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

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

magmatism in western indonesia the trapping of the sumba block and the gateways to the east of sundaland abstract the western sulawesi magmatic belt and the sunda banda arc define the eastern and south eastern margins of sundaland which is part of the relatively stable eurasian plate the eastern margin is bounded by the makassar strait which separates western sulawesi from kalimantan the initial opening of the makassar strait took place in the late cretaceous early tertiary leading to the opening of the pacific indian ocean gateway during neogene time recent studies indicate similarities in the late cretaceous paleogene stratigraphic sequence and calc alkali magmatism between sumba south sulawesi and south east kalimantan suggesting a sundaland origin for all of these areas the southward migration of sumba to the present frontal arc position of the sunda banda arc has occurred since late cretaceous paleocene time

petrologic isotopic and radiometric age constraints on the origin and tectonic history of the malino metamorphic complex nw sulawesi indonesia the malino metamorphic complex mmc is located at the western end of the north arm of sulawesi it consists of mica schists and gneisses derived from proximal turbidite and granitoid protoliths with intercalations of greenschist amphibolite marble and quartzite forming an e w elongated dome like structure bounded on all sides by faults the age of the mmc is constrained between devonian and early carboniferous this paleozoic age the presence of archean and proterozoic inherited zircons and the isotopic signature of the mica schists and gneisses indicate that the terrane was derived from the new guinea australian margin of gondwana similarities with basement rocks in the bird s head suggests a common origin greenschists forming a discontinuous selvage metamorphic carapace around the complex were derived from adjacent autochthonous paleogene formations the rocks of the mmc show a barrovian type progression from greenschist through epidote amphibolite to amphibolite facies p t estimations suggest a depth of burial of up to 27 30 km k ar and 40ar 39ar cooling ages of 23 11 ma and a 7 ma age for unconformably overlying volcanic rocks indicate that the complex was exhumed during the miocene two tectonic scenarios are considered 1 the continental fragment docked with sulawesi during the mesozoic and was exhumed as a metamorphic core complex during the miocene 2 it was subducted beneath the north arm during the late oligocene and then rapidly returned back to the surface

neogene basin history and paleomagnetism of a rifted and inverted forearc region on and offshore sumba eastern indonesia the island of sumba is located in the arc trench gap at the junction of the sunda and banda arcs to elucidate its enigmatic forearc position stratigraphic and paleomagnetic data were collected along three transects in central and east sumba sumba represents the emerged part of a terrane of se asian origin results of stratigraphic and paleomagnetic investigations on central and east sumba are presented and discussed in order to identify patterns of horizontal and vertical crustal movements a synthetic seismogram of the onshore record enables correlation with the offshore neogene e of sumba in order to interpret the data in a regional context the neogene basin history of sumba started when overall subsidence took place around the transition from early to middle miocene on east sumba above a basal condensed interval up to 600 m of middle late miocene volcaniclastic submarine fan deposits occur the sediments reflect island arc volcanism and are interpreted as parts of a prograding submarine fan system on a north dipping paleoslope fed by a volcanic source to the south of sumba biostratigraphic data indicate a high volcanic supply during the middle miocene and early middle tortonian a waning supply during the late tortonian and renewed supply during the messinian these patterns probably reflect variation in volcanic activity during the messinian calcilutites and foraminiferal chalks became dominant and deposition of these pelagic sediments continued into the early pliocene using the pattern of magnetic polarity changes in a selected interval the position of the mioceneepliocene boundary is better constrained seismic studies in the offshore area e of sumba van der werff 1995a b indicate the presence of an extensive tract of n prograding turbidites on sumba ridge savu basin the proposed correlation with the onshore record suggests lateral continuity from the island into the offshore area where the volcaniclastic submarine fan system presumably had its largest development subsidence proceeded so fast i m ka on east sumba that before the beginning of the late miocene sediments were deposited below the ccd this pattern suggests that the sumba region underwent strong crustal attenuation due to rifting as this was coupled with the arrival of volcaniclastics we envisage intra arc rifting coupled with the growth of a short lived volcanic arc further south its initiation at the transition of middle late miocene suggests interaction with the then approaching banda arc subduction system the paleomagnetic data indicate that during this process sumba was at a paleolatitude of 12 south and underwent a counter clockw ise rotation of 5 the reappearance of planktonic foraminiferal assemblages during the latest miocene and their present elevation above sea level suggest an average rate of post miocene uplift of 0 7 mm a emergence of sumba probably did not take place before 3 ma this inverted motion of the sumba terrane is probably coupled with the change to the present tectonic situation in which volcanic activity was resumed in the present inner arc after a non volcanic interval during the spreading stage 1 1997 elsevier science ltd

