RAPID EPISODIC CONTINENTAL GROWTH DURING SUPERCONTINENT DISPERSAL

Rapid growth of major segments of continental crust represent critical, but poorly understood, transitions in Earth History. Regions of rapid growth include the 2.2-1.9 Ga old Sveckokarelian shield, the 1.9-1.7 Ga old southwest USA, and the 0.9-0.6 Ga Arabian-Nubian shield. Also, the bulk of the Australian continent grew in two short bursts between 1.9-1.6 Ga and 0.53-0.23 Ga. Such rapid continental growth cannot be accommodated by conventional arc accretion models, nor by recent popular oceanic plateau accretion models.

A new model of continental growth is developed, based on the Lachlan Orogen, part of the Paleozoic east Gondwanan continental margin. Rare, but widespread, Cambrian basalts indicate initial development as a vast, west Pacific-style intraoceanic arc-backarc system upon which ubiquitous craton-derived Ordovician turbidites were deposited. Discrete Silurian-Devonian orogenic events involving repeated crustal thinning/thickening episodes, widespread silicic magmatism, and high-T low-P metamorphism, ultimately produced a stable 700-800 km wide orogenic belt in 60 Ma. Minor but persistent eruption of mafic rocks throughout the orogenic history indicates that semi-continuous arc/backarc-related lithospheric extension was the primary mechanism for granite magma generation and creation of sediment-filled repositories. Intermittent, transient episodes of transpressional crustal shortening aided in thickening and duplicating the crust. This generally extensional environment that produced sequential backarcs is termed an extensional accretionary orogen, and is a requisite for rapid continental growth.

Rapid, irreversible growth of this Paleozoic Gondwanan margin occurred during supercontinent dispersal, following breakup of Rodinia. Sequential backarc basins best develop during protracted slab rollback at large distances from mid-ocean ridges (eg., western Pacific), because the slab is relatively dense. When supercontinents exist, mid-ocean ridges generally are at greater distances from subduction zones, so a greater tendency exists for slab-rollback. Mantle wedge counter flow beneath the continental margin, induced by rollback, causes regional upper plate extension and development of backarc basins. Then, if other conditions are right, rapid, continental growth may begin.