4.18 – Oceanic Plateaus

TL;DR: The early magmatism of the Kerguelen Plateau is associated with the Early Cretaceous breakup of Gondwana and displays evidence of contamination with continental crust as mentioned in this paper.

Abstract: Oceanic plateaus are vast areas (> 0.1 × 106 km2) of overthickened oceanic crust (up to, and sometimes > 30 km) that are widely interpreted to have formed by decompression melting of hot mantle plumes. Oceanic plateaus have formed throughout most of Earth's history and, due to their excess crustal thicknesses, are difficult to subduct; typically, their uppermost sections are accreted to continental margins. In addition to providing a means of preserving sections of these plateaus in the geological record, the accretion of oceanic plateaus has been an important contributor to crustal growth throughout Earth's history. Although some Cretaceous oceanic plateaus have been partially accreted, most are still ‘in situ’ in the ocean basins, and studies of both the accreted portion of these plateaus and oceanic drilling have yielded considerable information about the composition, origin, and structure of oceanic plateaus. The Cretaceous Ontong Java Plateau is characterized by very homogeneous basaltic rocks that may reflect either a homogeneous mantle source region or, perhaps more likely, homogenization of heterogeneous melts during extensive (up to 30%) melting. By contrast, the Cretaceous Caribbean plateau was derived from a markedly heterogeneous mantle source region, and high-MgO lavas are relatively common. The early magmatism of the Kerguelen Plateau is associated with the Early Cretaceous breakup of Gondwana and displays evidence of contamination with continental crust. Unlike the other Cretaceous plateaus, the Kerguelen Plateau was constructed in different phases over a considerable period (>50 Ma), and crustal contamination declines with time as the Indian Ocean widens and the plateau becomes more oceanic in character. The Triassic Wrangellia Plateau formed in the Panthalassic Ocean and accreted to the western margin of North America shortly thereafter. The well-exposed basalts and picrites are markedly heterogeneous. Some of this heterogeneity is derived from the mantle plume source region, but some of the variable compositions are better explained by contamination with preexisting arc crust, which the lavas passed through. Throughout a considerable portion of Earth's history, oceanic plateau formation correlates with periods of environmental catastrophe characterized by oceanic anoxia, leading to black shale formation and mass extinction events. Such correlations are particularly evident in the Cretaceous, and can be partly attributed to the release of CO2 during oceanic plateau formation, resulting in a runaway greenhouse effect. Additionally, the elevation of sea level, as well as disruption of oceanic circulation patterns by displacement of seawater during plateau formation, contributed to increased environmental stress and biotic extinction at these times. Radiogenic isotope signatures of oceanic plateau volcanism can be found in the black shales that formed during these anoxic events, and this strengthens the causal link between oceanic anoxia and oceanic plateau formation.