reappraisal of early paleogene ccd curves foraminiferal assemblages and stable carbon isotopes across the carbonate facies of perth abyssal plain bulk carbonate content planktic and benthic foraminiferal assemblages stable isotope compositions of bulk carbonate and nuttallides truempyi benthic foraminifera and non carbonate mineralogy were examined across 30 m of carbonate rich paleogene sediment at deep sea drilling project dsdp site 259 on perth abyssal plain off western australia carbonate content mostly reflecting nannofossil abundance ranges from 3 to 80 and generally exceeds 50 between 35 and 57 mbsf a clay rich horizon with a carbonate content of about 37 occurs between 55 17 and 55 37 mbsf the carbonate rich interval spans planktic foraminiferal zones p4c to p6b 57 52 ma with the clay rich horizon near the base of our zone p5 upper p6b throughout the studied interval benthic species dominate foraminiferal assemblages with scarce planktic foraminifera usually of poor preservation and limited species diversity a prominent benthic foraminiferal extinction event bfee occurs across the clay rich horizon with an influx of large acarinina immediately above the 13c records of bulk carbonate and n truempyi exhibit trends similar to those observed in upper paleocene lower eocene 57 52 ma sediment from other locations two successive decreases in bulk carbonate and n truempyi 13c of 0 5 and 1 0 characterize the interval at and immediately above the bfee despite major changes in carbonate content foraminiferal assemblages and carbon isotopes the mineralogy of the non carbonate fraction consistently comprises expanding clay heulandite zeolite quartz feldspar sodic or calcic minor mica and pyrolusite mno2 the uniformity of this mineral assemblage suggests that site 259 received similar non carbonate sediment before during and after pelagic carbonate deposition the carbonate plug at site 259 probably represents a drop in the ccd from 57 to 52 51 ma as also recognized at other locations

late cenozoic geodynamic evolution of eastern indonesia this paper presents an internally and globally consistent model of plate evolution in eastern indonesia from middle miocene to present time it is centered on the banda sea region located in the triple junction area between the pacific philippine australia and south east asia plates the geological and geophysical data available from indonesia were until recently insufficient to define a unique plate tectonic model in this paper the new data taken into account clearly restrict the possible interpretations owing to a great number of geological geophysical and geochemical studies the major plate boundaries the sunda banda subduction zone to the south the tarera aiduna fault zone and the seram thrust to the east and the sorong fault zone and molucca sea collision zone to the north are now clearly identified the age of the major tectonic structures is also better known geodetic measurements well constrain the present time plate kinematics we also consider the deformation history within eastern indonesia where numerous short lived microplates and their related microcontinents successively accreted to the asiatic margin moreover magnetic anomalies identification of the north and south banda sea basins allows a precise kinematic reconstruction of the back arc opening we used the plates software to test the coherency of our model presented as a series of 4 plate reconstruction maps from 13 ma to the present finally the origin of oceanic domains restored by our reconstruction is discussed

formation and tectonic evolution of the cretaceous jurassic muslim bagh ophiolitic complex pakistan implications for the composite tectonic setting of ophiolites the muslim bagh ophiolitic complex balochistan pakistan is comprised of an upper and lower nappe and represents one of a number of ophiolites in this region which mark the boundary between the indian and eurasian plates these ophiolites were obducted onto the indian continental margin around the late cretaceous prior to the main collision between the indian and eurasian plates the upper nappe contains mantle sequence rocks with numerous isolated gabbro plutons which we show are fed by dolerite dykes each pluton has a transitional dunite rich zone at its base and new geochemical data suggest a similar mantle source region for both the plutons and dykes in contrast the lower nappe consists of pillow basalts deep marine sediments and a me lange of ophiolitic rocks the rocks of the upper nappe have a geochemical signature consistent with formation in an island arc environment whereas the basalts of the lower nappe contain no subduction component and are most likely to have formed at a mid ocean ridge the basalts and sediments of the lower nappe have been intruded by oceanic alkaline igneous rocks during the northward drift of the indian plate the two nappes of the muslim bagh ophiolitic complex are thus distinctively different in terms of their age lithology and tectonic setting the recognition of composite ophiolites such as this has an important bearing on the identification and interpretation of ophiolites where the plate tectonic setting is less well resolved

palaeomagnetism of the balantak ophiolite sulawesi abstract palaeomagnetic data from the east arm of sulawesi provide constraints on the cretaceous and subsequent history of a fragment of the tectonic mosaic of a complex region close to the junction between se asia the indo australian plate and the western pacific microplates primary thermoremanence has been demonstrated in samples from 23 sites in a tilted cretaceous palaeogene lava sequence in the balantak ophiolite at batusimpang on the eastern tip of the east arm of sulawesi its direction d 60 5 i 32 1 95 5 5 indicates formation at 17 4 s and 60 of subsequent clockwise rotation the primany mode of remanence is deduced from nrm trm comparisons low temperature cycling and other rock magnetic tests point to sd magnetite as the principal remanence carrier supporting evidence for the palaeolatitude and northward movement of the east arm is derived from other lavas at binsil and from the boba cherts but lack of tectonic control limits the interpretation the contrast between these results and the subequatorial origin of contemporary rocks on nearly halmahera is consistent with subduction of indian ocean lithosphere beneath the sunda margin in the late mesozoic and early tertiary large differences in declination of remanence in the e sulawesi rocks indicate large clockwise and anticlockwise rotations of tectonic blocks only tens of kilometres across

getting over continent ocean boundaries the idea of a simple linear boundary between continental and oceanic crust at extended continental margins is widely recognized to be an oversimplification despite this such boundaries continue to be mapped because of their perceived utility in palinspastic and plate kinematic reconstructions to examine whether this perception is justified we review the data and models on which basis continent ocean boundaries are interpreted and map a set of such interpretations worldwide from more than 150 publications the maps show that the location of the continent ocean boundary is rarely consistently estimated within the 10 100 km observational uncertainty that might be expected of the geophysical data used for doing so that this is the case regardless of whether the transition zone behind the boundary is classified as magma rich or magma poor and that the geographical separation of estimates exceeds the width of single study continent ocean transition zones the average of global maximum separations across sets of three or more estimates is large 167 km and mostly a consequence of interpretations published over the last decade we interpret this to indicate an extra component of uncertainty that is related to authors understanding of the range of features that are interpretable at extended continental margins we go on to discuss the implications of this uncertainty for palinspastic and plate kinematic modelling using examples from the literature and from the south atlantic ocean we conclude that a precise continent ocean boundary concept with locational uncertainty defined from the ensembles is of limited value for palinspastic reconstructions because the reconstruction process tends to bunch the ensemble within a region that is i of similar width to the observational uncertainties associated with continent ocean boundary estimates ii narrower than the regions of uncertainty about rotated features implied by the propagation of uncertainties from plate rotation parameters and iii coincident within all the above uncertainties with the more easily mapped continental shelf gravity anomaly secondly we conclude that estimated continent ocean boundaries are of limited use in developing or testing plate kinematic reconstructions because i reconstructions built using them as markers do not within uncertainty limits defined from the ensembles differ greatly from those using more easily determined bathymetric or gravity anomaly contours and ii because it is impossible to segment and date them with useful precision to use as markers of the edges of rigid oceanic lithosphere outside of the constraints of a pre existing plate kinematic model

evidence for a submerged and displaced continental borderland north banda sea indonesia the banda sea is separated into two oceanic fragments the south and north banda basins by a large intervening region of ridges and basins called the banda ridges the north and south banda basins appear to be trapped pieces of pre tertiary oceanic crust whereas the banda ridges are a submerged and displaced continental borderland structurally very similar to the southern california offshore borderland analysis of dredged rocks from the banda ridges suggests an origin from western irian jaya

