ORIGIN OF OCEANIC CRUST

Because pillow lava is the rock type that invariably appears from beneath the abyssal sediments that flank the exposed parts of Mid-Ocean ridges, it is generally considered that ocean crust beneath the uppermost veneer of relatively unconsolidated sediment of oceanic abyssal plains is formed of pillow
lava.

The deeper parts of oceanic crust cannot however be so directly examined, and ideas concerning its nature were originally based on a simple comparison of the P-wave velocity structure of oceanic crust with the measured P-wave characteristics of a range of ultramafic, mafic and felsic rock types. One possible example of an up-faulted but otherwise in-situ section of oceanic crust however does occur on Macquarrie Island, located on the Alpine fault between New Zealand and Antarctica. The section here is formed of pillow basalt, diabase, gabbro, troctolite, and ultramafic rocks.
     Similar sequences (known as ophiolites) are also found throughout the world, either perched on the edges of continental crust as in the case of Oman, Papua-New Guinea, New Caledonia, Oregon, Cyprus, or Cuba, or located within orogenic belts such as the Canadian Cordillera, the Appalachians of Western Newfoundland and Quebec, the Caledonides of Norway, the Alpine 'eugeosyncline', the southern Andes, and the Urals.
    Although at one time ophiolites were thought to represent giant basaltic effusions poured out onto an 'eugeosynclinal' sea-floor, Reinhard's study (1969) of the Semail ophiolite of Oman, and later studies of the Bay of Islands in Western Newfoundland and elswhere revealed that they are more likely to have formed in magma chambers located beneath mid-ocean ridges, or in association with rifts developed within island arcs. They are invariably composed of a lower unit of mantle ultramafic tectonite (lherzolite/harzburgite/dunite) and an upper unit of pillow lava overlain by deep-sea sediment. In between, from top to bottom, the oceanic crust is composed of an enigmatic unit of diabase in the form of vertical dikes, commonlyexhibiting one-sided chilled margins; massive gabbro; and layered gabbro and ultramafic cumulates (igneous sediments) deposited on the mantle peridotite floor of the magma chamber.
    The magma occupying the magma chamber beneath oceanic ridges is wedge shaped in form and tends to cool vertically at the top of the magma chamber and laterally in the lower reaches of the chamber. Consequently, as the magma crystallizes, the roof of the magma chamber grows downwards, whereas the floor grows upwards as igneous sediments accumulate and are transported laterally in piggy-back fashion on the laterally spreading mantle peridotite. In the lateral reaches of the chamber the roof gabbro eventually comes into contact with the cumulate unit, and both floor and roof units are carried away from the ridge on the laterally migrating underlying mantle conveyor belt. The `sheeted diabase' unit of ophiolites is uniquely characteristic of oceanic rocks formed at a spreading centre. The mechanism of formation of the unit is related to the existence of a tensional stress system at spreading centres and the steady-state nature of the spreading.

Overhead sequence:
Crustal section of the ocean floor off the east coast of Argentina
The seismic structure of the Atlantic (09ocesct.gif)
Compressional wave velocities for granite and gabbro
Distribution of Ophiolites: location of MacQuarrie Island; (09ophloc.gif)
Geology Map of the north part of Macquarrie Island
Layered cumulates and sheeted diabase, Macquarrie Island
Global distribution of ultramafic-gabbro-diabase-pillow lava complexes (ophiolites)
Ophiolites in geosynclinal theory
The 1959 Alpine "laccolith" model of ophiolite
Location of Oman (09ophloc.gif)
Reinhardt - the Oman ophiolite and the discovery of sheeted_diabase. (09ophiol.gif)
The Bay of Islands complex - mantle peridotite; lherzolite; harzburgite; cumulate peridotite cumulate gabbro; roof gabbro; sheeted diabase. (09wnewf.gif)
Magma chamber model of ocean crust formation. (09ocform.gif)

FIGURES

The P-wave velocity structure of oceanic crust.07ocesct.gif

Measured P-wave of granite and gabbro.

Global distribution of ophiolite complexes and the location of MacQuarrie Island.09ophloc.gif

Macquarrie Island.09macq.jpg

Macquarrie Island diabase.09macdiab.jpg

Macquarrie Island troctolite.09mactroct.jpg

Map showing location of the Semail ophiolite of Oman perched on the northern margin of the Arabian Shield

Geological map of Newfoundland - the Bay of Islands ophiolite of western Newfoundland.09nfdlnd.gif

Location of ophiolite in the Alpine orogenic system according to the eugeosyncline model.09ophiol.gif

The giant basaltic effusion model for ophiolites.09ophiol.gif

The Semail ophiolite of Oman.

The Bay of Islands ophiolite complex of Western Newfoundland.

Formation of oceanic crust at an oceanic spreading centre.

Return to beginning.

Click here to return to course outline.

http://www.uwo.ca/earth/ugrad/courses.html