CONTINENTAL VERSUS CRUSTAL GROWTH: UNDERSTANDING THE PARADOX

Continental crust growth models vary widely, with extremes that advocate dominantly Early Archean growth, and dominantly post-Archean growth, respectively. These calculations have given little consideration to the fundamental constraint imposed on crustal growth and preservation by the presence or absence of durable lithopheric mantle. Systematic mapping of the Sub-Continental Lithospheric Mantle (SCLM) has been undertaken over the last 7 years, by integration of geophysical, geological, and geochemical data on the crust and lithospheric mantle. The resultant maps describe lithospheric composition, architecture and history, enabling a 4D understanding of continental evolution. This mapping suggests that ≥70% of SCLM may have an Archean parentage. Most preserved Proterozoic crust overlies Archean SCLM that has been variably refertilised and metasomatised by mantle melts associated with convergent margin, post-collisional, and mantle plume processes. This suggests that consideration of lithospheric preservation and recycling is crucial to understanding Earth evolution, including the concepts of crustal, as against continental, growth. In a recycling model, ancient, rigid, bouyant SCLM survives the rifting and accretionary processes of supercontinent cycles, whilst juvenile, fertile, dense SCLM typical of island arcs is largely returned to the convecting mantle via delamination. Significant areas of preserved post-Archean juvenile crust can be attributed to the complexities of plate tectonic processes. These include:- (i) obduction; and (ii) resurfacing. The latter may may involve mantle-derived magmatism during regional extension, or follow rifting and detachment of variable (upper- to whole-) crustal thicknesses. Post-Archean continental net growth is likely near zero. Plate tectonic processes implicit in supercontinent cycles redistribute continental lithosphere via continental collisions and microcontinent migrations.