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Guy R. Cochrane Brian T. R. Lewis Kevin J. McClain 《Pure and Applied Geophysics》1988,128(3-4):767-800
Seismic reflection and refraction data off Washington and Oregon are used to determine the style of sediment deformation and to infer the physical properties of accreted sediments on the lower slope. Onshore-offshore seismic refraction data off Washington are used to determine the location of the trench, or where the plate bending starts.We find that off Washington the subduction zone is characterized by a trench whose physiographic expression is buried under several kilometers of sediments and is tens of kilometers landward of the lower slope, which is accreting seaward as the result of the offscraping of sediments.Seismic reflection data support previous observations that offscraping occurs along seaward and landward dipping thrust faults. Refraction data indicate that a sediment package thrust up along a seaward dipping fault (off Washington) was not measurably changed in velocity with respect to a Cascadia basin section. However a package uplifted by thrusting along a landward dipping fault (off Oregon) did have increased velocity. It is suggested that the increased velocities off Oregon could be the result of erosion and exposure of more deeply buried and compacted sediments, rather than the result of dewatering due to tectonic stress. Off Washington the sensitivity of velocity to porosity and resolution of the seismic method does not preclude dewatering due to tectonic stress, but it does limit the degree of dewatering.In the deeper parts of the lower slope section off Washington and Oregon velocities as high as 3 to 4 km/sec are found. Heat flow data indicate that the temperatures in this high velocity regime are greater than 100°C. It is hypothesized that lithification related to clay diagenesis may be partly responsible for the high velocities, rather than simply compaction. It also appears that the high velocity sediments are subducted while the unlithified low velocity sediments are offscraped. 相似文献
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The Gnargoo structure is located on the Gascoyne Platform, Southern Carnarvon Basin, Western Australia, and is buried beneath about 500 m of Cretaceous and younger strata. The structure is interpreted as being of possible impact origin from major geophysical and morphometric signatures, characteristic of impact deformation, and its remarkable similarities with the proven Woodleigh impact structure, about 275 km to the south on the Gascoyne Platform. These similarities include: a circular Bouguer anomaly (slightly less well-defined at Gnargoo than at Woodleigh); a central structurally uplifted area comprising a buried dome with a central uplifted plug; and the lack of a significant magnetic anomaly. Gnargoo shows a weakly defined inner 10 km-diameter circular Bouguer anomaly surrounded by a broadly circular zone, ～75 km in diameter. The north?–?south Bouguer anomaly lineament of the Giralia Range (a regional topographic and structural feature) terminates abruptly against the outer circular zone which is, in turn, intersected on the eastern flank by the Wandagee Fault. A <?28 km-diameter layered sedimentary dome of Ordovician to Lower Permian strata, surrounding a cone-shaped, central uplift plug of 7?–?10 km diameter, are inferred from the seismic data. Seismic-reflection data indicate a minimum central structural uplift of 1.5 km, as compared to a model uplift of 7.3 km calculated from the outer structural diameter. An interpretation of Gnargoo in terms of a plutonic or volcanic caldera/ring origin is unlikely as these features display less regular geometry, are typically smaller and no volcanic rocks are known in the onshore Gascoyne Platform. An interpretation of Gnargoo as a salt dome is likewise unlikely because salt structures tend to have irregular geometry, and no extensive evaporite units are known in the Southern Carnarvon Basin. Morphometric estimates of the rim-to-rim diameter based on seismic data for the central dome correspond to the observed diameter deduced from gravity data, and fall within the range of morphometric parameters of known impact structures. The age of Gnargoo is constrained between the deformed Lower Permian target rocks and unconformably overlying undeformed Lower Cretaceous strata. Because of its large dimensions, if Gnargoo is an impact structure, it may have influenced an environmental catastrophe during this period. 相似文献
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A. D. Albani P. C. Rickwood P. G. Quilty J. W. Tayton 《Australian Journal of Earth Sciences》2013,60(6):681-694
Shallow seismic-reflection profiling and proton-precession magnetometry have been conducted over the continental shelf off Sydney between Broken Bay (in the north) and Bate Bay (in the south) as well as over part of the upper continental slope in the area east of the coastline between Narrabeen and Port Hacking. In this area, four major paleodrainage channels incise the bedrock, within the coastal estuaries and on the inner and mid-shelf:
the Hawkesbury paleo-watercourse, which is joined offshore by four lesser paleochannels, that we call the Newport, Mona Vale, Narrabeen and Long Reef paleo-watercourses;
the Parramatta paleo-watercourse, which is joined by the Bondi paleo-watercourse;
the Botany paleo-watercourse and
the Georges paleo-watercourse, which is joined under the Kurnell Peninsula land isthmus by the Cooks paleo-watercourse, and then by the Hacking paleo-watercourses in Bate Bay.
the ancestral Hawkesbury River;
the minor Peak paleo-watercourse east of Coogee, which is discernible at depths shallower than the ?100 m isobath and is maintained to depths greater than ?210 m; and
the minor Island paleo-watercourse whose head terminates at ～?140 m east of Bronte, lies entirely below the LGM paleo-shoreline, and can be traced to a depth of ?260 m.
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