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

magmatic record of neoarchean arc polarity reversal from the dengfeng segment of the central orogenic belt north china craton abstract the dengfeng segment of the archean central orogenic belt in the north china craton ncc includes circa 2 66 2 5 ga metavolcanics metasedimentary and plutonic rocks all units are intruded by 2 5 2 4 ga mafic dykes and a suite of coeval circa 2 5 2 45 ga potassic granites and leucogranitic veins mafic dykes are characterized by mixed alkaline and calc alkaline affinities enriched lree and negative nb and zr anomalies indicating a lithospheric mantle source region potassic granitic dykes exhibit highly fractionated lree patterns and negative hfse anomalies and positive zircon ehf t values which are similar to those of ttg gneisses in the area leucogranitic veins cutting across the diorites are characterized by high contents of sio2 and k2o lree enriched patterns positive eu and sr anomalies negative nb and ti anomalies we suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts combined with other reported 2 5 ga mafic dykes and potassic granites from the ncc we propose that they demonstrate an ancient example of arc polarity reversal and provide a record of the formation of a new plate boundary in the archean the underplating of the mafic dyke source beneath the newly accreted eastern block may have generated the widespread 2 5 2 47 ht lp metamorphism with anticlockwise p t paths in the eastern block

tectonic history of sumba island indonesia since the late cretaceous and its rapid escape into the forearc in the miocene abstract the island of sumba lies within the sunda banda forearc and contains volcanic plutonic and volcaniclastic rocks that record a history of arc volcanism extending from late cretaceous 80 ma or earlier to oligocene 31 ma time during the late cretaceous sumba formed part of a great indonesian volcanic arc system near southeastern eurasia between late cretaceous and early miocene time sumba remained part of that arc system which ceased to be volcanically active by 31 ma during the late miocene 18 ma sumba as part of the relict arc system was situated near the present site of alor and wetar at 16 ma sumba was torn from the relict arc and began to move at a velocity of 50 mm yr in a west south westerly direction volcanism along the modern banda arc soon followed by 7 ma sumba finally came to rest after having moved 450 km into the forearc we interpret the rapid motion of sumba into the forearc as a discrete block to have been a response to the collision between australia and indonesia in the area of timor the savu basin possibly originated as an arc crestal rift which then opened behind sumba as the latter moved westward over the past 16 my more than 3 km of sediment have been deposited in the savu basin by erosion from the surrounding islands sumba stopped moving to the west south west at 7 ma after reaching approximately its present location since then the island has ceased to be a discrete tectonic fragment and has behaved as an integral part of the forearc during the past 7 my compressional tectonics within the forearc has led to elevation of the rocks exposed on sumba by as much as 4 km

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

deformation and sedimentation along a developing terrane suture eastern sunda forearc indonesia the collision of the eastern sunda arc with northwest australia has resulted in the development of a suture between the sumba ridge and sawu timor terranes along a zone of intraforearc convergence the developing suture varies from the low angle sawu thrust with attendant mud diapirs in the sumba basin to high angle reverse faults near a basement high of the underthrust sumba ridge terrane bottom currents associated with the flow of pacific ocean deep water into the indian ocean have eroded the terranes and subsequently deposited the detritus in an assemblage of contourites along the suture this study reveals the high structural variability of a terrane suture and the oceanographic influence on the deposition of overlap assemblages

differing controls over the cenozoic pb and nd isotope evolution of deepwater in the central north pacific ocean dissolved pb and nd isotope ratios of seawater as recorded by chemical marine precipitates such as hydrogenetic ferromanganese crusts have been used as paleoenvironmental proxy tracers for the north pacific however all ferromanganese crusts studied so far have either been subject to phosphatization or hydrothermal influence in their old part or only the young parts have been analyzed thus the pb and nd isotope compositions of north pacific deep waters prior to 20 ma are not well constrained we present new results for three ferromanganese crusts one of which cj01 shows no phosphatization and is located far away from the epr its age is inferred to be 75 ma and thus provides for the first time an opportunity to trace the nd and pb isotope evolution of central north pacific seawater back to the latest cretaceous the three crusts no matter whether phosphatized or not display very similar pb and nd isotope trends with age suggesting no modification of the pb and nd isotope distribution by post depositional phosphatization our data suggest that dissolved pb in deep waters of the central north pacific over the cenozoic and latest cretaceous has mainly been derived from eolian dust and only to a minor extent from weathering of island arcs for pb these trends broadly resemble the pb isotope evolution of the eolian silicate dust fraction of core ll44 gpc3 in the central north pacific we suggest that the isotope evolution of dissolved pb in central north pacific seawater has been mainly controlled by pb released from eolian dust from north america prior to 50 ma and after 40 ma from asia in contrast the nd isotope time series of the crusts are by no means similar to the nd isotope evolution of the silicate dust fraction in core gpc3 suggesting a decoupling from the pb and negligible contributions from dust to the dissolved nd in the central north pacific deep water the rise of nd isotope ratios of pacific seawater during the cenozoic has most likely been caused by the increasing volcanic activity and erosion of the volcanic arcs around the pacific

