Crustal structure of the Peruvian continental margin : results from wide-angle seismic studies

Durch Modellierung von seismischen Weitwinkeldaten konnten die Struktur, sowie die P-Wellen Geschwindigkeiten des Untergrundes der subduzierenden ozeanischen Nazca Platte und der kontinentalen Süd-Amerikanischen Platte von 8S bis 15S am peruanischen Kontinentalrand verifiziert werden. Drei detaillierte Profile entlang der Subduktionszone und ein Querprofil ueber das Lima Becken werden in dieser Arbeit vorgestellt. Die ozeanische Kruste der Nazca Platte ist mit einer duennen pelagischen Sedimentschicht bedeckt, die von 0 bis 200 m variiert. Die ozeanische Kruste im Messgebiet hat durchschnittlich eine Maechtigkeit von 6.4 km, die im Bereich des Trujillo Trogs bei 8S, einer Grabenstruktur, auf 4 km ausduennt. Die abtauchende Platte kann anhand der Daten, bis in eine Tiefe von 25 km beobachtet werden. Ein Subduktionskanal von 50 m Maechtigkeit, und der P-Wellen Geschwindigkeit von 4.5 km/s, wird mit der Methode der Wellenfeld-Modellierung nachgewiesen. Die Geschwindigkteis-Modelle zeigen, dass sich ein Akkretionskeil ausgebildet hat, welcher mit dem steilen unteren Kontinentalhang in Verbindung gebracht wird. Terrigene Sedimente werden dort abgelagert, welche durch niedrige P-Wellen Geschwindigkeiten charakterisiert sind. Diese lagern an eine landeinwaerts liegende Backstop Struktur, die aus kontinentalem Basement aufgebaut ist, und mit seismischen P-Wellen Geschwindigkeiten von 3.5 bis 6.0 km/s charakterisiert ist. Die Subduktionsprozesse vor Peru werden von tektonischer und basaler Erosion dominiert mit Perioden erhoehter Erosion durch die Nazca-Ruecken-Subduktion. Anzeichen dafuer sind der grosse Winkel zwischen Kontinentalhang und subduzierender Platte, lateral variierende Neigungswinkel entlang des Kontinentalhanges, variierende Materialeigenschaften der kontinentalen Platte und unterschiedliche Maechtigkeiten der Forearc Becken entlang des Kontinentalrandes

Morphological structures relate to the location and extent of the seismogenic zone - bathymetric studies of the Sunda margin, Indonesia

Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. Morphological structures from multibeam bathymetric data across the forearc relate with the extent of the seismogenic zone (SZ). Off Java and the Lesser Sunda islands the Indo-Australian plate subducts almost normal underneath the oceanic plate of the Indonesian archipelago. Landward of the trench, the outer wedge of the slope break is ~50 km uniformly wide with uniform bathymetric gradients. The slope of the outer wedge is locally cut by one/two steeper ridges of ~5 km extent. The sharp slope break corresponds to the updip limit of the SZ, which is also associated with the seawardmost part of the outer arc high. Landward of the slope break we find narrow, uniform outer arc ridges. The landward termination of these ridges coincides with the downdip limit of the SZ. The intersection of the shallow upper plate mantle with the subduction thrust fault marks the downdip limit of the SZ beneath the forearc. Off Sumatra the Indo-Australian plate subducts obliquely underneath the continental part of the Indonesian Sunda margin. Landward of the trench, the outer wedge varies, being mostly ~70 km wide, in some areas narrowing to 50 km width. The lower slope bathymetric gradients are steep. The outer wedge slope is made up of several steeper ridges of ~5 km extent. The slope break is only locally sharp, and corresponds to the updip limit of the SZ. The outer arc ridges off Sumatra are, in comparison with the forearc structures off Java and the Lesser Sunda islands, wider and partly elevated above sea level forming the Mentawai forearc islands. The downdip limit of the SZ coincides with the intersection of a deeper upper plate mantle with the subduction thrust fault beneath the forearc. Sunda Strait marks a transition zone between the Sumatra and Java margins. Seafloor morphology enables the identification of the seismogenic zone (SZ) across the entire Sunda margin. The SZ is uniformly wide for the Sumatra margin and narrows off Sunda Strait. Sunda Strait is the transition between the Sumatra margin and the uniformly narrow extent of the SZ of the Java/Lesser Sunda margin. Comparing the Java and Lesser Sunda islands with the Sumatra margin we find the differences along the Sunda margin, especially the wider extent of the SZ off Sumatra, producing larger earthquakes, to result from the combination of various causes: The sediment income on the oceanic incoming plate and the subduction direction; we attribute a major role to the continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault. Off Sumatra the SZ is up to more than twice as wide as off Java/Lesser Sunda islands, enlarging the unstable regime off Sumatra and thus the risk of sudden stress release in a great earthquake

