Geological Long-Range Inclined Asdic Research Articles

Glacigenic debris-flows on the Bear Island Trough-Mouth Fan, Barents Sea margin

Unlike low-latitude river-fed submarine fans, high-latitude trough-mouth fans (TMFs) are dominated by stacked sequences of mainly diamictic glacigenic debris-flows (e.g. Vorren et al. 1989; Dowdeswell et al. 1996; King et al. 1996). Individual debris-flow deposits have a lobe-shaped geometry and can be traced for up to about 200 km down continental slopes of very low gradient (<1°). A number of TMFs, of which the Bear Island Fan is the largest (Fig. 1), have been identified around the margins of the Norwegian–Greenland Sea (Laberg & Vorren 1995). Fig. 1. ( a ) Side-scan sonar imagery of the northern part of the Bear Island TMF. Individual debris-flow lobes are shown in dark grey tones (adapted from Taylor et al. 2002). Acquisition system Geological LOng-Range Inclined Asdic (GLORIA) side-scan sonar. Frequency 6.5 kHz. ( b ) Observed and ( c ) interpreted side-scan sonar imagery of a debris-flow lobe with possible compressional ridges on the northern Bear Island Fan (adapted from Vogt et al. 1993). Acquisition system SeaMARC II side-scan sonar. Frequency 11–12 kHz. ( d ) Oblique view of debris-flow lobes identified in side-scan sonar imagery of the Bear Island Fan, draped onto IBCAO …

Slope instability and sediment redistribution in the Rockall Trough: constraints from GLORIA

AbstractA recent GLORIA (Geological LOng Range Inclined Asdic) sidescan survey covered 200 000 km2 of the sea bed in the Irish Rockall Trough. It revealed a range of sedimentary features on the trough floor and its steep (&gt;6°) margins. The western margin is characterized by large-scale (of the order of hundreds of kilometres in length) downslope mass movement. Smaller-scale slides and slumps (tens of kilometres across) occur on the eastern margin, but they are subordinate to canyon, channel and fan systems. The western and central parts of the trough floor contain the Feni Sediment Ridge, a 600 km long contourite sediment build-up covered by large sediment waves trending sub-parallel to the dominant modern current pattern. Strong, northward-flowing bottom currents are thought to have eroded the base of the slope in the east and redeposited the sediments on the western margin and the trough floor. Mass wasting and terrigenous sediment input through canyons is regarded as the primary source of sediment in the region. The increase in the degree and frequency of canyon incision along the NE margin of the trough reflects increased terrigenous input from the Irish mainland and a possible glacial influence on the basin margin. The GLORIA images reflect a broad interplay of alongslope and downslope sediment transport processes in the Rockall Trough with sediments sourced from the NE margin and redistributed by currents along the western margin. Although alongslope and downslope processes are the major controlling factors, basin subsidence, Quaternary glaciations and glacio-eustatic sea-level fluctuations have also influenced the pattern of sedimentation in the Rockall Trough.

Comparison of flat-topped stellate seamounts on Earth's seafloor with stellate domes on Venus using side-scan sonar and Magellan synthetic aperture radar

The diameters of many domes on Venus imaged by the Magellan synthetic aperture radar (SAR) are an order of magnitude larger than terrestrial subaerial domes; however, surveys of the seafloor using sonar imaging systems such as GLORIA (Geological Long Range Inclined Asdic) and SeaBEAM have revealed flat-topped volcanic features with similar diameters, volumes and slope angles. Suggested analogies between domes on Venus and seamounts on the northern East Pacific Rise and off Hawaii have been tested by D.K. Smith [J. Volcanol. Geotherm. Res. 73 (1996) 47–64] using comparisons of size and shape and significant differences identified. GLORIA images of flat-topped seamounts near Hawaii show them to have stellate planforms. Over half of the total population of 375 domes on Venus have been modified and many have stellate planforms. We have focused on the similarities in stellate planforms between flat-topped seamounts and modified domes on Venus. We have used GLORIA sonar and Magellan radar data to constrain the formation and evolution of both flat-topped seamounts on the ocean floor and volcanic domes on Venus to see if the processes that formed them are similar. We propose that flat-topped seamounts on the Hawaiian Ridge are built on sediments overlying the oceanic crust and that available evidence supports three processes that can form stellate margins: (1) radial dike intrusions related to flank rift zones; (2) dike intrusions which cause slope failures; and (3) gravitational slope failures unrelated to intrusions. For Venus, where low erosion rates and the absence of a sediment layer results in lavas being emplaced directly onto the crust, the available evidence supports stellate margins originating by gravitational slope failures unrelated to diking, though intrusions cannot be ruled out with existing data.

