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

observed volume fluxes and mixing in the dardanelles strait 1 pairs of moorings equipped with current profilers were deployed at each end of the dardanelles strait and remained in place for over 13 months current observations were able to resolve well the exchange flow and volume fluxes volume fluxes showed distinct temporal variability in upper and lower layers especially evident on synoptic time scales the synoptic flux variability in the upper layer was coherent with the local atmospheric forcing and the bottom pressure anomaly gradient while the flux variations in the lower layer were related to the bottom pressure anomaly gradient estimated volume flux values were often two or more times larger than their respective annual means annual upper layer flux means were 25 66 10 3 and 36 68 10 3 sv whereas the lower layer averages were 14 02 10 3 and 31 67 10 3 sv for the marmara and aegean sections respectively the fluxes also showed that there was a net low salinity water outflow to the aegean sea and that they varied weakly on longer time scales monthly to seasonal high salinity water fluxes 39 psu were used to calculate strait averaged vertical eddy diffusivities which ranged between 10 4 and 10 2 m2 s 1 additionally microstructure observations were used to evaluate vertical eddy diffusivities these estimates indicated that mixing in the strait varied spatially and temporarily and it was dependent on complex strait geometry exchange flow status and partially on meteorological conditions large values of eddy diffusivities with a depth averaged mean of 1 3 10 2 m2 s 1 and vigorous mixing were found in the nara pass the narrowest section in the dardanelles strait

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

seismotectonics of the central segment of the indonesian arc abstract in this paper a revision of seismicity affecting the central segment of the indonesian island arc ranging between 110 and 126 e is presented using the areal and in depth distribution of seismic activity strain release maps and focal mechanisms lateral changes in the wadati benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries the seismicity pattern indicates that the sumbawa flores wetar sector shows different forms of behaviour with respect to the adjacent sectors these include driving mechanism inclination and continuity of the subducting slab and subduction features this area therefore seems to be isolated from the sunda and banda arcs by two principal boundaries these having a nearly n s trend in the bali region and with a nearly e w trend in the region ranging between wetar northern timor and tanibar the first boundary characterized by an absolute minimum of seismic activity at all ranges of depth has been interpreted in terms of subduction of the roo rise aseismic bathymetric ridge for the second boundary dividing a northwardly steeply dipping slab from an e w subducting slab dipping with an angle of 30 40 a tear in the upper part depth less than 300 km and a hinge fault system in the deepest part of the lithosphere have been proposed from the analysis of focal mechanisms of shallow earthquakes it was inferred that the central part of the indonesian arc is subject to a vortex shaped stress field centred on the savu basin in this model the compressive axes appear to rotate counterclockwise from sw to nne in the sumba sumbawa western flores region and clockwise from w to nnw in the timor eastern flores zone to interpret these features on the basis of seismological evidence a lateral discontinuity in the arc trench system close to sumba a collision between sumba and sumbawa and a rotation towards the north northeast of sumba have been suggested the proposed structural discontinuity trending nw se may represent a major transcurrent fault zone through which the australian continental lithosphere comes into contact with the indian oceanic lithosphere

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

short wavelength high amplitude gravity anomalies around the banda sea and the collapse of the sulawesi orogen abstract in eastern indonesia high density ophiolitic rocks outcropping on islands surrounding the banda sea are in many cases associated with strong gravity anomalies and steep gravity gradients however the relationships are not always straightforward bouguer gravity levels and gradients over the extensive east sulawesi ophiolite are generally relatively low although short wavelength high amplitude anomalies indicate rapid changes in thickness of high density rocks in a few places in the banda arc most local positive anomalies due to ophiolites are superimposed on a steep regional gravity gradient but in one case in western seram there is a distinct and important spatial separation between the two on buru west of seram a gradient of more than 10 mgal km testifies to the presence of very dense rocks in the near subsurface despite the absence of ophiolites in the outcrop gravity variations and ophiolite distribution around the banda sea are compatible with extension having occurred in the sulawesi region following and as a result of oligo miocene collision with an australian derived microcontinent similar histories have been proposed for many mediterranean deep basins of similar size shape and character and emplacement of some of the high density masses in the banda arc has probably resembled at least the later stages in the emplacement of peridotite massifs in the rif betic belt in both areas the present close association of the ultramafic rocks and their associated local anomalies with a strong regional gravity gradient is largely coincidental

