Pieces of Evidence of Tectonically Dissected Caldera Setting that Controls the Ulubelu-Waypanas Geothermal System

Abstract

Ulubelu-Waypanas geothermal prospect is located in the southernmost Province of Sumatera Island, Lampung Province. It consists of a tertiary to quaternary volcanic complex and a prominent depression structure along an NW-SE trend axis. Several authors have attempted to describe the geologic field situation as volcano-tectonic depression (Trianggo et al., 2015), strongly influenced by overlapping NW-SE parallel to Sumatera Fault System (Husein et al., 2015). Siahaan et al. (2015) identified the geological framework of Ulubelu lies in lineaments patterns, some fault segments, collapse structures and circular features from Radar and Imagery Landsat study.

This paper describes the geothermal field setting based on recent geological field mapping, borehole data, and geophysical survey, especially the advanced modelling of gravity method. According to a recent study, Strong evidence of the Caldera environment was found in the vicinity of the Ulubelu-Waypanas Geothermal System. Initially, an advanced remote sensing map identified a Semi-Circular collapse structure on the west to the south side of the field. Clarifications were made by visiting the caldera wall to find pieces of evidence of such volcanic structure existence, and subsurface data from wells inside the Ulubelu depression was also evaluated to support the study. Furthermore, Geophysics investigations were deployed to reveal the caldera structure using the gravity method.

The result of the joint study reveals that there is strong evidence of caldera structure in the area. It also displays a series of strong post-caldera tectonic activities. Significant faults and systematic joints inside the caldera are well recorded in the morphology and the surface outcrop, respectively. The caldera itself is named in the volcano-stratigraphical Units as Sula Volcanic. It is described in a geological map done by Masdjuk (1989) as composed of andesitic composition and Tertiary age (4.5 mya). The complete Bouguer anomaly map depicts the caldera wall as a circular higher Bouguer anomaly, enclosing the graben's lower gravity anomaly inside. The gravity model estimated Sula's depression as having a thickness of approximately 2500 m, shown as a low-density body that fills the higher density caldera. The volcanism that follows later may initially be controlled by the formation of caldera structure and recurring tectonic events. The most prominent structure formed inside the caldera is the Pagar Alam normal fault. It is the main feature of the graben structure inside the Sula Caldera that acts as a conduit for the geothermal system of Ulubelu. The combination of the Sula Caldera product, and the following volcanism, together with faulting and fracturing that occur post-Sula Caldera due to tectonic activity, is a perfect recipe for Ulubelu-Waypanas geothermal system to be established.