Interaction of strong lower and weak middle crust during lithospheric extension in western New Zealand

Abstract

Exhumed sections of the middle and lower crust in western New Zealand reveal how deformation was partitioned within a thermally and rheologically evolving crustal column during Cretaceous continental extension. Structural data, P‐T determinations, and U‐Pb geochronology from central Fiordland and the Paparoa Range in Westland show that extension initiated in the lower crust by ∼114 Ma as a period of arc‐related magmatism waned. Initially, deformation was localized into areas that were weakened by heat and magma. However, these hot, weak zones were ephemeral. During the period 114–111 Ma, lower crustal fabrics record a rapid progression from magmatic flow to high‐temperature deformation at the garnet‐granulite facies (T > 700°C, P = 12 kbar) to cooler deformation at the upper amphibolite facies (T = 550–650°C, P = 7–9 kbar). Lower crustal cooling and compositional contrasts between mafic granulites and hydrous metasedimentary material resulted in a middle crust that was weak relative to the lower crust. Between circa 111 and circa 90 Ma, focused subhorizontal flow and vertical thinning in a weak middle crust led to the collapse of the upper crust and the unroofing of midcrustal material. During this period, arrays of conjugate‐style shear zones transferred displacements vertically and horizontally through the crust, resulting in a structural style that resembles crustal‐scale boudinage. The New Zealand example of continental extension shows that a weak middle crust and a relatively cool, highly viscous lower crust can result in a localized style of extension, including the formation of metamorphic core complexes that exhume the middle crust but not the lower crust.

Evaluating the importance of metamorphism in the foundering of continental crust

“…The WFO comprises a series of intermediate to mafic plutons emplaced inboard of the continental margin at various depths (0.9–1.8 GPa) 22 . The exposed WFO sequence is tectonically disrupted, with plutons emplaced at 1.2 GPa now structurally juxtaposed with plutons emplaced at 1.8 GPa 22,29 . The WFO is dominated by gabbroic to monzodioritic protoliths, with the presence of distinct Na-Ca clinopyroxenes reflecting different emplacement depths 21,22,24,31 .…”

Cretaceous metamorphism, magmatism and shearing in the Waipuna Valley, directly south of the Reefton Goldfield

“…Coney 1980;Lister & Davis 1989). Formation of Cretaceous extensional faults zones in western New Zealand immediately preceded the opening of the Tasman Sea at 85 Ma (Gibson et al 1988;Scott & Cooper 2006;Klepeis et al 2007). The best studied of these fault zones occur within the Paparoa Metamorphic Core Complex in Westland Spell et al 2000;Klepeis et al 2007;Sagar & Palin 2011;Schulte et al 2014) (Fig.…”

Plutonic rocks of the Median Batholith in eastern and central Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

“…u-pb zircon lA-icp-MS dating of a leucogranite plug that cuts the grebe Mylonite Zone (ou75687), and which is inferred to emanate from the west Arm leucogranite, indicates an emplacement age of 116.3 ± 1.3 Ma . younger plutonic rocks in Fiordland are restricted to pegmatite dikes of unknown affinity with emplacement ages of 113.4 ± 1.7,102.1 ± 1.8, and 88.4 ± 1.2 Ma (Klepeis et al 2007;King et al 2008) and a Sr-rich trondhjemite dike (Scott & cooper 2006). Flowers et al (2005) present a u-pb titanite thermochronological cooling age of 128 ± 8 Ma from a sample within the pluton at west Arm.…”