THE RELATIONSHIP BETWEEN DEFORMATION AND METAMORPHISM IN CONTRASTING HIGH STRAIN ZONES IN THE LOWER CRUST OF A CONTINENTAL ARC, FIORDLAND, NEW ZEALAND

Field mapping and structural analyses in an exhumed section of lower crust in central Fiordland reveal the relationship between metamorphism and the formation of high strain zones at the root of a Cretaceous continental arc. A 45 km cross section of Breaksea Sound highlights two contrasting styles following the emplacement of the (125-123 Ma) Breaksea (BO) and (118-115 Ma) Malaspina (MO) orthogneisses.

The first period (125-107 Ma) of deformation and metamorphism produced broad (affecting >200 km²) high-strain zones during episodic partial melting and recrystallization at the granulite and eclogite facies. These high-strain zones exhibit linear fabrics, isoclinal folds of igneous layering, and asymmetric pressure shadows of matrix plagioclase around aggregates of garnet and pyroxene. Subgrains in pyroxene suggest high-T (≥850°C) subsolidus deformation. Plagioclase exhibits a strong LPO, subgrains, and lobate boundaries, suggesting it was the weakest phase and accommodated the most strain through climb-accommodated dislocation creep. Structural comparisons indicate that this period involved widely different styles of deformation, including diapirism in the older BO and heterogeneous vertical and horizontal flow in the younger MO.

The second period (106-88 Ma) of deformation and metamorphism produced sets of discrete upper-amphibolite facies shear zones (affecting ~80 km²) localized along some but not all contacts between MO, BO, and host gneiss. These record a progressive localization of strain linked to cooling, hydration and retrogression of the lower crust, resulting in the breakdown of garnet and pyroxene to form hornblende, biotite, fine-grained plagioclase and quartz. Plagioclase lacks a strong LPO and is statically recrystallized; whereas, hornblende exhibits a strong LPO. Thus, deformation during this phase involved fluid-assisted diffusion creep whereby hornblende and biotite accommodated most of the strain. Similar lineation orientations, and kinematic patterns suggest all retrograde shear zones record subhorizontal flow during regional NE-SW extension.

These contrasting styles of high strain zones and their respective microstructures help illustrate the relationship between strain distribution and deformation mechanisms at the lower crustal root of a continental arc.