hatton bank northwest u k continental margin structure summary the continent ocean transition near hatton bank was studied using a dense grid of single ship and two ship multichannel seismic mcs profiles extensive oceanward dipping reflectors in a sequence of igneous rocks are developed in the upper crust across the entire margin at the landward shallowest end the dipping reflectors overlie continental crust while at the seaward end they are formed above oceanic crust beneath the central and lower part of the margin is a mid crustal layer approximately 5 km thick that could be either stretched and thinned continental crust or maybe newly formed igneous crust generated at the same time as the dipping reflector sequence beneath this mid crustal layer and above a well defined seismic moho which rises from 27 km continental end to 15 km oceanic end across the margin the present lower crust comprises a 10 15 km thick lens of material with a seismic velocity of 7 3 to 7 4 kds we interpret the present lower crustal lens as underplated igneous rocks left after extraction of the extruded basaltic lavas a considerable quantity of new material has been added to the crust under the rifted margin the present moho is a new boundary formed during creation of the margin and cannot therefore be used to determine the amount of thinning

deep structure of the northern kerguelen plateau and hotspot related activity summary seismic refraction profiles were carried out in 1983 and 1987 throughout the kerguelen isles southern indian ocean terres australes antarctiques franqaises taaf and thereafter at sea on the kerguelen heard plateau during the md66 keobs cruise in 1991 these profiles substantiate the existence of oceanic type crust beneath the kerguelen heard plateau stretching from 46 s to 55 s including the archipelago seismic velocities within both structures are in the range of those encountered in standard oceanic crust however the kerguelen isles and the kerguelen heard plateau differ strikingly in their velocity depth structure unlike the kerguelen isles the thickening of the crust below the kerguelen heard plateau is caused by a 17 km thick layer 3 velocities of 7 4 km s 1 or so within the transition to mantle zone below the kerguelen isles are ascribed to the lower crust intruded and or underplated by upper mantle material the crust mantle boundary below the kerguelen heard plateau is abrupt and devoid of any underplated material the difference in structure between the northern edge of the kerguelen plateau including the archipelago and the kerguelen heard plateau may be related to variability of the time dependent hotspot activity the kerguelen heard plateau was emplaced during the cretaceous time 110 ma when the volcanic output rate of the kerguelen plateau and the ninetyeast ridge was high as well as high potential temperature the northernmost kerguelen plateau and the archipelago were emplaced during tertiary time 40 45 ma as the volcanic output rate reduced furthermore intraplate volcanism continued in the kerguelen archipelago for at least 40 ma the isostatic compensation of the kerguelen isles and the kerguelen heard plateau is achieved by low density mantle material as shown by refraction and geoid studies the velocity depth structure below the kerguelen isles is similar to that found below intraplate oceanic islands such as hawaii despite the differences in age the crust below iceland 0ma and the kerguelen plateau 100 120 ma are strikingly akin the similarity between the kerguelen heard plateau and iceland then strongly supports a similar origin for both structures the kerguelen heard plateau being a fossil equivalent of present day iceland crustal thickening beneath the kerguelen heard plateau which may result from an iceland type setting i e an active spreading centre over a hotspot is mostly produced by thickening of layer 3 layer 2 representing 25 per cent only of the thickness of the igneous crust the kerguelen isles despite the initial volcanism near the active southeast indian ridge behave as a midplate volcanic island and are definitely not representative of the whole kerguelen plateau structure

