whole layer convection in a heterogeneous planetary mantle 1 geochemical observations demonstrate that earth s mantle is heterogeneous but the sizes forms and characteristics of its reservoirs are not constrained using laboratory experiments we have systematically studied a simple nonhomogeneous system where two layers of miscible fluids with different densities depths and viscosities are subjected to a destabilizing temperature contrast when the buoyancy number b i e the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly is lower than 0 3 0 5 the whole layer regime develops with a deformed interface and convective patterns over the whole tank depth the system systematically evolves toward one fluid rayleigh benard convection because of stirring however the two isolated fluids can persist for very long time compared to the characteristic timescale of thermal convection and give rise to numerous transient behaviors of particular interest for planetary mantles are the pulsatory dynamics where the interface between the two layers deforms in large domes moving up and down quasiperiodically according to the scaling laws derived from the experiments such a mechanism could indeed provide a simple and single physical explanation for the superswells observed at present on earth and more generally for the long term episodicity in planetary interiors observed in geological records

slumping and sliding in miocene and recent developing arc basins onshore and offshore sumba indonesia abstract neogene slidemasses in eastern sumba are discussed and compared to neogene and quaternary analogues in seismic profiles from the offshore lombok and savu basins the onshore examples were deposited in deep marine base of slope environments within the reach of large amounts of clastics derived from a volcanic arc i e in a setting comparable to that of the recent slides onshore three types of slides are distinguished ranging from mud clast rich debris flows type 1 their possible seismic origin is discussed elongate lenticular slumps type 2 to over 100 m thick slide masses of intimately mixed and folded strata together with rafts of less deformed sediments type 3 the offshore analogues may be twice as thick both in the field and in the seismic profiles such slides are found at various superimposed levels five superimposed units can be distinguished in a seismic profile in the savu basin near the westernmost extension of the offshore sumba ridge a giant blocky slide occurs all slide masses are located on slopes with increasing gradients due to differential vertical movements tectonically induced oversteepening is therefore considered a main cause of failure in outcrops the upper surfaces of the slides are almost flat this contrasts with a hummocky topography seen in some of the offshore examples

structural expression of forearc crust uplift due to subducting asperity abstract new structural observations and mapping of reefal terraces carried out both on the field and on satellite remote sensing data indicate that sumba island is presently undergoing a large amount of extension associated with a significant regional uplift this crustal uplift may have been created by a major thrust emerging in the south of the island the uplift which partly accommodated the australian plate south west banda arc convergence is associated with the general northeastward tilt of the island the consequent anomalous positive topography along the southern coast of the island is being compensated by significant tectonic erosion along large scale curvilinear normal faults in the southeastern half of the island the most important expression of this gravitational collapse is located at the receding side of an advancing circular dome showing striking similarities with accretionary wedges being affected by seamount subduction the part of the forearc basin known as the savu basin is moderately deformed mostly in its central part and appears to act as a rigid buttress in the convergence between the banda arc and the australian plate as a result the convergence appears to be transferred northward within the actively shortening back arc domain which goes from the north of the flores island to the southwest of the timor block the convergent plate boundary shows a transition between a stable domain west of sumba and a tectonized domain east of sumba the latter coinciding with the subduction of the outer australian passive margin the subduction of the ocean continent boundary of the austral indian plate below the banda arc since the lower pliocene may have incorporated some crustal fragments in the plunging benioff zone most likely the integration of the stretched continental lithosphere in the subduction zone caused the uplift the entire forearc domain exhibiting inherited structures of the upper plate

three dimensional numerical simulation on the interaction of solitary waves and porous breakwaters abstract a three dimensional 3d large eddy simulation model with macroscopic model equations of porous flow is proposed to investigate solitary waves interacting with permeable breakwaters the major objective of this paper is twofold first we seek to evaluate the present model through the comparison with available simulated and measured data in the literature the second aim given the 3d nature of flow past a permeable breakwater the variations of permeable breakwater modeled on both macroscopic and microscopic scales are examined first validation is carried out with experiments on solitary wave propagation in a 3d wave basin and then runup on a vertical permeable breakwater with a gap in the lateral direction a satisfactory agreement on the free surface elevation time series is obtained between model and measured results second we replicate the experiments on a solitary wave interaction with a submerged permeable breakwater in a two dimensional narrow wave flume the porous medium is composed of spheres with a uniform size and arranged in a non staggered regular pattern such that the porous medium can thus be modeled on macroscopic and microscopic scales the numerical calculations indicate that the results obtained with macroscopic and microscopic modeling both fit the measurements fairly well in terms of the free surface elevations and velocity fields specifically the microscopic modeling better simulates detailed phenomena such as flow injection from the porous medium and the initial stage of the formation of the main vortex in the leeward face of the obstacle after the solitary wave completely propagates over the permeable object the discrepancies between macroscopic and microscopic model results are insignificant more accurate 3d results are used to determine the trajectories of fluid particles around the porous object to help understand the possible sediment movements in suspensions

