Tectonic and stratigraphic evolution of the Sarulla graben geothermal area, North Sumatra, Indonesia

The Sarulla graben is a composite Plio-Pleistocene basin developed along the northwest striking, dextral-slip Sumatra fault in a region where the fault coincides with the Sumatra volcanic arc. Offset of the 0.27±0.03 Ma Tor Sibohi rhyodacite dome by an active strand of the Sumatra fault, the Tor Sibohi fault (TSF), indicates a slip rate of about 9 mm/y. This value is lower than previous regional estimates of ∼25–30 mm/y for Holocene slip on the Sumatra fault determined from stream offsets in the Taratung region. This discrepancy may be due to (1) a difference between Holocene and late Quaternary rates of slip and (2) additional slip on other faults in the Sarulla area. Since the magnitude of undated stream offsets along the TSF in the Sarulla area is similar to those in the Taratung area, the discrepancy is likely to be due largely to a change in slip rate over time. Within the Sarulla area, major volcanic centers include the Sibualbuali stratavolcano (∼0.7–0.3 Ma), the Hopong caldera (∼1.5 Ma), and the Namora-I-Langit dacitic dome field (0.8–0.1 Ma). These centers generated the majority of the ash-flow tuffs and tuffaceous sediments filling the Sarulla graben, and appear to have been localized by structural features related to the Sumatra fault zone. Four geothermal systems within the Sarulla area are closely linked to major faults and volcanic centers. In three of the systems, reservoir permeability is clearly dominated by specific structures within the Sumatra fault system. In the fourth geothermal system, Namora-I-Langit geothermal field, permeability may be locally influenced by faults, but highly permeable fractures are widely distributed