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

structure and early history of the labuan basin south indian ocean recent multichannel seismic reflection data from the labuan basin in the southern indian ocean are used to reevaluate older single channel data from this region together they throw light on the structure and evolution of this basin situated between the older than 100 ma southern kerguelen plateau and the younger than 43 ma australian antarctic basin the labuan basin is a deep extensive basement depression more than 350 000 km2 in area located adjacent to the eastern margin of the southern kerguelen plateau in contrast to the nearby kerguelen plateau no prominent reflectors are observed within the basement of the labuan basin this suggests a different mechanism of formation from that of the kerguelen plateau which is believed to have been formed by thick sequences of large lava flows the basement surface of most of the labuan basin is presently quite rough as the result of a tectonic event which created prominent tilted block structures and turned it into a large northwest southeast trending syncline there is a 1 1 5 km elevation difference between the labuan basin and the australian antarctic basin created at the southeast indian ridge as well as a large difference in sedimentary thickness between them indicating that the labuan basin is significantly older the boundary of the labuan basin with the kerguelen plateau is generally a steep and somewhat linear feature which appears to be of tectonic origin it seems to result from two extensive tectonic episodes dated at 96 ma and 75 68 ma associated with the prerift phase of plate breakup between australia and antarctica the basement of the labuan basin was created 130 100 ma ago at about the same time than the kerguelen plateau an extensive tectonic episode to the south of the labuan basin initiated the formation of the boundary between the southern kerguelen plateau and the labuan basin this tectonic episode could be linked to the beginning of seafloor spreading between australia and antarctica 96 ma ago sediments were regularly deposited in the basin until 75 68 ma at which time an important extensional tectonic event occurred it involved two large northwest southeast uplifts one centered on the kerguelen plateau affected the western part of the labuan basin while the other of which only the western half is observed affected the eastern part of the labuan basin this tectonic episode seems to correspond to the prerifting episode leading to the breakup between the kerguelen plateau labuan basin and broken ridge diamantina zone at 43 ma after this tectonic episode the sediments of the postuplift megasequence filled topographic lows formed by the normal faults and the tilted blocks the diamantina zone which prior to 43 ma was continuous with the labuan basin can be expected to have a similar tectonic history in particular the rough basement of the diamantina zone can be due to postformation tectonism rather than to slow spreading as previously suggested

basin evolution in oceanic volcanic plateaux seismic reflection evidence from the kerguelen plateau south indian ocean abstract multi channel seismic reflection data from the kerguelen plateau south indian ocean are used to describe the evolution of sedimentary basins on this large feature a number of sedimentary basins are present the largest of which the raggatt basin is over 150 km wide all the sedimentary basins are underlain by basement which shows internal reflection indicative of volcanic flows these reflections define basement wedges and synforms overlain by a marked erosional unconformity which developed early in the history of the plateau intra basement synforms acquired their initial shape during the emplacement of the plateau before the erosional episode the lateral continuity of the reflectors across most of the basement synforms suggests that the volcanic sources of the plateau lie outside of them the basement synforms may represent low areas between elevated volcanoes subsidence of large volcanoes may have caused the subsidence of these basins as flexural moats if so superposition of the subsidence of several volcanoes on the plateau has resulted in the variable shape of the basins it also induced subsidence of basins relative to some adjacent volcanoes this relative subsidence seems to have been active throughout the lifetime of this early cretaceous plateau as indicated by onlaps within the sediments and the small bathymetric deeps associated with the basins the observation that the sources of the volcanic flows lie outside the basement synforms is not consistent with existing models which explain the origin of dipping basement reflectors along passive margins

structure and evolution of the kerguelen heard plateau indian ocean deduced from seismic stratigraphy studies abstract the structure and evolution of the kerguelen heard plateau the northern domain of the kerguelen gaussberg ridge is derived from stratigraphic interpretation of multichannel seismic profiles the kerguelen heard plateau was created between 130 and 100 ma at or near an active spreading center in the gap between the antarctica australia plate and the india plate from the early late cretaceous about 100 ma to the eocene 42 45 ma the kerguelen heard plateau was a shallow marine structure continuously subsiding at a rate of about 20 m ma 1 and covered by pelagic and shelf sediments at 42 45 ma the kerguelen heard plateau and broken ridge were clearly separated by sea floor spreading at the southeast indian ridge from 42 45 ma to the miocene a major gap of sedimentation occurred later the kerguelen heard plateau was covered by pelagic sediments interbedded with thick clastic sedimentary layers coming essentially from kerguelen island

