Postcaldera volcanism and hydrothermal activity revealed by autonomous underwater vehicle surveys in Myojin Knoll caldera, Izu‐Ogasawara arc

Abstract

Myojin Knoll caldera, one of the submarine silicic calderas lying on the volcanic front of the northern Izu‐Ogasawara arc, has attracted increasing attention since the discovery of a large hydrothermal field called the Sunrise deposit. Although numerous submersible surveys have been conducted in Myojin Knoll caldera, they have not sufficiently explored areas to produce a complete picture of the caldera and understand the origin of the Sunrise deposit. We conducted comprehensive deep‐sea surveys using an autonomous underwater vehicle and obtained high‐resolution bathymetric and magnetic data and sonar images from ~70% of the caldera. The detailed bathymetric map revealed that faulting and magma eruptions, possibly associated with an inflation‐deflation cycle of the magma reservoir during postcaldera volcanism, had generally occurred in the caldera wall. The main dome of the central cone was covered with lava flows and exhibits exogenous growth, which is unusual for rhyolitic domes. The magnetization distribution in the central cone indicates preferential magma intrusion along a NW‐SE direction. It is presumed that magma migrated along this direction and formed a rhyolite dome at the foot of the southeastern caldera wall, where the Sunrise deposit occurs. The Sunrise deposit is composed mainly of three ridges extending in slope directions and covers ~400 × ~400 m. Magnetization reduction in the deposit area is small, indicating that the alteration zone beneath the Sunrise deposit is slanting rather than vertical. It is presumed that several slanting and near‐vertical volcanic vents serve as pathways of hydrothermal fluid in Myojin Knoll caldera.

The Eruption of Submarine Rhyolite Lavas and Domes in the Deep Ocean – Havre 2012, Kermadec Arc

“…For example, Allen et al (2010) utilized ROV to document the morphological and textural differences among rhyolite lava domes at Sumisu-I knoll, Izu-Bonin arc. At Myojin-knoll caldera, 70 km farther north, Honsho et al (2016) produced a highresolution AUV map of an intracaldera rhyolite dome. A series of studies in the Manus Basin, Papua New Guinea, including highresolution bathymetry, seafloor observation, and sampling by underwater vehicles as well as drilling (Ocean Drilling Program Leg 193), provided a detailed description of a deep submarine dacite dome and lava complex on the modern sea floor (Bartetzko et al, 2003;Paulick et al, 2004;Binns et al, 2007;Thal et al, 2014).…”

Self-potential mapping using an autonomous underwater vehicle for the Sunrise deposit, Izu-Ogasawara arc, southern Japan

“…The location of the Sunrise deposit (Iizasa et al 1999) is marked by the thick dashed curve. The locations of a low-magnetization area, ridge-like structures, and scarp structures (Honsho et al 2016a) are marked, respectively, by a dot-dashed curve, thin dotted curves, and inverted Ts visual survey conducted using a submersible (Iizasa et al 1999) approximated the deposit size as 400 m × 400 m. The mineral assemblage of this deposit is classified as Kuroko type: recovered samples consist of Cu-Zn-Pb sulfides with high Ag (approx. 1200 ppm) and Au (approx.…”

“…70%) observed vents were dead according to Iizasa et al (1999), which led them to infer that this hydrothermal site was in a decline stage at that time. Later, in 2014, an acoustic survey detected signals of fluid discharge above the deposit (Honsho et al 2016a). The system remained active in 2014.…”

“…Results of a detailed morphological survey using acoustic sounding suggest that the Sunrise deposit was formed in association with post-caldera volcanism that occurred after the main caldera collapse (Honsho et al 2016a): ridge-like volcanic structures cut the caldera wall. Three ridge-like structures of post-caldera volcanism origin run along the NW-SE orientation inside of the Sunrise deposit (Honsho et al 2016a). They are correlated with the distribution of hydrothermal vents found by Iizasa et al (1999); the most active vent, designated as Daimyojin, is located near the southern end of the westernmost ridge.…”

“…The salient benefit of the self-potential method in marine environments is that it responds specifically to the presence of hydrothermal deposits, which is a unique feature that is unavailable from other geophysical methods. For instance, the geomagnetic method (e.g., Caratori Tontini et al 2012a, b;Fujii et al 2015;Gee et al 2001;Honsho et al 2013Honsho et al , 2016aSzitkar et al 2014a, b;Tivey and Dyment 2013;Tivey and Johnson 2002), which is regarded as an important method for exploring marine hydrothermal ore deposits (e.g., Urabe et al 2015), responds to any magnetized/less-magnetized body. Actually, the observed magnetization is high (e.g., Gee et al 2001;Honsho et al 2016b) or low (e.g., Honsho et al 2016a) depending on the mineral assemblage (e.g., Körner 1994) as well as the degree of hydrothermal alteration.…”