new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

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

using oxygen isotopes to establish freshwater sources in bedford basin nova scotia a northwestern atlantic fjord abstract a weekly time series of oxygen isotope 18 o measurements was collected over a 16 month period from near surface 1 m and near bottom 60 m waters of bedford basin a coastal fjord adjacent to the scotian shelf off eastern canada the time series was complemented with 18 o measurements of local precipitation rain and snow river and wastewater runoff the isotopic composition of precipitation displayed strong seasonality with an average volume weighted 18 o value of 5 39 0 96 for summer and a depleted value of 10 37 2 96 over winter winter precipitation exhibited more depleted and variable 18 o of solid precipitation relative to rainfall the annual amount weighted average 18 o of sackville river discharge 6 49 0 82 was not statistically different from precipitation 7 24 0 92 but exhibited less seasonal variation freshwater end members zero salinity intercepts estimated from annual and seasonal regressions of 18 o versus salinity s for bedford basin near surface samples were consistent with the 18 o of summer precipitation and the annual amount weighted average for the sackville river however the isotopically depleted signature of winter precipitation was not observed clearly in near surface waters of bedford basin which might reflect isotope enrichment during sublimation from accumulated snowfall prior to melting and discharge or retention and mixing within the drainage basin in near bottom waters most of the 18 o s variation average freshwater end member 7 47 2 17 could be explained by vertical mixing with near surface waters average freshwater end member 6 23 0 34 and hence with locally derived freshwater however the near bottom 18 o s variation suggested an additional contribution of a freshwater end member with a 18 o of 15 55 2 3 consistent with a remotely derived freshwater end member identified previously for the scotian shelf residuals from a long term regression of 18 o s were generally within the range expected due to analytical uncertainty 0 05 however near surface waters exhibited seasonal variability of small amplitude which was consistent with the timing and 18 o variability of local freshwater inputs

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

damped regional scale stress inversions methodology and examples for southern california and the coalinga aftershock sequence 1 we present a new focal mechanism stress inversion technique to produce regional scale models of stress orientation containing the minimum complexity necessary to fit the data current practice is to divide a region into small subareas and to independently fit a stress tensor to the focal mechanisms of each subarea this procedure may lead to apparent spatial variability that is actually an artifact of overfitting noisy data or nonuniquely fitting data that does not completely constrain the stress tensor to remove these artifacts while retaining any stress variations that are strongly required by the data we devise a damped inversion method to simultaneously invert for stress in all subareas while minimizing the difference in stress between adjacent subareas this method is conceptually similar to other geophysical inverse techniques that incorporate damping such as seismic tomography in checkerboard tests the damped inversion removes the stress rotation artifacts exhibited by an undamped inversion while resolving sharper true stress rotations than a simple smoothed model or a moving window inversion we show an example of a spatially damped stress field for southern california the methodology can also be used to study temporal stress changes and an example for the coalinga california aftershock sequence is shown we recommend use of the damped inversion technique for any study examining spatial or temporal variations in the stress field

heterogeneous strain regime at the west of the ogasawara plateau in the western pacific ocean from inversion of earthquake focal mechanisms abstract the ogasawara plateau has a major control on the geology of the region between the izu bonin and mariana subduction zones this region is important for understanding active collisional zones we attempt to illustrate the stress field in the wadati benioff zone of this region through seismic estimations 49 events m 3 5 are chosen for waveform inversion by the isola software to distinguish the stress field according to available focal mechanism solutions the heterogeneous stress field parameters are determined for several depth ranges based on the epicentral distribution and seismic stress field the subduction system at the west of the ogasawara plateau can be divided into three segments separated by 26 5 n and 24 n at depths of 0 50 km in the outer trench slope the slab is characterized by a varying stress filed the best fit stress model with horizontal 3 and vertical 1 is related to the plate bending and preexisting faults for the central segment the solutions present a compressional stress regime in the outer trench slope without preexisting faults induced by the resistance of the buoyant plateau the solutions for the depth range of 0 50 km in the forearc region 1 coincides with the plate convergence and 3 dips steeply to the west in the northern and central segments controlled by the friction effect and convergence of the plates however this depth range in the southern segment shows an extensional stress range associated with subduction of older slab segment relatively low rate of plate convergence and the morphological adjustment of the slab between different segments at depths of 50 200 km the inhomogeneous stress field exhibits a complicated extensional stress system with strike slip focal mechanisms at depths of 200 400 km the seismicity presents a pronounced seismic gap and the stress field cannot be determined the seismic gap may be attributed to the missing or tear of the subducting slab which is caused by the subduction of the buoyant ogasawara plateau the mechanism solutions are characterized by an inhomogeneous strain regime below 400 km driven by gravity and strong contortion of the slab and resistance to penetration of the 660 km discontinuity in contrast there are little earthquakes below 400 km in the southern region of the ogasawara plateau

