The Prismed II cruise, carried out on board the research vessel "l'Atalante", surveyed the Nile deep-sea fan, using multibeam bathymetry, acoustic imagery and high resolution seismics. The Nile deep-sea fan can be divided into three morphological, structural and sedimentological provinces. The western Province is characterized by a nearly uniform sedimentary apron, dessected by a network of meandering distributary channels. Their lengths can exceed 200 km and their orientation is northwestwards. They are flanked by sedimentary levees, several tens of meters to one hundred meters high. Their tight meanders seem similar to the meanders of the Rhone, the Amazon, or the Zaire deep-sea fans. Some of the channels, especially in the western part of this province are recent, while others seem much older. The upper continental slope is characterized by sedimentary instability, expressed in numerous growth-faults and slumps. These synsedimentary faults are closely linked to salt domes, which ascend from the Messinian evaporitic layer underneath. The vertical throw of the faults reaches 200 m, and the thickness of the sediments that they offset exceeds 2000 m. The sedimentary apron is built of layers which display sometimes chaotic acoustic signatures representing probably debris flow deposits. In the distal section of this province, the compressive tectonic regime of the adjacent Mediterranean Ridge leads to a series of folds, with reverse faults in places. The central province is distinguished by numerous growth-faults, which extend along a stretch of 200 km, with throws of 100-200 m. In the southwestern part of this province, some channels display internal sedimentary structures, wich suggest western migration of the sedimentary distributary system of the deep-sea fan. The eastern province is abruptly separated from the central province by a NNW-SSE trending fault system that extends for more than 200 km and is nearly 100 km wide. This fault system also forms the southwestern boundary of Eratosthenes Seamount. The seismic profiles indicate that the faults are deep, the offset along them is probably strike-slip, and the fault planes are commonly penetrated by Messinian salt diapirs. These features can be linked to the possible northern extension of the Suez Rift. In spite of this active tectonic regime, Nilotic distributary deep-sea channels can be discerned. But most of them appear dislocated, their orientation apparently controlled by the faults. In places it seems that the channels are linked to the dissolution of the salt diapirs, and apparently brine pools may occur there in places. In the eastern border of the eastern province, the Sinai continental margin is marked by a conspicuous escarpment of more than 500 m high. Another series of deep-sea channels are encountered east of Eratosthenes Seamount. These channels seem associated with sedimentary supply originating from the central Levant area.
The Eratosthenes seamount and surroundings situated in the Eastern Mediterranean have been surveyed during the PRISMED II cruise (February 1998), using Simrad EM12D swath mapping and acoustic imagery, high speed seismic reflexion, 3.5 kHz echo sounding, gravity and magnetic measurements. From the resulting new and accurate data, the morphology, the surficial structure of the seamount and the relationships with the surrounding structures are precised. The Eratosthenes seamount is a large (120 x 80 km) submarine plateau which dominates the Nile Deep sea fan to the South and separated to the North from Cyprus by a narrow East-West trending flat abyssal plain. This uplifted block is an elliptical structure rising at a water-depth of about 700 m and towers the surroundings which lie at depths of more than 2000 m. The seamount is flat-topped, slightly inclined northward and sliced by a series of East-West trending normal faults which affect both the thin Plio-Quaternary sediments and the underlying strong reflectors attributed to Miocene or older. The northern flank of the seamount, widely affected by normal faulting dipping to the North, is progressively downthrown and plunges beneath a flat plain bottomed by a wedge-shaped infilling made of Plio-Quaternary sediments. This E-W trending basin which marks the contact zone between the Eratosthenes seamount and the Cyprus thrust zone shows a double deepening polarity. Northward the wedge thickens toward the Cyprus margin where it is thrusted by highly deformed Plio-Quaternary sediments and to the West the Plio-Quaternary infilling increases rapidly and the basin widens to connect with the abyssal plain. Evidence of compressive deformation of the Plio-Quaternary sediments is clearly identified along the foot of Cyprus margin. The southern flank of the seamount is gently sloping and unaffected by faulting except along its Southeastern edge where it is crosscut by a N150E trending fault zone. The contact zone between the southward dipping reflectors of Eratosthenes and the Nile Deep sea sediments is faulted and marked by a scarp formed by disturbed material including Plio-Quaternary sediments mixed with evaporitic material. The Erastosthenes seamount interpreted as a continental fragment of the African Craton constituted by Mesozoic carbonates (Mart et al., 1997) overlain by a thin Plio-Quaternary sedimentary blanket is pushed northward and thrusted beneath the Southern Cyprus margin (Robertson et al., 1995). The intense normal faulting which affects its northern flank is a reaction to its downward pull under Cyprus while the compressional deformation attests of the present thrusting along the Cyprus margin.
Mart Y, Robertson AHF, Woodside JM & ODP Leg 160 shipboard science party, C. R. Acad. Sci, 324, 127-134, (1997).
Robertson AHF, Kidd RB, Ivanov MK, Limonov AF, Woodside JM, Galindo-Zaldivar J, Nieto L & the Scientific Party of the 1993 TTR-3 Cruise, Terra Nova, 7, 254-264, (1995).
Geophysical surveys PRISMED1 (1993) and, more explicitly, PRISMED2 (1998) provided evidence for mud extrusion over wide areas of the central part of the Mediterranean Ridge accretionary prism between Libya and Crete. In a zone of incipient continent-continent collision between Africa in the south and Eurasia in the north, a c200 km wide and several km thick wedge has been accreted. Recent acoustic imagery, bathymetry, and MCSeismic profiling allowed to discriminate between three different mud domains. First, the 'outer belt' adjacent to the deformation front off Libya consists of small mud volcanoes (generally less than 1 km in diameter) which most likely relate to tectonic dewatering of the toe of the accretionary prism. Second, some 150 km north of the deformation front, namely at the backthrusted central part of the prism, a series of bigger mud volcanoes (up to c 5 km across) forms the 'inner belt' (with the prominent 'Olimpi field' with numerous active mud volcanoes). These features are believed to result from ascent of overpressured mud from décollement depth. A third product of expulsion of liquefied mud are three circular mud pies southeast of Crete. The features are well imaged both on acoustic backscatter and bathymetric maps and indicate a size of c12-25 km in diameter. Both seismic lines and bathymetry data illustrate both positive and negative reliefs of approximately 100-150 m relative to the surrounding seafloor, which is most likely a function of the time of extrusion. The biggest feature has a c150 m deeper water depth than the surrounding seafloor, suggesting that a collapse may have occurred after expulsion of the mud some time ago. The extruded volume of the three mud pies, as estimated from depth-migrated MCS lines, exceeds by far the amount of mud expelled along the 'inner' and 'outer' mud volcano belts. The local thickness of the seismically opaque mud varies from c100 m to more than 600 m. The origin of the mud is largely unknown, but may most likely be connected with transtensional faults cross-cutting the accretionary prism parallel to its regional strike (SW-NE).
