Episodic continental growth models: afterthoughts and extensions

Although there are still problems that need to be resolved with episodic models for continental growth, a large number of geochemical and geophysical observations can be explained with these models. The models have in common episodic collapse of subducted slabs through the 660 km seismic discontinuity, mantle convection changing from layered to whole mantle, and each collapse correlates with major episodes of crustal formation. Isotopic age distributions of juvenile continental crust suggest that crustal formation episodes at 2.7, 1.9, and 1.2 Ga may have been very short, each less than or equal to 100 My in length. The supercontinent cycle probably operates independently of slab avalanches at the 660 km seismic discontinuity, except for the first supercontinent at 2.7 Ga, which may have formed in response to the first slab avalanche. The production rate of continental crust may increase during slab avalanches in response to increased production rate of oceanic plateaus and of subduction-related crust. Crustal recycling rate may drop significantly below crustal production rate during slab avalanches due to the formation of supercontinents, which trap juvenile crust. If supercontinents are responsible for the preservation of large amounts of juvenile continental crust, then they play an important role in the growth of continental crust with time. During each of three superevents at 2.7, 1.9, and 1.2 Ga, the mantle may have been well mixed, thus losing geochemical evidence for enriched mantle sources. (C) 2000 Elsevier Science B.V. All rights reserved.