ALBERT

Description: For assessing the tsunamigenic potential, the mechanical strength, composition and fabric of forearc slope and accreted sediments from the Nankai Trough offshore SW-Japan have been investigated (NanTroSEIZE; IODP expeditions 315, 316, 333). Triaxial testing of shallow whole round samples (maximum depth of 130 mbsf) at confining pressures of 0.4-1.0 MPa, room temperature, strain rates of approx. 10-3 to 10-6s-1, and up to 64% axial strain (Stipp et al., 2013) revealed mechanically and structurally weak samples from the upper and middle forearc slope of the accretionary prism and strong samples from the accretionary prism toe (Stipp et al., 2013). In order to constrain these results, three additional experiments on samples from greater depth (211-221 mbsf) at confining pressures of 3.0-8.0 MPa, room temperature, strain rates of approx. 10-5s-1, and up to 54% axial strain were carried out. For these tests a digitally controlled servo-hydraulic triaxial appartus with a maximum load of 100 kN was used. Correspondingly to the shallow samples, these experiments show a deviatoric peak stress after only a few percent strain (〈10%) and a continuous stress decrease after this maximum combined with a continuous increase in pore pressure indicative of structurally weak behavior for the two forearc slope samples. The sample from the prism toe area, however, displays a continuous stress increase together with a decrease in pore pressure towards high strain indicative of structurally strong behavior. Synchrotron texture and composition analysis of the experimentally deformed and undeformed samples using the Rietveld refinement program MAUD indicates an increasing strength of the illite and kaolinite textures with increasing depth down to 523 m below sea floor corresponding to a shape preferred mineral alignment due to compaction (Schumann et al., 2014). Experimentally deformed samples have generally stronger textures than related undeformed core samples, and they show increasing strength of the illite and kaolinite textures with increasing axial strain. Mechanically weak samples have a bulk clay plus calcite content of 31-65 vol.-% and most of their illite, kaolinite, smectite and calcite (001)-pole figures have maxima 〉1.5 mrd (multiples of a random distribution). Mechanically strong samples, which were deformed to approximately the same amount of strain (up to 40%) have no calcite and a bulk clay content of 24-36 vol.-%. Illite, kaolinite and smectite (001)-pole figure maxima are predominantly 〈1.5 mrd. The synchrotron textures indicate that the mineral fabric as a whole (clay and also calcite grains) becomes preferentially oriented in the mechanically weak samples. Reorientation of the mineral grains is an important cause of strain weakening and contraction, persisting to high compressive strains. In contrast, the strong samples from the accretionary prism toe keep their microfabric up to fairly high compressive strain, allowing for strain hardening and dilation. This soft sediment hardening tends to involve increasingly large volumes of sediment into the imposed deformation, permitting strain dissipation as long as the sediments are homogeneous. Deformation will tend to localize into structurally weak sediments if they occur within the lithological sequence. Such weak sediments, which soften further with increasing strain, predominate in the cover units of the forearc slope and around the existing megasplay fault. They may either provoke mechanical runaway situations allowing for earthquake rupture, surface breakage and tsunami generation, or slope destabilization and resulting submarine mass wasting.

Description: Highlights • Crustal structure of Walvis Ridge reveals high seismic velocities in the lower crust intruding the African continent. • This modified crust is localized to approx. 100 × 100 km within the continent. • No indication for a large plume head observed The opening of the South Atlantic is a classical example for a plume related continental breakup. Flood basalts are present on both conjugate margins as well as aseismic ridges connecting them with the current plume location at Tristan da Cunha. To determine the effect of the proposed plume head on the continental crust, we acquired wide-angle seismic data at the junction of the Walvis Ridge with the African continent and modelled the P-wave velocity structure in a forward approach. The profile extends 430. km along the ridge and continues onshore to a length of 720. km. Crustal velocities beneath the Walvis Ridge vary between 5.5. km/s and 7.0. km/s, a typical range for oceanic crust. The crustal thickness of 22. km, however, is approximately three times larger than of normal oceanic crust. The continent-ocean transition is characterized by 30. km thick crust with strong lateral velocity variations in the upper crust and a high-velocity lower crust (HVLC), where velocities reach up to 7.5. km/s. The HVLC is 100 to 130. km wider at the Walvis Ridge than it is farther south, and impinges onto the continental crust of the Kaoko fold belt. Such high seismic velocities indicate Mg-rich igneous material intruded into the continental crust during the initial rifting stage. However, the remaining continental crust seems unaffected by intrusions and the root of the 40. km-thick crust of the Kaoko belt is not thermally abraded. We conclude that the plume head did not modify the continental crust on a large scale, but caused rather local effects. Thus, it seems unlikely that a plume drove or initiated the breakup process. We further propose that the plume already existed underneath the continent prior to the breakup, and ponded melt erupted at emerging rift structures providing the magma for continental flood basalts.

