Ari SETIAWAN, Sismanto SISMANTO, Richard LEWERISSA, Subagyo PRAMUMIJOYO

The igneous rock intrusion beneath Ambon and Seram islands, eastern Indonesia, based on the integration of gravity and magnetic inversion: its implications for geothermal energy resources

Ambon and Seram islands in eastern Indonesia are located in the convergence zone of three major plates, causing this area to have many geothermal manifestations on the surface. A subsurface geological study on Ambon and Seram has been conducted based on the integration of gravity and magnetic inversion modeling to determine the intrusion of igneous rock as a potential source of geothermal energy. The gravity and magnetic inversion utilized gravity and magnetic anomaly data from the World Gravity Map (WGM) and the Earth Magnetic Anomaly Grid (EMAG), as well as the CRUST 1.0 global earth crust model, as the constraints. In general, gravity and magnetic anomalies in the study area are compatible, in which the southern part of Ambon is dominated by high anomalies and the northern part is covered by low anomalies associated with the Seram trench. The high anomaly is thought to be related to the intrusion of igneous rocks underneath the surface that have cooling and are rich in magnetic minerals, while the low anomaly isrelated to the destruction zone due to subduction in the north of Seram. The gravity inversion shows the distribution of subsurface rock density ranging from 1.86 g cm–3 to 2.82 g cm–3, while magnetic inversion produces rock susceptibility contrast ranging from –0.0034 SI to 0.0058 SI. Based on Curie point depth analysis of magnetic data, the bottom depth of the magnetic source is estimated at 25 km. The3D subsurface model of Ambon and Seram based on gravity and magnetic inversion shows a massive and evident intrusion pattern with high rock density and susceptibility contrast down to a depth of 10 km. The distribution of high density and susceptibility is thought to have implications for geothermal resources in Ambon and Seram.

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