin and lake st croix can be compared since the absolute field paleointensity is known a priori the effects of the magnetic recording assemblage can be isolated anhysteretic remanent magnetization arm is the best choice among normalization parameters for the lake pepin sediments as the natural remanent magnetization nrm normalized by arm shows no coherence with magnetic grain size proxies and yields a record of relative paleointensity peaks and troughs whose amplitudes are very similar to those in the archeomagnetic arcmag and lake st croix data sets features with a wavelength of 1000 years are correlative between the three paleointensity records nrm normalized by saturation isothermal remanent magnetization sirm and by susceptibility shows grain size dependences that cause errors in the amplitudes of paleointensity features nrm sirm and nrm are not coherent with their normalizers but are both strongly coherent with independent grain size proxies such as the median destructive field of the nrm and hcr we successfully removed the grain size dependences from nrm sirm by applying a correction function based on the linear relationship between normalized intensity and the median destructive field of the nrm

rock magnetism of late neogene and pleistocene deep sea sediments relationship to sediment source diagenetic processes and sediment lithology we use bivariate scatter plots to illustrate variations in selected rock magnetic properties low field susceptibility anhysteretic and isothermal remanence of late neogene and pleistocene deep sea sediments from 16 sites in the arctic ocean north atlantic equatorial atlantic and north pacific ocean and the arabian sea our intention is to examine the ability of the rock magnetic properties to differentiate the sediments according to factors such as lithology geographical area and the dominant mode of terrigenous sedimentation which at these sites is via ice rafting via bottom currents or via eolian processes overall correlations between sediment magnetic properties and gross lithology is poor and factors such as the source and transport path of terrigenous sediment and detrital magnetic minerals together with the action of reductive diagenetic processes are the major controls on the magnetic properties on the bivariate scatter plots sites with major ice rafted contributions tend to have high sedimentary ferrimagnetic mineral concentrations relatively coarse ferrimagnetic grain sizes and scattered sample point distributions in contrast sites where we infer significant bottom current supply of terrigenous material have tightly grouped sample point distributions carbonate sediments in which the terrigenous component is supplied by eolian processes tend to have a broad range of magnetic mineral concentration caused by glacial interglacial fluctuations in carbonate accumulation and eolian activity sediments containing significant volcanogenic material have high concentrations of relatively coarse grained ferrimagnetic material reductive diagenesis is a significant determinant of sediment magnetic properties in high productivity areas and has the effect of preferentially removing the fine grained ferrimagnetic fraction causing a coarsening of the ferrimagnetic grain size distribution and a rise in the antiferromagnetic ferrimagnetic ratio

structural evolution of bondoc burias area south luzon philippines from seismic data abstract recently acquired 2 d seismic profiles in the offshore area between bondoc peninsula and burias island south luzon philippines are interpreted in the context of known structural styles observed onshore and in relation to paleo and neo tectonic regimes in the region two distinct seismic sequences can be distinguished relative to their structural style and tectonic significance the top of a lower sequence shows strongly reflective properties this unit correlative to late oligocene to early miocene limestone bodies observed onshore in bondoc peninsula and burias island is affected by intense to moderate superposed folding and thrust faulting an upper sequence correlative to a two member turbiditic and shallower marine clastic deposit widely exposed in bondoc peninsula is affected by thrust faulting and deformation associated with overturned tight folding the onshore equivalents of these two seismic sequences form the core of a bent anticlinorium that twists from a nw se axis in onshore bondoc into a n s axis southwards into the southern tip of the peninsula then back to a nw se axis in the offshore region further to the south overlying this structural core is a relatively less deformed sequence where syn sedimentary half grabens are still preserved in places this complex structural style is the result of a series of several tectonic events occurring from the eocene to the present involving carbonate build up deep water turbidite deposition consequent compression folding and faulting and late event half graben controlled deposition some resulting structures are indicative of tectonic inversion processes positive and negative which may prove to be potentially favorable in the search for structural plays in the area

deformation and stress states along the central segmentof the philippine fault implications to wrench fault tectonics abstract this paper discusses the space time evolution of deformation and states of stress along the central segment of the left lateral philippine fault from bondoc peninsula to leyte island this segment is characterized by deformations and stress states which do not always conform to predictions of classical wrench tectonics directions of the maximum principal stress axis 1 are in some cases oriented perpendicular to the fault trace contradicting the classical fault strike 1 angle of 30 45 in other cases it is the minimum axis 3 that orients perpendicular to the fault or in other cases parallel to it rotations of 1 directions as functions of time and space are observed all along the fault more complex stress regimes are observed in areas where the philippine fault intersects with two other major left lateral strike slip faults namely the legaspi lineament in the ragay gulf area and the sibuyan sea fault in masbate these observations strongly underline the urgence of prudence and precaution in applying wrench fault tectonic principles in natural examples

