Accordion vs. quantum tectonics: insights into continental growth processes from the Paleozoic of eastern Gondwanan

The Early Paleozoic Lachlan Fold Belt of eastern Australia is widely regarded as an ancient convergent plate margin beneath which paleo-Pacific (Panthalassic) oceanic lithosphere was continuously subducted. It is cited as the type example of a retreating accretionary orogeny. However, sandstone compositions, the sedimentological nature and timing of chert accumulation and overall stratigraphic architecture are not necessarily consistent with this model. We suggest an alternative explanation for growth of Gondwanan continental margin. Oceanic lithosphere outboard of the passive Gondwana continental margin was subducted beneath an extensive intra-oceanic island arc that now crops out as an allochthonous element (Macquarie arc) within the fold belt. Once intervening oceanic lithosphere was eliminated this arc collided with, and was emplaced upon the Gondwana margin. Recognition of four such events along this margin through the Phanerozoic suggests it is a significant mechanism for continental growth.