Documenting Lower-Crustal Polyphase Cooling Associated with Cretaceous Continental Breakup and Tertiary Rifting, Western Fiordland, New Zealand

Plate reconstructions document a polyphase rifting history of the Zealandia microcontinent, from Cretaceous continental breakup, to Tertiary intraplate rifting (Lamarche et al., 1997; Gaina et al., 1998; Sutherland, 1999; Cande and Stock, 2004; Schellart et al., 2006; Furlong and Kamp, 2013). Cretaceous extension led to collapse of a thickened arc in Zealandia (Klepeis et al., 2004; Stowell et al., 2014), followed by opening of the Tasman Sea (Gaina et al., 1998). Following a ~60 Ma period of tectonic quiescence, paleogeographic maps illustrate a rift system that bifurcated around an undeforming, rigid Fiordland block (Nathan et al., 1986; Turnbull et al., 1993; King and Thrasher, 1996; Turnbull et al., 2010). However, identification of several major and minor intra-Fiordland transtensional faults (Turnbull et al., 2010; Newman, 2014) suggest that the Fiordland block is much more complex than previously thought, although the amount of slip and direct relation to exhumation remain unknown. Notwithstanding a >60 Ma tectonic history of progressive rifting and crustal thinning in Zealandia, we still have a limited understanding of how the lower-crustal portion of Fiordland responded to polyphase rifting. The objective of this study is to use LASS rutile U-Pb chronology to investigate the thermal evolution of the lower crust of Western Fiordland in response to Cretaceous continental breakup and Tertiary rifting. Results from three-hundred and eighty-seven rutile 206Pb/238U dates are classified into two groups: 1) Eocene-Oligocene homogeneous age populations (group EO), and 2) overdispersed populations in which 206Pb/238U dates span >> 10 Ma and do not define a single age population (group OD). Integrating these dates with diffusion equations for simple cooling (Dodson, 1973) and reheating (Watson and Cherniak, 2013) demonstrate that the Group EO rutile U-Pb system was totally reset at T ≥ 655 °C during propagation of Tertiary intraplate rifting. At distances of 7-60 km from Group EO, the Group OD rutile U-Pb system experienced partial resetting during open-system behavior over temperatures of ~460-585 °C. Although Tertiary paleodepths remain uncertain, rutile U-Pb dates and calculated reheating temperatures suggest that the lower crust of Western Fiordland was not fully exhumed during the Cretaceous, and illustrates the importance of investigating lower-crustal exhumation with respect to major intra-Fiordland transtensional faults (re)activated in the Tertiary.