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

collisional melange development geologic associations of active melange forming processes with exhumed melange facies in the western banda orogen indonesia analysis of block assemblages matrix clay mineral composition and microfauna and offshore seismic reflection profiles reveal that the bobonaro melange in the timor region is sourced from mud rich australian continental margin sequences that are remobilized during accretion and form various facies depending on the structural conditions of emplacement melange facies include broken formation matrix rich mud injections and classic mixed block in clay facies each of these are distinguished by varying degrees of remobilization mixing and dispersion at different structural positions across the orogenic wedge the most important structural control is whether melange was generated beneath or in front of upper plate banda forearc basement banda terrane at the present collisional deformation front in the timor trough seismic reflection profiles show that melange forms mostly by stratal detachment and fluid assisted remobilization above a basal decollement propagating laterally along overpressured jurassic to cretaceous clay rich sequences of the distal australian continental margin the broken clay rich material injects upward through faults to form intrusive bodies at the base of slope cover sediment to form mud ridges at the surface similar patterns of stratal disruption are exposed onshore in the pliocene kolbano fold and thrust wedge of southern timor which is structurally contiguous with the timor trough deformation front melange in the kolbano mountains is mostly broken formation and matrix rich injections of mud from jurassic and cretaceous units deformation mechanisms include intense layer parallel extension associated with emplacement by mud diapirs that rise from near the decollement upward to the surface along fault conduits in the hinterland of the orogenic wedge east timor and northern west timor melange is dominantly of mixed block in clay facies with large blocks derived from roof thrust sheets of intermixed banda terrane and maubisse formation units at the structural base of these thrust sheets is the sonnebait disruption zone sdz which formed the initial suture between the banda terrane and structurally underthrust australian continental margin sequences during the late miocene to early pliocene the thickest accumulations of mixed block in clay melange in the timor region are found at the southern edge outlet of the sdz near the central and viqueque synorogenic basins the extent of block dispersion and mixing in the sdz is indicative of intense shear strains perhaps induced by an oversupply of accretable material when the suture zone was clogged by underthrusting of the australian continental margin

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

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

geology of the kai islands implications for the evolution of the aru trough and weber basin banda arc indonesia abstract the kai archipelago is situated at the apex of the 180 bend in the banda island arc topographically the islands occupy a forearc position between the aru trough to the east and the weber forearc basin to the west however the structural style of the eastern kai islands is very different from the rest of the banda forearc being dominated by normal faults downthrowing to the aru trough with no sign of earlier compressive forearc deformation the aru trough is an extensional feature although it is in direct bathymetric continuity with the compressional timor tanimbar trough to the south the banda arc thrust front does not continue simply northward from the timor tanimbar trough but steps westward as a result of extension in the aru trough the thrust front runs n s through the kai group separating an inactive accretionary complex to the west from active extension in the east the weber basin results from e w extension with pre existing thrust faults probably reactivated in extension as low angle normal faults both the compressional and extensional deformation phases have occurred since the pliocene a period of less than 5 my

emplacement of the east sulawesi ophiolite evidence from subophiolite metamorphic rocks abstract a newly discovered high temperature metamorphic sole at the base of the extensive east sulawesi ophiolite in sulawesi indonesia yields new structural and petrological data with which to constrain the emplacement history the metamorphic sole is composed of a thin sequence of garnet and epidote amphibolite and basal greenschist metaclastics east dipping tectonite fabrics in the metamorphic sole amphibolite are coincident with those in subjacent basement mica schist and superjacent peridotite and indicate orthogonal east to west emplacement of the ophiolite in central sulawesi metabasite in the upper levels of the sole has morb geochemical characteristics was recrystallized at temperatures of about 700 c and yielded k ar radiometric ages of approximately 30 ma initial detachment of the ophiolite probably occurred at a ridge segment transform intersection in a back arc basin at the south east margin of sundaland detachment closely preceded nw directed overthrusting and orthogonal obduction on to the sundaland continental margin in the oligocene sinistral movement on a paleo transform boundary now the lawanopo fault may have facilitated obduction although the westernmost segments of ophiolite may represent younger more proximal slices of a marginal basin those further east probably represent successively older more distal slices of underthrust indian ocean back arc or forearc

