upper miocene to lower pliocene sigaboy formation turbidites on the pujada peninsula mindanao philippines internal structures composition depositional elements and reservoir characteristics abstract turbidites of the upper miocene to lower pliocene sigaboy formation on the pujada peninsula southeastern mindanao philippines potentially provide a superb record how a forearc basin may fill up investigation of the basin sequences allows the provenance of the material to be established and hence the data are useful for tectonic models of the sub region in addition information obtained from the basin may give insights that are useful for petroleum exploration which mindanao is an important area the sigaboy formation is exposed for 45 km along the western coast of pujada peninsula the unit directly overlies various levels within the pujada ophiolite various facies within the formation identified from the detailed stratigraphic logs of six major outcrops represent channelized and unchannelized deposits of submarine fans that were deposited largely by turbidity currents field and petrographic evidence suggest that the portions of the sangihe arc and its uplifted outer arc ridge now forming the pujada peninsula provided the main source of detritus that was shed into the forearc basin this event probably followed the initial subduction of the snellius plateau halmahera arc in mindanao latest middle miocene and prior to the initiation of the philippine trench in the region

evidence of active backthrusting at the ne margin of mentawai islands sw sumatra summary the indo australian plate subducts obliquely beneath the sunda plate leading to a slip partitioning into pure thrust and strike slip motion just in the last 5 yr three pure thrust earthquakes of mw 8 4 have occurred along this subduction interface the great sumatra fault traversing the sumatra continental block takes up a significant part of the strike slip motion but the mentawai fault bounding the ne margin of mentawai islands has been suggested to accommodate a part of the strike slip motion although the great sumatra fault is active no seismicity has been observed along the mentawai fault using a combination of high resolution seismic reflection and bathymetry data here we show that the mentawai fault seems to be characterized by active sw dipping backthrusts the presence of recent steeply dipping thrust earthquakes suggests that these faults should be active combined with results from north in 2004 earthquake region and south of this study area our results suggest that backthrusting should play an important role in forearc evolution sw of sumatra we also observed several mass wasting sites at ne margin of the mentawai islands which could be erosional features or landslides triggered by earthquake activities localized uplift along the steeply dipping backthrusts at the ne margin of mentawai islands in the fully locked region could pose serious seismic and tsunami risks to the sw coast of sumatra in the future

lake michigan water levels and the development of holocene beach ridge complexes at two rivers wisconsin stratigraphic geomorphic and radiocarbon evidence two beach ridge complexes formed at two rivers wisconsin on the west shore of lake michigan during the past 5700 yr auger drilling closely spaced vibracoring and subsurface geophysical profiling using ground penetrating radar along two east west transects across ridges and swales indicate that several facies of beach sediment are capped with dune sand 14c ages were obtained from basal peat deposits collected from swales between successive beach ridges associated lake levels were interpreted from the elevation of coarse foreshore deposits observed on the east basinward side of the next lakeward beach ridge 14c ages geomorphic relationships and stratigraphic interpretations indicate that progradation and water level drop followed a major transgression lake level fluctuated on a scale of 1 7 2 m during the past 3000 yr

mentawai fault zone off sumatra a new key to the geodynamics of western indonesia the geodynamic evolution of the western part of the sunda arc is controlled by the change from frontal subduction of the indo australian plate along java to oblique subduction along sumatra this obliquity gives rise to the sumatra fault zone that links the accretionary zone of the andaman sea to the sunda strait previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south consequently it has been proposed that the area between the subduction zone and the fault zone i e the sumatra sliver platelet was deformed an oceanographic cruise on the indonesian ship r v baruna jaya iii was designed to study this area seismic reflection data reveal the existence of a major strike slip fault parallel to the sumatra fault zone that we called the mentawai fault zone located in the fore arc area just east of the mentawai islands it is at least 600 km long thus the sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction

a guide to stratigraphical procedure the continuing development of stratigraphical concepts and the application of new methods has led the stratigraphy committee of the geological society of london to present a new guide to stratigraphical procedure some of the principal types of stratigraphy are outlined and guidelines and recommendations are given on procedure practice and terminological usage

