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Rifting and Ocean Basins

The physiography of the Atlantic.

        Zones of rising hot material deep within the mantle are known as asthenospheric mantle 'plumes' or 'hot spots'.  Attempts to map the thermal anomalies associated with the plumes have been made on the assumption that mantle temperatures inversely correlate with mantle seismic velocities, the science being known as 'Global Tomography'. As in the case of mid-ocean ridges, the rising plumes of mantle material undergo decompression melting, the magma so-produced errupting at  the Earth's surface as a shield volcanoe or a volcanic plateau.  In oceanic areas the existence of a plume may be remarked  by the trace 'burnt' into the migrating oceanic crust, as in the case of the Hawaiian islands.

          Where continents have aggregated into 'super-continents', they form an insulating surface layer, the existence of which favours the formation of thermal anomalies in the underlying mantle asthenosphere. Should the anomalies become gravitationaly unstable, their rise towards the surface of the earth causes colder, higher density lithosphere to be replaced by hotter less dense  asthenosphere, and material near the surface of the Earth  will become 'thermally elevated' relative to surrounding areas. At  the same time, the lateral flow of the plume material may cause the lithosphere to undergo extension causing the  formation of rift zones at the surface of the Earth, e.g. the Afar triangle. Classic  present day rift systems in various stages of development  include the Tibesti region of North Africa (early volcanic dome  stage), Rhine Valley of Germany (early dome and rift stage),  Kenya rift (rift stage), and the Red Sea (oceanic stage).

        Once rifting has been initiated, subsidence (enlargement) of the newly formed continental margins takes  place in two phases: 1) an initial subsidence related to extension (25m.y.), and 2) a more prolonged (200 m.y.)  thermal subsidence as the thermal anomaly generated by the  plume is dissipated by movement of the margin away from  the plume. The early extensional phase of rifting may be  marked by the deposition of red-beds and evaporites, and the extrusion of relatively alkaline and Ti-rich volcanic rocks.

         Developing oceans are classified as 'passive' (Atlantic), 'active' (Pacific),  and 'marginal'  (Sea of Japan; Phillipines).   'Passive' margins are characterized by the presence of shallow  water deposits (cross-bedded sandstones; massive  carbonates) on the continental platform, and thick turbidite  accumulations (graded sandstones; proximal and distal  Bouma cycle) of the continental slope and rise, whereas 'active' margins are dominated by material of volcanogenic origin.

        'Passive' margin sediments are all derived from a continental source and are  characterized commonly by the presence of 'floating mica'.  The more abyssal regions of mature oceans accumulate wind blown 'brown clay' (northern and western Pacific),  and 'calcareous and siliceous oozes' (cherts), whereas 'glacio-marine' sediment dominates the peri-polar regions of the Earth.

        Sedimentation at 'active' margins is largely influenced by the  presence of volcanic arcs, and largely accumulates in sediment traps between the arc and the subduction slip zone (frontal arc basins), and in 'marginal'  oceans behind the arc. In 'passive', 'active' and 'marginal' oceans, oceanic crust is  formed in extensional environments delineated by oceanic ridges.fs

         Economic deposits in ocean basins include  hydrocarbon accumulations in continental margin rift basins,  e.g. Hibernia), nickel associated with deep-sea hydrogenous  nodules formed in the abyssal regions of oceans, and base-metal sulphides associated with 'black  smokers' located along oceanic ridges.

Overhead sequence:
        Seismic structure of the Kenya Rift (07rift.gif).
        Global tomography map.
        The oceanic Hawaiian hot spot chain.
        Continental rifting of the Afar triangle (07afar.gif).
        Structure of the Atlantic Passive continental margins  (Sawyer et al.,1982, Geology, 10,  p.134).
        Passive, active and marginal ocean basins  (06wldmap.gif).
        Physiographic section of the Atlantic margins -  continental-shelf; slope and rise; abyssal plain; mid-oceanic  ridge.
        Turbidity currents - graded-bedding; Bouma cycle.
        Seismic character of the South American Atlantic  margin.
        Distribution of deep sea sediments - Brown clay;  oozes (cherts); glacial marine; hydrogenous manganese  nodules. (07dpsea.gif).

    Essay Questions:

   1)  Describe the steps involved in the break up of a 'Supercontinent'?
    2) Describe the physiographic components of a rifted continent - ocean plate.



Structural Provinces of North America.

The physiography of the Atlantic.

Active and Passive oceans.

Active and Passive margins.

Rifting and mantle plumes.

The Afar Triangle, Red Sea region.

Sediments of Deep Ocean Basins.


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