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  • 1
    ISSN: 1437-3262
    Keywords: Accretionary prisms ; Convergent margins Peru ; Analogue modeling ; Tectonics ; Accretion ; Tectonic erosion ; Marine geophysics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences
    Notes: Abstract Reflection seismic data from the Peruvian continental margin at 12° S clearly reveal an accretionary wedge and buttress. Sandbox experiments applying the physical concept of the Coulomb theory allow the systematic investigation of the growth and deformation of such an accretionary structure. The style of deformation of the buttress and the internal structure of the wedge is observed in the sandbox models. The possibility of underplating material beneath the buttress and the amount of tectonic erosion depend on the physical properties of the materials, mainly internal friction, cohesion and basal friction. Boundary conditions such as the height of the subduction gate and the thickness of incoming sand also constrain the style of growth of the model accretionary structure. The configurations of two experiments were closely scaled to reflection seismic depth sections across the Peruvian margin. A deformable buttress constructed of compacted rock powder is introduced to replicate the basement rock which allows deformation similar to that in the seismic data. With the sandbox models it is possible to verify a proposed accretionary history derived from seismic and borehole data. The models also help in understanding the mechanisms which control the amount of accretion, subduction and underplating as a function of physical properties, boundary conditions and the duration of convergence.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Book , NonPeerReviewed
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  • 3
    Publication Date: 2019-07-17
    Description: The lower boundary of the methane hydrate stability zone in continental margin sedi-mentsis often marked by a strong, phase reversed reflection subparallel to the seafloor,called the bottom simulating reflector (BSR). High resolution multichannel seismic(MCS) data from the Yaquina Basin offshore Peru at 8 deg S show a BSR that is vary-inglaterally in amplitude as well as in continuity. The amplitudes of the reflectionsabove the BSR also vary with the appearance of the BSR. Where the BSR is strong,the reflections above it are weaker compared to areas where the BSR is weak. Andalthough the strong part of the BSR is underlain immediately by strong reflections,reflections several hundred meters beneath the BSR appear weaker than those wherethe BSR is weak. This variation indicates significant heterogeneity in the distribu-tionof gas and gas hydrate in this area. Chemoherms observed at the Yaquina Basinsea floor indicate the presence of free gas in the sediments up to the seafloor. Thepresence of gas and gas hydrate within the sediment sequence significantly influencesthe P-wave velocity in the affected layers. Therefore a detailed analysis of velocityvariations enables to understand the apparently different conditions for the formationof gas hydrate along the BSR and the migration paths of the free gas. Ocean bot-tomseismometer (OBS) data from profiles coincident with the MCS data can providesuch detailed velocity depth information. Velocity analysis from OBS data included2D-ray tracing and 1D-interval-velocity analysis by means of DIX-inversion. In orderto find a trade-off between vertical resolution and minimization of errors caused bythe sensitivity of the DIX formula to velocity variations in thin layers, the data haveundergone a Kirchhoff wave-equation datuming and adjacent coherence filtering wasapplied to the data to eliminate the one sided travel path through the water columnof the OBS-observations. The derived velocity structure confirms the interpretation ofthe reflection pattern in terms of gas and gas hydrate distribution.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2019-07-17
    Description: MCS data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Intricate lateral variations of the reflection pattern at the assumed base of the GHSZ in terms of continuity, reflection amplitude, and signal attenuation underneath are observed, as well as the occurrence of paleo-BSR. Phase reversed reflections at an erosional unconformity above the BSR indicate free gas within the GHSZ. In order to further constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. The results from 2D-forward modelling and interactive velocity analysis show consistentresults. They exhibit a low velocity layer almost directly beneath the seafloor. Another low velocity layer with less than 1.5 km/s is present between the unconformity and the BSR.. In the vicinity and beneath prominent chemoherms, high velocities have been observed between the BSR and seafloor. Heat flux values calculated on the basis of the velocity-depth functions increase with decreasing amplitude of the bottom simulating reflector and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is present under parts of the BSR, and within the hydrate stability zone, particularly under the sea floor and under the erosional unconformity. The higher interval velocities near and beneath the chemoherms are suspected to be caused either by thick gas hydrate lenses or a significant amount of precipitated carbonate within the sediment or a combination of both. The hypothesis of a paleo-BSR that reflects the uplift of the base of the GHSZ caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 5
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2018-05-07
    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.
    Keywords: Salt flow; sedimentary basins
    Language: English
    Type: article , publishedVersion
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  • 7
    Publication Date: 2016-11-15
    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.
    Type: Article , PeerReviewed
    Format: text
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  • 8
    Publication Date: 2014-06-28
    Description: Measurement of magnetic vector or tensor quantities, namely of field or field gradient, delivers more details of the underlying geological setting in geomagnetic prospection than a scalar measurement of a single component or of the scalar total magnetic intensity. Currently, highest measurement resolutions are achievable with superconducting quantum interference device (SQUID)-based systems. Due to technological limitations, it is necessary to suppress the parasitic magnetic field response from the SQUID gradiometer signals, which are a superposition of one tensor component and all three orthogonal magnetic field components. This in turn requires an accurate estimation of the local magnetic field. Such a measurement can itself be achieved via three additional orthogonal SQUID reference magnetometers. It is the calibration of such a SQUID reference vector magnetometer system that is the subject of this paper. A number of vector magnetometer calibration methods are described in the literature. We present two methods that we have implemented and compared, for their suitability of rapid data processing and integration into a full tensor magnetic gradiometry, SQUID-based, system. We conclude that the calibration routines must necessarily model fabrication misalignments, field offset and scale factors, and include comparison with a reference magnetic field. In order to enable fast processing on site, the software must be able to function as a stand-alone toolbox.
    Keywords: Geomagnetism, Rock Magnetism and Palaeomagnetism
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 9
    Publication Date: 2013-06-10
    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.
    Type: Article , PeerReviewed
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  • 10
    Publication Date: 2013-11-05
    Type: Conference or Workshop Item , NonPeerReviewed
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