Mapping oil seeps on the sea floor by Gloria side-scan sonar images—A case study from the northern Gulf of Mexico

An oil seep site in the northern Gulf of Mexico is characterized by high backscattering levels on the GLORIA (Geological Long-Range Inclined Asdic) side-scan sonar images against a background of low backscattering. The high backscattering from the oil-seep area are most likely caused by a combination of small-scale roughness and porosity reduction due to the precipitation of CaCO3 formed during biodegradation of the oil-seep. Geostatistical methods have been applied to analyze side-scan images from both oil seep and nonseep areas. The results show that GLORIA images from oil seep areas can be distinguished from nonseep areas in terms of local histograms, variograms, and textural patterns. Pixels from seep and nonseep areas cluster into distinct groups on a textural feature plot. GLORIA side-scan images could be used as a reconnaissance tool to delineate oil and gas seep sites on the seafloor and thereby, reduce the dry-hole risk of petroleum exploration in deep-water frontier areas.

The tectonic evolution of the Indian Ocean Triple Junction, anomaly 6 to present

The tectonic evolution of the Indian Ocean ridge‐ridge‐ridge triple junction, the junction of the Central Indian Ridge (CIR), Southeast Indian Ridge (SEIR) and Southwest Indian Ridge (SWIR), is inferred by mapping structures along its traces on the three plates with a long‐range side scan sonar, Geological Long‐Range Inclined Asdic (GLORIA). The sonar images to the west of the triple junction show two different styles of tectonic evolution. Some sections of the trace on the African plate are marked by a SWIR‐facing scarp containing fine lineaments that are probably normal faults. These correspond with sections of the trace on the Antarctic plate of the same age that have a sharp intersection of SWIR and SEIR abyssal hills that is to be expected from a simple ridge‐ridge‐ridge triple junction evolution (a “herringbone” pattern of isochrons). In contrast there are other sections of the trace on the African plate that have a blocky structure with no definite termination of CIR abyssal hills against those of the SWIR. The corresponding regions of the Antarctic plate show large 10‐ to 30‐km‐long overlapping fault scarps, tapering to the northeast and ending in asymmetric valleys to the southwest. These major normal faults are each offset by 1–6 km, forming an en echelon series. The two different styles of tectonic structures indicate two different modes of evolution, although why the triple junction has evolved with two different modes is unclear. Periods of asymmetric spreading in the CIR magnetic anomaly sequence collected on the African plate may possibly indicate that the CIR and SEIR have been offset by a fracture zone at the triple junction during one of these two modes. Echo sounder traverses on the Antarctic and African plates commonly show a steady shallowing of the CIR and SEIR seafloor toward the traces, a steep scarp facing the SWIR and a broad marginal deep at the base of this scarp. This rift flank shape resembles that of the SWIR valley's rift shoulders close to the present triple junction (Mitchell, 1991a), suggesting that the mechanical evolution has been similar to the present day and the relief due to the rift flank uplift has been preserved by the cooling and strengthening of the lithosphere with age away from the ridge axes. If the shape of the rift shoulder uplift along the trace on the Antarctic plate is interpreted with a simple flexural model involving an end‐loaded elastic plate, the inferred rigidity (1019‐1021 N m or 2–6 km effective elastic thickness) is consistent with the suggestion that the isostatic uplift occurred in young, weak lithosphere, close to the triple junction. Interestingly, the observed ∼1‐km relief of the trace escarpment and the estimated rigidity suggest that the uplift was produced by a load (2–5 × 1011 N/m) that is equivalent to half the present cross‐sectional area of the SWIR valley 35 km from the triple junction.

GLORIA survey of the oceanic-continental transition of the Havre-Taupo back-arc basin

A GLORIA (Geological Long-Range Inclined Asdic) side-scan sonar survey, covering 23,000 km2, provides the first complete imagery of an active and contiguous, oceanic to continental back-arc system, namely, the Havre Trough to Taupo Volcanic Zone (TVZ) New Zealand. Havre Trough tectonism and volcanism relates to a series of laterally discontinuous, mutiple spreading rifts which terminate southward at the 3-km-deep Ngatoro Basin. A 45-km sinistral offset attributed toen echelon synthetic shearing separates the basin from the actively spreading TVZ. Sonographs reveal a youthful and complex volcanic seascape with 20 newly discovered seamounts, whereas flanking regions are mantled with a largely featureless mud blanket.

