During the cruise with S.V. EXPLORA within the Ross Sea on the second marine-geophysical expedition of the Federal Institute for Geosciences and Natural Resources (BGR) to Antarctica, in total 6,745 km of magnetic, gravity and digital reflection seismic lines and additionally 1,400 km gravity lines were acquired in the period from January 10th to March 2nd 1980. On 43 stations sonobuoy refraction measurements have been carried out. The main results are: (1) In the eastern part of the Ross Sea Shelf two striking discontinuities have been identified in the reflection seismics representing gaps in the sediments at the turn-over of the Upper Miocene to the Pliocene (ca. 7 mio years B.P.) and between the Middle and Upper Miocene (ca. 11.5 mio. years B.P.) according to results of DSDP boreholes. (2) In the southern part of the Ross Sea Shelf the basement is uncovered at depths over 700m due to a thrust of the shelf ice recently. (3) A structural unit extends alongside the meridian of 180° separating the Ross Sea into two different geologic regions. This unit is characterised by two basement highs with seismic velocities exceeding 5 km/sec. (4) In relation with the GANOVEX expedition two profiles have been measured off northern Victoria Land which indicate two large faults with a faulting amount of 2 km. Another area is characterised by intrusive and volcanic bodies.
Main target of the project GIGICS (Cooperative German-Indonesian Geoscientific Investigations in the Celebes Sea) is the investigation of the internal crustal structure and the plate tectonic evolution of the Celebes Sea and its active continental margins off Mindanao and Northern Sulawesi. These investigations were carried out during the cruise SO98 of RV SONNE by the Federal Institute for Geosciences and Natural Resources (BGR), Hannover; the German Research Centre for Geosciences (GFZ), Potsdam; the GEOMAR, Kiel; the Institute of Oceanography (IfM), Hamburg; the Mines and Geoscience Bureau, Manila; the Agency for the Assessment and Application of Technology, Jakarta, and the Institute of Oceanography, Wormley. The cruise SO98 consisted of three legs of two weeks duration and one leg of four weeks duration. The total amount of data acquired during the cruise were: - 3,300 km of multichannel reflection seismics, - over 6,800 km of gravimetric and magnetic data and approximately 10.000 km of swath bathymetric and sediment echosounder data, - 3 wideangle-/refractionseismic profiles, each of 120 - 150 km length, - geological, geochemical sampling and oceanographical measurements at a total of 37 stations. During the cruise SO98 a widespaced but regular grid of magnetic and gravimetric profiles were acquired in the eastern part of the Celebes Sea from which up to then reliable data were very sparse. WEISSEL (1980) recognized in the western Celebes Sea WSW-ENE striking magnetic lineations, which he interpreted as chrons 18 - 20 (39 - 43 Ma according to the timescale of HARLAND et al. (1990)). The data from cruise SO98 show that there is no continuation of these anomalies to the east. In the eastern part the magnetic field of the Celebes Sea is less clear and much more disturbed. Nevertheless, E-W-striking anomalies are recognizable. Because amplitudes of local magnetic anomalies are higher than the lineations, the correlation of these lineations with the magnetic reversal scale is still somewhat ambiguous. The gravity map compiled from the measured gravimetric data shows elongated positive anomalies in the eastern part of the Celebes Sea. Exceptions occur at the deep sea trenches off North Sulawesi (North Sulawesi Trench) and Mindanao (Cotabatu Trench) and at the Sulu Archipelago where strong negative gravity anomalies were found. A remarkable NW-striking gravity high of up to 60 mgal was found in the central eastern part of the Celebes Sea. Gravimetric modelling suggests that this high can be correlated with the gravimetric effect of the Molucca Sea Plate subducting from the east under the Sangihe Arc. The reflection seismic data from the northern part of the Celebes Sea show indications for a juvenile subduction of oceanic Celebes Sea crust under the Sulu Archipelago. The oceanic crust bends down towards the Sulu Arc with angles between 2° and 5° and the sedimentary sequence above is deformed indicating a compressional stress regime. With the exception of two linear arranged seamount-like basement highs the Celebes Sea is dominated by two different oceanic crustal types showing distinct differences in the topography. The first one is showing a very similar reflection seismic pattern as it is found for oceanic crust of the Atlantic (HINZ et al., 1994). This type is characterized by a small-scale block-faulted relief of the top basement and a low reflectivity in lower crustal levels typically related as to be accreted at slow to intermediate