paleomagnetic field variation with strong negative inclination during the brunhes chron at the banda sea equatorial southwestern pacific abstract we reconstruct the earth magnetic field in the brunhes epoch at the banda sea by studying the paleomagnetic data from core md012380 collected during the international marine global change study images vii cruise in 2001 magnetic analysis is carried out for whole core with a sampling spacing of 1 cm by using u channel magnetic susceptibility nature remanent magnetization nrm anhysteretic remanent magnetization arm and isothermal remanent magnetization irm are measured in our paleomagnetic experiment results show the low latitude geomagnetic field variation at the banda sea during the last 820 kyr except for the brunhes matuyama boundary bmb there is no clear signal of reverse events in paleo inclination and paleo declination patterns however the synthetic paleointensity curve displays the asymmetrical saw tooth pattern that can be used for determining reverse events and shows a maximum intensity drop at the bmb the characteristics of paleointensity provide a useful tool to identify reverse signals and improve the difficulties from only using inclination and declination patterns especially at low latitude with the help of paleointensity inclination and declination we have identified five reverse events furthermore if we consider the secular variation effect we think that the strong negative inclination observed in our study may be the zonal time averaged field with paleo secular variation rather than non dipole effect within the brunhes epoch

modes of bed load movement on a smooth rigid surface sediment grains transported as supply limited bed load on a rigid surface move either discretely or collectively as bed forms with significantly different effective grain speeds and active storage volumes the adopted mode has implications for sediment sorting and heavy mineral placer formation dispersal of grain associated pollutants and accumulation and flushing of sediment deposits in unlined canals and sewers the threshold condition between the two modes has been established for a smooth surface from flume experiments with different sediment types flow conditions and sediment supply rates this is expressed in terms of a relationship between the sediment movability number a dimensionless bed load parameter and a grain shear reynolds number

neogene sediment thickness and miocene basin floor fan systems of the celebes sea abstract in the celebes sea basin the middle miocene turbidites were correlated from odp site 767 throughout the studied area differences in their regional thickness variations and distribution indicate two source areas the middle miocene turbidite fan complexes of the central and southern celebes sea basin are controlled by the paleo tarakan delta system the tectonic events and the basin floor morphology respectively the main source area for the time correlative turbidites along the southern sulu arc is assumed to be mindanao the correlation of the middle miocene to pleistocene sequences exhibit tentative ages for the development of the accretionary wedges along the cotabato trench and along the north sulawesi subduction a post middle miocene to pre pliocene age is inferred for the cotabato wedge and a plio pleistocene age is assumed for the north sulawesi wedge

magnetic lineations constraints for the back arc opening of the late neogene south banda basin eastern indonesia abstract the south banda basin is located within eastern indonesia near the triple junction between the eurasian pacific and indo australian plates it is underlain by oceanic crust but its origin and age were not well established it has been interpreted as a mesozoic trapped piece of indian ocean or as a cretaceous eocene basin related to the celebes and sulu basins but a neogene back arc origin was also considered recent geochemical and geochronological studies strongly support the latter hypothesis in this paper we present a new analysis of the magnetic data from the eastern part of the south banda basin the damar basin we used magnetic field measurements collected during nine oceanographic cruises from various institutions looking for magnetic correlation in the time span given from recent geochronological data the comparison between measured profiles and theoretical profiles deduced from the reversals of earth s magnetic field during neogene time allows us to infer an opening of south banda basin during late miocene early pliocene time from 6 5 to 3 5 ma magnetic lineations 2an 2ar 3n 3r 3an and possibly 3ar are recognized with an extinct spreading centre trending ene wsw at least five segments are identified each segment being separated by inactive transform faults perpendicular to the extinct spreading centre a half spreading rate of about 3 cm yr is calculated based on spacing of magnetic lineations the opening history of the basin is discussed the cause of cessation of spreading is likely the arc continent collision dated at about 3 ma however the onset of opening is less determined we suggest that damar basin began to open at about 6 5 ma during magnetic period 3an as an intra arc basin separating the banda volcanic arc to the south from the incipient lucipara volcanic arc to the north the latter was probably created at the beginning of rifting of damar basin as shown by both magnetic and geochronological data the young age of the south banda sea basin is contradictory with its great depth we discuss this problem in view of thermal and tectonic considerations we finally conclude that our magnetic model allows us to more precisely describe the opening of the basin

temporal association of arc continent collision progressive magma contamination in arc volcanism and formation of gold rich massive sulphide deposits on wetar island banda arc whole rock 87sr 86sr and 18o analyses of volcanic rocks and 3he 4he analyses of sulphides and sulphates from mineralized rocks on wetar indonesia indicate a variable contribution of assimilated crustal material or sediment sourced from the subducted australian craton to the south these new data support the idea of progressive source contamination with precisely dated events showing that wetar island hosts the most extreme examples of crustal assimilation in the region the increased continental contamination occurs during the pliocene zanclian to piacenzian during distinct magmatic events between 5 and 4 ma and at 2 4 ma when 87sr 86sr ratios in unaltered lavas with whole rock 18o values between 5 7 and 9 6 increase from 0 707484 to extreme radiogenic values of 0 711656 the earlier of these magmatic events is important in the generation of the hydrothermal systems responsible for the mineralization recorded on wetar samples from this yield radiogenic 3he 4he ratios between 0 5 and 1 4 r ra similar to the data from volcanic rocks on nearby romang the later magmatic event coincides with the arrival of the australian continental margin at the subduction zone along the banda arc progressive incorporation of continental sourced components into the source region below the wetar island edifice coincides with the formation of gold rich volcanogenic massive sulphide deposits hosted within the contaminated volcanic pile

historical biogeography of the cicadas of wallacea new guinea and the west pacific a geotectonic explanation abstract the present day distribution patterns of the cicadas of wallacea new guinea and the west pacific reflect the extremely complex geotectonic history of that area the patterns found in two unrelated monophyletic groups of cicadas insecta homoptera cicadoidea the subtribe cosmopsaltriaria and the tribe chlorocystini with its sister tribe prasiini are analysed and related to data from the geological literature the two groups have a comparable distributional range over sulawesi maluku new guinea the bismarck archipelago and the east melanesian archipelagos to tonga and samoa the occurrence of endemic species species groups and genera in congruently delimited parts of this range made us recognize 14 areas of endemism the areas of endemism coincide with geological entities which have been isolated during a considerable period of time most of the areas originated from oceanic island arcs which developed as a result of subduction along the southern and western edges of the pacific tectonic plate others are rifted microcontinents concentrations of species and especially endemic species of the various cicada genera in different areas of endemism suggest that each of these genera evolved in isolation on one of the afore mentioned geological entities these entities can therefore be regarded as source areas for the genera the island arc fragmented as a result of collisions first with the asian and later with the australian tectonic plate and several of its parts reamalgamated at the northern craton of the australian plate where they now form the greater part of the island of new guinea after reamalgamation of the arc fragments the cicadas of the respective genera that had evolved on these fragments could disperse into adjoining areas but since cicadas are poor dispersers the source areas of the genera can still be recognized in present day distribution patterns the generic relationships in the two groups of cicadas under study indicate area relationships that comply with the latest palaeogeographic reconstructions the main vicariant speciation events in the cicada cladograms correspond with the presumed sequences of fragmentation of the island arc cicada phylogeny and distribution combined reflect the historic area relationships rather than the present ones

