Processed seismic data of Cruise SO49 1987

The SONNE cruise SO-49/1 from 6th April to 7th May 1987 was designed to investigate the Cotabato subduction zone off Mindanao and the geological structure of the eastern part of the Sulu Sea including the convergent continental margins off Zamboanga Peninsula, Negros, and Panay by a geophysical survey. On the 1st leg multichannel seismic reflection measurements were carried out in parallel with magnetic, gravimetric, sea beam and 3.5 kHz subbottom profiler measurements on 16 lines with a total length of 2,700 km. The SONNE cruise SO-49/1 was financed by the Federal Ministry of Research and Technology (BMFT). The geophysical survey in the Celebes Sea and in the Sulu Sea was carried out as a co-operative project by the Federal Institute for Geosciences and Natural Resources (BGR), the Bureau of Mines and Geoscience (BMG) and the Bureau of Energy Development (BED). 16 German scientists and technicians and 4 Philippine scientists attended SONNE cruise SO-49/1. The seismic lines surveyed across the Cotabato Trench/Celebes Sea and the Sulu Trench/Sulu Sea illustrate the active deformation of the layered sediments of the Celebes Sea and the SE Sulu Basin along the trenches: The seismic data suggest an active development of imbricate thrust sheets at the toe of the accretionary wedges and a simultaneous duplex-kind shortening within the wedges above the downgoing oceanic crust of the Celebes Sea and the SE Sulu Basin. The surface of the downgoing oceanic crust forms a major detachment plane or sole thrust. By these processes mass is added to the accretionary wedges resulting in thickening and growing of the wedges. The sedimentary apron overlaying the wedge is only mildly affected by these processes because the surface of the accretionary wedges forms a roof thrust. The collected geophysical data suggest that the oceanic SE Sulu Basin previously extended northward into Panay Island. It was closed by eastward subduction of oceanic crust beneath the upthrusted/updomed Cagayan Ridge. The Negros Trench, a 4.000 to 5,000 m deep bathymetric depression, is thought to represent the collision suture of the opposed subduction systems. The Cagayan Ridge which divides the Sulu Sea into the NW Sulu Basin and the SE Sulu Basin continues into the Antique Ridge of Panay. Approximately 45 suitable and problem-oriented sampling locations have been defined and documented for the subsequent geological and geochemical program by on-board analysis and interpretation of the seismic near trace records and the recordings of the 3.5 kHz subbottom profiler and the sea beam system. On cruise SO49/2 from 10th May to 21st June 1987, the research vessel SONNE of the Federal Republic of Germany undertook geoscience cruises in the South China Sea. The multidisciplinary study of the tectonic and natural resources of the region was a cooperative project between the Federal Institute for Geosciences and Natural Resources (BGR) and the Second Institute of Oceanography (SIO) in the frame of the Agreement between the State Oceanic Administration of the People's Republic of China and the Federal Ministry for Research and Technology of the Federal Republic of Germany on Cooperation in Marine Science and Technology. The first part of cruise SO49/2 was primarily to acquire multichannel seismic data, together with gravity, magnetic, sea beam, and 3.5 kHz measurements, and consisted of 4,112 km of traverses across the deep eastern and western sub-basins of the South China Sea from the Dangerous Grounds to the Chinese continental margin. The observed complex crustal deformation in the Southwestern South China Sea basin and in particular deep intracrustal reflection suggest a large-scale simple-shear kinematic mechanism for the development of at least the western sub-basins. The second part of cruise SO49/2 had primarily geological, geochemical and geothermal objectives and 21 dredge stations, 17 geochemical stations and 6 heat flow stations were carried out. The aims of the sampling were firstly to determine the lithologies and ages of the seismic sequences, and secondly to collect unconsolidated sediments for geochemical study of sorbed hydrocarbon gases in combination with heat flow measurements. Late Oligocene shallow-water carbonates dredged from 700 m to 2700 m of water depth indicate a strong subsidence of the investigated area. The underlying basement consists of continental crust with basaltic intrusions. The hydrocarbon gases of the outer continental slope originated by thermogenic processes from source rocks with a predominantly high maturity of the organic substances.