The Rhodes Basin is a deep (>4400 m) basin lying between the mountains of southwest Turkey and the Island of Rhodes, immediately to the east of Rhodes and in continuity with the Pliny and Strabo Trenches along the eastern Hellenic Arc. Detailed geophysical mapping of the Florence Rise during the 1998 French PRISMED-II Expedition indicates that it is a post-Miocene basin which has probably developed both by the same rifting that created the grabens north of Rhodes and to the east (e.g. Finike Basin), and by the continuing transform tectonics along the Pliny/Strabo Trench system and extending, according to seismicity, into southwestern Turkey. There are no significant Messinian evaporites in the basin and the post-Miocene sediment thickness is less than 1 km. The Plio-quaternary sediments in the basin are more or less undeformed; however, the underlying unit, presumed to be Cretaceous to Miocene rocks of Hellenide/Tauride affinity, shows increasing deformation towards the north and west, especially north of a low asymmetric ENE-WSW rise dividing the basin into two subbasins at about 35N45. In the south the lower unit is observed to have contorted stratification beneath an angular unconformity. Strike-slip faults define the western margin of the basin and NE-SW through the northern basin where a flower structure is observed. The northeastern margin is inferred to be faulted on the basis of a NW-SE through the western part of the Anaximander Mountains. Sediments from the Mediterranean Ridge are invading the basin from the south in the form of a nappe with thrusts clearly seen through the toe of the nappe.
During the PRISMED II cruise carried out on board the research vessel L'Atalante in February 1998, more than 50000 Km2 of the sea floor of the central Mediterranean Ridge (between Southern Crete and the lower Libyan continental margin) have been mapped at a 100% coverage. The data include swath bathymetry, acoustic imagery and about 3000 km of continuous seismic reflexion, 3.5 KHz, gravity and magnetic measurements.
This presentation focuses mainly on the tectonics of this deformed sedimentary wedge which is apparently involved in the incipient continent-continent collision between the Southern Aegean microplate and the African continental margin.Various distinct structural domains are distinguished on the basis of their backscattering pattern and sub-surface deformation. They are from south to north: (a) An outer domain directly overthrusting the Libyan margin and characterised by strong surface deformation, both resulting from sub-surface ductile deformation (short wave-length folds) and deep brittle deformation (strike-slip faults, thrusts). (b) A relatively flat central area, characterised by a globally weak backscattering appears to be deformed in more or less linear belts. To the north, this area shows a series of high backscattering patches correlating with important mud and fluid expulsions (Gavdos, Olympi and United Nations mud volcanoes fields). (c) The central domain is bounded to the north by a long, curved topographic step in, the so-called inner escarpment. The elevated position is related to backthrust faulting of the central area against a rigid Cretan backstop. (d) Finally, the Mediterranean Ridge faces northward a deeper basin-plateau-like area which displays a disturbed thick Plio-Quaternary sedimentary section overlying Messinian deposits. This area, which is disconnected from the Cretan continental margin by the Pliny trench system, is interpreted as a continental backstop.
The correlation between surficial data (backscattering and bathymetry) and seismic reflexion profiles leads to three main results. First, the activation of an indenting process, probably responsible of a crustal deformation in front of the Libyan promontory and a general lateral escape within the accretionary complex. Second, the superposition of three distincts types of deformation: (i) large scale, probably crustal, tectonics, (ii) transtensional tectonics, most likely related to the lateral escape generated by the indenting process and (iii) ductile tectonics, more or less identified within the whole accretionary wedge. Finally, it appears that the structural style of both the outer Mediterranean Ridge and its backstop is closely related to the presence and distribution of Messinian evaporitic formations at depth.
The Vema fracture zone is the northernmost of a set of major transform faults which displace the Mid Atlantic Ridge (MAR) at 10°50' N by over 320 km. It is paralleled on its southern flank by a prominent transverse ridge, the summit of which shoals up to 450 m below the sea level near the western ridge transform intersection. The north facing wall of the Vema transverse ridge exposes a relatively complete and undisturbed sliver of upper oceanic lithosphere. Consolidated pelagic limestones locally encrust the exposed igneous rocks, while shallow water carbonate platforms, up to 600 m thick, cap the summit of the transverse ridge. The Vema transverse ridge originated by vertical movements that have uplifted a flexured sliver of oceanic lithosphere. Multichannel seismic reflection profiles taken across the transverse ridge and the transform valley give evidence of recent transpression in the valley, and of extension across the transverse ridge and in the area south of it. Vertical movements might be due to these transpressive and transtensional stresses, related to small changes in spreading direction. Pelagic, as well as shallow water limestones recovered from the transverse ridge, will provide age constraints on the emplacement history of the lithospheric sliver exposed on the transverse ridge.
