Abstract
Regional geological investigation has been carried out since 1998 to evaluate geothermal resources in the Bajawa City and its surrounding areas of Flores Island as a part of the Research Cooperation Project on the Exploration of Small-scale Geothermal Resources in the Eastern Part of Indonesia (ESSEI Project). Since 4Ma, volcanic activity occurred in two volcanic terrains: the central area and southern coast. Both of terrains form an element of the en echelon island structure characteristic from Flores to Alor Islands. An 800 m uplift occurred in the two terrains during the past 2.5 million years. In the central area, the Welas caldera formed at about 2.5 Ma. After its post caldera volcanism, the volcanic activity has been almost extinct. On the southern coast, volcanic activity has continued from 4 Ma to the present. The most conspicuous event was the appearance of the Bajawa rift zone that was related to the north-south left-lateral shear stress accommodated between the north moving Australian accretion block in the east and relatively fixed Sundaland block in the west. The Bajawa rift zone was initiated by the formation of a NNW-SSE elongated volcano probably during 0.8-0.2 Ma. It was followed by the collapse of the eastern flanks from its crest. The collapsed area produced the Bajawa Cinder Cone Complex that consists of more than 60 cinder cones aligned 20 km along the NNW-SSE trending rift zone. Although the majority of volcanic rocks in the study area is basaltic and tholeiitic, all the effusive rocks from the Bajawa rift zone including the volcanic rocks of the elongated volcano and Bajawa Cinder Cones Complex are andesitic, calc alkaline and very homogenous in composition. This suggests a connected dike swarm magma chamber beneath alignments of numerous cones of the Bajawa Cinder Cone Complex. Three steaming grounds and several high temperature hot springs in the study area are closely associated with the Bajawa rift zone magma system as a heat source.
