DIRECT DATING OF EXHUMATIONAL FABRICS IN THE ANITA SHEAR ZONE, FIORDLAND, NEW ZEALAND

The Anita Shear Zone (ASZ) is a 4 km wide by 20 km long ductile shear zone that parallels the trace of the Alpine Fault in northern Fiordland. Host rocks to the ASZ consist of paragneiss, orthogneiss and metaperidotite, all of which contain penetrative mylonitic foliations that formed during two kinematically unrelated episodes of amphibolite facies deformation. Early fabrics contain rutile and are characterized by subhorizontal foliations that show top-down-to-the-SE displacements. Peak metamorphic conditions are 11.9 ± 1.1 kbar and 581 ± 34ºC (Klepeis et al. 1999). This fabric is truncated and transposed by a steep to subvertical fabric that formed during dextral displacement in a transpressional flow regime. Metamorphic titanite is a common, foliation-parallel accessory phase. Peak metamorphic conditions associated with the subvertical fabric are 8.7 ± 1.2 kbar and 587 ± 42ºC (Klepeis et al. 1999). These fabrics are interpreted to record extension, followed by transpression and exhumation of the deep crust during the transition from arc flare up at 124-114 Ma to extensional orogenic collapse beginning at 108-106 Ma (Klepeis et al., 1999; Czertowicz et al. 2016); however, fabrics remain undated casting uncertainty on the significance and timing of shearing and exhumation in the ASZ.

Here, we present LASS results from metamorphic zircon, titanite and rutile to clarify the metamorphic and exhumation history of the ASZ. Zircons from two paragneisses yield primarily Paleozoic and Proterozoic dates, and both also contain a population of low Th/U (<0.15) grains that yield dates of ca. 288 Ma. A calc-silicate gneiss yielded only Cretaceous metamorphic zircons with dates ranging from 93 to 119 Ma. Deconvolution of these dates yielded populations of 97.6 ± 0.8 Ma and 111.6 ± 0.6 Ma. In contrast, titanites from 5 samples yielded Miocene dates: 4 ± 23 Ma, 8.5 ± 3.7 Ma, 9.1 ± 1.4 Ma, 10.5 ± 0.6 Ma, and 19.0 ± 1.3 Ma. Scarce mid-Cretaceous and Paleocene dates are present in two of the five samples; however, dominant populations in all samples are Miocene, suggesting a major episode of recrystallization/resetting at that time. Rutile dates from two samples also give Miocene dates of 9.0 ± 3.4 Ma and 10.0 ± 1.0 Ma. These data suggest that both fabrics record a Miocene transpressional event associated with dextral movement along the Alpine Fault.