DEFINITION OF OCEANIC PLATEAUX ACCRETION

Oceanic plateaux are thicker than normal oceanic crust

formed at mid-ocean ridges: they are more buoyant and relatively

unsubductable, forming potential sources of accreted

oceanic material to the continental crust at convergent plate

boundaries. Accretion of oceanic plateaux has been proposed

as a mechanism of crustal growth in a number of orogenic

belts, including Archean, Proterozoic, and Phanerozoic examples.

Oceanic plateaux are interpreted to form from plumes

or plume heads that come from the lower mantle (D’’) or the

415-mile (670-km) discontinuity, and they may occur either

within the interior of plates, or interact with the upper mantle

convective/magmatic system and occur along mid-ocean

ridges. Oceanic plateaux may be sites of komatiite formation

preserved in Phanerozoic through Archean mountain belts,

based on a correlation of allochthonous komatiites and high-

MgO lavas of Gorgona Island, Curaçao, and in the Romeral

fault zone, with the Cretaceous Caribbean oceanic plateau.

Portions of several komatiite-bearing Archean greenstone

belts have been interpreted as pieces of dismembered

Archean oceanic plateaux. For instance, parts of several

greenstone belts in the southern Zimbabwe craton are

allochthonous and show a similar magmatic sequence including

a lower komatiitic unit overlain by several kilometers of

tholeiitic pillow basalts. These may represent a circa 2.7 Ga

oceanic plateau dismembered during a collision between the

passive margin sequence developed on the southern margin of

the Zimbabwe craton and an exotic crustal fragment preserved

south of the suture-like Umtali line.

The accretion of oceanic plateaux and normal oceanic

crust in arc environments may cause a back-stepping of the

subduction zone. As the accretionary complex grows, it is

overprinted by calc-alkaline magmatism as the arc migrates

through the former subduction complex. Further magmatic

and structural events can be caused by late ridge subduction

and strike-slip segmentation of the arc. Average geochemical

compositions of the continental crust, however, are not consistent

with ocean plateau accretion alone.

Parts of many Archean, Proterozoic, and Phanerozoic

greenstone belts interpreted as oceanic plateau fragments are

overprinted by arc magmatism, suggesting that they either

formed the basement of intra-oceanic island arcs, or they

have been intruded by arc magmas following their accretion.

Perhaps the upper and lower continental crusts have grown

through the accretion of oceanic island arcs and ocean

plateaus, respectively. Accreted oceanic plateaux may form a

significant component of the continental crust, although most

are structurally disrupted and overprinted by arc magmatism.

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