Episodic methane concentrations at seep sites on the upper slope Opouawe Bank, southern Hikurangi Margin, New Zealand

Along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere. With numerous cold seeps already known, the convergent Hikurangi Margin east of North Island, New Zealand, was selected as a target area for further detailed, multidisciplinary investigation of cold seeps within the New Vents and associated projects. Methane and temperature sensors (METS) were deployed at selected seep sites on the Opouawe Bank off the southeastern tip of North Island and near the southern end of the imbricate-thrust Hikurangi Margin, together with seismic ocean bottom stations. They remained in place for about 48 h while seismic data were collected. The seeps were associated with seep-related seismic structures. Methane concentrations were differing by an order of magnitude between neighbouring stations. The large differences at sites only 300 m apart, demonstrate that the seeps were small scale structures, and that plumes of discharged methane were very localised within the bottom water. High methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow. Methane discharge from the seafloor was episodic, which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides. The strong semi-diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater. Despite dispersal by currents, fluid flow through fissures, fractures, and faults close to the METS positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the METS. Small earthquakes do not appear to be correlated with seawater methane anomalies

The structures beneath submarine methane seeps : seismic evidence from Opouawe Bank, Hikurangi Margin, New Zealand

The role of methane in the global bio-geo-system is one of the most important issues of present-day research. Cold seeps, where methane leaves the seafloor and enters the water column, provide valuable evidence of subsurface methane paths. Within the New Vents project we investigate cold seeps and seep structures at the Hikurangi Margin, east of New Zealand. In the area of Opouawe Bank, offshore the southern tip of the North Island, numerous extremely active seeps have been discovered. High-resolution seismic sections show a variety of seep structures. We see seismic chimneys either characterised by high-amplitude reflections or by acoustic turbidity and faults presumably acting as fluid pathways. The bathymetric expression of the seeps also varies: There are seeps exhibiting a flat seafloor as well as a seep located in a depression and small mounds. The images of the 3.5 kHz Parasound system reveal the ear-surface structure of the vent sites. While highamplitude spots within the uppermost 50 m below the seafloor (bsf) are observed at the majority of the seep structures, indicating gas hydrate and/or authigenic carbonate formations with an accumulation of free gas underneath, a few seep structures are characterised by the complete absence of reflections, indicating a high gas content without the formation of a gas trap by hydrates or carbonates. The factors controlling seep formation have been analysed with respect to seep location, seep structure, water depth, seafloor morphology, faults and gas hydrate distribution. The results indicate that the revailing structural control for seep formation at Opouawe Bank is the presence of numerous minor faults piercing the base of the gas hydrate stability zone

Bathymetry of the Indonesian Sunda margin-relating morphological features of the upper plate slopes to the location and extent of the seismogenic zone

Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. Morphological structures in multibeam bathymetric data across the forearc relate with the extent of the seismogenic zone. Its updip limit corresponds to the slope break, most distinct off Java and Lesser Sunda islands, where we find coincident narrow, uniform, continuous outer arc ridges. Their landward termination and a shallow upper plate mantle mark the downdip limit of the seismogenic zone. In contrast the outer arc ridges off Sumatra are wider and partly elevated above sea level forming the forearc islands. The downdip limit of the seismogenic zone coincides with a deeper upper plate mantle. Sunda Strait marks a transition zone between the Sumatra and Java margins. We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/oceanic upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault