Short-wavelength, high-amplitude gravity anomalies around the Banda Sea, and the collapse of the Sulawesi orogen

Abstract

In eastern Indonesia, high-density ophiolitic rocks outcropping on islands surrounding the Banda Sea are in many cases associated with strong gravity anomalies and steep gravity gradients. However, the relationships are not always straightforward. Bouguer gravity levels and gradients over the extensive East Sulawesi Ophiolite are generally relatively low, although short-wavelength, high amplitude anomalies indicate rapid changes in thickness of high-density rocks in a few places. In the Banda Arc, most local positive anomalies due to ophiolites are superimposed on a steep regional gravity gradient but in one case, in western Seram, there is a distinct and important spatial separation between the two. On Buru, west of Seram, a gradient of more than 10mGal/km testifies to the presence of very dense rocks in the near subsurface, despite the absence of ophiolites in the outcrop.Gravity variations and ophiolite distribution around the Banda Sea are compatible with extension having occurred in the Sulawesi region following, and as a result of, Oligo–Miocene collision with an Australian-derived microcontinent. Similar histories have been proposed for many Mediterranean deep basins of similar size, shape and character, and emplacement of some of the high-density masses in the Banda Arc has probably resembled at least the later stages in the emplacement of peridotite massifs in the Rif-Betic belt. In both areas the present close association of the ultramafic rocks and their associated local anomalies with a strong regional gravity gradient is largely coincidental.