earthquake induced crustal gravitational potential energy change in the philippine area abstract the crustal gravitational potential energy change gpe caused by earthquakes in the philippine area from january 1976 to november 2011 was estimated in this study the active convergence between the philippine sea plate and the sundaland eurasian margin is reflected by the greatest gains in gpe along the philippine negros and cotabato trenches whereas the manila trench is covered by a gpe loss pattern although the philippine mobile belt pmb itself is actually affected by the ongoing collision and subduction processes almost the entire philippine fault zone is dominated by gpe loss revealing a slightly extensional environment along the fault the time evolution of the cumulated gpe for different segments along the philippine archipelago shows distinct patterns due to the numerous large underthrusting events that have occurred along the philippine trench the cumulated gpe is regularly increasing in its most southern segment however in the middle segments where the palawan block enters into collision with the pmb the increase in cumulated gpe is relatively small in the most northern segment where the north luzon is located a decrease of cumulated gpe demonstrates that the seismic characteristic of the manila trench is dissimilar from other subduction systems in the world we suggest that the collision of both the palawan block and the benham rise with the pmb promotes the rotation of the pmb and facilitates the northward escape of the northeastern luzon resulting in a decrease of cumulated gpe in the northern philippines

determination of stress from slip data faults and folds a new technique is derived to invert slickenside data for the stress field that caused the faulting episode this inversion is simplified by the assumption that the magnitude of the tangential traction on the various fault planes at the time of rupture is similar study of three normal faulting regimes shows that the inversion derived with this assumption yields results that closely match older inversions that did not include the assumption hence the assumption may be valid and is shown to be justified by analyzing a simple fracture criterion application of slip data inversions is extended from faulting regimes to the slip on bedding plane faults in folding regimes comparison of the inversion results with the geometry of the folds shows this application to be successful greatly increasing the number of data sets that can be used to find the paleostress field

plate convergence measured by gps across the sundaland philippine sea plate deformed boundary the philippines and eastern indonesia summary the western boundary of the philippine sea ph plate in the philippines and eastern indonesia corresponds to a wide deformation zone that includes the stretched continental margin of sundaland the philippine mobile belt pmb extending from luzon to the molucca sea and a mosaic of continental blocks around the ph australia sunda triple junction the gps geodyssea data are used to decipher the present kinematics of this complex area in the philippines the overall scheme is quite simple two opposing rotations on either side of the left lateral philippine fault clockwise to the southwest and counterclockwise to the northeast transfer 55 per cent of the ph sundaland convergence from the manila trench to the northwest to the philippine trench to the southeast further south 80 per cent of the ph sunda convergence is absorbed in the double subduction system of the molucca sea and less than 20 per cent along both continental margins of northern borneo finally within the triple junction area between the sundaland ph and australia plates from sulawesi to irian jaya preferential subduction of the celebes sea induces clockwise rotation of the sulu block which is escaping toward the diminishing celebes sea oceanic space from the eastwardadvancing ph plate to the south we identify an undeformed banda block that rotates counterclockwise with respect to australia and clockwise with respect to sundaland the kinematics of this block can be defined and enable us to compute the rates of southward subduction of the banda block within the flores trench and of eastward convergence of the makassar straits with the banda block the analysis made in this paper confirms that this deformation is compatible with the eastward motion of sundaland with respect to eurasia determined by the geodyssea programme but is not compatible with the assumption that sundaland belongs to eurasia as was often assumed prior to this study

island arc carbonates characterization and recognition in the ancient geologic record abstract carbonates of island arc origin that are preserved in paleozoic mesozoic terranes of the north american cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs this study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities models are developed that show patterns in the long term evolution of shallow marine organisms and the construction evolution and demise of carbonate platforms in island arcs a suite of criteria is identified that may be used to differentiate island arc carbonates from limestones that accumulated in other platform settings biogeographic isolation prolonged subsidence steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism impoverished normal marine faunas complex provincial affinities and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences fringing and barrier reefs at the shelf margin extensive lagoonal deposits and rapid lateral and vertical facies changes although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform and particular facies types may not be represented e g fringing and barrier reefs may be replaced by sand shoals at the platform margin many characteristics have potential value for identifying carbonates of island arc origin in the ancient rock record apart from being associated with calc alkaline volcanic and volcaniclastic assemblages the most valuable suite of features for recognizing island arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities extraordinary thicknesses of platform shallow marine and periplatform carbonates and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones recognizing that an ensemble of features is distinctive within island arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins

present day crustal deformation along the philippine fault in luzon philippines abstract the philippine fault results from the oblique convergence between the philippine sea plate and the sunda block eurasian plate the fault exhibits left lateral slip and transects the philippine archipelago from the northwest corner of luzon to the southeast end of mindanao for about 1200 km to better understand fault slip behavior along the philippine fault eight gps surveys were conducted from 1996 to 2008 in the luzon region we combine the 12 yr survey mode gps data in the luzon region and continuous gps data in taiwan along with additional 15 international gnss service sites in the asia pacific region and use the gamit globk software to calculate site coordinates we then estimate the site velocity from position time series by linear regression our results show that the horizontal velocities with respect to the sunda block gradually decrease from north to south along the western luzon at rates of 85 49 mm yr in the west northwest direction this feature also implies a southward decrease of convergence rate along the manila trench significant internal deformation is observed near the philippine fault using a two dimensional elastic dislocation model and gps velocities we invert for fault geometries and back slip rates of the philippine fault the results indicate that the back slip rates on the philippine fault increase from north to south with the rates of 22 37 and 40 mm yr respectively on the northern central and southern segments the inferred long term fault slip rates of 24 40 mm yr are very close to back slip rates on locked fault segments suggesting the philippine fault is fully locked the stress tensor inversions from earthquake focal mechanisms indicate a transpressional regime in the luzon area directions of 1 axes and maximum horizontal compressive axes are between 90 and 110 consistent with major tectonic features in the philippines the high angle between 1 axes and the philippine fault in central luzon suggests a weak fault zone possibly associated with fluid pressure