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

multiple sources for volcanic rocks dredged from the western australian rifted margin abstract igneous rock samples recovered during the 2008 09 australian southwest margin marine reconnaissance survey ga 2476 off the western australian coast from perth to the wallaby plateau come from a variety of tectonic settings reflecting a complex magmatic history of this margin cretaceous rhyolite and basaltic andesite samples most likely erupted in a continental extensional setting while younger volcanic samples reflect a transition to a mature passive continental margin with the generation of mid ocean ridge basalt during the last phase of volcanism a series of volcanic cones and mounds with ocean island basalt geochemical signatures formed along the margin the pre rift sedimentary rocks sampled on the wallaby plateau support the viewpoint that it is an extended block of continental crust overlain by syn rift and post rift volcanic rocks

the banda celebes sulu basin a trapped piece of cretaceous eocene oceanic crust the banda celebes and sulu basins are poorly understood marginal seas located at the junction of the eurasian indian australian and pacific philippine sea plates the incompleteness of the data sets the complex geology of the surrounding islands and the late cenozoic evolution involving subduction rifting transform faulting and island arc collision have complicated tectonic interpretation of the region the banda basin is underlain by oceanic crust1 3 bowin et al 2 and lapouille et al 3 have suggested that it is a trapped oceanic basin which was once continuous with the late jurassic argo abyssal plain off north west australia similarly the celebes and sulu basins are also underlain by oceanic crust4 6 on the basis of newly identified magnetic reversal ages fig 1 heat flow data and on land geological studies we suggest that the celebes and sulu seas may have been continuous with the banda basin this cretaceous eocene basin has been dissected into its present configuration by tertiary tectonic processes

multidisciplinary research on fault activity in the western part of the north anatolian fault zone abstract westward migration of large earthquakes along the north anatolian fault zone seems to have been suspended after the 1967 mudurnu valley earthquake which may imply the formation of a seismic gap however the westward extension of the north anatolian fault zone has not been clearly defined two branches have been proposed but which one of them is the true extension is a matter of controversy in this paper we review some of the results which we have obtained so far from field work in order to derive some basic information as to the characteristic features of the respective branches the northern branch which we call the izmit sapanca fault zone is characterized by a wide graben like structure in the deformation zone however the topography corresponding to folding prevails which implies compressional deformation in late quaternary the southern branch which we call the iznik mekece fault is a simple continuous strand and is displaced right laterally there also exist holocene offset features indicating that strike slip faulting has been dominant temporary seismic observations were made twice for one month in 1986 and for two months in 1988 during these two observation periods no significant seismic activity was found in the vicinity of the iznik mekece fault whereas many small earthquakes occurring at the izmit sapanca fault zone were recorded although the observation period is rather short we conclude that current seismic activity at the iznik mekece fault is very low in view of some geological evidence showing the past activity of this fault we interpret low seismic activity as indicating that seismicity is currently at the stage of quiescence we attempted to estimate recurrence intervals of large earthquakes at the iznik mekece fault by excavating trenches at two locations we carefully selected trench sites but they were not good enough to provide crucial information the only result is that the latest earthquake at the eastern portion of the iznik mekece fault must have taken place 200 500 years b p trenching was also attempted at a location where the surface rupture associated with the 1967 mudurnu valley earthquake is clearly recognized we found evidence showing the last two faulting events the most marked feature is extensive magnetic anomalies along the iznik mekece fault detailed features of the observed anomalies indicate that they are closely related to the fault structure in particular weak magnetization zones are found to correspond to fractured zones these magnetic anomalies can be interpreted in terms of dyke like structures similar fault related anomalies have been found in the distributions of electric potential and radon concentration in soil gas these anomalies provide important information as to a strategy for continuous measurements fault related anomalies should be taken into consideration when an effective way is pursued for detecting anomalous changes prior to faulting

p wave velocity structure of the crust and its relationship to the occurrence of the 1999 izmit turkey earthquake and aftershocks we determined hypocenters and focal mechanism solutions of aftershocks and three dimensional p wave velocity structure in and around the focal area of the 1999 izmit earthquake m w 7 4 aftershocks form a 170 km long narrow zone trending in an east west direction along the northern branch of the north anatolian fault zone nafz they are not homogeneously distributed in the whole aftershock area but consist of several clusters there are three significant clusters in the aftershock activity 1 near the hypocenter of the mainshock 2 near 29 2 e in the marmara sea and 3 east of 30 4 e focal mechanism solutions of aftershocks have various types but they are similar to each other within each cluster the p wave velocity structure obtained has a distinct low velocity area to the west of the mainshock hypocenter the mainshock rupture area estimated by yagi and kikuchi 2000 lies outside this low velocity area there exists a high velocity anomaly to the east of 30 4 e this high velocity area lies below the aftershock cluster distributed to the east of 30 4 e this high velocity anomaly extends to the shallower depth of the southern branch iznik mekece fault of nafz the anatolian earthquake sequence that had migrated westward for the past 60 yr did not propagate into this southern branch this suggests the possibility that the anatolian earthquake sequence progressed to the west exploiting an area that might break more easily

reconstruction of the east africa and antarctica continental margins the early jurassic separation of antarctica from africa plays an important role in our understanding of the dispersal of gondwana and pangea previous reconstruction models contain overlaps and gaps in the restored margins that reflect difficulties in accurately delineating the continent ocean boundary cob and determining the amount and distribution of extended continental crust this study focuses on the evolution of the african margin adjacent to the mozambique basin and the conjugate antarctic margin near the riiser larsen sea satellite derived gravity data have been used to trace the orientations and landward limits of fracture zones a 3 d gravity inversion has produced a crustal thickness model that reliably quantifies the extent and amount of stretched crust crustal thicknesses together with fracture zone terminations reveal cobs that are significantly closer to the african and antarctic coasts than previously recognized correlation of fracture zone azimuths and identified cobs suggests antarctica began drifting away from africa at approximately 171 ma in a roughly sse direction an areal balancing method has been used to restore the crust to a uniform prerift thickness so as to perform a nonrigid reconstruction for both nonvolcanic and volcanic margins both margins reveal a trend of increasing extension from east to west our results suggest africa underwent extension of 60 120 km while antarctic crust was stretched by 105 180 km various models tested to determine the direction of extension during rifting suggest that antarctica moved away from africa in a wnw ese direction during the period between 184 and 171 ma prior to the onset of seafloor spreading

40ar 39ar constraints on obduction of the seram ultramafic complex consequences for the evolution of the southern banda sea on kaibobo w seram obduction of hot oceanic lithosphere produced high grade metamorphism and granite in overthrust continental crust 40ar 39ar plateau ages of 5 65 6 0 ma and 5 4 ma of muscovite and biotite from the sole among the youngest so far measured on single crystals indicate complete resetting of the ar isotope system chloritized biotite crystals gave disturbed spectra and younger ages single fusion on biotite from the granite gave an age of 5 5 ma time limits for the obduction exhumation history of the kaibobo complex and sole are approximated by simple 1d forward p t t modelling constrained by pt data and 40ar 39ar thermochronology c 400 c at 6 ma c 320 c at 5 5 ma best fit results indicate that the decelerating post emplacement exhumation of ophiolite and sole began 8 ma ago the rate during the first 1 ma of this uplift exceeded 7 5 mm a 1 undoing 8 ma of migration in the sw pacific and australian regimes of motion back tracks kaibobo to the site where obduction emplacement ended near the southeastern corner of the banda sea plate similar compositions and time settings of the kaibobo and n timor ophiolites lead us to postulate that during the early miocene slow rate spreading occurred in the oceanic lithosphere of the southern banda sea south of the current volcanic arc

