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

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

substorms fundamental observational features distinction from other disturbances and external triggering fundamental observational signatures of substorms and other auroral zone disturbances are summarized in this review it is shown that at least three distinct types of major geomagnetic disturbance have been identified in the auroral zones and it is suggested that care should be taken in distinguishing substorms from the other types of disturbance when reaching observational conclusions concerning substorms substorms can be uniquely identified by the formation of the current wedge and by the associated westward electrojet and region of active aurora that move poleward within the ionosphere during the expansion phase periods of enhanced convection have been observed to continue without substorms for as long as the interplanetary magnetic field imf remains stably southward they can have al signatures that look very much like intense substorms but these signatures result from strong dp 2 ionospheric currents driven by enhanced convection rather than from a nightside westward electrojet a third type of disturbance is characterized by short intervals of enhanced equatorward flow within the ionosphere and enhanced auroral emissions these enhancements move equatorward from near the polar cap boundary and they may be associated with several minute long enhancements of earthward flow within the plasma sheet studies that have examined the triggering of substorm expansions by imf changes are also reviewed when considered together and substorms are distinguished from the other disturbances these studies imply that most and perhaps all expansions are triggered by imf changes these changes include both northward turnings and reductions in the magnitude of the y component if most substorms are indeed externally triggered as suggested here then substorms must generally not be the result of an internal magnetospheric instability

dynamics of the substorm expansive phase meridian scanning photometer msp and magnetometer data from the canadian auroral network for the open program unified study canopus ground based array have been used to study the dynamics of the substorm expansive phase the relative latitudinal motions of the msp 630 0 557 7 and 486 1 nm emissions for eight isolated events have been studied the data show the expansive phase to comprise three stages an explosive tens of seconds onset rapid poleward motion of 557 7 nm emissions of the order of a few minutes and a period of slower tens of minutes poleward moving 630 0 nm emissions we interpret the rapid poleward motion of the 557 7 nm data in terms of a region of instability expanding rapidly down the magnetotail possibly accelerating plasma sheet electrons as it proceeds all events show that lobe flux reconnection occurs after near earth onset with typical time delays between 1 and 5 min the extremely short time interval between near earth onset and the beginning of lobe flux reconnection presents a severe observational constraint on any substorm model which attempts to explain the substorm expansive phase magnetometer data from the canopus churchill line have been used to study the dynamics of the substorm electrojets during the growth and early expansive phases of the substorm the borders of these electrojets closely follow the motion of the msp data during the growth and expansive phases at the instant of expansive phase onset the growth phase electrojets as observed along the churchill meridian line disappear being replaced by the substorm westward electrojet associated with the current wedge the equatorward border of the substorm westward electrojet is embedded within the proton aurora emissions and appears at expansive phase onset 2 equatorward of the growth phase electrojets further showing the near earth proximity of expansive phase onset no indication of reconnection on closed field lines prior to expansive phase onset was observed in either the magnetometer or the photometer data

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

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

on the use of photometer data to map dynamics of the magnetotail current sheet during substorm growth phase the canadian auroral network for the open program unified study canopus is a large scale ground based instrument array of remote sensing equipment monitoring the high latitude ionosphere from the north central to the northwest portion of north america it provides realtime coverage of auroral events such as magnetospheric substorms in this paper the variation of magnetotail thickness is estimated during substorm growth phase by using a simple magnetic field model and data from the meridian scanning photometers in the canopus array an important parameter in the modeling process is the parameter defined as the square root of the minimum curvature radius to maximum larmor radius ratio through use of this parameter we are able to grossly determine the expected proton precipitation regions in the ionosphere two examples presented february 9 1995 and march 9 1995 show that crosstail current sheet thinning from 2 0 1 r e can occur during the course of a growth phase within the framework of our model it is demonstrated that the magnetotail field line stretching associated with thinning of the current sheet is correlated with the equatorward motion of hp auroral emissions also demonstrated is the earthward movement of the inner edge of the plasma sheet to within 6 r e

