Subsurface structure of the Myojin Knoll pumiceous volcano obtained from multichannel seismic reflection data

Yoshiyuki Kaneda,Ayumi Okuda,Kensaku Tamaki,Tetsuro Tsuru,Tetsuo No

doi:10.1186/bf03352820

Abstract

Abstract The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P-wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.

Highlights

The tectonic evolution of the Izu-Ogasawara (Bonin) Arc located at the eastern rim of the Philippine Sea has been well studied by geological and geophysical observations (e.g., Honza and Tamaki, 1985; Suyehiro et al, 1996; Takahashi et al, 2006), while much of formation histories of submarine volcanoes on the volcanic front of the Arc remain unexplained
We demonstrate a tectonic structure of the Myojin Knoll from newly collected deep-penetrating multichannel seismic (MCS) data and its velocity analysis, verify the presence of stratified pumice layers within the caldera wall, and discuss the mode of caldera formation
Based on the structural interpretation of the detailed velocity analysis of MCS data, it can be concluded that the Myojin Knoll caldera wall is mainly composed of thick pumiceous volcanic breccia

Summary

Introduction

The tectonic evolution of the Izu-Ogasawara (Bonin) Arc located at the eastern rim of the Philippine Sea has been well studied by geological and geophysical observations (e.g., Honza and Tamaki, 1985; Suyehiro et al, 1996; Takahashi et al, 2006), while much of formation histories of submarine volcanoes on the volcanic front of the Arc remain unexplained. We demonstrate a tectonic structure of the Myojin Knoll from newly collected deep-penetrating multichannel seismic (MCS) data and its velocity analysis, verify the presence of stratified pumice layers within the caldera wall, and discuss the mode of caldera formation. Unit 1 is the shallowest unit and is characterized by subparallel internal reflections with a relatively weak amplitude and somewhat transparent seismic facies It is approximately 200 m thick at the northern caldera rim, becoming gradually thinner outwards from the rim. This unit is characterized by well-stratified internal reflections with a high amplitude, showing limited distribution only in the depressions of the volcanic basement (the acoustic basement) It has a maximum thickness of approximately 300 m in the northern area.

Lithological Correlation of Seismic Units with Surface Geology and Well Data

Findings

Discussion

Conclusion