Continental growth and craton stabilization

Many of the geologic features that comprise Protero-zoic belts (Section 11.4.1) indicate that the continental lithosphere achieved widespread tectonic stability during this Eon. Tectonic stability refers to the general resistance of the cratons to large-scale lithospheric recycling processes. The results of seismic and petrologic studies (Sections 11.3.1, 11.3.3) and numerical modeling (Lenardic et al., 2000; King, 2005) all suggest that compositional buoyancy and a highly viscous cratonic mantle explain why the cratons have been preserved for billions of years. These properties, and isolation from the deeper convecting mantle, have allowed the mantle lithosphere to maintain its mechanical integrity and to resist large-scale subduction, delamination and/or erosion from below. Phanerozoic tectonic processes have resulted in some recycling of continental lithosphere (e.g. Sections 10.2.4, 10.4.5, 10.6.2), however the scale of this process relative to the size of the cratons is small.

The cores of the first continents appear to have reached a sufficient size and thickness to resist being returned back into the mantle by subduction or delam-ination some 3 billion years ago. Collerson & Kamber (1999) used measurements of Nb/Th and Nb/U ratios to infer the net production rate of continental crust since 3.8 Ga. This method exploits differences in the behavior of these elements during the partial melting and chemical depletion of the mantle. The different ratios potentially provide information on the extent of the chemical depletion and the amount of continental crust that was present on Earth at different times. This work and the results of isotopic age determinations (Fig. 11.13) suggest that crust production was episodic with rapid net growth at 2.7, 1.9, and 1.2 Ga and slower growth afterward (Condie, 2000; Rino et al., 2004). Each of these pulses may have been short, lasting <100 Ma

Fig. 11.13 Plot showing the distribution of U-Pb zircon ages in continental crust (after Condie, 1998, with permission from Elsevier).

Fig. 11.13 Plot showing the distribution of U-Pb zircon ages in continental crust (after Condie, 1998, with permission from Elsevier).

(Condie, 2000). On the basis of available data, Condie (2005b) concluded that 39% of the continental crust formed in the Archean, 31% in the Early Proterozoic, 12% in the Middle-Late Proterozoic, and 18% in the Phanerozoic.

Two of the most important mechanisms of Late Archean and Early Proterozoic continental growth and cratonic root evolution were magma addition (Section 9.8) and terrane accretion (Section 10.6). Several authors (e.g. Condie, 1998; Wyman & Kerrich, 2002) have suggested that the ascent of buoyant mafic material in mantle plumes may have initiated crust formation and may have either initiated or modified root formation during periods of rapid net growth (Section 11.3.3). Schmitz et al. (2004) linked the formation and stabilization of the Archean Kaapvaal craton in South Africa to subduction, arc magmatism, and terrane accretion at 2.9 Ga. In this and most of the other cratons, isotopic ages from mantle xenoliths and various crustal assemblages indicate that chemical depletion in the mantle lithosphere was coupled to accretionary processes in the overlying crust (Pearson et al., 2002). This broad correspondence is strong evidence that the crust and the underlying lithospheric mantle formed more or less contemporaneously and have remained mechanically coupled since at least the Late Archean. A progressive decrease in the degree of depletion in the lithospheric mantle since the Archean (Fig. 11.14) indicates that the Archean-Proterozoic boundary represents a major shift in the nature of lithosphere-forming processes, with more gradual changes occurring during the Phanero-zoic (O'Reilly et al., 2001). The most obvious driving mechanism of this change is the secular cooling of the Earth (Section 11.2). In addition, processes related to subduction, collision, terrane accretion, and magma addition also helped to form and stabilize the continental lithosphere.

Whereas these and many other investigations have identified some of the processes that contributed to the formation and stabilization of Archean cratons, numerous questions still remain. Reconciling the composition of craton roots determined from petrologic studies with the results of seismic velocity studies is problematic (King, 2005). There are many uncertainties about how stability can be achieved for billions of years without suffering mechanical erosion and recycling in the presence of subduction and mantle convection. Another problem is that the strength of mantle materials required to stabilize craton roots in numerical experiments exceeds the strength estimates of these materials

Fig. 11.14 CaO-Al2O3 plot showing the range of subcontinental lithospheric mantle (SCLM) compositions for selected cratons that have been matched with ages of the youngest tectonothermal events in the overlying crust (after O'Reilly et al., 2001, with permission from the Geological Society of America). Compositions have been calculated from garnetxenocrysts (Gnt). Xenolith averages shown for comparison. Plot shows that newly formed subcontinental lithospheric mantle has become progressively less depleted in Al and Ca contents from Archean through Proterozoic and Phanerozoic time. Garnet peridotite xenoliths from young extensional areas (e.g. eastern China, Vitim in the Baikal region of Russia, and Zabargad Island in the Red Sea) are geochemically similar to primitive mantle, indicating very low degrees of melt depletion.

derived from laboratory measurements (Lenardic et al., 2003). These issues, and the extent to which the cratonic mantle interacts with and influences the pattern of mantle convection, presently are unsolved. Improved resolution of the structure, age and geochemical evolution of the continental crust and lithospheric mantle promise to help geoscientists resolve these problems in the future.

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  • jorja davidson
    How craton stabilised?
    2 years ago

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