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

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

spatial variation of the crustal stress field along the ryukyu taiwan luzon convergent boundary 1 we applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the philippine sea plate our results indicate that the crustal stress along the ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction including the tokara channel an extensional stress regime is observed along the entire ryukyu back arc implying that back arc rifting may have extended northward to kyushu a triangular area near the southernmost terminus of the ryukyu arc is characterized by a unique stress signature the eastern boundary of this ryukyu taiwan stress transition coincides with the 123 e meridian where the gagua ridge intercepts the ryukyu trench whereas its western boundary agrees remarkably well with the border between the postcollision and waning collision domains in northern taiwan the taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the lukang magnetization high lmh suggesting that the lmh may be critical in controlling the local stress distribution the stress signature of the luzon arc taiwan collision reaches as far south as 19 5 n the tectonic stress along the manila trench luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in slab membrane stress since this extensional regime is observed only south of 22 n it probably marks the northern limit of the contemporary boundary between the subduction along the manila trench and the collision in taiwan

stress orientations in subduction zones and the strength of subduction megathrust faults subduction zone megathrust faults produce most of the world s largest earthquakes although the physical properties of these faults are difficult to observe directly their frictional strength can be estimated indirectly by constraining the orientations of the stresses that act on them a global investigation of stress orientations in subduction zones finds that the maximum compressive stress axis plunges systematically trenchward consistently making an angle of 45 to 60 with respect to the subduction megathrust fault these angles indicate that the megathrust fault is not substantially weaker than its surroundings together with several other lines of evidence this implies that subduction zone megathrusts are weak faults in a low stress environment the deforming outer accretionary wedge may decouple the stress state along the megathrust from the constraints of the free surface

plate boundary forces are not enough second and third order stress patterns highlighted in the world stress map database 1 the world stress map project compiles a global database of contemporary tectonic stress information of the earth s crust early releases of the world stress map project demonstrated the existence of first order plate scale stress fields controlled by plate boundary forces and second order regional stress fields controlled by major intraplate stress sources such as mountain belts and zones of widespread glacial rebound the 2005 release of the world stress map project database provides for some areas high data density that enables us to investigate third order local stress field variations and the forces controlling them such as active faults local inclusions detachment horizons and density contrasts these forces act as major controls on the stress field orientations when the magnitudes of the horizontal stresses are close to isotropic we present and discuss examples for venezuela australia romania brunei western europe and southern italy where a substantial increase of data records demonstrates some of the additional factors controlling regional and local stress patterns

plate convergence at the westernmost philippine sea plate abstract to understand the convergent characteristics of the westernmost plate boundary between the philippine sea plate psp and eurasian plate ep we have calculated the stress states of plate motion by focal mechanisms cataloged by the harvard centroid moment tensor solutions harvard cmt and the broadband array in taiwan bats moment tensor 251 focal mechanisms are used to determine the azimuths of the principal stress axes we first used all the data to derive the mean stress tensor of the study area the inversion result shows that the stress regime has a maximum compression along the direction of azimuth n299 this result is consistent with the general direction of the rigid plate motion between the psp and ep in the study area in order to understand the spatial variation of the regional stress pattern we divided the study area into six sub areas blocks a to f based on the feature of the free air gravity anomaly we compare the compressive directions obtained from the stress inversion with the plate motions calculated by the euler pole and the global positioning system gps analysis as a result the azimuth of the maximum stress axis 1 generally agrees with the directions of the theoretical plate motion and gps velocity vectors except block c lanhsu region and block f ilan plain region the discrepancy of convergent direction near the ilan plain region is probably caused by the rifting of the okinawa trough the deviation of the 1 azimuth in the lanhsu region could be attributed to a southwestward extrusion of the luzon arc la block between 21 n and 22 n whose northern boundary may be associated with the right lateral ne sw trending fault i e huatung fault hf along the taitung canyon comparing the 1 stress patterns between block c and block d great strain energy along hf may not be completely released yet alternatively the upper crust of block c may significantly have decoupled from its lower crust or uppermost mantle

the generation of high sr y plutons following late jurassic arc arc collision blue mountains province ne oregon abstract high sr y plutons sr y 40 occupy large areas in ancient and modern orogenic belts yet considerable controversy exists regarding mechanisms of their generation the tectonic settings in which they form and their relationship to contractional deformation through time in the blue mountains province ne oregon a suite of late jurassic 148 145 ma high sr y plutons intrude middle jurassic 162 157 ma low sr y 600 ppm and sr y values 40 and positive initial epsilon hf values 10 5 to 7 8 2 weighted average these geochemical and isotopic results are consistent with geochemical models suggesting derivation from partial melting of island arc crust in the presence of a plagioclase poor to absent clinopyroxene hornblende garnet bearing source depths 35 40 km we propose that the transition from low sr y to high sr y magmatism resulted from orogenic thickening of island arc crust in the dixie butte area during late jurassic arc arc collision between the olds ferry and wallowa island arcs at 159 154 ma this fundamental change in crustal structure influenced post orogenic magmatism and resulted in a relatively brief 3 myr 148 145 ma episode of high sr y magmatism other high sr y plutons occur throughout the us sector of the western north american cordillera e g salmon river suture zone klamath mountains peninsular ranges and closely follow major contractional events involving arc arc and arc continent collisions