geochemical and strontium isotopic characteristics of parental aleutian arc magmas evidence from the basaltic lavas of atka eighteen flows from a basal stratigraphic sequence on the aleutian island of atka were analyzed for major elements trace elements and initial 87sr 86sr ratios petrographically these lavas contain abundant plagioclase 24 45 and lesser amounts of olivine 7 magnetite and clinopyroxene phenocrysts compositionally the lavas are high alumina 20wt basalts 48 51 wt sio2 with low tio2 1 and mgo 5 within the section compositional variations for all major elements are quite small while mgo content correlates with olivine phenocryst contents no such relationship exists between the other oxides and phenocryst content these lavas are characterized by 8 10 ppm rb high sr 610 669 ppm 308 348 ppm ba and very constant zr 23 29 ppm and sc 23 29 ppm abundances ni and cr display extremely large compositional ranges 12 118 ppm and 12 213 ppm respectively no correlation exists between trace element concentrations and phenocryst contents strontium isotopic ratios show a small but significant range 0 70314 0 70345 and are slightly elevated with respect to typical morb no systematic correlation between stratigraphic position and petrography or geochemistry is evident ree abundances measured on six samples are lree enriched la yb n 2 20 2 81 and display similar chondrite normalized patterns one sample has a slight positive eu anomaly but the other lavas do not compared to other aleutian basalts of similar silica content these lavas are less lree enriched and have lower overall abundances the geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature published phase relations indicate fractionation of a more mgo rich magma could not have produced these lavas the high al2o3 and low mgo and compatible element abundances suggest a predominantly oceanic crustal source for parental high alumina basalts

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

seismological evidence for shallow thrusting north of the timor trough summary the eastern sunda arc in indonesia is the site of subduction of the continental crust of australia and its shallow seismicity is manifested by infrequent large ms 7 earthquakes that occur near the volcanic arc within the upper plate rather than as interplate events beneath the forearc in the same region volcanic activity has ceased and small shallow earthquakes are rare we examine two of the larger earthquakes the events of 1962 may 15 ms 7 2 and 1977 august 27 ms 7 0 the 1977 event occurred north of central timor and the inversion of long period p and sh waves indicates a nearly pure thrust mechanism at a depth of 10 km below sea level and a seismic moment of 4 1 1026 dyne cm the p wave nodal planes for this solution strike roughly parallel to the mapped trace of a thrust zone the wetar thrust zone at the southern margin of the backarc basin and the south nodal dipping plane is consistent with the sense of thrusting on this thrust zone a broad zone of compressional deformation is evident in seismic reflection profiles over the epicentral region of the 1977 event and gravity data suggest that a small amount of either isostatically uncompensated crustal thickening or flexure of the underthrusting plate has occurred the 1962 earthquake occurred approximately 300 km north east of the 1977 earthquake and near the eastern end of the volcanically quiet section of the banda arc the similarity of the few available waveforms to those at the same stations for the 1977 event suggests a similar source mechanism modelling of pdif waveforms for the 1962 event indicates a depth of less than 40 km and a preferred centroidal depth of 20 km for which a seismic moment of 5 1026 dyne cm is estimated these fault plane solutions indicate that most of the seismic energy released by the collision between australia and the sunda arc is generated by faulting within the overriding plate and we infer that a major change in the plate geometry is imminent we suggest that most of the relative convergence between the plates is accommodated by thrusting of the marginal basin the banda sea southward beneath the arc and that the present geometry represents the initial stage of the reversal of the arc s polarity

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 styles of an accretionary wedge south of the island of sumba indonesia revealed by seamarc ii side scan sonar the accretionary wedge south of sumba island indonesia is in the early stages of continent island arc collision australian continental shelf sediments are being actively accreted to the sunda arc at the timor trough we have correlated seamarc ii side scan images and bathymetry with 3 5 khz and seismic reflection profiles to construct a structural interpretation of this zone deformation is concentrated on the lower slope of the accretionary wedge within 15 to 25 km of the thrust front above which the strain rate appears to decrease abruptly three distinct structural styles are developed in the survey area the characteristics of the easternmost region are 1 deformation in a fold and thrust geometry 2 presence of mud volcanoes and mud ridges parallel to the thrust front indicating elevated fluid pressure and 3 the development of a long decollement extending 10 to 15 km from the thrust front in contrast the central region shows a more homogeneous thickening resulting mainly from closely spaced faulting seaward verging thrust faults and conjugate strike slip faults are the main structures in this segment of the accretionary wedge mud volcanoes and a very long decollement are absent in the central region we present a hypothesis in which structural style is controlled by the length of the decollement developed in front of the thrust front short thick thrust sheets may favor uniform shortening whereas long thin sheets may be unstable to buckling and deform by folding high fluid pressure may facilitate propagation of the decollement in contrast to the accretion occurring in the other two regions the western part of the accretionary wedge is in the process of being indented and redeformed by a basement ridge

aleutian tholeiitic and calc alkaline magma series i the mafic phenocrysts diagnostic mafic silicate assemblages in a continuous spectrum of aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the aleutian arc crust tectonic segmentation of the arc exerts a primary control on the variable mixing fractional crystallization and possible assimilation undergone by the magmas end members of the continuum are termed calc alkaline ca and tholeiitic th ca volcanic rocks e g buldir and moffett volcanoes have low feo mgo ratios and contain compositionally diverse phenocryst populations indicating magma mixing their ni and cr rich magnesian olivine and clinopyroxene come from mantle derived mafic olivine basalts that have mixed with more fractionated magmas at mid to lower crustal levels immediately preceding eruption high al amphibole is associated with the mafic end member in contrast th lavas e g okmok and westdahl volcanoes have high feo mgo ratios and contain little evidence for mixing evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma these lavas contain groundmass olivine fo 40 50 and lack ca poor pyroxene aleutian volcanic rocks with intermediate feo mgo ratios are termed transitional tholeiitic tth and calc alkaline tca tca magmas are common e g moffett adagdak great sitkin and kasatochi volcanoes and have resulted from mixing of high al basalt with more evolved magmas they contain amphibole high and low al or orthopyroxene or both and are similar to the japanese hypersthene series tth magmas e g okmok and westdahl contain orthopyroxene or pigeonite or both and show some indication of upper crustal mixing they are mineralogically similar to the japanese pigeonite series high al basalt lacks mg rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite the low pressure mineral assemblage of high al basalt results from crystallization at higher crustal levels

