Episodic growth of juvenile crust and catastrophic events in the mantle

Abstract

Episodic growth of continental crust and supercontinents at 2.7,1.9,and 1.2 Ga may be caused by superevents in the mantle as descending slabs pile up at the 660-km seismic discontinuity and then catastrophically sink into the lower mantle. A superevent cycle involves supercontinent breakup that initiates both slab avalanches and the onset of formation of a new supercontinent; arrival of slabs at the D" layer triggers mantle plumes that rise and bombard the base of lithosphere producing juvenile crust trapped in the growing supercontinent; and shielding of the mantle beneath the new supercontinent results in a mantle upwelling that eventually breaks the supercontinent as the cycle starts over. Superevents comprise three or four events each of 50-80 My duration, each of which may reflect slab avalanches at different locations and times at the 660-km discontinuity. Superplume events in the late Paleozoic and Mid-Cretaceous may have been caused by minor slab avalanches as the 660-km discontinuity became more permeable to the passage of slabs. The total duration of a superevent cycle decreases with time probably reflecting the cooling of the mantle