spreading ridges. This type is found in the western, northern and southern part of the investigated area. In the eastern and especially in the southeastern part the igneous crust shows a very different image. The reflection of the top of the basement is less distinct and of lower frequency. The relief is very much smoother than in the previous type. This reflection seismic image indicates a volcanic/magmatic overprinting of the oceanic crust in this part of the Celebes Sea. Another target of cruise SO98 was the area of the active continental margin off North Sulawesi and its accretionary complex. The internal structure of the accretionary complex should be investigated to decide whether this active margin is also of the 'splinter-type' or not. During former geophysical cruises with RV SONNE oceanic crustal splinters were discovered in the accretionary wedges of the Sulu Sea and off Costa Rica (e.g. HINZ et al., 1991). From our reflection seismic measurements this active continental margin is morphologically subdivided into three units and consists of two accretionary complexes of different internal structural style: the lower and middle continental slope is underlain by an intensively thrusted, sedimentary accretionary wedge. This wedge was most probably formed during the last 5 Ma. Landward of this wedge an older and seismically very complex accretionary unit is present which is overlain at its landward termination by a sedimentary fore-arc basin. Within this older accretionary complex, units with a strong, low frequency reflection pattern were found which are interpreted to represent crustal splinters of igneous oceanic or ophiolitic nature. This interpretation is supported by our gravity and magnetic data. The magnetic profiles show an increase of the magnetic field towards the north arm of Sulawesi across the continental margin. This increase of the magnetic field suggests an increase of magnetized material within the older accretionary wedge towards the northern arm of Sulawesi where ophiolites are emplaced. During the interpretation of the reflection seismic data of the project GIGICS BSR's (bottom simulating reflectors) were discovered for the first time along the active continental margin of North-Sulawesi. BSR's are the seismic expression of a velocity decrease at the bottom of a gas hydrate zone. The distribution and depth of the BSR's correlates with the geochemical and geothermal results. Radiometric age dating and geochemical analyses from pillow basalts of a seamount from the southeastern Celebes Sea indicate hot-spot activity in this part of the Celebes Sea during or shortly after the formation of the oceanic crust approximately at 43 Ma ago. Three NW-striking ridges or seamount-chains in the northeastern Celebes Sea were mapped and investigated in detail. They are thought to represent a wrench fault system extending through the northeastern Celebes Sea. At the flank of one of these ridges a strongly alterated plagioclase-olivine basalt sample was dredged which was overlain by non-fossiliferous clay stone. A similar lithostratigraphic sequence was drilled during ODP leg 124 (RANGIN et al., 1990). The geochemical composition of these basalts is different from typical MORB. The existence of a large crustal splinter within the accretionary wedge off southwestern Mindanao obviously is responsible for a high thermal conductivity which in turn could have enhanced heat flow (108.1 mW/m2) and methanogenesis (405 ppb). The heat flow of 103.0 mW/m2 at the deformation front of the Mindanao wedge and the high methane concentration of 5.555 ppb suggests tectonically induced fluid transport within the wedge. High methane concentrations between 8.044 and 49.006 ppb at the lower slope off Sulawesi and in the North Sulawesi Trench are accompanied by high heat flow values of up to 100.5 mW/m2. Heat flow is significantly lower upslope (31.3 mW/m2). This general heat flow distribution pattern is seen over a large portion of the accretionary wedge. The elevated heat flow values and high methane concentrations near the deformation front most likely result from heat transport by fluids squeezed out from vertically and laterally compacting sediments. The reduced heat flow towards the coast is compatible either with a cooling effect of slow subduction of the oceanic crust, or stacking of cool slabs of compacted sediments. A subduction of oceanic crust with a heat flow around 60 mW/m2 over a period of more than 3 million years would have produced the low heat flow values of the upper slope if the wedge consists of claystone with a low thermal conductivity (1.2 - 1.7 W/mK). Even in the low-heat flow area isolated fluid venting is possible. Lateral variations in the heat flow pattern (e.g. broadening of the anomalies in the west) may be due to different thermal regimes within the subducted crust.
