Tectonic evolution of Macquarie Island: extensional structures and block rotations in oceanic crust

Three distinct tectonic periods are recognized on Macquarie Island. D1 is a protracted period of palaeo-N–S extension, encompassing initial crust formation at the Proto-Macquarie Spreading Ridge (PMSR), over-printing by late-stage dykes, and a wide range of extensional and dilational structures formed in the near- to off-axis environment. Extensional structures in the lower-crust include fractures, serpentine veinlets, brittle faults, semi-ductile shear zones and ductile mylonites. All were formed in a stress regime with sub-vertical σ1 and sub-horizontal N-trending σ3 and are consistent with pure extension in a spreading ridge environment. A minor set of orthogonal fractures and serpentine veinlets suggest a component of along-axis extension during D1. Early D1 tilting of 20–58° around horizontal axes parallel to the ridge axis accompanied growth faults and major differential block uplifts in the near-axis environment. Superimposed on, and in the waning stages of D1, was a period of palaeo-NE–SW extension (D2) with dolerite dyke emplacement. D2 occurred in a transtensional regime, during transition from extension at the spreading ridge to a transcurrent plate margin (D3). D3 constitutes dextral transcurrent movements at the NNE-trending Indo-Australian/Pacific plate margin, active from approximately 10.5 Ma to the present day. Both palaeo-stress analysis of D3 faults and neotectonic fault scarp geometry indicate dominantly strike–slip and rare thrust events, with NE-trending σ1 compatible with dextral transpression. Sharp angular discordances in the palaeo-seafloor fabric (D1 dyke trend), document clockwise rotation of km-scale crustal blocks around vertical axes during D3, entirely consistent with dextral transpression at the plate margin.