5:15 PM - 6:45 PM
[SCG48-P28] Heat flow distribution in the southwestern Okinawa Trough
Keywords:heat flow, OKinawa Trough, backarc basin rifting, seafloor spreading
Heat flow measurements were carried out in the southwestern Okinawa Trough, during research cruises KH-23-11 (Hakuho-Maru, lead by Makoto Otsubo, December 28, 2023 to January 11, 2024), and KS-24-01 (Shinsei-Maru, lead by Ayanori Misawa, January 25 to February 3, 2024). We report here these results, together with those of the previously conducted cruise KH-21-3 (lead by Makoto Ohtsubo) and others.
The Okinawa Trough is a back-arc basin located on the Eurasian Plate and has undergone intermittent extention (rifting) since 2 million years ago. Otsubo et al. (2021 AGU) consider this to be an early stage of thinning of the continental crust, but it is not clear how the crust becomes thinner during this stage and what factors promote it. In particular, the thermal structure near the axis gives critical constraints on the extension mode, but until now there have been almost no measured data.
During the KH-23-11 cruise, we used a 4m-piston corer with seven temperature sensors attached to the coring pipe to measure temperature gradients simultaneously with core sampling. A total of four data were obtained inside the Yonaguni rift and Yaeyama rift. Unfortunately, another heat flow probe (3 m long) fell to the seafloor due to wire breakage during the operation.
During the KS-24-01 cruise, diving surveys using submersible Hyper Dolphin were conducted in a small mound in a rift (24_55’N, 123_55’E), south of the Yonaguni rift, and three data were obtained using a 1m-long heat flow probe. The heat flow probe dropped during the KH-23-11 cruise was visually located, and one heat flow measurement was made there.
A total of three data were obtained in the Yonaguni rift near 123°30'E. They are 40 mW/m^2 at the northern end of the axis and 58 mW/m^2 at the southern end. Near the westernmost are (123_5'N), the heat flow exceeds 700 mW/m^2.
In the Yaeyama rift, four measurements were made on the axial flat and near the normal fault at the southern end of the axis, all at about 60 mW/m^2. These values are almost consistent with the existing values (about 20 points), although two of the existing values exceed 100 mW/m^2. It should be noted that a hydrothermal biological community was found in the Yaeyama Central Knoll (near 124_20'E), and the heat flow at the base of the knoll was found to be slightly lower than the others.
In the rift where the submersible survey was conducted, a series of seamounts extend east-west in the mid-axis, and the heat flow were measured at the margin of the seamounts. Both the low heat flow (~30 mW/m2) and high heat flow (>200 mW/m2) were obtained. It may reflect the difference in the timing of formation if there was volcanic/hydrothermal activity in the graben.
We estimate that heat flow is generally less than 60 mW/m^2 within the rift zone of the southwestern Okinawa Trough. Heat flow is high near seamounts/knolls, probably due to magmatic and hydrothermal activity. Structural interpretation of seismic reflection surveys and chemical analysis of cores are also planned for this study. Together with the heat flow results, we plan to explore the situation at the onset of the extention.
The Okinawa Trough is a back-arc basin located on the Eurasian Plate and has undergone intermittent extention (rifting) since 2 million years ago. Otsubo et al. (2021 AGU) consider this to be an early stage of thinning of the continental crust, but it is not clear how the crust becomes thinner during this stage and what factors promote it. In particular, the thermal structure near the axis gives critical constraints on the extension mode, but until now there have been almost no measured data.
During the KH-23-11 cruise, we used a 4m-piston corer with seven temperature sensors attached to the coring pipe to measure temperature gradients simultaneously with core sampling. A total of four data were obtained inside the Yonaguni rift and Yaeyama rift. Unfortunately, another heat flow probe (3 m long) fell to the seafloor due to wire breakage during the operation.
During the KS-24-01 cruise, diving surveys using submersible Hyper Dolphin were conducted in a small mound in a rift (24_55’N, 123_55’E), south of the Yonaguni rift, and three data were obtained using a 1m-long heat flow probe. The heat flow probe dropped during the KH-23-11 cruise was visually located, and one heat flow measurement was made there.
A total of three data were obtained in the Yonaguni rift near 123°30'E. They are 40 mW/m^2 at the northern end of the axis and 58 mW/m^2 at the southern end. Near the westernmost are (123_5'N), the heat flow exceeds 700 mW/m^2.
In the Yaeyama rift, four measurements were made on the axial flat and near the normal fault at the southern end of the axis, all at about 60 mW/m^2. These values are almost consistent with the existing values (about 20 points), although two of the existing values exceed 100 mW/m^2. It should be noted that a hydrothermal biological community was found in the Yaeyama Central Knoll (near 124_20'E), and the heat flow at the base of the knoll was found to be slightly lower than the others.
In the rift where the submersible survey was conducted, a series of seamounts extend east-west in the mid-axis, and the heat flow were measured at the margin of the seamounts. Both the low heat flow (~30 mW/m2) and high heat flow (>200 mW/m2) were obtained. It may reflect the difference in the timing of formation if there was volcanic/hydrothermal activity in the graben.
We estimate that heat flow is generally less than 60 mW/m^2 within the rift zone of the southwestern Okinawa Trough. Heat flow is high near seamounts/knolls, probably due to magmatic and hydrothermal activity. Structural interpretation of seismic reflection surveys and chemical analysis of cores are also planned for this study. Together with the heat flow results, we plan to explore the situation at the onset of the extention.