constraints on the crustal nature and tectonic history of the kerguelen plateau from comparative magnetic modeling using magsat data abstract the kerguelen plateau in the southern indian ocean is associated with the broken ridge plateau because of their location with respect to the southeast indian ridge the kerguelen plateau lies south of the mid ocean ridge and the broken ridge plateau is located at a nearly equal distance to the north both plateaus have positive magsat crustal magnetic anomalies associated with them comparative modeling of these anomalies places constraints on the crustal nature the thermal structure and the tectonic history of the plateaus our results support the theory that both plateaus are underlain by oceanic crust are structurally and lithologically similar and were formed as a single structure which has since been split by seafloor spreading active volcanoes on the northern part of the kerguelen plateau suggest that it lies on a hotspot which has raised the curie isotherm to within the crust of the plateau decreasing the crustal volume contributing to the magnetic anomaly and thus decreasing the amplitude of the anomaly

seismic stratigraphy and evolution of the raggatt basin southern kerguelen plateau six major seismic stratigraphic sequences in the raggatt basin on the southern kerguelen plateau overlie a basement complex of cretaceous or greater age the complex includes dipping reflectors which were apparently folded and eroded before the raggatt basin developed the seismic stratigraphic sequences include a basal unit f which fills depressions in basement a thick unit e which has a mounded upper surface volcanic or carbonate mounds a depression filling unit d a thick unit c which is partly middle to late eocene and two post eocene units a and b which are relatively thin and more limited in areal extent than the underlying sequences a mid or late cretaceous erosional episode was followed by subsidence and basin development interrupted by major erosion in the mid tertiary late cenozoic sedimentation was affected by vigorous ocean currents

structure of the kerguelen plateau province from seasat altimetry and seismic reflection data the kerguelen plateau in the southern indian ocean stands 2 4 km above the adjacent sea floor and is 2 500 km long fig 1 1 2 seasat provided a unique data set for deriving the free air gravity field of the region fig 2 3 5 here we report the results of a new analysis of the plateau province s structure employing both seasat and newly acquired multichannel seismic mcs data6 7 for ground truth the northern sector is characterized by volcanism and a sedimentary basin the southern sector by a broad anticlinal arch major faulting and a sedimentary basin and the eastern sector by an abyssal basin labuan and bounding ridge william s the three sectors argue for a more complex tectonic evolution of the feature than has been previously proposed

structure and tectonic history of the southern kerguelen plateau indian ocean deduced from seismic reflection data early single channel and recent multichannel seismic reflection data together reveal major tectonic events in the history of the southern kerguelen plateau skp the skp was created by early cretaceous volcanism which was mainly subaerial we estimate the age of the adjacent ocean basins to be similar to that of the plateau following termination of the main volcanism the skp began to subside and to accumulate sediment a lower sedimentary megasequence was deposited at this time primarily in depressions on the plateau at about 88 ma tectonism affected the eastern raggatt basin this tectonism may be related to faulting which created the steep eastern margin of the skp at about 72 ma predominant tectonic activity characterized by widespread extension and uplift forming the tilted block morphology occurred over large areas of the skp the extension centered along several nw se trending rift systems in places creating well preserved axial rifts the largest structure of this kind the central skp uplift lies along the center of the skp its relief increases from the southernmost part of the plateau toward the nw the northernmost part of this uplift the banzare bank was elevated during rifting above sea level and was eroded a second extensional structure the sw uplift lies in the sw corner of the plateau and other smaller structures may also be present the extension appears to have culminated in the initiation of the southeast indian ridge at 43 ma but at least in the raggatt basin the two events were not continuous tectonic subsidence that was associated with the extension corresponds to deposition of a second megasequence estimated to be about 1000 m thick on banzare bank the subparallel trend of the rift systems on the skp to the southeast indian ridge and their timing suggest that the rearrangement of spreading in the south indian ocean at 43 ma was not solely the result of the collision of india with asia it started earlier in association with other plate motions in the area