evolution of a permo triassic sedimentary melange grindstone terrane east central oregon the grindstone terrane in east central oregon is one of the few areas in western north america where large blocks of unmetamorphosed devonian mississippian and permian limestones are inter mixed with permian and lower triassic radiolarian chert and pennsylvanian permian and triassic volcaniclastic rocks although originally described as parts of a coherent succession we interpret the grindstone rocks to be a sedimentary melange composed of paleozoic limestone slide and slump blocks that became detached from a carbonate shelf fringing a volcanic knoll or edifice in late permian to middle triassic time and were intermixed with permian and triassic slope to basinal clastic and volcaniclastic rocks in a forearc basin setting paleogeographic affinities of the grindstone limestone faunas and volcaniclastic debris in the limestone and clastic rocks all indicate deposition in proximity to an island arc system near the north american craton the grindstone terrane deposits are unconformably overlain by upper triassic to middle jurassic sequences of the izee terrane although lithologic and faunal differences indicate that the grindstone and izee terranes together represent a tectonic block separate from the adjacent baker terrane all three terranes were juxtaposed by late triassic or early jurassic time we recommend reduction to informal status for the coffee creek mississippian spotted ridge pennsylvanian and coyote butte permian formations because 1 they cannot be mapped or traced beyond limited areas and 2 these older rocks are chaotically inter mixed with younger chert and volcaniclastic rocks new biostratigraphic data indicate that the bulk of the spotted ridge volcaniclastic rocks represent a part of the triassic vester formation of the izee terrane

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

geologic and paleomagnetic evidence for a possible miocene collision in western panay central philippines the palawan metamorphic terrane represented by paleozoic basement on the northwestern tip of panay central philippines collided with the remainder of the island between late oligocene and middle miocene time subduction continued north and south of the collision zone along the manila and negros trenches respectively evidence for the collision is 1 the juxtaposition along a major fault of old melange terrane against highly metamorphosed paleozoic continental sediments similar to those exposed on palawan island itself and 2 miocene thrusting in the melange terrane oriented parallel to the suture paleomagnetic studies show that since the early miocene the island of panay has rotated 20 clockwise declination 20 9 inclination 22 1 for six sites whereas the northern philippines has rotated counterclockwise these contrasting rotations are consistent with a collision of the philippine arc with a northeastward converging palawan block

plate coupling along the manila subduction zone between taiwan and northern luzon abstract we use gps data trench parallel gravity anomaly tpga and bathymetry to infer plate coupling patterns along the manila subduction zone using a block model and a fault geometry constrained by seismicity we simultaneously solve for the location of euler pole and angular velocity between the sunda and luzon blocks as well as the slip deficit rate on plate interface our estimates show that the euler pole between the sunda and luzon blocks is situated at southern palawan near 8 3 n and 119 4 e with the angular velocity of 4 6 myr 1 the estimated convergence rate along the manila trench continuously decreases southward from 91 mm yr at the northern tip of luzon to 55 mm yr north of mindoro the inversion of gps data reveals partially locked fault patches extending from the west luzon trough to the east of scarborough seamount chain the slip deficit rate in this region is in the range of 20 30 mm yr corresponding to a coupling ratio of 0 4 however the fault slip behavior is not well resolved near the north luzon trough based on a good correlation between locations of large subduction zone earthquakes and areas possessing gravity low we investigate a variety of tpga based plate coupling models assuming different scaling between tpga values and plate coupling ratios the tpga based plate coupling models offer plausible rupture scenarios which are not constrained by current gps data the partially locked fault zone near 15 16 5 n may be associated with the subducted scarborough seamount wherein oceanic floor is highly fractured the great subduction zone earthquake propagates beneath the scarborough seamount seems to be unlikely the densification of gps network in central luzon and seafloor geodetic observations close to trench axis are crucial to distinguish the detailed fault coupling patterns and the role of subducted seamounts

role of fluids in faulting inferred from stress field signatures the stress orientation signature of weak faults containing high pressure fluids has been observed for segments of the san andreas fault system in southern california the inferred lithostatic fluid pressures extend into the surrounding relatively intact rock in a zone scaling with the width of the interseismic strain accumulation repeated strain related fracturing and crack sealing may have created low permeability barriers that seal fluids into the network of currently active fractures