quaternary raised coral reef terraces on sumba island indonesia a spectacular sequence of coral reef terraces six steps broader than 500 meters and many minor substeps is developed near cape laundi sumba island between an ancient patch reef 475 meters high and sea level several raised reefs have been dated with the electron spin resonance and the uranium series dating methods the uplift trend deduced from these reefs is 0 5 millimeter per year most terraces although polycyclic in origin appear to correspond to specific interglacial stages with the oldest terrace formed 1 million years ago this puts them among the longest and most complete mid quaternary terrace sequences

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

geochemical and tectonic relationships in the east indonesian arc continent collision region implications for the subduction of the australian passive margin abstract van bergen m j vroon p z and hoogewerff j a 1993 geochemical and tectonic relationships in the east indonesian arc continent collision region implications for the subduction of the australian passive margin in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 97 116 variations in the isotopic signatures of volcanics along the east sunda banda arc reflect changes in the nature and amount of sedimentary material supplied by the northeast indian ocean floor and the adjacent australian passive continental margin which form the two major domains of the indian ocean plate that approach the arc system a compilation of isotopic data for 200 500 km long arc sectors shows that the trend in magmatic signatures follows distinct subduction collision stages reached by the corresponding oceanic and continental margin sections entering the trench system maximum amounts of magma source contamination are inferred for volcanics near an extinct sector north of timor where the australian continent started to collide with the arc first pb nd isotopic source mixing models point to contamination by sediments with variations in composition similar to observed along arc changes in sediments entering the trench the results indicate an increasing contribution of subducted continental material in the direction of the collision region mass balance calculations considering the magmatic output and minimum input of subducted continental material required to generate the composition of the volcanic arc in the collision region are difficult to reconcile with subduction of ocean floor sediments alone thicknesses of sediments presently covering oceanic crust near the margin are close to calculated thicknesses of the sediments fluxed into the trench and magmatically returned to the arc crust but cannot account for the additional volumes of material accreted on the overriding plate in the same period of time it is inferred that leading portions of the australian continental margin have reached magma generation zones in the easternmost sunda arc and western banda arc which implies subduction to depths greater than 100 km

rise and fall of the eastern great indonesian arc recorded by the assembly dispersion and accretion of the banda terrane timor abstract new age petrochemical and structural data indicate that the banda terrane is a remnant of a jurassic to eocene arc trench system that formed the eastern part of the great indonesian arc the arc system rifted apart during eocene to miocene supra subduction zone sea floor spreading which dispersed ridges of banda terrane embedded in young oceanic crust as far south as sumba and timor in timor the banda terrane is well exposed as high level thrust sheets that were detached from the edge of the banda sea upper plate and uplifted by collision with the passive margin of nw australia the thrust sheets contain a distinctive assemblage of medium grade metamorphic rocks overlain by cretaceous to miocene forearc basin deposits new u pb age data presented here indicate igneous zircons are less than 162 ma with a cluster of ages at 83 ma and 35 ma 40ar 39ar plateau ages of various mineral phases from metamorphic units all cluster at between 32 38 ma these data yield a cooling curve that shows exhumation from around 550 c to the surface between 36 28 ma after this time there is no evidence of metamorphism of the banda terrane including its accretion to the edge of the australian continental margin during the pliocene these data link the banda terrane to similar rocks and events documented throughout the eastern edge of the sunda shelf and the banda sea floor

