EVOLUTION OF THE TOBA CALDERA COMPLEX IN A RESTRAINING BEND OF THE GREAT SUMATRA FAULT

The Toba caldera complex, North Sumatra, Indonesia, was the source of one of Earth’s largest rhyolite ash-flow eruptions at 0.075 Ma, with known volcanic activity dating back only ~1.2 m.y. The 100x30-km topographic depression sits astride a broad region of uplift in the NW extension of the Sunda arc. Toba is distinguished from other volcanoes in the arc by its large size and its silicic nature, and because it is centered ~30-40 km NE of the right-lateral, Great Sumatra Fault (GSF) whereas most other Sumatran volcanoes approximately follow the GSF trace. The GSF bends near Toba: small circles that trace its path to the NW and SE of Toba intersect along the E margin of the Toba depression, and correspond with linear topographic features probably associated with older faults. We interpret this to indicate that the GSF may have once bent more sharply, and that Toba may have originated within, and may owe its unique character to, a restraining bend in the fault. This contractional setting may have hindered eruption of ascending, subduction-related melts, forcing them to coalesce and to evolve to more silicic compositions. Associated crustal thickening would have facilitated interaction between ascending melts and crust, accounting for the strong crustal isotopic signature present in Toba magmas. Consistent with this proposal are suggestions by others that the GSF didn’t exist prior to ~2 Ma, and that the youngest fault segment is the one now adjacent to Toba. Perhaps recent shifting of the GSF to this position relieved stresses associated with the restraining bend, leading to crustal extension, caldera collapse, and to the cataclysmic eruption of 0.075 Ma.