the onset of movement on the philippine fault in eastern mindanao a transition from a collision to a strike slip environment the spatial and temporal evolution of eastern mindanao from a zone of active arc arc collision to one of pure strike slip environment is addressed this is made possible by tracing the philippine fault confined to a well defined and narrow zone of deformation and by differentiating from it earlier collision related compressional features whose deformation is distributed over a broad zone a diachronous unconformity marking the end of collision related deformation reflects the southward propagating nature of the collision and provides limits on the age of initiation of the philippine fault this diachronism suggests that the initiation of the philippine fault is related to the completion of the collision we show the rapidity at which formation and propagation of the fault follows the locking of the collision zone we likewise demonstrate that the philippine fault reactivates pre existing collisional features in cutting across the southern pacific cordillera

analysis of crustal deformation in luzon philippines using geodetic observations and earthquake focal mechanisms we utilize regional gps velocities from luzon philippines with focal mechanism data from the harvard centroid moment tensor cmt catalog to constrain tectonic deformation in the complex plate boundary zone between the philippine sea plate and eurasia the sundaland block processed satellite imagery and digital elevation models are used with existing gravity anomaly seismicity and geologic maps to define a suite of six elastic blocks geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault locking parameters for each of the tectonic blocks and faults comprising luzon major tectonic structures that were found to absorb the plate convergence include the manila trench 20 100 mm yr 1 and east luzon trough 9 15 mm yr 1 philippine trench 29 34 mm yr 1 which accommodate eastward and westward subduction beneath luzon respectively the left lateral strike slip philippine fault 20 40 mm yr 1 and its northward extensions the northern cordillera fault 17 37 mm yr 1 transtension and the digdig fault 17 27 mm yr 1 transpression the macolod corridor a zone of active volcanism crustal thinning extension and extensive normal and strike slip faulting in southwestern luzon is associated with left lateral transtensional slip of 5 10 mm yr 1 the marikina fault which separates the central luzon block from the southwestern luzon block reveals 10 12 mm yr 1 of left lateral transpression our analysis suggests that much of the philippine fault and associated splays are locked to partly coupled while the manila and philippine trenches appear to be poorly coupled luzon is best characterized as a tectonically active plate boundary zone comprising six mobile elastic tectonic blocks between two active subduction zones the philippine fault and associated intra arc faults accommodate much of the trenchparallel component of relative plate motion 2006 elsevier b v all rights reserved

the western central luzon volcanic arc the philippines two arcs divided by rifting abstract the western central luzon arc is apparently a complex zone of volcanism and tectonism the volcanoes of the northern segment of the arc the bataan arc ba fall along two semi parallel lineaments the western bataan lineament wbl and the eastern bataan lineament ebl this northern segment is cut off from the southern extension of the arc by a ne sw cross arc zone of volcanism the macolod corridor mc the volcanoes of the southern segment of the arc mindoro arc ma fall along two parallel lineaments the western mindoro lineament wml and the eastern mindoro lineament eml samples collected from the mc ebl and ma are alkali and lil enriched compared with the wbl the enrichment of the ebl is believed to be the result of the release of smaller amounts of hydrous rich fluids within the mantle below the ebl than below the wbl this process generated smaller degrees of partial melting below the ebl as compared to the wbl which resulted in the higher concentrations of lil elements in the ebl magmas as compared to the wbl magmas the alkali and lil enriched nature of the mc is believed to be the result of small degrees of partial melting during rifting the enriched nature of the ma is thought to be due to slivers of crustal material from the palawan mindoro block being incorporated into the mantle during subduction this region has the highest radiogenic sr values found in the philippines the mc may represent a pull apart rift zone between the philippine and west luzon shear zones and the transform between the manila and philippine trenches

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

structural setting of the bicol basin and kinematic analysis of fractures on mayon volcano philippines mayon volcano is located within the bicol basin in the southeastern region of luzon philippines the basin is contained within a releasing bend associated with the curvature of the central segment of the left lateral philippine fault a structural analysis of the faults that traverse the bicol basin was conducted through reviews of existing literature seismic data interpretation and remote sensing in particular those structures that may have influenced the growth and evolution of mayon dynamic analysis of the aforementioned structures reveals a regional transtensional stress regime for the bicol basin field investigation on mayon s slopes revealed the existence of fractures along its western and eastern flanks these structures on mayon have the same geometry as the dominant faults found within the bicol basin interpretation of these fractures indicates a tectonic control over their origin the identification of fractures on mayon s slopes is the first description of such features on its edifice these new data highlight the probable landslide hazards associated with an oversteepened volcano that may have been further destabilized by tectonic activity

