Thermal evolution of the Sisters shear zone, southern New Zealand; Formation of the Great South Basin and onset of Pacific‐Antarctic spreading

Abstract

The separation of Zealandia from West Antarctica was the final stage in the Cretaceous breakup of the Gondwana Pacific margin. Continental extension resulting in formation of the Great South Basin and thinning of the Campbell Plateau leading to development of the Pacific‐Antarctic spreading ridge was partially accommodated along the Sisters shear zone. This east‐northeast striking brittle‐ductile structure exposed along the southeast coast of Stewart Island, New Zealand, is a greenschist facies extensional shear zone that separates a hanging wall of chloritic, brecciated granites, and undeformed conglomerate from a footwall of mylonitic Carboniferous and Early Cretaceous granites. This complex structure exhibits bivergent kinematics and can be subdivided into a northern and southern segment. The 40Ar/39Ar thermochronology indicates that cooling of the shear zone footwall began at ∼94 Ma with accelerated cooling over the interval ∼89–82 Ma. Structural and thermochronological data indicate a spatial and temporal link between the Sisters shear zone, initial sedimentation within the offshore Great South Basin, extension of the Campbell Plateau, and initiation of the Pacific‐Antarctic spreading ridge.

Kinematic, finite strain and vorticity analysis of the Sisters Shear Zone, Stewart Island, New Zealand

“…bivergent symmetry in the SW and monovergent symmetry in the NE. Kula et al (2009) did not report any quantitative structural data to support their along-strike differences in shear zone kinematics and overall geometry. Muscovite, biotite and feldspar 40 Ar/ 39 Ar by Kula et al (2009) suggest that footwall cooling started at~94 Ma with accelerated cooling between 89 and 82 Ma.…”

“…Kula et al (2009) reported along-strike differences in the kinematics of the Sister Shear Zone (SSZ), which is separating the Pegasus core complex from overlying sediments of the Great South Basin (Fig. 2).…”

“…The SSZ is a NE-striking low-angle extensional shear zone running along the southeastern edge of Stewart Island (Fig. 2) (Allibone and Tulloch, 2008;Kula et al, 2009). The rocks in the shear zone are granitoids variably deformed that include breccia, protomylonite, mylonite, and ultramylonite (Kula et al, 2009).…”

“…The southwest part of the SSZ is supposed to have a top-N displacement that is thought to be associated with a hypothetical top-S shear zone giving the structure an overall bivergent symmetry. The northeast part of the SSZ is supposed to be a monovergent top-S shear zone (Kula et al, 2009, their Figs. 2 and Kula et al, 2009).…”

“…The northeast part of the SSZ is supposed to be a monovergent top-S shear zone (Kula et al, 2009, their Figs. 2 and Kula et al, 2009). Note that basin architecture in Great South Basin is characterized by two orthogonal sets of normal faults.…”