in situ δ7li li ca and mg ca analyses of synthetic aragonites in situ secondary ion mass spectrometry sims analyses of 7li li ca and mg ca were performed on five synthetic aragonite samples precipitated from seawater at 25 c at different rates the compositions of 7li in bulk aragonites and experimental fluids were measured by multicollector inductively coupled plasma mass spectrometry mc icp ms both techniques yielded similar 7li in aragonite when sims analyses were corrected to calcium carbonate reference materials fractionation factors 7li 6li range from 0 9895 to 0 9923 which translates to a fractionation between aragonite and fluid from 10 5 to 7 7 the within sample 7li range determined by sims is up to 27 exceeding the difference between bulk 7li analyses of different aragonite precipitates moreover the centers of aragonite hemispherical bundles spherulites are enriched in li ca and mg ca relative to spherulite fibers by up to factors of 2 and 8 respectively the li ca and mg ca ratios of spherulite fibers increase with aragonite precipitation rate these results suggest that precipitation rate is a potentially important consideration when using li isotopes and elemental ratios in natural carbonates as a proxy for seawater composition and temperature

paleomagnetism of borneo abstract the paleomagnetism of borneo remains controversial although the preponderance of results both from the island itself and from the surrounding regions suggest that counterclockwise ccw rotation has taken place ccw rotations are seen in minor intrusions in sarawak sabah and kalimantan which increase systematically with the age of the intrusion to a maximum value of 51 8 3 7 the rotation can be no older than 25 ma which is the age of the intrusion showing the maximum rotation the rotation appears to have neared completion by 10 ma similar ccw rotations are seen in sites from peninsular malaysia through borneo to sulawesi the celebes sea and palawan in the philippines but the ages of these rotations are for the most part unknown in mesozoic rocks in kalimantan and sarawak a stronger declination rotation of nearly 90 ccw is recorded at seven sites including sites which pass fold and reversal tests this strong rotation is no older than youngest cretaceous and although seen over a wide region in borneo it is not seen in peninsular malaysia nor in the celebes sea or palawan where only the weaker ccw rotation is seen the widespread occurrence of this strong rotation in western borneo suggests that it is essentially a rigid plate or microplate rotation and not a series of local rotations caused by distributed shear in limited deformation zones the rotation of borneo appears to be a consequence of convergence between the australian and eurasian plates which is accommodated by subduction along the northwest margin of borneo

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

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

multibeam study of the flores backarc thrust belt indonesia using the seamarc ii seafloor mapping tool in conjunction with closely spaced seismic reflection profiles we have mapped a segment of the flores back arc thrust zone structural irregularities along the deformation front of the thrust zone result from changing stratigraphy and basement structure of the lower plate ne trending faults cutting the outer slope of the flores basin are easily mapped because they truncate dense drainage patterns obliquely mud diapirs probably indicating elevated fluid pressures have formed throughout the accretionary wedge but appear to be concentrated as do back arc thrust earthquakes at the ends of thrust faults the overall orientation of the deformation front of the accretionary wedge is 100 suggesting a nne sense of thrust motion and supporting an origin of the thrust zone by collision of the arc with australia rather than by magmatic forcing or gravitational sliding or spreading orientations of faults and folds close to the arc are not consistent with the trends of the frontal thrusts however and the difference may be due to either different initial orientations or to later rotations of structural features as the accretionary wedge grew