u pb dating of a terminal pliocene coral from the indonesian seaway abstract pristine detrital platygyra corals were discovered in an exhumed package of syn orogenic marine sediments on the island of timor in the eastern indonesian region and dated using u pb techniques a single coral from the upper part of the sequence yields a 238 u 206 pb 207 pb 206 pb concordia age of 2 66 0 14 2 ma that is supported by coral 87 sr 86 sr chemostratigraphy and foraminiferal biostratigraphy from bounding strata minor u series disequilibrium is best explained by u mobility within the last 150 ka as pore water chemistry was altered during exhumation and is unlikely to have affected 238 u 206 pb and the apparent sample age by more than 1 2 the ability to date corals beyond the limits of 14 c and u th techniques provides the opportunity to improve the temporal resolution of associated marine chronostratigraphic records in this instance we refine the timing of timor s emergence from beneath the waters of the indonesian seaway is and the initiation of turbiditic deposition at the study site to between ca 3 35 and 2 66 ma these results have implications for the evolution of topography and is oceanic pathways in the active orogenic belts along the northern fringes of the australian plate

tectonics of east indonesia abstract the complex processes resulting in basin formation and inversion during the tertiary tectonic evolution of indonesia owe their origin to the interplay between the indian eurasian australian and pacific plates within the framework of these major plates and their relative motions some microplates may be recognized whose tectonic development was constrained by motions of the large plates treated as small ridged microplates their boundaries define the sites of basin formation their dynamic development results in the formation and subsequent inversion of basins throughout the region this paper outlines the tectonic framework of indonesia throughout tertiary time in a series of reconstructions and considers the constraints that this implies for microplate motions and basin histories

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

a one million year long sequence of marine terraces on sumba island indonesia abstract an exceptional sequence of raised coral reef terraces is visible at cape laundi on the north coast of the island of sumba in the savu sea in which at least eleven terraces wider than 100 m six of them even being over half a kilometer wide are geomorphologically distinguishable between an ancient patch reef now at 475 m and the present sea level esr date estimations of almost unrecrystallized corals most of them in situ make possible a tentative identification of the terraces corresponding to isotope stages 15 ca 600 ka ago and 9 ca 330 ka ago when the uplift trend deduced from the present altitude of dated terraces 0 49 0 01 mm yr is extrapolated to the whole raised section most geomorphological features appear to correspond to interglacial stages of the astronomically calibrated oxygen isotope record up to stage 27 ca 0 99 ma beyond the matuyama jaramillo geomagnetic boundary the upper terraces would in this case be twice as old as previously estimated ca 0 5 ma main terraces seem to correspond to high sea levels at certain interglacial stages and substages but in some cases the action of low sea levels at glacial stages cannot be ruled out for terraces between 30 and 70 m in altitude th u and esr dates reveal that aragonitic corals at similar elevations may differ in age as much as 100 ka this can be explained by taking into account a polycyclic origin for several terraces reconstructions available of changes in the eustatic sea level during the last 250 kyr and the fact that coral reef development may have been scattered at some periods only without totally covering subjacent older formations the sumba sequence of marine terraces at cape laundi is among the longest and most complete for the past million years reported in the literature its relatively good state of preservation will probably give impetus to a number of new studies on the chemical physical isotopic morphological palaeoclimatic pedological ecological and diagenetic changes which have occurred during the past one million years

mesozoic cenozoic rift drift sequence of asian fragments from gondwanaland abstract the university of cambridge atlas map plotting computer program has been used to plot a reconstruction of the dispersal of continental blocks from eastern gondwanaland from the late jurassic 160 ma until the late miocene 10 ma the geological basis for the reconstruction rests on the concept of south tibet burma western thailand malaya and sumatra having been part of the eastern gondwana continent during the late palaeozoic and early mesozoic the reconstruction is also based on the view that these asian continental blocks were rifted from eastern gondwanaland in the late jurassic this rifting episode gave rise to the continental margin of central new guinea and northern australia it was associated with the sea floor spreading anomalies preserved in the floor of the northeastern indian ocean the model considers the indication of crustal uplift in the middle jurassic followed by profound subsidence in the cretaceous and palaeogene exposed in the islands of the outer banda arc from seram to timor and the similar jurassic cretaceous facies in eastern sulawesi implies that these areas formed part of this rifted northern margin of australia in the jurassic subsequent tectonic collision and associated major strike slip movements have deformed and disrupted these rock sequences during the cainozoic the date of the rifting that removed south tibet burma western thailand and malaya from gondwanaland is controversial the indications in favour of the rifting episode having occurred in the jurassic rather than in the permo triassic are stratigraphie structural observations on the northwestern australian shelf in the banda arc and in central new guinea the identification of the continental blocks removed by this rifting is based partly on stratigraphie correlation with the banda arc and new guinea and partly on indications that the age of collision of tibet burma western thailand and malaya with asia was a late cretaceous event and not triassic jurassic as is generally held there are very few reliable land based palaeomagnetic measurements to constrain the movements of these dispersed continental fragments from gondwanaland the computer generated map reconstructions put forward here fit the available indian ocean sea floor spreading data although they are not unique solutions much of the critical ocean floor data associated with the rifting and dispersal of these continental fragments has been destroyed in the tectonic collision processes that occurred at the asian continental margin

