Search

Terraces on the modern seafloor are defined as topographically flat areas above the active submarine channel thalweg but within the confines of the channel-belt. They have been described from many modern submarine channels, but the controls on terrace distribution, evolution and stacking patterns are not well understood. In this study, we describe the architecture of the Mahin and Avon channel-belts and their associated terraces, located offshore Nigeria towards the northwest of the Niger Delta. The studied channel sections are < 10 km apart up-dip and converge downslope. They are on slopes with similar gradients, yet they have significantly different morphologies indicating that the interplay between sedimentary processes and channel evolution must be different. The surfaces defining the bases of the terrace bodies have been mapped along both channels using high-resolution 3D seismic data. Spectral decomposition of the data reveals subtle variations in seismic character that highlight sedimentological detail that can otherwise not be recognized, allowing us to suggest the processes responsible for terrace formation and terrace body composition. The contrasting evolution of the two channels is reflected in the morphology and architecture of their terraces. While the Mahin terrace bodies show a predictable pattern, typically consisting of stacked channel-fill and overbank deposits in a circular planform shape (shape controlled by cut-off channel bends), the deposits of the Avon terrace bodies up-dip of the confluence with the Mahin are dominated by overbank deposits, with the planform terrace shape heavily controlled by the topography of the underlying channel-belt deposits. This study shows how spatially and temporally associated channels can have markedly different architectures. The evolution of the channel and the abundance and stacking patterns of compositional elements within terrace bodies are shown to be linked. [ABSTRACT FROM AUTHOR]

The Gop and Laxmi Basins lying off Northwest India have been assigned ambiguous crustal types and evolution mechanisms. The Chagos–Laccadive Ridge (CLR) complex lying along the southwest coast of India has been attributed to different evolutionary processes. Late Cretaceous seafloor spreading between India and Africa formed the Mascarene Basin, and the plate reconstruction models depict unequal crustal accretion in this basin. Re-interpretation of magnetic data in the Gop and Laxmi Basins suggests that the underlying oceanic crust was accreted contemporaneously from 79 Ma at slow half spreading rates (0.6 to 1.5 cm/yr) separating the Seychelles–Laxmi Ridge complex from India. The spreading ridge became extinct at 71 Ma in the central region between 19–20°N and 65.5–67°E, and at 68.7 Ma in the Gop Basin. Extinction progressed southwards with time until 64.1 Ma at ~ 14.5°N in the Laxmi Basin. This spreading probably limited the seafloor spreading in the northern Mascarene Basin, while normal to fast spreading continued in the south. The fracture zone FZ2 acted as a major boundary between the northern and southern Mascarene Basin, and may have also restricted the southward propagation of the Laxmi Basin spreading ridge. Plate reconstruction models generated in this study accommodate several microplates between Madagascar and India, and trace their movements over time within the India–Africa spreading corridor. Furthermore, these models suggest the formation of the Seychelles–Laxmi Ridge microplate by 79 Ma in a complex three plate system. This microplate may have existed up to 64.1 Ma and experienced an anticlockwise rotation of about 50°. Under the influence of the Deccan hotspot activity, the Laxmi Basin spreading ridge jumped southwestward splitting the Seychelles–Laxmi Ridge complex and initiating the Carlsberg Ridge. The presence of about 160 km of oceanic crust in the Gop and Laxmi Basins partially reduces the space problem faced in the earlier plate reconstruction models. [ABSTRACT FROM AUTHOR]

Abstract: At least 70% of the volcanism on Earth occurs along the 65,000km network of mid-ocean ridge (MOR) spreading centers. Within these dynamic environments, the highest fluxes of heat, chemicals, and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions. However, because underwater volcanoes are difficult and expensive to access, researchers are rarely, if ever, in the right place at the right time to characterize these events. Therefore, our knowledge is limited about the linkages among hydrothermal, chemical and biological processes during seafloor formation and crustal evolution. To make significant advancements in understanding the evolution of MOR environments, the United States and Canada have invested in the first plate-scale submarine cabled observatory linked through the global Internet. Spanning the Juan de Fuca tectonic plate, these two networks include >1700km of cable and 14 subsea terminals that provide 8–10kW power and 10Gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column — resulting in a 24/7/365 presence in the oceans. Data and imagery are available in real- to near-real time. The initial experimental sites for monitoring volcanic processes include the MOR volcanoes called Axial Seamount and the Endeavour Segment that are located on the Juan de Fuca Ridge. Axial, a hot-spot influenced seamount, is the most robust volcano along the ridge rising nearly 1400m above the surrounding seafloor and it has erupted twice in the last 15years. In contrast, the Endeavour Segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known. A non-eruptive spreading event lasting 6years was documented at Endeavour between 1999 and 2005. Hydrothermal venting intensity, chemistry, and temperature, as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events. This paper presents the similarities and differences between the Axial and Endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies. The U.S. has made a 25-year commitment for sustained observations using the cabled infrastructure. The highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences, robotics, genomics and sensor miniaturization. Comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins. [Copyright &y& Elsevier]

Abstract: Pockmarks in the Oslofjord, Norway, show no evidence of recent expulsion of gas or water, but have still experienced reduced sedimentation rates through the Holocene. They are therefore suitable model systems for studying the currents and sedimentation in pockmarks. Mapping of sediment in a single pockmark using a large number of precisely positioned cores shows thickening of an unconsolidated, almost flocculent layer inside the pockmark. X-ray fluorescence logging of anthropogenic metals indicates higher sedimentation rates and mass accumulation rates inside the pockmark with respect to the surrounding seafloor. Furthermore, the cores taken near the pockmark centre contained relatively coarser material than those from outside the depression. Sediment traps collected substantially more sediments inside than outside of the pockmark when positioned close to the seafloor. This agrees with resuspension of sediments from the seafloor being enhanced inside the pockmark. Moreover, the traps deployed in the pockmark intercepted relatively coarser sediments. Acoustic backscatter near the seabed varies in a diurnal cycle, not related to the tide. This is interpreted as a probable result of vertical migration of zooplankton or fish. Inside the pockmark there is also a diurnal cycle in current velocity with high values in the daytime, possibly because of swimming fish. The data indicate that pockmarks collect particles, which may be largely due to deposition of sediments transported as bedload. This process should be dominant for pockmarks found in proximity to fluvial coarse-grained sediment input. The suspended fine material, however, is kept in suspension by turbulence and possibly biological activity, and can be transported away before settling. Whether an inactive pockmark will behave as a sediment depocentre or will be maintained by the action of currents, is likely to depend on the relative importance of bedload versus suspended load transport. [Copyright &y& Elsevier]