a detrital heavy mineral viewpoint on sediment provenance and tropical weathering in se asia abstract understanding heavy mineral preservation is important for interpreting generation pathways provenance and geochemistry of sediments despite this many assumptions about heavy mineral stability are based on ancient strata and few studies consider modern sediments particularly those in tectonically active tropical areas such as se asia we report new heavy mineral data on 69 river sand samples from the malay peninsula and sumatra in which one aim was to find provenance indicators specific to these areas identifications were performed using optical microscopy and confirmed with sem eds in the malay peninsula heavy minerals record granitic and contact metamorphic provenance variable amounts of zircon tourmaline hornblende andalusite epidote monazite rutile and titanite and minor amounts of pyroxene apatite anatase garnet diaspore colourless spinel cassiterite and allanite are typical of this source area the composition of assemblages from sumatra indicates contributions from two major sources the modern volcanic arc i and the basement ii abundant pyroxene particularly hypersthene up to 70 is diagnostic of the volcanic arc source vesuvianite garnet andalusite tourmaline chrome spinel rutile anatase and corundum are present only in small amounts

tertiary stratigraphy and basin evolution southern sabah malaysian borneo abstract new mapping and dating of strata in the southern part of the central sabah basin in northern borneo has made it possible to revise the lithostratigraphy and chronostratigraphy of the area the recognition in the field of an early miocene regional unconformity which may be equivalent to the deep regional unconformity recognised offshore has allowed the development of a stratigraphic framework of groups and formations which correspond to stages in the sedimentary basin development of the area below the early miocene unconformity lies ophiolitic basement which is overlain by an accretionary complex of eocene age and a late paleogene deep water succession which formed in a fore arc basin the late paleogene deposits underwent syn depositional deformation including the development of extensive melanges all of which can be demonstrated to lie below the unconformity in this area some localised limestone deposition occurred during a period of uplift and erosion in the early miocene following which there was an influx of clastic sediments deposited in delta and pro deltaic environments in the middle miocene these deltaic to shallow marine deposits are now recognised as forming two coarsening upward successions mapped as the tanjong and kapilit formations the total thickness of these two formations in the central sabah basin amounts to 6000 m only half of the previous estimates although the total stratigraphic thickness of cenozoic clastic strata in sabah may be more than 20 000 m

plate kinematics origin and tectonic emplacement of supra subduction ophiolites in se asia abstract a unique feature of the circum pacific orogenic belts is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust in se asia a majority of the ophiolites appear to have originated at convergent margins and specifically in backarc or island arc settings which evolved either along the edge of the sunda eurasia and australian cratons or within the philippine sea plate these ophiolites were later accreted to continental margins during the tertiary because of fast relative plate velocities tectonic regimes at the active margins of these three plates also changed rapidly strain partitioning associated with oblique convergence caused arc trench systems to move further away from the locus of their accretion we distinguish relatively autochthonous ophiolites resulting from the shortening of marginal basins such as the present day south china sea or the coral sea and highly displaced ophiolites developed in oblique convergent margins where they were dismantled transported and locally severely sheared during final docking in peri cratonic mobile belts i e the philippine mobile belt we find a series of oceanic basins which have been slightly deformed and uplifted varying lithologies and geochemical compositions of tectonic units in these basins as well as their age discrepancies suggest important displacements along major wrench faults we have used plate tectonic reconstructions to restore the former backarc basins and island arcs characterized by known petro geochemical data to their original location and their former tectonic settings some of the ophiolites occurring in front of the sunda plate represent supra subduction zone basins formed along the australian craton margin during the mesozoic the philippine sea basin the huatung basin south of taiwan and composite ophiolitic basements of the philippines and halmahera may represent remnants of such marginal basins the portion of the philippine sea plate carrying the taiwan philippine arc and its composite ophiolitic continental crustal basement might have actually originated in a different setting closer to that of the papua new guinea ophiolite and then have been displaced rapidly as a result of shearing associated with fast oblique convergence

