Delineation of basement structure from gravity data: example from Nara basin, Japan

Abstract

The basement topography of the Nara basin in southwest Japan is estimated applying a gravity-geological method. In this method, gravity anomaly values are directly correlated with the bedrock topography. Several wells penetrating the basement rocks served as important control points in the process of delineating the basement topography of the entire basin. The Bouguer gravity anomaly values in the Nara basin vary from 16 to 44 mgal, and are generally low (<20 mgal) in its northeastern and northwestern parts. The highest anomaly values are observed in the mid-western and southeastern parts, which correspond to the basement rock outcrops. The anomaly field exhibits a positive feature as a whole. There are three distinct gravity ridges. One of them is extending roughly in N–S direction in the northwestern part. These anomalies are supposed to be originated from intra-basement lithological variations. The eastern and northwestern parts exhibit almost N–S trending high-gradient zones attributable to the presence of shear zones. At the locations where the altitude of basement is known, the basement topography was obtained from the gravitygeological method by comparing the residual Bouguer anomalies (obtained after subtracting the regional effect from the Bouguer anomalies) with the basement altitudes. There is a high correlation coefficient between the residual gravity and known bedrock altitude. The results also show that the altitude of basement topography is the lowest in the northeastern part of the basin and the rocks form a NE–SW trending subsurface ridge south of the Yamato River.

Journal of Nepal Geological Society, 2006, Vol. 33 pp.39-44