BGR Bundesanstalt für Geowissenschaften und Rohstoffe

Marine seismic survey area: Off Middle- / South America - Reflection seismic

Refraction seismic

Working area: Off Middle- / South America
(1) Costa Rica(2) Chile(3) Brasil(4) Argentine
BGR04 (Ak. Karpinskiy) 2004
SO81 (Sonne) 1992SO104 (Sonne) 1995BGR90 / M14(Meteor/Prospekta) 1990BGR87 (Explora) 1987
BGR92 (Longva) 1992 (3D)SO - 161 (Sonne) 2001SO85 (Sonne) 1993
BGR99 (Prof. Polshkov) 1999BGR 98 (Ak. Lazarev) 1998/1999



Working area: OFF Middle- / South AmericaWorking area: OFF Middle- / South America Source: BGR



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Cruise SO81 1992

Vessel Sonne

Costa Rica

Multichannel seismic data acquisition parameters:
Total length of lines1.900 km 
Sampling rate4 ms
Record length12 000 ms
Streamerdigital - 48 channels

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Cruise BGR92 1992

Vessel Longva

Costa Rica 3D

Multichannel seismic data acquisition parameters:
Total length of lines5.500 km 3D
Sampling rate4 ms
Record length15 000 ms
Streamerdigital - 60 channels

The SO-81 cruise a well as other geophysical investigations revealed that the basic structure of Pacific continental margin of Costa Rica is a wedge-shaped unit of high-velocity rock characterized by high-amplitude reflections at the top and at the base. It is covered by a sedimentary apron, fronted by a small accretionary prism and underlain by a distinct sequence of high-amplitude-low frequency reflections. This unit features strong lateral variations, which can be explained by subducting seamounts as well as underplating of basal removed material from the margin wedge and/or tectonism associated with transform faults. The high lateral variability let BGR to complement the 2-D seismic coverage at this margin by a 3-D survey. The 3-D survey was also designed to acquire a data set for an ODP proposal for deep drilling into the seismogenic zone.

The greatly improved images of the structural architecture of a small part of the active divergent margin landward of the Middle America Trench off Costa Rica, produced by the 3-D acquisition and processing until depth migration clearly demonstrate a subdivision of the margin wedge into a seaward (lower) and a landward (upper) wedge. The upper margin wedge very probably represents material that continues with the adjacent onshore Nicoya Complex and consists of mid-ocean ridge basalt (Jurassic), deep-sea sedimentary rocks and of a late Cretaceous basaltic suite. The thinned lower margin wedge is characterized by a more irregular relief and by a different internal seismic image suggesting a different nature of the lower margin wedge. We interpret the lower margin wedge to consist of an oceanic crustal section formed from the Galápagos plume tail which was subsequently detached during collision of the Eocene-aged crust and attached against and to some extent beneath the Caribbean Nicoya Complex, i.e. the upper margin wedge.

Perspective view of the top of the high-velocity wedge structure as a result of the workstation-based interpretation of the depth-migrated 3-D data.Perspective view of the top of the high-velocity wedge structure as a result of the workstation-based interpretation of the depth-migrated 3-D data. Source: BGR

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Cruise BGR99 1999

Vessel Prof. Polshkov

Costa Rica

Multichannel seismic data acquisition parameters:
Total length of lines3.004 km 3D1.200 km 943 km 860 km
Sampling rate4 ms 1 ms 1 ms
Record length14 000 ms 12 000 ms 7 000 ms
Streamerdigital - 360 channelsdigital - 480 channelsdigital - 420 channels

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Cruise SO104 1995

Vessel SONNE

Off Chile

Cruise SO104-CINCA-LogoSource: BGR

Summary

The main goal of the CINCA programme was to investigate the structure and evolution of the continental margin of North Chile. It was postulated that subduction erosion is active, here.

Our results support the hypothesis of subduction erosion basically due to the surface geometries and deep-reaching fracturing of the subducting oceanic crust, lack of an accretionary wedge, the pronounced fractured and extensional character of the continental slope as well as its continuing subsidence since at least Miocene demonstrated by geological sampling. According to the eastward migration of the volcanic arc the ancient continental margin in Early Jurassic times at about 200 Ma bp was probably located at about 200 km west of its modern position and the formerly existing continental crust in this region was removed. The related processes are still not well understood, and it is also unclear whether they are invariable and whether the related margin configuration remained unchanged through time. We assume, however, that the basic configuration and character was more or less stationary at least since about 30 Ma when the High Andes formed, and the pre-existing arid conditions were even enforced. Because also the incoming oceanic plate bears almost no sediments on its top the Peru-Chile trench is extremely sedimentary staved off North Chile. This situation differs basically from that in the southern Andes.