The Sonne Cruise SO122 was carried out by the Federal Institute for Geosciences and Natural Resources (BGR, Hannover) from 3rd August to 9th September 1997, in cooperation with GEOMAR (Kiel), the National Institute of Oceanography (NIO, Karachi) and the Hydrocarbon Development Institute of Pakistan (HDIP). During the joint project with R/V SONNE the Makran accretionary wedge off Pakistan should have been investigated in detail with multi-channel reflection seismics, magnetics and gravimetry. Intense fishery offshore Pakistan forced a change of the area of investigation to the south with the following objectives: investigation of the crustal structure and occurrence of the bottom simulating reflector (BSR) in the Makran accretionary wedge; investigation of the structure of the Murray Ridge System in order to reconstruct the geodynamic evolution of the eastern Indian Plate margin; determination of the origin of the crust underlying the Indus Fan and reconnaissance of the sedimentary history of the Indus Fan in order to reveal the uplift and erosion history of the Himalayas.
The CINCA marine geoscience investigations on the convergent continental margin of Chile between 19°S and 33°30'S were accomplished during three legs of RV SONNE cruise SO-104, from 22. July to 15. October 1995. The objectives of the first leg are to contribute to an understanding of the geological architecture and of the tectonic mechanism in the area of the Chile convergent zone through a geophysical assessment of the tectonic structures of the Chile continental margin and the adjacent oceanic Nazca plate. During the first leg from 22. July to 24. August 1995 multichannel seismic reflection data with BGR's new digital streamer were collected along a systematic grid with a total traverse length of 4,494 km simultaneously with the acquisition of magnetic, gravimetric, Hydrosweep and Parasound data over a total traverse length of 7,012 km. GFZ's mobile land array of 12 seismic stations recorded the air gun shots fired by RV SONNE within the CINCA area. Three seismic lines were surveyed between 32°30'S and 33°30'S in the area of the CONDOR project. Here, the surface of the downbending oceanic crust is smooth. The 5,000 m to 6,000 m deep trench floor is underlain by sediments, in excess 2,500 m thick. The inner trench slope consists of a landward thickening accretionary wedge which terminates against a body forming the base of a fore arc basin near Valparaiso. The principal area of the CINCA project extends between 19°S and 26°S and comprises the convergent continental margin, the Peru-Chile trench and the seaward adjacent part of the Nazca plate up to approximately 75°W longitude. The tectonic regime of these units of the CINCA area is very different from the tectonic system of the respective units of the CONDOR area. The Eocene-aged and sediment-starved oceanic crust of the Nazca plate becomes blockfaulted when approaching the outer trench slope break. The 50 km to 70 km wide outer trench slope is characterized by a complex system of horst and graben structures in the CINCA area probably resulting from the strong downbending. Steep fault scarps forming the flanks of the horsts reach vertical offsets varying between few hundreds of metres to 1,000 m, and locally even more. The 7,000 m to 8,l00 m deep trench is very narrow and mostly sediment-starved in the CINCA area. Morphology and architecture of the continental margin of the CINCA area are controlled by planar and listric faulting and tilted blocks of inferred continental nature, which apparently slid down into the trench. The inferred continental blocks, overlying a reflective mass, are covered by sediments of presumably turbiditic nature. An accretionary wedge is difficult to define on the seismic single channel records from the CINCA area. However, processed seismic data show a deep reflective mass underlying the downfaulted blocks of inferred continental nature. This deep reflective mass is interpreted to consist of a tectonically eroded and underplated continental crust-basalt melange forming the transition between the downfaulted continental upper plate and the subducting oceanic lower plate. Complex structural highs of still unknown origin and nature have been observed on the upper continental slope at 20°S, 24°S and 25°S. The northernmost structural high represents the seaward termination of the Iquique fore arc basin. The accuracy of the acquired gravity and bathymetric data is very good, i.e. better than 1 mGal and less than 10 m. The Chile trench is associated with strong negative gravity anomalies, and the continental margin is characterized by several positive and negative gravity anomalies of varying size and amount. The first results of magnetic modeling show, that the intensive blockfaulting of the oceanic crust across the outer trench slope causes no loss of the magnetization of the oceanic crust. The air gun shots fired by RV SONNE in 50 m intervals along 17 seismic traverses were recorded by GFZ's mobile land array in the coastal area of Chile. Good quality data were obtained out to about 100 km distance and in some cases even out to about 150 km.