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

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

melange versus forearc contributions to sedimentation and uplift during rapid denudation of a young banda forearc continent collisional belt abstract new sedimentary geochemistry and petrographic analyses provide the most extensive sedimentary documentation yet of the rapid denudation of the young timor orogen the data from three basins including two widely separated well dated sections of the synorogenic megasequence of timor leste and a re dated dsdp 262 constrain the source and timing of detrital sediment flux during forearc continent collision along the timor sector of the banda arc the exhumed synorogenic piggy back basins formed above a melange unit that developed at the expense of a weak stratigraphic horizon in the mesozoic stratigraphy and was exhumed to the sea floor in latest messinian time following an interval of deep marine chalky marl sedimentation an increasingly muddy sediment flux indicates that the island of timor became emergent and shed sediment by 4 5 ma comparison of exhumed sections with similar patterns in the dsdp262 chemistry suggests that the sediment source was probably located some 50 60 km distant from the basin which is consistent with the aileu region of timor leste that shows an appropriate exhumation history all sedimentation between 4 5 and 3 2 ma was probably derived from a low relief rapidly eroding and mudstone dominated landscape with geochemical affinities to the triassic mudstone derived synorogenic melange the melange unit overlies and surrounds the banda terrane and was presumably structurally emplaced by propagation of a decollement through the triassic rocks during the collision after 3 2 ma sedimentation was dominated by hard rock lithologies of the banda terrane consisting of forearc cover and basement the latter including elements of metamafic rocks and metapelites this phase of sedimentation was accompanied by rapid uplift which may have been partly driven by a transient imbalance between rock uplift and denudation as resistant lithologies emerged from below melange like mudstone previous work has suggested that the timing of collision in timor leste and west timor was substantially different our reevaluation of dsdp 262 facies migration history in the context of the re dating presented here favours a relatively synchronous onset of uplift in both halves of the island but with different partitioning of strain between the foreland and hinterland in each half of the island

tertiary evolution of the eastern indonesia collision complex abstract eastern indonesia is the zone of interaction between three converging megaplates eurasia the pacific and indo australia the geological basis for interpretations of the tertiary tectonic evolution of eastern indonesia is reviewed and a series of plate tectonic reconstructions for this region at 5 million year intervals covering the last 35 million years is presented the oldest reconstruction predates the onset of regional collisional deformation at this time a simple plate configuration is interpreted consisting of the northward moving australian continent approaching an approximately e w oriented southward facing subduction zone extending from the southern margin of the eurasian continent eastwards into the pacific oceanic domain beginning at about 30 ma the australian continental margin commenced collision with the subduction zone along its entire palinspastically restored northern margin from sulawesi in the west to papua new guinea in the east from this time until ca 24 ma the australian continent indented the former arc trend with the northward convergence of australia absorbed at the palaeo northern boundary of the philippine sea plate the present day palau kyushu ridge at ca 24 ma the present day pattern of oblique convergence between the northern margin of australia and the philippine sea plate began to develop at about this time a large portion of the palaeogene colliding volcanic arc the future eastern philippines began to detach from the northern continental margin by left lateral strike slip from ca 18 ma oblique southward directed subduction commenced at the maramuni arc in northern new guinea at ca 12 ma the sorong fault zone strike slip system developed effectively separating the philippines from the indonesian tectonic domain the sorong fault zone became inactive at ca 6 ma since which time the tectonics of eastern indonesia has been dominated by the anticlockwise rotation of the bird s head structural block by some 30 40 contemporaneously with post 18 ma tectonism the banda arc subduction collision system developed off the northwestern margin of the australian continent convergence between indo australia and eurasia was accommodated initially by northward subduction of the indian ocean and subsequently since ca 8 ma by the development of a second phase of arc continent collision around the former passive continental margin of nw australia

