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

Three distinct tectonic periods are recognized on Macquarie Island. D-1 is a protracted period of palaeo-N-S extension, encompassing initial crust for mation at the Proto-Macquarie Spreading Ridge (PMSR), over-printing by late -stage dykes, and a wide range of extensional and dilational structures for med in the near- to off-axis environment. Extensional structures in the low er-crust include fractures, serpentine veinlets, brittle faults, semi-ducti le shear zones and ductile mylonites. All were formed in a stress regime wi th sub-vertical sigma (1) and subhorizontal N-trending sigma (3) and are co nsistent with pure extension in a spreading ridge environment. A minor set of orthogonal fractures and serpentine veinlets suggest a component of alon g-axis extension during D-1. Early D-1 tilting of 20-58 degrees around hori zontal axes parallel to the ridge axis accompanied growth faults and major differential block uplifts in the near-axis environment. Superimposed on, a nd in the waning stages of D-1, was a period of palaeo-NE-SW extension (D-2 ) with dolerite dyke emplacement. D-2 occurred in a transtensional regime, during transition from extension at the spreading ridge to a transcurrent p late margin (D-3). D-3 constitutes dextral transcurrent movements at the NN E-trending Indo-Australian/Pacific plate margin, active from approximately 10.5 Ma to the present day. Both palaeo-stress analysis of D-3 faults and n eotectonic fault scarp geometry indicate dominantly strike-slip and rare th rust events, with NE-trending sigma (1) compatible with dextral transpressi on. Sharp angular discordances in the palaeo-seafloor fabric (D-1 dyke tren d), document clockwise rotation of km-scale crustal blocks around vertical axes during D-3, entirely consistent with dextral transpression at the plat e margin. (C) 2001 Elsevier Science Ltd. All rights reserved.