late cenozoic geodynamic evolution of eastern indonesia this paper presents an internally and globally consistent model of plate evolution in eastern indonesia from middle miocene to present time it is centered on the banda sea region located in the triple junction area between the pacific philippine australia and south east asia plates the geological and geophysical data available from indonesia were until recently insufficient to define a unique plate tectonic model in this paper the new data taken into account clearly restrict the possible interpretations owing to a great number of geological geophysical and geochemical studies the major plate boundaries the sunda banda subduction zone to the south the tarera aiduna fault zone and the seram thrust to the east and the sorong fault zone and molucca sea collision zone to the north are now clearly identified the age of the major tectonic structures is also better known geodetic measurements well constrain the present time plate kinematics we also consider the deformation history within eastern indonesia where numerous short lived microplates and their related microcontinents successively accreted to the asiatic margin moreover magnetic anomalies identification of the north and south banda sea basins allows a precise kinematic reconstruction of the back arc opening we used the plates software to test the coherency of our model presented as a series of 4 plate reconstruction maps from 13 ma to the present finally the origin of oceanic domains restored by our reconstruction is discussed

tectonometamorphic evolution of seram and ambon eastern indonesia insights from 40ar 39ar geochronology the island of seram eastern indonesia experienced a complex neogene history of multiple metamorphic and deformational events driven by australia se asia collision geological mapping and structural and petrographic analysis has identified two main phases in the island s tectonic metamorphic and magmatic evolution 1 an initial episode of extreme extension that exhumed hot lherzolites from the subcontinental lithospheric mantle and drove ultrahigh temperature metamorphism and melting of adjacent continental crust and 2 subsequent episodes of extensional detachment faulting and strike slip faulting that further exhumed granulites and mantle rocks across seram and ambon here we present the results of sixteen 40ar 39ar furnace step heating experiments on white mica biotite and phlogopite for a suite of twelve rocks that were targeted to further unravel seram s tectonic and metamorphic history despite a wide lithological and structural diversity among the samples there is a remarkable degree of correlation between the 40ar 39ar ages recorded by different rock types situated in different structural settings recording thermal events at 16 ma 5 7 ma 4 5 ma and 3 4 ma these frequently measured ages are defined in most instances by two or more 40ar 39ar ages that are identical within error at 16 ma a major kyanite grade metamorphic event affected the tehoru formation across western and central seram coincident with ultrahigh temperature metamorphism and melting of granulite facies rocks comprising the kobipoto complex and the intrusion of lamprophyres later at 5 7 ma kobipoto complex rocks were exhumed beneath extensional detachment faults on the kaibobo peninsula of western seram heating and shearing adjacent tehoru formation schists to form taunusa complex gneisses then at 4 5 ma 40ar 39ar ages record deformation within the kawa shear zone central seram and overprinting of detachment faults in western seram finally at 3 4 ma kobipoto complex migmatites were exhumed on ambon at the same time as deformation within the kawa shear zone and further overprinting of detachments in western seram these ages support there having been multiple synchronised episodes of high temperature extension and strike slip faulting interpreted to be the result of western seram having been ripped off from se sulawesi extended and dragged east by subduction rollback of the banda slab

evidence for a submerged and displaced continental borderland north banda sea indonesia the banda sea is separated into two oceanic fragments the south and north banda basins by a large intervening region of ridges and basins called the banda ridges the north and south banda basins appear to be trapped pieces of pre tertiary oceanic crust whereas the banda ridges are a submerged and displaced continental borderland structurally very similar to the southern california offshore borderland analysis of dredged rocks from the banda ridges suggests an origin from western irian jaya

fine details of the wadati benioff zone under indonesia and its geodynamic implications using about 5200 handpicked p s pp sp pcp and scp phases from digitally recorded seismograms together with international seismological centre reported phases we obtain improved hypocentral locations for 1790 earthquakes 983 of them having 90 confidence limits 50 km in the period 1962 to september 1996 along the indonesian subduction zone we use an improved joint hypocenter determination method in which the solution is more stable than in the original one we show that for deep earthquakes use of the core reflected phases is as good as use of depth phases and since such core phases often have larger amplitudes and sharper onsets than the depth phases their arrival times can be read very accurately the positions of the relocated hypocenters show that 1 a portion of the indonesian arc between 110 e and 123 e longitude and deeper than 500 km is dipping southward at an angle of 75 that is in a direction opposite to the upper part of the north dipping slab and 2 east of about 108 e the seismic zone is wider near 670 km than near the 500 km depth the first suggests southward lateral flow in the mantle relative to the plate motion vector here from the contortion of the seismic zone along the eastern portion of the indonesian arc we can estimate the average lateral shear strain rate in the 300 to 670 km depth range to be of the order of 10 16s 1 over the last 10 20 myr

