the angat ophiolite luzon philippines lithology structure and problems in age interpretation abstract the angat ophiolite is an incompletely unroofed and structurally dissected ophiolitic mass consisting of layered and massive gabbros diabase sheeted dikes tonalites and pillow basalts mapping in the montalban quadrangle has revealed complex structural relationships between pillow basalts and overlying sediments which hinder an accurate determination of ophiolite age sparse radiolarian data suggest a cretaceous age but undifferentiated as to early or late cretaceous the structural deformation of the ophiolite is probably due in part to strike slip faulting during early to middle miocene time this date being constrained by deformed sediments in fault contact with the ophiolite initial geochemical data from sheeted dikes and pillow lavas clearly related to the ophiolite show unusually high silica contents and trace element discrimination suggests that different magma types may have been associated with the ophiolite pillow basalts that are probably genetically unrelated to ophiolite were also mapped close to the angat ophiolite ages and tectonic settings determined from sedimentary rocks overlying these basalts should not be used to constrain age and paleotectonic interpretations of the ophiolite

sedimentation in pull apart basins active examples in eastern turkey the erzincan susehri niksar and lake hazar basins are actively evolving pull apart basins in eastern turkey their structural drainage and depositional systems generally support a pull apart basin sedimentation model based on ancient pull apart basin deposits this model involves marginal coarse grained sedimentation adjacent to master faults axial fine grained sedimentation away from master faults and high sedimentation rates however studies of the turkish pull apart basins suggest the following refinements in the model 1 detailed sedimentary facies patterns and the dominant facies of a pull apart basin vary according to basin shape climate drainage character and volcanic activity depositional environments near fault bounded margins include alluvial fan fan delta and alluvial apron depositional environments in central areas away from fault bounded margins include active braid plain inactive braid plain meander plain salt playa marsh lacustrine mouth bar delta and volcanic 2 i

multiple large earthquakes in the past 1500 years on a fault in metropolitan manila the philippines the first 14 c based paleoseismic study of an active fault in the philippines shows that a right lateral fault on the northeast edge of metropolitan manila poses a greater seismic hazard than previously thought faulted hillslope colluvium stream channel alluvium and debris flow deposits exposed in trenches across the northern part of the west marikina valley fault record two or three surface faulting events three eroded clay rich soil b horizons suggest thousands of years between surface faulting events whereas 14 c ages on detrital charcoal constrain the entire stratigraphic sequence to the past 1300 1700 years we rely on the 14 c ages to infer faulting recurrence of hundreds rather than thousands of years minimal soil development and modern 14 c ages from colluvium overlying a faulted debris flow deposit in a nearby stream exposure point to a historic age for a probable third or fourth most recent faulting event

kinematics of the philippine sea plate in this study new instantaneous rotation vectors describing the motion of the philippine sea plate relative to surrounding plates are determined that most nearly satisfy all available geological geophysical and seismological observations along the plate margins pacific philippine and caroline pacific poles are generated from a set of trial eurasia philippine poles of rotation using published determinations of the eurasia pacific and caroline philippine poles the pacific philippine caroline pacific and eurasia philippine poles are then checked for agreement with 1 slip vectors determined from focal mechanisms of earthquakes along the ryukyu trench and the nankai trough 2 convergence at the sagami trough 3 extension in the mariana trough 4 extension behind the bonin trench 5 extension in the western sorol trough and 6 convergence at the mussau trench trial eurasia philippine poles are eliminated if predicted plate motions do not match observed plate motions the locations and rotation rates of the three best fitting poles are eurasia philippine 37 0 n 141 0 e 1 60 ma pacific philippine 3 8 n 130 4 e 1 68 ma caroline pacific 16 9 n 141 4 e 0 54 ma the plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the philippine pacific and caroline plates intersect intermediate and deep focus earthquake hypocenters along the southern mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the mariana trough disappears the absence of volcanism and intermediate depth seismicity along the southernmost mariana yap and palau sections of the plate boundary is attributed to very low convergence rates a new focal mechanism solution along the trench south of guam is the first clear underthrusting solution found along the mariana trench and agrees with the convergence direction implied by the pacific philippine pole of rotation no clear underthrusting focal mechanism solutions were found along the yap and palau trenches although three high angle thrust solutions demonstrate that compression is occurring in the upper plate perpendicular to the plate boundary

