Bouguer gravity anomalies and the three-dimensional density structure of a thick mudstone area: A case study of southwestern Taiwan

Gravity surveying is one of the best methods to deduce the subsurface structures in areas covered by thick mudstone in southwestern Taiwan where seismic data analysis is limited. We obtained regional and residual gravity anomalies from the Bouguer anomaly map using the wavelength filtering method to image the gravitational effects of subsurface sources at different depths. The distribution of the residual gravity anomalies corresponds well with the locations of faults and anticlines. Through the change of residual gravity anomalies due to sources at different depth, we estimated the inclination and extension of the anomalous bodies at different depths. Then, the three-dimensional density structure was inferred from inversion of the gravity data to reveal the distribution of mud diapirs and changes of depth to the basement. The positive residual gravity anomalies of approximately 1–2 mGal indicate that the material density of the diapirs is 0.1–0.2 g/cm³ higher than the surrounding rock. The origin of positive residual gravity anomalies can be roughly divided into three types corresponding to the possible subsurface structures. The depth of the positive gravity anomaly below the major anticlines is approximately 4–5 km, corresponding to the bottom of the mudstone formation, from which the mud diapirs may originate. The residual gravity anomaly pattern also indicates a connection of the Chegualin and Youchang faults and the nearly vertical Zuochen fault above 10 km depth. Finally, the low gravity in the Pingtung Plain results from the high-density contrasts on both sides of the two major faults and the deepening of the basement in the plain. The high-angle Chauchou fault, one of two major faults, was also identified at shallow depths based on analysis of the gravity data.