Investigating Timescales of Resurgence at Large Calderas: \ud Post-74 ka Activity at Toba Caldera, Sumatra, Indonesia

Graduation date:2017The Toba Caldera Complex is the youngest resurgent caldera in the last 100 kyrs, formed from four overlapping eruptions starting 1.2 Myrs ago. The last caldera-forming eruption, the Youngest Toba Tuff eruption, occurred ~74 kyrs ago, emitting 2800 km3 of ash and pumice into the atmosphere and forming the caldera outline seen today. The amount of ejecta released into the atmosphere potentially affected the global climate and regional human evolution. The youth of this caldera system has made it the perfect natural laboratory for investigating resurgence, the last stage in the caldera cycle, associated with volcanic effusions and structural deformation and uplift. Organic-rich sediments found on the uplifted caldera floor provide a well-preserved and detailed history of the resurgent uplift, while lava extrusions along faults running through and around the caldera provide geological context for the initiation of resurgent activity at Toba Caldera. New data reveal details of the post-caldera history at the Earth’s youngest resurgent supervolcano, Toba caldera in Sumatra. Resurgence after the caldera-forming ∼74 kyrs ago Youngest Toba Tuff eruption uplifted the caldera floor as a resurgent dome, Samosir Island, capped with 100 m of lake sediments. 14C age data from the uppermost datable sediments reveal that Samosir Island was submerged beneath lake level (∼900 m a.s.l) at 33 kyrs ago. Since then, Samosir experienced 700 m of uplift as a tilted block dipping to the west. 14C ages and elevations of sediment along a transect of Samosir reveal that minimum uplift rates were ∼5.6 cm/year from ∼33.7 to 22.5 kyrs ago, but diminished to ∼0.7 cm/year after 22.5 kyrs ago. Localised uplift activity continued along the eastern coast of Samosir associated with volcanic effusions, with 14C ages of sediments found blanketing lava domes as young as ~2.7 kyrs ago, producing localized uplift rates of ~1.9 cm/yr. Zircon U-Th crystallization and (U-Th)/He ages reveal resurgence commenced at 69.7 ± 4.5 kyrs ago progressing westward across the caldera, as reflected by post-caldera effusive lava eruptions and uplifted lake sediment. Previous geochronology for four of these lava dome localities (North Samosir, Tuk Tuk Samosir, North Pardepur, South Pardepur) by combined U-Th-disequilibrium/(U-Th)/He zircon geochronology yielded eruption ages ranging from 69.7 ± 4.5 kyrs ago to 56.9 ± 3.9 kyrs ago, implying resurgent volcanic activity started almost immediately after the climactic eruption and continued for ~20,000 years. 40Ar/39Ar ages from sanidine and plagioclase feldspar crystals from the same lava domes have returned ages that are contemporaneous with the climactic eruption, between 74.3 ± 0.4 and 71.4 ± 8.2 kyrs ago for North Samosir, Tuk Tuk Samosir, and South Pardepur, with only North Pardepur showing a substantially younger 40Ar/39Ar crystallization age of 23.6 ± 9.8 kyrs ago. These ages overlap with the (U-Th)/He ages, but the younger error weighted average (U-Th)/He eruption ages imply that the process of resurgence is more complex. Using eruption temperatures and a simple argon diffusion model, the difference between the weighted mean (U-Th)/He age and stacked plateau 40Ar/39Ar ages can be explained by a difference in closure temperatures between argon and helium. Sanidine and plagioclase feldspar crystals from three lava domes record the YTT eruption age and are interpreted to be antecrysts that remained in the remnant magma system below argon closure temperature (Tc) in sanidine/plagioclase (350°C). In contrast, the younger North Pardepur lava dome 40Ar/39Ar inverse isochron plagioclase age of 47.9 ± 11.1 kyrs ago overlaps with the (U-Th)/He age, implying that plagioclase crystals are either autocrysts, formed during the eruption of the North Pardepur dome, or YTT antecrysts that have been reset due to their host magma remaining above argon Tc for >700 yrs. Matrix glass analyses represented on the quartz-albite-orthoclase (Qz-Ab-Or) ternary plot indicate that the Samosir dome magmas equilibrated over a deeper range of pressures (200-50 MPa) than the YTT (100-50 MPa), while the Pardepur dome magmas record the lowest pressures (50-0.1 MPa). Fault and drainage analysis of the resurgent dome shows that the southern half of Samosir Island is much more dissected than the northern half of the island. Drainage basins in the south are much more developed, with very little sediment cover left and basement rock observed to be exposed in some areas. Using drainage basin analysis tools in ArcGIS and the Geomorph Tools program in Matlab, the stream networks were analysed and channel profiles were created. Combining the identified locations of knickpoints along these channel profiles with assumed erodibility paramters, the minimum amount of time needed for the drainage basins to develop was calculated. Based on these calculations, drainage basins in the north and south took the same amount of time to develop. This means that the difference in drainage development is a result of another factor. One possible explanation is that pre-existing faults may underlie the thick intracaldera YTT deposit on the southern half of Samosir. These faults may have been from the previous half of the OTT resurgent dome, Uluan, which Samosir has appeared to take the place of. Pre-existing faults could have resulted in areas of weakness that were exploited during drainage basin development, thus making the southern drainage basins develop faster than the northern drainage basins. A more detailed study is required, however, to fully develop this potential hypothesis. A major active stratovolcano north of Toba, Sinabung, shows strong geochemical kinship with Toba, through 87Sr/86Sr isotopes, whole rock chemistry, and 238U-230Th disequilibrium ages of zircons from recent eruption products. This suggests that Toba’s climactic magma reservoir, which was tapped during resurgence, may extend beneath Sinabung and is being tapped during current Sinabung eruptions. This implies that resurgence at the Toba Caldera system may have continued as eruptions at Sinabung