Two models of the subduction erosion process were analyzed in the light of the comprehensive new CINCA results: A - major contributions from the continental slope top side and the frontal slope unit, and B - almost all contributions from the base of the continental wedge. Due to higher compatibility with the acquired geoscientific data we favour model B although still some uncertainties exist. The terrace-like shape of the continental slope and a structural outer high at its eastern border were observed throughout the entire study area. They are obviously produced by different impact of fluid processes effectuating stronger erosion at the base of the middle slope and hence stronger subsidence.

Compared to other convergent subduction erosion prone margins and to the probably non-erosive South Chilean margin a number of questions still are open. Therefore we view the present results within the scope of on-going research work that should include also the South Chilean and Middle American active margins. Considering North Chile, ocean deep drilling and additional geological seafloor sampling are highly recommended to obtain a higher degree of certainty with respect to the erosive processes and the role of fluids therein.

Interpreted seismic data along a seismic line (W-E cross section parallel 22°10'S) Interpreted seismic data along a seismic line (W-E cross section parallel 22°10'S)  Source: BGR

Multichannel seismic data acquisition parameters:
Total length of lines4.500 km
Sampling rate4 ms
Record length15 000 ms
Streamerdigital - 120 channels

Publications:

Kösters, M., H.-J. Götze, S. Schmidt, J. Fritsch, M. Araneda: The Gravity Field of the Continent-Ocean Transition at the Western Continental Margin of South America. - EOS, Trans., AGU, Vol. 78, pp. 13 - 16.

v. Huene, R., J. Corvalán, E.R. Flueh, K. Hinz, J. Korstgard, C.R. Ranero, W. Weinrebe and the CONDOR Scientists (1997): Tectonic control of subducting Juan Fernández Ridge on the Andean margin near Valparaiso, Chile. - Tectonics, vol. 16, 3, p. 474 - 488.

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Cruise SO - 161 2001 Leg 2 & 3

Vessel SONNE

Off Chile

Multichannel seismic data acquisition parameters:
Total length of lines5.294 km
Sampling rate4 ms resp. 1 ms
Record length14 332 ms
Streamerdigital - 480 channels

Summary

Within the frame of the comprehensive SPOC project (Subduction Processes off Chile) the SONNE cruises SO161 Leg 2 and 3 have been conducted between october 16th and november 29th, 2001, off central Chile between 28° and 44° S. In that period some 5,300 km were surveyed with multichannel seismic (MCS) reflection, magnetic, gravity, high-resolution bathymetric and echographic methods. In addition, approximately 3,900 km were surveyed with the same spectrum of methods but without MCS. The total number of 2D profiles was 48.

Target was the variation of the subduction properties between the convergent oceanic Nazca and continental Southamerica plates and the different conditions that might influence the subduction process as there are: (1) age of the oceanic crust, (2) its structure and composition, (3) its sedimentary cover, (4) its thermal state, (5) the subduction angle and obliquity, and (6) the terrigenous sediment afflux from the continent. Furthermore, special focus was given to the subduction front, the subduction interface, the structure of the slope as well as to the forearc basin structure and history, and the general distribution of gas hydrate indicating bottom simulating reflectors (BSR's). The results are to be compared with previous studies of the Chilean active margin, e.g. CONDOR (SO 101 and 103) and CINCA (SO 104).

The SPOC target area was subdivided into three sub-areas A,B and C. One area was chosen for a detailed survey by aid of a narrowly spaced grid and for a close link with a lot of partners. This area is characterized by a distinctly different margin type south of it is assumed. Moreover, the subducting portion of the aseismic Juan Fernandez Ridge is located in that area representing another important target of the survey. Advantageous conditions enabled the survey of an east-west profile south of Chiloé Island, providing a section through the submerged coastal Cordillera into the flooded longitudinal valley.