magnetic lineations constraints for the back arc opening of the late neogene south banda basin eastern indonesia abstract the south banda basin is located within eastern indonesia near the triple junction between the eurasian pacific and indo australian plates it is underlain by oceanic crust but its origin and age were not well established it has been interpreted as a mesozoic trapped piece of indian ocean or as a cretaceous eocene basin related to the celebes and sulu basins but a neogene back arc origin was also considered recent geochemical and geochronological studies strongly support the latter hypothesis in this paper we present a new analysis of the magnetic data from the eastern part of the south banda basin the damar basin we used magnetic field measurements collected during nine oceanographic cruises from various institutions looking for magnetic correlation in the time span given from recent geochronological data the comparison between measured profiles and theoretical profiles deduced from the reversals of earth s magnetic field during neogene time allows us to infer an opening of south banda basin during late miocene early pliocene time from 6 5 to 3 5 ma magnetic lineations 2an 2ar 3n 3r 3an and possibly 3ar are recognized with an extinct spreading centre trending ene wsw at least five segments are identified each segment being separated by inactive transform faults perpendicular to the extinct spreading centre a half spreading rate of about 3 cm yr is calculated based on spacing of magnetic lineations the opening history of the basin is discussed the cause of cessation of spreading is likely the arc continent collision dated at about 3 ma however the onset of opening is less determined we suggest that damar basin began to open at about 6 5 ma during magnetic period 3an as an intra arc basin separating the banda volcanic arc to the south from the incipient lucipara volcanic arc to the north the latter was probably created at the beginning of rifting of damar basin as shown by both magnetic and geochronological data the young age of the south banda sea basin is contradictory with its great depth we discuss this problem in view of thermal and tectonic considerations we finally conclude that our magnetic model allows us to more precisely describe the opening of the basin

geodynamics complex subduction eastern indonesia hosts one of the most complex and fascinating tectonic systems on the planet palaeogeographical reconstructions indicate that subduction and deformation of a single slab of oceanic crust created the complicated configuration

gps measurements of crustal deformation within the pacific australia plate boundary zone in irian jaya indonesia abstract global positioning system gps measurements made in 1991 1992 and 1993 provide preliminary estimates of slip distribution between the australian and pacific plates in irian jaya indonesia we interpret the gps results with constraints from earthquake mechanisms and slip vectors recent marine surveys and geology three gps sites in southeastern irian jaya show motions that are within 10 mm yr of the expected motion of australia a coast to coast n s baseline along 140 5 e crosses all known onland regions of active deformation but reveals no more than 15 mm yr of shortening and 20 mm yr of left lateral shear in the 27 month period the remaining 40 mm yr of expected convergence between the pacific and australian plates probably occurs at the new guinea trough gps sites on the island of biak at 136 e and at sorong near the western tip of bird s head at 131 e both move 90 100 mm yr in a wsw direction relative to irian jaya but less than 15 mm yr relative to each other these sites are on either side of the sorong fault and demonstrate that it is not presently the major boundary between the australian and pacific plates instead the plate boundary is now south of the sorong and biak sites earthquakes suggest possible structures that accomodate motion between bird s head and australia but the relative importance of them remains uncertain

sunda banda arc transition incipient continent island arc collision northwest australia the eastern sunda arc represents one of the few regions globally where the early stages of continent arc collision can be studied we studied along the western limit of the collision zone at the sunda banda arc transition where the australian margin collides with the banda island arc causing widespread back arc thrusting we present integrated results of a refraction wide angle reflection tomography gravity modeling and multichannel reflection seismic imaging using data acquired in 2006 southeast of sumba island the composite structural model reveals the previously unresolved deep geometry of the collision zone changes in crustal structure encompass the 10 12 km thick australian basement in the south and the 22 24 kmthick sumba ridge in the north where backthrusting of the 130 km wide accretionary prism is documented the structural diversity along this transect could be characteristic of young collisional systems at the transition from oceanic subduction to continent arc collision citation shulgin a h kopp c mueller e lueschen l planert m engels e r flueh a krabbenhoeft and y djajadihardja 2009 sunda banda arc transition incipient continent island arc collision northwest australia geophys res lett 36 l10304 doi 10 1029 2009gl037533

