Investigation of geological structure over caldera hosted geothermal system using geological study and remote sensing analysis

The Blawan-Ijen volcanic complex is located in the easternmost of Java Modern Volcanic Arc, overlying the Kendeng Basin. It is characterized as caldera hosted geothermal system. The presence of geothermal system in this area is indicated by a few thermal manifestations such as fumarole, hot springs, and hydrothermal alteration. Tectonically, the area is located in the margin of Java Block and Sumba Block. Thus, they trigger a sinistral shear deformation due to the movement velocity difference. Structural studies are performed to get understanding of the geothermal system by utilizing remote sensing analysis and field observation. The studies use ASTER Global Digital Elevation Model to observe the geological structure by evaluating the morphological manifestation in this area. Furthermore, this tentative structure is validated with field evidence obtained from field observation. The observation also gives additional information about the structural kinematics that can be used to recognize the structure characteristic related to geothermal system. Four groups of structural lineaments are identified from remote sensing analysis: N-S trend, NW-SE trend, E-W trend, NE-SW trend, and circular caldera structure. These structures are indicated by the alignment of valleys and volcanoes around the caldera. In the northern area, the shape of caldera structure is clearly identified from remote sensing analysis. While in the southern area, it is not clearly noticeable because it is covered by younger volcanic product such as Mt Pendil, Mt Rante, Mt Genteng and the other products. All the hotsprings located in the northern area are classified as an outflow by its chemical composition. The outflow zone follows the N-S fault, indicating that the recent geothermal activity is controlled by the tectonic structure. Whereas, an upflow zone is probably located in the southern area. It is supported by the fact that numerous young volcanics are emerged in the southern caldera which shows the high thermal activity in this area. Moreover, the caldera structure is not directly correlated to the presence of Blawan hotspring. The structure might be acts as a path for natural recharge and young volcanic formations.