Coupled extrusion of sub-arc lithospheric mantle and lower crust during orogen collapse: a case study from Fiordland, New Zealand

The Anita Peridotite is a similar to 20 km long by 1 km wide exhumed fragment of spinel facies sub-arc lithospheric mantle that is enclosed entirely within the <= 4 km wide ductile Anita Shear Zone, and bounded by quartzofeldspathic lower crustal gneisses in Fiordland, south-western New Zealand. Deformation textures, grain growth calculations and thermodynamic modelling results indicate the mylonitic peridotite fabric formed during rapid cooling, and therefore likely during extrusion. However, insights into the exhumation process are gained through examination of aluminous garnetbearing meta-sedimentary gneisses also enclosed within the shear zone. P-T calculations indicate that prior to mylonitization the gneisses enclosing the peridotite equilibrated at 675-746 degrees C in the sillimanite stability field (stage I), before being buried to near the base of thickened arc crust (stage II; similar to 686 +/- 26 degrees C and 10.7 +/- 0.8 kbar). From this point on, the peridotite unit and the quartzofeldspathic rocks share a deformation history involving extensive recrystallization (stage III) within the Anita Shear Zone. Coupled exhumation of these portions of lower crust and upper mantle occurred during regional thinning of over-thickened lithosphere at c. 104 Ma (U-Pb zircon). Our favoured model for the exhumation process involves heterogeneous transpressive deformation within the translithospheric Anita Shear Zone, which provided a conduit for ductile extrusion through the crust.