determining the sula block kinematics in the triple junction area in indonesia by gps summary the point of convergence of the eurasian philippine and australian plates is situated adjacent to the island of sulawesi indonesia the relative plate velocities are estimated by nuvel1 to be 7 to 9 cm yr 1 the complex tectonic mechanism of the triple junction has been studied over a two year period in the course of the geodyssea southeast asian project the gps investigations concentrate on measurements of both the sulawesi eastern indonesia part of the inter regional geodyssea network and a local subnetwork on sulawesi motions derived using data from the subnetwork con rm what the results of the inter regional geodyssea network have suggested that is that current deformation is high and there are distinct deformation domains in the study area on sulawesi the tectonic mechanism of the triple junction has been analysed using a rigid microblock model the triple junction area can best be interpreted as a headland of the australian plate de ected by its collision with the philippine plate thereby identifying the driving forces of the current deformation the northern part is dominated by the sula domain which shows clockwise rotation to the south it is connected to the australian plate by an ensemble of microblocks undergoing counterclockwise rotation in addition to the above our tectonic model permits the determination of the local in uence of two large earthquakes m 7 8 1996 january 1 and m 7 0 1996 july 22 on the motion of the station tomini north sulawesi more observations and a denser gps network are planned in order to study the behaviour of the palu koro fault the main fault on the western limit of the sula block

a note on the depth recurrence and strain release of large vrancea earthquakes the large 1940 vrancea event with body wave magnitude mb 7 3 was investigated in terms of the depth of rupture initiation and extent it was found that the rupture started at 150 km depth and propagated downwards the ruptured zones of the 1977 1986 and 1990 vrancea events were estimated from the distribution of aftershocks located with the joint hypocentre determination method their depth ranges are 90 110 km 130 150 km and 70 90 km respectively because the body wave magnitudes mb were saturated for the last three events and the surface wave magnitudes ms were underestimated not being corrected for depth we chose the moment magnitudes to quantify the four events 7 7 1940 7 4 1977 7 1 1986 and 6 9 1990 respectively the depth interval 110 130 km is found to be unbroken since at least 1838 or 1802 and therefore is expected to accommodate the next large moment release mw 7 0 to 7 4 the strain in the vertical and in the se nw directions is about 0 0003 giving a moderate strain rate of about 10 13 s 1 a correlation between the depth interval of the large events and the year of occurrence in a century is suggested offering a physical basis to the observed quasi cyclicity

new paleomagnetic investigations of the emeishan basalts in ne yunnan southwestern china constraints on eruption history abstract paleomagnetic and magnetostratigraphic investigations have been conducted on two consecutive sections of the emeishan trap at shuimo sm and dadi dd in northeastern yunnan province southwestern china rock magnetic analysis indicates that the magnetic carrier of the basalts is dominantly low ti magnetite stepwise thermal demagnetization isolates a stable characteristic remanent magnetization chrm at high temperatures 400 585 c the chrms 301 323 at sm section are northeasterly upward of normal polarity with a mean direction of ds is 11 0 5 2 n 12 k 53 7 95 6 0 at dd section the chrms are southwesterly downward of reversed polarity with a mean direction of 205 9 10 0 n 5 k 78 4 95 8 7 the chrms from the basaltic gravels from the overlying conglomerate layer in late permian xuanwei formation at dd section pass the conglomerate test suggesting the primary nature of magnetization together with other magnetostratigraphic studies e g n r pattern of the emeishan basalts and the known radiometric ages ca 260 ma it suggests that the eruption period of the emeishan large igneous province is less than 3 myr

