Yoshifumi Nogi

Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free-air gravity anomaly data from eight Japan Agency for Marine-Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along-axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume-like mantle upwelling under the spreading axis, and the other is a magma-starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet-like mantle upwelling under the spreading axis. Different features in the MBA for off-axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.

Magnetic anomaly lineations and fracture zones deduced from vector magnetic anomalies in the West Enderby Basin

Abstract Vector geomagnetic anomaly field data obtained in the West Enderby Basin. Strikes of two-dimensional magnetic structures at the positions of their boundaries were determined using these data. The magnetic strikes were divided into two groups based on structural change and magnetic polarity change. A change in structural trend occurred around 500 km off the Antarctic coast to the east of Gunnerus Ridge. Approximately NNE-SSW structural trends dominate in the northern portion of the study area. In contrast, in the southern part, structural trends range between NW-SE and NNW-SSE directions. The latter trend is dominant just seaward of the continental slope of Antarctica, and is almost perpendicular to the coastline. ENE-WSW and E-W magnetic anomaly lineation trends, possibly belonging to the Mesozoic magnetic anomaly lineation sequence, are also observed in the southern part. Therefore, we surmise that the NW-SE and NNW-SSE-trending structures east of Gunnerus Ridge correspond to fracture zones formed during Mesozoic time and reflect the initial Gondwana break-up trend.

The Mantle Dynamics, the Crustal Formation, and the Hydrothermal Activity of the Southern Mariana Trough Back-Arc Basin

The Southern Mariana Trough back-arc basin is a currently active back arc basin, and it has fast spreading morphologic and geophysical characteristics, suggesting an additional magma supply, even though the full spreading rate is categorized as slow spreading. Five hydrothermal vent sites have been found within 5 km around the spreading axis at 13°N. The Japanese TAIGA Project selected this area as one of three integrated target sites, and TAIGA Project members conducted series of JAMSTEC research cruises for different types of geophysical surveys, together with dive observation and samplings by the submersible Shinkai 6500. We reviewed the results from these geophysical surveys and the volcanic rock samples to summarize the products from the TAIGA Project. The results provide strong constraints on the mantle dynamics and the crustal formation at the Southern Mariana Trough back-arc basin; all the results support that they are influenced by hydration derived from the subducting slab with accompanying the additional magma supply. Furthermore, the results from the geophysical and geological surveys for the five hydrothermal vent sites provide characteristic features on the hydrothermal activity and the features are different between on-axis and off-axis hydrothermal sites. The on-axis hydrothermal site is associated with an episodic diking event followed by fissures in a fourth order ridge segment, and its duration and size vary depending on the episodic diking event and on the fissures following. In contrast, the formation of the off-axis hydrothermal sites is closely related to the residual heat from the volcanism rather than tectonic stresses accompanied by faults, and the off-axis hydrothermal activity is for a long period and in a large scale. We summarized all the evidence to propose our scenario of the mantle dynamics, the crustal formation, and the hydrothermal activity of the Southern Mariana Trough back-arc basin.

Examination of Volcanic Activity: AUV and Submersible Observations of Fine-Scale Lava Flow Distributions Along the Southern Mariana Trough Spreading Axis

A high-resolution acoustic investigation using the AUV Urashima has revealed detailed volcanic and tectonic features along the neo-volcanic zone of the intermediate-rate spreading Southern Mariana Trough, where the high magma flux forms fast-spreading type axial high morphology. Side-scan sonar imagery suggests that the survey area mainly consists of two types of terrain: high-backscattering lumpy terrain occupies the majority of the neo-volcanic zone, and low-backscattering terrain is scattered over the entire area to form various bathymetric features. Visual observations by the submersible Shinkai 6500 show that the former corresponds to bulbous pillow lava and the latter to jumbled or wrinkled sheet lavas. The estimated proportion of sheet lava with respect to study area is approximately 10 %. Pillow lavas are flatly distributed and do not form the pillow mounds that are common in the slow-spreading Mid-Atlantic Ridge. Furthermore, we did not observe any pillars, collapse features, or axial summit troughs, all of which are frequently reported in the fast-spreading East Pacific Rise.