late jurassic early cretaceous evolution of the eastern indian ocean adjacent to northwest australia over 9700 km of new aeromagnetic data were acquired off the northwest coast of australia and combined with existing magnetic data to map magnetic isochrons in the eastern indian ocean the isochrons were used to constrain a tectonic model of the evolution of the seafloor in the argo cuvier and gascoyne abyssal plains a complete set of anomalies from m26 through m16 was found in the argo abyssal plain trending generally n70 e spreading commenced in the center of the basin at or prior to m26 and propagated outward until at least m24 time anomalies m10 mo recording the separation of australia and india were found in the cuvier and gascoyne abyssal plains with a trend of about n30 e a significant crustal age discontinuity occurs in the vicinity of the joey rise where the two lineation sets converge because there appears to be no overlap of isochron ages in the two groups it is not necessary to postulate that a triple junction existed off northwest australia as has been previously suggested at m4 m5 time a 10 clockwise change in spreading direction occurred on the cuvier gascoyne spreading system this event triggered ridge jumps that transferred two pieces of the indian plate to the australian plate overlapping spreading on the forming and dying ridges curved fracture zones and lineations as well as fanned lineation trends suggest that the ridge jumps occurred by ridge propagation and that the transferred lithospheric blocks behaved as microplates for a brief interval of approximately 1 2 m y

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

a numerical model of earthquake induced piezomagnetic anomalies a numerical model for evaluating the order of magnitude of earthquake induced piezomagnetic anomalies is proposed the stress field associated with normal or reverse faulting with a 10 mpa mean stress drop is determined by mean of displacement discontinuity method the induced piezomagnetic tensor is then used to predict the associated magnetic field variations observed at ground surface the model shows that for earthquakes occurring in homogeneous magnetic masses with piezomagnetic properties of andesite rocks the associated anomalies are of the order of 0 2 nt for faults with a 5 km dip slip and 2 nt when the dip slip is 20 km long the variation of magnetic field for the case of a piezomagnetic dyke intruding a nonmagnetic body is found to be much greater for a 4 km thick dyke parallel to the fault and located 4 km from it the observed ground surface magnetic field variation is of the order of 10 nt these results outline that induced piezomagnetic effects strongly depend on the stress drop and stress distribution along and outside the fault the location of the earthquake source with respect to the piezomagnetic body and the piezomagnetic properties of the rock

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

cross section restoration and one dimensional basin modeling of the central subbasin in the southern kunsan basin yellow sea the petroleum system of the kunsan basin in the northern south yellow sea basin is not well known compared to other continental rift basins in the yellow sea despite its substantial hydrocarbon potential restoration of two depth converted seismic profiles across the central subbasin in the southern kunsan basin shows that extension was interrupted by inversions in the late oligocene middle miocene that created anticlinal structures one dimensional basin modeling of the iih 1xa well suggests that hydrocarbon expulsion in the northeastern margin of the depocenter of the central subbasin peaked in the early oligocene predating the inversions hydrocarbon generation at the dummy well location in the depocenter of the subbasin began in the late paleocene most source rocks in the depocenter passed the main expulsion phase except for the shallowest source rocks hydrocarbons generated from the depocenter are likely to have migrated southward toward the anticlinal structure and faults away from the traps along the northern and northeastern margins of the depocenter because the basin fill strata are dipping north faulting that continued during the rift phase middle miocene of the subbasin probably acted as conduits for the escape of hydrocarbons thus the anticlinal structure and associated faults to the south of the dummy well may trap hydrocarbons that have been charged from the shallow source rocks in the depocenter since the middle miocene

arc and backarc geochemical signatures of the proto philippine sea plate insights from the petrography and geochemistry of the samar ophiolite volcanic section abstract remnants of a cretaceous lithosphere are found at the peripheries of the west philippine basin these mesozoic fragments preserve arc affinity and include the amami plateau the east halmahera ophiolite and the ophiolites along the eastern margin of the philippine archipelago the eastern margin of the philippines is composed of early to late cretaceous ophiolites and ophiolitic complexes that exhibit strong subduction imprints the early late cretaceous samar ophiolite in the central philippines forms part of this eastern belt recent surveys in southern samar revealed the presence of peridotites gabbros and massive and pillowed flows of the samar ophiolite major trace and rare earth element signatures of the volcanic rock samples indicate moderate to strong subduction related influences and formation in an island arc setting in contrast to other similarly aged ophiolites along the eastern margin of the archipelago those to the north of samar island exhibit weak to almost mid oceanic ridge characteristics these differences in the geochemical signatures of the ophiolites and ophiolitic complexes along eastern philippines require a re evaluation of a previous model suggesting that these ophiolites originated from a single oceanic lithosphere possibly the proto philippine sea plate

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

application of gauss s method to magnetic anomalies of dipping dikes quite a few computerized interpretation schemes on the inversion of a magnetic profile across a dike have been reported recently mcgrath and hood 1970 described an essentially trial and error method in which each of the dike parameters is varied in turn by an arbitrary amount until a set of best fit parameters is found rao et al 1973 described a method of differential correction to improve the initially assumed dike parameters similar works may include those by moo 1965 hall 1968 bosum 1968 and others

a north indonesian basin geo thermal and hydrocarbon generation histories abstract geohistory thermal history oil generation and accumulation have been examined in an early pliocene basin which is located offshore north of indonesia the basinal sediments are 4500 m thick and were deposited in about 5 million years leading to development of high excess fluid pressures up to 170 atm at 2000 m mainly due to strong undercompaction the successive deltaic platforms in the basin are controlled by a major growth fault system which is probably related to a large overpressured shale diapir a one dimensional study of two wells was undertaken using an inverse non linear scheme to determine dynamic and thermal parameters and then to reconstruct the geohistories and thermal histories at the well sites the two thermal histories suggest a major heating event about 4 million years before present probably tied to revewed volcanism which boosted the maturation of organic matter so that the development of hydrocarbons was massively enhanced without the heating event hydrocarbon generation would have been very unlikely in such a young basin

experimental flume study of the deposition of heavy minerals in a simulated witwatersrand sandstone unconformity the concentration of heavy minerals in a sedimentary deposit depends inter alia on the composition of the source material local hydraulic conditions during deposition sizes and densities of the sediment grains and sizes of the grains relative to the bed roughness effects of these factors have been investigated in a laboratory flume by depositing pure heavy mineral and mixed heavy and light mineral sediments on a stationary granular bed experiments with pure heavy minerals indicate that once a minimum mobility condition is exceeded deposits will be fairly extensive and uniform and only slightly affected by the flow condition in all experiments deposits were generally finer grained than the supply sediments and in some cases were bimodal the concentrations and total amounts of heavy mineral in the deposits were influenced by the concentrations in the supply mixtures and were strongly dependent on the sizes of the light mineral grains the results show that settling velocity is a poor indicator of hydraulic equivalence while critical shear velocity appears to be a good indicator

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

formation of arcs and backarc basins inferred from the tectonic evolution of southeast asia since the late cretaceous abstract results of the geological and geophysical surveys in the daito ridges and basin in the northern west philippine basin suggest that the daito ridge was an arc facing toward the south from the late cretaceous to the early tertiary the late cretaceous and tertiary history of southeast asia is evaluated based on these data in the daito ridges and basins and reconstructed based on overall plate kinematics that have operated in this area during the late cretaceous the daito ridge and the east philippine islands were positioned along the boundary between the indian and pacific plates the western half of the philippines setting on the indian plate approached from the south and collided with the east philippine daito arc either during the latest paleocene or the earliest eocene it is inferred that the bulk of the philippine archipelago rotated clockwise and borneo spun counterclockwise during the tertiary from the reconstruction the formation of backarc basins and their spreading direction are assessed as a result some primary causes and significant characteristics are suggested for the opening of backarc basins in southeast asia first opening of some backarc basins commenced with or was triggered by collisions second backarc basins opened approximately parallel to oceanic plate motion third the formation of some backarc basins was triggered by the approach of a hot spreading center fourth the spreading mode or direction of backarc basins was greatly affected by the configuration of the surrounding continent and was also rearranged to spread approximately parallel to oceanic plate motion the formation of backarc basins and their spreading direction can be reasonably explained by plate kinematics however the generative force responsible for their formation is possibly within the subduction system particularly to form horizontal tensional force in backarc side

