Structure Boundary Separating using Gravity Data for Central and Southern Parts of the East Java Basin Beneath the Quaternary Volcanic Deposits

Stratigraphic interpretation shows that the relationship between Cenozoic volcanic rocks deposited in the Southern Mountains and marine clastic volcanic rocks in the Kendeng Basin possibly forms a stratigraphic transition bordered by the major fault that is yet to be defined. The thick Quaternary volcanic deposits obstruct field observation on the surface. Miocene outcrops in limited presence are found on the volcanic slope, positioned relatively more to the north than the same outcrop. This feature indicates major thrusting in southern Java that moved it to the north. The gravity data analysis was used to obtain the field observation and identify the subsurface geology. Total Horizontal Derivative analysis of the gravity data showed a major fault boundary with a relative west-east direction separating the high Bouguer anomaly area in the south from a large negative area of about -50 mGal in the north. Second Vertical Derivative analysis reconstructed the fault slope direction and showed a south-facing slope with a large angle. Based on the combination of the surface data and the subsurface analysis, the boundary is a major fault with a west-east direction on the south side of the Quaternary volcanic range and a south-facing slope. It is interpreted as the result of the compressive tectonics that formed the fold-thrust belt system.