p and s wave travel time tomography of the se asia australia collision zone abstract the southeast se asia australia collision zone is one of the most tectonically active and seismogenic regions in the world here we present new 3 d p and s wave velocity models of the crust and upper mantle by applying regional earthquake travel time tomography to global catalogue data we first re locate earthquakes provided by the standard isc reviewed and isc ehb catalogues using a non linear oct tree scheme a machine learning algorithm that clusters earthquakes depending on their spatiotemporal density was then applied to significantly improve the consistency of travel time picks we used the fast marching tomography software package to retrieve 3 d velocity and interface structures from starting 1 d velocity and moho models synthetic resolution and sensitivity tests demonstrate that the final models are robust with p wave speed variations 130 km horizontal resolution generally recovered more robustly than s wave speed variations 220 km horizontal resolution the retrieved crust and mantle anomalies offer a new perspective on the broad scale tectonic setting and underlying mantle architecture of se asia while we observe clear evidence of subducted slabs as high velocity anomalies penetrating into the mantle along the sunda arc banda arc and halmahera arc we also see evidence for slab gaps or holes in the vicinity of east java in the banda arc we image the slab as a single curved subduction zone furthermore a high velocity region in the mantle lithosphere connects northern australia with timor and west papua the s wave model shows broad scale features similar to those of the p wave model with mantle earthquakes generally distributed within high velocity slabs the high velocity mantle connection between northern australia and the eastern margin of the sunda arc is also present in the s wave model while the s wave model has a lower resolution than the p wave model due to the availability of fewer paths it nonetheless provides new and complementary insights into the structure of the upper mantle beneath southeast asia

erratum a replacement name for lanthanocephalus modesto rubidge and welman 2002 not lanthanocephalus williams and starmer 2000 ref canadian journal of earth sciences 39 12 1755 1765 we recently proposed the generic name lanthanocephalus for a new dicynodont therapsid from the

seismic response to recent tectonic processes in the banda arc region abstract internal structure and recent tectonics of the geologically complex banda arc region latitude 1 9 s longitude 124 135 e have been studied by means of the analysis of global seismological data relocated hypocentral determinations of the international seismological center ehb data and fault plane solutions of the global centroid moment tensor project have been used together with previously published information on regional geology and dynamics the analysis considered earthquakes in the lithospheric wedges above subducting slabs and earthquakes in the uppermost portions of the wadati benioff zones focal depth

seismic moment release during slab rupture beneath the banda sea summary the highest intermediate depth moment release rates in indonesia occur in the slab beneath the largely submerged segment of the banda arc in the banda sea to the east of roma termed the damar zone the most active western part of this zone is characterized by downdip extension with moment release rates 10 18 nm yr 1 per 50 km strike length implying the slab is stretching at 10 14 s 1 consistent with near complete slab decoupling across the 100 200 km depth range differential vertical stretching along the length of the damar zone is consistent with a slab rupture front at 100 200 km depth beneath roma propagating eastwards at 100 km myr 1 complexities in the slab deformation field are revealed by a narrow zone of anomalous in plane p axis trends beneath damar where subhorizontal constriction suggests extreme stress concentrations 100 km ahead of the slab rupture front such stress concentrations may explain the anomalously deep ocean gateways in this region in which case ongoing slab rupture may have played a key role in modulating the indonesian throughflow in the banda sea over the last few million years

a seismological study of the eastern new guinea and the western solomon sea regions and its tectonic implications summary we use new and pre existing seismological data to study the neotectonics and the state of stress in the seismic zones of eastern papua new guinea and the western solomon sea regions earthquake hypocentres for the period 1964 90 were relocated using the joint hypocentre determination jhd technique we show that the t axes of earthquakes in the 100 250km depth range beneath eastern papua new guinea are aligned parallel to the seismic zone with no seismicity below 250 km this intermediate depth seismic zone forms an inverted u shaped zone with limbs dipping to the north and south and is interpreted to be seismicity along the subducted solomon sea plate the northern and southern limbs of the seismic zone are connected by a flattened zone of seismicity which is marked by predominantly strike slip and a few normal fault mechanisms we suggest that the solomon sea plate at depth beneath the finisterre mountains is no longer influenced by the tectonic forces acting at the surface but breaking up and sinking under its own gravitational forces a significant north dipping seismic zone is imaged above the deeper seismic zone this feature characterized by thrust mechanisms extrapolates to the surface along the ramu markham valley and is attributed to the ramu markham fault rmf the rmf has an average dip of about 40 to the north east but its detailed geometry would appear to be complex the rmf is interpreted to ramp steeply in the uppermost 10 20 km flattening out sharply to a dip of 15 at a depth of about 20km the rmf marks the suture between the finisterre terrane and the australia new guinea plate and may extend to a depth of 90km beneath the western limit of the finisterre mountains there is a clear gap in seismicity between the rmf and the deeper solomon sea plate seismic zone beneath the finisterre mountains which together with the alignment of the t axes of earthquakes along the seismic zone at depths 100km indicates that these are indeed two distinct features the seismicity in the depth range 50 80 km along the new britain arc follows the trend of the 2000m bathymetric contour of the new britain trench and the finsch deep far more closely than it does the trend of the surface plate boundary indicating that the increase in the distance between the solomon sea south bismarck plate boundary and the active volcanos along the new britain arc west of 6 83 150 o e is only a shallow phenomenon and does not reflect the geometry of the solomon sea plate at depths greater than 50 km