the structural evolution of the timor collision complex eastern indonesia abstract the structural style of timor is consistent with foreland fold belt processes and zones of frontal accretion and underplating can be recognized in the australian parautochthonous sequence the parautochthon is overlain by an allochthonous sequence which corresponds to the pre collisional oceanic forearc in northern timor the parautochthon has been underplated directly beneath the allochthonous basement unlike previous interpretations of timor in terms of foreland fold belt processes the timor foldbelt is here interpreted as having evolved in a fairly straightforward way from the pre collisional forearc by the sequential addition of thrust slices of australian continental crust to the front and base of the developing collision complex

ocean trench blocked and obliterated by banda forearc collision with australian proximal continental slope abstract the bathymetry and abrupt changes in earthquake seismicity around the eastern end of the java trench suggest it is now blocked south east of sumba by the australian jurassic rifted continental margin forming the largely submarine roti savu ridge plate reconstructions have demonstrated that from at least 45 ma the java trench continued far to the east of sumba from about 12 ma the eastern part of the java trench called banda trench continued as the active plate boundary located between what was to become timor island then part of the australian proximal continental slope and the banda volcanic arc this banda trench began to be obliterated by continental margin arc collision between about 3 5 and 2 ma the present position of the defunct banda trench can be located by use of plate reconstructions earthquake seismology deep reflection seismology dsdp 262 results and geological mapping as being buried under the para autochthon below the foothills of southern timor locating the former trench guides the location of the apparently missing large southern part of the banda forearc that was carried over the australian continental margin during the final stage of the period of subduction of that continental margin that lasted from about 12 ma to about 3 5 ma tectonic collision is defined and distinguished from subduction and rollback collision in the southern part of the banda arc was initiated when the overriding forearc basement of the upper plate reached the proximal part of the australian continental slope of the lower plate and subduction stopped collision is characterised by fold and thrust deformation associated with the development of structurally high decollements this collision deformed the basement and cover of the forearc accretionary prism of the upper plate with part of the unsubducted australian cover rock sequences from the lower plate together with parts of the forearc basement they now form the exposed banda orogen the conversion of the northern flank of the timor trough from being the distal part of the banda forearc accretionary prism carried over the australian continental margin into a foreland basin was initiated by the cessation of subduction and simultaneous onset of collisional tectonics this reinterpretation of the locked eastern end of the java trench proposes that from its termination south of sumba to at least as far east as timor and probably far beyond the java banda trench and forearc overrode the subducting australian proximal continental slope locally to within 60 km of the shelf break part of the proximal forearc s accretionary prism together with part of the proximal continental slope cover sequence were detached and thrust northwards over the java banda trench and forearc by up to 80 km along the southwards dipping savu thrust and wetar suture these reinterpretations explain the present absence of any discernible subduction ocean trench in the southern banda arc and the narrowness of the forearc reduced to 30 km at atauro north of east timor

plate rupture by hydraulic fracture resulting in overthrusting lithospheric plates bending into subduction zones may become ruptured by hydraulic fracturing initiated by the injection of mobilized sediments thick poorly indurated water laden sediments of the proximal continental rise are the most obvious potential source for the large volume of material required for this process rupture of the descending plate may result in the subducted oceanic part of the lithosphere becoming detached and sinking independently while the more buoyant continental part rises by isostatic rebound we show here that applying this model to eastern indonesia provides an explanation for the banda arc geometry the apparent absence of a normal forearc and for the position of the thrust sheets relative to the benioff zone1 we also report that it can account for the short phase of deformation of the australian continental margin preceding its uplift of 3 km ref 1

