OS33B-0582 1340h
Marine Geology of the Southwestern San Juan Islands: New Insights From Multibeam Imagery and Processed Aeromagnetic Data
The San Juan Islands, located in the seismically active northern Puget Sound, have a complicated and not yet fully understood geologic history. This study is among the first marine geologic mapping efforts within the San Juan Islands, filling an important gap in an otherwise well-studied region. Existing geologic and geophysical data were combined with interpretations of new multibeam bathymetry and backscatter seafloor imagery to construct a seamless onshore - offshore geologic map of the southwestern San Juan Islands. Simrad EM1002 (95 kHz) and Reson 8101 (240 kHz) multibeam bathymetry and backscatter data were collected between October 2000 and November 2003 within Haro Strait, northeastern Strait of Juan de Fuca, and San Juan Channel. Sun-shaded images of the processed data reveal a complex network of faulted and fractured bedrock exposures, deep glaciated channels, Pleistocene glacial sediments, and dynamic bedforms. Distinct slump morphologies in multibeam and backscatter imagery suggest active slumping of recent sediments at the mouth of San Juan Channel. A number of previously inferred offshore geologic structures were extended and constrained based on distinct linear bedrock features visible in the multibeam imagery. Aeromagnetic data collected by the U.S. Geological Survey in 1997 were processed to accentuate short-wavelength, presumably shallow, magnetic sources. The resultant derivative aeromagnetic map reveals a number of areas with distinctive anomaly patterns. Gradients in magnetic anomalies often corresponded with fault traces identified in high-resolution multibeam imagery and may reflect slight magnetic susceptibility contrasts across fault contacts. Aeromagnetic data also constrain two tectonostratigraphic terrane boundaries not identified in multibeam imagery: 1) the Buck Bay fault, which separates the Lopez Structural Complex and Decatur terrane from the underlying Constitution Formation, and 2) the Haro fault separating the Deadman Bay terrane of the San Juan Thrust system from the Wrangellia terrane on Vancouver Island.
OS33B-0583 1340h
Relations Between Basement Tectonics, Sediment Deformation and Fluid Flow at the Eastern Juan de Fuca Ridge Flank: Results From Very High Resolution Seismic Data
In summer 2004, IODP Leg 301 was drilled on the eastern flank of the Juan de Fuca Ridge as a successor of ODP Leg 168 (1996). The main subject of the new leg was to study the properties of an active ridge flank hydrothermal system, which is for a big part isolated beneath a low permeable sediment cover. During the Leg 301 seismic pre-site survey, which was carried out during R/V Sonne Cruise SO 149 (2000), this sediment cover was imaged in a multi-frequency seismic data set at high and very high resolution. Folds, faults, and the layer structure were carefully mapped to show that forced folding is a dominant process affecting sediment physical properties and fluid flow. By comparing data from areas of different basement age and integrating seismic and borehole data, the importance of basement tectonics on sediment deformation and fluid flow, depending on crustal age and sedimentation processes is investigated. Preliminary results show that growth faults characterized by upwardly decreasing offsets reflect off-axis tectonic crustal reorganization. Growth fault propagation at Second Ridge ceased about 0.28 Ma ago at the base of a major sediment transport channel, whereas tectonic activity at First Ridge was more recent. Forced folding at First Ridge controls creation and maintenance of a subsurface plumbing system for vertical fluid flow through the sediments. Indications for forced folding are also found at Second Ridge though vertical fluid flow likely is much less affected.
OS33B-0584 1340h
Morphotectonic fabrics of major spreading centers, subduction zones and transform faults in the southern hemisphere -Beagle 2003 cruise by R/V {\it Mirai} the round trip in the southern hemisphere-
Precise bathymetric surveys of the world ocean has been made it possible to establish continental drift, sea-floor spearheading and plate tectonics. However the data have mostly been restricted from the oceans in the northern hemisphere. The precise submarine bathymetric data from the southern hemisphere are quite scarce because of the long distance from the major countries and of being always severe sea conditions with bad weather for the bathymetric swath mapping. Well-surveyed areas of the southern hemisphere are the Indian triple junction, SW and SE Indian Ridges, Southern East Pacific Rise from 13\deg S to 20\deg S and Scotia arc-trench-backarc system. We had not been known about the vast area of the southern hemisphere. We had a bathymetric survey around the southern hemisphere during the cruise of BEAGLE 2003. The project BEAGLE 2003 started from Brisbane, on 2003 Aug and ended at Fremantle, on Feb 2004 with a great success. In the South Pacific Ocean the line crosses on the 28\deg S - 32.5\deg S latitude from Brisbane to Valparaiso via Papeete and in the South Atlantic Ocean on the 30\deg S latitude from Santos to Cape Town and in the Indian Ocean on the 20\deg S latitude from Madagascar to Fremantle via Mauritius Island, respectively. We had a precise bathymetric survey by using the narrow multi beam system (SeaBeam 2112) that is installed in the JAMSTEC R/V {\it Mirai}. Bathymetric survey lines cross the Tonga-Kermadec Arc-Trench and backarc system, Louisville Ridge, East Pacific Rise spreading center, Peru-Chile Trench, southern Mid-Atlantic Ridge, Central Indian Ridge, Ninety East ridge and many other oceanic plateaus and seamounts. We had obtained a narrow band data on the morphologic features of the ridges, trenches, seamounts and oceanic plateaus and we compared the data with those from ETOPO2 that was distributed by Sandwell and Smith (1996). These data offer significant information on the plate tectonics and plume tectonics if we think about the morphology on the global scale.
