Fig. 3

Sumatra, with the Indian Ocean to the south and west and continental Southeast Asia to the north and east. The map shows free-air gravity derived from satellite altimetry offshore (Sandwell et al., 2021) and Bouguer gravity onshore. Bouguer gravity is largely controlled by crustal geology and crustal thickness, free-air gravity adds additional emphasis to bathymetric features such as the Sumatra Trench because the sediment cover, although denser than sea water, is significantly less dense than the basement rocks. The thin white lines are bathymetric contours, the thick yellow line is the trace of the Sumatra Fault System. IFZ: investigator Fracture Zone; SS: Sunda Strait. The inset shows the long-wavelength gravity field in the western and central parts of the Indo-Pacific gateway and is dominated by a gravity high that is interpreted as a consequence of the abundance of unassimilated relics of subduction within the asthenosphere. Illustration from Milsom & Walker (2005, Fig. 3.1)