Evidence for crustal thickening and shortening of the overriding plate during incipient plate/plate subduction

Abstract

Evidence from a study of the Macquarie Ridge Complex and comparison with other plate boundaries displaying varying development of intraplate compressional structures, indicate that thickening and shorting of the overriding plate occur during incipient subduction. The Macquarie Ridge Complex has been produced by a tectonic history oblique compression between the Indian/Australian and Pacific plates at their common boundary south of New Zealand. In the central Macquarie Ridge Complex intraplate transpressive forces have resulted in the uplift of oceanic crust of the Indian/Australian plate to form the Macquarie Ridge and incipient subduction of the Pacific oceanic plate at the Macquarie Trench. The uplift has been effected by thickening and shortening of the oceanic crust of the overriding Indian/Australian plate accompanied by shallow seismicity at the ridge, whereas the Pacific plate at the trench has not been thickened. Post-1964 earthquakes of the Macquarie Ridge Complex form a single narrow band some tens of kilometres wide. In the central Macquarie Ridge Complex the band of seismicity is confined to the ridge and not the trench, suggesting that crustal thickening rather than subduction is the preferred style of tectonism for incipient subduction zones. Fault plane solutions indicate dextral motion that is consistent with anticlockwise rotation of the Pacific plate relative to the Australian plate, and with transpression and incipient subduction in the central segment of the Complex.