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

the wetar back arc thrust belt eastern indonesia the effect of accretion against an irregularly shaped arc the 350 km long east west trending wetar back arc thrust belt in eastern indonesia is a result of the collision of australia with the indonesian island arc along the northwest margin of wetar island a short 50 km section of the thrust belt trends northeast coincident with the offset of the indonesian arc by the newly discovered left lateral wetar atauro fault which runs along the shelf region and trends northeast the wetar atauro fault may be viewed as a large scale lateral ramp or wrench fault separating the eastern wetar thrust belt from the western wetar thrust belt the interaction of strike slip faulting and back arc thrust faulting creates several arc blocks whose geometry strongly affects the structure of the deformed wedge of sediment accreting in the wetar back arc thrust belt the varying orientations of the arc backstop make the wetar back arc thrust belt a perfect laboratory for the study of oblique convergence along most of its length the wetar thrust belt parallels the arc slope however along the northeast trending offset in the thrust belt seamarc ii side scan sonar plus seismic reflection data show that the trend of the thrust belt is not parallel to the trend of the arc slope rather it is intermediate between the trend of the arc slope and the perpendicular to the expected convergence direction the side scan images allow us to map the geometry of four main thrust faults the wetar liran atauro and alor faults the wetar liran and atauro faults trend northeast parallel to the thrust front and do not have an en echelon relationship the westernmost fault in the survey area the alor fault trends almost east west the orientation of the wetar atauro fault is consistent with a maximum principal stress within the arc oriented between 8 and 23 west of north depending on the internal coefficient of friction this agrees well with the p axes of nearby earthquake focal mechanisms which trend consistently west of north suggesting that the regional principal tectonic compression is oriented west of north because the thrust belt is oblique to the arc slope we infer the structural directions are influenced by both the arc backstop and by the regional tectonic stress the tectonic compression due to arc continent collision in this region may be modified by the arc geometry in several ways 1 the stress necessary to support the arc topography may be significant close to the arc 2 the change in thickness of the elastic part of the lithosphere may cause a concentration of tectonic stress and 3 the difference in material properties between the relatively rigid arc bulldozer and the weak basin sedimentary fill may be an important factor in producing a thrust belt that conforms roughly to the shape of the arc the structure of the wetar back arc thrust zone demonstrates that the development of small rigid blocks within a major collision zone may produce complex structural patterns and local directions of shortening that are highly oblique to the main direction of plate convergence

evidence for the jurassic arc volcanism of the lolotoi complex timor tectonic implications abstract we report the first sensitive high resolution ion microprobe shrimp u pb zircon ages with geochemical data from metavolcanic rocks in the lolotoi complex timor the zircon u pb ages of two andesitic metavolcanic rocks yield a permissible range of the middle jurassic extrusion from 177 ma to 174 ma the geochemical data indicate that the origins of the basaltic and andesitic metavolcanic rocks are products of prolonged oceanic crust and arc magmatism respectively they are originated from partial melting of lherzolites providing an insight into the tectonic evolution of the forearc basements of the banda volcanic arc thus parts of the banda forearc basement are pieces of allochthonous oceanic basalts and jurassic arc related andesites accreted to the sundaland during the closure of mesotethys and are incorporated later into the great indonesian volcanic arc system along the southeastern margin of the sundaland

the effects of flowline length evolution on chemistry δ18o profiles from penny ice cap baffin island canada the isotopic and chemical signatures for ice age and holocene ice from summit greenland and penny ice cap baffin island canada are compared the usual pattern of low 180 high ca2 and high cl is presented in the summit records but penny ice cap has lower than present cl in its ice age ice a simple extension of the hansson model hansson 1994 is developed and used to simulate these signatures the low ice age cl from penny ice cap is explained by having the ice age ice originating many thou sands of km inland near the centre of the laurentide ice sheet and much further from the marine sources summit s flowlines all start close to the present site the penny ice cap early holocene 180 s had to be corrected to offset the laurentide meltwater distortion the analysis suggests that presently the summit and penny ice cap marine impurity ori ginates about 500 km away and that presently penny ice cap receives a significant amountoflocal continental impurity

