Implications of gravity data from east Kalimantan and the Makassar Straits: a solution to the origin of the Makassar Straits?

Recent free-air gravity data covering the Makassar Straits is integrated wi th Bouguer gravity data from onshore East Kalimantan to provide new insight s into the basement structure of the region, Onshore Kalimantan, gravity hi ghs on the northern margin of the Kutai Basin trend NNE-SSW and N-S and cor respond with the axes of inverted Eocene half-grabens. NW-SE trending lows correspond to deep seated basement weaknesses reactivated as normal faults during the Tertiary. An intra-basin gravity high trending NNE-SSW, the Kuta i Lakes Gravity High, is modelled as folded high density Paleogene sediment s Banked by syn-inversion synclines infilled with low density sediments. Of fshore Kalimantan, the Makassar Straits include two basins offset by an en- echelon fault zone, suggestive of an extensional origin. The regional signa ture of the free-air anomaly data mirrors the bathymetry, but this effect c an be reduced by the use of filters in order to examine the basin architect ure. The free-air gravity minimum in the Makassar Strait is only -20 mGal, much smaller than that appropriate for a foreland basin, and more indicativ e of an extensional basin, The steepness of the gradients on the flanks of the basins indicates fault control of their margins, A regional 2D profile across the North Makassar Basin suggests the presence of attenuated crust ( <14 km) in the basis axis at the present day, whereas flexural backstrippin g implies the presence of oceanic crust of middle Eocene ag. The presence o f oceanic crust in the North Makassar Straits Basin has implications for re gional plate tectonic models, (C) 1999 Elsevier Science Ltd. All rights res erved.