RHEOLOGICAL TRANSITIONS AND STRAIN LOCALIZATION IN LOWER CRUSTAL SHEAR ZONES: EVIDENCE FROM THE ROOT OF A CONTINENTAL ARC EXPOSED IN FIORDLAND, NEW ZEALAND

We use field relationships, microstructure, and electron backscatter diffraction data to investigate the rheological changes promoting strain localization in the lower crust of a continental arc in Fiordland, New Zealand. The rocks record a Cretaceous history of subduction and contraction followed by extension as an outboard arc of Jurassic and older rock was underthrust beneath Gondwana. This event increased magmatism, thickened the crust to ≥65 km, and created steep shear zones up to 15 km thick through which magmas and crustal melts moved.

At 45-65 km depth, plutons composed mostly of hornblende diorite record three rheological transitions that promoted strain localization. The first involved a change from hypersolidus flow in melt-rich magmas to deformation close to the solidus when dislocation creep of plagioclase and pyroxene occurred in diffuse zones near the tops of crystallizing plutons. Subsequent deformation was partitioned into plagioclase. The second transition occurred as crustal melts infiltrated high-strain zones rich in plagioclase after strain partitioning. The timing of infiltration is indicated by K-feldspar-rich melt pseudomorphs, which occur in the interstitial spaces between recrystallized plagioclase. The third transition occurred as melts drained from plagioclase-rich shear zones and deformation became localized outside the former leucosome, where fine-grained (d = ~14 μm) plagioclase records the onset of grain boundary sliding (GBS). At this stage the change in plagioclase deformation mechanism from dislocation creep to GBS had a stronger effect on the rheology than the presence of melt.

We infer a process whereby crystal-plastic deformation in lower crustal shear zones reduces plagioclase grain size and promotes melt infiltration. Melt-infiltration further weakens the crust and localizes strain. As melts are extracted melt vol% decreases to <7%, resulting in a rheological switch where leucosome (former melt) is strong and grain size sensitive GBS dominates and localizes strain in previously recrystallized areas. The data indicate initial strain localization was achieved by crystal-plastic deformation of both plagioclase and pyroxene, and later enhanced by both melt infiltration and a change in plagioclase deformation mechanism as melts drained from the lower crust.