In the scope of International Geoscientific Programs and in close cooperation with PETRONAS, Malaysia, the Federal Institute for Geosciences and Natural Resources (BGR) carried out a geophysical survey on the continental margin off Sabah during the period from July 20 to August 10, 1986, using the PRAKLA-SEISMOS vessel EXPLORA chartered by the BGR. The research cruise is a continuation of BGR’s marine geoscientific studies in the South China Sea and in the Sulu Sea with the German research vessels VALDIVIA (1977), SONNE (1982/83) and EXPLORA (1984).The previous investigations provided new information regarding the geological and tectonic history of the southern part of the South China Sea in the context of plate tectonics. According to BGR’s interpretation of the large amount of collected geophysical, geological and geochemical data the widely accepted hypotheses of the presence of an ancient subduction zone beneath the Sabah-Palawan Trough should be revised.
The main objective of the EXPLORA cruise was to search for an Oligocene-Early Miocene carbonate platform off Sabah and in the western Sulu Sea.
On the continental margin off Sabah 27 lines were surveyed with gravity and multichannel reflection seismics and partly with magnetics, with a total length of 3 126 km. A strong reflector interpreted as reflector BLUE of previous BGR cruises off Palawan was recognizable beneath the Sabah Trough in depth between 4 and 7 sec (TWT). The isochrones of the reflector strike approximately 50°N. The surface of the reflector dips with 2 to 3 degree towards southeast. Toward the north-western part of the Sabah Trough the reflector is disrupted by a basement high.
According to the preliminary interpretation of the gravity data, the prominent free-air anomaly associated with the Sabah Trough and adjacent areas has the same shape as the anomaly observed across the Palawan Trough. Therefore it is concluded that the Sabah Trough is underlain by a thinned continental crust.