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Deep seismic profiling across the Tornquist Zone — project BABEL, southern part
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ABSTRACTS 18TH MEETING
waves generated every 50 m by powerful airguns. Several papers have already been published or are to be published on the most important findings. In the northern part of the BABEL defined as all lines north of Aland the main target was defined to be the transition zone between the central Svecofennian and northern Svecofennian subprovinces, in particular the Skellefte belt in Sweden and its possible continuation into Finland. Crustal reflections on two parallel profiles in this zone show a clear northeastward dip in the south and southwestward dip in the northern part of the area. The former reflections can be traced down to 20 sec TWT. We interpret these reflectors as relics of an old subduction zone operating 1.89-1.86 Ga ago. The conductive northeastward dipping
body found on land, south of the Skellefteh district can be projected nearly perfectly on the seismic section supporting the idea that the Skellefte~ conductive body is an accreditionary prism formed from sedimentary rocks rich in carbon or saline fluids. Upper crustal reflectors are generally less developed than deeper ones. An exception is the area of the Central Scandinavian Dolerite Group, where numerous subhorizontal reflectors occur with a signature similar to those from dolerite sills in the Siljan area. Wide angle observations are presently being processed at the participating universities. They will fill the gaps between the Fennolora line and the Finnish land profiles and will help to better define Moho depths and the internal structure of the crust.
Deep seismic profiling across the Tornquist Zone - project BABEL, southern part Niels Ballinga and BABEL Working Group ~ aDepartment of Earth Sciences, Geophysical Laboratory, University of Aarhus, Finlandsgade 8, DK-8200 Arhus N, Denmark
Project BABEL (Baltic and Bothnian Echoes from the Lithosphere) is a collaborative effort of British, Swedish, Finnish, Danish, and German geoscientists to acquire deep-crustal reflection profiles and coincident wide-angle/refraction observations in the Baltic Sea and Gulf of Bothnia for the study of the structure and development of the Precambrian Baltic Shield and its transition to the Phanerozoic crust of western Europe across the Tornquist Zone. A total of 2270 km of reflection profiles were collected in the autumn of 1989 with record ~BABEL Working Group members are given in the first Working Group publication: Nature, 458:34-38 (Nov. 1990).
lengths of 18-25 s two-way travel time and with land station seismometer recordings of the airgun shots at about 50 sites. The data are of very good quality, and the project is highly successful in obtaining detailed observations also from the deepest parts of the crust and the uppermost mantle. This paper deals with BABEL lines A and B in the southern part of the Baltic Sea and especially line A crossing the Tornquist Zone (TZ) near the island of Bornholm. Principal regional crustal and geologic-tectonic problems of this region are outlined and first result of the combined reflection and refraction observations are presented and discussed. Resuits include multichannel wide-angle/refrac-
75
NORDIC ASSOCIATION FOR APPLIED GEOPHYSICS
tion observations from the island of Bornholm and preliminary 1-D and 2-D velocity-depth models. The pattern of deep crustal reflectivity varies significantly from the southernmost shield and platform units to the Tornquist Zone and the Danish and German Basins. Although processing and modelling results are preliminary, data clearly show that different tectonic units
exhibit different seismic and structural characteristics. Subhorizontal reflectors and bands of lower crustal reflectors are dominant in the younger tectonic units SW of the TZ contrasting with generally dipping reflectors, cross dipping and diffraction patterns of reflectivity in the shield units NE of the TZ. Geodynamic implications such as collisional and extensional tectonics are briefly discussed.
Stacking of wide-angle data from the BABEL project BABEL Working Group' and Trine Dahl-Jensen c/o Department of Geophysics, Universityof Uppsala, Box 556, 75122 Uppsala, Sweden
The BABEL project (Baltic And Bothnian Echoes from the Lithosphere) is designed to study the Baltic shield and its transition from the younger crust of Europe with both deep near-vertical reflection seismic profiles and wide angle data. The dataset was recorded during three weeks in September/October 1989 and consists of many forms of wide angle data reversed long-range refraction profiles, fan profiles and array data - in addition to the near-vertical data. On several locations the long-range refraction profiles were recorded on an array of geophones for offsets up to 80 km, producing multifold data that can be CDP-
IBABEL Working Group members are given in the first Working Group publication: Nature, 458:34-38 (Nov. 1990).
sorted and stacked. At such locations near-vertical stacks, wide angle stacks and refraction lines can be combined. The wide angle stacks are sensitive to to stacking velocities far deeper than the near-vertical stacks, and can help to combine velocity models from the long-range refraction profiles with the near-vertical data. Energy from the airgun array is predominantly P-wave energy, but at offsets larger than approximately 20 km sufficient energy is converted into S-wave energy at the seabottom to enable recordings of S-wave reflections from the lower crust. Thus also wide angle S-wave stacks can be produced, and Poisson's ratio can be estimated as 0.24 using P- and S-wave stacking velocities from matched P- and S-reflections from the lower crust.