The first bathymetric image of the Makran accretionary wedge off Pakistan obtained from SONNE cruise 123 Hydrosweep data revealed a strike slip fault as the most prominent tectonic feature. The sinistral strike slip fault named Sonne fault is cutting the whole Makran accretionary wedge striking northwest-southeast and continuing in the abyssal plane. We indentified this strike slip fault as a plate boundary, separating a micro-plate from the Arabian plate, named Ormara plate. The Ormara micro plate has a triangular shape with all three types of plate boundaries and a triple junction at all its corners. The identification of the Ormara Plate was exclusively from bathymetry, however, several other data including the distribution of seismicity, the different dip angle of the Arabian and Ormara plates, the variations in distance between the trench and the arc volcanoes, and the presence respectivly the distribution of mud volcanoes and gas lenses confirm our hypothesis. In general, seismicity in the Makran subduction zone is suprisingly low compared to other active margins, however, a magnitude 8.1 event in 1945 revealed the potential for very large intraplate thrusts earthquakes. The Sonne fault as a plate boundary separates the western and the eastern segments of different seismicity. The western segment is characterized by very low seismicity and the eastern segment shows a significantly higher seismicity. The offset of the accretionary ridges running nearly perpendicular to the convergence direction increase with distance from the trench. This indicates that the Sonne fault has been active either over a long period or being reactived several times. The convergence rate between the Ormara and Eurasian plates is greater than that between the Arabian and Eurasian plates which is also evident from geodetic data. Detailed numerical analyses of stress and strain of this region should confirm our observation.
SEISCAN is not about collecting data and freely distributing it. It is about providing the opportunity to archive securely without cost. The project also addresses everyone's responsibility for preserving information for the future. SEISCAN is designed to rescue the early single channel line-scan seismic records, a concept derived from difficulties tracking down old seismic records from the deep-water North Atlantic.
The National Geophysical Data Center at Boulder held photographic prints of the early Lamont-Doherty Geological Observatory seismic sections. They had decided to scan these photographs to establish a new image database. It was evident that the same need existed throughout Europe. This led to successful MAST-3 funding of SEISCAN (MAS3-CT97-0101) in October 1997. The project uses a Pan-European hardware and software strategy - Sun Ultra workstations, Colortrac A0 scanners and 'Cameleo' imaging software from Caldera Graphics in Strasbourg, France.
The track coverage scanned to date (50,000 plus line km) includes seismic sections from academic institutions across the EC, and some outside it! The target region covers oceanic waters from the shelf edge out to the EEZ limit. Design of a metadata skeleton strategy enables contributors to provide basic information to the project on their holdings in these waters with minimum effort. Thereafter SEISCAN manages the task and returns the original records with CD-ROM images. A Project Information Booklet is available from SEISCAN.
Data in any form is personal to either a funding body, collecting institution or, in some cases, an individual. For the project to succeed, confidence was required in the team, in their integrity and their dedication to security. The umbrella of EC funding was best placed to provide the framework for this. All members of the SEISCAN project stress that their interest is to secure these data for the future and not to gain access to it. That will remain the prerogative of the data owners, on whose co-operation, the project, and the future of these data depends.
High resolution chirp sonar data and piston cores have been collected from the Lomonosov Ridge (85-90°N; 130-155 °E). The acoustic stratigraphy shows substantial erosion on the ridge crest, relatively close to the sediment surface. More than 50 m of the stratigraphic section is estimated to be missing in the most eroded parts. Five selected piston cores, located along a nearly north-south transect along the ridge crest, have been analyzed in detail with respect to physical properties, magnetic properties, shear strength, and calcareous nannofossil biostratigraphy, permitting inter-core correlation and the establishment of an age model. All cores were retrieved in the vicinity of chirp sonar profiles in order to attain control of the depositional environment at the site and to integrate acoustic data with core data. After the erosional phase(s) the sediment deposition appears to be near continuous on the Lomonosov Ridge crest from the base of OIS 5. The erosional phase(s), caused either by ice grounding at ca 1000 m below the present sea level or by bottom currents, terminated during the transition between OIS 6 and 5. If ice grounding was the cause, extensive glaciation of the northern Siberian shelf areas during OIS 6 may have produced the candidate icebergs that scraped the top of the Lomonosov Ridge crest.
The NW-SE Bivrost Lineament, along the landward prolongation of the oceanic Bivrost Fracture zone, forms the northern boundary of the Vøring basin (mid Norway continental shelf). The lineament appears as a large saddle zone between the structuraly elevated Lofoten margin and the Vøring basin extending east to the Nordland Ridge. The character and the evolution of the lineament are studied by mapping faults patterns and sediment distribution on either side. The geometrical and structural styles are different on either side of the lineament, and the base Cretaceous, base Tertiary, top Paleocene unconformities illustrate the variation in vertical relative motion between the Naglfar region and the Røst High before the continental break-up, near the Paleocene-Eocene transition. Our preliminary interpretation suggests the importance of the Bivrost lineament zone during the Late Creataceous and Paleocene times, when it may be linked to local pull-appart basins or an accomodation zone.
The Pac/Aus plate boundary south of New Zealand runs, from north to south, along the intracontinental transpressive Alpine Fault, the oblique (60°) Puysegur subduction and the intraoceanic transpressive Puysegur fault. A Tectonic analysis of the Puysegur margin, based on marine geophysical data indicates: 1) The Alpine Fault extends across the western Fiordland margin (SW South Island) and connects at sharp angle (35°) with the Puysegur subduction front. The transition is accommodated by a tear in the Australian plate along a crustal discontinuity that aligns with the Alpine Fault. 2) The Puysegur subduction, also well developed northward, becomes incipient southward. 3) Transfer of plate motion from the subduction interface to the Puysegur fault occurs through a 150 km wide transpressive deformation zone located immediately south of the continental Fiordland margin. 4) We demonstrate that the northern part of the relay zone develops between the Alpine and Puysegur faults, prior to subduction inception, concomitantly with the Alpine Fault initiation.
These results help to constrain a plate kinematic reconstruction. 1) In Eocene time, plate motion induces opening of the SE Tasman Basin across New Zealand continent. In South Island, rifting occurs East of Fiordland. 2) In Oligocene time, strike slip motion initiates in southern New-Zealand, propagates southward and creates the Puysegur Fault. To the south, basin opening becomes more and more oblique to accommodate motion changes. 3) at 23 My the Alpine Fault initiates West of Fiordland. A transpressive relay south of Fiordland links the Alpine and Puysegur Faults. 4) Around 20 My, the oceanic crust of the basin is laterally transferred along the relay zone and eventually faces the continental crust of Fiordland. The western passive margin of the basin progressively aligns with the Alpine Fault. 5) Between 20 and 15 My, the Puysegur subduction initiates along the relay zone, and the passive margin structures control the development of the Alpine Fault. 6) Around 11 My transpression increases along the Puysegur Fault and leads to the Puysegur ridge uplift. Spreading ceases in the basin and strike-slip motion occurs along the plate boundary. 7) In Pliocene time, subduction propagates along the Puysegur ridge toe and isolates the western part of the ridge from the Australian plate.