a mid permian chert event widespread deposition of biogenic siliceous sediments in coastal island arc and oceanic basins abstract radiolarian and conodont of permian siliceous rocks from twenty three areas in teh the circum pacific and mediterranean regions reveal a widespread permian chert event during the middle leonardian to wordian radiolarian and or sponge spicule rich siliceous sediments accumulated beneath high productivity zones in coastal island arc and oceanic basins most of these deposits now crop out in fault bounded accreted terranes biogenic siliceous sediments did not accumulate in terranes lying beneath infertile waters including the marine sequences in terranes of northern and central alaska the permian chert event is coeval with major phosphorite deposition along the western margin of pangea phosphoria formation and related deposits a well known analogue for this event is middle miocene deposition of biogenic siliceous sediments beneath high productivity zones in many parts of the pacific and concurrent deposition of phosphatic as well as siliceous sediments in basins along the coast of california interrelated factors associated with both the miocene and permian depositional events include plate reorientations small sea level rises and cool polar waters

stress permutations three dimensional distinct element analysis accounts for a common phenomenon in brittle tectonics 1 using three dimensional 3 d distinct element modeling we explored a variety of simulations to characterize and interpret the stress permutations in brittle tectonics stress inversions of fault slip data or earthquake focal mechanisms often revealed such permutations the main aim of our study is to produce simple mechanically consistent 3 d models that account for these switches between the principal stress axes 1 2 or 2 3 even with simple boundary conditions the stress changes induced by variations in rheology are large enough to modify the local tectonic behavior and to produce permutations of principal stress axes rather than simple directional changes of stress axes which exist but often remain limited the relative variations in principal stress magnitudes are the major cause of permutations 1 2 and 2 3 in nature permutations being left apart the orientations of axes often remain tightly clustered in our experiments we adopted a ratio 2 3 1 3 of 0 5 which makes permutations difficult low favors 2 3 permutations high favors 1 2 permutations and we explored a variety of tectonic situations involving compression extension and strike slip our experiments indicate that the major causes of stress permutations are the heterogeneity of the brittle deformation e g intact rock massifs between heavily faulted deformation zones and the anisotropy of the mechanical properties that results from fracturing and faulting which concur to modify the mechanical balance inside the analyzed volume and to produce stress permutations our models show that contrasts and anisotropy in rock properties favor stress permutations of major importance is the existence of relatively resistant zones at tips of the deformed zones acting as channels where stress concentrates and switches occur because in nature such zones move in time and space it is not surprising that stress permutations are so pervasive

temporal and spatial variation of stress field in taiwan from 1991 to 2007 insights from comprehensive first motion focal mechanism catalog article i nfo the total amount of 4 761 focal mechanisms was determined based on p wave first motion polarities from 1991 to 2007 in taiwan region this dataset offers us a good opportunity to examine temporal and spatial variability of the stress field we find that the spatial variations of stress axes are mainly controlled by tectonic structures while the temporal changes are greatly influenced by the chi chi earthquake the orientation of the maximum horizontal compressive stress axes sh shows a general agreement with the direction of plate motion between a depth range of 0 30 km the 20 anticlockwise rotation of sh from the longitudinal valley lv to western taiwan is probably caused by the left lateral motion on the longitudinal valley fault lvf that has consumed part of the oblique motion of plate convergence on the other hand part of the oblique convergence is transferred into the central range and the hsuehshan range judging from counterclockwise rotation of sh from east to west and strike slip faulting in the hsuehshan range most events with a depth greater than 30 km occur offshore eastern taiwan and the azimuth of sh is close to e w directed different from nw se directed at shallow depths this may infer the existence of the transition of lithosphere rheology in offshore eastern taiwan the trends of sh in the depth of 0 10 km are strongly affected by the coseismic stress change of the chi chi earthquake in the northern half of the chi chi rupture area the trends of sh rotate 30 clockwise and the stress ratio increased by a factor of six after the mainshock the orientations of sh still differ by 30 in 2007 comparing to that in the period before the chi chi earthquake the variation of sh trend is more diverse in the southern half of the rupture area showing 20 counterclockwise rotation immediately after the 1999 mainshock followed by a clockwise rotation the trend of sh returns to the pre seismic direction of 110 in 2001 these notable changes of sh before and after the chi chi mainshock suggests that the magnitude of background stress in the rupture area is close to the coseismic stress drop we also recognize a significant 10 counterclockwise rotation of sh in the entire chi chi rupture area between 1991 and 1999 before the earthquake took place however to the south of the chi chi rupture the trends of sh remain little changed before and after the chi chi earthquake