cenozoic evolution of the savu basin indonesia forearc basin response to arc continent collision abstract a seismo stratigraphic analysis of the savu basin relates the evolution of a complexly evolved forearc basin to the late miocene recent collision with australia initial early miocene subsidence of the outer forearc basin and the subsequent development of a middle late miocene volcanic proto arc located in the south of the basin both resulted from the late oligocene early miocene eastward propagation of the java timor trench block faulting of the relatively thick continental forearc basement probably determined the rather irregular and scattered pattern of middle late miocene volcanic centres just north of the accretionary prism the forearc basement is flexed down and reshaped into a trenchward dipping backstop that facilitates backthrusting of the accretionary prism the southern forearc basement probably acted as a barrier against compression the thickness of this continental basement is suggested to be a critical factor in the response of the forearc to continent collision owing to its strength the savu basin did not react to collision by the development of northward verging inter arc thrusts in contrast it responded to the underthrusting of continental crust by reactivation of the basement ridges this resulted in the differentiation of the forearc basin into a presently extinct and uplifted miocene south savu basin and a pliocene recent active north savu basin the simultaneous late miocene recent uplift of large segments of the outer forearc and subsidence of the north savu basin is interpreted to result from tectonic underplating of continental material and uplift caused by the buoyancy of the subducted continental crust the relatively recent decoupling of this crust from the oceanic slab and the subsequent isostatic rebound may superimpose the initial effects of post collision extension on arc continent collision

deformation and exhumation in timor distinct stages of a young orogeny abstract timor island in the outer banda arc bordering the timor sea preserves the orogenic product of an arc continent collision between the australian plate and the banda arc that commenced after 10 9 9 8 ma gts2004 but emerged above sea level only 3 1 ma ago the orogenic pile includes large tracts of material from the australian margin including the permian to middle jurassic gondwana megasequence and the late jurassic to early late miocene australian margin megasequence which occur in thrust slices in addition material from the banda arc side of the plate margin referred to as the banda terrane occurs throughout the island and includes both seafloor metamorphosed igneous material and cover sediments also in thrust sheets however the distribution of thrust slices is unclear in many areas perhaps because only the uppermost nappes of the thrust pile are currently emergent and also because the thrust piles have been disrupted by later high angle faulting evidence from east timor suggests that the major break between deformed pre collisional strata and the relatively undeformed overlying deposits was during the late miocene 9 8 5 5 ma we present evidence for the timing of three distinct phases of orogenic development as determined from east timor including initial collision and emplacement of the early nappes creating loading and diapirism within the 9 8 5 5 ma interval a tectonic quiet interval 5 5 ma 4 5 ma that extended for about a million years during the middle of the collision and may represent the time of locking of the subduction system and a post 4 5 ma phase of uplift unroofing and further diapirism in response to isostatic rebound our conclusions offer an alternative model for the evolution of this part of the banda arc

rates of neogene and quaternary tectonic movements in the southern banda arc based on micropalaeontology it is generally considered that the outer banda arc islands are composed in part of strongly deformed sediments that accumulated at the north australian continental margin the onset and completion of folding nappe emplacement imbrication and uplift that produced the islands of the outer banda arc have been dated by means of planktonic foraminifera the dating of the onset of uplift of timor from a deep submarine position at the end of the neogene nappe emplacement to mountains now reaching nearly 3 km high indicates the rate of post collision uplift to have been initially 3 mm yr then it slowed to about 1 5mm yr where the australian continental margin meets the eastern end of the present java trench there are indications that the australian margin has overridden the trench in the timor region by 240 km micropalaeontological dating of the neogene collision between the australian continental slope proximal rise and the trench provides constraints for checking the overriding hypothesis by calculating rates of convergence derived from plate motions 7 5cm yr against the rates based on the tectonic model 7 4cm yr after nappe emplacement possibly faster than 6 25 cm yr shortening of the continental crust appears to have migrated towards the australian continent so that the shelf itself has become involved in the imbrication with shortening of the cover rocks between the nappes and the present shelf edge amounting to about 40 km during the last 2 ma