estimation of the magnitudes and epicenters of philippine historical earthquakes abstract the magnitudes and epicenters of philippine earthquakes from 1589 to 1895 are estimated based on the review evaluation and interpretation of historical accounts and descriptions the first step involves the determination of magnitude felt area relations for the philippines for use in the magnitude estimation data used were the earthquake reports of 86 recent shallow events with well described effects and known magnitude values intensities are assigned according to the modified mercalli intensity scale of i to xii the areas enclosed by intensities iii to ix a iii to a ix are measured and related to magnitude values the most robust relations are found for magnitudes relating to a vi a vii a viii and a ix historical earthquake data are obtained from primary sources in libraries in the philippines and spain most of these accounts were made by spanish priests and officials stationed in the philippines during the 15th to 19th centuries more than 3000 events are catalogued interpreted and their intensities determined by considering the possible effects of local site conditions type of construction and the number and locations of existing towns to assess completeness of reporting of these events 485 earthquakes with the largest number of accounts or with at least a minimum report of damage are selected the historical epicenters are estimated based on the resulting generalized isoseismal maps augmented by information on recent seismicity and location of known tectonic structures their magnitudes are estimated by using the previously determined magnitude felt area equations for recent events although historical epicenters are mostly found to lie on known tectonic structures a few however are found to lie along structures that show not much activity during the instrumented period a comparison of the magnitude distributions of historical and recent events showed that only the period 1850 to 1900 may be considered well reported in terms of magnitude distribution each earthquake is evaluated for its quality of determination based on the number of intensity reports earlier than 1850 the data collected are few and most earthquakes had fewer than ten reports good quality reports began to be collected from 1850 partly correlative to an increase in the number of towns and partly to the start of a systematized collection of earthquake accounts by the manila observatory parameters of these well reported earthquakes may be used for conducting various seismological studies examples of how the parameters of poorly reported events were arrived at are also discussed

gravity changes and internal processes some results obtained from observations at three volcanoes temporal gravity changes provide information about mass and or density variations within and below the volcano edifice three active volcanoes have been under investigation each of them related to a plate boundary mayon luzon philippines merapi java indonesia and galeras colombia the observed gravity changes are smaller than previously expected but significant for the three volcanoes under investigation and within the observation period mainly the increase of gravity is observed ranging from 1 000 nm 2 to 1 600 nms 2 unexpectedly the gravity increase is confined to a rather small area with radii of 5 to 8 km around the summit at mayon and merapi the parallel gps measurements yield no significant elevation changes this is crucial for the interpretation as the internal pressure variations do not lead to significant deformation at the surface thus the classical mogi model for a shallow extending magma reservoir cannot apply to confine the possible models the attraction due to changes of groundwater level or soil moisture is estimated along the slope of merapi exemplarily by 2 d modelling mass redistribution or density changes were evaluated within the vent as well as deeper fluid processes to explain the gravity variations the results are compared to the model incorporating the additional effect of elastic deformation

paleointensity studies on holocene pleistocene sediments from lake escondido argentina abstract relative changes in geomagnetic field intensity over the last 16 000 years bp were recovered from the study of four cores obtained from lake escondido south argentina rock magnetic analysis shows that the main magnetic mineral is magnetite the concentrations being mainly between 0 01 and 0 1 and the grain size between 1 and 8 m in order to fulfil the criteria for assessing the reliability of paleointensity data derived from sediments rev geophys 31 1993 319 the samples whose concentrations and size are beyond this range were rejected the remanent magnetisation at 20 mt nrm20 mt was normalised using the anhysteric remanent magnetisation arm100 mt the saturation of the isothermal remanent magnetisation sirm and the low field magnetic susceptibility k coherence function analysis indicates that the records are not significantly affected by local environmental conditions this suggests that the variations in normalised remanence are mostly likely due to geomagnetic paleointensity fluctuations arm100 mt is the best normaliser for the lake escondido sediments the record of relative paleointensity nrm20 mt arm100 mt shows peaks and troughs whose amplitudes are similar to those in the st lawrence estuary lake baikal lake pepin and larsen a ice shelf data set and a stack of archeomagnetic data given as virtual axial dipole moment vadm

can relative paleointensities be determined from the normalized magnetization of the wind blown loess of china although the wind blown loess and interbedded paleosol sequence in the loess plateau of china has been extensively studied and regarded as one of the most complete terrestrial records of both the geomagnetic field and climatic changes during the quaternary little attention has been paid to it as a source of relative paleointensity in this study we examine the malan loess l1 the last glacial sediments in the lingtai section to determine whether loess is able to reliably record the relative paleointensity toward that end both the conventional normalizing method as well as the pseudo thellier method tauxe et al 1995 were used in conjunction with the examination of the rock magnetic properties and natural remanent magnetization nrm rock magnetic properties of l1 show the uniformity in terms of magnetic mineralogy and grain size suggesting that they may be suitable for relative paleointensity studies normalized remanences derived from l1 show highs between both 72 66 ka and 62 51 ka and lows near 63 ka 42 ka and between 20 and 10 ka comparable results using different normalization parameters suggest that large amplitude variation of normalized magnetization may reflect the intensity variation of the geomagnetic field however comparisons of the normalized magnetization with contemporaneous marine records show some dissimilarities in particular the intensity low at 20 10 ka corresponding to the last glacial maximum is not compatible with the sint 800 composite record on the other hand spectral coherence analyses of middle and high frequency components of the normalized magnetizations suggest some climatic influence thus this apparent discrepancy may be explained by climatic changes in the loess plateau and the resulting effect on the nrm