revisiting the structure age and evolution of the wharton basin to better understand subduction under indonesia understanding the subduction processes along the sunda trench requires detailed constraints on the subducting lithosphere we build a detailed tectonic map of the wharton basin based on reinterpretation of satellite derived gravity anomalies and marine magnetic anomalies the wharton basin is characterized by a fossil ridge dated 36 5 ma offset by n s fracture zones magnetic anomalies 18 to 34 38 84 ma are identified on both flanks although a large part of the basin has been subducted we analyze the past plate kinematic evolution of the wharton basin by two plate india australia and three plate india australia antarctica reconstructions despite the diffuse plate boundaries within the indo australian plate for the last 20 ma we obtain finite rotation parameters that we apply to reconstruct the subducted wharton basin and constrain the thickness buoyancy and rheology of the subducting plate the lower subductability of younger lithosphere off sumatra has important consequences on the morphology with a shallower trench forearc islands and a significant inward deviation of the subduction system this deviation decreases in the youngest area where the wharton fossil spreading center enters subduction the discontinuous magmatic crust and serpentinized upper mantle consequences of the slow spreading rates at which this area was formed weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism deeper effects include the possible creation of asthenospheric windows beneath the andaman sea in relation to the long offset fracture zones and east of 105 e as a result of subduction of the spreading center

re interpretation of the age and environment of deposition of paleogene turbidites in the andaman and nicobar islands western sunda arc abstract turbidites composed of sandstone shale alternations on the kalipur shibpur coast north andaman island classified under the palaeocene eocene mithakhari group in several recent papers are identified as and compared with the type section of the oligocene andaman flysch exposed at corbyn s cove south andaman island the kalipur turbidites were interpreted as the inner fan and the latter as the distal mid fan facies of a forearc submarine fan the turbidites of these two locations are separated by 250 km detailed studies concerning the identification and comparison have however revealed significant differences and the turbidite outcrops are not continuous between these two localities turbidites at kalipur shibpur and adjacent areas are part of a melange terrane normally gritty and coarse grained massive to locally graded bedded calcareous intercalated interstratified with conglomerates and reefoidal limestones and show a framework composition varying from volcanolithic to lithic poor arkosic sandstones deposited in several isolated basins fed by transverse supply of detritus from an accreted and uplifted ophiolite and arc massif the kalipur shibpur coast exposes marginally deformed sandstone dominated turbidites coherent units containing abundant ichnotraces late palaeocene foraminifera and rip up shale clasts indicating deposition in shallow water accretionary slope basins south of kalipur the ramnagar coast exposes mud rich turbidites showing accretion related deformation these deformed turbidites are interpreted as offscraped trench deposits further south on the rampur coast turbidites intercalated with reefoidal limestones containing late palaeocene foraminifera indicate deposition on the upper slope or on top of the accretionary slope basin in contrast classical bouma sequence bearing sandstone shale turbidites at corbyn s cove are part of a continuous outcrop belt of siliciclastic turbidites lack fossils carbonate facies and conglomerates and consist of compositionally uniform greywackes deposited in an open deep sea fan fed by axially transported detritus derived from the continental blocks of western burma these key differences together with previous mapping and stratigraphic studies confirm the incorrect identification of andaman flysch in north andaman island in recent papers the turbidites of these two locations were neither produced by the same sediment gravity flows nor deposited in the same forearc fan during the oligocene instead they were derived from different palaeographic domains deposited in different tectonic and sedimentary environments and also at different times this provides new insights into the paleogene turbidite deposition in this part of western sunda arc

collisional emplacement history of the naga andaman ophiolites and the position of the eastern indian suture abstract dismembered late mesozoic ophiolites occur in two parallel belts along the eastern margin of the indian plate the eastern belt closely following the magmatic arc of the central burma basin coincides with a zone of high gravity it is considered to mark a zone of steeply dipping mafic ultramafic rocks and continental metamorphic rocks which are the locus of two closely juxtaposed sutures in contrast the western belt which follows the eastern margin of the indo burma range and the andaman outer island arc broadly follows a zone of negative gravity anomalies here the ophiolites occur mainly as rootless subhorizontal bodies overlying eocene oligocene flyschoid sediments two sets of ophiolites that were accreted during the early cretaceous and mid eocene are juxtaposed in this belt these are inferred to be westward propagated nappes from the eastern belt emplaced during the late oligocene collision between the burmese and indo burma andaman microcontinents ophiolite occurrences in the andaman islands belong to the western belt and are generally interpreted as upthrust oceanic crust accreted due to prolonged subduction activity to the west of the island arc this phase of subduction began only in the late miocene and thus could not have produced the ophiolitic rocks which were accreted in the late early eocene