Im Rahmen des weit gespannten SPOC-Projektes (Subduction Processes off Chile) wurden zwischen dem 16.10. und 29.11.2001 vor Zentralchile zwischen 28° und 44°S die SONNE-Fahrten SO161 Leg 2 und 3 durchgeführt. In diesem Zeitraum wurden etwa 5.300 km mit vielkanal-/reflexionsseismischen (MCS), magnetischen, gravimetrischen, hochauflösenden bathymetrischen und echographischen Methoden vermessen. Hinzu kamen etwa 3.900 km mit denselben Verfahren, jedoch ohne MCS. Die Gesamtzahl der 2D-Profile betrug 48. Auf vier Ost-West-Traversen wurden amphibische (d. h. in Land-/See-Kombination) weitwinkel-/refraktionsseismische Beobachtungen durchgeführt. Darüber hinaus wurden die ausgesandten marin-seismischen Signale an Land von einem zweidimensionalen seismischen Mobilstationsnetz, das zwischen 37° und 39°S aufgestellt war, aufgezeichnet. Ziel der Untersuchungen war die Veränderlichkeit der Subduktionseigenschaften zwischen den konvergenten Nazca (ozeanisch) und Südamerika (kontinental) Platten sowie die verschiedenen Begleiterscheinungen, die den Subduktionsprozess beeinflussen können wie: (1) Alter der ozeanischen Kruste, (2) ihre Struktur und ihr Aufbau, (3) ihre sedimentäre Bedeckung, (4) ihr thermischer Zustand, (5) Subduktionswinkel und -schiefe sowie (6) der terrigene Sedimentzufluss von der Kontinentseite. Weiterhin waren die Subduktionsfront, die Subduktionsflächen, die Struktur des Kontinenthanges ebenso wie Struktur und Entwicklungsgeschichte der Forearcbecken und die Verteilung von Gashydrat anzeigenden "Meeresboden simulierenden Reflektoren" (BSRs) Gegenstand der Untersuchungen. Die Ergebnisse sollen mit früheren Studien am aktiven chilenischen Kontinentalrand in Vergleich gesetzt werden, z. B. mit CONDOR (SO101 und 103) und CINCA (SO104). Das SPOC-Zielgebiet war in zwei Untergebiete A und B unterteilt. A erstreckt sich von 36°S bis 40°S und war ausersehen für eine detaillierte Untersuchung mit Hilfe eines engabständigen Profilnetzes sowie für eine enge Verbindung mit den landseitigen Aktivitäten des Sonderforschungsbereichs SFB 267 ("Deformationsprozesse in den Anden") der Deutschen Forschungsgemeinschaft (DFG), des GeoForschungsZentrums Potsdam, der FU Berlin und anderer Partner. Dieses Gebiet ist charakterisiert durch einen deutlich anderen Kontinentrandtyp und andere Eigenschaften des Tiefseegrabens als die Gebiete nördlich davon. Gebiet B wurde ausgewählt, weil in diesem Bereich eine Übergangszone oder eine Grenze zwischen dem subduktionserosiven Kontinentrandtyp vor Nordchile und dem Typ, der im Süden vorherrscht, vermutet wird. Darüber hinaus tritt in dieser Region der Juan Fernandez Rücken in die Subduktionszone ein, der ebenfalls ein wichtiges Erkundungsziel des Projektes ist. Günstige Umstände erlaubten die Vermessung eines Ost-West-Profils südlich von Chiloé zwischen 43°S und 44°S, das über die abgesunkene Küstenkordillere bis in das geflutete Valle Longitudinal verläuft. Dieses Gebiet wird als "C" bezeichnet. Anhand einer ersten an Bord durchgeführten Interpretationen ergeben sich für Fahrtabschnitt 2 und 3 folgende Resultate: In beiden Gebieten A und B konnte kein "Subduction Bulge" (Outer High) festgestellt werden. Möglicherweise reichen die Profile hierfür nicht weit genug. Die sedimentäre Bedeckung der ozeanischen Kruste ist sehr dünn und die ozeanische Krustendicke im allgemeinen sehr "normal" mit ungefähr 7 km, abgeleitet aus relativ schwachen Moho-Reflexionen. In Gebiet B konnte eine bis jetzt magnetisch unkartierte Region vermessen werden, woraus sich recht sichere Alter für die ozeanische Kruste ergeben sowie starke Anhaltspunkte dafür, dass die Challenger Fracture Zone westlich des Untersuchungsgebietes aufhört. Die Arbeiten im Gebiet "C" erbrachten wertvolle Information über die Trench-Morphologie und das bis jetzt einzige MCS-Profil südlich von Chiloé, welches einen sehr breiten Trench anzeigt und weitgehend die Extrapolation der Eigenschaften, die im Gebiet A angetroffen wurden, nach Süden bis etwa 44°S erlaubt.
