| Title | Geothermal Systems Constrained by the Sumatran Fault and Its Pull-Apart Basins in Sumatra, Western Indonesia |
|---|---|
| Authors | Hirofumi Muraoka, Masaaki Takahashi, Herry Sundhoro, Sjafra Dwipa, Yoshio Soeda, Manabu Momita, Kanichi Shimada |
| Year | 2010 |
| Conference | World Geothermal Congress |
| Keywords | pull-apart basin, Indonesia, Sumatra, Sumatran fault, clustered volcanoes, brittle-plastic transition, geothermal system, geothermal resource |
| Abstract | Two types of geothermal systems occur in Sumatra, western Indonesia; one is situated on the slope of volcanic edifices and the other is situated in pull-apart basins along the Sumatran fault. The latter type is formed as thirteen pull-apart basins by the special tectonic setting where the major strike-slip Sumatran fault and volcanic zone coexist in Sumatra. The latter type gives extra reservoir frameworks in Sumatra, duplicating geothermal resources in this Island. All the thirteen pull-apart basins along the Sumatran fault are distributed near clustered volcanoes, but rarely inside of the areas. Fault segments are often warped from the general trend of the Sumatran fault with their closing to the volcanic clusters. Fault segments penetrating the volcanic clusters often exhibit “z”-shaped drags. The genesis of pull-apart basins is explained by the depth distribution of the brittle-plastic transition along the Sumatran fault. With the increasing obliquity of the subduction, segments of the strike-slip faults commence nucleation in non-volcanic areas where the brittle layer is thick. These segments are growing toward the volcanic clusters where the brittle layer is very thin and initially inhibits fault segment expansion. The tips of the fault segments are warping near the volcanic cluster where clockwise rotation deformation occurs by the thin brittle crust layer and underlying dehydration front of the brittle-plastic transition. Finally, new segments penetrate the volcanic clusters and form pull-apart basins together with the pre-existing warped tips of the fault segments. When the length of thirteen pull-apart basins is plotted against the latitude along the Sumatran fault, the length tends to be larger to the northwest and smaller to the southeast. This tendency is broadly consistent with known slip rates of the Sumatran fault. Boundary normal faults of pull-apart basins play an important role as major discharge zones for geothermal fluid, because the extensional stress is concentrated in the boundary normal faults. The vapor-dominated type geothermal resources may be common on the volcanic edifices and the water-dominated type geothermal resources with the neutral chloride type or bicarbonate type thermal water may be common in the pull-apart basins. |