neotectonics of the sumatran fault indonesia the 1900 km long trench parallel sumatran fault accommodates a significant amount of the right lateral component of oblique convergence between the eurasian and indian australian plates from 10 n to 7 s our detailed map of the fault compiled from topographic maps and stereographic aerial photographs shows that unlike many other great strike slip faults the sumatran fault is highly segmented cross strike width of step overs between the 19 major subaerial segments is commonly many kilometers the influence of these step overs on historical seismic source dimensions suggests that the dimensions of future events will also be influenced by fault geometry geomorphic offsets along the fault range as high as 20 km and may represent the total offset across the fault if this is so other structures must have accommodated much of the dextral component of oblique convergence during the past few million years our analysis of stretching of the forearc region near the southern tip of sumatra constrains the combined dextral slip on the sumatran and mentawai faults to be no more than 100 km in the past few million years the shape and location of the sumatran fault and the active volcanic arc are highly correlated with the shape and character of the underlying subducting oceanic lithosphere nonetheless active volcanic centers of the sumatran volcanic arc have not influenced noticeably the geometry of the active sumatran fault on the basis of its geologic history and pattern of deformation we divide the sumatran plate margin into northern central and southern domains we support previous proposals that the geometry and character of the subducting investigator fracture zone are affecting the shape and evolution of the sumatran fault system within the central domain the southern domain is the most regular the sumatran fault there comprises six right stepping segments this pattern indicates that the overall trend of the fault deviates 4 clockwise from the slip vector between the two blocks it separates the regularity of this section and its association with the portion of the subduction zone that generated the giant m w 9 earthquake of 1833 suggest that a geometrically simple subducting slab results in both simple strike slip faulting and unusually large subduction earthquakes

tectonic features of the southern sumatra western java forearc of indonesia 1 multichannel reflection seismic profiles along the active sunda arc where the indo australian plate subducts under the overriding eurasian margin revealed two accretionary wedges the inner wedge i is of assumed paleogene age and the outer wedge ii is of neogene to recent age the inner wedge i is composed of tectonic flakes stretching from southeast sumatra across the sunda strait to northwest java implying a similar plate tectonic regime in these areas at the time of flake development during upper oligocene today wedge i forms the outer arc high and the backstop for the younger outer wedge ii the missing outer arc high of the southern sunda strait is explained by a combination of neogene transtension due to a clockwise rotation of sumatra with respect to java and by arc parallel strike slip movements the rotation created transtensional pull apart basins along the western sunda strait semangka graben as opposed to transpression and inversion on the eastern sunda strait within the new detected krakatau basin the arc parallel transpressional mentawai strike slip fault zone mfz was correlated from the sumatra forearc basin to the northwest java forearc basin off the sunda strait northward bending branches of the mfz are connected with the right lateral sumatra fault zone sfz along the volcanic arc segment on sumatra it is speculated that the sfz was attached to the cimandiri pelabuhan ratu strike slip fault of java prior to the presumed rotation of sumatra and that since the late lower miocene the main slip movement shifted from the volcanic arc position to the forearc basin area due to increasingly oblique plate convergence

some fundamental considerations in the dating of glauconies a comment on a test of the reliability of rbsr dates for selected glauconite morphologies of the upper cretaceous navesink formation of new jersey by r l montag and d e seidemann in an attempt to understand the geochemical behaviour of rb and sr in glauconies see below used for dating purpose montag and seidemann 1 stated several hypotheses in contradiction with some facts generally known to the specialists in these difficult problems in their introduction these authors define the word glauconite as a mineral but unfortunately also use it to describe the green sedimentary pellets in which glauconitic minerals occur the term has unfortunately come to be used loosely to cover two concepts on the one hand a concept of facies something green usually in pelletal form lying in a marine sediment on the other hand a mineralogic concept which designates a mica 2 the two concepts cover diverse possibilities this duality of significance leads us amongst others 3 to propose to solve this confusion by using glaucony or pelletal glauconite for the facies and glauconitic minerals for the authigenic components of the green pellets 4 these glauconies may be more or less evolved and fundamentally contain two kinds of components 1 t h e inherited ones which originate from the initial substrate of glauconitization quartz mica clay calcareous debris 5 there is no glauconitization without this preliminary substrate 2 the authigenic ones which are the glauconitic minerals ranging from a smectitic end member component to a micaceous endmember only the latter should actually be named glauconite the suffix ite characterizes a mineral this is a convention other conventions are possible and we do not intend to impose a solution but it is certain that the confusion of the two concepts and withirl each concept the non recognition of their diversity can only lead to complete confusion in the application to geochronology ages of glauconies are not generally 10 20 younger than the age of sedimentation this old assumption was based on the study of deeply buried samples from north america which had been rejuvenated or on a biased time scale 6 numerous recent studies have shown that the main problems with the dating of giauconies are in chronological order 1 the zero time problem see tisserant and odin 7 and fig 1 2 possible influences from tectonic and deep burial 8 and 3 possible changes due to weathering 9 whatever the colour or the form the green pellets may lack reliability for geochronology due to the three main causes listed above if the light green pellets give apparent ages which are often different from the age of sedimentation it is well recognized today that this is not related to their colour but to the fact that either they are little evolved or that