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

seismic stratigraphy and cenozoic evolution of the lombok forearc basin eastern sunda arc abstract on the basis of single and multi channel seismic reflection data the stratigraphic framework and geologic history of the lombok forearc basin is discussed the lombok basin is built on the southeast sunda shelf continental margin and probably underlain by basement of thinned rifted continental crust it represents a broad ridged residual forearc basin which is still in an early stage of evolution five seismo stratigraphic sequences reflecting the cenozoic history and development of the forearc basin south of the islands of bali and lombok were identified the lowermost sequence 1 probably is composed of synrift deposits of paleogene age which predate the initiation of the present convergent margin sequences 2 and 3 reflect two major phases of tectonic evolution of the accretionary prism between the late oligocene and mid miocene sequences 4 and 5 represent slope front fill deposits and reflect both volcanic activity as well as tectonic uplift of the magmatic arc from mid miocene times onwards by the late miocene increased convergence between the subducting indian and overlying asian plates resulted in a stronger mechanical coupling between both plates this was expressed in the southern forearc basin by folding of the oldest basin fill deposits the present tectonic activity in the lombok basin is governed by the late pliocene collision of the accretionary prism with the scott marginal and roo rise oceanic plateaus this resulted in uplift of both the outer arc ridge and the southern part of the forearc basement

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

postcollision extension in arc continent collision zones eastern indonesia postcollisional extension superimposed on arc continent collision complexes is a common feature in the orogenic belts of eastern indonesia in the southern banda arc the arc continent collision event is essentially a pliocene and younger feature but already an element of extension is being superimposed on the compressional structures of the collision complex it is thus likely that the extension commences within a very short period 5 m y after the initiation of collision related compressional deformation a similar history of fold belt development immediately followed by extension can be inferred for the lengguru fold belt and the adjacent wandamen wondiwoi terranes of irian jaya and for the gulf of bone in sulawesi it is suggested that the extension results from decoupling of the subducting oceanic lithosphere from the unsubductable continental lithosphere and that the superimposition of extension is a virtually unavoidable consequence of the arc continent collision

grain size dependence of initial susceptibility and saturation magnetization related parameters of four natural magnetites in the psd md range summary initial susceptibility coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain size fractions of a natural magnetite and titanomagnetite both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite both of which show exsolution phenomena it was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic fe rich inclusions these inclusions are magnetically screened in coarse grains only for the lower grain sizes studied they have an appreciable influence on the data the size of the fractions varied from 5 to 250 m the parameters and several parameter ratios are plotted as a function of grain size several of these parameters can be used as mineralogical and grain size indicators the varying degree of exsolution of the minerals is reflected in the gradients of the plots from plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size variation of the various parameters with grain size is explained in terms of a gradual transition from large psd grains to md grains without there being any sharp transition for titanomagnetite x 0 55 the transition takes place at grain sizes that are about 15 m larger than for magnetite

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

paratethys pacing of the messinian salinity crisis low salinity waters contributing to gypsum precipitation during the so called messinian salinity crisis msc 5 97 5 33 myr ago reduced exchange with the atlantic ocean caused the mediterranean to develop into a saline giant wherein 1 million km3 of evaporites gypsum and halite were deposited despite decades of research it is still poorly understood exactly how and where in the water column these evaporites formed gypsum formation commonly requires enhanced dry conditions evaporation exceeding precipitation but recent studies also suggested major freshwater inputs into the mediterranean during msc gypsum formation here we use strontium isotope ratios of ostracods to show that low saline water from the paratethys seas actually contributed to the precipitation of mediterranean evaporites this apparent paradox urges for an alternative mechanism underlying gypsum precipitation we propose that paratethys inflow would enhance stratification in the mediterranean and result in a low salinity surface water layer with high ca cl and so4 cl ratios we show that evaporation of this surface water can become saturated in gypsum at a salinity of 40 in line with salinities reported from fluid inclusions in msc evaporites

