ALBERT

Description: We evaluate different hypotheses concerning the formation of a peculiar, flat-topped ridge at Rock Garden, offshore of the North Island of New Zealand. The coincidence of the ridge bathymetry with the depth at which gas hydrate stability intersects the seafloor has been previously used to propose that processes at the top of gas hydrate stability may cause seafloor erosion, giving rise to the flat ridge morphology. Two mechanisms that lead to increased fluid pressure (and sediment weakening) have previously been proposed: (1) periodic formation (association) and dissociation of gas hydrates during seafloor temperature fluctuations; and (2) dissociation of gas hydrates at the base of gas hydrate stability during ridge uplift. We use numerical models to test these hypotheses, as well as to evaluate whether the ridge morphology can develop by tectonic deformation during subduction of a seamount, without any involvement from gas hydrates. We apply a commonly-used 1D approach to model gas hydrate formation and dissociation, and develop a 2D mechanical model to evaluate tectonic deformation. Our results indicate that: (1) Tectonics (subduction of a seamount) may cause a temporary flat ridge morphology to develop, but this evolves over time and is unlikely to provide the main explanation for the ridge morphology; (2) Where high methane flux overwhelms the anaerobic oxidation of methane via sulphate reduction near the seafloor, short-period temperature fluctuations (but on timescales of years, not months as proposed originally) in the bottom water can lead to periodic association and dissociation of a small percentage of gas hydrate in the top of the sediment column. However, the effect of this on sediment strength is likely to be small, as evidenced by the negligible change in computed effective pressure; (3) The most likely mechanism to cause sediment weakening, leading to seafloor erosion, results from the interaction of gas hydrate stability with tectonic uplift of the ridge, provided bulk permeability strongly decreases with increasing hydrate content. Rather than overpressure developing from dissociation of hydrates at the base of gas hydrate stability (as previously thought), we found that the weakening is caused by focusing of gas hydrate formation at shallow sediment levels. This creates large fluid pressures and can lead to negative effective pressures near the seafloor, reducing the sediment strength.

Description: Bottom-simulating reflections (BSRs) are probably the most commonly used indicators for gas hydrates in marine sediments. It is now widely accepted that BSRs are primarily caused by free gas beneath gas-hydrate-bearing sediments. However, our insight into BSR formation to date is mostly limited to theoretical studies. Two endmember processes have been suggested to supply free gas for BSR formation: (i) dissociation of gas hydrates and (ii) migration of methane from below. During a recent campaign of the German Research Vessel Sonne off the shore of Peru, we detected BSRs at locations undergoing both tectonic subsidence and non-sedimentation or seafloor erosion. Tectonic subsidence (and additionally perhaps seafloor erosion) causes the base of gas hydrate stability to migrate downward with respect to gas-hydrate-bearing sediments. This process rules out dissociation of gas hydrates as a source of free gas for BSRs at these locations. Instead, free gas at BSRs is predicted to be absorbed into the gas hydrate stability zone. BSRs appear to be confined to locations where the subsurface structure suggests focusing of fluid flow. We investigated the seafloor at one of these locations with a TV sled and observed fields of rounded boulders and slab-like rocks, which we interpreted as authigenic carbonates. Authigenic carbonates are precipitations typically found at cold vents with methane expulsion. We retrieved a small carbonate-cemented sediment sample from the seafloor above a BSR about 20 km away. This supported our interpretation that the observed slabs and boulders were carbonates. All these observations suggest that BSRs in Lima Basin are maintained predominantly by gas that is supplied from below, demonstrating that this endmember process for BSR formation exists in nature. Results from Ocean Drilling Program Leg 112 showed that methane for gas hydrate formation on the Peru lower slope and the methane in hydrocarbon gases on the upper slope is mostly of biogenic origin. The δ13C composition of the recovered carbonate cement was consistent with biologic methane production below the seafloor (although possibly above the BSR). We speculate that the gas for BSR formation in Lima Basin also is mainly biogenic methane. This would suggest the biologic productivity beneath the gas hydrate zone in Lima Basin to be relatively high in order to supply enough methane to maintain BSRs.

