Geologic Interpretation of Gravity Anomaly at the Hohi Geothermal Field, Central Kyushu

Abstract

Gravity anomalies at the Hohi geothermal field, central Kyushu, reflect the density contrast between the pre-Tertiary basement rocks and the Cenozoic sedimentary and volcanic rocks. Recent volcanic rocks tend to distribute around the areas of local gravity high anomalies. Hatchobaru, well known geothermal field, is located in one of such areas. The basement structures analyzed from gravity anomalies can be calssified into two types. One is active and the other is dormant. The active structures are inferred from common characteristics with active faults, linear features of drainage pattern, or water divide, which are formed through recent tectonic movements. The dormant structures have little in common with above mentioned tectonic features suggesting ceased geologic movements. The tectonic movements which have formed basement structure originated in the Tertiary as the displacement of basement analyzed from gravity anomaly is too large to be accounted for by the rate of the Quaternary tectonic movements. Three gravity basins at Kokonoe, Kujuu, and Shonai have mutually common characteristics such as linear boundary in parallel with each other, steep gradient of gravity on boundary, comparable depth, and similar areal extent, suggesting common tectonic origin, although the only Kokonoe basin has been supposed to have close relation with volcanic activities.