tertiary and quaternary magmatism in mindanao andleyte philippines geochronology geochemistry and tectonic setting abstract a set of 230 igneous rock samples from leyte and mindanao philippines has beenstudied for their major and trace element chemistry mineralogy and geochronology 40k 40ar dating several volcanic sectors have been distinguished according to their geographic position tectonic framework and geologic history a in leyte the ophiolitic basement at 50 ma is morb type but characterized by negative nb 0 anomalies typical of back arc basalts miocene 20 and 11 ma and late pliocene to quaternary magmas 2 5 0 4 show typical island are talc alkaline characteristics in eastern mindanao eocene 47 46 ma magmatic rocks are arc tholeiitic while those of miocene 18 6 ma and early pliocene 4 5 ma age are generally talc alkaline dacites of 12 ma age have adakitic compositions the young 3 ma volcanics in surigao and north davao are mostly adakitic although talc alkaline basalts and basaltic andesites dated at 0 9 ma overlie adakites in surigao b in daguma oligocene 30 ma arc tholeiitic diorites intrude older arc sequences miocene0 17 0 7 7 ma dikes and flows in both daguma and sarangani peninsula are mostly talc alkaline andesites although minor adakites dated back to 18 ma have been found overlying undated breccias in daguma in southeast daguma the tale alkaline volcanic substratum of pliocene recent volcanoes is 8 6 ma old quaternary 1 83 ma to present andesitec and dacitic volcanics in mt parker and mt matutum southwest daguma are either talc alkaline or adakitic while mt blit northwest daguma is built up by 2 5 ma old hfse enriched basalts 1 ma old normal talc alkaline andesites and 0 8 ma old adakites c in central mindanao miocene andesites yield ages of 20 and 16 ma pliocene 2 5 ma 0 volcanism is dominated by talc alkaline and hfse enriched basalts and basaltic andesites quaternary lavas i ma to present range in character from tale alkaline to shoshonitic and exhibit wide variations in their trace element behavior d miocene lavas of 19 11 ma age in zamboanga are mostly talc alkaline though a lone 0 18 ma old adakite was also encountered pliocene quaternary basalts and basaltic andesites dated back to 2 2 1 ma are hfse enriched such that some display positive nb anomalies in spidergrams adakites of 3 8 0 7 ma age are located nearer the sulu trench the compositional variety of tertiary recent island arc magmatism in leyte and mindanao couldbe linked to the tectonic settings prevailing during each magmatic phase and is here utilized to refine paleokinematic reconstruction of the tectonic history of the philippine archipelago the identification of adakites at certain time periods is used to date timings of subduction initiations and arc polarity reversals major and abrupt changes in geochemical compositions are well exhibited by the pliocene pleistocene volcanism involving the collision related shift from talc alkaline to shoshonitic volcanism in central mindanao and adakite production in all the volcanic sectors defined the origin of the hfse enrichments among lavas younger than 3 ma could be attributed to the presence in their mantle source of an oib component or alternatively to the enrichment of this mantle source through percolation of slab derived melts

geological setting of the rapu rapu gold rich volcanogenic massive sulfide deposits albay province philippines gold rich fe cu zn volcanogenic massive sulfide deposits occur within strata of probable jurassic age on rapu rapu island in albay province philippines massive sulfides at the ungay malobago and hixbar deposits are spatially associated with dacitic volcanic rocks within a highly deformed sequence of mafic volcanic and quartzofeldspathic sedimentary rocks the massive sulfide deposits formed at the stratigraphic contact between footwall dacites and hangingwall mafic volcanic and quartzofeldspathic rocks the deposits and their host strata have undergone regional metamorphism with strong penetrative deformation metamorphic mineral assemblages and textural evidence suggest that peak metamorphism was upper greenschist to lower amphibolite grade and syn d1 deformation based on the age of regional metamorphism deformation is inferred to be mid tertiary in age deformation at rapu rapu resulted in reorientation of the strata into a broad antiform with strong shallow plunging elongation fabrics overturning of the volcanic sequence that hosts the ungay malobago deposit and complex folding of the mineralized zones the present highly linear form of the ungay malobago deposit is mainly a product of this ductile strain

earthquakes along the ryukyu kyushu arc strain segmentation lateral compression and the thermomechanical state of the plate interface we systematically determined the focal depths and mechanisms of 49 large to moderate sized earthquakes mb 5 4 that occurred along the ryukyu kyushu arc since 1963 by inverting the waveforms and amplitudes of p and sh wave trains at teleseismic distances the results are sufficiently precise to delineate seismogenic structures near the plate interface in detail revealing features not predicted by plate kinematics in contrast to previous studies of this arc and those along other subduction zones shallow seismicity along the plate interface is systematic showing two distinct layers of activities the second layer of seismicity is delineated by a few earthquakes that occurred at depths between 50 and 65 km some 10 20 km directly beneath the seismogenic portion of the interplate thrust zone these earthquakes indicate lateral compressional strain within the subducted slab as their p axes are subparallel to the local strike of the arc not perpendicular to the arc as one would expect from the direction of plate convergence the occurrence of these events cannot be accounted for by membrane stress due to the geometry of the subducted slab to our knowledge similar earthquakes occurred only beneath the northern indoburman ranges and along the banda arc where subducted slabs as part of the indian plate are being dragged northward with their northern edges bumping into east west trending collision zones nearby by drawing an analogy between the tectonic settings of these three regions we interpret events beneath the plate interface along the ryukyu kyushu arc as a consequence of ongoing collision between the philippine sea plate and eurasia near taiwan the interplate thrust zone is largely aseismic down to a depth of approximately 30 km a large number of earthquakes showing low angle thrust faulting commence at this depth and are accompanied by two events that show antithetic thrust faulting at a slightly shallower depth of 20 25 km these observations suggest that the strength of the plate interface increases significantly below the depth of proximately 25 5 km based upon available heat flow measurements between the trench axis and the volcanic arc we estimated the temperature field and magnitude of shear traction along the interplate thrust zone the deepest interplate earthquakes 40 50 km correspond to temperatures of approximately 730 980 c comparable to the limiting temperature of intraplate mantle earthquakes 800 c and that of intermediate and deep focus earthquakes at wadati benioff zones potential temperature 900 k therefore the cessation of seismicity in general is probably controlled by similar temperature conditions regardless of the tectonic settings of source regions the magnitude of shear traction along the seismogenic portion 40 50 km depth of the interplate thrust zone seems to be of the order of 100 mpa which in turn implies a low average coefficient of friction of only 0 10 0 05 considerably less than those observed for laboratory specimens the inception of intermediate focus earthquakes within the subducted slab is at a depth of about 100 km whereas shallow earthquakes show no apparent variations along the entire arc intermediate depth earthquakes indicate downdip extension along the northern end of the arc near kyushu but abruptly change to downdip compression along the rest of the arc all available evidence indicates that this is a spatial pattern unrelated to earthquake cycles at the moment tectonic interpretation of such a distinct strain segmentation along the arc is enigmatic without the presence of clear discontinuities in the age of the subducted slab the sudden switch in the strain field of the slab is difficult to explain by the continuously varying rate of subduction along the arc in any case this pattern of strain segmentation requires the presence of a major accommodation structure such as a tear fault in the subducted slab near the tokara channel appendix is available with entire article on microfiche order from american geophysical union 2000 florida ave n w washington dc 20009 document b91 004 2 50 payment must accompany order