continental margin origin for cretaceous radiolarian cherts in western timor in timor fig 1 widespread outcrops of cretaceous cherts belonging to the wai bua and cretaceous kolbano facies have been interpreted as deep sea deposits that probably accumulated on the australian continental rise1 i consider here another but less well known group of cretaceous cherts exposed in western timor at the base of the palelo group which outcrops in mountain massifs with metamorphic rocks of the mutis complex although the observed contacts between the two units are faulted2 3 the palelo group contains fragments of metamorphic rocks similar to those of the mutis complex4 5 suggesting an original unconformable relationship geochemical data6 7 from metasedimentary gneisses in the boi massif suggest that metamorphism occurred at pressures and temperatures likely to prevail deep in the continental crust i suggest that the cretaceous radiolarian cherts were deposited on a continental margin as the first sediments of a forearc basin this is the first reported occurrence of radiolarian cherts deposited in this tectonic sedimentary environment and contrasts strongly with other cretaceous radiolarian cherts exposed on the island the evidence is crucial to understanding the tectonic evolution of the region because it is clear that the two groups of cherts were derived from different sides of the continent island arc collision zone which makes up the banda arcs the palelo group and the rocks it is associated with were derived from the south east asian side which has had an active margin since at least the early cretaceous

cordierite bearing schists and gneisses from timor eastern indonesia p t conditions of metamorphism and tectonic implications in the boi massif of western timor the mutis complex which is equivalent to the lolotoi complex of east timor is composed of two lithostratigraphical components various basement schists and gneisses and the dismembered remnants of an ophiolite cordierite bearing pelitic schists and gneisses carry an early mineral assemblage of biotite garnet plagioclase al silicate but contain no prograde muscovite sillimanite occurs in a textural mode which suggests that it replaced and pseudomorphed kyanite at an early stage and some specimens of pelitic schist contain tiny kyanite relics in plagioclase textural relations between and mineral chemistries of ferro magnesian phases in these pelitic chists and gneisses suggest that two discontinuous reactions and additional continuous compositional changes have been overstepped possibly with concomitant anatexis as a result of decrease in pload during high temperature metamorphism the simplified reactions are garnet and or biotite sillimanite quartz cordierite hercynite ilmenite excess components p t conditions during the development of the early mineral assemblage in the pelitic gneisses are estimated to have been p 10 kbar and t 750 c based upon the plagioclase garnet al silicate quartz geobarometer and the garnet biotite geothermometer p t conditions during the subsequent development of cordierite bearing mineral assemblages in the pelitic gneisses are estimated to have been p 5 kbar and t 700 c with xh2o 0 5 based upon the fe content of cordierite occurring in the assemblage quartz plagioclase sillimanite biotite garnet cordierite coexisting with melt final equilibration between some of the phases suggests that conditions dropped to p 2 3 kbar and t 600 c a similar exhumation p t path is suggested for the pelitic schists with early metamorphic conditions of p 6 2 kbar and t 745 c and subsequent development of cordierite under conditions in the range p 3 4 kbar and t 600 700 c the tectonic implications of these p t estimates are discussed and it is concluded that the p t path followed by these rocks was caused by decompression during rifting and synmetamorphic ophiolite emplacement resulting from processes during the initiation and development of a convergent plate junction located in southeast asia during late jurassic to cretaceous time

mesozoic and late tertiary submarine fan sequences and their tectonic significance sumba indonesia abstract the indonesian island of sumba occupies a critical zone which separates a normal subduction complex java trench system from a continent island arc collision zone banda arc sumba is considered by some authors to be a microcontinental fragment although in this context its pre fragmentation origin is uncertain detailed studies of the stratigraphy of sumba however throw light on the origin and subsequent geological history of the island and surrounding areas on available evidence the oldest outcropping rocks are of cretaceous age and locally contain a tropical tethyan marine fauna they comprise dark coloured sometimes carbonaceous and often volcanogenic mudstones sandstones gravels and diamictites pervasively intruded by andesitic and dacitic dykes and locally intruded by granodiorite plutons these sediments are siliclastic and appear to have had a continental provenance the cretaceous rocks in many areas compromise part of a major submarine fan complex which at least in south central sumba prograded towards the southwest and south shallow marine and non marine paleogene sediments and volcanic agglomerates lie unconformably above the uplifted tilted and eroded cretaceous strata these are overlain in turn by the widely outcropping miocene sumba formation an oceanic sequence with island arc affinities accessible outcrops of the sumba formation comprise platform carbonates with abundant reef developments in west sumba and a major submarine fan pelagic chalk complex in east sumba the fan complex which extends beneath the southern savu sea north of sumba is dominated by large scale slumps olistostromes and thin and thick bedded turbidites turbidite sands and gravels invariably are volcaniclastic containing components derived from a typical inner volcanic arc complex paleocurrent indicators are difficult to find in the sumba formation thus it is uncertain at this stage whether the volcaniclastic turbidites were derived from the present volcanic arc or from an undetermined source which may since have subsided the portion of the fan complex which underlies east sumba however has undergone vertical uplift since its emplacement of between one and several kilometres uplift is actively in progress at present possibly due to underthrusting by northward moving australian continental crust in this context the island of sumba comprises an uplifted north tilted and subaerially exposed part of the forearc basin of the indonesian arc this uplift can be related to initial stages of continental collision