Description: Pockmarks are variably sized crater-like structures that occur in young continental margin sediments. They are formed by gas eruptions and/or long-term release of fluid or gas. So far no pockmarks were known from the Pacific coast of South America between 51°S and 55°S. This article documents an extensive and previously unknown pockmark field in the Seno Otway (Otway Sound, 52°S) with multibeam bathymetry and parametric echosounding as well as sediment drill cores. Up to 31 pockmarks per square kilometer occur in water depths of 50 to 〉100 m in late glacial and Holocene sediments. They are up to 150 m wide and 10 m deep. Below and near the pockmarks, echosounder profiles image acoustic blanking as well as gas chimneys often crosscutting the 20 to 〉30 m thick glacial sediments above the acoustic basement, in particular along fault zones. Upward-migrating gas is trapped within the sediment strata, forming dome-like features. Two 5 m long piston cores from inside and outside a typical pockmark give no evidence for gas storage within the uppermost sediments. The inside core recovered poorly sorted glacial sediment, indicating reworking and re-deposition after several explosive events. The outside core documents an undisturbed stratigraphic sequence since ~15 ka. Many buried paleo-pockmarks occur directly below a prominent seismic reflector marking the mega-outflow event of the Seno Otway at 14.3 ka, lowering the proglacial lake level by about 80 m. This decompression would have led to frequent eruptions of gas trapped in reservoirs below the glacial sediments. However, the sediment fill of pockmarks formed after this event suggests recurrent events throughout the Holocene until today. Most pockmarks occur above folded hydrocarbon-bearing Upper Cretaceous and Paleogene rocks near the western margin of the Magallanes Basin, constraining them as likely source rocks for thermogenic gas.

Description: This chapter documents the primary procedures and methods employed by the operational and scientific groups during the offshore and onshore phases of International Ocean Discovery Program (IODP) Expedition 357. This information concerns only shipboard and Onshore Science Party (OSP) methods described in the site chapters. Methods for postexpedition research conducted on Expedition 357 samples and data will be described in individual scientific contributions. Detailed drilling and engineering operations are described in the Operations section of each site chapter.

Description: International Ocean Discovery Program (IODP) Expedition 357 successfully cored an east–west transect across the southern wall of Atlantis Massif on the western flank of the Mid-Atlantic Ridge (MAR) to study the links between serpentinization processes and microbial activity in the shallow subsurface of highly altered ultra- mafic and mafic sequences that have been uplifted to the seafloor along a major detachment fault zone. The primary goals of this ex- pedition were to (1) examine the role of serpentinization in driving hydrothermal systems, sustaining microbial communities, and se- questering carbon; (2) characterize the tectonomagmatic processes that lead to lithospheric heterogeneities and detachment faulting; and (3) assess how abiotic and biotic processes change with varia- tions in rock type and progressive exposure on the seafloor. To ac- complish these objectives, we developed a coring and sampling strategy centered on the use of seabed drills—the first time that such systems have been used in the scientific ocean drilling pro- grams. This technology was chosen in the hope of achieving high recovery of the carbonate cap sequences and intact contact and de- formation relationships. The expedition plans also included several engineering developments to assess geochemical parameters during drilling; sample bottom water before, during, and after drilling; sup- ply synthetic tracers during drilling for contamination assessment; acquire in situ electrical resistivity and magnetic susceptibility mea- surements for assessing fractures, fluid flow, and extent of ser- pentinization; and seal boreholes to provide opportunities for future experiments. (...)

Description: Highlights • Late stage volcanism covers old oceanic crust north of the Florianopolis Fracture Zone. • No influence of fracture zone on formation of Walvis Ridge at 6° E. • Walvis Ridge at 6° E erupted in deep water environment. Abstract The Walvis Ridge is one of the major hotspot trails in the South Atlantic and a classical example for volcanic island chains. Two models compete about the origin of the ridge: It is either the result of a deep mantle plume or active fracture zones above mantle inhomogeneities. Among other things crustal information is needed to constrain the models. Here, we provide such constraint with a 480 km long P-wave velocity model of the deep crustal structure of the eastern Walvis Ridge at 6° E. According to our data the Walvis Ridge stretches across the Florianopolis Fracture Zone into the Angola Basin. Here, we observe a basement high and thick basaltic layers covering the oceanic crust and the fracture zone. We found two crustal roots along the profile: one is located beneath the ridge crest, the other one beneath the northern basement high in the Angola Basin. The crustal thickness reaches 18 km and 12 km and the lower crustal velocities are 7.2 km/s and 7.4 km/s, respectively. The bathymetric expression of the ridge along the profile is less pronounced than closer to shore, which is mainly attributable to the absence of a thick layer of volcanic debris, rather than to reduced crustal thickness below the basement surface. Therefore, this part of the ridge was never or only briefly subaerially exposed. The crustal structure suggests that the ridge and the fracture zone formed independently of each other. The oceanic crust north of the fracture zone, which is buried underneath the basalt layer, is younger than the reconstructed age of hotspot volcanism of the Walvis Ridge. We interpret these structures north of the fracture zone to be at least partly a product of late stage volcanism.