A deep crustal shear zone exposed in western Fiordland, New Zealand

Abstract

An approximately 180-km-long, deep crustal shear zone in western Fiordland, New Zealand, has been shown to have formed during continental extension between 116 and 100 Ma. The western Fiordland shear zone forms a major tectonic boundary between two sets of gneisses with unrelated deformation histories. A detailed study of the northern part of the shear zone has revealed that the shear zone formed at depths of-40 km (approximately 12 kbar and 680øC) and was originally very gently dipping or subhorizontal prior to subsequent upright folding. The current thickness (postfolding) of the shear zone is -3-4 km. Abundant asymmetric structures in the shear zone indicate noncoaxial deformation and a topto-the-NE sense of shear. Unlike ductile shear zones in metamorphic core complexes from elsewhere in the word, there is no evidence that decompression of the shear zone rocks or footwall rocks occurred during the active life of the shear zone, suggesting that displacement along the shear zone by noncoaxial deformation must have been largely horizontal. This shear zone provides important evidence for the nature of deep crustal deformation occurring during continental extension. Zealand: Cretaceous extension associated

Shear zones in the upper mantle: evidence from alpine- and ophiolite-type peridotite massifs

“…The recrystallized grain size varies from coarse (>4 mm), medium (1-4 mm) to fine (1-0.1 mm). The term mylonite is used to describe a cohesive fault (or shear zone) rock with strong foliation Wood (1972); Hill (1995) Boudier et al (1984; Brodie & Rutter (1987); Skrotzki et al…”

Early Cretaceous dextral transpressional deformation within the Median Batholith, Stewart Island, New Zealand

“…These structures on Stewart Island are therefore likely to be mid-upper crustal manifestations of the contractional deformation event which resulted in loading of the Western Fiordland orthogneiss. The possibility that some further movement occurred on the Freshwater Fault System during extension (Tulloch & Kimbrough 1989;Spell et al 1999), and unroofing of the Western Fiordland Orthogneiss from c. 110 Ma (Gibson et al 1988;Gibson & Ireland 1995;Hill 1995;Flowers et al 2005;Scott & cooper 2006), cannot be ruled out as the timing of cessation of movement on this structure is poorly constrained. However, no obvious younger mineral elongation lineation that might be associated with extensional reactivation of the Freshwater Fault System was observed.…”

“…Y. Bradshaw , 1990Clarke et al 2000;Daczko et al 2001Daczko et al , 2002Klepeis et al 2004;Flowers et al 2005;Marcotte et al 2005). Extension and the development of metamorphic core complexes in NelsonWestland and Fiordland followed between c. 110 and 90 Ma (Gibson et al 1988;Tulloch & Kimbrough 1989;Gibson & Ireland 1995;Hill 1995;Klepeis et al 1999;Spell et al 2000;Scott & Cooper 2006). East of Stewart Island, further extension was associated with opening of the Great South Basin between c. 90 and 85 Ma (Cook et al 1999; Kula et al 116 New Zealand Journal of Geology andGeophysics, 2008, Vol.…”

Gneissic rocks of the Bonar Range, central Westland, New Zealand

“…Possible origins of this fault could be related to Cretaceous extension, as originally suggested by Tulloch (1995), and/or a Cenozoic uplift associated with the current plate boundary. Certainly the presence of pre-79 Ma mylonitic rocks, a top to the northeast-east shear sense, and a change in metamorphic grade are features consistent with Early Cretaceous extensional detachment faulting inferred for the Paparoa Range (Tulloch & Kimbrough 1989) and Fiordland (Gibson et al 1988;Hill 1995). Dating of mylonitic micas is required to substantiate Early Cretaceous tectonism, but the nearby Rb-Sr Early Cretaceous resetting ages of Aronson (1968) do lend support.…”