stratigraphic constraints on suture models for eastern indonesia abstract although collision in eastern indonesia is now accreting the australian continent to southeast asia the small north and south banda oceanic basins within the suture zone are interpreted as late cenozoic extensional features stratigraphic columns from the surrounding islands conform to one of three generalised patterns two of which can be related to the margins of se asia sundaland and the australian continent respectively the third system which is dominant in the outer banda arc and eastern sulawesi is associated with a microcontinent that was rifted from australia in the jurassic drifted northwards ahead of australia in the cretaceous and collided with the sundaland margin in the paleogene subsequent collapse of the resulting collision orogen led to rapid extension and the formation of the banda sea behind the outer banda arc thrust belt eastern indonesia thus duplicates a pattern familiar in the mediterranean the tertiary compressional structures of the region cannot be explained solely in terms of the most recent collision which began only in the pliocene

tertiary tectonic and magmatic evolution of western sulawesi and the makassar strait indonesia evidence for a miocene continent continent collision

global trench migration velocities and slab migration induced upper mantle volume fluxes constraints to find an earth reference frame based on minimizing viscous dissipation sincetheadventofplatetectonicsdifferentglobalreferenceframeshavebeenusedtodescribethemotionofplatesandtrenches thedifferencein plate motion and trench migration between different reference frames can be substantial up to 4 cm yr this study presents an overview of trench migration velocities for all the mature and incipient subduction zones on earth as calculated in eight different global reference frames calculations show that irrespective of the reference frame 1 trench retreat always dominates over trench advance with 62 78 of the 244 trench segments retreating 2 the mean and median trench velocity are always positive retreating and within the range 1 3 1 5 cm yr and 0 9 1 3 cm yr respectively 3 rapid trench retreat is only observed close to lateral slab edges b1500 km and 4 trench retreat is always slow far from slab edges n2000km thesecalculationsarepredictedbygeodynamicmodelswithavaryingslabwidth inwhichplatemotion trenchmotionandmantleflow result from subduction ofdense slabs suggesting that trenchmotion is indeedprimarily drivenby slab buoyancyforces andthat proximity to alateral slab edge exerts a dominant control on the trench migration velocity despite these four general conclusions significant differences in velocities between such reference frames remain it is therefore important to determine which reference frame most likely describes the true absolute velocities togetanunderstandingoftheforcesdrivingplatetectonicsandmantleconvection itishereproposedthat basedonfluiddynamicconsiderationsand predictions from geodynamic modelling the best candidate is the one which optimises the number of trench segments that retreat minimizes the trench perpendicular trench migration velocity vt in the centre of wide n4000 km subduction zones maximizes the number of retreating trench segments located within 2000 km of the closest lateral slab edge minimizes the average of the absolute of the trench perpendicular trench migration velocity vt forallsubductionzonesonearth andminimizestheglobaluppermantletoroidalvolumeflux to thatresultsfromtrenchmigration and associated lateral slab migration i e slab rollback or slab roll forward calculations show that these conditions are best met in one particular indo atlantic hotspot reference frame where 75 of the subduction zones retreat vt in the centre of wide subduction zones ranges between 3 5 and 1 8 cm yr 83 of the trench segments located within 2000 km of the closest lateral slab edge retreat the average of vt is 2 1 cm yr and to 456 km 3 yr lower limit and 539 km 3 yr upper limit inclusion of all the incipient subduction zones on earth results in slightly greater fluxes of 465 km 3 yr lower limit and 569 km 3 yr upper limit it is also found that this reference frame is close to minimizing the total sub lithospheric uppermantlevolumeflux k associatedwithmotionofcontinentalkeelslocatedbelowthemajorcratons itisstressed however that kisanorder of magnitude smaller than to and thus of subordinate importance in conclusion the indo atlantic hotspot reference frame appears preferable for calculating plate velocities and plate boundary velocities

