GPS compared to long-term geologic motion of the north arm of Sulawesi

Geodetic data acquired in the north arm of Sulawesi are compared with geologic and palaeomagnetic data from north Sulawesi and the Celebes Sea. The objective of this study is to assess whether the long-term motion deduced from palaeomagnetic and geological studies is coherent with the extrapolation of the current motions measured by GPS. Palaeomagnetic data that have been collected in the north arm of Sulawesi indicate that since 5 Ma about 20 to 25 degrees of semi-rigid rotation occurred about a pole located north of Manado. This suggests 200 to 250 km of left-lateral displacement along the Palu-Koro fault with a displacement rate of 4 cm/year. A rather similar displacement on the Palu fault is derived using the distribution of the asymmetric magnetic anomalies of the Celebes seafloor. The distribution implies that 200 to 250 km of oceanic crust was subducted at the north Sulawesi trench. Another marker for the rotation is derived from the opening of the Gulf of Tomini and the NW migration of the calc-alkaline subduction-related volcanism. The far-field observation by GPS of 4 cm/year of left-lateral strike-slip motion over 17 months and the 5 year average of 3.4 +/- 0.3 cm/year on a transect across the Palu fault fit well with the geological observations that indicate a motion of 4 to 5 cm/year of the north arm. We therefore conclude that the current rates deduced from GPS measurements approximate the long-term rates and may hence be extrapolated over a few million years. (C) 1998 Elsevier Science B.V. All rights reserved.