structural evolution of backthrusting in the mentawai fault zone offshore sumatran forearc 1 we present the interpretation of newly acquired high quality industry standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the mentawai fault zone mfz the mfz lies along the boundary between the accretionary wedge and the proposed continental backstop this zone exhibits arcuate ridges on the seafloor convex toward the east beneath these ridges the structures developed as landward vergent imbricated backthrusts in the inner part of the accretionary wedge and higher angle backthrusts that deformed the forearc basin sediments in the forearc high anticlines were developed due to the seaward vergent forearc high thrusts originating in the accretionary wedge the imbricated backthrusts may have initiated during the early middle miocene contemporaneously with the slide and back rotation of forearc high thrusts in the late miocene the higher angle backthrusts were initiated continuous contraction induced the frontal higher angle backthrusts and formed a fold thrust belt toward the east during the pliocene the folds and thrusts were disturbed by diapirs and mud volcanoes backthrusting and fold thrust belts developed in the mfz may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern sumatra margin the backthrusts along the mfz are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small

holocene coastal deformation in simeulue and nias indonesia west of sumatra indonesia the islands of nias and simeulue as well as their smaller neighbours display elevated coral reefs and abandoned intertidal platforms faunas from three sections indicate a late cainozoic age for the reefs unaltered mollusc shells from nineteen fossil platforms were dated by first order 14c analysis the localities sampled were 0 5 to 3 5 m above normal high tide apart from one non finite result the ages ranged between modern 300 yrs and 5800 300 yrs b p the resulting average uplift rates are 0 3 1 0 mm yr no simple relationship between sample age and height is apparent but the bulk of the ages fall within three periods 0 800 yrs b p 2000 3400 yrs b p and 4500 5800 yrs b p suggesting that episodes of uplift were separated by periods of quiescence and that they were seismic in origin the pattern of deposition at the key section on nias supports this inference evidence of recent subsidence in eastern simeulue shows that uplift was locally accompanied by tilting the pattern and rate of movement are consistent with the position of the islands on the outer arc ridge of the sunda arc and with the long term uplift rates derived from palaeontological sources the inverse darwin coral reef model proposed here for the development of reefs on the flanks of rising neogene cores may also be applicable elsewhere

u pb zircon age of the andaman ophiolite implications for the beginning of subduction beneath the andaman sumatra arc abstract the andaman ophiolites form the basement of the andaman islands which is a part of the outer forearc that links the indo burma accretionary complex to the north with the java sumatra trench arc system to the se upper mantle harzburgite and dunite are overlain by a cumulate peridotite gabbro complex high level intrusive rocks and a tholeiitic volcanic series the upper crust in the south andaman ophiolite shows also a prominent andesite dacite volcanic suite suggesting arc volcanism built onto ocean crust u pb zircon dating of a trondhjemitic rock from chiriya tapu in south andaman island using laser ablation inductively coupled mass spectrometry reveals an age of crustal formation of 95 2 ma the trondhjemites have geochemistry comparable with that of plagiogranites associated with ophiolite complexes and e nd values around 7 further confirm that they are derived from depleted mantle melts basaltic pillow lava and basaltic dykes that cut the trondhjemites have mid ocean ridge basalt like trace element geochemistry the new data show that the andaman volcanic arc was built on cenomanian ophiolite oceanic crust and that subduction was initiated at this time along tethys at least from cyprus through oman to the andaman islands

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

indo burma range a belt of accreted microcontinents ophiolites and mesozoic paleogene flyschoid sediments this study provides an insight into the lithotectonic evolution of the n s trending indo burma range ibr constituting the southern flank of the himalayan syntaxis paleogene flyschoid sediments disang barail that represent a shallow marine to deltaic environment mainly comprise the west central sector of ibr possibly resting upon a continental base on the east these sequences are tectonically flanked by the eocene olistostromal facies of the disang which developed through accretion of trench sediments during the subduction the shelf and trench facies sequences of the disang underwent overthrusting from the east giving rise to two ophiolite suites naga hills lower ophiolite nhlo andvictoria hills upper ophiolite vhuo but with different accretion history the ophiolite and ophiolite cover rock package were subsequently overthrusted by the proterozoic metamorphic sequence originated from the burmese continent the nhlo suite of late jurassic to early eocene age is unconformably overlain by mid eocene shallow marine ophiolite derived clastics on the south the vhuo of mesozoic age is structurally underlain by continental metamorphic rocks the entire package in victoria hills is unconformably overlain by shallow marine late albian sediments both the ophiolite suites and the sandwiched continental metamorphic rocks are thrust westward over the paleogene shelf sediments these dismembered ophiolites and continental metamorphic rocks suggest thin skinned tectonic detachment processes in ibr as reflected from the presence of klippe of continental metamorphic rocks over the nhlo and the flyschoid disang floor sediments and half windows exposing the disang beneath the nhlo

a new stratigraphy for the islands of the sumatran forearc indonesia abstract the sumatran forearc of western indonesia contains a number of islands where extensive exposures of basement rocks and their sedimentary cover may be examined the islands such as nias and the batu islands are located along the outer edge of the sumatran forearc whilst others including the banyak island group and pini island lie within the forearc basin detailed sedimentological palaeontological and palaeobathymetric data from the tertiary strata from the forearc region require a new stratigraphy as previous stratigraphic schemes have not explained the variations across the region adequately this stratigraphy developed initially from detailed data collected on nias and the banyak islands can fully account for the successions of sedimentary rocks on the banyak and batu islands and siberut and explains many of the apparent inconsistencies between previous stratigraphies a basement complex and six new formations are formally defined in this paper important sedimentological differences between these formations represent key stages in the evolution of the outer part of the sumatran forearc studies of the basement rocks across the forearc area suggest the basement is inhomogeneous large intact sections of ophiolitic material occur in some areas whilst there is evidence for both oceanic and continental basement in others such heterogeneity is to be expected in a long lived obliquely convergent margin in oligocene earliest miocene times extension of the heterogeneous basement is inferred through indirect evidence palaeobathymetric data from the oyo formation indicates that the initial deposition in the newly formed extensional sub basins on nias was in most areas deep marine in many cases below the ccd detailed biostratigraphic analyses and structural and geochronological studies indicate a major early miocene unconformity in the western lahewa sub basin and parts of central nias mujoi sub basin this unconformity was developed as a direct result of a period of basin inversion that affected western parts of nias whilst sub aerial erosion occurred in parts of western nias conformable deposition of the gawo and olodano formation continued in the gomo and eastern parts of the mujoi sub basins the shallow marine sedimentary rocks of the olodano formation tended to accumulate on intra sub basinal highs whose position was controlled by active faults that transected the sub basins the sedimentary record reveals that the lower and middle miocene phases of differential uplift and subsidence had ceased by the late miocene a massive influx of himalayan derived bengal fan sediments reached the sunda trench in the sumatra area in the late middle miocene continued addition of bengal fan material to the accretionary wedge south west of nias resulted in steady plate deflection and subsidence through flexural processes in forearc basin areas the flexural consequence of increased load added to the prism and associated subsidence history is documented by the sedimentary record on nias where the shallow marine olodano formation passes up into the neritic to upper bathyal lahomie formation the pliocene unconformity which is observed over all areas studied in the forearc is well constrained by structural biostratigraphic and sedimentological studies the unconformity represents the initiation of a major phase of uplift and deformation that continues to the present day rapid uplift of the outer arc ridge and deformation of the prism during the pliocene led to increased subsidence landward of the deformation the rapid subsidence of the forearc basin landward of the outer arc ridge has contributed greatly to the apparent differences between the forearc basin and the outer arc ridge at the present day two areas with remarkably similar pre pliocene histories now have remarkably different physiographies