paleomagnetism of younger volcanics from western timor indonesia abstract paleomagnetic research has been carried out on volcanic rocks from two formations exposed in western timor indonesia i e the metan formation of eocene age and the manamas formation of late miocene age because of the chaotic tectonic setting of the rocks at the metan formation it was the inclination data from five lavas only that could be interpreted in the final analysis of the characteristic remanence directions chrm with i 33 6 a 95 4 1 the metan volcanics most probably extruded originally in the northern hemisphere at a latitude of about 17 the volcanics may have extruded on a crustal unit which includes a continental fragment that became detached from gondwana in mesozoic times and subsequently shifted to the southeast asian continent this crustal unit broke away from southeast asia in eocene times moved towards the south and collided 3 ma ago with the frontal part of the australian continental margin the manamas formation of late miocene age includes obducted volcanics consisting of both deep sea pillow lavas and island arc volcanics with chrm directions of d 132 6 i 15 8 no a 95 because 2 sites only and d 295 9 i 26 3 a 95 2 8 respectively the declinations of the chrm directions derived from the rocks from two localities show counterclockwise deviations of approximately 45 and 60 respectively relative to the present earth s field direction the deviating directions can be explained in terms of a counterclockwise rotation of timor which may have begun as early as 3 ma ago and which could be a result of the obliquely northward protruding australian continent where the originally west east directed banda arc rotated in a counterclockwise sense it is also possible that during the obduction the lava piles performed counterclockwise rotations rock magnetic analyses show that magnetite or and titanomagnetite is the main carrier of remanence

transverse intra arc rifting palaeogene wagwater belt jamaica abstract the wagwater belt in eastern jamaica is an exceptionally well exposed example of a transverse intra arc rift formed during an abrupt transition from convergent to strike slip tectonics the belt consists of the uplifted wagwater half graben containing at least 5 6 km of coarse clastic terrestrial sedimentary rocks which is overlain and flanked by a much wider basin filled with about 1 2 km of upward fining marine sedimentary rocks neogene strike slip movements structurally inverted the half graben and exposed thick sections of both the half graben and overlying basin early palaeocene early eocene rifting preceded the middle eocene formation of oceanic crust in the cayman trough pull apart basin and affected a 50000 km 2 area near the eastern distal end of the pull apart fifteen half grabens identified in jamaica and in offshore areas using seismic profiling range in strike from north east to north west rifts in jamaica are presently oblique to the generally east west strike of plutons and sedimentary facies belts within a maestrichtian palaeocene island arc based on their apparent relationship to the strike of the enclosing palaeo arc we interpret the jamaican rifts as transverse intra arc rifts following the deposition of palaeocene redbeds and evaporites wagwater formation in the half graben subsidence was widespread in a marine sedimentary basin this marine basin was filled in early eocene time by marine conglomerates port maria and albany members of the richmond formation sandstone and siltstones roadside member and finally in the latest early eocene to earliest middle eocene time by a calcareous mudstone langley member a bimodal suite of volcanic rocks newcastle and nutfield volcanics was erupted during the deposition of the wagwater formation and the roadside member the structure and tectonic setting of transverse intra arc basins in jamaica is somewhat similar to rifts of the neogene pannonian basin in hungary in both areas an abrupt transition between convergent and strike slip tectonics resulted in transverse intra arc rifts striking almost perpendicular to a dying volcanic arc transverse intra arc rifts may be common features of small arc fringed plates such as the caribbean and pannonian plates that respond quickly to collision by strike slip motion towards an unopposed oceanic free face

fission track evidence for the source of accreted sandstones barbados a part of the accretionary wedge associated with the lesser antilles arc system is exposed in an area of approximately 50 km in northeastern barbados zircon separates from sandstones collected from the scotland district barbados w i were analyzed using the fission track technique in order to assess the spectrum of source areas from which these terrigenous sediments were derived and to better constrain the timing of deposition results yielded a mixture of ages with strong groupings from 25 80 ma 200 350 ma and greater than 500 ma the youngest population indicates that some of the scotland beds previously dated by paleontologic methods as eocene may actually be as young as late oligocene possible source areas include the lesser antilles arc the netherlands venezuelan antilles arc and the caribbean mountains of venezuela the 200 350 ma population may reflect partially annealed cratonic material an andean component and or material associated with a triassic rifting event the oldest zircons 500 ma and metamict zircons were likely derived from the south american craton detrital feldspars separated from sandstone at the base of chalky mount were analyzed by the 40ar 39ar age spectrum technique results yield what is interpreted to be a slow cooling gradient from 1350 ma to 925 ma and provides additional evidence of a cratonic source for the accreted sediments based on results from this study and paleogeographical constraints it is proposed that source areas for the scotland sandstones of barbados may have included the guayana shield central cordillera uplifted areas of coastal south america and the lesser antilles volcanic arc we speculate that in late oligocene time an immense proto orinoco river system flowed in a northeasterly direction from the shield through the northern venezuelan mountain system cordillera de la costa and araya paria finally emerging into the caribbean sea in the area now occupied by unare depression sediments comprising this deltaic complex and deep sea fan were either deposited directly into the trench or onto the sea floor where they were soon after caught up in the accretionary wedge of the lesser antilles arc system this article contains supplementary material

was the laramide orogeny related to subduction of an oceanic plateau numerous models have been presented to explain the late cretaceous early cenozoic laramide orogeny which affected the foreland region of the western cordillera within the us1 the most attractive models invoke low angle subduction2 4 which can develop for various reasons5 6 evidence from south america indicates that one such reason may be the subduction of buoyant ocean floor we here extend the low angle subduction models by attributing the shallowing of the subduction angle during the laramide orogeny to subduction of a large oceanic plateau of anomalously thick and buoyant oceanic crust pacific plate history indicates that a twin to the hess rise may represent the oceanic plateau that triggered laramide events