This study allows to identify 4 geodynamical factors that control initiation of subduction along a strike-slip plate boundary: 1) A transpressive relay between two strike-slip segments localise compressive deformation. 2) Strike slip motion at the relay zone eventually juxtapose crusts with different thicknesses and densities. These parameters appear to be necessary for subduction initiation. 3 and 4) progressive increase of plate convergence and, inherited structures favour subduction initiation and control its propagation.
The EDUL cruise sampled 89 sites along the neo-volcanic zone of the ultra-slow spreading Southwest Indian ridge (SWIR, Half spreading: 7 mm/year) from 69°E, near the Rodriguez triple junction (RTJ) to 49°E, 300 km West of the Gallieni fracture zone. The depth of the axial ridge decreases from 5500 to 2000 meters, from East to West. A change in its morphotectonic segmentation occurs near 62°E, close to the Melville Fracture Zone (61°E) between 49° and 70°E. Basaltic glasses show unusually high Na2O and TiO2 similar to the Cayman Trough and the Antarctic-Australian Discordance. Most samples are depleted MORB with (La/Sm)n<1.
Between 61°E, Melville fracture zone (MFZ) and RTJ, the basaltic glasses show higher values for Na8.0, Zr and lower values for Ti8.0 than those to the West of the MFZ and those of typical N-MORB of Atlantic and Pacific ocean. These compositional features are unique for MORB. They also display higher (Ce/Sm)n, Zr/Y than the region to the west of the MFZ. Between 49° and MFZ, basaltic glasses define a compositional group which is part of the global correlation of N-MORB, for Fe8.0, Na8.0 et Ti8.0. The ratios such as (Ce/Sm)n, Zr/Y and Na8.0 show a coherent evolution with axial depth. They decrease with the increasing depth from east to west. These chemical variations can be interpreted as the result of a gradual increase in the extent of melting.
These data may indicate a difference of source between the two regions, associated with a different thermal state of the mantle and/or a complex liquid line of descent related to propagation of the SWIR into the older oceanic lithosphere of the Indian ocean. In particular, as a consequence of the low extent of melting from the region to the east of the MFZ, the unusual MORB compositions permit the identification of a source component unique for the Indian ocean.
The pioneering Pb isotopic study of Fe-Mn nodules by Chow and Patterson (1959) demonstrated the existence of Pb isotopic variability in the oceans. Von Blankenburg et al. (1996) suggested that Pb is efficiently mixed in the Pacific based upon the uniform Pb isotopic compositions found in outermost layers of Pacific Fe-Mn crusts. This view contrasts with the observed NE-SW Pacific trend reported by the previous authors. To assess the extent to which Pb exhibits isotopic variability or is well-mixed in the Pacific, we performed high precision (2<sigma>ext < 100 ppm) Pb isotopic analyses using a Pb triple spike, together with Nd isotopic measurements, on surface scrapings of forty-five Fe-Mn nodules and crusts covering the whole Pacific basin. Pb isotope ratios display small but none the less resolvable variations with 206Pb/204Pb = 18.52 to 18.84, 207Pb/204Pb = 15.55 to 15.65, and 208Pb/204Pb = 38.16 to 38.85. Three geographic Pb isotopic provinces are distinguished: (1) the NW Pacific has the least radiogenic Pb, (2) the Central Pacific has intermediate Pb isotope ratios, while those in (3) the South Pacific are the most radiogenic. Superimposed on these regional variations, there is asystematic increase in 206Pb/204Pb from the NW toward the SE Pacific, confirming the NW-SE trend originally observed by Chow and Patterson (1959). Low Pb isotope ratios seen near ocean ridges and distinctive Pbisotopic compositions close to the basin margins suggest some control by local Pb inputs. <epsilon>Nd values are more radiogenic in the North compared to the South Pacific, in agreement with previous studies (Albarède and Goldstein, 1992). As water depth increases, <epsilon>Nd values decrease while206Pb/204Pb ratios increase in the NW and NE Pacific but appear independent of depth in the South Pacific. In Pb isotope space, two distinct geographically-related linear arrays are observed, indicating the presence of at least four distinct Pb sources to the Pacific ocean. The first array is defined by relatively constant 207Pb/204Pb ratios and increasing 206Pb/204Pb ratios from the NW toward the NE Pacific; the second array, represented by samples from the South and central Pacific, overlaps that defined by Fe-Mn nodules from the Pacific sector of the Circum-Antarctic ocean (Abouchami and Goldstein, 1995), reflecting the incursion of Southern Component Water (SCW). There is no evidence for a SCW influence in the NW Pacific sector based upon available data. Thus, the fact that Pacific Pb exhibits less isotopic variability than Atlantic Pb is not due to efficient mixing in the Pacific. Rather, it is clearly due to the relative isotopic homogeneity of Pb sources contributing to the Pacific basin compared to those in the Atlantic.
The Florence Rise is a gentle seafloor rise extending west from Cyprus to the Anaximander Mountains. Detailed geophysical mapping of the Florence Rise during the 1998 French PRISMED-II Expedition indicates that it does not currently act as an accretionary prism associated with the Cyprus arc as had previously been thought. It was probably a positive bathymetric feature in late Miocene time because it has no significant deposits of Messinian evaporites in contrast to the basins to the north and south, and there is an angular unconformity in places at the base of the Pliocene sediments. A core ridge structure narrows and deepens westward and may now be subsiding. It is bounded to the south by a wrench zone and on parts of the north flank by thrusting. Current tectonic deformation is related to the adjustment along the Florence Rise of the collisional/compressional plate interaction between the westward moving Cyprus and the northeast moving African plate. A large proportion of this is passive superficial deformation caused by the invasion of thick Herodotus Basin and Mediterranean Ridge sediments from the southwest into the area of the Florence Rise. The rise appears to be broken into two or three sections by NE-SW strike-slip faults which also bound the highest standing part to the northwest and southeast. The tectonic situation is in keeping with the westward movement (escape) of the Anatolian plate (and Cyprus) and relative east-northeast to west-southwest convergence between the Anatolian plate and the African plate. The relative pole of rotation for this convergence is not likely to be too far to the south, which implies that the associated deformation is relatively slow.