tectonic diapiric and sedimentary chaotic rocks of the rakhine coast western myanmar abstract the western margin of myanmar is the northern extension the active sunda india eurasia subduction zone coastal regions and offshore islands have remarkable exposures of chaotic rock terranes along wave cut terraces that allow characteristics of tectonic sedimentary and diapiric melanges to be recognized tectonic shear zones tectonic melanges contain fragments of cretaceous ophiolites chrome spinel bearing peridotites and radiolarian cherts that are in contact with thrust packets of eocene turbidite units broken formations the turbidites contain shale rich beds that have been sheared during soft sediment deformation sedimentary broken formations and are sandwiched between undeformed thick sandy beds these are mass transport deposits mtds that most likely formed during deposition of the initial detritus of the himalayan orogenic zone probably trench slope basins on the accretionary prism the ophiolitic and turbiditic thrust slices have been exhumed and are currently being intruded by active mud volcanoes that bring fragments of units up from depth to the surface forming diapiric melanges these diapiric melange bodies contain only small fragments

collision melange and circular basins in north borneo a genetic link abstract attention has been given to the relevance of chaotic rock units including tectonic sedimentary and diapiric melanges in the evolution of subduction related accretionary complexes but they have often been perceived as relicts in suture zones northwest borneo west sabah was evidently a subduction complex which experienced collision caused by the proto south china sea and a consequence of post collision intrusion currently expressed as a 4000 metre high mountain together with gravity tectonics offshore of palawan and borneo the continuous deformation of the accretionary wedge from the collision to the post collision stages is however a matter of contention along with the timing as subduction ceased this study integrates the seismic profiles field observation and topographic interpretation to show that the backstop is composed of ultramafic rocks and a vast amount of severely sheared pillow basalts and radiolarite wide shear bands also exhibit metric phacoid shaped blocks of basalt and radiolarite successor basins cover these collision features to the west and through a system of circular basins overlying a shale matrix engulfing metric blocks of basalts radiolarite and sandstone the chaotic rock unit was occasionally brought to the surface and the overlying strata are deformed as circular synclines and anticlines we therefore suspect that the mechanism of exposing the melange involved a diapiric process that pushed this low viscosity material upward accordingly the entire process illustrates a tectonic evolution involving gravity tectonics from the end of the collision to the post collision setting of northern borneo

the palaeogene forearc basin of the eastern alps and western carpathians subduction erosion and basin evolution scarce palaeogene sediment remnants in the eastern alps and western carpathians are interpreted as remains of a continuous forearc basin new apatite fission track geochronological data corroborate mild paleocene eocene exhumation and relief formation in the eastern alps palinspastic restoration and nine palaeogeographical maps of the eastern alps and western carpathians ranging from the paleocene to the late oligocene epoch illustrate west to east migration of subsidence in the forearc basin subsidence isochrons indicate that oblique subduction of the european plate below the adriatic plate was responsible for forearc basin migration at a rate of 8 mm a 1 the periadriatic lineament was formed as a result of shearing by oblique subduction the neogene to recent sumatra forearc basin is an analogue for the evolution of the east alpine west carpathian forearc basin

