17:15 〜 18:45
[SCG48-P15] Preliminary report of KH-23-9 cruise: new datasets to understand the Southern Mariana region
キーワード：南部マリアナ地域、かんらん岩、広域拡大、チャレンジャー海淵
In this contribution, we will introduce the preliminary results of KH-23-9 cruise of R/V Hakuho-maru conducted early winter in 2023 to study the Southern Mariana region.
The 3,000 km long Izu-Bonin-Mariana (IBM) arc system is an outstanding example of an intraoceanic convergent plate margin. Among the entire IBM arc system, recent studies revealed that the Southern Mariana region is in a diffuse extensional field, the pervasive volcano-tectonic fabrics being normal at high angles to the trench axis [e.g., Martinez et al., JGR, 2018]. These pervasive volcano-tectonic fabrics are manifesting the young (~3 million years ago) forearc rifts on the landward slope of the Southern Mariana Trench, breaking the sequence of older (middle Eocene to middle Miocene) volcanic rocks located along the upper parts of the landward slope of the Southern Mariana Trench [Ribeiro et al., Island Arc, 2013; Stern et al., Island Arc, 2014]. The phenomenon of forearc rifting is quite unique in that young rifting occurs very close to the trench axis. In the Southern Mariana region, mantle peridotite is also widely exposed in the deeper part of the region due to the predominant tectonic erosion of the overriding plate. We now further interpret that the Shinkai Seep Field (SSF), a serpentinite-hosted cold seep associating with abundant chemosynthetic biological communities in the landward slope of the Southern Mariana Trench approximately 80 km northeast of the Challenger Deep [Ohara et al., PNAS, 2012; Okumura et al., G3, 2016], can be sustained by this diffuse forearc rifting.
Although our understanding of the Southern Mariana region is becoming robust including the presence of diffuse forearc rifting, most of our knowledge comes from the data and samples from the area east of the Challenger Deep. The major objectives of KH-23-9 cruise were therefore (1) to collect rock samples and (2) to obtain comprehensive geophysical data of the landward slope of the Southern Mariana Trench from the area to the west of the Challenger Deep which has only poorly explored to date. In addition, ancillary objectives of this cruise were (3) to test the Hakuho-maru’s newly installed multibeam echo sounder Kongsberg EM124 at the Challenger Deep [see Tamura et al., this session], (4) to collect benthic animals and (5) to conduct fishery observations to further advance the understanding of Japanese eel spawning activities. All of the cruise objectives were successfully attained during the cruise, conducting 18 dredges, ~90 hours of geophysical mapping covering approximately 350 km east-west of the landward slope of the Southern Mariana Trench, soundings at the Challenger Deep, collecting benthic animals, and fishery observations.
One of the significant results of this cruise was to recovery of fresh glassy basalt at ~142ºE, indicating that diffuse forearc rifting occurs as far west as at this longitude. Previously, fresh glassy basalt was only known as far west as at the longitude of ~143ºE, so the discovery from this cruise will further advance our understanding of the extent of diffuse extension in the Southern Mariana region.
The 3,000 km long Izu-Bonin-Mariana (IBM) arc system is an outstanding example of an intraoceanic convergent plate margin. Among the entire IBM arc system, recent studies revealed that the Southern Mariana region is in a diffuse extensional field, the pervasive volcano-tectonic fabrics being normal at high angles to the trench axis [e.g., Martinez et al., JGR, 2018]. These pervasive volcano-tectonic fabrics are manifesting the young (~3 million years ago) forearc rifts on the landward slope of the Southern Mariana Trench, breaking the sequence of older (middle Eocene to middle Miocene) volcanic rocks located along the upper parts of the landward slope of the Southern Mariana Trench [Ribeiro et al., Island Arc, 2013; Stern et al., Island Arc, 2014]. The phenomenon of forearc rifting is quite unique in that young rifting occurs very close to the trench axis. In the Southern Mariana region, mantle peridotite is also widely exposed in the deeper part of the region due to the predominant tectonic erosion of the overriding plate. We now further interpret that the Shinkai Seep Field (SSF), a serpentinite-hosted cold seep associating with abundant chemosynthetic biological communities in the landward slope of the Southern Mariana Trench approximately 80 km northeast of the Challenger Deep [Ohara et al., PNAS, 2012; Okumura et al., G3, 2016], can be sustained by this diffuse forearc rifting.
Although our understanding of the Southern Mariana region is becoming robust including the presence of diffuse forearc rifting, most of our knowledge comes from the data and samples from the area east of the Challenger Deep. The major objectives of KH-23-9 cruise were therefore (1) to collect rock samples and (2) to obtain comprehensive geophysical data of the landward slope of the Southern Mariana Trench from the area to the west of the Challenger Deep which has only poorly explored to date. In addition, ancillary objectives of this cruise were (3) to test the Hakuho-maru’s newly installed multibeam echo sounder Kongsberg EM124 at the Challenger Deep [see Tamura et al., this session], (4) to collect benthic animals and (5) to conduct fishery observations to further advance the understanding of Japanese eel spawning activities. All of the cruise objectives were successfully attained during the cruise, conducting 18 dredges, ~90 hours of geophysical mapping covering approximately 350 km east-west of the landward slope of the Southern Mariana Trench, soundings at the Challenger Deep, collecting benthic animals, and fishery observations.
One of the significant results of this cruise was to recovery of fresh glassy basalt at ~142ºE, indicating that diffuse forearc rifting occurs as far west as at this longitude. Previously, fresh glassy basalt was only known as far west as at the longitude of ~143ºE, so the discovery from this cruise will further advance our understanding of the extent of diffuse extension in the Southern Mariana region.