Validación de la propuesta

Field observations and structural correlations reveal that a third phase of deformation (D3) is preserved as narrow (≤10 m-thick) greenschist-facies, gently-moderately dipping, strike-slip or normal-sense shear zones. These structures are localized near the Cozette-Misty plutonic contact near Camp 2, in a screen of the Cozette pluton and along the eastern margin of the Misty pluton at Coronation Saddle, and in an exposure of the Cozette pluton north of Te Au Saddle. These shear zones deform S₂/L₂, and are themselves reactivated by fourth-phase brittle and semi-brittle faults.

S3/L3 shear zones have well-constrained cross-cutting relationships with the other deformation events presented in Table 1, but the tectonic environment between the cessation of movement along the D2 MSZ at c. 120 Ma (Schwartz et al., 2015) and the occurrence of D₄ [perhaps as young as 7-9 Ma based on muscovite and biotite ages reported by Nathan et al. (2000) at Milford Sound] leaves a very large time frame during which D3 could have occurred. In Fiordland during this time frame, the tectonic environment transitioned from transpressional with emplacement of the WFO and SPS, to extensional collapse and rifting of Gondwana, to transtension, plate boundary reorganization and the transition to the modern dextral transpressive setting. In the

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absence of absolute age data, we rely on the geometric, kinematic, and textural characteristics of S3/L3 shear zones to place them within Fiordland’s tectonic history between the Early Cretaceous and late Tertiary.

Structures within Fiordland recording transpression or convergence involving the WFO prior to orogenic collapse are dominantly steeply dipping, including the Indecision Creek and George Sound shear zones (Marcotte et al., 2005; Klepeis et al., 2004). The exception is the Caswell fold and thrust belt (Fig. 3.2), which records arc-normal displacements as folds and thrusts that sole into a subhorizontal shear zone at the upper contact of the WFO with its host rock (Daczko et al., 2002; Klepeis et al., 2004).

Daczko et al. (2002) document mylonitic and ultramylonitic foliations with gently-plunging, west-trending mineral lineations in a zone of imbricate thrusts at Caswell Sound, consistent with the mylonitic-ultramylonitic textures in our S3/L3 shear zones.

Like S₃/L₃ shear zones, the imbricate thrusts deform the WFO and its host rock, but the mineral assemblage associated with deformation at Caswell Sound consists of garnet, biotite, plagioclase, K-feldspar, and quartz, consistent with granulite facies conditions at the WFO-host rock contact (Daczko et al., 2002). Mineral growth during D₃ in our field area is limited to biotite and limited chlorite, and dynamic recrystallization of quartz and feldspar is accommodated dominantly via BLG and SGR in quartz, and core-mantle structures in feldspar. These observations are more consistent with deformation at greenschist facies conditions, in the realm of 450-500 °C (Stipp et al., 2002; Passchier and Trouw, 2005).

Although the geometry of S3 and L3 are compatible with the zone of imbricate thrust faulting described by Daczko et al. (2002), and both structures are localized along

the margin of the WFO, we do not observe evidence of mineral growth or microstructures compatible with granulite facies conditions, and the kinematics of our S3/L3 shear zones show normal components of offset, with top-down-to-the west/southwest sense of shear.

This is opposite of the thrust sense observed in the Caswell fold-thrust belt. For these reasons we suggest that D3 is unrelated to Cretaceous contractional or transpressional deformation.

Like S3/L3 shear zones in our field area, extensional structures in Fiordland have a normal component of deformation on gently dipping shear zones. Included are the Doubtful Sound, Resolution Island, and Mount Irene shear zones (DSSZ, RISZ, MISZ) (Gibson et al., 1988; Gibson and Ireland, 1995; Klepeis et al., 2007; Scott and Cooper, 2006) (Fig. 3.2). The DSSZ and RISZ are upper amphibolite facies, mylonitic shear zones separating the WFO (footwall) from its dominantly Paleozoic host rock. Both shear zones preserve top-down-to-the northeast and top-down-to-the southwest sense of shear.

Although S3/L3 shear zones preserve evidence of lower-grade, greenschist facies deformation, the geometry and kinematics of the DSSZ and RISZ are compatible with structures formed during our D₃ event.

The MISZ is approximately along-strike of our D3 shear zones (Fig. 3.3), and records amphibolite–greenschist facies conditions on a shear zone recording top-down-to-the southwest sense of shear (Scott and Cooper, 2006). The MISZ deforms an early foliation in the Robin gneiss and Irene Complex in the same way our S3/L3 shear zones deform S₂. Scott and Cooper (2006) call upon differences in equilibrium assemblages in the upper and lower plates of the shear zone to suggest that displacements are consistent with a metamorphic core complex model. Although the timing of our D₃ event may

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coincide with inferred movement along the Mount Irene shear zone, we see no evidence for different metamorphic grade across S3/L3 shear zones, and their narrow, anastomosing geometry is inconsistent with a single detachment accommodating large displacements as interpreted by Scott and Cooper (2006)

Even though our S3/L3 shear zones are narrow structures, likely with minimal displacements, the similarities in kinematics and geometry with the DSSZ, RISZ, and MSZ suggests that S3/L3 shear zones formed during extensional orogenic collapse during the Early Cretaceous. When paired with evidence of syn-tectonic dikes within the DSSZ dated at 101.4 ± 1.7 Ma (Klepeis et al., 2016) and 102.1 ± 1.8 Ma (Klepeis et al., 2007), and a syn-kinematic dike in the MSZ dated at c. 108 Ma (Scott and Cooper, 2006), it is conceivable that D₃ occurred at broadly the same time in our field area.

Finally, it is worthwhile to consider the relationship between D3 in our field area and Tertiary faults in Fiordland. Evidence of transtensional deformation related to rifting is limited in Fiordland, but Newman (2014) identified a fault system compatible with an approximately east-west trending extension direction. This is subparallel to our L3

mineral stretching lineation, and like transtensional faults identified by Newman (2014), our S3/L3 shear zones are extensively reactivated to accommodate younger brittle and semi-brittle faulting. Although the transtensional faults are generally much steeper than our S3/L3 shear zones, the extension directions and relative timing are compatible and we therefore do not rule out the possibility that our faults represent a transtensional phase of Fiordland’s history prior to the development of the modern, dextral transpressive tectonic regime.