STRAIN LOCALIZATION AND EXHUMATION OF THE LOWER CRUST DURING CRETACEOUS TO TERTIARY EXTENSION AND TRANSTENSION IN FIORDLAND, NEW ZEALAND

The lower crustal root of a Mesozoic magmatic arc exposed in Fiordland records a transition from subduction and contraction along the Gondwana margin to extension that led to opening of the Tasman Sea at ~83 Ma. A second transition occurred in the Eocene as a new spreading center formed south of New Zealand. Structural data, zircon ²⁰⁶Pb/²³⁸U ion probe (Stanford-USGS SHRIMP-RG) ages, and ²⁰⁶Pb/²³⁸U titanite LASS-ICP-MS dates allowed us to investigate strain localization and exhumation processes in the arc during these transitions.

The first transition followed a magma flare-up from 118-115 Ma that was accompanied by granulite facies metamorphism and lower crustal melting. During this period, until ~111 Ma, melt-present lower crustal deformation was highly disorganized, distributed over at least 1200 km², and decoupled from the upper and middle crust. By ~111 Ma, titanite ages show cooling from >850°C to <650ºC as deformation began to focus into three ≤0.5 km-thick extensional shear zones that record hydration, horizontal flow, and crystal-plastic deformation. Zircon ages on syn- and post-tectonic dikes in these shear zones show a migration of deformation toward the hot back arc region over ~20 Ma. From NE to SW the shear zones were abandoned at 105.8 ± 0.7 Ma, 97.8 ± 1.3 Ma, and 89.3 ± 1.2 Ma, and were followed by extension outside Fiordland. Despite evidence that extension in the upper and lower crust was coupled during this period, the migration prevented the formation of crustal-scale detachments that deeply exhumed the lower crust.

The second transition occurred in the Late Eocene with the development of strike-slip and normal faults that formed a sinistral transtensional system along with spreading centers south of Fiordland. These faults reactivated older structures and provide another mechanism for partially exhuming the lower crust. Final exhumation occurred during late Cenozoic transpression and erosion.

These results show a lower crust initially weakened by magma, heat, and melting. Later, fluid activity, reaction softening, and a reactivation of inherited structures controlled deformation. Although the magnitudes of extension and exhumation still must be determined, the data show how changing styles of deformation resulted in a piecemeal exhumation of the lower crust over tens of millions of years.