DOI: 10.5061/dryad.kh189326h
title: The initiation and growth of transpressional shear zones through continental arc lithosphere, southwest New Zealand

description: 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.
description: 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 <sup>202</sup>Hg, <sup>204</sup>(Pb+Hg),
 <sup>206</sup>Pb, <sup>207</sup>Pb,
 <sup>208</sup>Pb, <sup>232</sup>Th, and
 <sup>238</sup>U. All isotopes were collected in counting mode
 with the exception of <sup>232</sup>Th and
 <sup>238</sup>U, 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 <sup>207</sup>Pb/<sup>206</sup>Pb and
 <sup>238</sup>U/<sup>206</sup>Pb 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
 <sup>206</sup>Pb/<sup>238</sup>U date for zircons
 &lt;1100 Ma, and the
 <sup>207</sup>Pb/<sup>206</sup>Pb date for zircons
 &gt;1100 Ma. For zircon thermometry, Ti
 concentrations were collected simultaneously with
 <sup>206</sup>U/<sup>238</sup>U 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).