Regional Geothermal Geology of the Ngada District, Central Flores, Indonesia

Abstract

Geological investigations of geothermal resources in the Ngada District, Flores Island, Indonesia have been conducted as a part of a cooperative project. Subaerial volcanism started in the area about 4 Ma, reflecting the local collisional tectonics. Welas caldera appeared at about 2.5 Ma along the central axis of the island where 600 m of uplift has occurred since 2.5 Ma. On the southern coast, volcanism has continued until the present with Bajawa caldera appearing around 0.15 Ma. Bajawa caldera is a sort of a small rift valley where fragmentary caldera walls are not closed and elongated north-south, indicating a dike-shaped magma chamber. Although the majority of volcanic rocks in the study area are basaltic and tholeiitic, the Bajawa caldera magma system is andesitic and calc-alkaline. Chemical compositions of the caldera-forming tuff and post-caldera cones show little variation. This suggests a homogeneous, dike-shaped magma chamber beneath Bajawa caldera. Four major geothermal systems in the study area, Mataloko, Nage, Wolo Bobo and Mengeruda derive their heat from the Bajawa caldera magma system.