ABSTRACTS 18TH MEETING
waves generated every 50 m by powerful airguns. Several papers have already been published or are to be published on the most important findings. In the northern part of the BABEL defined as all lines north of Aland the main target was defined to be the transition zone between the central Svecofennian and northern Svecofennian subprovinces, in particular the Skellefte belt in Sweden and its possible continuation into Finland. Crustal reflections on two parallel profiles in this zone show a clear northeastward dip in the south and southwestward dip in the northern part of the area. The former reflections can be traced down to 20 sec TWT. We interpret these reflectors as relics of an old subduction zone operating 1.89-1.86 Ga ago. The conductive northeastward dipping
body found on land, south of the Skellefteh district can be projected nearly perfectly on the seismic section supporting the idea that the Skellefte~ conductive body is an accreditionary prism formed from sedimentary rocks rich in carbon or saline fluids. Upper crustal reflectors are generally less developed than deeper ones. An exception is the area of the Central Scandinavian Dolerite Group, where numerous subhorizontal reflectors occur with a signature similar to those from dolerite sills in the Siljan area. Wide angle observations are presently being processed at the participating universities. They will fill the gaps between the Fennolora line and the Finnish land profiles and will help to better define Moho depths and the internal structure of the crust.
Deep seismic profiling across the Tornquist Zone - project BABEL, southern part Niels Ballinga and BABEL Working Group ~ aDepartment of Earth Sciences, Geophysical Laboratory, University of Aarhus, Finlandsgade 8, DK-8200 Arhus N, Denmark
Project BABEL (Baltic and Bothnian Echoes from the Lithosphere) is a collaborative effort of British, Swedish, Finnish, Danish, and German geoscientists to acquire deep-crustal reflection profiles and coincident wide-angle/refraction observations in the Baltic Sea and Gulf of Bothnia for the study of the structure and development of the Precambrian Baltic Shield and its transition to the Phanerozoic crust of western Europe across the Tornquist Zone. A total of 2270 km of reflection profiles were collected in the autumn of 1989 with record ~BABEL Working Group members are given in the first Working Group publication: Nature, 458:34-38 (Nov. 1990).
lengths of 18-25 s two-way travel time and with land station seismometer recordings of the airgun shots at about 50 sites. The data are of very good quality, and the project is highly successful in obtaining detailed observations also from the deepest parts of the crust and the uppermost mantle. This paper deals with BABEL lines A and B in the southern part of the Baltic Sea and especially line A crossing the Tornquist Zone (TZ) near the island of Bornholm. Principal regional crustal and geologic-tectonic problems of this region are outlined and first result of the combined reflection and refraction observations are presented and discussed. Resuits include multichannel wide-angle/refrac-
75
NORDIC ASSOCIATION FOR APPLIED GEOPHYSICS
tion observations from the island of Bornholm and preliminary 1-D and 2-D velocity-depth models. The pattern of deep crustal reflectivity varies significantly from the southernmost shield and platform units to the Tornquist Zone and the Danish and German Basins. Although processing and modelling results are preliminary, data clearly show that different tectonic units
exhibit different seismic and structural characteristics. Subhorizontal reflectors and bands of lower crustal reflectors are dominant in the younger tectonic units SW of the TZ contrasting with generally dipping reflectors, cross dipping and diffraction patterns of reflectivity in the shield units NE of the TZ. Geodynamic implications such as collisional and extensional tectonics are briefly discussed.
Stacking of wide-angle data from the BABEL project BABEL Working Group' and Trine Dahl-Jensen c/o Department of Geophysics, Universityof Uppsala, Box 556, 75122 Uppsala, Sweden
The BABEL project (Baltic And Bothnian Echoes from the Lithosphere) is designed to study the Baltic shield and its transition from the younger crust of Europe with both deep near-vertical reflection seismic profiles and wide angle data. The dataset was recorded during three weeks in September/October 1989 and consists of many forms of wide angle data reversed long-range refraction profiles, fan profiles and array data - in addition to the near-vertical data. On several locations the long-range refraction profiles were recorded on an array of geophones for offsets up to 80 km, producing multifold data that can be CDP-
IBABEL Working Group members are given in the first Working Group publication: Nature, 458:34-38 (Nov. 1990).
sorted and stacked. At such locations near-vertical stacks, wide angle stacks and refraction lines can be combined. The wide angle stacks are sensitive to to stacking velocities far deeper than the near-vertical stacks, and can help to combine velocity models from the long-range refraction profiles with the near-vertical data. Energy from the airgun array is predominantly P-wave energy, but at offsets larger than approximately 20 km sufficient energy is converted into S-wave energy at the seabottom to enable recordings of S-wave reflections from the lower crust. Thus also wide angle S-wave stacks can be produced, and Poisson's ratio can be estimated as 0.24 using P- and S-wave stacking velocities from matched P- and S-reflections from the lower crust.