mapping magma sources in the east sunda banda arcs indonesia constraints from helium isotopes abstract we report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from flores in the east sunda arc through the inactive segment between the arcs to banda island at the extreme of the contiguous banda arc in the east sunda arc 3 he 4 he ratios r vary between 4 5 ra r a air 3 he 4 he for the leucitic batu tara volcano to a remarkable low of pure radiogenic helium 0 0075 ra for werung at the southern tip of lomblen island lavas from the inactive zone which represents the locus of collision of the australian continent with the arc have a narrower range in r r a from 3 9 for kisu in the straits of pantar to 1 0 for romang island our one locality banda island in the banda arc gives the highest r r a ratio 3 1 observed along this arc to date the results are consistent with the involvement of crustal material in magma genesis throughout the east sunda banda arcs as far west as iya in central flores we combine these helium isotope results with published and on going strontium isotope studies and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the australian continent rather degassing of australian continental crust appears to be the dominant process controlling the helium budget the he sr isotope systematics also provide a framework to account for the areal pattern of 3 he 4 he in this part of the arcs the radiogenic crustal component is diluted with mantle helium both in a down dip direction and with increasing lateral distance from the collision zone these factors result in an excellent first order relationship between the 3 he 4 he ratio degree of he sr enrichment relative to the postulated mantle endmember and alkalinity of the erupted lavas such a relationship has a direct bearing on models of the tectonic evolution of the collision zone and on the observation that helium isotopes are decoupled from strontium and other geochemical signatures along the banda arc

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

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

comment on on the steric and mass induced contributions to the annual sea level variations in the mediterranean sea by david garcia et al 1 garcia et al 2006 hereafter referred to as gcdvg present an analysis of mass induced sea level variation slvmass of the mediterranean sea based on satellite altimetry ocean modeling and gravity recovery and climate experiment grace gravity measurements in particular gcdvg find consistency between altimeter sea level variations corrected for steric effects and changes in oceanic mass derived from gravity recovery and climate experiment grace gcdvg show that the annual cycle of the sea level variability slvtotal is dominated by changes in its steric part slvsteric and that the basin average of mass slvmass is maximum in february they conclude that when the sea level is rising falling the mediterranean sea is actually losing gaining mass 2 gcdvg appear to use an unrealistically large estimate of steric sea level variability slvsteric and neglect changes in continental water storage that can significantly affect grace measurements over the mediterranean sea these issues are examined in turn below by reviewing relevant individual basin averages of the mediterranean sea in comparison to results of other studies in particular while also finding consistency between altimetry and grace measurements fenoglio marc et al 2006 hereafter referred to as fkb in a similar study of the mediterranean sea find steric effects only accounting for about 60 of the total annual sea level variability and the annual cycle of slvmass having a maximum in november

unraveling the roles of asymmetric uplift normal faulting and groundwater flow to drainage rearrangement in an emerging karstic landscape landscape adjustment to tectonic lithologic and climatic forcing leads to drainage reorganization and migration of divides the respective contribution of these forcings especially on carbonate landscapes is not well defined here we have addressed this issue by combining field observations satellite image interpretation and dem quantitative analysis to assess drainage response to spatially heterogeneous rainfall asymmetric uplift and normal faulting on an emerging carbonated platform sumba island indonesia we map geomorphic markers of fluvial dynamics and drainage rearrangement and compute a parameter that incorporates the contributions of unevenly distributed precipitation and asymmetric uplift to estimate erosional disequilibrium across drainage divides we find that asymmetric emergence of sumba island created an initial parallel drainage asymmetric across a divide that propagates landwards soon after establishing itself on the emerging slopes this drainage was disturbed by normal faulting which has become the main force driving drainage rearrangement vertical offsets across normal fault scarps first triggered aggradation within valleys over the hanging walls and then disconnected upstream reaches from downstream reaches leading to the formation of wind gaps atop the fault scarps and upstream perched sedimentary basins the defeat of rivers by growing fault scarps was catalysed by the possibility for surface water to be rerouted near the fault scarps into underground water networks inside the underlying carbonates at the end of the process the opposite drainage across the main water divide captured the struggling drainage capture mechanisms include initial groundwater capture of the perched alluvial aquifers followed by ground sapping at the head of the opposite drainage and surface stream diversion by avulsion finally normal faulting is the main driving force of drainage rearrangement allowing avulsion and karstic rerouting whereas asymmetric uplift and climate forcings have shown a low efficiency the role of karstification is more ambiguous catalyzing or inhibiting drainage rearrangement

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

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

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