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

origin of the sula platform eastern indonesia the sula platform is a fragment of continental crust that is generally regarded as a piece of new guinea that was detached from western irian jaya in late cenozoic time the assumption is based on the apparent correlation of the stratigraphy of the sula platform with that of western irian jaya new data from the sula platform and western irian jaya show that this correlation is probably not valid the best correlation of the sula platform stratigraphy with that of the former northern margin of the australian craton is found in central papua new guinea between long 141 and 145 e more than 1200 km farther east than any previously proposed site of origin implying a displacement of more than 2500 km

geology of a miocene collision complex buton eastern indonesia the island of buton in eastern indonesia is part of a neogene collision zone that encompasses much of the eastern margin of sulawesi miocene collision of microcontinents with a west dipping subduction zone emplaced the tukang besi platform tbp against buton and the sula platform sp against the east arm of sulawesi and thrust large ophiolite sheets eastward over the imbricated margins of the platforms reconnaissance observations of the resulting collision complex in buton wolio complex indicate that imbricate west dipping thrust sheets and overturned folds are the predominant structural element although later steep faults offset the imbricate stack and largely control present map patterns the wolio complex consists of the sedimentary turumbia sequence peridotite and two distinct groups of metamorphic rocks the turumbia sequence makes up most of the eastern part of the wolio complex and consists mostly of deep water limestone ranging in age from late triassic through late eocene or oligocene it is interpreted as a deep water facies of the western tbp margin ophiolitic rocks are represented in present outcrop only by massive peridotite in the western part of the wolio complex but the compositions of clasts in conglomerates overlying the wolio complex indicate that a full ophiolite succession was exposed to erosion during the collision metabasite and metachert ranging in grade from greenschist to amphibolite facies are found locally in fault contact with peridotite and are interpreted as the remnants of a metamorphic sole developed at the base of the ophiolite pelitic phyllite and quartzite in northeastern buton probably represent a slice of the continental basement of the tbp for similar rocks have been dredged from the northeast margin of the tbp and also form the pre mesozoic basement of the sp the middle to upper miocene tondo formation in buton consists of clastic strata derived from uplift and erosion of the wolio complex placing an upper limit on the age of tbp collision we infer a middle miocene time of collision with oblique convergence continuing into late miocene time in contrast sparse biostratigraphic data place the age of the sp east arm collision in the late miocene separate microcontinents may have collided successively with different parts of the buton sulawesi convergent margin or a single large microcontinent may have been fragmented during oblique collision

structure and collision history of the buton continental fragment eastern indonesia although commonly reservoired within tertiary rocks hydrocarbons in eastern indonesia have generally been sourced from mesozoic sediments deposited on the continental margin of northern australasia fragments of this margin are now widely dispersed as allochthonous terranes throughout the area one of the most far traveled examples being the island of buton southeast of sulawesi asphalt reserves on buton support a significant local industry and exploration continues for oil and natural gas in common with other prospective australian derived terranes around the margins of the banda sea buton is now separated from australia by the active plate boundary marked by the java trench and the collision trace along the timor tanimbar and seram troughs buton differs from these other terranes in its distance from that boundary and its consequent insulation from the effects of the pliocene pleistocene collision between australia and the banda arcs reconstruction of the geological history of the buton terrane thus has an important role in guiding future exploration in the other australasian fragments in eastern indonesia geophysical studies of the buton region have used seismic reflection gravity and magnetic including paleomagnetic techniques seismic reflection images generally record extension rather than compression as dominating the recent history of the area gravity data define the present day western limits of the buton terrane and suggest that in the east the terrane includes the almost entirely submerged tukang besi platform the gravity surveys also demonstrate that the ophiolitic rocks exposed on buton are not attached to deep roots but are thin and isolated overthrust sheets therefore they do not mark a terrane boundary and their presence has little bearing on the prospectivity of the area paleomagnetic results document the independent movements of thrust sheets on buton during the pliocene pleistocene the combined data from buton record its separation from australia as part of a microcontinental block in the jurassic or late triassic followed by collision with the eurasian margin in southeastern sulawesi in the oligocene or early miocene collision was followed by extension as in sulawesi itself producing minor separation of tukang besi from buton and much greater dispersion of other fragments of the microcontinent some of which have since been incorporated in the new collision zone in the outer banda arc the oil seeps and asphalt deposits of buton are proof that hydrocarbons in the banda arc fragments can be sourced from within these fragments and are not necessarily derived from the underthrusting australian margin