Some results of Leg 2 and 3 are:

In all areas A, B and C no subduction bulge (outer high) in the oceanic crust was visible perhaps due to the shortness of the profiles. The sedimentary cover of the oceanic crust is exceptionally thin, and the crustal thickness is generally quite "normal" with around 7 km derived from relatively weak Moho reflections. In area B a so far magnetically unmapped region was filled providing reliable ages of the oceanic crust, and suggesting that the Challenger Fracture Zone abruptly terminates west of the area of investigation. The survey in area C yielded valuable information on the trench morphology. The so far unique MCS profile south of Chiloé island shows a very wide trench and allows to extrapolate the general conditions encountered an area A southward to approximately 44° S. It can be stated that the situation is in sharp contrast to the basin structures detected by industry profiles further north in the Golfo de Corcovado.

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Cruise BGR90 1990

Vessel Meteor / Prospekta

Off Brasil

Multichannel seismic data acquisition parameters:
Total length of lines4.930 km Prospekta: 330 km
Sampling rate4 ms
Record length15 000 ms
Streamerdigital - 80 channels

Summary

The geophysical investigations of Leg M14/3 were designed to study the variability of the igneous oceanic crust in the western South Atlantic.

Twenty-two multichannel seismic lines with a total length of 4,930 km were surveyed in parallel with magnetic, gravimetric, Hydrosweep and Parasound measurements along selected traverses. In addition two-ship seismic measurements were conducted by FS METEOR and R/V PROSPEKTA in the area of the Rio Grande Rise. During this two-ship experiment four Expanded Spread Profiles (ESP) were obtained, and a 630 km long and 4 km wide traverse was surveyed by using a two-ship-parallel reflection seismic survey arrangement. The following results were obtained:

  1. It appears that there was a magmatic/volcanic-tectonic episodicity in the midoceanic ridge processes during the Late Cretaceous: A distinct change of the oceanic crustal structure was observed at magnetic lineations # 34/33 in the Brazil basin. Older oceanic crust that accreted before the emplacement of # 34 is characterized by a flat surface, and by an internally divergent pattern of reflectors having ubiquitous seaward dips within an approximately 150 km wide zone landward of lineation # 34.
    Seaward of lineation # 33 the oceanic crust which accreted during the late Upper Cretaceous is intensively blockfaulted.

  2. Wedge-shaped basement structures having a divergent, seaward-dipping reflection pattern are present on the Rio Grande at lineation # 34.

  3. Strong magnetic anomalies with amplitudes of 300 to 400 nT and wavelengths of 100 km are present on the São Paulo Plateau.

  4. Hydrosweep measurements and interpretation of magnetic data reveal the presence of a fracture zone between 14° and 15°S/25° and 26° W.

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Cruise BGR87 1987

Vessel Explora

Off Argentine

Multichannel seismic data acquisition parameters:
Total length of lines3.254 km
Sampling rate4 ms resp. 1 ms
Record length14 000 ms
Streamerdigital - 60 channels

Summary

In the period from 22nd December, 1987 to 15th January, 1988 a geophysical reconnaissance survey has been carried out with S.V. EXPLORA on the Argentine Eastern continental margin. A total of 3,254 km of digital seismic reflection profiles in parallel with gravimetric and in part magnetic measurements, and 13 sonobuoy refraction profiles were recorded during this survey.

The general aim of the survey was to search the Argentine eastern continental margin between 37°S and 47°S for evidence of continent-ocean boundary structures previously recognized by us off South Africa. The following preliminary results were obtained and will be discussed in this report:

  1. Five regional seismic markers/unconformities have been observed, named from bottom to top AR V to AR I.

  2. Two units are recognizable on all reflection seismic records: A buried lower unit the top of which is marked by the distinct ‘AR V’ unconformity of presumably Beriasian/Valangian age, and a tectonically undisturbed upper sedimentary unit.

  3. The dominant feature of the lower unit is a 50 km to 100 km broad wedge-shaped body characterized by an internally divergent pattern of reflection horizons having seaward dip. The seismic characteristics and recent ODP drilling is consistent with the wedge being formed from extrusive basaltic rocks.

  4. The more than 5000 m thick wedge is parallel with the shelf edge and can be traced continuously for 1200 km. Its landward pinchout coincides with the magnetic slope anomaly ‘G’.

  5. A giant contourite mound of Neogene age has been recognized in the southeastern part of the survey area.

  6. Bottom simulating reflectors have been recognized. Their occurrence is associated with the contourite mound.

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Cruise SO85 1993

Vessel SONNE

Argentine

SONNE cruise SO-85 was designed to study the variability of the oceanic crustal structure along flow-line profiles over Early Tertiary through Early Cretaceous old crust in the Cape and Argentine basins of the South Atlantic.