formation of interior basins associated with curved faults in alaska the positions of certain nonmarine sedimentary basins in central and southern alaska are spatially related to the geometry of adjacent curved faults these interior basins occur along the northern convex side of these fault systems which change trend from northwest in canada and eastern alaska to east west in central alaska and to northeast in western alaska the basins probably initiated in early to middle tertiary time and deposition within all basins continued through the quaternary broadly synchronous with tertiary strike slip displacements along the tintina denali and border ranges fault systems we examine the spatial and temporal relationship between basins and faults by using a mechanical model of curved strike slip faults based on boundary elements we relate slip along the curved faults to tertiary pacific north american plate motions by using principal stress directions inferred from the plate motion directions locations of the predicted mean stress reductions around the fault bends caused by fault slip correlate well with positions of the interior basins our results suggest that basin subsidence and location were controlled primarily by the geometry of adjacent curved fault systems and the state of stress in alaska during the tertiary mechanical interaction between curved alaskan fault systems increases the tendency for basin subsidence stresses inferred from early tertiary plate motion vectors are not oriented correctly to drive right lateral slip along kaltag farewell and parallel faults in western alaska furthermore these stress states would have produced left lateral displacements for which there is no evidence along these western faults in their present orientations we find that a combination of slightly rotated fault geometry and low friction along the faults appears to facilitate right lateral slip along the faults in western alaska our results show that deformation well inside the continent can be explained in terms of the pacific north american plate motions thus tertiary plate motions have resulted in a broad zone of fault slip uplift and subsidence in the nonrigid north american plate producing several large basins

a geophysical study of the manila trench luzon philippines 2 fore arc basin structural and stratigraphic evolution the pattern of sediment distribution within the manila trench subduction zone west of luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic hemipelagic sediment layer and oceanic basement in the northern part of the manila trench these turbidites were transported from the uplifted collision zone of taiwan southward along the manila trench by gravity controlled processes similarly thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the north palawan block and the central philippines sedimentation within the manila trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis in contrast the fore arc basin system of the manila trench receives sediment through submarine canyons incised into the west luzon continental slope the rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench the size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench a thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region this generalization may apply to other convergent plate margins as well the uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins this variability results in the formation of several different types of fore arc morphology along the manila trench the overall seismic stratigraphy of the west luzon trough fore arc basin is remarkably uniform with seismic sequences continuous for up to 110 km along strike the unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long term approximately 10 ma uplift of the accretionary prism and relatively short term approximately 1 2 ma variations in sediment supply to the fore arc basin modulated by eustatic sea level changes

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

evolution of a forearc basin luzon central valley philippines the cenozoic history of the 14 km thick luzon central valley sequence illustrates the development of a forearc basin forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations the luzon basin is floored by oceanic crust on the seaward western side and older accreted terranes on the arc eastern side initial sedimentation on this oceanic crust occurred during early tertiary northward translation and emplacement of the crust as an ophiolite along a strike slip or oblique slip zone the basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite a sequence of arc derived sediments at least 26 000 ft 8 km thick was shed into the astern arc side of the basin during late paleogene to quaternary convergence along the western margin of luzon by the middle miocene the central valley became a continuous elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths nonmarine deposition began in central portions of the basin in the pliocene and migrated with time both north and south along the basin axis late miocene to holocene movement along the philippine fault zone caused uplift and folding of adjacent parts of the basin exploration models for the central valley predict gas prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred the hydrocarbon potential of the central valley has not been determined adequately

interseismic deformation and moment deficit along the manila subduction zone and the philippine fault system we examine interseismic coupling of the manila subduction zone and fault activity in the luzon area using a block model constrained by gps data collected from 1998 to 2015 estimated long term slip rates along the manila subduction zone show a gradual southward decrease from 90 100 mm yr at the northwest tip of luzon to 65 80 mm yr at the southern portion of the manila trench we provide two block models models a and b to illustrate possible realizations of coupling along the manila trench which may be used to infer future earthquake rupture scenarios model a shows a low coupling ratio of 0 34 offshore western luzon and continuous creeping on the plate interface at latitudes 18 19 n model b includes the north luzon trough fault and shows prevalent coupling on the plate interface with a coupling ratio of 0 48 both models fit gps velocities well though they have significantly different tectonic implications the accumulated strain along the manila subduction zone at latitudes 15 19 n could be balanced by earthquakes with composite magnitudes of mw 8 8 9 2 assuming recurrence intervals of 500 1000 years gps observations are consistent with full locking of the majority of active faults in luzon to a depth of 20 km inferred moments of large inland earthquakes in luzon fall in the range of mw 6 9 7 6 assuming a recurrence interval of 100 years