the sumba enigma is sumba a diapiric fore arc nappe in process of formation abstract the anomalous updomed morphological expression of sumba island its enigmatic lack of strong neogene deformation and the northward morphological indentation of southern sumbawa and flores require explanation the stratigraphy of sumba may be correlated with the cretaceous to miocene part of the timor allochthon the sedimentary and eruptive rock succession in sumba shows remarkable similarities with the allochthonous palelo wiluba and cablac deposits of timor in both islands the cretaceous parts of these sequences are regarded as characteristic of fore arc deposits built on thin continental crust the timor nappe is interpreted as a 5 km thick tectonic flake of the banda fore arc thrust onto the australian continental margin in the mid pliocene collision the postulated sumba nappe has not yet been thrust onto the australian margin which in the sumba region has not yet converged as close to the arc as in the timor area the postulated sumba nappe is interpreted as a diapiric elongated dome of the sunda fore arc that is being squeezed by the converging margin of australia against the volcanic islands of sumbawa and flores the absence of indications on the seismic reflection profiles for the presence of the thrust fault of the sumba nappe may perhaps be explained by the thrusts being nearly horizontal within flat lying strata the savu thrust is correlated with the probably older pre late pliocene wetar suture as a major southward dipping lithospheric rupture east of 124 e this suture does not seem to have moved much since the mid pliocene collision that emplaced the nappes on timor however microearthquake data suggest some activity is continuing

crustal structure of the outer banda arc new free air gravity evidence abstract marine gravity data from the banda arc in the vicinity of western timor and the central savu sea integrated with geophysical and geological information support the postulated subduction of australian continental crust beneath the timor trough and the island of timor gravity analysis also indicates the presence of a mass deficit in the upper mantle or crust beneath the entire savu sea this mass deficit is interpreted to be related to a combination of 1 1 subducted lighter continental crust 2 2 downward flexing of the upper crustal plate in the forearc area and 3 3 possibly anomalous low density upper mantle below the savu sea the north sumba ridge a bathymetric and structural high dividing the savu sea is underlain by high density material possibly related to an older volcanic arc located between the present day volcanic arc and timor the crust beneath the northern savu basin appears nearly oceanic in character but thickens beneath the modern volcanic arc finally our analysis suggests thickening of crust beneath timor forming a root in relation to the savu sea crust to the north

microearthquake seismicity and fault plane solutions related to arc continent collision in the eastern sunda arc indonesia the collision of australia with the eastern sunda and banda arcs of indonesia represents a modern example of the early stages of arc continent collision in order to obtain a more detailed view of the tectonics of the collision zone a microearthquake survey was conducted around the savu sea the region that encompasses the transition from subduction of indian ocean lithosphere on the west to collision or subduction of australian continental litho sphere on the east intermediate depth earth quakes were concentrated near pantar island the easternmost active volcano of the sunda arc and outline a northwest dipping zone that strikes n65oe from 70 to 150 km depth the seismic zone probably marks the western edge of the collision zone and its more northerly strike and the orientations of nodal planes from fault plane solutions indicate a 25 o bend in the subducted slab to a depth of at least 150 km because the convergence rate and the timing of collision from geologic observations on timor suggest that the oart of the slab now at 150 km depth was at the trench when australia first collided with timor we infer that continental crust or at least rifted continental margin crust caused the bend in the convergent margin and has been subducted to 150 km depth fault plane solutions of several events show nearly vertical nodal planes that trend parallel to the strike of the seismic zone along which a northwest side down sense of dis placement is indicated these events are concentrated at the southern top side of the seismic zone and suggest that the subducted lithosphere is presently detaching in the 50 100 km depth range beneath the eastern savu sea fault plane solutions for earthquakes within the upper part of the detaching slab 70 100 km depth have p and t axes whose trends lie parallel to the strike of the seismic zone and horizontal southeast trending b axes these earthquakes are interpreted as due to stresses caused by the bend in the subducting plate