This paper presents the continuation of an integrated geophysical-geological study of the western continental shelf of Sardinia (Fais et al., 1996) in its southern part. The studied area overlooks the Sulcis archipelago (Islands of Sulcis) and is located over a submerged crustal block that is separated from south-western Sardinia by a system of extensional faults. This stretch of shelf is affected by the magnetic anomalies under study and is clearly subdivided into two parts: an internal part where volcanic rocks underlie a weak Quaternary cover, and an external part where Middle-Upper Miocene marine sediments underlie the Quaternary. Outwards, in the upper part of the continental margin, the Quaternary sediments prograde over weak thicknesses of Miocene sediments that in turn overlie the pre-Oligo-Miocene rocks (Lecca et al., 1986; Thomas et al., 1988). As is known, the continental crust of Sardinia is made up of variously metamorphosed sedimentary rocks and granitoid rocks dating back to Hercynian Orogeny. Mesozoic and Palaeogene sequences of moderate thickness, marginally involved by deformative Laramic and Pyrenean events, overlie these Hercynian rocks. The system of extensional faults that separates the block of the continental margin from the rest of south-western Sardinia was affected by andesitic and rhyolitic calc-alkaline volcanism in the Lower-Middle Miocene. This volcanism gave rise to the emplacement of andesites inside Sulcis basins as early as the end of the Oligocene - beginning of the Miocene. The volcanism continued in the Lower-Middle Miocene, with andesites and ignimbrites that occupy more external, i.e. western, positions.The study area is covered by an aeromagnetic survey of Sardinia that was carried out by the Compagnie General de Geophysique (CGG) on behalf of AGIP and Ente Mineranio Sardo (E.M.Sa.). The interpretation of the aeromagnetic data was carried out by the analytic signal method and by Werner and Euler deconvolution techniques. In the presented geological context these methods showed a high resolution power that allowed to define lateral contacts corresponding to magnetic susceptibility discontinuities. The results of these inversion techniques (depth and horizontal location of the magnetic causative bodies) have been used as constraints for the 2.5 D modelling. Moreover this area is covered by reflection seismic lines from various surveys, such as Flexotir, Air-gun, Sparker, by which it has been possible to recognise the main structural and stratigraphic elements. The integrated analysis of aeromagnetic and seismic data together with the on-land geological knowledge allowed a more consistent interpretation of the main structural features of the south-western sector of the Sardinian shelf. The magnetic anomalies in the studied area are attributable in the first place to the presence of andesitic bodies and secondly to the presence of extensional fault zones.
Fais S, Klingelee E & Lecca L, Marine Geology, 133, 203-222, (1996).
Lecca L, Carboni S, Scarteddu R, Sechi F, Tilocca G & Pisano S, Mem. Soc. Geol. It, 36, 31-40, (1986).
Thomas B, Gennesseaux M & Lecca L, Marine Geology, 83, 31-41, (1988).
In May 1998 during the Margau cruise, R/V Marion Dufresne collected ~12.000 km of geophysical data along the Southwest Australian margin and in the Diamantina Zone between 108E and 120E. These data include swath-bathymetry, reflectivity, magnetic, and gravity data, and few single-channel seismic profiles. These data unveil in great details the structure and the geometry of the transition zone between the Australian continental margin and the oceanic crust in the Australian-Antarctic Basin. Basement rocks were also recovered along two transects across the continent-ocean boundary (COB) (Beslier et al., 1999).
Three E-W oriented structural units are observed; from north to south they are: 1) the steep continental slopes of Australia and Naturaliste Plateau (NP), 2) a flat and sedimented zone, 150-200 km wide, and 3) a rough zone, 100 km wide, corresponding to the Diamantina Zone and its extension south of Australia. Theses elongated areas are segmented by two major fracture zones, striking NW-SE. To the east, the Leeuwin FZ separates Australia from the NP; to the west, the Naturaliste FZ bounds the NP. The fracture zone trends match the Early Cretaceous spreading direction in the Perth Basin (north of the NP) as well as the Late Jurassic transfer faults along the south-Australian margin. The two fracture zones abut against the rough Diamantina Zone. South of the NP, and east of the Naturaliste FZ, a SW-NE seafloor tectonic fabric similar to abyssal hills is observed, suggesting the presence of Mesozoic oceanic crust.
The Diamantina Zone is characterised by an east-west oriented fabric made of a series of discontinuous ridges, 20 to 50 km wide and culminating at 3.3 to 3.6 km, separated by 6 km deep trenches. The southern trench, marking the limit between the Diamantina Zone and the Eocene oceanic crust, is continuous and narrow (~20 km) and the deepest (6.5 km). The northern trench is made of a series of parallel and discontinuous segments. The continuity and the E-W structural grain of the Diamantina Zone between 108E and 120E suggest a N-S episode of extension or of ultra-slow seafloor spreading between Australia and Antarctica.
These observations and the presence of mantle outcrops in the sedimented zone at the foot of the continental slope and in the Diamantina zone suggest that amagmatic extension and/or ultra-slow spreading occurred after continental break-up and before oceanic accretion initiated. The width of the COB (150-200 km) implies that the continental break-up occurred much earlier than the oldest magnetic anomalies in the Australian-Antarctic Basin (chron 34 - 83 Ma). Opening of the eastern Indian Ocean started by two phases of tectonic extension initially along a NW-SE direction (with dextral shear motion along the Leeuwin FZ) and then N-S.