OS33B-0585 1340h
A Shear Strength Characteristics in Deep-sea Sediment From the Clarion-Clipperton Fracture Zone, Northeast Equatorial Pacific
Deep-sea surface sediments, acquired by multiple corer from 69 stations in the Clarion-Clipperton fracture zone of the northeast equatorial Pacific, were analyzed for shear strength properties to understand their sedimentological process. The pelagic red clay from northern part of study area shows low average shear strength (4.4 kPa), while the siliceous sediment from middle area shows high (6.3 kPa). The calcareous sediment from southern area shows very low average shear strength (3.4 kPa), and transitional sediment between middle and southern area shows intermediate value (3.8 kPa) between siliceous and calcareous sediment. The depth profiles of average shear strength of pelagic red clay show gradual increment with depth due to decrease of water content with depth by general consolidation process. On the other, abrupt increment of average shear strength with depth in siliceous sediment is related to sedimentary hiatus. The very low shear strength in calcareous sediment is linked to very high sedimentation rate of southern area compared with other study area.
OS33B-0586 1340h
Digital Image Processing Techniques for Enhancement and Classification of MR1 Side Scan Sonar Imagery and Preliminary Results of Manganese Nodule Occurrence between the Clarion and Clipperton Fracture Zones, NE Equatorial Pacific
The recent growth in the production rate of digital side scan sonar images, coupled with the rapid expansion of systematic seafloor exploration programs, has created a need for fast and quantitative means of processing seafloor imagery. A number of numerical techniques used to enhance and classify imagery produced by long range side scan sonar (MR1) in the Clarion and Clippertion Fracture Zones, NE equatorial Pacific. Side scan sonar imagery is traditionally interpreted visually and qualitatively by experts. Textural Analysis enables a more objective approach, supplementing the interpreter with reliable quantitative results. Grey-level co-occurrence matrices describe numerically textual information and detect subtle details invisible to the human eyes. The area between the Clarion and Clipperton fracture zones (NE equatorial Pacific) is one of the highest manganese nodule abundance in the world oceans. A detailed analysis of MR1 sonar images and ground truth - free-fall grab (FFG) data in the area, reveals a close relationship between sonar characters of seafloor and manganese nodule occurrence. The close relationship between distribution of sonar imagery and manganese nodule abundance implies that seafloor topography and sediment thickness are important controlling factors for occurrence of manganese nodules.
OS33B-0587 1340h
Sediment Accumulation Rates of Late Quaternary Deposits in San Pedro Basin, the Gulf of Santa Catalina, and San Diego Trough, Offshore Southern California
A multiyear program of seismic-reflection profiling and sediment coring has focused on understanding the history of late Quaternary sedimentation within the inner basins of the California Borderland. The objective of this study has been to develop a high-resolution seismic stratigraphy primarily for the Last Glacial Maximum and the Holocene that can be used to understand depositional processes, sediment budgets, and deformation within the basins. The Santa Barbara and Santa Monica Basins in the northern Borderland are closed basins and both were cored during the Ocean Drilling Program (ODP) in support of paleoclimatic studies. In contrast, the inner basins in the southern Borderland that are the focus of this report are open-ended basins that have not been the subject of scientific drilling. We present the preliminary results of sediment coring at 21 sites in San Pedro Basin, the Gulf of Santa Catalina, and San Diego Trough. Initial estimates of sediment-accumulation rates for these basins are based on 48 previously unpublished radiocarbon dates. During the Holocene, average sediment-accumulation rates are generally less than 0.5 m/ky on the basin floors where turbidite deposition locally continued at reduced rates from those of the OIS 2-lowstand interval. This rate is nearly an order of magnitude less than was documented for the Holocene by ODP coring at Site 1015 in Santa Monica Basin reflecting, in part, the loss of sediment in the non-closed basins. Background hemipelagic rates range from 0.01 to 0.1 m/ky, with the lowest rates on mid-basin highs, e.g., Lasuen Knoll. More dating is currently underway for some of the core sites to determine the rate of change in sediment accumulation during the Holocene.