the tectonic emplacement of sumba in the sunda banda arc paleomagnetic and geochemical evidence from the early miocene jawila volcanics the island of sumba is a continental fragment in the fore arc region near the transition between the sunda arc and banda arc in southeastern indonesia paleomagnetic and geochemical evidence from the early miocene volcanics of the jawila formation in western sumba constrain the final drift stage and tectonic emplacement of the island the lavas range from predominantly andesites to dacites and display textural evidence for a weak metamorphism rock magnetic and mineral chemical data point to pseudo single to multi domain titanojmagnetite fe ti o with grain sizes up to 10 pm as the main carrier of remanence the jawila formation reveals a chrm direction with declination 4 6 inclination 19 2 c 9 9 and a paleolatitude of 9 9 s which corroborates earlier results chamalaun and sunata 1982 taking paleomagnetic evidence from other formations on the island into account we conclude that the sumba fragment has occupied approximately its present position since the miocene the talc alkaline affinity and trace element signatures of the lavas point to an origin in an arc environment this occurrence of subduction related volcanic activity in the early miocene on sumba implies that a volcanic arc existed south of the present day east sunda arc or that the island was located within the latter arc between sumbawa and eastern flores and still had a minor southward drift to cover

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

in search of the latest danian event in a paleobathymetric transect off kasserine island north central tunisia danian lower paleocene sequences in north central tunisia are dominantly composed of marls and shales but a conspicuous indurated glauconite bearing marker bed is associated with the p3a p3b transition this glauconite bed is considered to correlate with the latest danian event lde described from the nile basin in egypt with the top chron c27n event atlantic and pacific oceans and with the cie ds1 zumaia spain the lde is thought to reflect a short period of global warming similar to the paleocene eocene thermal maximum but of lesser magnitude the presence of a glauconitic bed at the p3a p3b transition in tunisia suggests that the sequence is condensed which is confirmed by planktic foraminifer and nannofossil biostratigraphy and by the absence of the typical lde beds found in most egyptian sections benthic foraminiferal assemblages were quantitatively analyzed in three sections in north central tunisia sidi nasseur garn halfaya elles in order to characterize the paleoenvironmental evolution during the late danian and compare this with sections in egypt the benthic foraminiferal assemblages indicate that the tunisian sections were located along a depth and a paleoenvironmental gradient on the shelf north of kasserine island the reconstructed depth range is comparable with sections in egypt and encompasses middle neritic sidi nasseur section to shallow upper bathyal depths elles section with up to 16 gavelinella beccariiformis below the p3a p3b like in egypt assemblage shifts across the p3a p3b subzonal transition indicate shallowing and a transition to a more eutrophic paleoenvironment characterized by relatively high abundances of buliminids and stainforthia sp the 13c and 18o records generated on well preserved specimens of the ostracode species bairdia failed to demonstrate the presence of the carbon isotope excursion cie associated with the lde at the base of subzone p3b a condensed section or a hiatus associated with the glauconite bed would explain why the cie is not recorded in the tunisian sections

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

nature and distribution of deformation across the banda arc australian collision zone at timor recently acquired seismic reflection and seamarc ii side scan and swath bathymetry profiles near timor show that the banda arc australia collision zone has a tectonic framework similar to that of a typical oceanic subduction system deformation is occurring at present most intensely at the foot of the inner slope of the timor trough this deformation front is discontinuously advancing southward as new thrust slices develop within the subducted australian margin strata in contrast present deformation is apparently negligible in the savu basin the complex fore arc basin north of timor a possible significant exception is a postulated right lateral northeast trending fault zone offsetting the outer arc high between savu and roti although back arc thrusting has been documented north of the volcanic arc this component of convergence is minor compared with the scale of ongoing deformation in the timor trough the detailed nature of these surveys has also led to the recognition of along strike variations in deformation in the timor trough and in the savu basin these variations may be related to the variable degree of involvement of the australian continental margin along the arc

new constraints on the timing of flexural deformation along the northern australian margin implications for arc continent collision and the development of the timor trough abstract numerous extensional faults offset the passive margin strata of the northern bonaparte basin this extensional deformation has been attributed to lithospheric flexure of the descending australian plate in an overall convergence setting here we use an extensive 2d and 3d seismic dataset calibrated with well biostratigraphy and strontium sr isotope age data to constrain the timing of deformation along the northern australian margin during the neogene analysis of fault throw and differential thickness variations give new insights on the propagation and slip history of the faults along dip throw profiles exhibit d shape distributions skewed towards the top positive throw gradients above the throw maxima coinciding with intervals of growth strata indicate multiphase fault activity results indicate that post rift extensional deformation initiated during the latest miocene ca 6 ma the development of the modern timor trough as a foreland basin and cartier trough also commenced during this period a second episode of increased tectonic activity occurred around the pliocene quaternary boundary ca 3 ma and the deformation continued intermittently to the present day these new results are in agreement with the timing of initiation of collision between the australian plate and the banda arc and uplift of the timor island recently derived from stratigraphic analysis in timor these regional tectonic events have profoundly affected the paleogeography of the timor sea and may explain major changes in oceanic circulation and climate during the neogene

