Abstract
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 ka Youngest Toba Tuff eruption uplifted the caldera floor as a resurgent dome, Samosir Island, capped with 100m of lake sediments. 14C age data from the uppermost datable sediments reveal that Samosir Island was submerged beneath lake level (~900m a.s.l) ~33.7 ky. Since then, Samosir experienced 700m of uplift as a tilted block dipping to the west. Using 14C ages and elevations of sediment along a transect of Samosir reveal that minimum uplift rates were ~4.9 cm/yr from ~33.7 to 22.5 ka, but diminished to ~0.7 cm/yr after 22.5ka. Thermo-mechanical models informed by these rates reveal that detumescence does not produce the uplift nor the uplift rates estimated for Samosir. However, models calculating the effect of volume change of the magma reservoir within a temperature-dependent viscoelastic host rock reveal that a single pulse of ~475 km3 of magma produces a better fit to the uplift data than a constant flux. Reproducing the uplift rates require more sophisticated models. Motivation for resurgent uplift of the caldera floor is rebound of remnant magma as the system re-established magmastatic and isostatic equilibrium after the caldera collapse. Previous assertions that the caldera floor was apparently at 400m a.s.l or lower requires that uplift must have initiated between sometime between 33.7 ka and 74 ka at a minimum average uplift rate of ~1.1 cm/ year. The change in uplift rate from pre-33.7 ka to immediately post-33.7 ka suggests a role for deep recharge augmenting rebound. Average minimum rates of resurgent uplift at Toba are at least an order of magnitude slower than net rates of restlessness at currently active calderas. This connotes a distinction between resurgence and “restlessness” controlled by different processes, scales of process, and controlling variables.
Highlights
Large silicic calderas with diameters of several tens of kilometers such as Yellowstone, Toba, Valles, and Long Valley Caldera, are loci of the most explosive eruptions on Earth (Miller and Wark, 2008; Mastin et al, 2014)
14C age data of post-caldera lake sediments in the Toba caldera reveal a complex post-caldera resurgence history. These data indicate that Samosir Island, the upper surface of which was the floor of the caldera that collapsed 74 ka during the Young Toba Tuff eruption, was submerged beneath lake level (900 m a.s.l) ∼33.7 ky
Post caldera resurgence of Samosir has resulted in ∼700 m of uplift in the east, while the western edge remained at or below lake level producing a tilted block dipping to the west
Summary
Large silicic calderas with diameters of several tens of kilometers such as Yellowstone, Toba, Valles, and Long Valley Caldera, are loci of the most explosive eruptions on Earth (Miller and Wark, 2008; Mastin et al, 2014). They distinguish a set of shallow mechanisms such as fluid (hydrothermal) activity, small intrusions and ring dykes that produce short term, episodic uplift from larger-scale deeper mechanisms, such as magmatic readjustments and intrusions that would result in slower uplift rates that act on a much longer-term Testing this dichotomy requires finding a system that faithfully preserves the geological record of resurgence, but is youthful enough that the time scale can be resolved with available chronometers. A single 14C date of 33,090 ± 570 years ago, obtained from near the top of the lake sediment stratigraphy shows that the resurgent dome was under water at this time (Chesner, 2012) Based on this one age, Chesner (2012) calculates a minimum 1.8 cm/year rate of uplift. Based on this one age, Chesner (2012) calculates a minimum 1.8 cm/year rate of uplift. Chesner and Rose (1991) proposed that either magmatic pressure or regional detumescence caused the postYTT resurgent uplift
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