OS33B-0588 1340h
The Application of Seismic Array Techniques to Image UXO-Contaminated Littoral Environments
We investigate the application of seismic array techniques to increase the energy radiation and resolution of seismic waves in littoral areas to improve the success rate of detecting UXO in contaminated underwater sites. The investigation is carried out based on numerical modeling, including 2-D finite difference modeling and 3-D analytical solutions of the problem. In addition to various UXO orientations, we also modeled the presence of clutter in the subsurface. An array of 31 source and receiver elements was located floating in the water as well as sited on the seafloor, which allowed the comparison between single source-receiver combinations and beam-forming techniques. The numerical forward modeling involved noise-free and noisy data as well as interferences by free surface reflections (off the water-air interface), which produced the strongest phases on the seismograms. The inversion of the scattered seismic energy was performed using a 2-D eikonal solver (curved rays), which stacked and located the recorded amplitudes in space to determine the location of the UXO. The inversion also included the determination of the best fitting velocity model for the bay mud. The results of the 2-D modeling indicated that a single, horizontally oriented, UXO could be well detected as a function of depth and horizontal location. In the case of the source-receiver array being placed on the seafloor, the edges of the UXO were resolved indicating its horizontal extent, while the top of the UXO was correctly located. The cases of a second, vertically oriented, UXO and clutter located 0.1 m next to the first UXO, produced similar results. In each case the two objects produced slight interference in the backscattered seismic signal, yet the resolution of the seismic wave was still good enough to resolve the two objects from each other. The introduction of a rippled water-seafloor interface during the forward modeling didn't change the results for the case of a floating source-receiver array even though a flat bottom was assumed in the inversion approach, which showed the robustness of the inversion approach.
OS33B-0589 1340h
Bioturbation in near-surface sediments from western Clarion-Clipperton zones: evidence from excess $^{210}$Pb measurement
In order to evaluate bioturbation in sediments from the western part of Clarion-Clipperton fracture zones, northeast tropical Pacific, excess $^{210}$Pb profiles in sediments cores collected with multiple corers during R/V DAYANGYIHAO Environmental Program Cruise in 1998 were measured by direct gamma assay using Ortec GWL series HPGe well detector. Based on a steady-state diffusion model of excess $^{210}$Pb profiles, it is calculated that bioturbation mixing depths and biodiffusion coefficients are 16 cm and 2.75cm$^{2}$/a in station ES9801 (145\deg 25'40"W, 8\deg 31'06"N, 5374 m in water depth), and 6 cm and 0.26 cm$^{2}$/a in station WS9805 (154\deg 04'31"W, 10\deg 03'48"N, 5140 m in water depth), respectively. The bioturbation mixing depth and mixing intensity becomes higher from west to east in the survey area. Besides, mean density of {\it polychaete} and total organic carbon (TOC) content are also higher in station ES9801 than that in station WS9805. It is proposed that bioturbation is directly controlled by species and abundance of benthic fauna as well as the organic matter content in the sediments.
OS33B-0590 1340h
Multi-Scale Analysis of Morphological Parameters on the Modern Congo Deep-Sea Channel
The overreaching motivation for this study is to improve understanding of underwater sedimentological processes that govern construction and organisation of siliciclastic deposits on abyssal plains. These processes have been poorly constrained so far, due to a lack of modern high-resolution bathymetric, seismic, and lithological data. Recent advances in the resolution of subsurface imagery now reveal the existence of large river-fed submarine fans which reach 10 km in thickness and extend more than 1000 km in length (e.g. Amazon, Mississippi, Indus, Congo systems). These huge sedimentary units result from the development of highly meandering channel-levee systems that spread out across the seafloor. Relationships between the different morphological parameters extracted from bathymetric data also improved the channel-levee growth models. In Congo basin, multibeam bathymetry and high resolution 2D seismic acquired during the ZAIANGO project (Total - Ifremer cooperation) have covered almost the entire fan surface, from the shelf to the deep basin and have provided better constraints on the fan architecture and morphology. Using multi-scale wavelet analysis, we show that the morphological parameters (levees and channel width and depth, longitudinal channel profile .) clearly record the successive phases of channel development and bifurcation. Hydrodynamic-related parameters (wavelength, sinuosity, gradient, width and depth of channel) reveal a periodic evolution linked to the dynamic of turbidity currents. Recurrent changes in the channel slope (wavelength = 200 km) are interpreted as the result of periodic variations of the stream power along channel.