an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence the orientations of fault planes and slip directions indicated by a population of earthquake focal mechanisms can be used to determine best fit regional principal stress directions and r 2 1 3 1 a measure of relative stress magnitudes under the assumption of uniform stress in the source region this analysis allows for the possibility that failure occurs on preexisting zones of weakness of any orientation in the inversion we perform a grid search of stress models to find the one which requires the smallest total rotation of all the fault planes that is needed to match the observed and predicted slip directions the method allows for errors in orientations of both the fault planes and slip directions we have an objective means for identifying the more likely of the two possible fault planes from each focal mechanism relative to a given stress model thus we do not face the problem of ambiguity of nodal planes which plagues other analyses of this kind by using a grid search of stress models rather than a linearization scheme we are able to perform a realistic error analysis and thus establish confidence limits for the preferred regional stresses the method can be used to investigate possible stress inhomogeneities during earthquake sequences by analyzing subsets of the data population the technique has been applied to 76 events from the san fernando earthquake sequence for which we have found best fit stresses plunge and azimuth 1 7 187 2 27 281 3 62 84 and r 0 65 the average misfit between this stress model and all the data is about 8 and all but eight of the aftershocks have misfits of less than 20 these values are considerably less than the uncertainty of the focal mechanism determinations therefore significant stress inhomogeneities are not required by the data our analysis does not support the suggestion of a change in stresses during the aftershock sequence as proposed by others on the basis of an apparent change in focal mechanisms

genetic relations between the central and southern philippine trench and the sangihe trench we surveyed the junction between the central and southern philippine trench and the sangihe trench near 6on using swath bathymetry gravity and magnetics these data along with seismicity allow us to discuss the genetic relations between these trenches and the forces acting on converging plates our final model favors the northern extension of the halmahera arc up to 8on with three segments offset left laterally along nw se transform faults accretion of the northern segment to mindanao island 4 to 5 m y ago resulted in the failure within the philippine sea plate east of the arc initiation of the philippine trench between 7on and 10on agrees with the maximum recorded depth of the philippine trench floor 10 000 m below sea level and philippine sea slab 200 km south of 6on trench junction another segment of the arc is being subducted beneath the sangihe margin while south of 3on the southern segment of the halmahera arc is still active the rapid southward shallowing of the trench floor along the southern philippine trench the type of faulting affecting both sides of the trench the lack of significant interplate seismicity and the concentration of the seismicity beneath the miangas talaud ridge are interpreted as a slowing down of the subduction along this branch of the philippine trench compared with the rest of the subduction zone the sangihe deformation front has been recognized up to 7on but seems active only south of 6on

crust and upper mantle structure and its tectonic implications in the south china sea and adjacent regions abstract we present a 3d s velocity model for the crust and upper mantle of the south china sea and the surrounding regions constrained from the analysis of over 12 000 of fundamental rayleigh wave dispersion curves between 10 s and 150 s periods the lateral resolution was found to vary from 2 to 4 with the increasing period over the study region a robust scheme of debayle and sambridge allowed us to conduct the tomographic inversion efficiently for massive datasets group velocity maps varying with period show lateral heterogeneities well related to the geological and tectonic features in the study region the 3d s velocity model was constructed from the 1d structure inversion of the tomographic group velocity dispersion curves at each node the obtained average crustal structure is similar to the prem model while the average mantle velocity is typically lower than the global average the complicated 3d structures reveal three prominent features correlated with geological divisions sea basin regions island and arc regions and continental regions the derived crustal and lithospheric thicknesses range from 15 to 50 km and from 60 to 140 km respectively with the thinnest in the south china sea the thickest in eastern tibet and the yangtze block and the medium in the south china fold belt indochina and island arc regions our results further confirm that 1 a mesozoic subduction zone which is interpreted as the tectonic weak zone during the paleogene exists along the south china margin 2 the influence of the indochina extrusion along the red river fault is limited for the south china sea region 3 there is a slab remnant of the proto south china sea beneath borneo new findings suggest that the mesozoic subduction zone should be built into any evolution model for the region as well as the other two major tectonic boundaries of the red river fault and proto south china sea subduction zone