GLORIA: the second decade

Report of a meeting jointly sponsored by the Marine Studies Group of the Geological Society and the Challenger Society for Marine Science held at Burlington House on 1 February 1989. T. J. G. Francis, the convenor, opened the meeting by briefly summarizing the development of the GLORIA (Geological Long Range Inclined Asdic) long range sidescan sonar at the Institute of Oceanographic Sciences since the mid-nineteen sixties. GLORIA has established itself in the nineteen eighties as the ideal tool for surveying Exclusive Economic Zones (EEZs). GLORIA surveys were carried out for 241 days in 1988 from two different ships, 140 of which were spent surveying the United States EEZ with the U.S. Geological Survey. By the end of 1988, some 13 million km 2 of ocean floor has been surveyed, which is about 4% of the ocean deeper than 200m. The scale of USGS activity with GLORIA was equivalent to all UK deep Ocean geology and geophysics. R. C. Searle (IOSDL, Wormley) spoke on the contribution of GLORIA to understanding mid-oceanic ridge tectonics. After reviewing a wide range of observations from the Atlantic, Pacific and Indian Oceans, he reported the results of the recent survey of the whole of the Easter Microplate camed out from RRS Charles Darwin last November. This revealed for the first time the complex plate boundaries, spreading fabric and associated propagating rifts and transform faults of the whole of this unusual feature. D. Twichell (USGS, Woods Hole) spoke about the integration of digital bathymetry (SEABEAM) and sidescan (GLORIA) data with particular reference to the Gulf of Mexico. Gridding of the two data sets was done on the USGS Mini Image Processing System (MIPS). Since the sidescan backscatter

Small‐scale mapping of the exclusive economic zone using wide‐swath side‐scan sonar

Abstract The U.S. Geological Survey (USGS) started a program in April 1984 to map the deep‐water (greater than 200 m) areas of the U.S. Exclusive Economic Zone (EEZ) at a reconnaissance scale as a first effort to develop a geologic understanding of the new national offshore territory. This effort was in response to President Reagan's EEZ proclamation in March 1983, which extended U.S. jurisdiction over an area extending 200 nautical miles offshore of the United States and U.S. trust territories. This USGS mapping effort is a cooperative effort with the Institute of Oceanographic Sciences (IOS) of the United Kingdom and uses a unique side‐scan sonar system named GLORIA (Geological Long‐Range Inclined Asdic) developed by IOS. To date, over 3,500,000 km2 have been mapped off the west coast, the east coast, the Island of Hawaii, Puerto Rico, and the U.S. Virgin Islands and in the Gulf of Mexico and the Bering Sea. All of these surveys have been highlighted by discoveries of major geologic features that contri...

GLORIA II Sonograph Mosaic of the Western U.S. Exclusive Economic Zone

In 1983 the United States declared sovereign rights and jurisdiction over living and nonliving resources in an area extending 200 nautical miles (370 km) seaward from its shores. In response to the establishment of this Exclusive Economic Zone (EEZ), the U.S. Geological Survey (USGS) has implemented a program, called EEZ‐Scan, to systematically map the EEZ, using the Geological Long‐ Range Inclined ASDIC (GLORIA) II longrange side scan sonar system developed by the Institute of Oceanographic Sciences (IOS) of Great Britain [Somers et al, 1978]. The first part of the EEZ‐Scan field program was completed in the summer of 1984, when USGS and IOS scientists surveyed the EEZ off the western conterminous United States aboard the British research vessel Farnella (Figure 1). The west coast survey, requiring 96 days of ship time and four separate legs, has resulted in virtually total sonograph coverage of the sea floor from the continental shelf break to the 200‐nautical mile limit between the Mexican and Canadian borders, an area of about 850,000 km2 . Other data collected on the cruises included two‐channel digital seismic reflection and 3.5‐kHz highresolution and 10‐kHz bathymetric profiles, as well as towed magnetometer data along approximately 20,000 km of trackline spaced nominally at 30‐km intervals.

Geomorphic Features off Southern California as Seen by GLORIA Side-Scan Sonar System: ABSTRACT

Approximately 165,000 km2 of the sea floor off southern California was mapped during May 1984, as part of a USGS/IOS cooperative program to study the newly proclaimed Exclusive Economic Zone (EEZ) of the United States Pacific margin. The area was insonified using the Geological Long-Range Inclined Asdic (GLORIA), a long-range side-scan sonar system. Images were corrected for water-column velocity anomalies, for along-track distortions caused by variations in ship speed, and for slant-range distortions caused by acoustic ray travel paths. A photo-mosaic of the overlapping sonographs has been compiled at a scale of 1:375,000. The basins of the inner California continental borderland are characterized by both sinuous channel and fan complexes and by feathery acoustic patterns indicating active sediment transport. In contrast, outer borderland basins appear to be more sediment starved, exhibit large areas of sediment failure, and show significant structural influence. West of Patton Escarpment, the sonographs are dominated by acoustic patterns showing volcanic ridges and seamounts and by deposits of the Monterey and Arguello Fans. Arguello Fan, for example, exhibits multiple sinuous channels that have transported sediment 60 km south from the canyon mouth. These channels coalesce into a single 100-km long, westward-meandering channel that terminates in a 600-m deep box canyon. A zone of sediment failure is identifiable on the north levee of an upper fan channel. Tectonic trends associated with oceanic basement are highlighted by the terminus of the west-trending Murray Fracture Zone and by the prevailing northeast trend of volcanic ridge and seamount chains. End_of_Article - Last_Page 251------------