subduction in eastern indonesia how many slabs abstract seismicity associated with arc continent collision in eastern indonesia testifies to past north directed subduction of indian ocean lithosphere beneath the banda sea the complex patterns of deep seismicity have been cited as evidence for simultaneous south directed subduction at the northern margin of the sea but this interpretation has not been universally accepted recently available recomputations of hypocentre locations have provided increased resolution of eastern indonesian wadati benioff zones wbzs shallow to intermediate depth seismic activity around the banda arc appears to support models involving subduction of two separate and distinct lithospheric slabs but between 150 and 500 km the wbz has a continuous shoehorn shape this shape confirms the presence of subducted lithosphere beneath seram in the north as well as beneath timor in the south is incompatible with independent subduction of two unconnected plates and implies rapid eastwards retreat of the subduction trace across a now vanished northern spur of the indian ocean this roll back is unlikely to have been driven by local gravitational forces alone and may have been sustained by injection behind the banda slab of asthenospheric material escaping from the molucca sea arc arc collision

orogeny in arc continent collision the banda arc and western new guinea eastern indonesia contains a modern continent island arc collision that demonstrates how a complex juxtaposition of continental and oceanic elements can occur in an orogenic belt shallow earthquakes show that strike slip faulting transports large crustal blocks into out of and along the collision zone while crustal shortening and thickening occur by steep angle thrust faulting near the strike slip faulting whereas strike slip faulting is paramount in positioning australian continental rocks so as to enclose the oceanic and island arc rocks of eastern indonesia its role may be overshadowed by the contemporaneous thrusting and may confuse geologic interpretations of the resulting orogenic belt

magmatic evolution of sulawesi indonesia constraints on the cenozoic geodynamic history of the sundaland active margin abstract tertiary and quaternary magmatic rocks from west sulawesi record the complex history of part of the sundaland margin where subduction and collision have been and are still active the present study based on petrographic data major and trace element chemistry and 40 k 40 ar dating aims to document the age and chemical characteristics of the magmatic formations from west sulawesi and to determine the corresponding constraints on the geodynamic evolution of the sundaland border the west sulawesi magmatic province includes the south arm of sulawesi ujung pandang area the western part of central sulawesi with the toraja and palu areas and finally the north arm extending from palu to manado which includes the tolitoli and manado areas paleocene magmatic activity seems to be restricted to an episode of calc alkaline magmatism in the ujung pandang area 61 59 ma the major eocene 50 40 ma magmatic event is tholeiitic and is documented in all areas except in ujung pandang it led to the emplacement of tholeiitic pillow lavas and basaltic dykes of back arc basin bab affinity these rocks are potential equivalents to the celebes sea basaltic basement from oligocene to miocene magmatic eruptions produced successively island arc tholeiitic iat and calc alkaline ca rock series the youngest iat activity occurred around 18 ma in the central part palu area and around 14 ma in the north arm tolitoli area while ca magmas were emplaced in the north arm at ca 18 ma tolitoli and manado areas typical calc alkaline activity resumed only in the north arm tolitoli and manado areas during the late miocene 9 ma and is still active in the manado region in other areas palu toraja and ujung pandang areas an important and widespread magmatic event occurred between 13 and 10 ma and emplaced k rich magmas either silica undersaturated alkali potassic basalts ak ultrapotassic basanites uk or shoshonites sh k rich activity continued in the south until the pleistocene 0 77 ma with alkali potassic ultrapotassic and shoshonitic magmas in central sulawesi toraja and palu areas the most recent magmatic event occurred between 6 5 and 0 6 ma the corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits all these rocks are acidic in character sio 2 60 with trace element and isotopic signatures sr nd pb typical of a strong continental imprint the most striking tectonic implication of this magmatic evolution is that west sulawesi can no longer be considered as a typical magmatic arc as previously assumed with the exception of the manado area beneath which subduction is still active calc alkaline and island arc tholeiitic lavas and plutonics are volumetrically minor with respect to k rich magmas their occurrence through time is also fairly restricted mostly to the period between 30 and 15 ma another important feature is the occurrence of island arc tholeiitic and calc alkaline magmas crosscutting an older terrane of bab affinity the tinombo formation manado tolitoli and palu areas as this formation is being regarded as an equivalent to the celebes sea floor the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the nw dipping subduction beneath the north arm the late miocene high k magmatic activity in central and south sulawesi reflects the prevalence of a post collisional tectonic regime following the docking of microcontinents of australian origin to central sulawesi during neogene times the incompatible element enriched character of these high k rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event such a model cannot account for the plio pleistocene cak magmatism of central sulawesi the acidic composition of which does not support a derivation from an ultrabasic source the trace element patterns of the cak rocks are very similar to those of the high grade metamorphics of central sulawesi suggesting that the latter might represent their possible source such an anatectic model implies a collisional to post collisional tectonic regime limited to central sulawesi while a post subduction regime prevailed in the south