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

a neogene back arc origin for the banda sea basins geochemical and geochronological constraints from the banda ridges east indonesia abstract dredgings conducted during the french indonesian cruises banda sea ii and iii collected volcanic rocks from several ridges of the banda sea area tukang besi ridge site 218 lucipara ridge sites 214 and 305 nieuwerkerk emperor of china sites 219 and 220 with the exception of one 46 ma old n morb type basalt thought to belong to an ophiolitic complex k ar and ar ar datings indicate that all the dredged volcanics are neogene they range in age from ca 10 ma tukang besi back arc basalts to 8 7 ma nieuwerkerk emperor of china calc alkaline andesites and to 7 3 ma lucipara oib type transitional basalts and cordierite bearing andesites radiogenic isotopic signatures of andesites are consistent with an afc assimilation coupled with fractional crystallization process involving assimilation of continental crust 8 3 ma old calc alkaline volcanic activity is also recorded on the wetar segment an inactive part of the east sunda arc and corresponding isotopic compositions are also consistent with an afc process involving continental crust these features suggest that lucipara nieuwerkerk emperor of china ridges and the wetar segment were representing a single volcanic arc 8 7 ma ago the corresponding calc alkaline activity was related to the subduction of the indian oceanic lithosphere beneath continental blocks of australian origin back arc opening processes occurred from 6 to 3 ma as a multi rift opening for the wetar basin and as a single rift opening for the damar basin while subduction related magmatism was still active in the wetar segment volcanic activity stopped at 7 ma in the nieuwerkerk emperor of china ridge on lucipara ridge 6 3 ma volcanic activity emplaced concomitantly transitional basalts and cordierite bearing andesites the mineralogical and chemical features of the latter are consistent with an afc process involving assimilation of continental crust by mantle derived basaltic magmas the end of magmatic activity on both volcanic segments at 3 ma is thought to result from the collision of timor with the wetar segment of the sunda arc

garnet peridotite and associated high grade rocks from sulawesi indonesia abstract the effects of collision between three major plates define the geological development of eastern indonesia garnet peridotite and associated granulite facies contact rocks are described from two sites within the valley of the active palu koro left lateral strike slip fault crossing central sulawesi disrupted parts of a medium to low grade metamorphic complex intruded by neogene granite occur on both sides of the fault thermobarometry on minerals and fluids in the garnet peridotite reveals a re equilibration path from a depth of 60 km upward chemistry points to metasomatic effects isolated trace element enrichment by a co 2 rich liquid and fluid in a peridotite of oceanic affiliation the granulite shows an increase in t and incipient melting at the arrival of peridotite the sequence of fluid inclusions of an evolving co 2 ch 4 n 2 h 2 o bearing fluid defines a concave decompression path suggesting rapid uplift trace element chemistry of granulite with basaltic to peraluminious rhyolitic composition indicates island arc affinity the described history may well reflect the processes beneath a mantled gneiss dome present as a coeval metamorphic aureole around the garnet peridotite outcrops

late cretaceous early tertiary convergence between the indian and arabian plates recorded in ophiolites and related sediments abstract remnants of ocean floor forming the eastern ophiolite belt in oman and the western ophiolite belt in pakistan have a common plate tectonic history culminating in emplacement at the cretaceous tertiary boundary fragments of ocean floor in these two belts have ages between 150 and 65 ma and recorded tectonic events in the early indian ocean at 150 ma 130 120 ma 110 100 ma and 70 65 ma new radiometric and chronostratigraphic ages and paleomagnetic and sedimentary information are used to relocate these ophiolites in the frame of the evolving indian ocean which was characterised by the stepwise breakup of gondwana at 158 ma east and west gondwana 130 ma southern atlantic breakup of east gondwana 95 84 ma madagascar and india seychelles 65 ma india and seychelles and finally at 40 ma rifting between africa and arabia the 150 ma old oceanic rocks of masirah island in oman originally formed the extension of the basins now preserved along the eastern edge of the afro arabian plate masirah drifted together with india seychelles when a new ridge formed at approximately 130 ma separating microplates e g kabul block from the northern edge of greater india parts of this new ocean were later emplaced to form the western ophiolite belt in pakistan consumption of oceanic crust and emplacement of ophiolites during the closure of the neotethys occurred not only north of india seychelles but also along two or three subduction zones between the afro arabian plate and india seychelles documented by the formation of metamorphic soles beneath ophiolites relics of ocean floor formed approximately 65 70 ma ago now form the upper units in the western ophiolite belt of pakistan where they overlie an accretionary prism of pillow lavas of dominantly aptian albian age and sedimentary rocks the breakup of india and the seychelles at 65 ma was associated with the eruption of flood basalts deccan and seychelles and enhanced counter clockwise movement of india this breakup possibly caused the late jurassic early cretaceous fragments of ocean floor e g masirah to be thrust onto the eastern edge of oman the ophiolites which were emplaced onto the western edge of india drifted as part of this plate farther north where collision with eurasia and the accreted microplates occurred in the eocene 55 ma

ocean crust vein mineral deposition rb sr ages u th pb geochemistry and duration of circulation at dsdp sites 261 462 and 516 abstract cation exchange experiments ammonium acetate and cation resin on celadonite smectite vein minerals from three dsdp holes demonstrate selective removal of common sr relative to rb and radiogenic sr this technique increases the rb sr ratio by factors of 2 3 to 22 without significantly altering the age of the minerals allowing easier and more precise dating of such vein minerals the ages determined by this technique site 261 121 4 1 6m y site 462a 105 1 2 8m y site 516f 69 9 2 4m y are 34 54 and 18 m y younger respectively than the age of crust formation at the site in the case of site 462a the young age is clearly related to off ridge emplacement of a massive sill flow complex at the other sites either the hydrothermal circulation systems persisted longer than for normal crust 10 15 m y or were reactivated by off ridge igneous activity celadonites show u and pb contents and pb isotopic compositions little changed from their basalt precursors while th contents are significantly lower celadonites thus have unusually high alkali u th ratios and low th u ratios if this celadonite alteration signature is significantly imprinted on oceanic crust as a whole it will lead to very distinctive pb isotope signatures for any hot spot magmas which contain a component of aged subducted recycled oceanic crust initial sr isotope ratios of ocean crust vein minerals smectite celadonite zeolite calcite are intermediate between primary basalt values and contemporary sea water values and indicate formation under seawaterdominated systems with effective water rock ratios of 20 200