lower plate structure and upper plate deformational segmentation at the sunda banda arc transition indonesia the sunda banda arc transition at the eastern termination of the sunda margin indonesia represents a unique natural laboratory to study the effects of lower plate variability on upper plate deformational segmentation neighboring margin segments display a high degree of structural diversity of the incoming plate transition from an oceanic to a continental lower plate presence absence of an oceanic plateau variability of subducting seafloor morphology as well as a wide range of corresponding fore arc structures including a large sedimentary basin and an accretionary prism outer arc high of variable size and shape here we present results of a combined analysis of seismic wide angle refraction multichannel streamer and gravity data recorded in two trench normal corridors located offshore the islands of lombok 116 e and sumba 119 e on the incoming plate the results reveal a 8 6 9 0 km thick oceanic crust which is progressively faulted and altered when approaching the trench where upper mantle velocities are reduced to 7 5 km s the outer arc high located between the trench and the fore arc basin is characterized by sedimentary type velocities vp 5 5 km s down to the top of the subducting slab 13 km depth the oceanic slab can be traced over 70 100 km distance beneath the fore arc a shallow serpentinized mantle wedge at 16 km depth offshore lombok is absent offshore sumba where our models reveal the transition to the collisional regime farther to the east and to the sumba block in the north our results allow a detailed view into the complex structure of both the deeper and shallower portions of the eastern sunda margin

rifting of the northern margin of the australian continent and the origin of some microcontinents in eastern indonesia abstract continental australia is bounded on the east south and west sides by passive margins and the geological histories of these are well documented falvey and mutter 1981 the northern margin of the australian continent is now an active collision margin its previous history as a passive margin has rarely been examined this paper shows how the late palaeozoic to mesozoic sequence which forms the northern margin of the australian continent in the island of new guinea is readily related to the tectonic stages of a rift drift sequence rifting start of breakup began at about 230 m y ago at the permian triassic boundary the onset of seafloor spreading is marked by a post breakup unconformity and ranges in age along the northern margin of the continent from 185 m y in papua new guinea to 170 m y in irian jaya from there the age of the post breakup unconformity continues to young in a southwesterly direction along the western margin of the australian continent reflecting the opening of the indian ocean off western australia the timing of the onset of spreading in central papua new guinea is consistent with the timing of the initiation of spreading in the proto pacific ocean proposed by nur and ben avraham 1977 by the end of the jurassic the northern margin of the australian continent faced a seaway which linked the proto indian and proto pacific oceans this newly formed ocean was separated from the pre existing oceans of the neo tethys and panthalassa by a screen of continents or microcontinents the identity of this screen is discussed and it is suggested that part of it is preserved in the microcontinents of eastern indonesia

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

structure evolution and tectonic activity of the eastern sunda forearc indonesia from marine seismic investigations forearc structures of the eastern sunda arc are studied by new multichannel reflection seismic profiling we image a high along strike variability of the subducting oceanic plate the interface between subducting and overriding plate the accretionary wedge the outer arc high and forearc basins we highlight ongoing tectonic activity of the entire outer arc high active out of sequence thrust faults connecting the plate interface with the seafloor slope basins showing tilted sedimentary sequences on the outer arc high vertical displacement of young seafloor sediments and tilted sedimentary sequences in the lombok forearc basin while frontal accretion plays a minor role the growth of the outer arc high is mainly attributed to oceanic sediments and crustal fragments which are attached to the base of the upper plate and recycled within the forearc we image ongoing large scale duplex formation of the oceanic crust the incoming oceanic crust is dissected by normal faulting into 5 10 km wide blocks within a 50 70 km wide belt seaward of the deep sea trench these blocks determine the geometry and evolution of duplexes attached to the base of the overriding plate landward of the trench long lasting and ongoing subsidence of the lombok basin is documented by distinct seismic sequences in the lombok basin we image mud diapirs fed from deeply buried sediments which may have been mobilized by rising fluids we propose a wrench fault system in the eastern lombok forearc basin that decouples the subduction regime of the sunda arc from the continent island arc collision regime of the western banda arc the observed tectonic activity of the entire forearc system reflects a high earthquake and tsunami hazard similar to the western part of the sunda arc

reinterpretation of the ordovician rotations in nw argentina and northern chile a consequence of the precordillera collision early paleozoic paleomagnetic data from nw argentina and northern chile have shown large systematic rotations within two domains one composed of the western puna that yields very large up to 80 counter clockwise rotations and the other formed by the famatina ranges and the eastern puna that shows 40 clockwise rotations around vertical axes in several locations lack of significant rotations in younger rocks constrains this kinematic pattern to have occurred during the paleozoic previous tectonic models have explained these rotations as indicative of rigid body rotations of large para autochthonous crustal blocks or terranes a different but simple tectonic model that accounts for this pattern is presented in which rotations are associated to crustal shortening and tectonic escape due to the collision of the allochthonous terrane of precordillera in the late ordovician this collision should have generated dextral shear zones in the back arc region of the convergent sw gondwana margin where systematic domino like clockwise rotations of small crustal blocks accommodate crustal shortening the western puna block bordering the precordillera terrane to the north might have rotated counterclockwise as an independent microplate due to tectonic escape processes in a fashion similar to the present day relationship between the anatolia block and the arabian microplate

the java convergent margin structure seismogenesis and subduction processes the java margin is characterized by a distinct variation in lower to upper plate material transfer and recurring catastrophic tsunamogenic earthquakes both processes are closely linked to the subduction of oceanic basement relief resulting in varying degrees of fore arc deformation tomographic models of refraction seismic profiles and reflection seismic lines in combination with high resolution multibeam bathymetric data reveal the variability in the deep structure and deformation of the java fore arc shallow subduction processes are governed by the sediment supply in the trench as well as by the nature and fabric of the oceanic lithosphere the deep structure of the fore arc reveals a shallow upper plate crust mantle transition present along the entire java margin section the serpentinized fore arc mantle wedge governs the depth extent of the seismogenic zone here which is narrower compared to its sumatran analogue in addition offshore central java high relief oceanic basement features potentially act as asperities as well as barriers to seismic rupture limiting the possible magnitude of subduction thrust earthquakes however the potential for geohazards in particular tsunamis is high along the entire margin this results from tsunamogenic earthquakes ubiquitous splay faults and potentially tsunamogenic landslides which further increase the risk of future tsunamis