dipping basement reflectors along volcanic passive margins new insight using data from the kerguelen plateau multichannel seismic reflection data from the southern kerguelen plateau show many dipping basement reflectors associated with volcanic flows these reflectors are quite similar in their shape to seaward dipping basement reflectors observed along volcanic passive margins on the kerguelen plateau the sources are updip of the basement reflectors in the presently extinct and eroded volcanoes we suggest that the same source reflector geometry may also apply to the seaward dipping basement reflectors observed along passive margins we interpret these reflectors to be the result of volcanism on the passive margin which flowed in all directions into the newly created ocean basin at an early spreading stage

structure profonde du mont ross d apres la refraction sismique iles kerguelen ocean indien austral mont ross is the main volcanic feature of the kerguelen archipelago terres australes et antarctiques francaises this newly formed volcano buildup over 2 ma provides us with an outstanding model

the aegir ridge structure of an extinct spreading axis in 1988 a reflection seismic survey was carried out by the alfred wegener institute for polar and marine research to study structure and development of the aegir ridge an extinct spreading axis in the norway basin a number of volcanolike structures were detected in the central ridge the emerging lava flows dip away from their source in ridge strike perpendicular to ridge strike we observed lava flows dipping towards the ridge center this pattern might represent a more general flow pattern at mid ocean ridges after spreading ceased sedimentation resulted in the deposition of 800 ms two way time twt to 1300 ms twt about 900 1400 m sediments in the central ridge distribution of the sediments indicate subsidence of the central ridge probably due to cooling after termination of spreading this subsidence started no later than late oligocene or early miocene we detected an area where migration of a fluid phase led to obstruction of the sedimentary layering and the formation of mounds and faults at the ocean bottom several sources for the fluid phase are discussed

seismic stratigraphy of the flexural moat flanking the hawaiian islands the elastic thickness of oceanic lithosphere based on flexure studies in the region of surface loads is two to three times smaller than its seismic thickness1 thus as a load ages the thickness of the mechanically supportive part of the lithosphere must decrease from its short term seismic thickness to its long term mechanical thickness extrapolation of data from experimental rock mechanics suggests that thinning occurs by brittle failure in the uppermost part of the lithosphere and thermally activated creep in its lower part2 modelling shows that for load ages less than about 1 myr flexural moats change shape and the peripheral bulges migrate in towards the load3 these transient changes in flexure are best manifest we believe in the stratigraphical sequences that infill flexural moats the largest thicknesses of sediments occur in moats flanking orogenic belts4 5 however the stratigraphical record in these basins is complicated by the effects of compaction sub aerial erosion and sea level changes and by the fact that the loading history extends over very long periods of time here we describe a multichannel seismic reflection profile across a flexural moat flanking the volcanic load of oahu in the hawaiian islands the advantages of oahu are that it is located in the interior of a plate the load is 2 7 myr old and formed within 1 myr6 and the flexural moat was not significantly affected by sea level changes and erosion we show that the observed stratigraphical patterns cannot be explained by changes in material influx load distribution and rapid relaxation of the lithosphere to its long term mechanical thickness rather the observed patterns suggest a prolonged relaxation 1 myr due to either local or regional re heating of the lithosphere this re heating has important implications for the relationship between elastic thickness and age of the lithosphere at the time of loadings

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

basement reflectors in the kerguelen plateau south indian ocean indications for the structure and early history of the plateau the kerguelen plateau is a large structure in the south indian ocean of which the nature oceanic or continental and origin are not known in this work multichannel seismic reflection surveys particularly in the southern domain of the plateau are used to study its origin internal basement reflections characterize most of the investigated area basement reflections together with other geological and geophysical information indicate that the entire plateau is underlain by a volcanic basement suggesting that both the northern and the southern domains have an oceanic origin basement areas with no internal reflections are associated mostly with basement ridges basement reflector wedges associated with volcanic flows from the elevated volcanoes appear on the flanks of these ridges we suggest that the basement ridges are the extinct volcanic sources of the southern kerguelen plateau typical volcanic morphology is absent in the southern domain because it was removed during the erosional phase known to have occurred following the emplacement of the plateau and which is apparent in the seismic data two types of volcanic sources are identified in the southern domain isolated volcanoes and linear sources this pattern is somewhat similar to the volcanic pattern in iceland suggesting that the kerguelen plateau may have been a similar structure related to excessive volcanism associated with a hot spot on a mid ocean spreading ridge in the early cretaceous south indian ocean

the kerguelen plateau records from a long living composite microcontinent abstract the kerguelen plateau lies in the southern indian ocean and is classically considered as a large igneous magmatic province lip emplaced above an oceanic crust the interpretation of the gravimetric bathymetric anomalies shows that the plateau is structured by nnw and nw trending en echelon fractures along its north eastern border and by wnw trending faults along its western side which separate very differentiated zones i e elan bank and enderby basin a geochemical contamination by continental material has been evidenced for all the studied magmatic rocks of the southern and central part of the plateau only the basalts of the northern zone recording the signature of ridge emitted magmas the facies evolution of the overlying sediments also shows that the deposition began in a subaerial environment for the southern part of the plateau and that the subsidence notably increased after 60 ma all these evidences suggest that the southern part of the kerguelen plateau is underlain by stretched continental crust as it is the case for elan bank and that the central kerguelen plateau is most probably also underlain by attenuated continental crust in reconstructions the south and central kerguelen plateaus resulted from the interaction between these remnants of continental crust with the kerguelen plume while the latter interacted with the south east indian ridge to form the north kerguelen plateau we thus propose a cinematic scenario characterised by the formation of a composite microcontinent composed of fragments of stretched continental crust separated by ridge propagators which were emplaced successively or contemporaneously between india and antarctica during the rifting process being triggered by the kerguelen plume emplacement

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

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

a seismic refraction survey in the kerguelen isles southern indian ocean summary two 65 km long on land refraction lines with shot detonated at sea were implemented in the courbet peninsula eastern region of the kerguelen archipelago terres australes et antarctiques francaises southern indian ocean the first profile pi oriented ne sw runs from cap ratmanoff toward mont ross and the second one p2 from pointe suzanne se of the courbet peninsula through the val studer data from both profiles were supplemented by using the bouguer gravity anomaly map not previously studied the data combined standard travel time interpretation wide angle reflected wave study synthetic seismograms and s wave analysis the velocity depth behaviour shows that the mean crustal thickness ranges from 14 to kerguelen resembles those observed beneath aseismic ridges i e the crozet rise and the madagascar ridge average velocity of 5 5 km s on both lines is in the range of those determined for oceanic layer 2 which is there 8 9 km thick average velocity of 6 6 km s is in the range of velocities within oceanic layer 3 the transition to mantle velocity 7 95 km s is best modelled by a positive velocity gradient within the crust this feature is similar to that observed on structures generated near spreading centres these new data com bined with geological and geochemical investigations in the archipelago support an oceanic origin for the kerguelen heard ridge and also broken ridge both structures were joined 40myr ago as shown by the magnetic anomaly pattern refraction studies invalidate the assumption that isostatic compensation is achieved by a 23 km deep crustal root but substantiate the i 17km only varying with the locality the structure of the crust beneath

structure and tectonic setting of the 77 e and 75 e grabens kerguelen plateau south indian ocean the central kerguelen plateau south indian ocean is characterized by abundant north south striking normal faults which comprise two prominent north south rifts known as the 77 e and 75 e grabens the 77 e graben is a well defined structure which extends over some 800 km from the eastern margin of the kerguelen plateau to about 58 5 s over most of its length it is associated with a 10 30 km wide axial rift and with a 100 150 km wide uplift the 75 e graben is less well documented but the available data suggest that its dimensions and internal structure resemble that of the 77 e graben in the better documented 77 e graben six rift segments 50 100 km long are identified faulting is more developed at the northern and southern ends of the 77 e graben possibly resulting from the interaction with other rifts to the north the 77 e graben abuts the highly faulted eastern margin of the kerguelen plateau and the northern part of an even larger rift zone the plate boundary rift zone which extends along the boundary with the australian antarctic basin to the south the 77 e graben adjoins the northwestern end of the southern kerguelen plateau rift zone the 77 e and 75 e grabens and the other rift zones on the kerguelen plateau appear to have been formed at approximately the same time between 72 and 60 ma they are all part of an important extensional phase which occurred in the region and mark the beginning of the process which led to the development of the plate boundary rift zone into the southeast indian ridge between 46 and 43 ma the north south trend of the 77 e and 75 e grabens is different from that of the other rift zones which are oriented northwest southeast this geometry suggests that some strike slip motion may have occurred along the north south trending grabens as a result of extension on the northwest southeast trending rifts particularly the southern kerguelen plateau rift zone however since near surface extension estimates for the southern kerguelen plateau rift zone are small the strike slip motion along the 77 e graben must be equivalently small also the available seismic data from this graben do not show typical seismic characteristics of a strike slip environment such as zones of compression or flower structures thus the results of this work are inconsistent with any model for the development of the south indian ocean which requires significant amount of transform motion on the 77 e or 75 e grabens finally the seismic data from this area provide a unique opportunity to compare rifting on an oceanic plateau environment with continental rifting we find great similarities between the two processes in the segmentation of the rifts the asymmetric cross section and the associated shoulder uplifts

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

the diamonds of south australia abstract diamonds in south australia occur in kimberlites at eurelia orroroo and in placer deposits which include the springfield basin and the historic echunga goldfield to identify the kimberlitic and mantle sources of the placer diamonds and to determine any possible connections between the placer diamonds and the diamonds from the eurelia kimberlites we examined the physical and compositional characteristics and the mineral inclusion content of 122 diamonds from the springfield basin and 43 diamonds from kimberlites at eurelia additional morphological data for three echunga diamonds are also given most of the diamonds from the springfield basin are similar to the diamonds from eurelia with respect to their crystal shapes surface textures and colors the diamond populations from both areas are characterized by a high abundance of low nitrogen

seismic stratigraphy of the raggatt basin southern kerguelen plateau tectonic and paleoceanographic implications during cenozoic and late mesozoic time sediment accumulated in the raggatt basin on the southern kerguelen plateau we describe the seismic stratigraphy of the raggatt basin utilizing multichannel seismic mcs data obtained by the bureau of mineral resources geology and geophysics australia and the institut de physique du globe de strasbourg france seven major seismic stratigraphic sequences in the raggatt basin overlie a basement complex of early cretaceous age the underlying basement complex is characterized by two types of seismic images acoustic basement and a layered basement which includes dipping reflectors the seismic stratigraphic sequences include terrestrial and shallow water units k1 and k2 of early to late cretaceous age which fill depressions in the basement a thick unit k3 of late cretaceous to paleocene age which is of mixed shallow water and open marine facies and in places has a mounded carbonate upper surface a depression filling pelagic deposit p1 of late cretaceous to eocene age a thick pelagic unit p2 which is mainly eocene in age and two post eocene pelagic sequences pn1 and nq1 which are relatively thin and more limited in areal extent than the underlying sequences highlights in the geologic history of the raggatt basin include formation and erosion of the basement complex in a subaerial or shallow water environment in early cretaceous time differential subsidence probably thermal in origin and development of carbonate mounds and a major western boundary transform fault in late cretaceous time renewed subsidence near the cretaceous tertiary boundary possibly related to formation of the labuan basin diamantina zone by sea floor spreading and vigorous antarctic circumpolar current activity beginning by oligocene time probably related to the breakup of the northern kerguelen plateau and broken ridge

new findings from the kerguelen plateau new geophysical data confirm the existence of marked structural and morphological differences between the northern and southern sectors of the kerguelen plateau a major sedimentary basin raggatt basin covers an area of at least 30 000 km2 on the eastern part of the southern plateau a deep oceanic basin labuan basin with extremely rough basement lies adjacent to the southeastern flank of the plateau we suggest that this may be the former western extension or conjugate feature of the diamantina zone samples of rock obtained by free fall grabs suggest that the southern kerguelen plateau contains continental crust