trace element characteristics of different pyrite types in mesoarchaean to palaeoproterozoic placer deposits a comparative in situ la icp ms trace element study on pyrite from three different variably auriferous archaean to palaeoproterozoic palaeoplacer deposits in the ouro fino syncline quadrilatero ferrifero brazil the elliot lake area north of lake huron se canada and several deposits within the witwatersrand basin south africa revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits especially the ni and au concentrations as well as co ni and mo ni ratios were found to be systematically different in detrital compact detrital porous and post sedimentary hydrothermal pyrite grains from different source areas high co ni ratios and low au concentrations are typical of post sedimentary pyrite which is hydrothermal in origin in contrast relatively low co ni ratios and high au contents characterise detrital porous banded and concentric pyrite grains au 1 ppm which are syn sedimentary in origin in the elliot lake area and the witwatersrand basin detrital compact rounded pyrite is characterised by high co ni ratios which is in agreement with derivation from a hydrothermal source low au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks in contrast derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the ouro fino syncline because of low to intermediate co ni ratios high au contents may indicate a genetic relationship between pyrite and gold there systematic differences exist between the three areas with respect to au ni co mo and cu distributions in detrital pyrite which reflects differences in the provenance a predominantly mafic ultramafic source is indicated for the ouro fino a felsic source for the elliot lake and a mixed felsic mafic provenance for the witwatersrand pyrite populations independently of pyrite type the higher au endowment of the studied witwatersrand and ouro fino conglomerates are also reflected by an overall higher au concentration in the respective pyrite grains compared to the relatively au poor samples from elliot lake in general a strong positive correlation between au and pb levels in the various pyrite grains is noted analogous to pb which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete pbs phases au is considered to be present mainly in the form of discrete au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice

solar and lunar geomagnetic variations in the northwestern part of turkey summary hourly mean values of x y and z components of the geomagnetic field from two observatories and the total intensity f of the geomagnetic field from eight monitoring stations were analysed to study the behaviour of solar and lunar geomagnetic variations in the northwestern part of turkey the chapman miller method was applied to the data first as a whole and then by grouping it to lloyd s seasons to calculate seasonal dependence of the harmonics amplitudes of solar and lunar harmonics increase from winter months to summer months the first two harmonics of both solar and lunar origins were calculated annually to compare with sunspot relative numbers to see annual and semi annual variation the solar harmonics show very good sunspot dependence but lunar harmonics do not it means that both ionospheric and ring current systems have an important role in the formation of solar harmonics but lunar harmonics are affected by ionospheric current system only in the northwestern part of turkey we would not expect and we do not find much ocean effect at these observatories since they are all remote from oceans with significant tides

paleo oceanographic and tectonic settings of early miocene reefs and associated carbonates of offshore southeast asia the late oligocene to early middle miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast asia a generally rising eustatic sea level during this period allowed the accumulation of thick reefal units an expansion of the coral algal regional facies belt north to japan and south to new zealand occurred during the early miocene as the result of a reduced oceanic latitudinal temperature gradient an episode of enhanced organic carbon burial is reflected in a positive dgr13c excursion from 17 5 to 13 5 ma increasing paleo oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen minimum zone marine oil prone or anic carbon was deposited in silled anoxic basins both bounded by and within island arc systems a combination of tectonic and paleo oceanographic factors therefore created conditions that favored contemporaneous deposition of organic carbon rich source beds and reef reservoirs during the latest early to early middle miocene paleogeographic and facies reconstructions for an 18 m y interval indicate that deposition occurred in three tectonic settings 1 passive continental margins 2 convergent plate boundaries and 3 obliquely convergent plate boundaries in all these settings the vertical motion histories of fault bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups their scal and morphology duration diagenesis and relationship to siliciclastics

la icp ms elemental mapping of pyrite an application to the palaeoproterozoic atmosphere a hydrothermal origin has been advocated for the genesis of rounded and subhedral euhedral pyrite in some neoarchaean palaeoproterozoic clastic sedimentary rocks thereby limiting their value as proxies for the composition of the early atmosphere we present laser ablation inductively coupled plasma mass spectrometry la icp ms element maps of rounded and euhedral subhedral pyrite grains from the basal conglomerates of the palaeoproterozoic mississagi formation huronian supergroup canada these maps reveal the common existence of rounded detrital cores and isolated grains variably modified by hydrothermal overgrowth the detrital origin of pyrite grains is demonstrated by the rounding of isolated grains and the presence of cores in subhedral euhedral grains rounding and breakage of grains is implied to have occurred via physical abrasion during sediment transport furthermore trace element maps expose truncated cores of originally rounded detrital fragments that also indicate abrasive rather than accretional or dissolutional rounding finally we document a complex core that consists of multiple grains suggestive of various source materials that were co deposited in the original placer textural and geochemical evidence further suggests the detrital porous pyrite had a diagenetic origin prior to fluvial transport grain size and shape analysis suggests that the large porous pyrite cores and grains may have had a shorter transport distance than the on average smaller compact rounded grains transport was prolonged from tributaries into the main valley and the high energy setting implies well aerated palaeoproterozoic river water since pyrite is unstable under oxidising conditions the frequent survival of detrital pyrite in the fluvial mississagi formation after sustained transport in the catchment basin and alluvial valley as well as aerated palaeoproterozoic river waters therefore implies that the contemporary atmosphere was oxygen deficient the depositional age of the mississagi formation is between 2 45 ga thessalon fm and 2 31 ga gordon lake fm and the presence of the detrital pyrite therefore suggests that the atmosphere was devoid of any significant amount of free oxygen at the time immediately preceding the great oxygenation event c 2017 elsevier b v all rights reserved

the sulu sea a back arc basin setting within a neogene collision zone abstract the southeast sulu sea is a small oceanic basin that opened in middle miocene time in a back arc setting along the sundaland margin during subduction of the celebes sea plate beneath the sulu arc this back arc extension post dates compressive tectonism present in this back arc area in early middle miocene time progressive and still incomplete closing of this basin initiated in late miocene time is the result of collision of exotic terranes with the sundaland margin this collision also induced flipping of the sulu subduction zone the neogene geodynamic evolution of the sulu sea provides a modern example of how young oceanic crust can be incorporated within an orogenic belt

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

the geological development of the passive margins of the exmouth plateau off northwest australia abstract the exmouth plateau is a very large sunken continental block off australia s northwest shelf whose culmination lies 800 m below sea level it is elongated in a northeasterly direction and bounded by abyssal plains on three sides its broad tectonic framework is controlled by mesozoic rifting and transform faulting which accompanied the breakup of australia and greater india after which the plateau sank to bathyal depths the pre breakup sequence consists of more than 5000 m of palaeozoic and early mesozoic shallow marine to terrestrial detrital sediments the post breakup sequence consists of about 2000 m of late mesozoic and cainozoic marine sediments mainly carbonates the northern margin came into existence in the callovian 155 m y ago when a continental fragment separated from it and moved off to the northwest an early phase of rifting gave rise to triassic jurassic intermediate and acid volcanics 213 192 m y ago which overlie a thick triassic paralic sequence steady subsidence north of an east west hinge line allowed several thousand metres of early and middle jurassic carbonates and coal measures to accumulate before breakup breakup occurred along a series of rifted and sheared segments and the margin is further complicated by northeast trending callovian horsts and grabens the horsts were planed off in late jurassic and early cretaceous times and the whole margin was covered by a few hundred metres of late cretaceous and cainozoic pelagic carbonate as it sank steadily to its present average depth of 2000 2500 m the northeast trending western margin formed by rifting in the neocomian 120 125 m y ago as greater india moved off to the northwest callovian normal faults parallel the margin a thick triassic paralic sequence is unconformably overlain by thin late jurassic and younger marine sequences indicating that the area was high in the early and middle jurassic thin late cretaceous and cainozoic pelagic carbonates cover the margin which now lies more than 2000 m below sea level the northwest trending southern margin formed by shearing in the neocomian at the same time as the western margin it is cut by northeast trending normal faults which formed in the late triassic and callovian and is paralleled by neocomian and later normal faults a thick triassic paralic sequence is unconformably overlain by a thick late jurassic and neocomian delta suggesting that the area was high in the early and middle jurassic but a depocentre before and afterwards there was thermal uplift of more than 1000 m during the neocomian period of shearing and igneous intrusions buttressed the margin later normal faulting lowered the outermost margin and turned the uplift into a marginal anticline trending northwest the anticline had sunk beneath the sea by late in the cretaceous and thereafter this margin was covered by a thin sequence of pelagic carbonate which now lies beneath more than 1500 m of water

