Resurgent Toba—field, chronologic, and model constraints on time scales and mechanisms of resurgence at large calderas

Abstract

Highlights• New data reveal for the first time a history of the last ∼33.7 ky of uplift of Samosir.• Minimum uplift rates were high (4.9 cm/year) for the first 11.2 ky but diminished after that to <1 cm/year for the last 22.5 ky.• Numerical modeling suggests that rebound of remnant magma augmented by deep recharge appears to be the most likely driver for uplift.• Detumescence makes a negligible contribution to resurgent uplift.• The volume of the resurgent dome is isostatically compensated by magma • Average rates of uplift at Toba are much lower than currently restless calderas indicating a distinction between resurgence and "restlessness".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 100 m of lake sediments. 14 C age data from the uppermost datable sediments reveal that Samosir Island was submerged beneath lake level (∼900 m a.s.l) at 33 ka. Since then, Samosir experienced 700 m of uplift as a tilted block dipping to the west. 14 C ages and elevations of sediment along a transect of Samosir reveal that minimum uplift rates were ∼4.9 cm/year from ∼33.7 to 22.5 ka, but diminished to ∼0.7 cm/year after 22.5 ka. 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 km 3 of magma produces a better fit to the uplift data than a constant flux. The cause of 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 400 m a.s.l or lower requires that uplift must have initiated between sometime between 33.7 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 de Silva et al.Resurgence at Toba caldera, Sumatra 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.

Post-supereruption recovery at Toba Caldera

“…The eruption of the Samosir lava domes along the Samosir fault, where uplift of at least 700 m is recorded1021 (Fig. 1), clearly demonstrates the link between the resurgent volcanism and structural uplift of Samosir and suggests that the contribution of magmatism to the uplift of the resurgent dome would have been greatest during the emplacement of the Samosir lava domes.…”

Protracted Multipulse Emplacement of a Postresurgent Pluton: The Case of Platoro Caldera Complex (Southern Rocky Mountain Volcanic Field, Colorado)

“…Many studies suggest that resurgence via symmetric and asymmetric flexural doming is accommodated by emplacement of shallow plutons (e.g., Kennedy et al, 2012;Nielson & Hulen, 1984;Self et al, 1986;Smith & Bailey, 1968;Steven & Lipman, 1976). Some resurgent calderas also include simple-shear-dominated piston or trapdoor resurgence of a rigid block (e.g., Carlino, 2012;Carlino et al, 2006;de Silva et al, 2015;Lipman, 2000;Lipman et al, 1993Lipman et al, , 1996Orsi et al, 1991Orsi et al, , 1996. In addition, late ring dikes and cone sheets may contribute to caldera resurgence (e.g., Saunders, 2004;Schirnick et al, 1999).…”

A geochemical and geophysical reappraisal to the significance of the recent unrest at Campi Flegrei caldera (Southern Italy)

“…It is generally accepted that volcanic calderas result from the rapid magma withdrawal associated with the collapse of magma chamber roof [e.g., Geyer et al ., ; Acocella et al ., ; both of them based on experimental approaches]. After the caldera formation, episodes of resurgence involve periods of unrest, which sometimes lead to new eruptions [ Newhall and Dzurisin , , presenting almost all the known historical unrest calderas in the World; Kennedy et al ., , related to Valles and Lake City (USA) calderas; Acocella et al ., , discussing several recent episodes of caldera unrest, from Rabaul (Papua, New Guinea) to Campi Flegrei (Italy); de Silva et al ., , concerning Toba caldera (Indonesia)]. Intense episodes of ground uplift, generally accompanied by seismic swarms and geochemical anomalies, characterize caldera unrest [e.g., Hill et al ., , reviewing the Long Valley (USA) unrest; Newhall and Dzurisin , ; De Natale et al ., , concerning the Campi Flegrei caldera].…”