OS33B-0591 1340h
Microbathymetric And Bathymetric Surveying - A Comparison Of Seafloor Topography At Porcupine Bank, West Of Ireland
One of the latest discoveries in the world oceans are carbonate structures in the North-East Atlantic. In the frameworks of several European projects, research vessel Polarstern and underwater robot Victor 6000 were engaged to explore these areas. The data described in this paper were collected during the expedition ARK XIX/3 between 16 - 19th June 2003. Bathymetric and microbathymetric data in parts of the Pelagia mound province, located on the northern Porcupine Bank, west of Ireland, were measured with two multibeam sonar systems deployed at different distances from the bottom. The compared models come from a Kongsberg Simrad EM 2000 multibeam sonar system and an Atlas Elektronik Hydrosweep DS-2 multibeam sonar system. Four of the measured areas were selected for a comparison. After necessary corrections of the data, Digital Terrain Models (DTM) were created using appropriate software. Can these DTMs be compared? What aspects influence the values of differences? Grid differences and correlations of the depth measurements provide answers to these questions.
OS33B-0592 1340h
The Lithosphere-Ocean-Atmosphere Seismo-Electromagnetic Transformer and Applications for the Seaquake Monitoring
To clear the physical nature of seismic electromagnetic (EM) signals observed in near sea regions a mathematical model of seismo-hydro-EM interaction in a lithosphere-ocean-atmosphere domain is formulated on the basis of principles of electrodynamics of moving continuous media. Generation and propagation of seismic, EM, temperature, and hydrodynamic waves caused by elastic displacements (main frequencies 0.1 to 1 Hz and amplitude and duration of the order of a few cm and sec respectively) in the upper mantle under the seafloor are traced numerically up to the low boundary of the ionosphere. The first measurable (50 pT) signal of the described seismic excitation (SE) of the ocean lithosphere arises in the form of ULF oscillations, in the frequency range of the SE, of the horizontal component of the magnetic field at the sea bottom's surface 3.5 sec after the beginning (t = 0) of the seaquake (the axes of polarization of the SE is approximately vertical). The seismic P wave caused by the SE arrives at the bottom a few sec later (depends on the focal depth). Let us note that the computed diffusive EM signal arises (t = 7 sec) at the sea surface together with the hydro-acoustic wave propagating upward from the sea bottom deformed by the seismic P wave. By runs with different reasonable geophysical characteristics of the medium and weak precursory SEs (see above), the magnetic signal amplitude is of the order of a few hundreds of pT at the sea surface and a of the order of a few tens of pT near the lower boundary of ionosphere, t=10 sec. The signal amplitude increases in proportion to the amplitude of a SE. So, the lithosphere-ocean-atmosphere system may be regarded as a seismo-EM transformer. The computed long (150 km) tsunami wave's amplitude far from a shore is about 15 cm only and EM signals (propagating in the atmosphere above the ocean with the light velocity) must be recorded. Basing on these and other numerical results (computed amplitudes, frequencies and velocities of elastic, EM, temperature and hydrodynamic waves are of the orders observed) the authors develop the project of the Lithosphere-Ocean-Atmosphere Multilevel Multidisciplinary Observatory (LOAMMO) including, among others: a bottom station (seismometry, magnetometry, thermometry, sounding), a moored ocean surface buoy (oceanography, satellite link) and an observation balloon (multidisciplinary gradient measurements, a long receiving antenna, the ionosphere sounding). We hope that the LOAMMO and multidisciplinary computations (above) will provide a stable detection of ionosphere Pc pulsations and EM signals of lithosphere origination as well as checking satellite prognostic data regarding earthquakes and seaquakes. The ocean lithosphere seismicity must be investigated and monitored not only because of earthquakes in near sea regions (especially Pacific segment of the Earth) and tsunamis but, as well, in view of increasing interest to global changes and, therefore, to powerful geodynamic processes under the Ocean floor covering about 75% of the Earth surface.
OS33B-0593 1340h
Design of a Dual-Pressure Port Penetration Probe for Pore Pressure Measurements in Ocean Drilling
We are building a tapered probe to measure pore pressure and hydraulic properties in low permeability sediments within boreholes. To minimize the time required for measurement we are using two pressure ports: one close to the narrow diameter tip and one on the large-diameter shaft. In-situ pore pressure is estimated within a short time by a two-point intersection method using the pressure dissipation recorded at the two pressure ports. A parametric study is presented to evaluate how possible changes in the probe geometry affects pore pressure dissipation at the pressure ports. The Strain Path Method (SPM) and a total stress soil model (MIT-T1) are used to predict the initial undrained pore pressure response upon insertion of series of hypothetical geometries. Uncoupled-isotropic consolidation is applied to simulate the subsequent excess pore pressure dissipation. The modeled response to insertion in normally consolidated Boston Blue Clay includes the following. Decreasing the length of the thin probe or increasing the diameter of the large shaft results in more rapid two-point intersection and a higher pressure at the two-point intersection. If the second pressure port is placed on the face of the tapered section, instead of the larger-diameter shaft, the intersection time is reduced. We are striving for a tool geometry that generates a consistent two-point intersection time that occurs as rapidly as possible in a range of soil properties.