bathymetric distribution of benthic foraminifera on the australian irian jaya continental margin eastern indonesia abstract we identified 164 taxa of benthic foraminifera in 35 selected box core top samples collected on the australian irian jaya continental margin in waterdepths between 60 and 2119 m along three systematically sampled transects across the banda arc the bathymetric distribution pattern of the benthic foraminiferal faunas is related to the oceanographic situation of this area where the watermasses of the indian ocean collide with the watermasses of the pacific ocean with the results of cluster analyses and empirical depth ranges of isobathyal taxa four faunal depth zones and four subzones can be distinguished 1 a the outer shelf biofacies 60 150 m corresponding to the photic oxycline zone of the indonesian surface waters and inhabited by a benthic foraminiferal association dominated by amphistegina lessonii operculina ammonoides heterolepa dutemplei and various miliolids 2 b the upper bathyal biofacies 150 400 m reflecting the aphotic deeper indonesian surface waters dominated by bolivina robusta heterolepa mediocris hanzawaia nipponica and lenticulina spp a major faunal break is situated at the lower boundary of this depth zone 3 c the middle bathyal biofacies 400 1500 m representing the indonesian intermediate waters with minimum oxygen contents dominated by bolivina robusta cassidulina carinata gavelinopsis lobatulus and sphaeroidina bulloides in this depth zone many taxa occur with only limited depth ranges on which four subzones c1 4 could be identified allowing for a local paleo bathymetric resolution of a few hundred meters 4 d the lower bathyal biofacies 1500 2120 m corresponds with the indonesian deep waters it is dominated by pullenia bulloides and other cosmopolitan deep water indicators such as epistominella exigua laticarinina pauperata oridorsalis umbonatus and planulina wuellerstorfi the sample fraction 250 m can be used to readily delineate the major faunal trends paleobathymetric resolution improves when the sample portion 125 m is used

subduction zone recycling processes and the rock record of crustal suture zonesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent offshore observations at modern ocean margin subduction zones omszs reveal that bodies of accreted material are commonly volumetrically small or missing that crustal thinning and subsidence 3 5 km has occurred and that most trench axes lie close 5 30 km to the seaward tapering edge of coastal basement rock onshore mapping commonly documents missing or only narrow terranes of former forearc rock and the inboard migration of the arc magmatic front these observations are evidence that subduction is accompanied by the removal of sediment and crustal material from the submerged forearc by the kindred tectonic processes respectively of sediment subduction and subduction erosion subduction erosion truncates the margin migrates the trench inboard at 2 5 km ma onshore observations at ancient crust suturing subduction zones csszs imply that collisional suturing is accompanied by sediment subduction and truncation of both upper and lower plates during a protracted period of suturing 20 50 millio

seismic evidence for broken oceanic crust in the 2004 sumatra earthquake epicentral region the 2004 sumatra earthquake was one of the largest events to occur in a subduction zone in the past 50 years seismic reflection data for this subduction zone reveal thrust faults cutting across the entire oceanic crust this observation coupled with the hypocentres of aftershocks suggests that the megathrust the interface between the indo australian plate and the sunda plate currently lies in the oceanic mantle

persistent elastic behavior above a megathrust rupture patch nias island west sumatra we quantify fore arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible elevated coral microatolls paleoreef flats and chenier plains show that the sumatran outer arc island of nias has experienced a complex pattern of relatively slow long term uplift and subsidence during the holocene epoch this same island rose up to 2 9 m during the mw 8 7 sunda megathrust rupture in 2005 the mismatch between the 2005 and holocene uplift patterns along with the overall low rates of holocene deformation reflects the dominance of elastic strain accumulation and release along this section of the sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long term plate convergence the fraction of 2005 uplift that will be retained permanently is generally 4 for sites that experienced more than 0 25 m of coseismic uplift average uplift rates since the mid holocene range from 1 5 to 0 2 mm a and are highest on the eastern coast of nias where coseismic uplift was nearly zero in 2005 the pattern of long term uplift and subsidence is consistent with slow deformation of nias along closely spaced folds in the north and trenchward dipping back thrusts in the southeast low holocene tectonic uplift rates provide for excellent geomorphic and stratigraphic preservation of the mid holocene relative sea level high which was under way by 7 3 ka and persisted until 2 ka

crustal structure of the sunda forearc region west of central sumatra from gravity data free air gravity anomalies near nias island include a 100 mgal low centered 10 20 km landward of the trench axis and a 80 mgal high over the outer arc ridge but also show a few large anomalies unrelated to topography a 90 mgal low is found on the structural high between the banyak islands and sumatra north of nias an 80 mgal rise at a gradient of 8 mgal km is modeled by a near surface body of high density material oceanic crust 4 km wide and tens of kilometers long this slab may be exposed on the southwest coast of nias where ultramafic bodies have been mapped and its northerly trend compared with northwesterly bathymetric trends may be related to other obliquely trending structures near here to a proposed strike slip fault which crosses nias and or to the basement high topped by the batu islands a 30 mgal free air low over the forearc basin can be modeled best if pre miocene melange or continental crust underlies the basin and it seems unlikely that the basin can be underlain by a large thickness of oceanic crust