The BGR Antarctic cruise 1996 from 29th December 1995 to 6th February 1996 with M.S. AKADEMIK NEMCHINOV was designed to acquire new marine geophysical data for a better understanding of the geological processes, timing, occurrence and location of rifts of the initial break-up of southern Gondwanaland. A total of 3,836 km of multichannel seismic reflection data have been collected in the areas of the Cosmonaut Sea, the Astrid Ridge, the Lazarev Sea and the southern Agulhas Plateau in parallel with magnetic and gravity measurements. In addition magnetic and gravity measurements were carried out on transit. Major new observations of the collected MCS data include: (1) Volcanic rocks play a major part in the construction of the Astrid Ridge and also of the Agulhas Plateau. (2) The early opening of the Lazarev Sea was associated with excessive volcanism resulting in the emplacement of a voluminous volcanic body characterized by an internally divergent pattern of seaward-dipping reflectors. (3) The Astrid Fracture Zone continues in form of a sediment-filled basement depression flanked by distinct basement highs into the Lazarev Sea, and apparently swings to the west parallel to the coast of Queen Maud Land. (4) The thickness of sediments in the Cosmonaut Sea overlying oceanic crust of inferred Early Cretaceous age is in excess of 4s (twt), i.e. about 6,000 m. Three regional seismic markers of inferred Cretaceous, Late Eocene-Oligocene and Middle Miocene ages subdivide the sedimentary column.
The 3rd cooperative BGR/SMNG Arctic cruise was designed to acquire new scietific data for a better understanding of temporal and spatial lithospheric variations during rifting and its influence on the tectonic and structural evolution of the continental crust of the Laptev Sea undergoing extension since at least the Early Tertiary, and for tackling open questions regarding the evolution of the submarine permafrost zone. Although conditions for seismic measurements were worse in 1997 than in 1993 and 1994, along 4,622 km of seismic traverses reflection seismic data and wide angle reflection/refraction data from 23 OBH-(ocean bottom hydrophone) stations were collected in the Laptev and East Siberian Sea. The most prominent rift basin is the Ust' Lena Rift, which is at least 300 km wide at latitude 75°N. The Cenozoic sedimentary cover exceeds 3 km everywhere, increasing up to 14 km at two locations. In the northern part of the shelf, the complex mainly N–S-trending Anisin Basin has a basin fill of up to 10 km thickness. The New Siberian Basin which is located in the northwestern part of the study area shows an up to 9 km thick graben fill. The Laptev Horst crust is locally subdivided into several tilted blocks by deep-reaching faults and there are several half grabens of smaller extent which divide the Laptev Horst into three parts: the North, the South and the East Laptev Horst. A major west dipping listric fault of at least 250 km length separates the Laptev Horst from the Ust' Lena Rift. Results from the seismological investigation indicate that recent extension is concentrated within the narrow rift basins of the eastern Laptev Sea. From wide-angle reflection/refraction seismic measurements the seismic velocities of the crustal layers were estimated along five profiles. The layers with velocities of up to 3.5 km/s apparently consist of predominantly Cenozoic sediments. The sedimentary section showing relatively high seismic velocities of 4.5 to 5.2 km/s might be interpreted as Late Paleozoic to Mesozoic deposits or overcompacted/cemented syn-rift deposits. In the eastern shelf area a layer beneath the acoustic basement was interpreted to represent Ordovician to Early Mesozoic carbonates. The lower crust in the area under study shows relatively uniform seismic velocities of about 6.0-6.8 km/s and the velocities estimated for the crust-mantle transition are in the range of 8.0 to 8.2 km/s. The origin of a several 100 m thick layer with a relative high velocity of 3 to 3.5 km/s directly beneath the seafloor was inferred as sub-sea permafrost.