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

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

development of pull apart basins a comparative study of well mapped active and ancient pull apart basins suggests a qualitative model for their continuous development pull aparts evolve from incipient to extremely developed basins through a sequence of closely related states new and compiled map data from several areas including the northern caribbean and turkey suggests the following stages in pull apart development 1 in rigid intracontinental strike slip plate boundary zones larger pull aparts nucleate at releasing fault bends along segments of the principal displacement strike slip fault zone which are oblique to the theoretical interplate slip lines 2 initial opening across releasing fault bends produces spindle shaped basins defined and often bisected by oblique slip faults connecting the discontinuous ends of the strike slip faults 3 increased strike slip offset produces basin shapes which we colloquially call lazy s shape for basins between sinistral faults and lazy z shape for basins between dextral faults 4

40ar 39ar laser probe dating by step heating and spot fusion of phengites from the dora maira nappe of the western alps italy 40ar 39ar laser probe dating of phengitic micas has been carried out by step heating and spot fusion procedures these micas represent successive deformation stages in the structural evolution of the internal dora maira nappe western alps single phengites from a gneiss affected by a single ductile strain under retrogressive conditions sample 99 1 display complete isotopic resetting with nearly homogeneous intracrystalline ar distribution and yield plateau ages of about 40 ma small clusters of phengites from an earlier foliation were selected from a polydeformed mica schist sample ptx3 they show a partial isotopic resetting in response to overprinting during the retrogressive deformation stage with a concentric age zoning from 68 ma on the rim to 87 ma in the core on one cleavage plane this zonation is fully consistent with the laser derived discordant age spectrum which ranges from 40 to 90 ma from low to high temperatures according to the deformation history of both samples these preliminary data suggest a deformation control on ar migration during recrystallization processes and they are consistent with the timing of the collisional evolution previously reported for southern dora maira units this study shows that the 40ar 39 ar continuous laser probe dating technique produces data accurate enough to discriminate several tectonometamorphic episodes recorded in single hand samples

the philippine wrench fault system in the ilocos foothills northwestern luzon philippines abstract luzon is part of the taiwan luzon mindoro mobile belt which marks the plate boundary between the eurasian and philippine sea plates on luzon the philippine fault runs parallel to the philippine archipelago for more than 1300 km and is an active left lateral strike slip fault which forms a braided system detailed field mapping together with structural and biostratigraphic analysis have been conducted in the ilocos foothills on the western flank of the central cordillera cenozoic magmatic arc this area on the northwestern tip of luzon appears to consist of three large tectonic units which are continuous for more than 200 km and bounded by major left lateral strike slip faults related to the philippine fault from west to east these units are as follows 1 1 the vigan laoag coastal strip vlcs consists of an ultramafic basement which is strongly tectonized and unconformably overlain by a relatively undeformed middle miocene to pliocene sedimentary cover 2 2 the vintar median strip vms is bounded to the west by the vigan aggao wrench fault which upthrusts westwards and to the east by the abra river strike slip fault this unit is composed of a thick 6 10 km late eocene to pliocene volcaniclastic sequence deposited in a basin which was fed mainly by the central cordillera but also until the early miocene by an ultramafic high composed of the vlcs basement this sequence is strongly folded 3 3 the eastern strip es is a late eocene to miocene volcaniclastic sequence intruded by oligocene and miocene rocks this structural pattern results from two major tectonic episodes 1 1 the late middle to late miocene episode comprises the abra river strike slip faulting this is characterized by the folding of a 20 km wide band and by the creation of a ridge transverse to the tectonic strips related to the abra river fault restraining bend this ridge seems to have absorbed the compressive deformation whereas the strike slip component was accommodated by the nearly pure strike slip faulting along the abra river fault 2 2 the pliocene to quaternary episode involved strike slip activity propagating westward from the abra river fault relatively inactive during this phase to the vigan aggao wrench fault the vigan aggao fault is located at the boundary between an ultramafic palaeo high highly deformed mesozoic ophiolite and a palaeobasin filled with a thick late eocene to miocene sedimentary sequence this particular location seems to attest to the recent use of an older major tectonic feature

