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

Abstract

The character, timing, and significance of deformation within the Median Batholith has been debated since at least 1967, with allochthonous and autochthonous models proposed to account for internal variations in the character of the batholith. Stewart Island provides excellent exposures of intrabatholithic structures, allowing many aspects of the deformation history within the batholith to be analysed, far removed from the effects of later deformation related to the current plate boundary. Median Batholith rocks in northern and central Stewart Island are deformed by three major structures: the Freshwater Fault System, Escarpment Fault, and Gutter Shear Zone. Lineation orientations, Al in hornblende geobarometry, and Ar‐Ar thermochronology indicate up to c. 7 km of NNE‐directed uplift of the hanging wall of the Escarpment Fault between c. 110 and 105 Ma. Unlike the Escarpment Fault, a wide range of mineral elongation lineation orientations, including many oblique to the strike and dip of related foliations, characterise both the Gutter Shear Zone and Freshwater Fault System. Lineation and limited sense of shear data indicate dextral‐reverse movement on both structures during development of their dominant ductile fabrics. Crosscutting and intrusive relationships indicate movement on the Freshwater Fault System after c. 130 Ma and on the Gutter Shear Zone between 120 and 112 Ma. The amount of movement on the Freshwater Fault System and Gutter Shear Zone remains largely unconstrained. However, the 342 ± 24 Ma age of a granite clast in a Paterson Group lithic tuff horizon at Abrahams Bay overlaps that of Carboniferous plutons in the block immediately south of the Freshwater Fault System, implying that the Paterson Group is little displaced from the basement rocks through which it was erupted. The three structures mapped on Stewart Island form part of a narrow transpressional mobile belt active within the Jurassic‐Cretaceous arc on the outboard margin of the Western Province during the later stages of arc development between c. 125 and 105 Ma. Deformation is largely confined to the area outboard of the Paleozoic metasedimentary basement in southern Stewart Island. In central Stewart Island, transpressional deformation began on the broad amphibolite facies Gutter Shear Zone. Subsequent deformation was focused along the narrower Escarpment Fault, which offset the c. 300°C isotherm in the upper crust. All three structures on Stewart Island likely form part of a regional‐scale network of transpressional shear zones and faults that includes the Indecision Creek and Grebe Shear Zones in eastern Fiordland. Transpressional deformation along these structures broadly coincided with loading of the Western Fiordland Orthogneiss in northern Fiordland by c. 6 kbar, indicating that contractional deformation affected all levels of the crust at this time.