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

Stable Strontium Isotope (δ88/86Sr) Fractionation in the Marine Realm: A Pilot Study

The determination of the isotopic composition of natural substances is an important field of research within isotope geochemistry. Especially the investigation of the alkaline earth element strontium (Sr) plays an important role in geological and geochemical research. In order to quantify the degree of natural stable Sr isotope fractionation a double spike technique was developed in the frame of this study. This technique allows the precise determination of natural Sr isotope fractionation without normalizing the 87Sr/86Sr to a fixed 88Sr/86Sr ratio in order to correct for instrumental mass fractionation. Variations in the stable Sr isotope ratio are presented in the common δ-notation in per mill [‰] deviation from standard material NIST SRM 987 (δ88/86Sr[‰]=((88Sr/86Sr)sample/(88Sr/86Sr)standard–1)∙1000). Measurements were carried out at the IFMGEOMAR in Kiel using a thermal ionization mass spectrometer (TIMS). Long term measurements of the coral standard JCp-1 and the seawater standard IAPSO resulted in δ88/86Sr=0.194±0.025‰ and δ88/86Sr=0.389±0.026‰ (2SD), respectively. This corresponds to an improvement of measurement precision of at least a factor of 2 when compared to multi collector inductively coupled plasma mass spectrometer (MC-ICP-MS) measurements using bracketing standard (FIETZKE and EISENHAUER, 2006). The precise determination of natural Sr isotope fractionation adds a new dimension to the well established radiogenic Sr isotope system. Seawater and marine carbonates show significant differences in their stable Sr isotopic composition which were not accessible by applying the radiogenic 87Sr/86Sr ratio alone. In order to constrain glacial/interglacial changes in the marine Sr budget the isotope composition of modern seawater and modern marine biogenic carbonates are compared with the corresponding values of river waters and hydrothermal solutions in a triple isotope plot (δ88/86Sr vs. 87Sr/86Sr). The Sr sources (87Sr/86Sr ~ 0.7106±0.0008, δ88/86Sr ~ 0.31±0.01‰) show a heavier isotopic composition compared to marine carbonates (87Sr/86Sr ~ 0.70926±0.00002, δ88/86Sr ~ 0.21±0.02‰), representing the main Sr sink. This reflects isotopic disequilibrium with respect to Sr inputs and outputs. In contrast to the modern ocean, isotope equilibrium between inputs and outputs was achieved during the last glacial maximum (10-30 kyr before present). This can be explained by invoking three times higher Sr inputs from a uniquely “glacial” source: weathering of shelf carbonates exposed at low sea levels. Our data are also consistent with the “weathering peak” hypothesis that invokes enhanced Sr inputs resulting from weathering of post-glacial abundant finegrained material left exposed by the retreating ice masses (VANCE et al., 2009). Furthermore, the temperature dependency of δ88/86Sr in cultured and temperature controlled (21°C to 29°C) warm water corals (Acropora sp.) was investigated. A strict linear trend like reported by (FIETZKE and EISENHAUER, 2006; RÜGGEBERG et al., 2008) could not be confirmed in this study. Our measurements rather revealed a nonlinear relationship between temperature and δ88/86Sr (δ88/86Sr=0.001∙T2 – 0.039∙T + 0.692, r2=0.47) whereas the Sr/Ca ratio shows the expected linear trend. Moreover, we determined δ88/86Sr-, δ18O- and Sr/Ca-ratios of a fossil (15 kyr B.P.) Porites sp. coral originating from Tahiti (French-Polynesia). The Sr/Ca as well as the isotope ratios shows a similar seasonal variability. Fossil Porites sp. (δ88/86Srmean=0.205±0.017‰, 2SEM) and recent Porites sp. represented in this study by the coral standard JCp-1 (δ88/86SrJCp-1=0.194±0.009‰, 2SEM) show connatural mean δ88/86Sr values. The average δ88/86Sr is obviously not affected by enhanced weathering and elevated Sr fluxes from exposed shelves during glacial times like it is the case for Sr/Ca elemental ratios. Therefore, stable Sr isotope fractionation can potentially serve as independent and unbiased parameter for reconstructing paleo-sea-surface-temperatures

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

Determination of radiogenic and stable strontium isotope ratios (87Sr/86Sr; δ88/86Sr) by thermal ionization mass spectrometry applying an 87Sr/84Sr double spike

Recent findings of natural strontium isotope fractionation have opened up a new field of research in non-traditional stable isotope geochemistry. While previous studies were based on data obtained by MC-ICP-MS we here present a novel approach combining thermal ionization mass spectrometry (TIMS) with the use of an 87Sr/84Sr double spike (DS). Our results for the IAPSO sea water and JCp-1 coral standards, respectively, are in accord with previously published data. The strontium isotope composition of the IAPSO sea water standard was determined as δ88/86Sr = 0.386(5)‰ (δ values relative to the SRM987), 87Sr/86Sr* = 0.709312(9) n = 10 and a corresponding conventionally normalized 87Sr/86Sr = 0.709168(7) (all uncertainties 2SEM). For the JCp-1 coral standard we obtained δ88/86Sr = 0.197(8)‰, 87Sr/86Sr* = 0.709237(2) and 87Sr/86Sr = 0.709164(5) n = 3. We show that by applying this DS-TIMS method the precision is improved by at least a factor of 2–3 when compared to MC-ICP-MS