stress fields about strike slip faults inferred from stylolites and tension gashes abstract in general the long axis of the tension gashes and stylolitic columns developed in limestones during a single phase of compressional deformation occur parallel to the direction of the maximum compressive stress 1 this is the case in the languedoc for structures developed in the jurassic limestones during the n s pyrenean compression exceptionnally however these microstructures turn in direction and become oblique even orthogonal one to the other probably as a consequence of a variation in intensity and direction of the stress field at the end of a microfault this mechanism also occurs in a larger scale structure involving segments of pre tectonic joints that act as en echelon microfaults in a brittle kink band equivalent to a peculiar type of potential wrench fault

holocene temperature fluctuations in the northern tibetan plateau arid central asia aca lies on a major climatic boundary between the mid latitude westerlies and the northwestern limit of the asian summer monsoon yet only a few high quality reconstructions exist for its climate history here we calibrate a new organic geochemical proxy for lake temperature and present a 45 yr resolution temperature record from hurleg lake at the eastern margin of the aca in the northern tibetan plateau combination with other proxy data from the same samples reveals a distinct warm dry climate association throughout the record which contrasts with the warm wet association found in the asian monsoon region this indicates that the climatic boundary between the westerly and the monsoon regimes has remained roughly in the same place throughout the holocene at least near our study site six millennial scale cold events are found within the past 9000 yr which approximately coincide with previously documented events of northern high latitude cooling and tropical drought this suggests a connection between the north atlantic and tropical monsoon climate systems via the westerly circulation finally we also observe an increase in regional climate variability after the mid holocene which we relate to changes in vegetation forest cover in the monsoon region through a land surface albedo feedback

role of strike slip faulting in the evolution of allochthonous terranes in the philippines concepts of allochthonous terrane transport and emplacement are dominated by the assumption that most terranes originate on the subducting plate collide with the upper plate and are emplaced there movement of terranes along the convergent margin is recognized but is generally attributed to postcollision slip in the northern philippines allochthonous terranes originate primarily within the arc system have been translated along it by strike slip faults and were emplaced by cessation of that slip the authors suggest that in the philippines some originally vertical strike slip boundaries may have evolved into shallow dipping sutures marked by fold and thrust systems this mode of terrane evolution may be more common than generally appreciated particularly in orogenic belts developed in response to oblique convergence

experimental fracture strain and subsidence patterns over en echelon strike slip faults implications for the structural evolution of pull apart basins abstract claybox experiments show that with progressive left lateral displacement on basal left stepping en echelon strike slip faults an extremely complicated zone of evolving fracture strain and subsidence patterns develops in the overlying clay the fracture pattern is more complex than predicted by theoretical models and consists of a progressively widening array of riedel shears conjugate riedel shears and tension gashes above basal master faults conjugate riedel shears rotate anticlockwise and lengthen forming s shaped fractures riedel shears rotate both anticlockwise and clockwise those that rotate anticlockwise eventually open as tension gashes other tension gashes open where the ends of riedel and conjugate riedel shears link strain contours in the deformed clay show an asymmetrical z shaped pattern over the stepover area between basal en echelon faults in the early stages of displacement strain is distributed penetratively over a progressively broadening zone in later stages of displacement strain is partitioned into a narrow middle zone maximum strain is sustained always in the middle of the sheared zone and increases exponentially with progressive displacement subsidence is accommodated in the stepover area by oblique slip on many riedel and conjugate riedel shears distributed throughout the stepover area subsidence y exhibits a linear relationship with master fault displacement x described by y 0 36 x 1 4 the experimental fracture strain and subsidence patterns in clay over en echelon faults can be used as interpretive and predictive tools with which to constrain interpretations of static field examples especially in pull apart basins on the crustal scale where many similarities with the models exist

structure and geological history of the lepanto cervantes releasing bend on the abra river fault luzon central cordillera philippines abstract the luzon central cordillera corresponds to the uplifted and tectonized magmatic arc associated with the manila trench subduction zone it is cut into n s trending strips by the horsetail like overprint of the philippine strike slip fault the median branch herein named the abra river fault splits away to the west from the dalton fault and runs north northwestward for 240 km through the range trending n150e in its southern part it undergoes a sinistral bend in the lepanto cervantes area and then strikes n s makes a second bend and runs north northeast up to the northwestern tip of the island in the bend area the compartment east of the fault has subsided forming a narrow strip along the southern lepanto segment and a wider trough parallel to the fault which stretches 25 km from cervantes the basement of the strip is position dependent corresponding either to the oligo miocene arc succession volcanoclastic sediments and dioritic intrusive or to the pre late eocene ophiolitic basement of the arc in the lepanto area two volcanic formations are preserved the oldest one the malaya formation is latest miocene in age 6 1 0 3 and 5 7 0 28 ma by k ar and represents the infill of the trough the youngest the mount pudso volcanics is holocene in age 0 5 ma by k ar the cervantes trough contains a thick reaching 1700 m volcanoclastic sequence dated as latest miocene early pliocene 3 7 0 18 6 2 0 28 ma by k ar the malaya formation on the western and southern sides the abra river fault separates the very deformed infill from the strongly uplifted pre late miocene basement on the east side the basement whose elevation increases gradually eastward is disconformably overlain by the mio pliocene sequence to the north the trough is slightly deformed the structural analysis of the lepanto cervantes strip together with the local and regional geological and geochronological data allow us to propose a three stage history the first stage during the early late miocene is the major left lateral wrench displacement of the abra river fault and the appearance of the bend under a transpressive regime the second stage from latest miocene to early pliocene and marked by strong volcanic activity comes with and or immediately follows the subsidence of the lepanto cervantes strip the last stage starting in the late pliocene is characterized by the e w foreshortening of the cervantes trough and the beginning of its erosion

geodynamic evolution of the taiwan luzon mindoro belt since the late eocene abstract the structural framework of the taiwan luzon mindoro belt or festoon is described following three major transects the luzon transect with active subduction and active island arc the taiwan transect with active collision the mindoro transect with active subduction and inactive collision based on this geological study and on available geophysical data a model for the geodynamic evolution of this portion of the philippine sea and eurasia plates boundary is proposed in a succession of reconstructions between the late eocene and the present the major geodynamic events are 1 1 beginning of the opening of the south china sea s c s in lower oligocene times contemporaneous with obduction of the zambales and angat ophiolites on luzon 2 2 subduction of a mesozoic oceanic basin along the proto manila trench from the upper oligocene to the lower miocene 3 3 obduction of the south china sea oceanic crust onto the chinese and reed bank calamian passive margins in middle miocene time 14 15 ma related to a major kinematic reorganization end of opening of the s c s 4 4 beginning of collision between the luzon microblock and the two margins of the s c s in the upper miocene 7 ma collision is still active in taiwan whereas it stopped in mindoro during the pliocene

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia abstract siliciclastic supersequences of the kalgoorlie and gindalbie terranes post date 2660 ma deformation uplift and erosion associated with the amalgamation of arc related terranes but were tectonically buried to greenschist or amphibolite facies during the orogenic shortening that accreted the eastern goldfields superterrane to the yilgarn craton the supersequences are synclinal remnants of late stage basins that were linear zones of subsidence adjacent to north trending strike slip faults fluvial facies merougil navajo jones creek scotty creek and yandal basins are dominated by sheet sandstones that represent the deposits of medial braid plains but proximal channel and bar complexes of cobble and boulder conglomerates are also preserved submarine facies kurrawang basin range from boulder to pebble conglomerates but are dominantly bouma turbidites with rare black shales and record the filling of a proximal canyon that fed a distal submarine fan axial depositional systems are preferentially preserved in all basins but transverse systems are preserved at the base of some fluvial deposits the gross vertical succession in all basins is from proximal up to distal the basins developed during two tectonic stages separated by an intervening period of uplift tilting and erosion with the southern kalgoorlie terrane recording change from a stage i fluvial to a stage ii deep marine basin stage ii fluvial deposits are preserved in the northern kalgoorlie terrane and in the gindalbie terrane sparse palaeocurrents establish sediment dispersal was to the south or southeast implying uplift to the north or northwest controlled palaeoslopes stacking of lithofacies is aggradational and is consistent with limited lateral migration of facies tracts in fault confined basins similar lithofacies and aggradational styles of facies stacking characterise syncollisional sequences of the cenozoic coastal range basin of taiwan and the present day syncollisional basins of southeastern taiwan are good analogues of late stage basins of the kalgoorlie and gindalbie terranes

the sulu sea a back arc basin setting within a neogene collision zone abstract the southeast sulu sea is a small oceanic basin that opened in middle miocene time in a back arc setting along the sundaland margin during subduction of the celebes sea plate beneath the sulu arc this back arc extension post dates compressive tectonism present in this back arc area in early middle miocene time progressive and still incomplete closing of this basin initiated in late miocene time is the result of collision of exotic terranes with the sundaland margin this collision also induced flipping of the sulu subduction zone the neogene geodynamic evolution of the sulu sea provides a modern example of how young oceanic crust can be incorporated within an orogenic belt

geochemistry and tectonic setting of the luzon arc philippines miocene 10 ma to recent volcanism is associated with eastward subduction along the manila trench for some 1 200 km from the coastal range in taiwan south to min doro we suggest calling this the luzon arc there are five distinct segments along this arc mindoro bataan northern luzon babuyan and taiwan the bataan and mindoro segments in the south are separated by a northeast southwest zone of volcanoes the northern luzon segment consists of pliocene to pleistocene necks and plugs that run along the entire western section of northern luzon the babuyan segment 10 ma to recent is made up of a larger number of active volcanoes than are other segments of the luzon arc the taiwan segment is composed entirely of extinct volcanoes in the coastal range and on the islands of lanhsu and lutao late miocene to 1 8 ma the nature of volcanism throughout the luzon are is clearly subduction related with calc alkaline affinities high th la ratios and negative nb ta and ti anomalies the earliest phase of volcanism so far recorded 10 ma however is tholeiitic island of calayan in the babuyan segment with mid oceanic ridge basalt like morb like tight rare earth element and large ion lithophile element concentrations and low th u ratios there are clear differences in sr and nd isotopic ratios among the samples from the luzon are samples from the southern segments mindoro through bataan fall within the mantle array with higher sr and lower nd isotopic values than those of morb pb isotope data indicate sediment involvement the isotopic values have been interpreted to result from source contamination by slab derived fluids bataan segment and by subducted crustal material from collision of the mindoro palawan terrane mindoro segment with the manila trench in contrast babuyan segment samples north have unusual island arc nd and sr isotopic ratios that fall below the mantle array they appear to be the result of sediment source contaminants or a unique mantle composition incorporation of sediments analyzed from eastern china and taiwan into a morb type mantle can explain the nd and sr isotopic ratios of the babuyan segment

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

experimental insights into the partitioning of motion within zones of oblique subduction abstract in zones of oblique convergence the motion is often partitioned into subduction and an associated strike slip component fitch 1972 in this paper we first describe the geodynamic setting of the philippine region as a case study of deformation by strike slip faulting behind parallel subduction zones then we present six small scale laboratory experiments with analogue materials to investigate how partitioning may be controlled by internal boundaries between oceanic lithosphere and continental or island arc lithosphere during the early stages of a typical experiment deformation is taken up on oblique thrusts subduction zones which root into internal boundaries paired strike slip faults develop at later stages thereafter subduction zones and paired strike slip faults operate simultaneously the partitioning occurs without any need for complex constraints on external boundaries

