Upper mantle electrical resistivity structure beneath the Southwest Indian Ridge 37ºE

第3回極域科学シンポジウム/第32回極域地学シンポジウム 11月30日（金） 国立極地研究所 ３階ラウン

High-resolution magnetic signature of active hydrothermal systems in the back-arc spreading region of the southern Mariana Trough

International audienceHigh-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites

Geochemical characteristics of back-arc basin lower crust and upper mantle at final spreading stage of Shikoku Basin: an example of Mado Megamullion

AbstractThis paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Author Posting. © Macmillan Publishers, 2008. This is the author's version of the work. It is posted here by permission of Macmillan Publishers for personal use, not for redistribution. The definitive version was published in Nature 455 (2008): 790-794, doi:10.1038/nature07333.The formation of oceanic detachment faults is well established from inactive, corrugated fault planes exposed on seafloor formed along ridges spreading at less than 80 km/My1-4. These faults can accommodate extension for up to 1-3 Myrs5, and are associated with one of two contrasting modes of accretion operating along the northern Mid-Atlantic Ridge (MAR). The first is symmetrical accretion, dominated by magmatic processes with subsidiary high-angle faulting and formation of abyssal hills on both flanks. The second is asymmetrical accretion involving an active detachment fault6 along one ridge flank. An examination of ~2500 km of the MAR between 12.5 and 35°N reveals asymmetrical accretion along almost half of the ridge. Hydrothermal activity identified to date in the study region is closely associated with asymmetrical accretion, which also exhibits high-levels of near continuous hydroacoustically and teleseismically recorded seismicity. Enhanced seismicity is probably generated along detachment faults accommodating a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which concentrates primarily at their ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. Both seismic and geochemical evidence indicate that the axial lithosphere is thicker and colder at asymmetrical sections of the ridge, either because associated hydrothermal circulation efficiently penetrates to greater depths, or because the rising mantle is cooler. We suggest that much of the variability in seafloor morphology, seismicity and basalt chemistry found along slow-spreading ridges can be thus attributed to the frequent involvement of detachments in oceanic lithospheric accretion.Supported by CNRS (JE), NSF (DKS, HS, JC, CL and SE), WHOI (JE, DKS, HS and JC), Harvard University (JE, CL and SE), Univ. of Leeds (JC), and MIT (JE)

Gravity processed data (NIPRORI Gravity Meter working area dataset) of RV HAKUHO MARU during cruise KH-01-3, trackline free-air gravity anomaly

Geophysical data (magnetics and gravity) were collected in the AAD (Australian-Antarctic Discordance) area by R/V Hakuho-maru KH-01-3 cruise Leg 3 in 2002. The focused survey area ranges from 123.5°E to 125.5°E and from 50°S and 48°S, along the Southeast Indian Ridge. The gravity measurement was done using a NIPR-ORI type shipboard gravitymeter

フィリピン海の背弧海盆の形成過程

京都大学0048新制・論文博士博士(理学)乙第10014号論理博第1348号新制||理||1079(附属図書館)UT51-99-D218(主査)教授 安藤 雅孝, 教授 尾池 和夫, 教授 中西 一郎学位規則第4条第2項該当Doctor of ScienceKyoto UniversityDA

Gravity processed data (NIPRORI Gravity Meter working area dataset) of RV HAKUHO MARU during cruise KH-01-3, gridded free-air gravity anomaly

Geophysical data (magnetics and gravity) were collected in the AAD (Australian-Antarctic Discordance) area by R/V Hakuho-maru KH-01-3 cruise Leg 3 in 2002. The focused survey area ranges from 123.5°E to 125.5°E and from 50°S and 48°S, along the Southeast Indian Ridge. The gravity measurement was done using a NIPR-ORI type shipboard gravitymeter

Magnetic processed data (Proton precession magnetometer working area dataset) of RV HAKUHO MARU during cruise KH-01-3, trackline magnetic anomaly

Geophysical data (magnetics and gravity) were collected in the AAD (Australian-Antarctic Discordance) area by R/V Hakuho-maru KH-01-3 cruise Leg 3 in 2002. The focused survey area ranges from 123.5°E to 125.5°E and from 50°S and 48°S, along the Southeast Indian Ridge. The total magnetic field was measured using a proton precession magnetometer towed behind the ship

Australian-Antarctic Discordance: bathymetry, magnetics, gravity

Multibeam bathymetry and track line geophysical data (magnetics and gravity) were collected in the AAD (Australian-Antarctic Discordance) area by R/V Hakuho-maru KH-01-3 cruise Leg 3 in 2002.The focused survey area ranges from 123.5°E to 125.5°E and from 50°S and 48°S, along the Southeast Indian Ridge. The multibeam bathymetry was also obtained in the adjacent areas.The bathymetry was collected by SeaBeam2120 (20kHz). The bathymetry data quality is not so high due to the bad sea condition. The total magnetic field was measured using a proton precession magnetometer towed behind the ship. The gravity measurement was done using a NIPR-ORI type shipboard gravitymeter. The main objective of the survey is to reveal the extent of the oceanic core complexes and to reconstruct the mid-ocean ridge process under the melt-poor environment