Description: Within the GEOPECO project (Geophysical Experiments at the Peruvian Continental Margin - investigations of tectonics, mechanics, gas hydrates and fluid transport) seismic refraction and reflection data were acquired during RV 'Sonne' cruise SO 146 along with bathymetric and gravimetric mapping, sea-floor sampling, observation of the ocean floor and heat flow measurements. The objectives were a quantitative characterization of the structures and geodynamics of the Peruvian section of the Andean subduction zone and the associated gas hydrate systems in regions with differing tectonic development. The oceanic Nazca Plate, which is approximately 28 to 38 million years new at the Peruvian trench, is subducting under the South American Plate. The Peruvian Continental Margin has been influenced over the last 8 million years by collision with the Nazca Ridge, a 400 km long and 50 km wide basement high. Collision migrated progressively from north to south, is presently in the area of 15°S and has influenced the area to the north in several ways. Six wide angle seismic profiles, each approximately 100nm long, were shot with three 32 liter Bolt-airguns over 9 to 14 OBH/S instruments at the Peruvian Margin. During the cruise a total amount of 127 OBH/S were successfully deployed showing high quality data. Forward modeling was performed to characterize the structure and the velocities of the different stages of the evolution of the margin after collision with the Nazca Ridge. The coincident reflection seismic profiles were used to constrain the structure and thickness of the upper layers. The resulting crustal cross sections reveal a rough surface and a thin sediment layer of the subducting oceanic Nazca Plate. The crust thickens beneath the Nazca Ridge. Its thickness also varies north and south of Mendana Fracture Zone (MFZ), which separates younger (~25 Ma old) from older (~35 Ma old) oceanic crust at about 11°S. There is no accretionary wedge where Nazca Ridge currently subducts. 3 Ma after the ridge has passed, a new accretionary prism is already set up with a width of 20 to 30 km and 4 to 5 km thickness which does not further increase in size as revealed by the profiles recorded further north of Nazca Ridge. This indicates that current subduction along the Peruvian Margin is non-accreting. The slope angle of the accretionary prism increases south of MFZ, whereas the profile north of MFZ shows a smaller slope angle. As the subducting Nazca Plate dips at about 6° on all profiles north of Nazca Ridge, the resulting taper is 12° to 17°, indicative of high basal friction and non-accretionary subduction. The horst and graben like structure and rough topography of the oceanic plate also substantiates non-accretionary even erosional subduction for the graben structures are filled with sediment before subduction. Two cross profiles from Lima Basin reveal the crustal structure of the continental slope. Lima Basin is some 80 km wide (along dip) and its thickness varies from 1 to 3 km below sea floor. Furthermore it shows an asymmetric shape and is divided into two parts by a basement high at the landward termination.

Description: Salt flow in sedimentary basins is mainly driven by differential loading and can be described by the concept of hydraulic head. A hydraulic head in the salt layer can be imposed by vertically displacing the salt layer (elevation head) or the weight of overburden sediments (pressure head). Basement faulting in salt-bearing extensional basins is widely acknowledged as a potential trigger for hydraulic heads and the growth of salt structures. In this study, scaled analogue experiments were designed to examine the kinematics of salt flow during the early evolution of a salt structure triggered by basement extension. In order to distinguish flow patterns driven by elevation head or by pressure head, we applied a short pulse of basement extension, which was followed by a long-lasting phase of sedimentation. During the experiments viscous silicone putty simulated ductile rock salt, and a PVC-beads/quartz-sand mixture was used to simulate a brittle supra-salt layer. In order to derive 2-D incremental displacement and strain patterns, the analogue experiments were monitored using an optical image correlation system (particle imaging velocimetry). By varying layer thicknesses and extension rates, the influence of these parameters on the kinematics of salt flow were tested. Model results reveal that significant flow can be triggered in the viscous layer by small-offset basement faulting. During basement extension downward flow occurs in the viscous layer above the basement fault tip. In contrast, upward flow takes place during post-extensional sediment accumulation. Flow patterns in the viscous material are characterized by channelized Poiseuille-type flow, which is associated with subsidence in regions of "salt" expulsion and surface uplift in regions of inflation of the viscous material. Inflation of the viscous material eventually leads to the formation of pillow structures adjacent to the basement faults (primary pillows). The subsidence of peripheral sinks adjacent to the primary pillow causes the formation of additional pillow structures at large distance from the basement fault (secondary pillows). The experimentally obtained structures resemble those of some natural extensional basins, e.g. the North German Basin or the Mid-Polish Trough, and can aid understanding of the kinematics and structural evolution of sedimentary basins characterized by the presence of salt structures.