Beslier M-O, Royer J-Y, Hill PJ & Margau Shipboard Scientific Party, Mantle exhumation at a rift zone: evidence for a wide Ocean-Continent Boundary along the Southwest Australian margin. J. Conf. Abs., 4, (1999).
The recent CALMAR survey (R/V Atalante, November 1997) was conducted in the eastern part of the Gulf of Lions in order to complete the bathymetric map of the Catalano-Languedocian margin using the SIMRAD EM12 Dual multibeam system. Simultaneously, seismic (GI gun) and 3.5 kHz acoustic data were collected. A prominent ridge situated at the base of the slope, off Pyrenean canyons, was mapped. The main morphological feature of this ridge consists of an undulating topography which reminds sediment waves fields known elsewhere in the world. Indeed, 3.5 kHz and seismic profiles display asymmetric undulating sedimentary structures characterised by the upslope migration of their crests and by erosion on their downstream steep sides. The ridge appears as a thick sedimentary body, named Pyreneo-Languedocian Sedimentary Ridge (PLSR), displaying typical facies of sedimentary levees with almost sub-parallel and continuous reflectors consistent with turbidity deposits. In this sector, seismic data show that the PLSR constitutes most of the Quaternary deposits. The supposed Plio-Quaternary boundary (G reflector) can be traced throughout the whole area and marks an important discontinuity. Underlying Pliocene deposits are affected by numerous listric faults linked with Messinian salt rollers. In opposition, Quaternary sediments are not so disturbed and exhibit an almost stratified seismic facies. CALMAR and previous surveys data allowed us to draw the isochron map of the G reflector, the isopach map of Quaternary deposits and the structural map of Plio-Quaternary faults. In the north- western part of the PLSR, these above described maps and bathymetric chart indicate a good correlation between the structural trends, the isochron of G reflector paleotopographic orientations and the ENE-WSW trending of sediment waves. A good correlation has also been observed between the western PLSR boundary and the «G» paleotopographic trends. This boundary runs almost parallel with the Messinian salt layer N-S trending Eastern limit clearly correlated with underlying basement lineaments. Finally, the northern limit of the PLSR corresponds to the Creus Cape Canyon and to its associated fault. Nevertheless, the influence of the structural inheritage appears to be restricted to the topographic trends and to the western and northern boundaries of the PLSR. Indeed, there is no obvious correlation between the geometry of the G reflector (paleotroughs) and the PLSR depocenter location which may have been mostly controlled by the orientation of sedimentary inputs through turbidity currents. The origin of these currents may be searched in Creus Cape Canyon overflows. The turbidity origin of PLSR deposits is quite credible but the undulating topography still remain a problem because contour currents can generate a similar morphology. Coring and high resolution seismic data have been recently acquired (PYLRIDGE cruise, R/V Tethys October 1998) and will complete our knowledge of the structure of recent PLSR deposits.
The recent PRISMED II cruise (R/V "L'Atalante") allowed to survey the Nile deep-sea fan and surroundings (Eastern Mediterranean Sea), using multibeam bathymetry, acoustic imagery (Simrad EM12 Dual) and high resolution seismics. The objective was a comprehensive study of the morphological, structural and sedimentary characteristics of this area involved in incipient continental collision between Africa and Europa.
Rock-salt layers generally display a gravitational anomaly in the lithological sequences. While the average density of most of the sedimentary rocks is 2.5-2.6 g/cm3, salt is characterized by a low density (2.2 g/cm3). Consequently, salt layers create a density inversion, and when the adequate conditions occur, they flow upwards to form diapirs. The structural meaning of the upward flow conditions is a weakness surface in the overlying sedimentary cover. Such surfaces are preferably normal faults, but salt diapirs are known to ascend along strike-slip and even thrust faults (Vendeville and Jackson, 1992).
This paper especially deals with the salt response to the interacting stresses generated both by the Nile deep-sea fan thin-skinned tectonics and the convergent or/and transcurrent geodynamics. On the upper continental slope of the Nile deep-sea fan, linear buried or outcropping salt diapirs have been evidenced during the survey, as well as numerous listric growth faults, roughly perpendicular to the slope line. These syn-sedimentary faults, with a vertical throw up to 200 m, affect the thick Plio-Quaternary sequence (up to 2000 m) and are deeply-rooted in the underlying layer of Messinian evaporites, acting as a "décollement". Some diapirs are associated with collapse structures, forming elongated basins approximately 150-200 m deep, suggesting salt dissolution or keystone grabens. The diapirs are oriented along two major trends: NNW-SSE and NE-SW. In places, the NE-SW diapirs are horizontally offset by NNW/SSE-trending faults, injected by evaporitic material. The latter diapirs appear to be passively developped along a transtensional tectonic direction while the former ones are firstly due to active gravitational gliding of the Plio-Quaternary cover above the décollement. This mechanism generates thin-skinned extensional tectonics and resulting structures such as normal growth faults, sedimentary rollover anticlines, stratigraphic wedges and salt rollers, that will be emphasized by the high Nilotic sedimentation such as for the Rhône Cone (Gaullier, 1993). The NNW-SSE trend in the Levant is probably related to the Red Sea and the associated diapirs appear parallel to the structural axis of the Suez rift. In the distal part of the Nile Cone, the compressive tectonic regime of the adjacent Mediterranean Ridge leads to a series of folds and reverse faults injected by salt material.
Gaullier V, Thèse de Doctorat de l'Université Paris VI, 330 p, (1993).
Vendeville BC & Jackson MPA, Mar. and Petrol. Geol, 9, 331-353, (1992).