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

multipoint study of a substorm on february 9 1995 an extended interval of strong northward interplanetary magnetic field imf was observed by the wind spacecraft located at an upstream distance of 193 re from february 8 10 1995 with a brief break of southward imf from 0200 to 0400 ut on february 9 this brief interval of southward imf led to an isolated substorm of moderate intensity 500 nt with expansion phase starting at 0431 ut this substorm may be triggered by the northward turning of the imf since its onset time matched well with the time expected for the arrival of the northward turning of the imf at earth the substorm activities were monitored by 11 spacecraft in space wind imp 8 geotail six geosynchronous satellites one dmsp satellite and freja and two networks of ground stations canopus and superdarn covering both the northern and southern hemispheres the extensive coverage of this event provides us with results 1 showing some unusual characteristics possibly related to the isolated nature of the substorm and 2 revealing some surprising features difficult to reconcile with the traditional substorm model in the first category is unusually long duration of the growth phase and the long time delay between substorm expansion onset and particle injection onset at the geosynchronous orbit in the second category is new evidence for multiple particle acceleration sites during substorm expansion and for sunward flow during the late expansion phase of a substorm being unrelated to a single acceleration site x line moving from the near earth tail to the more distant tail we also present observations which show the possible optical signature on the ground of bursty bulk flows in the magnetotail

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

dynamic deformation of etna volcano observed by satellite radar interferometry satellite radar interferometry of mt etna volcano sicily italy reveals a sequence of deformation characterized by deflation during the end of the 1993 eruption inflation from 1993 1995 with an increase in the inflation rate immediately before its resumed eruptive activity in late 1995 this was followed by very low deformation levels during the following year the source of the deformation changed from a depth of 9 km during deflation to more than 11 14 km during the subsequent inflation consistent with a model in which deflation at shallower levels is followed by inflation at greater depth as the volcano system recharges from below before its next eruption this study demonstrates that radar interferometry provides an important contribution towards understanding the dynamic deformation of volcanoes by revealing large scale changes in their pre eruption deformation rates radar interferometry could play an important role in volcano eruption monitoring

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

triggering of expansive phase intensifications of magnetospheric substorms by northward turnings of the interplanetary magnetic field over the past decade evidence has arisen suggesting that northward turnings of the interplanetary magnetic field after an interval of sustained southward imf can trigger the onset of major substorm expansive phase effects however there has been some question in the past as to just how many substorms are triggered and whether indeed the correlation of substorm onsets and northward turnings was just a matter of chance one reason for doubt was that the examples presented in the literature appeared during disturbed periods where the ims was fluctuating significantly on a time scale of less than one hour and many substorm expansive phase intensifications were observable in the time frame where the correlation between the imf and ground data was reported the purpose of this paper is to present three clear cut examples of isolated substorms where the reader may be able to convince himself of the validity of the reported triggering effect for these events it will be clear that the substorm current wedge triggered by the northward turning of the imf carries substantially larger currents than the driven system associated with the overall substorm event the role of the imf in externally triggering episodes of joule heating and particle precipitation within the substorm process will be discussed

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

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

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

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

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

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

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

north south structures in the midnight sector auroras as viewed by the viking imager during auroral substorms active auroras involving field aligned electric fields appear at the poleward border of the auroral oval equatorward of these active discrete auroras a diffuse region of luminosity develops which often features imbedded localized structures which can have distinctively north south orientations in this paper we study the behaviour of such longitudinally confined northsouth oriented auroral arc structures which may turn on and off sporadically during the substorm lifetime in particular we show observations which suggest that these structures may be the projection on the ionosphere of the drift paths of energetic electrons as they drift from the magnetotail into the more dipolar inner magnetosphere our results suggest that injection of auroral particles into the central plasma sheet occurs in quite localized regions over lengthy periods of time the injected particles drift earthward in the central plasma sheet where those electrons with pitch angles close to the loss cone are sporadically caused to precipitate into the ionosphere causing the fingers of luminosity north south aligned structures near the equatorward edge of the oval are believed to represent precipitation of a spatially localized population of energetic electrons for which the westward convective drift velocity in the evening sector nearly balances the eastward gradient and curvature drift velocity thus leading to purely earthward convective drift of the energetic electrons