analogous tectonic evolution of the ordovician foredeeps southern and central appalachians new stratigraphic data suggest that the diachronous evolution of the ordovician foredeeps in the southern and central appalachians was remarkably similar stratigraphic features that characterize the middle ordovician sevier basin in tennessee and the middle and late ordovician martinsbuirg basin in pennsylvania are in identical ascending order 1 disconformity on the knox group beekmantown group 2 shelf carbonates 3 slope deposits 4 submarine fan turbidites and 5 contourites and muddy turbidites we propose that diachronous attempted subduction of the north american craton beneath southeastern microplates and or volcanic arcs resulted in uplift and erosion of the western shelf followed by its rapid subsidence basinward migration of eastern and northeastern terrigenous source areas and associated submarine fan turbidites resulted from continued convergence and filled the basins finally tectonic stabilization and lowering of the source area is recorded by contourites and muddy turbidites the evolutionary model proposed for the sevier and martinsburg basins closely resembles present day tectonics of the timor foredeep and the adjoining sahul shelf north of australia similar comparisons have also been made for ordovician basins in the northern appalachians analogous tectonic mechanisms therefore appear to have operated diachronously along the eastern margin of north america from tennessee to new england and possibly to newfoundland during ordovician time

origin of the sumba forearc basement abstract an interpretation of acoustic basement structures in the eastern sunda western banda forearc region suggests structural continuity between sumba and northern timor the area between sumba and timor is delineated as a zone of continuous structures without major strike slip offsets these structures trend east west in the west but gradually change towards the east into ne sw trending structures a major ne sw trending discontinuity west of sumba between 117 30 and 118 30 e marks the transition between an intraoceanic volcanic arc system in the west and a volcanic arc continent collision zone in the east the extent of the sumba basement suggests either a common late jurassic rift drift history for both sumba and northern timor or an early miocene magmatic welding of two continental fragments of different origin resulting in structural continuity between two microplates the first explanation implies that sumba originated from the north west australian continental margin south of tethys from which it drifted away in the late jurassic the second hypothesis which is favoured proposes a northern provenance of sumba the island would then originate from the south east asian continent north of tethys from which it was detached in the eocene

the petroleum potential of inversion anticlines in the banda arc the banda arc in eastern indonesia and newly independent timor leste east timor is the zone of collision between the northern margin of the australian continent and an oceanic island arc system bordering the eurasian plate structurally the banda forearc is a fold and thrust belt consisting of the imbricated outer edge of the australian continent overlain locally by fragments of the precollisional oceanic forearc the banda forearc is an established petroleum province in the island of seram but the remainder of the forearc has so far yielded no further exploration success despite close geological similarity to seram a major disincentive to exploration in the banda forearc has been a common perception of structural complexity this complexity may however have been overstated and this paper presents a simple structural model for the evolution of the banda forearc key to this model is the recognition of basement involved inversion structures in the deeper parts of the collision complex the inverted graben basins are filled with permianjurassic continental margin sequences including high quality source rocks of late triassicearly jurassic age and contemporaneous potential reservoir sequences sealed by middlelate jurassic shales these jurassic shales also act as an important decollement horizon separating shallow level structural complexity from a simpler structural style beneath characterized by large inversion anticlines the oseil field in seram may be located in such an inversion anticline and comparable prospective structures are identified elsewhere in the banda forearc particularly in timor and the tanimbar islands

