Abstract

Apatite fission track data have been obtained from basement beneath the modern forearc basin in the Ciletuh region of SW Java, from the low grade metamorphic to granitic basement beneath the NW Java Basin, and from the Eocene through Pleistocene sedimentary succession contained in the Sunda-Asri basins, the NW Java Basin and the SW Java forearc basin.

Apatite fission track (FT) data from a total of 11 wells (four from offshore wells in the Sunda-Asri and seven onshore wells in the Northwest Java basins) show the usual pattern with increasing depth downhole of apparent age decreasing downward, eventually crossing over the stratigraphic age at the age range and temperature between 23 ± 7 My and 89 °C to 75 ± 31 My and 135 °C. Geohistory analysis indicates that in all of the wells there has been less than 500 m of burial of the lowermost parts of the wells during the past 10 My, and therefore the FT parameters are not out of equilibrium due to recent and rapid burial. Forward modelling of horizons for which fission tracks have been severely annealed indicates rapid increase in the geothermal gradient from 12 to 20 °C/km in the Sunda-Asri basin and 15 to 25 °C in the Northwest Java Basin since Plio-Pleistocene time. The probable origin of this increase in heat flow was associated with formation of the Neogene volcanic belt. The Ciletuh-Cimandiri region of SW Java, on the other hand, is structurally unusual in that the Eocene-Miocene sedimentary succession has been inverted, and basement exposed at the surface. Qualitative interpretation of the FT data in relation to the depositional ages of the sample horizons suggest that the sediments were heated to maximum temperatures in the range 70° – 95°C. Forward modelling implies 90° ± 10°C of cooling during the late Miocene-early Pliocene. This corresponds to about 3 km of inversion where the basement is exposed.