On the F.S. POLARSTERN cruise ANT-IV/3 (6th December, 1985 - 13th March, 1986) multichannel seismic measurements were carried out in parallel with magnetic and gravimetric measurements on 33 lines with a total length of 6,263 km. 3,350 km of the multichannel seismic lines have been processed aboard. The geophysical studies were designed to investigate the structure and geological development of the Weddell Sea continental margin from meridians zero to 60°W, and to define suitable and safe drilling locations for Leg 113 of the Ocean Drilling Program. The main results of the geophysical studies are: (1) The discovery of an approximately N50°E trending failed drift basin, following the trend of a negative magnetic anomaly and a positive gravity anomaly. (2) The discovery of two extensive wedge-shaped and symmetric basement units around a failed drift basin between longitudes 40°W and 20°W. The seismic characteristics, i.e. seismic velocities of 4 km/s and an internally divergent pattern of reflectors suggest that both wedges are formed from extrusive/intrusive volcanic rocks. (3) The confirmation of a major plate tectonic boundary trending approximately N80°E to N60°E, i.e. the EXPLORA-ANDENES escarpment. (4) The confirmation of a glaciogenic progradational wedge beneath the shelf of the Weddell Sea Embayment, made up of several thousand metres of sediments. (5) The definition of 13 suitable and safe drilling locations for ODP-Leg 113.
Between 08.11.1999 and 02.12.1999 the active convergent margin off Costa Rica was investigated using the S/V Professor Polshkov. The cruise had three scientific targets. Several seismic profiles in the dip-direction of the subduction zone were acquired to map the general variability of the accretionary wedge. Near the Jaco Scarp, a dense net of seismic profiles using a smaller seismic source should deliver information about the amount of gas hydrates within the shallow sub-surface. In an area of this wedge south of the Quepos Plateau densely spaced seismic lines were measured to prepare an ODP campaign (which was finished in 2011 as IODP Expedition 334).
The structure and tectonics of the Pacific margin of Costa Rica were studied by multichannel seismic measurements in parallel with gravity measurements and swathmapping from the Cocos Ridge to Nicoya Peninsula during R/V SONNE cruise SO81 legs 1 and 2 from 18th August to 15th September 1992. In addition geological sampling has been carried out. Dominant structural feature is the buried Costa Rica Terrrane (CRT), a complex and segmented, wedge-shaped unit characterized by relative high seismic velocities of 4 km/s. The thickness of this several tens of kilometres wide zone varies between 0.5 and 3 s (twt). The CRT forms the backstop against which the sediments of the subducting Cocos plate accrete resulting in accumulation of sedimentary mass beneath and in front of the CRT, as well as in simultaneous uplift and fracturing of the CRT. It appears that the distinct CRT is affected locally by raft tectonics, i.e. a form of thin-skinned extension by normal faulting from gravity sliding over a non-stretched oceanic crust. A unit is recognizable between the base of the CRT and the surface of the subducting oceanic crust on most of the seismic lines. This unit is thought to consist mostly of ductile pelagic to hemipelagic shales. Some segment boundaries of the CRT are associated with morphological furrows, 5 to 10 km wide and up to 30 km long running across the slope. We feel that the data acquired during SONNE cruise SO81, and the preliminary results at hand have already improved our knowledge on the geological processes of active continental margins. We are convinced that plausible concepts for the origin of tsunamis and asperities can be developed on the basis of the data collected during SONNE cruises SO81 and SO76. The research of both SONNE cruises are a contribution to the International Decade of Natural Desaster Reduction (IPNDR).
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