back arc thrusting in the eastern sunda arc indonesia a consequence of arc continent collision the structure of the eastern sunda back arc region is dominated by two large north directed thrusts the wetar and flores thrusts and one or more minor thrusts which may represent early stages of subduction polarity reversal of the arc the relationships between thrusting and magmatic activity surface slopes cross arc faulting uplift forearc structure and collision by the australian continental margin provide constraints on the likely driving mechanisms of thrust formation driving mechanisms are considered in two broad categories one set of possible mechanisms originates within the upper plate and includes gravity sliding gravity spreading and magmatic intrusion the other set focuses on stress propagation between the upper and lower plates and for this arc system the major mechanism is that of continental collision while no one mechanism explains the range of observations we see in the back arc region all of these aspects play some role in the development of the thrusts we can eliminate upper plate mechanisms as primary driving forces because slope effects are similar in areas of both thrusting and nonthrusting in addition the wetar thrust shows a negative relation between thrusting and volcanism and side looking airborn radar images show no evidence of postvolcanic rifting that could accompany intrusion without volcanism collision between the arc and the australian continent provides the clearest driving mechanism but it does not explain the discontinuous nature or the particular location of the thrusts we suggest a sequence that appears to account for the data and to provide insight into the controversies often associated with other such thrust belts thrusting was initiated in those areas where the crust of the forearc region was thick a factor that facilitated stress propagation across the arc and where the crust of the back arc was thin which facilitated back arc thrusting thrusting concentrated at the base of the slope where the slope stress is maximum the early thrusting may have been helped also by thermal weakening of the crust due to volcanism as convergence proceeded volcanism waned then ceased in the eastern part of the arc the crust then strengthened and thrusting required higher stress to maintain and propagate this later stage should show higher seismic energy release which appears to be the case for the wetar thrust region displacement on the flores thrust appears to be matched by deflection of the volcanic arc we have estimated shortening in the back arc from interpretation of reflection profiles and for the flores thrust the value of approximately 30 km is consistent with that estimated for deflection of the arc compressional deformation continues west of the collision zone across the bali basin and into java it is reported in ne sumatra as well this deformation involves much less convergence than the eastern sunda thrusts and it may be related to magmatic processes to subduction of oceanic plateaus such as the roo rise or in the case of the bali basin to efficient lateral propagation of the flores thrust

tectonic evolution of eastern indonesia and its bearing on the occurrence of hydrocarbons abstract the evolution of the eastern indonesian island arcs over the last 30 million years has been strongly affected by the northward drift of the australian continent and the westward thrust of the pacific plate as australia drifted northward new guinea first collided with the sepik island arc about 30 million years ago about 20 million years ago a major reorganization in the subduction pattern of the indonesian region took place resulting in the formation of an 8000 km long east west trending arc trench system stretching from the western tip of sumatra to buru and even further to the indonesian arc by way of java the lesser sunda islands timor tanimbar kai and seram prior to the arrival of the australian continent at the southeast asian continental margin a north south oriented sulawesi mindanao volcanic arc existed about 800 km east of borneo further to the southeast new guinea and sepik now welded into a bigger microcontinent collided with the inner melanesian island arc this opened the australian plate to the influence of the wnw moving pacific plate about 10 million years ago a south dipping subduction zone broke through north of irian jaya but no volcanism accompanied this process the most dramatic event in the geologic history of indonesia took place about 5 million years ago when the anticlockwise rotation of irian jaya and continuous northward movement of australia trapped the banda sea and caused tectonic shaving of the birdhead of new guinea along the sorong transform fault system subsequently buton and sula and other microcontinents collided with sulawesi and halmahera transforming the double island arcs into a k shaped form a small west dipping subduction zone developed in northern sulawesi accompanied by active volcanoes in minahasa and the sangihe island other small subduction zones with reverse polarities subsequently developed in northwest sulawesi and halmahera which can be held responsible for the generation of respectively the active una una volcano in the gulf of gorontalo central sulawesi and the volcanoes in the western halmahera arc active collision accompanied by emplacement of ophiolites is presently taking place between the west facing halmahera arc and the east facing minahasa sangihe arc the tectonic evolution of eastern indonesia which clearly envisaged a structural continuation of the sunda banda arc and the existence of the north south trending sulawesi mindanao arc during the tertiary period could conveniently be used to explain the hydrocarbon occurrence in complex paleosubduction or collision zones of timor seram and eastern sulawesi whereas oil and gas accumulation in the pull apart basins of irian jaya are found mostly in tertiary deposits the source rocks in the collision zones are likely to be of mesozoic age main exploration target areas in eastern indonesia are the intracratonic basins of the arafura shelf the marginal rift basins skirting the southern and eastern section of the banda arc the collision zones of timor seram east sulawesi and the thrustbelt of irian jaya

paleomagnetic evidence for the paleoposition of sumba island indonesia abstract jurassic paleomagnetic directions have been obtained for the sedimentary rocks in sumba the mean direction is 59 2 in declination and 44 2 in inclination the reliability of the direction is ascertained through thermal demagnetization and the presence of normal and reversed polarities comparison with the permian paleomagnetic direction of timor indicates that sumba was subjected to a clockwise rotation through 79 4 relative to timor since the jurassic when sumba is restored by a counter clockwise rotation the sumba pole is gradually approaching the jurassic pole of australia as well as the timor permian pole this implies that at least until the jurassic sumba and timor were situated at the australian continental margin and that sumba rotated clockwise during or after the jurassic