representation of a growth fault in a two dimensional quantitative model abstract a growth fault in a north indonesian basin has been modelled using the two dimensional fluid flow compaction model geopetii the location of the fault and permeability for fluid drive are of primary importance in the fluid pressure distribution in the basin using multiple vertical faults simulations of the fault have been carried out showing the importance of the growth history of the fault in influencing the excess pressure distribution the simulations which yield fluid pressure in agreement with the present day observations suggest that the fault has been dominantly closed to fluid flow since the time of its creation the modelling also led to the conclusion that the fault is not hydrodynamically connected to the surface at the present day but must have been at some time in the past to remove the overpressures developed through undercompaction which are due to the high sedimentation rate of the order of 1000 m ma characterizing the basin models using a single vertical fault suggest that the fault was open to fluid flow in a time interval ranging from 3 5 to 0 5 ma

placer formation in gravel bedded rivers a review the literature on placer formation processes within fluvial systems is widespread and ranges between detailed laboratory studies of the hydrodynamic segregation processes through to the intuitive interpretation of the distribution of minerals within geological sections however there are few if any comprehensive reviews of the literature surprisingly given the economic importance of placers the theoretical framework relating to the hydrodynamics of physical grain sorting is not well developed and there are relatively few detailed laboratory hydraulic investigations to inform theory in this wide ranging review the history of the development of principles of placer formation is explored as far as possible in a non technical fashion a consideration is given to the hydrodynamics of physical grain sorting above lower stage and upper stage bedforms and the typical internal sedimentary structures associated with placer concentrations are detailed finally examples of the depositional environment of diamond tin and gold placers are considered

a strategy of tectonomagnetic observation for monitoring possible precursors to earthquakes in the western part of the north anatolian fault zone turkey abstract in order to investigate changes in the geomagnetic field associated with seismic activity in the western part of the north anatolian fault zone nafz seven continuous measurement stations and eighteen repeated survey sites were set up in the inik geyve region taking into account the fault trace and its features simple differences in the total intensity between these continuous stations have been derived for detecting possible anomalous phenomena associated with earthquake occurrences theoretical studies imply that changes in the total intensity associated with fault activity are larger in the case of non uniform distribution of rock magnetization than those in the uniform case meanwhile inversion analyses of magnetic anomalies at the north anatolian fault zone nafz suggest a dyke like structure extending parallel to the fault strike this structure enables us to test our strategy based on non uniform magnetization for detecting pre and or co seismic changes in the geomagnetic field we made a theoretical estimation of geomagnetic changes for the case of a magnetic dyke structure intruding a non magnetic body parallel to the strike of a fault it turns out that unlike the uniform case the magnetic field arising from stress induced magnetization is enhanced a tectonomagnetic observation system talcing into account such a non uniform magnetization structure in the inik geyve region of the nafz is also described

geopressure evolution and hydrocarbon generation in a north indonesian basin two dimensional quatitative modelling abstract a two dimensional fluid flow compaction model was used to simulate geopressure evolution and hydrocarbon evolution in a 5 million year old basin in indonesia data taken from two wells on the cross section analysed showed the occurrence of large overpressure mainly due to undercompaction driven by the high sedimentation rate 4000 5000 m of deposition the entire section is controlled by a major growth fault system which has been modelled by a staircase like set of vertical instantaneous faults appearing progressively in time after calibration of the dynamic parameters controlling 1 the deposition of the sediments and 2 the evolution of the compaction process it was possible to reproduce the high excess fluid pressure in accordance with present day observations the important role of anomalously high temperature developed in connection with excess pressure formation is shown through effects on the expansion of water a major heating event suggested by a previous one dimensional study was introduced around three million years before present which has a major influence on the onset of maturation for the high gas potential organic matter suggesting the onset of oil gas generation in the last million years or so

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

tectonic setting of western pacific marginal basins abstract global kinematics as well as magnetic anomalies of marginal basins with continental geology and paleomagnetic data as additional constraints are used to present a set of reconstructions of the western pacific marginal basins between 56 ma and the present at key periods 56 43 32 20 12 and 3 ma our model accounts for the rapid motion of exotic terranes along the whole of the western pacific convergent zone marginal basins appear to open in a great variety of tectonic settings the two extreme examples being the mariana trough where trench suction may be the predominant driving force and the south china sea where intracontinental deformation appears to be the major mechanism the study of marginal basins is a 3 d problem which must take into account the whole tectonic context subduction related tectonics in cross section and upper plate deformation in map view and not only a 2 d problem the classical trench arc back arc problem

origin of marginal basins of the nw pacific and their plate tectonic reconstructions abstract geometry of basins can indicate their tectonic origin whether they are small or large the basins of bohai gulf south china sea east china sea japan sea andaman sea okhotsk sea and bering sea have typical geometry of dextral pull apart the java makassar celebes and sulu seas basins together with grabens in borneo also comprise a local dextral transform margin type basin system similar to the central and southern parts of the shanxi basin in geometry the overall configuration of the philippine sea resembles a typical sinistral transpressional pop up structure these marginal basins except the philippine sea basin generally have similar or compatible rift history in the cenozoic but there do be some differences in the rifting history between major basins or their sub basins due to local differences in tectonic settings rifting kinematics of each of these marginal basins can be explained by dextral pull apart or transtension these marginal basins except the philippine sea basin constitute a gigantic linked dextral pull apart basin system formation of the gigantic linked dextral pull apart basin system in the nw pacific is due to nne to ene ward motion of east eurasia this mainly was a response to the indo asia collision which started about 50 ma ago the displacement of east eurasia can be estimated using three aspects 1 the magnitude of pull apart of the dextral pull apart basin system 2 paleomagnetic data from eastern eurasia and the region around the arctic and 3 the shortening deficits in the large tibetan plateau all the three aspects indicate that there was a large amount 1000 to 1200 km of northward motion of the south china block and compatible movements of other blocks in eastern eurasia during the rifting period of the basin system such a large amount of motion of the eastern eurasia region contradicts any traditional rigid plate tectonic reconstruction but agrees with the more recent concepts of non rigidity of both continental and oceanic lithosphere over geological times based on these three estimates the method developed for restoration of diffuse deformation of the eurasian plate and the region around the arctic and the related kinematics of the marginal basins we present plate tectonic reconstructions of these marginal basins in global plate tectonic settings at the four key times 50 35 15 and 5 ma the plate tectonic reconstructions show that the first order rift stage and post rift stage of the marginal basins are correlated with the first order slow uplift stage and the rapid uplift stage of the tibetan plateau respectively the proto philippine sea basin was trapped as a sinistral transpressional pop up structure at a position that was 20 south of its present position at about 50 ma ago or earlier while the japan arc migrated eastward during the rifting period of the japan sea basin the shikoku basin opened and the parece vela basin widened

determination of the deep fault geometry along the southern branch of the north anatolian fault system by using resistivity and magnetic methods abstract the wenner lateral resistivity and magnetic dyke profiles carried out along the southern branch of the north anatolian fault system the use of magnetic dyke method indicates that the faults located in the deep parts of the continental crust were high angled and dipping towards north and the right lateral strike slip faults were the only active main fault types in the plio quaternary deposits besides a high angled strike slip fault and its secondary components form the geological contact between the basement rocks and the basin filling deposits nafs sb that is developed during plio quaternary neotectonic period is the only structural source during the formation of the deposits in the depression areas and recent times this zoning is related to nafs sb and developed depending on both the near vertical and 70 90 dipping strike slip faulting apart from the near vertical fault planes the faults dipping towards the depression areas developed due to the existence of the transtensional stresses in the strike slip faults