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

multiple ophiolite generation preserved in the northern philippines and the growth of an island arc complex abstract although all oceanic arcs grow through the addition of subduction generated magmas the geology of the northern philippines demonstrates that a major contribution to arc crustal growth can come from repeated episodic intra arc back arc and or fore arc oceanic crust generation with subsequent preservation of the basic ultrabasic units in the arc complex at least five episodes of oceanic crust generation are represented in the northern philippines by preserved ophiolitic sequences and recent intra arc seafloor spreading each episode is distinct in age as confirmed by modern dating techniques with the ages ranging from pre jurassic to quaternary although the philippines is widely regarded as an amalgamation of allochthonous terranes a review of the available data shows that there is currently no compelling evidence that these ophiolites are of exotic origin and that they have been tectonically accreted to the philippine arc complex rather the evidence suggests that most and possibly all of the ophiolites were generated as back arc fore arc or intra arc crust within the philippine arc complex hence there is a close spatial association of several ophiolitic terranes of diverse ages spanning 150 myr that formed as part of the arc complex such an association may have arisen from episodic generation of oceanic crust during periods of local extension in a suprasubduction zone setting which has experienced changing and possibly overlapping subduction from the east and west sides in the current reference frame disruption of the ophiolitic basement terranes has been and continues to be effected primarily by wrench faulting this style of arc growth has implications for the paleotectonic interpretation of ancient ophiolite arc terranes in continents and the petrologic evolution of island arcs

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

unsteady seepage analysis of wallace dam transient seepage patterns in the west dike station 58 00 of wallace dam are studied through a saturated unsaturated seepage analysis using a finite element solution technique numerical results obtained are presented and compared with field data further a simplified formulation of finite element equations for saturated unsaturated analysis is presented which increases computational efficiency considerably

polyphase tectonics at the southern tip of the manila trench mindoro tablas islands philippines abstract the southern termination of the manila trench within the south china sea continental margin in mindoro is marked by a complex polyphase tectonic fabric in the arc trench gap area onshore southern mindoro the active deformation front of the manila trench is marked by parallel folds and thrusts grading southward to n50 w trending left lateral strike slip faults this transpressive tectonic regime active at least since the late pliocene has overprinted the collision of an early miocene volcanic arc with the south china sea continental margin san jose platform the collision is postdated by deposition of the late miocene early pliocene elastics of the east mindoro basin the tectonic and geological framework of this arc which overlies a metamorphic basement and eocene elastics suggests that it was built on a drifted block of the south china sea continental margin

reconstructing the mesozoic early cenozoic evolution of northern philippines clues from palaeomagnetic studies on the ophiolitic basement of the central cordillera summary the first reliable palaeomagnetic data from the cretaceous to eocene ophiolitic basement rocks in the philippines are presented a total of 12 drill core sites from five localities in the central cordillera in northern luzon philippines were sampled eight drill core sites were from pillow basalts and four were from diabase feeder dykes combining the characteristic remanent magnetization direction from these sites gives a mean in situ direction of dec 162 2 i nc 21 4 95 17 0 k 21 1 and a tilt corrected direction of dec 159 3 inc 12 5 95 6 0 k 162 5 along with other lines of palaeomagnetic evidence this clustering suggests that the magnetization is primary and that the ophiolitic basement rocks of the central cordillera formed at subequatorial latitudes 6 3 n 3 1 this information

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

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

the fluvial and pyroclastic deposits of the cagayan basin northern luzon philippines an example of non marine volcaniclastic sedimentation in an interarc basin the cagayan basin of northern luzon an interarc basin 250 km long and 80 km wide contains a 900 m thick sequence of plio pleistocene fluvial and pyroclastic deposits these deposits are divided into two formations the ilagan and awidon mesa and three lithofacies associations the facies which are interpreted as meandering stream braided stream lahar and pyroclastic flow and fall deposits occur in a coarsening upward sequence meandering stream deposits interbedded with tuffs are overlain by braided stream deposits interbedded with coarser pyroclastic deposits lahars and ignimbrites the coarsening upward volcaniclastic deposits reflect the tectonic and volcanic evolution of the adjacent cordillera central volcanic arc uplift of the arc resulted in the progradation of coarser clastics further into the basin the development of an alluvial fan and migration of the basin depocentre away from the arc the coarsening of the pyroclastic deposits reflects the development of a more proximal calc alkaline volcanic belt in the maturing volcanic arc the cagayan basin sediments serve as an example of the type and sequence of non marine volcaniclastic sediments that may form in other interarc basins this is because the tectonic and volcanic processes which controlled sedimentation in the cagayan basin also affect other arc systems and will therefore control or significantly influence volcaniclastic sedimentation in other interarc basins