Multi-disciplinary (geological, petrographic, geochemical, geophysical, geochronological and isotopic) study of both Sub-Continental Lithospheric Mantle (SCLM) and overlying crust allows mapping of the architecture and tectonothermal evolution of the entire 60-280 km thick continental lithosphere. Our 20-year study finds that a) most (at least 59%, possibly much more) of the SCLM is of Archean origin; b) outside the cratons this ancient SCLM is subdivided into microcontinental blocks generally bounded by steep Trans-Lithospheric Faults (TLFs); c) the best preserved, oldest, and generally thickest, SCLM is the most depleted (Mg-rich, Fe-poor, low Al, Ca); d) The dominant process affecting the SCLM over time is refertilisation (including addition of Fe, Ca, Al; Griffin et al., 2009) resulting from the interaction with magmas derived from the convecting mantle, dominantly in magmatic arc settings; e) Refertilisation and associated thinning of the SCLM results in lower seismic velocity, increased density, and lower relative elastic thickness. Isotopic studies of the crust indicate that approximately 60% of today’s continental crust is material extracted from the convecting mantle in the Archean (e.g., Belousova et al., 2010). Relatively little of this may still exist as Archean crust – much of it has been repeatedly reworked into successively younger rocks, resulting in a gradual dilution of the old crustal component, or survives at mid- or lower-crust depths. Survival of this large Archean-derived crustal component is consistent with the existence of the ancient SCLM microcontinental blocks that can support and protect it though the eons. Post-Archean SCLM is rarely preserved through an orogenic cycle involving collision of sizable continental blocks. However, the crust may preserve significant volumes of post-Archean juvenile material within specific terranes that are generally smaller than microcontinents. This mapping offers a new, potentially revolutionary, understanding of continental structure and evolution. However, it also requires that large-scale projects in geoscience are strategic and integrative across subdiscipline boundaries and consider all scales from nano to global. Issues that need to be adequately addressed include: i) presence of widespread older deep basement in most arc terranes; ii) Re-Os model ages for mantle domains that are much older (including Archean) than the overlying crust; iii) contrasted growth of continents by collision addition of substantial, long-lived volumes of lithosphere hundreds of km across (and deep) rather than accretion of narrow ribbons of accreted juvenile island-arc terranes; iv) repeated partitioning of deformation such that the margins of such swaths are most intensely deformed and commonly cut by late-tectonic TLFs; v) domains of SCLM with anomalously high shear-wave velocity in post-Archean belts; vi) geodynamic influences on the isotopic characteristics of zircons; vii) the influence of paleotopography on detrital zircon sources; and viii) the inferred width and lifespan of oceanic plates.

Reference(s)

Belousova, EA, Kostitsyn, YA, Griffin, WL, Begg, GC, O’Reilly, SY and Pearson, NJ, 2010, Lithos 119, 457-466. Griffin, WL, O’Reilly, SY, Afonso, JC and Begg, GC, 2009, Journal of Petrology, 50, no.7, 1185-1204.