structural styles and tectonic evolution of the seram trough indonesia abstract the seram trough is located in the northern part of the banda arc australian collision zone in eastern indonesia and is currently the site of contraction between the bird s head of new guinea and seram island it has been interpreted as a subduction trench an intra continental thrust zone and foredeep and a zone of strike slip faulting recently acquired 2d seismic lines clarify its tectonic evolution and relationship to the bird s head folding in the early pliocene formed an anticlinorium running from misool to the onin peninsula of irian jaya and produced a newly recognised angular unconformity the unconformity truncates sediments as old as middle jurassic and is an ancient topographic surface with significant relief it was later folded and now dips south towards the trough where it is covered by up to 3 km of sediments initial tilting of the unconformity surface was accompanied by deposition of a transgressive sequence which can be traced into the trough this is overlain by two sequences which prograde towards the trough these sequences show progressive rotation of the unconformity surface gravitational displacement of sediments into the trough and thrusting which continues to the present day contraction occurred in the trough after the early pliocene and is younger than the previously suggested late miocene age thrust faults in the trough deform sediments deposited above the unconformity and detach at the unconformity surface on seram thrust faults repeat mesozoic miocene sequences and probably detach at their contact with metamorphic basement the detachment surface must cut through the mesozoic miocene sequence between seram and the trough this work suggests the seram trough is not a subduction trench but a foredeep produced in response to loading by the developing fold and thrust belt of seram with an associated peripheral bulge to the north the seram trough is interpreted to be a very young zone of thrusting within the australian continental margin

mantle structure beneath indonesia inferred from high resolution tomographic imaging we investigated mantle structure beneath the indonesian region by means of tomographic inversions of traveltime residuals of direct p and the surface reflected depth phases pp and pwp the hypocentres and phase data used in the inversions were derived from the reprocessing of data reported to international data centres which include data from the australian skippy project we used more than 6 0 10 data from more than 50 000 globally distributed earthquakes recorded at a subset of nearly 3000 seismological stations worldwide the use of depth phases improved the sampling of the upper mantle structure in particular beneath the back arc regions the mapping of distant aspherical mantle structure into the study region was minimized by combining a high resolution regional with a low resolution global inversion the inversion was linearized around the ak135 reference velocity model and a combination of minimum norm and gradient damping was applied to constrain the solution inversion of different subsets of the data and test inversions of synthetic data indicate that resolution is sufficient to substantiate the conclusions pertinent to the complex large scale structure of the subducted slab beneath the indonesian archipelago the tomographic images confirm previous evidence for regional slab penetration into the upper part of the lower mantle but suggest a more complex trajectory of mantle flow beneath indonesia the implied lateral variation in the shape of the upper mantle slab is generally in good agreement with inferences from seismicity the data reveal a seismic anomaly beneath the sunda arc which is detected to at least 1500 km in depth and forms the eastern end of a deep anomaly associated with the past subduction of lithosphere of the mesozoic tethys ocean below java the lithospheric slab is probably continuous from the earth s surface to the lower mantle but beneath sumatra the deep slab seems to be detached from the seismogenic slab the high resolution images suggest that the subducted slab is deflected in the transition zone beneath the banda arc and that the descending slabs form a spoon shaped structure north of the banda sea we detect the two opposing subducted slabs of the molucca sea plate this is consistent with previous studies but our images suggest that the westward subduction has a significantly steeper dip than the eastward subduction slab structure beneath the banda and molucca seas is explained in the context of the continent arc collision between australia and the southeast indonesian island arc and the arc arc collision between sangihe and halmahera inversion of traveltime residuals of direct s phases a novelty for this region largely confirms the inferences from the p wave data even though less detail is resolved