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

late cenozoic tectonic development of the southeast asian continental margin in the banda sea area abstract the late cenozoic tectonic development of the southeast asian continental margin around the banda sea is complicated by interaction with external geological elements southeast asian internal elements are the banda volcanic arc and previous older arcs external elements are the australian continental crust and the indian and pacific oceanic crusts these external elements are now trapped behind the banda volcanic arc three main geological events are responsible for the present configuration of the banda sea and adjacent areas 1 1 collision between the banda volcanic arc and australian continental crust 2 2 emplacement of the banda sea oceanic crust and 3 3 emplacement of microcontinents now present in and around the banda sea the geology of timor is fundamental to tectonic interpretations of the collision between the banda volcanic arc and australia differences exist in the tectonic interpretation of timor and include either overthrusting upthrusting or melange formation the present paper follows geological data contained in geological maps published by the geological survey of indonesia in which overthrust structures are clearly shown paleomagnetic and paleontologic data indicate that the overthrust units were from southern paleolatitudes depth and magnetic lineation data support the interpretation that the banda sea is underlain by old oceanic crust similar orientation of magnetic lineations in the banda sea and the argo abyssal plain suggests that they have the same origin geological data from northern banda microcontinents dredged samples from the banda lucipara ridges and comprehensive geological development of the northern banda arc support the interpretation that the microcontinents were translated left laterally westward from irian jaya

seismic history and seismotectonics of the sunda arc historic records of the last 300 years reveal two great interplate earthquakes 1833 mw 8 1861 mw 8 8 which ruptured major segments of the sumatra fore arc in western indonesia a significant percentage of interplate slip along this portion of the plate boundary appears to occur seismically the ends of these rupture zones are coincident with clusters of large ms 7 and moderate 6 ms 7 shocks and with heterogeneities in the plate interface as inferred from geologic and geophysical data the northern extent of the 1833 rupture zone is coincident with a group of moderate and large earthquakes a structural arch in the fore arc and a fracture zone on the subducted plate the southern portion is coincident with a very prominent cluster and the along strike extension of a sediment filled trough on the subducted plate prior to this study sumatra was characterized as relatively aseismic as inferred from the lack of great earthquakes in the instrumental record of this century java and the lesser sunda islands had major ms 6 earthquakes in the historic record but none are of the same magnitude as the great events near sumatra there is no clear evidence that shocks in the java fore arc were interplate events yet geophysical data indicate recent uplift of the trench slope break and a change in strike of the trench axis where the northernmost flank of the roo rise a prominent bathymetrie high appears to be interacting with the fore arc this feature was formed at a ridge crest and is more buoyant and thus more difficult to subduct than the surrounding seafloor there is no increase in seismicity associated with this tectonism however that may indicate a lack of contact between the crystalline portions of the plates at this time we infer that a majority of slip on the plate interface near java occurs aseismically the variation in occurrence of great interplate earthquakes along the sunda arc may be interpreted in terms of a model that attributes variation in interplate coupling to the age of the subducted lithosphere great interplate earthquakes occur near sumatra where the age of the youngest crust is 46 ma the characteristics of the sunda arc and analogies with pacific arcs imply that the entire length of sumatra has the potential to produce great thrust earthquakes its seismic potential should be considered high as there has been no recurrence of the great events of 1833 and 1861 in contrast no such events have been reported off java and the lesser sunda islands where the oldest crust is 152 ma the plate interface near java and the lesser sunda islands should be considered to have low seismic potential

the evolution of sumba island indonesia revisited in the light of new data on the geochronology and geochemistry of the magmatic rocks abstract the island of sumba presently located in the southern row of islands of the eastern nusa tenggara province of eastern indonesia has a unique position being part of the sunda banda magmatic arc and subduction system it represents a continental crustal fragment located at the boundary between the sunda oceanic subduction system and the australian arc continent collision system separating the savu basin from the lombok basin new data on magmatic rocks collected from sumba are presented in this paper including bulk rock major and trace element chemistry petrography and whole rock and mineral 40k 40ar ages three distinct calc alkaline magmatic episodes have been recorded during cretaceous paleogene all of them characterized by similar rock assemblages i e pyroclastic rocks basaltic andesitic lava flows and granodioritic intrusions they are i the santonian campanian episode 86 77 ma represented by volcanic and plutonic rock exposures in the masu complex in eastern sumba ii the maastrichtian thanetian episode 71 56 ma represented by the volcanic and plutonic units of sendikari bay tengairi bay and the tanadaro complex in central sumba and iii the lutetian rupelian episode 42 31 ma of which the products are exposed at lamboya and jawila in the western part of sumba no neogene magmatic activity has been recorded

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

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

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

the continental crust and its mineral deposits proceedings of a symposium held in honour of j tuzo wilson held at toronto may 1979 d w strangway editor geological association of canada special publication paper 20 the geological association of canada dept earth sciences univ of waterloo waterloo ont 1980 viii 804 pp no price is given

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