Albania is part of the Alpine Mediterranean shrivelled belt in the Dinarido-Albano-Helenic branch. Depending to the Jurassic-Cretaceous fold-formation stages, the outer and inner Albanides are distinguished. The huge ophiolitic complex, the Mirdita zone, with about of 4200 km2 extension, is the main feature of the inner Albanides. The gravity map compiling for all the Albanide territory urged a great contribution to settling some significant problems, for the compilation of the: new Geological Map, Tectonical Map and Metallogenic Map, of Albania at 1:200000 scale and especially on the interpretation of the geotectonical position of the ophiolitic complex. The ultramafic rocks occupy the main place in the ophiolitic complex both in superficial distribution and especially in to the depth, therefore the density taken for Bouguer anomaly calculation made possible to acquire some clear anomalies of the gravity, above ultramafic massifs. The interpretation of the gravity anomaly made possible to judge about their continuity in deeper strata and to have an idea of their relationships to the surrounding rocks. The gravity anomaly with the high intensity, in the Mirdita zone, outlines the form, dimensions and the general characteristics of the ophiolites of this tectonical area. Five high positive gravity anomalies, epicentres which are set after one another in a chain-form according to the anomaly belt, beginning from ultrabasic massif of Tropoja-Kukes north-east area to that of Lura, Bulqiza areas to the Morava south-east sector, are fixed within area. Therefore, this belt has a submeridional direction in accordance with the general structure of the ophiolitic extension. This belt is nearly 220 km long, and from 10 to 50 km wide. The maximum intensity of the gravity anomalies is ~ 105 mGal. In the upper part, this belt takes a powerful turn of 600-700 in the form of a north-eastern-ward incline, going on beyond our border, in Yugoslavian territory, while in the south, it continues to the Greek territory. The gravity positive anomalies acquired within this belt, are generally asymmetric, and in all the cases, their eastern flank has a higher gradient than the western one, that has an irregular outline. It is most marked especially in the sectors continuing southward of Tropoja- Kukes ultrabasic massif. The gravity Bouguer anomaly of the Bulqiza ultrabasic massif has an outline nearly similar to the massif frameworks, except it's in northern-western part, in Burrel basin direction. Such a precise outlining of the anomaly, fits perfectly with that of the massif one, and reflects as well the structural features of the massif itself and of its relationships to the adjacent rocks. In the north and southwards, the thickness of this massif is increasingly reduced. The Bulqiza massif is related northward to that Lura according to a very slim thickness of ultrabasic rocks localized in the western sector, while southwards, it is considerably reduced up to a closure. The gravity anomaly form of the Lura massif and its westward continuation helps us to judge about the continuity of the ophiolitic formation under the limestone deposits. A gravity anomaly is acquired further southward Bulqiza ultrabasic massif and it is caused by the presence of the Shebenik ultrabasic massif. The anomaly acquired in the Shebenik massif is extended beyond its border, in west, in the direction of the Kuturman massif. This reveals an extension of rocks with higher density under the Neogenic deposits in the west and further in the south. The magnetic observations above the Neogenic sediments reveal that the ophiolitic formations continue further under these deposits up to the Kuturman and Shpat massif further in the south. The most powerful anomaly acquired over the ophiolitic belt of the Mirdita zone in Albanides, is on the north-eastern margin of the territory on the Kam-Tropoja ultrabasic massif. The gravity anomaly in Kam-Tropoja massif (with 70 mGal amplitude) continues in the ex Yugoslavia, southwest-northeastward. According to the gravity data, the southern part of the Albanides ophiolitic belt, has a more limited thickness and it keeps developing southwards in Helenide. The anomaly of gravity field in Mirdita tectonic zone of Albanides reflects the ophiolitic complex development in this zone. The changing anomaly intensity is related to their changing extension thickness. Proceeding from this fact, we come to the conclusion that the highest ophiolite thickness is localised in the east and it is reduced westwards. These reductions of the ophiolitic thickness are not uniform. Thus, even based on this gravity inversion in various sections, a reduced trend of the ophiolitic thickness southward can be judged. While, in some southern sectors, the ophiolites situated on the surface cause no distinct gravity anomaly.
A compact digital recorder for seismic surveys (SEDIS III) has been developed at GeoPro GmbH, Hamburg during the last five years. It can be integrated in an Ocean Bottom Seismograph (OBS) for offshore studies or record as a stand-alone seismic land station. The system can record continuously up to one month and has been used in arrays of 10 - 100 units in various on-/offshore seismic investigations and microearthquake studies. A large campaign has been performed in Cyprus in cooperation between GeoPro, Geophysics Institute of Hamburg University and Geological Survey Department, Nikosia. Since 1994 ten local on-/offshore arrays of 10 - 40 units operated over 2 - 3 months periods. Instrument spacing was 3 - 8 km resulting in a coverage of 300 - 1800 km2. Hundreds of events of magnitudes between 0.7 and 6.8 (Paphos earthquake 9. Oct. 1996) were recorded and their hypocenters were located, allowing definition of several active faults and estimation of the seismic risk of the region. The SEDIS III system offers an excellent tool for monitoring the seismicity at mid-ocean ridges and defining tectonic and magmatic processes which are expressed by microearthquakes. Following a testphase using temporal seismic arrays, deployment of a permanent seismic network is envisaged, where data are transferred online from the seafloor observatory to a land based data center.
Large igneous provinces, formed by powerfull but geologically brief pulses of magmatic activity, has been identified on the Earth surface. These large continental or oceanic flood basalts provide information about mantle compositions and dynamics which are not reflected by mid-oceanic ridge volcanism and therefore are important for the understanding of the geological history of the Earth.
The Kerguelen Plateau with Broken Ridge in the Indian Ocean is one of the world's largest submerged oceanic plateaus. The origin and subsequent geologic and tectonic history of this imposing feature is still a matter of debate. French, American and Australian single-channel and multichannel seismic reflection data, acquired between 1970 and 1991, and drilling results obtained in 1988 during ODP Leg 119 and 120, provide a fair understanding of the structure, tectonic setting, sediment distribution, nature and age of the plateau. Most of the interpretations are based on geophysical and geological data collected in the Southern Kerguelen Plateau.
Important questions are still open: does the Kerguelen Plateau represent the surfacing of a mantle plume or did it form in the embryonic Indian Ocean in response to break-up of Eastern Gondwana over an old pre-existing thermal anomaly? How much magma was erupted; how long did it take to form the plateau; was the growth of the plateau episodic or continuous; is there an age progression along the plateau? What is the mechanism of the Kerguelen Plateau growth and the relative importance of vertical versus lateral accretion?
