Geology, geochemistry and geochronology of the Bajawa area, central Flores, Indonesia: Geologic structure and evolution of the Bajawa depression

摘要

The Bajawa area is located in the central part of Flores Island in eastern Indonesia. The area is widely underlain by Pliocene to Pleistocene voluminous volcanic products and includes a depression called the Bajawa depression. It extends 13-16 km east-west and is more than 12 km north-south with well-preserved steep cliffs up to 300-400 m in height at the western margin. Stratigraphy of the study area consists of older volcanic rocks (V1), Bajawa volcanic rocks (Bv), products of cinder cone (C1, C2), Aimere tuff (At) and products of the Inerie volcano (Ie) in ascending order. Evolution of the Bajawa area is divided into four stages: (1) In the Pliocene to Early Pleistocene, the Bajawa area became a site of intense volcanic activity to form a broad volcanic edifice that comprised the older volcanic rocks (V1) ranging from basalt to andesite in the tholeiite series. (2) Bajawa depression occurred on the central highland of the volcanic edifice formed during stage 1. Displacement of subsidence is estimated to have been more than 400 m. (3) The depression was widely filled by Bajawa volcanic rocks (Bv) consisting of andesite lavas and volcaniclastic rocks. Subsequently, andesitic cone volcanism occurred along linear fractures and formed some cinder cones (C1, C2). This volcanism was caused by calc-alkaline magma. (4) A large stratovolcano (Inerie volcano) consisting of tholeiitic andesite (Ie) grew at the southwestern margin of the depression. Based on the lineament on the satellite imagery, alignments of the cinder cones and linear discontinuities of resistivity, it is considered that the formation of the Bajawa depression results from the subsidence of the pre-depression rocks that is segmented by north-south, northwest-southeast and northeast-southwest trending faults. The north-south trending fault in the western part may be interpreted as the bounding fault that defined the western rim of the subsidence.