prominent magnetic anomaly along the continent ocean boundary between the northwestern margin of australia exmouth and scott plateaus and the argo abyssal plain abstract a prominent positive magnetic anomaly with amplitude as great as 1400 nt lies along the lower slope between the northern exmouth plateau characterized seismically by faulted layered reflectors and the argo abyssal plain characterized by hyperbolic reflectors because the slope of the northern exmouth plateau is floored by continental crust as shown by triassic and early jurassic volcanics and sediments deposited before the middle jurassic inception of spreading in the argo abyssal plain the anomaly lies along the continent ocean boundary cob to the northeast of the northern exmouth plateau the anomaly here with an amplitude no greater than 700 nt is traced along the lower slope of the rowley terrace and scott plateau and across the expanded continental rise north of 13 s and indicates that the rowley terrace and the bulk of the scott plateau are continental and that only the lower part of the rise north of 13 s is oceanic in this area an oceanic ridge rises above the adjacent breakup unconformity in the scott plateau to impound a deep sedimentary basin like that impounded by the outer voring plateau off norway the cob anomaly is modelled as a shallow two dimensional magnetic body with rectangular cross section and 40 80 km wide that extends up to 20 km seaward of the cob and is interpreted as a complex of rift related dykes in the continental crust and adjacent oceanic crust

geodynamic patterns of ophiolites and marginal basins in the indonesian and new guinea regions analysis of spatial temporal geological and geochemical patterns of ophiolites in the indonesian and new guinea region indicates a strong correlation with marginal basin development and closure the spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones suprasubduction zone ssz spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages most ophiolite slabs in the indonesian and new guinea region represent fragments of oceanic lithosphere with a subduction zone component as indicated by the common refractory petrochemistry of the mantle sequence and occurrence of boninite age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes collisions with buoyant lithosphere transfo parts of these ocean basins into ophiolites the connection between ophiolites and marginal basins is strongest where parts of actively spreading ssz basins are partially represented as ophiolites in collision zones many of the various tectonic models proposed for the origin and tectonic evolution of ophiolites are supported by plate interactions in the indonesian and new guinea region dewey bird 1970 silver smith 1983 moores et al 1984 searle stevens 1984 dilek moores 1990 harris 1992 dickinson et at 1996 however the geolo gical associations of these interactions remain poorly resolved allowing different parts of the region to be used as modern analogues for almost any tectonic scenario simple explanations based on plate kinematics alone fail to account for the increasing levels of complexity observed in the indonesian and new guinea region new ideas and supporting evidence for mantle extrusion mechanisms e g flower et at 2001 also rely heavily on descriptions of plate interactions in this region doglioni et at 1999 and raise new questions about how these processes may relate to the spatial and temporal pattern of ophiolites this paper provides a synthesis of the temporal and spatial patterns and tectonic associations of marginal basins and ophiolites throughout the indonesian and new guinea region it explores the relations between various plate boundary processes and ophiolite genesis and emplacement such as the role of strain partitioning trench rollback subduction polarity asymmetry astheno spheric flow and plate kinematics

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

the dispersion of magnetite bedload tracer across a gravel point bar and the development of heavy mineral placers economic concentrations of heavy minerals are often associated with fluvial point bars but prospecting models identifying the heaviest concentrations are poorly developed consequently the dispersal and storage of a heavy mineral bedload tracer magnetite across a rapidly evolving point bar was studied using magnetic susceptibility as a surrogate measure of magnetite concentration the bar head was a preferential area for the development of a placer owing to a lag accumulation of magnetite over an armoured bed surface in contrast when viewed in plan the bar platform and bar tail were regions of tracer dilution owing to downstream dispersion and mixing with shale in the vertical as the bar top rapidly aggraded however in section false bottom placers developed along bedding planes were evident the latter consisted of thin layers of concentrated magnetite resulting from the passage of bedload sheets consisting of a mix of shale and magnetite moving repeatedly from the bar head to accrete over the bar tail differential density sorting of magnetite and shale occurred during transport and deposition such that the heavier magnetite tended to accumulate as a visible concentrated bed layer later to be over run by layers of shale sediment in which magnetite was present diffusely however the placer thickness was greater than that visible because finer fractions of magnetite from each concentrated layer infiltrated the interstices of the top of the shale bed below the placer thickness the infiltration potential of the sediment bed and the actual rate of infiltration of the tracer were determined by fitting a mathematical function to measured variation in magnetic susceptibility with depth in the sediment body finally a simple mathematical model described in the literature as reproducing the plan view of flow and topographic patterns in river bends was found to reproduce patterns of depth velocity shear stress and competence in the point bar environment which were in accordance with the interpretation of the field data it was concluded that such a model when linked to entrainment functions for sediments of mixed density might be suitable for prospecting for economic heavy minerals in the point bar environment

stratigraphic evolution of mesozoic continental margin and oceanic sequences northwest australia and northern himalayas abstract during mesozoic time the northwest australian continental margin and the thakkhola region of north central nepal were adjacent passive margins off northeast gondwana the relatively sediment starved basins provide excellent opportunities for studying the early structural and depositional evolution of the indian ocean during the triassic jurassic rifting stages neo tethys and the jurassic cretaceous transition from rifting to drifting nepalese and australian paleomagnetic studies indicate subtropical paleolatitudes during the late triassic to early jurassic but higher latitudes of 35 40 s during the late jurassic to early cretaceous prior to the sustained northward flight of australia and greater india in the late cretaceous to cenozoic the latitudinal position of the basins and type of sediment are related principal continental margin formation along the northern edge of eastern gondwana started in late permian to triassic time by late triassic early jurassic time platform carbonates with thin lagoonal shales were laid down in a subtropical climate the carnian to rhaetian siliciclastics and carbonates show repeated shallowing upward sequences subsequent southward drift of greater india and australia during mid jurassic time replaced carbonates with more siliciclastic sediment input widespread erosion was caused during local uplift of parts of the northwest australian continental margin as a result of jurassic late rift block faulting a mid callovian early oxfordian hiatus in nepal is a submarine condensed sequence and non tectonic in origin in nepal the overlying 250 m thick organic rich dark shales which are correlated to the oxfordian kimmeridgian clays of circum atlantic hydrocarbon bearing basins can be traced along the northern himalayan range these shales probably represent an extensive continental slope deposit formed in a high productivity of organics belt the diverse foraminiferal microfauna of the belt was previously only known from boreal laurasia the callovian break up uncorformity off northwest australia is actually a post rift unconformity and precedes the onset of seafloor spreading by about 5 10 m y seafloor spreading leading to formation of the present indian ocean started in the argo abyssal plain around 155 ma ago in late jurassic time australia and greater india including northern nepal separated as early as in the late valanginian about 130 ma ago altered ash layers off northwest australia record an important volcanic phase in berriasian valanginian time mafic volcaniclastics in nepalese deltaic sediments also testify to continental margin volcanic activity which may be a precursor to the slightly younger rajmahal traps in eastern india an important tectonic event off northwest australia took place in aptian time 120 115 ma ago when hemipelagic sedimentation changed gradually into more pelagic sedimentation with sharply decreasing rates of deposition in nepal this marks the transition from coarse to fine grained organic rich terrigenous clastics leading eventually to more carbonate bearing strata during the mid cretaceous global sealevel highstand