display and quantitative assessment of distributions of earthquake focal mechanisms summary this paper describes quantitative methods for evaluating the distribution of focal mechanism orientations of groups of earthquakes and shows how to display this distribution on a triangle diagram we present a 2 based statistical test for determining whether two sets of focal mechanisms are drawn from distinct populations we apply these methods to 3625 better determined mechanisms for shallow earthquakes in the harvard centroid moment tensor cmt catalogue we describe the distributions of mechanisms in the whole catalogue and for catalogue subsets in some specific tectonic environments in addition we explore the geographical locations of mechanisms with orientations that occur relatively infrequently that is mechanisms that are unlike thrust normal or strike slip mechanisms such mechanisms are relatively rare along mid ocean ridges and in oceanic subduction zones the majority of these unusual mechanisms occur along plate boundaries where crustal thickness is highly variable and in regions where the plate convergence direction becomes oblique and thus relative motion changes from convergence to transform motion

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

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

philippine fault a key for philippine kinematics on the basis of new geologic data and a kinematic analysis we establish a simple kinematic model in which the motion between the philippine sea plate and eurasia is distributed on two boundaries the philippine trench and the philippine fault this model predicts a velocity of 2 to 2 5 cm yr along the fault geologic data from the visayas provide an age of 2 to 4 ma for the fault an age in good agreement with the date of the beginning of subduction in the philippine trench the origin of the philippine fault would thus be the flip of subduction from west to east after the locking of convergence to the west by the collision of the philippine mobile belt with the eurasian margin

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

transition from oblique subduction to collision earthquakes in the southernmost ryukyu arc taiwan region tectonic characteristics of the region between taiwan and the southernmost ryukyu arc are inferred from a detailed analysis of local seismicity and source parameters of 62 recent earthquakes of 5 5 mb 6 6 five major seismogenic structures can be delineated the collision seismic zone csz the interface seismic zone isz the wadati benioff seismic zone wbsz the lateral compression seismic zone lcsz and the okinawa seismic zone osz in the csz located along the east coast of taiwan and offshore earthquake focal mechanisms show horizontal p axes distributed in two directions 287 10 and 333 16 possibly reflecting a strain partition associated with the relative plate convergence between the eurasia plate and the philippine sea plate the corresponding seismic strain tensor indicates a maximum compressive strain rate of 1 2 10 7 yr 1 along 293 and a comparable extension in vertical direction presumably resulted from plate collision in the region the geometry of the isz which is distorted significantly at its westernmost end can be approximated by a north dipping plane that is gradually pushed northward with increasing dip the seismogenic portion of the interface spans a short depth range from 10 km to 35 km a clear pattern of earthquake slip partition is observed the average slip vector residual is as large as 35 seismic strain patterns within the subducted philippine sea slab show predominantly downdip extension between 80 and 120 km and downdip compression at 270 km different from the pattern of strain segmentation observed for the rest of the ryukyu arc where the northern and southern portions are dominated by downdip extension and compression respectively owing to the large convergence obliquity the slab is descending at a rate significantly slower near taiwan than in the southern ryukyu thus we interpret the appearance of downdip extension within the subducted lithosphere as a combined result of oblique subduction and the slab s negative buoyancy a number of thrust or oblique strike slip earthquakes between isz and wbsz show a consistent pattern of lateral compression with p axes oriented roughly parallel to the local strike of the trench arc system they are probably due to the compressive strain originated from the collision and transmitted laterally within the lithosphere shallow normal faulting earthquakes show successive rotation of t axes from approximately n s in the okinawa trough to approximately e w in northeast taiwan possibly as a result of interaction between the extension from the opening of the okinawa trough and the compression from collision one normal event january 18 1991 mb 5 9 occurred in the central range with t axis roughly parallel to the structural trend of taiwan implying that the nature of the orogeny in taiwan has changed from thin skinned deformation to lithospheric collision involving the whole crust and uppermost mantle

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

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

structural variety and tectonic evolution of strike slip basins related to the philippine fault system northern luzon philippines the northern part of the philippine fault zone in luzon corresponds to a complex braided system of left lateral strike slip faults the nw oriented main active branch which emerges from the philippine sea splits into an array of north striking splays responsible for the tectonic evolution of the central cordillera this complex fault pattern has favored local stress field variations strike slip basins have evolved in this framework along or in the vicinity of the main splays of the philippine fault apart from the classical pull apart tectonics on a releasing fault termination overlap or bend we describe other mechanisms such as the strike slip tilting or the warping of a strip limited by two strike slip faults the strike slip basins are good recorders of the evolution of the philippine fault system those located along the north striking cordilleran faults individualized in late early pliocene to pleistocene time when the present day philippine fault initiated but their main control is the fault shape acquired in upper middle miocene time the recent tectonic evolution of the fault system is best recorded in central luzon where the active basins trace an asymmetrical v shape with the longest branch trending ne parallel to the east luzon trough and the shortest one trending nw related to the philippine fault both the fault pattern and the basin distribution demonstrate the influence of the benham rise in the tectonic evolution of luzon the structural setting is interpreted as the result of an early miocene collision event between the benham rise and the eastern margin of luzon and subsequent inception of the nw striking strand of the philippine fault the present locations of the basins result from the interaction between the structural heritage and the present day regional plate motion