Deep imaging seismic reflection data in parallel with magnetic, gravimetric and hydroacoustic measurements have been collected along a 1,405 km long flow-line traverse across the Cape basin, and along three flow-line traverses across the Argentine basin with a total length of 4,255 km. A special geophysical survey was carried out on the Argentine continental margin.

Although bad weather hampered the geophysical measurements in the Argentine basin, some new and intriguing observations were made:

  • the presence of an elongated wedge of seaward dipping reflectors, often associated with the distinct magnetic anomaly ‘G’, was confirmed on both, the South African and Argentine continental margins;
  • the sequence of seaward-dipping reflectors consists of two wedges. The younger wedge rests partly on the older one;
  • the surface of the oceanic crust lies more deep in the Argentine basin than in the Cape basin;
  • the structure of the oceanic crust is more variable in time and space within the Argentine and Cape basins than within the Angola and Brazil basins;
  • despite this, it appears that the oceanic crust generated during the younger half of the Cretaceous magnetic quiet period up to C 33r is characterized by relative high reflectivity in lower crustal levels;
  • high-amplitude magnetic anomalies C 33 through C 34 are apparently correlatable with oceanic crustal segments characterized by common seismic features, e.g. a smooth and scarped surface of the oceanic crust;
  • an elongated, positive gravity anomaly exists at about 58°E between 43.5°E and 47°E.

The cruise SO-85 has been supported by the Federal Ministry of Research and Technology (BMFT).

Multichannel seismic data acquisition parameters:
Total length of lines7.000 km
Sampling rate4 ms resp. 1 ms
Record length10 000 ms
Streamerdigital - 48 channels

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Cruise BGR98 1998

Vessel M/V Akademik Lazarev

Argentine

In 1998/1999 the BGR in coorperation with YPF and Hydrografica Naval conducted three seismic surveys along the passive continental margine of Argentine. The seismic vessel MV Akademik Lazarev which is operated by SMNG was used for these cruises.

Although the seafloor spreading history of the South Atlantic Ocean is very well defined, the development of the continental margins poses still great problems to a profound understanding. When break-up of Gondwana started in Middle Jurassic, South Africa and South America remained connected. Their break-up started in Early Cretaceous, possibly propagating from south to north. Before break-up, huge tholeiitic flood basalts were extruded near the future rift, namely the Paraná basalts in Brazil and the Etendeka basalts in Namibia. It is likely that these basalts are caused by a large mantle plume. As the flood basalts were predecessors to the break-up, often a causal relationship is assumed. The dipping reflector sequences along large parts of the continental margins of South America and South Africa are interpreted as indications of the plume influence. Investigations along many passive margins have meanwhile shown that dipping reflector sequences exist along most of them. These so-called volcanic margins are the normal ones, the non-volcanic margins are the exception.

For many years, it had been assumed that the hydrocarbon potential of volcanic margins is very low. The discovery of the Kudu gas field has demonstrated that this assumption is not valid in general. This increases our interest in continuing the investigation of volcanic passive margins and the variability of their structure along strike.

Argentine Continental MarginArgentine Continental Margin Source: BGR



BGR has carried out already two marine geophysical cruises off Argentina, namely with M/V EXPLORA in 1985 and with FS SONNE in 1993. These surveys had shown locally the existence of threefold dipping reflector sequences (Hinz et al., 1999). Furthermore, it had turned out that not everywhere they are accompanied by the strong magnetic G anomaly (Rabinowitz & LaBrecque, 1979).









The main objectives of the BGR98 cruise were:

  • Investigation and mapping of the zone of the seaward dipping reflectors (SDR) (reflection + refraction seismic)
  • investigation of the sedimentary column within this zone (reflection seismic)
  • setting up a seismostratigraphy based on the seismic records obtained during the cruise and the information from the previous cruises (BGR87, SO85) as well as on additional well information
  • comparison of the argentine seismic data with seismic data of the conjugate volcanic margin off Namibia (BGR 87)

The study area is located offshore Argentine, extending from about 37° 20' S to 45° 40' S and from about 61° 10' W to 52° 00' W. It comprises the Colorado Basin as well as the outermost shelf, slope and rise of the continental margin with water depths up to 5000 m.

