GRAVITY DATA INTERPRETATION OF PRE-TERTIARY AND TERTIARY ROCKS IN NORTHERN KEBUMEN, CENTRAL JAVA

• Main Author: Nizar Ahmady, Faridz
• Format: Theses
• Language: Indonesia
• Subjects: Ilmu kebumian
• Online Access: https://digilib.itb.ac.id/gdl/view/37110
• Institution: Institut Teknologi Bandung

Karangsambung is a district in Northern Kebumen where the basement rock of Luk-Ulo Melange Complex (LMC) is exposed on the surface. LMC is characterized by various lithological background and structural geology as tectonic mélange complex. Based on geological study, LMC became the evidence of tectonic evolution that occurred in the Southern Sundaland, especially the shifting of subduction of Indo-Australian plate beneath Eurasian plate in Cretaceous Age. Due to lack of geophysical data, the subsurface condition in Karangsambung area is still in open discussion. Therefore, this study is focused on gravity data acquisition and processing in order to have better understanding of subsurface condition beneath Karangsambung area. The gravity survey was designed along main profile across Karangsambung area and 116 gravity data had been obtained with means inter-station spacing about 150m. Data processing were started by applying standard gravity correction to generate Complete Bouguer Anomaly (CBA). Overall, CBA value is varying from 68.36 to 98.9 mGal. Near surface density od 2.48 g/cm3 was estimated using Parasnis and Nettleton method. Furthermore, frequency domain filter with cut-off wavelength of 6414m was used to separate residual-regional sinyal from CBA. The result yields residual and regional anomaly are varying from -2.6 to 3.1 mGal and 71.3-98.6 mGal, respectively. The inversion method based on forward modelling process of CBA and residual data was used to create the density model representing the subsurface condition. The results of forward modelling of both of CBA and residual data showed the geological condition beneath the research area up to 10 km and 2 km depth, respectively. Based on those density models and also combined with geological data, both tectonic evolution and geological structure in the Karangsambung area were developed and interpreted. Overall, the interpretation showed that the geological structure in the research area is mainly composed by faults, especially related to the tectonic contact between Pre-tertiary and Tertiary rocks.