The initiation and growth of transpressional shear zones through continental arc lithosphere, southwest New Zealand

Structural analyses combined with new U-Pb zircon and titanite geochronology show how two Early Cretaceous transpressional shear zones initiated and grew through a nearly complete section of continental arc crust during oblique convergence. Both shear zones reactivated Carboniferous faults that penetrated the upper mantle below Zealandia’s Median Batholith but show opposite growth patterns and dissimilar relationships with respect to arc magmatism. The Grebe-Indecision Creek shear zone was magma-starved and first reactivated at ~136 Ma as an oblique-reverse fault, along which an outboard batholith partially subducted beneath Gondwana. This system nucleated at or above ~20 km depth and propagated downward at 2-3 mm yr-1, accumulating at least 35-45 km of horizontal (arc-normal) shortening by ~124 Ma. In contrast, the magma-rich George Sound shear zone first reactivated in the lower crust (~55 km depth) at ~124 Ma and grew upward at ~3 mm yr-1, reaching the upper crust by ~110 Ma. In this..., Zircons were dated at the USGS-Stanford Ion Microprobe Laboratory at Stanford University, California and at California State University Northridge. Methods for ion probe dating closely follow those described in Schwartz et al. (2017) and Coble et al. (2018). In both cases, zircons were separated following standard methods involving crushing, pulverizing with jaw crusher and disk mill, and density separation on a Wilfley gold table and with heavy liquids. Samples were processed through a Frantz isodynamic separator (side tilt = 5°, front tilt = 20°) at 1.5 amps to remove magnetic (non-zircon) minerals. Zircons were poured onto double sided tape and mounted in epoxy, ground and polished. They were then imaged on a Gatan MiniCL detector attached to a FEI Quanta 600 SEM at California State University Northridge.
For LA-SF-ICPMS U-Pb zircon geochronology, uranium-lead ratios were collected using a ThermoScientific Element2 SF-ICPMS coupled with a Teledyne Cetec Analyte G2 Excimer Laser (oper...,