Multichannel seismic data acquisition parameters:
Total length of lines12.000 km
Sampling rate2 ms resp. 1 ms
Record length15 000 ms
Streamerdigital - 360 channels

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Cruise BGR04 ARGURU (2004)

Vessel MV Ak. Aleksandr Karpinskiy

Argentine

From 19th November to 19th December 2004 BGR conducted a marine geophysical cruise between 34°S and 36°S off Uruguay and between 46°S and 50°S off Argentine. The main research objective was to contribute to a better understanding of the initial breakup and the early opening of the South Atlantic. In continuation of our former work on the South Atlantic continental margins off Argentina, Brazil, Uruguay, Namibia and South Africa marine geophysical research (multi-channel seismics, refraction-/wide-angle reflection seismics, magnetics and gravity) was performed in close cooperation with the Argentine and Uruguayan authorities Comisión Nacional del Límite Exterior de la Plataforma Continental (COPLA) of Argentina and Servicio de Oceanograficia, Hidrograficia y Meteorologia de la Armada (SOHMA) of Uruguay. Multi-channel seismic lines with a total length of 3,754 km and additional 3540 km with the other geophysical methods were acquired . Along two lines refraction-/wide-angle reflection seismic work was carried out.
The preliminary analyses of the new seismic data show different images of the crustal structures between Uruguay and southern Argentine with regard to the distribution and volume of offshore volcanic rocks (seaward dipping reflector sequences, SDRS) along the South American Atlantic margin. On the northern profiles between 34°S and 36°S one single well developed wedge of SDRS is present. Although the landward termination (‘feather edge’) on most of the lines is masked by multiples the average total width of the wedge across the margin seems to be 90 – 100 km and is very constant for this margin segment. This is strong contrast to the results from former cruises (BGR87, SO85 and BGR98) which covered the area between 38°S and 45°S. There, the SDRS showed distinct multiple wedges which in some places extend over 120 km across the continental slope.
The investigation of the sedimentary section yielded that in the area off Uruguay widespread bottom simulating reflectors (BSR) are present. This indications for stable gas hydrates cover a total area of 7000 km2.
One major aim of the cruise was to cover the transition between a volcanic passive margin and a non-volcanic passive resp. sheared margin. This was accomplished in the southern part of the investigated area. Two EW-trending profiles across the Argentine shelf into the Argentine Basin still show indications for SDRS but these structures are only 25 – 30 km wide. The profiles which extend from the NE to the SW crossing the Agulhas-Falkland Fracture Zone (AFFZ) onto the Falkland Plateau show the typical trend of a sheared margin. At the northern rim of the Falkland Plateau a set of small pre-rift half grabens were found indicating pre-rift extensional tectonic phases.
The magnetic data in the area off Uruguay show lineations which are preliminary interpreted as chrons M0 to M3. This might indicate that the first (oldest) oceanic crust was created at a time around the magnetic polarity reversal between the normal interval M4 and the reversed interval M3 (126-127 Ma). Together with existing data from previous cruises this indicates that the breakup of the South Atlantic started further South because there magnetic chrons back to M9 (130 Ma) were identified. In the southernmost part of the margin at 47°S only the magnetic lineations M0 to M4 were identified in the oceanic domain Nevertheless, it is likely that between M4 and the assumed position of the continent ocean boundary/transition (COB/COT) older oceanic crust exists that for some reasons does not show correlatable lineations.
The the free-air gravity map is dominated by the main topographic and structural features in the survey area. Rifted continental margins are characterized by prominent free-air gravity anomalies elongated parallel to the ocean-continent transition. The continental slope is considerably steeper in the North off Uruguay than in the South and thus the gravity high is much more pronounced in the North than in the South.
The simple Bouguer anomaly map also shows the difference between the more gentle and wider continental slope in the South and the steeper slope in the North. The lowest Bouguer gravity values are found in the area of the basins on the continental shelf. Especially the Salado Basin in the prolongation of the Rio de la Plata and the Colorado Basin at about 40°S are indicated by Bouguer gravity anomaly highs. The interpretation by forward density modelling shows, however, the presence of SDRS units in the North of relative high density in the area of the continental slope. Whereas the modelling shows no indications for such volcanic bodies in the South. Although the MCS data indicate a small SDRS wedge but this body may be too small to cause an anomaly.

Multichannel seismic data acquisition parameters:
Total length of lines3.754 km
Sampling rate4 ms
Record length14 000 ms
Streamerdigital - 138 channels

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Contact

    
Dr. Volkmar Damm
Phone: +49-(0)511-643-3226
Fax: +49-(0)511-643-3661

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