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 latter system, magmatism influenced shear zone architecture and drove its growth while subduction and oblique convergence ended. As magma entered the roots of the system and began to solidify, deformation was driven out of the lower crust and into the middle crust where the system widened by a factor of three when fold-thrust belts formed on either side of a steep, central transpressional shear zone. This study illustrates how the reactivation of inherited structural weaknesses localizes deformation at all depths in the lithosphere and shows how magma-deformation feedbacks influence shear zone connectivity and built a batholith from the bottom up.

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 (operating at a wavelength of 193 nm). Prior to analysis the Element2 was tuned using the NIST 612 glass standard to optimize signal intensity and stability. Laser beam diameter was ~25 microns at 10 Hz and 75-100% power. Ablation was performed in a HelEx II Active 2-Volume Cell and sample aerosol was transported with He carrier gas through Teflon-lined tubing, where it was mixed with Ar gas before introduction to the plasma torch. Flow rates for Ar and He gases were as follows: Ar cooling gas (16.0 NL/min), Ar auxiliary gas (1.0 NL/min), He carrier gas (~0.3-0.5 NL/min), Ar sample gas (1.1-1.3 NL/min). Isotope data were collected in E-scan mode with magnet set at mass 202, and RF Power at 1245 W. Isotopes measured include ²⁰²Hg, ²⁰⁴(Pb+Hg), ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U. All isotopes were collected in counting mode with the exception of ²³²Th and ²³⁸U, which were collected in analogue mode. Analyses were conducted in a ~40-minute time resolved analysis mode. Each zircon analysis consisted of a 20-second integration with the laser firing on sample, and a 20 second delay to purge the previous sample and move to the next sample. Approximate depth of the ablation pit was ~20-30 microns.

The primary standard, 91500, was analyzed every 10-15 analyses to correct for in-run fractionation of Pb/U and Pb isotopes. A second zircon standard, Temora-2 was analyzed every ~10 analyses to assess reproducibility of the data. Total uncertainties (analytical + systematic) were determined by Iolite. U-Pb analysis of Temora-2 during all analytical sessions yielded concordant results and error-weighted average ages of 414.5 ± 1.0 Ma (n=113) which is within close agreement of the accepted ages of 416.8-418.4 Ma (Black et al., 2004; Mattinson, 2010).

U-Pb isotopic data were plotted using IsoplotR (Vermeesch, 2018). Corrections for minor amounts common Pb in zircon were made following methods of Tera and Wasserburg (1972) using measured ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios and an age-appropriate Pb isotopic composition of Stacey and Kramers (1975). Zircons with large common Pb corrections (e.g., analyses interpreted as having ~30% or greater contribution from common Pb) were discarded from further consideration. Zircon dates are reported using the ²⁰⁶Pb/²³⁸U date for zircons <1100 Ma, and the ²⁰⁷Pb/²⁰⁶Pb date for zircons >1100 Ma.

For zircon thermometry, Ti concentrations were collected simultaneously with ²⁰⁶U/²³⁸U isotopic ages using SHRIMP-RG measurements. The Ferry and Watson (2007) calibration was used to calculate the temperatures of zircon growth. Most of the temperatures record magmatic crystallization temperatures, rather than metamorphic conditions. Other details of this calculation are described in Schwartz et al. (2017). 

For titanite, 206Pb/238U isotopes, geochemistry (La, Sm, Yb, U, Th) and Zr-in-titanite temperatures were collected using laser-ablation–split-stream–inductively coupled plasma–mass spectrometry (LASS-ICP-MS) at the University of California Santa Barbara following methods outlined in Kylander-Clark et al. (2013), Schwartz et al. (2016), and Buriticá et al. (2019). The data were corrected for common Pb using a regression on the Tera-Wasserburg Pb/U isochron following methods discussed in Buriticá et al. (2019). For titanite thermometry, trace elements were measured simultaneously with U-Pb isotopes. Model Zr-in-titanite temperatures were calculated using the methods of Hayden et al. (2008). Given the uncertainties in trace element measurements, assumed pressures, and activities, all temperatures have an uncertainty of ±50°C. These and other aspects of the calculation are described in detail by Buriticá et al. (2019).