New multichannel seismic reflection data were collected in March 1998 in the Northern Kerguelen Plateau by the French research vessel Marion Dufresne (KERIMIS cruise). These data, coupled with the previous surveys, provided detailed location for basement dredges and drilling sites. Five transects, representing some 2000 km and intersecting most of the existing lines have been shot across the deep Kerguelen-Heard Basin and over several large bathymetric highs located to the south and to the west of the Kerguelen Archipelago.
To answer the questions listed above and also to complete the sampling of the Cenozoic and Mesozoic sedimentary sections new drilling have been performed on the Kerguelen Plateau between December 1998 and February 1999 (ODP, Leg 183). The northern sites have been located on the basis of the new geophysical and geological data recently collected by the French over the Northern Kerguelen Plateau.
A 100 km-wide zone with blocks of continental crust, mantle rock, and the first products of oceanic accretion characterize the ocean-continent transition zone (OCT) of West Iberia margin (Whitmarsh et al., 1998). Reconsolidation tests on basal sediments in Hole 897D, which is located on the western side of the OCT, and geological data indicated low yield and high pore pressure (Karig, 1996). Opportunity to test the validity and extent of high pore pressure was afforded by new drillings at the OCT of West Iberia during Ocean Drilling Program (ODP) Leg 173 (Whitmarsh et al., 1998). A 20-cm-long claystone drill-sample was collected from 619 meters depth in Hole 1070A (20 km west of Hole 897D) from which four cylindrical subsamples were cored.
The reconsolidation tests were performed in a large triaxial cell driven by a computer-controlled servo-hydraulic load frame. It employs four controllers that can alter the effective stress state in the triaxial cell, and a range of instruments measures the volumetric, axial, and radial deformation of the sample.
The results suggest that the presence of expandable clays in the drill-core controls its mechanical behavior under deformation. All samples display an initial swelling phase although the confinement was increased. This is followed by a well-defined yield point and subsequent swelling pressure, which is a measure of the effective minimum principal stress (Skempton, 1961). At stresses above the swelling pressure, the samples are being compressed very rapidly.
The sediment has very low yield stress and swelling pressure, in the range from 1.2 to 1.4 MPa, which are very low for the depth from which the sample is collected (619 m). These stresses are in the same order in the basal sediment section of Hole 897D. In Hole 897D, Karig (1996) suggested that the elevated porosity and high pore-fluid pressure are associated with fluid expulsion from the intensely fractured and veined basement and/or the massive breccia unit, which overlies it. The low yield stress implied that the source of high-pressure fluid must have persisted until recently, because the basal units should otherwise have become fully or normally consolidated. The results reported here, support the interpretation by Karig (1996), and a high pore-fluid pressure zone is now inferred to exist in the basal sediment layer in two boreholes within the OCT of West Iberia. High fluid pressure in fault zones lowers the effective normal stress, and also the brittle strength, so that they may become approximately independent of depth along the fault zone (e.g. Rice, 1992).
Karig DE, Proc. ODP Sci. Results, 149, 363-373, (1996).
Rice JR, In B Evans & T-F Wong, Fault mechanics and transport properties of rocks, Academic press ltd: London, 475-503, (1992).
Skempton AW, Proc. 5Th Conf. Soil Mech. Paris, 351-357, (1961).
Whitmarsh RB, Beslier M-O, Wallace PJ, et al, Proc. ODP init. Repts, 173, 493 pp, (1998).
Seven basement highs were sampled during ODP legs 149 and 173 along a transect across the ocean-continent transition (OCT) of the Iberia Abyssal Plain passive margin offshore Portugal. The objective was to precise the nature and evolution of the basement in this zone of continental break-up and incipient oceanic accretion. Samples drilled during ODP leg 149 (1993) and during ODP leg 173 (1997) show that the OCT is a zone where spinel and plagioclase bearing peridotites where tectonically exhumed and stretched at the end of continental breakup and before the biginning of oceanic accretion.
The eastern limit of the OCT is underlined by a strtuctural high where a strong seimic reflector called the H-reflector croscutting the top of the basement is interprted as a major syn-rift contact. The 3 drilling sites located on this high suggest that amphibolites (Site1067) sheared gabbro (Site 900) and serpentinized peridotites (Site 1068) are superposed and that the H-reflector is the crust-mantle boundary. At Site 1067 the cored amphibolite consist of Hbl + Plg + Qt + Ap + Zr + Fe-oxide assemblage plus secondary Chl + Sph + Calc. The texture of the amphibolite evolves from magmatic at the bottom of the cored section to mylonitic at the top through a brecciated zone in the medium part of the section.
The calculated P-T conditions from amphibolites evidence a high T (650±50°C) - medium P (8±1 to 5,5±1 kbar) stage and a retrograde evolution under greenschist facies conditions (350±50°C; 4-2 kbar). This evolution is compatible with that of the gabbro of Site 900 which underwent a high T° (amphibolite to granulite facies conditions) medium P (>4 kbar) deformation that ended under grennschiste facies conditions around 136,4 Ma (Cornen et al., 1996; Feraud et al., 1996). At site 1067, in the upper mylonitic section acidic granulite consisting of Grt + KF + Plg + Zr assemblage occur as ribbons concordant to and folded with the foliation of the host amphibolite.
Geochemical and geochronological studies in progress are an attempt (1) to confirm the syn-rift underplated origin of the mafic rocks (2) to determine the origin and the evolution of the acidic granulites and their relationships with the mafic rocks. Do they represent associated syn-rift differenciated melts products, or do they represent pre-rift (hercynian) continental crust tectonically associated to the mafic rocks during the rifting along the major ductile shear zone cored at the top of the amphibolite?
Feraud G, Beslier MO, Cornen G, Whitmarsh R.B. Sawyer, D.S., Klaus, A. and Masson, D.G. (Eds.), Proc. ODP, Sci. Results, College Station, TX (Ocean Drilling Program), 149, 489-496, (1996).
Cornen G, Beslier MO, & Girardeau J, Whitmarsh R.B. Sawyer, D.S., Klaus, A. and Masson, D.G. (Eds.), Proc. ODP, Sci. Results, College Station, TX (Ocean Drilling Program), 149, 449-470, (1996).
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