Macroscopic geological structures of the Napier and Rayner Complexes, East Antarctica

Abstract This paper presents a form-line map of the Napier and Rayner Complexes, East Antarctica, constructed from attitude data for foliations shown on published geological maps, and discusses the macroscopic geological structures. The form-line map shows that the two complexes consist of several, structurally distinct, units or blocks bounded by east–west-, NE–SW- and NW–SE-striking faults. The major boundary between the two complexes, as indicated on the published geological maps, is a structural discontinuity shown as a large fault on the form-line map. On the form-line map, east–west- and NE–SW-trending folds are abundant and NW–SE-trending ones occur locally in both complexes. North–south-trending folds are also abundant in the Napier Complex. Dome-and-basin fold patterns on a regional scale occur in some regions. The regional strikes, macroscopic structures, and the major boundary between the two complexes are considered to have resulted from the same later deformation episode. The form-line map and distribution map of key mineral assemblages show that the Napier Complex is not uniform and includes at least two types of metamorphic units or fragments of the Archaean crust that were formed through distinct P–T–t evolutionary processes and divided by several faults.

Brief Report of Side-Scan Sonar Imagery Observations of the Archaean, Pika, and Urashima Hydrothermal Sites

A high-resolution acoustic investigation using AUV-Urashima reveals the geological features of three off-axis hydrothermal sites at the Southern Mariana Trough. They are developed ~5 km in distance from backarc spreading axis. The Archaean site is developed at the foot of axial horst and forms 60 m-high mound. Hydrothermal chimneys in the site are arrayed along the ridge of the mound. Our acoustic observations detect small structures along the ridge which could be interpreted as the chimneys. To the south of the Archaean mound, the seafloor is characterized by rough and elongated fabrics approximately trending in NE–SW direction, which coincides with the strike of background seafloor slope. Visual observation indicates that the area consists of lava tube slightly covered by sediment. The Pika and the Urashima sites are developed on top and at foot of a ~1,800 m-high off-axis knoll. Unprocessed sidescan sonar imagery above the Pika and the Urashima sites shows anomalous backscattering signatures in water column. A series of hills with convex shape develops on the southwestern slope of the off-axis knoll. It shows unique facies that is rough surface with high-backscattering intensity on the sonar imagery. Tube lavas are recognized in corresponding seafloor by visual observation.

Discovery and characterization of a new hydrothermal vent based on magnetic and acoustic surveys

A new hydrothermal vent site in the southern Mariana Trough has been discovered using acoustic and magnetic surveys. Two stage surveys by the autonomous underwater vehicle (AUV) Urashima (Japan Agency for Marine-Earth Science and Technology, JAMSTEC) and the manned submersible Shinkai 6500 (JAMSTEC) were conducted in 2009 and 2010, respectively. In the first stage, we detected a clear magnetization low that extends from a previously known hydrothermal vent site, the Pika site, by using a vector magnetometer attached to the AUV. The acoustic signals suggest the presence of hydrothermal plumes emanating from the seafloor within the water column of the 120kHz side scan sonogram; 10 m scale chimney like structures in the 400-kHz multibeam bathymetry data were also detected in the area of low magnetization. These observations strongly suggest the presence of an unseen hydrothermal vent. The subsequent submersible dive discovered a new vent site, which was named the Urashima site [1]. The calculated absolute magnetization using the submersible-attached magnetometer succeeded in estimating the extent of hydrothermally altered zone for both the Pika and Urashima sites. It should be emphasized that the search for a hydrothermal vent site was a direct consequence of the geophysical surveys undertaken in the first stage. Our method can bridge the gap between conventional hydrothermal plume surveys (resolution on a scale of 103 m) and visual/photographic surveys (resolution on a scale of 100 m) and can precisely delineate the hydrothermally altered zone.