5:15 PM - 7:15 PM
[SCG55-P17] Geophysical Observations During Hakuho-maru KH-19-6 Leg 4 and Preliminary Analysis of the Phoenix Ridge Spreading Process
Keywords:Geophysical observations, Antarctic-Phoenix Ridge, Near-ridge seamount
In the KH-19-6 Leg4 cruise, geophysical observations of bathymetry, geomagnetism, gravity, and SBP were conducted along the Drake Passage crossing survey line from South America (Chile) to the Antarctic Peninsula. In particular, we obtained a survey line crossing the plate boundary that subducts into the South Shetland Trench, passing from the already inactive Antarctic-Phoenix Ridge through the former Phoenix Plate. The data obtained in this cruise crossed the spreading axis P1, which is the farthest axis from the Trench on the Antarctic Peninsula side, and is thought to record the oldest history within the former Phoenix Plate. Therefore, these data are expected to provide important information for clarifying the process from the spreading of the Phoenix Plate to its subduction.
The Antarctic-Phoenix Ridge is a spreading axis that exists between the Antarctic Plate and the Phoenix Plate, and already inactive. The former Phoenix Plate was formed at this Antarctic-Phoenix Ridge, and although most of it has now subducted beneath the Antarctic Plate, part of it remains to the west of the Antarctic Peninsula, preserving traces of the spreading axis and the history of the subduction process.
In this study, we carried out the following four types of observations. First, we performed bathymetric surveys using a multibeam echosounder (SeaBeam 3020, L3 Communications ELAC Nautik) to obtain detailed water depth distribution and topographic data. Next, we measured three-component marine magnetics using the three-component magnetometers SBM-89 (GAUSS) and STCM-KOBE (SFG-1211, Tierra Technica). For gravity observations, we used a shipboard gravimeter (D-004, LaCoste & Romberg, ZLS Co.) and a portable gravimeter (CG-5 AUTOGRAV, SCHINTREX) in combination to determine absolute gravity values. In addition, to understand the shallow sub-seafloor structure, we conducted acoustic surveys using an SBP (Sub-Bottom Profiler, Bathy2010, SyQwest Inc.).
As a result of the bathymetric and SBP observations, we confirmed the continuous presence of three seamounts and two fragment-like rises around 58°40′S, indicating the possibility of near-ridge seamounts. Generally, near-ridge seamounts are considered to be formed by localized mantle activity near the spreading axis, and because there have been few survey examples to date, we will proceed with verification while comparing them to previous studies.
Furthermore, from the geomagnetic analysis, we recognized magnetic anomalies corresponding to Chron 3A–6 (approximately 6–20 Ma) on the east side of the ridge axis, suggesting the possibility that the spreading rate gradually decreased in stages toward cessation. Additionally, by comparing these magnetic anomalies with the distribution of the chain of seamounts and fragment-like rises, it is considered that we can examine how the duration of volcanic activity and the eruption sites relate to their distance from the spreading axis and the flow of the asthenosphere.
Based on these findings, when referencing the ETOPO2022 global bathymetric data, the other seamount chains like near-ridge seamounts were also confirmed around Drake Passage, and it was suggested that they might have been influenced by asthenospheric flow and changes in plate motion. In this presentation, we will show the analysis results derived from these observation data and discuss the spreading process generated from the P1 axis of the Phoenix Ridge and the origin of the seamount chains.
The Antarctic-Phoenix Ridge is a spreading axis that exists between the Antarctic Plate and the Phoenix Plate, and already inactive. The former Phoenix Plate was formed at this Antarctic-Phoenix Ridge, and although most of it has now subducted beneath the Antarctic Plate, part of it remains to the west of the Antarctic Peninsula, preserving traces of the spreading axis and the history of the subduction process.
In this study, we carried out the following four types of observations. First, we performed bathymetric surveys using a multibeam echosounder (SeaBeam 3020, L3 Communications ELAC Nautik) to obtain detailed water depth distribution and topographic data. Next, we measured three-component marine magnetics using the three-component magnetometers SBM-89 (GAUSS) and STCM-KOBE (SFG-1211, Tierra Technica). For gravity observations, we used a shipboard gravimeter (D-004, LaCoste & Romberg, ZLS Co.) and a portable gravimeter (CG-5 AUTOGRAV, SCHINTREX) in combination to determine absolute gravity values. In addition, to understand the shallow sub-seafloor structure, we conducted acoustic surveys using an SBP (Sub-Bottom Profiler, Bathy2010, SyQwest Inc.).
As a result of the bathymetric and SBP observations, we confirmed the continuous presence of three seamounts and two fragment-like rises around 58°40′S, indicating the possibility of near-ridge seamounts. Generally, near-ridge seamounts are considered to be formed by localized mantle activity near the spreading axis, and because there have been few survey examples to date, we will proceed with verification while comparing them to previous studies.
Furthermore, from the geomagnetic analysis, we recognized magnetic anomalies corresponding to Chron 3A–6 (approximately 6–20 Ma) on the east side of the ridge axis, suggesting the possibility that the spreading rate gradually decreased in stages toward cessation. Additionally, by comparing these magnetic anomalies with the distribution of the chain of seamounts and fragment-like rises, it is considered that we can examine how the duration of volcanic activity and the eruption sites relate to their distance from the spreading axis and the flow of the asthenosphere.
Based on these findings, when referencing the ETOPO2022 global bathymetric data, the other seamount chains like near-ridge seamounts were also confirmed around Drake Passage, and it was suggested that they might have been influenced by asthenospheric flow and changes in plate motion. In this presentation, we will show the analysis results derived from these observation data and discuss the spreading process generated from the P1 axis of the Phoenix Ridge and the origin of the seamount chains.