Magnetic Anomalies Over Oceanic Ridges

A recent survey of intense east-west magnetic anomalies between the Galapagos Islands and South America revealed a relatively small area with a pattern of symmetry about an axis that resembles the pattern of the great oceanic rises. A sea-floor spreading rate of 3.2 cm/yr was indicated. The relationship of this pattern to the East Pacific rise suggests a different type of mantle convection and crustal spreading.

Sea-floor spreading—Another rift

New bathymetric, gravity, and magnetic data in the Ross Sea are presented. The bathymetric data show that the entire Ross Sea continental shelf has a mean depth of about 500 meters with the eastern half systematically deeper than the western half. Subdued ridges and valleys characterize the relief of the shelf and trend roughly northsouth east of the 180° meridian and northeast-southwest west of it. These features appear to be primarily the result of erosion by past movement of a partially grounded Ross Ice Shelf. The continental slope is tectonically complex especially to the west of Iselin Bank. The most dramatic geophysical feature of the Ross Sea shelf is the linear positive gravity anomaly belt trending north-south across the western Ross Sea. This positive gravity belt is about 700 km long, 50 to 100 km wide, and has an amplitude of up to 80 mgal. We interpret it as marking the position of an ancient rift, probably active during the separation of Australia and New Zealand from Australia. The major sedimentary basin underlying the eastern Ross Sea is not marked by any significant local or regional gravity anomalies and probably contains highly compacted sediments. Large magnetic anomalies over the Ross Sea are few and these are local in extent.

/pdf/a-geophysical-study-of-the-ross-sea-antarctica-4w88wgcoi6.pdf

A Geophysical Study of the Ross Sea, Antarctica

Seismic profiler data from the continental shelf of the Ross Sea show numerous areas where gently dipping sediments with a total thickness of about 2 km are truncated at the sea floor or a few hundred meters below the sea floor. A series of basement (5.5 km/sec) ‘highs’, which outcrop at the sea floor in the south and lie approximately along the 180th meridian, separates the shelf into two regions. Iselin bank lies on the northern extension of this line. The profiler and sonobuoy data show that the eastern shelf is formed by a sedimentary basin containing up to 4 km of sediments. The western shelf is more complex. Profiler data indicate gentle synclines and anticlines that plunge northward and overlie, in the south, older structures that trend ENE. Sonobuoy data from the shelf show a sharp increase in the sediment thickness in an elongate region parallel to 177°E. The increase in sediment thickness, caused possibly by the presence of N-S faulting, is not apparent in the profiler data. Over the whole shelf sonobuoy results show a linear increase in sediment velocity with depth. The geological trends seen on the shelf are not consistent with the northwesterly regional strikes mapped in Victoria Land or in western Marie Byrd Land. The continental rise in the east is marked by a sharp seaward increase in the basement depth, from about 4.5 to 6.5 km, at the base of the rise. In contrast, the western sector of the continental rise is more complex with basement ridges, whereas the depth to basement does not drop below about 4.5 km.

Seismic profiler and sonobuoy measurements in Ross Sea, Antarctica

Seismic profiler data recorded by USNS Eltanin in the vicinity of New Zealand are used to delineate the major features of the sediment distribution in the region, particularly on the marginal plateaus. Sonobuoy data have given basement depths in some areas where the sediment cover is too thick to be profiled and also information on sediment velocities, enabling estimates to be made of sediment thicknesses. A strong reflector, probably Late Cretaceous-Early Tertiary in age, occurs over a large portion of the survey area. Flat-lying sediments of Tertiary age, thicker than 2 km, occur in the morphological depressions of New Caledonia Basin, southern Hikurangi Trench, and Solander Trough. Bounty Trough however has a relatively thin sediment cover. Deep sedimentary troughs, also containing at least 2 km of sediments, exist at the south-west of Campbell Plateau, along the southern margin of Pukaki Rise, and along the eastern margin of South Island from south of Banks Peninsula to Snares Depression.

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1977.10430730

Marine seismic measurements in the New Zealand region

During the summers of 1958–1959 and 1959–1960, total magnetic field surveys were made with ship-towed nuclear spin magnetometers between New Zealand and the Ross Sea. The total field contours are given for the region between New Zealand and 70°S and for the Ross Sea area. Magnetic anomalies in the different bathymetric regions are discussed, and some possible interpretations given. In general, smooth magnetic profiles are found in the shallow water to the north of the deep ocean basin, and in the basin itself a typical pattern of deep-water magnetic anomalies is found. A ridge, over which both bathymetric and magnetic profiles are rough, is crossed before entering the Ross Sea. A smooth magnetic profile in the Ross Sea shows the absence of any disturbing igneous rock close to the surface.

Total magnetic field surveys between New Zealand and the Ross Sea

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Total magnetic field surveys between New Zealand and the Ross Sea