Crustal thickening of the Early Cretaceous paleo-Pacific Gondwana margin

The cause of 7 to 25 km of Early Cretaceous thickening of parts of the New Zealand Gondwana margin has been argued to be due to tectonic or magmatic processes. New insights into this event have been gained from examination of the Grebe Mylonite Zone, an extensive amphibolite facies Early Cretaceous ductile shear zone that separates volcanic arc rocks of the Outboard Median Batholith from metasedimentary and intrusive rocks of the Western Province–Inboard Median Batholith at Lake Manapouri in southwest New Zealand. Ar–Ar and U–Pb geochronology shows that here exhumation of the Outboard Median Batholith began ≥ 128 Ma, syn-kinematic magmatism was taking place at 121 Ma, and measurable displacement had ceased by ≥ 116 Ma. Although the mylonite zone is currently subvertical, a combination of thermochronological cooling profiles, lineation orientations, and back-rotation of tilted Tertiary unconformities indicate that it most likely formed as an oblique sinistral-reverse fault that dipped 60°E and therefore displaced the Outboard Median Batholith over the margin of the Western Province. However, the high original angle of this and other compressional shear zones in analogous structural positions, the apparent absence of a regional thrust or nappe sheet(s), and the subtly different ages of thickening for different areas indicate that tectonic loading can have played only one part in the crustal thickening event. Another factor was the contemporaneous emplacement of > 30,000 km3 of mafic to felsic magma, which was likely accommodated by vertical depression of the middle and lower crust. Tectonic and magmatic processes leading to crustal thickening were, therefore, not mutually exclusive.