HR: 0830h
AN: S41D-0120    [PDF]
TI: Marine Gravity Measurements at the Subduction Zone offshore Central Chile
AU: * Heyde, I
EM: heyde.i@bgr.de
AF: BGR Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, 30655 Germany
AU: Kopp, H
EM: hkopp@geomar.de
AF: GEOMAR Research Center for Marine Geosciences, Wischhofstrasse 1-3, Kiel, 24148 Germany
AU: Reichert, C
EM: christian.reichert@bgr.de
AF: BGR Federal Institute for Geosciences and Natural Resources, Stilleweg 2, Hannover, 30655 Germany
AB: Gravity measurements were carried out during RV SONNE cruise SO-161 (SPOC) in late 2001 between 28$\deg$S and 44$\deg$S offshore Central Chile along a total length of about 17500 km. The mean accuracy of the data measured with the seagravimeter system KSS31M of BGR is better than 1 mGal. Further foreign marine gravity data were not included due to their considerable lower accuracy. Additional marine gravity data derived from satellite altimetry are needed to augment our data from the survey area. The SPOC data set was compared with 3 different satellite gravity data compilations. The data set with the best statistical results for the gravity differences was used for further gravity map compilations. The map of the freeair gravity is dominated by the anomalies of the main topographic features in the survey area. In the W the oceanic crust of the subducting Nazca Plate is characterized by weak positve gravity anomalies. Landward the anomalies decrease rapidly to less than minus 150 mGal in the Chilean trench. Further towards the coast extends a broad zone of alternating positve and negative freeair gravity anomalies. These could be interpreted either in terms of morphology of the continental slope or heterogeneous density distribution in the upper crust. Additionally Bouguer gravity anomalies were calculated. The anomalies on the Nazca Plate are strongly positive with a clear south - north trending increase of values, which reflect the increasing age of the oceanic crust. The effect of isostatic compensation was calculated assuming Vening-Meinesz models with different parameters. The gravity effect of the isostatic compensation root was eliminated from the Bouguer gravity anomalies and serves as a residual field. The interpretation of isostatic residual fields in this complicated tectonic environment leads to the detection of a series of offshore basins. In the N and the centre of the survey area the distribution of the profiles is rather uniform. For these areas 3D density models were developed. The models consider the results of the multi channel reflection seismic and especially the refraction seismic lines. The models show the density structure of the accretionary wedge which expands from N to S, the continental slope and various basins near to the coast. The possibility of a subduction channel in the southern area is discussed. The effect and trace of fracture zones before and after subduction were investigated.
UR: http://www.bgr.de
DE: 8105 Continental margins and sedimentary basins
DE: 8122 Dynamics, gravity and tectonics
DE: 8159 Rheology--crust and lithosphere
DE: 9360 South America
SC: Seismology [S]
MN: 2003 Fall Meeting