Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

Abstract

Recovering the time-evolving relationship between arc magmatism and deformation, and the influence of anisotropies (inherited foliations, crustal-scale features, and thermal gradients), is critical for interpreting the location, timing, and geometry of transpressional structures in continental arcs. We investigated these themes of magma-deformation interactions and preexisting anisotropies within a middle- and lower-crustal section of Cretaceous arc crust coinciding with a Paleozoic boundary in central Fiordland, New Zealand. We present new structural mapping and results of Zr-in-titanite thermometry and U-Pb zircon and titanite geochronology from an Early Cretaceous batholith and its host rock. The data reveal how the expression of transpression in the middle and lower crust of a continental magmatic arc evolved during emplace­ment and crystallization of the ~2300 km2 lower-crustal Western Fiordland Orthogneiss (WFO) batholith. Two structures within Fiordland’s architecture of transpressional shear zones are identified. The gently dipping Misty shear zone records syn-magmatic oblique-sinistral thrust motion between ca. 123 and ca. 118 Ma, along the lower-crustal WFO Misty Pluton margin. The subho­rizontal South Adams Burn thrust records mid-crustal arc-normal shortening between ca. 114 and ca. 111 Ma. Both structures are localized within and reactivate a recently described >10 km-wide Paleozoic crustal boundary, and show that deformation migrated upwards between ca. 118 and ca. 114 Ma. WFO emplacement and crystallization (mainly 118–115 Ma) coincided with elevated (>750 °C) middle- and lower-crustal Zr-in-titanite temperatures and the onset of mid-crustal cooling at 5.9 ± 2.0 °C Ma−1 between ca. 118 and ca. 95 Ma. We suggest that reduced strength contrasts across lower-crustal plu­ton margins during crystallization caused deformation to migrate upwards into thermally weakened rocks of the mid-crust. The migration was accompanied by partitioning of deformation into domains of arc-normal shortening in Paleozoic metasedimentary rocks and domains that combined shorten­ing and strike-slip deformation in crustal-scale subvertical, transpressional shear zones previously documented in Fiordland. U-Pb titanite dates indicate Carboniferous–Cretaceous (re)crystallization, consistent with reactivation of the inherited boundary. Our results show that spatio-temporal patterns of transpression are influenced by magma emplacement and crystallization and by the thermal structure of a reactivated boundary.

The Initiation and Growth of Transpressional Shear Zones Through Continental Arc Lithosphere, Southwest New Zealand

“…By ∼136 Ma, D 1 deformation had localized within this structure shortly following a period of magma infiltration. This magmatism was accompanied by high‐grade metamorphism that penetrated far into the interior of Gondwana, effectively weakening a thick lower crustal layer (Figure 11; Blatchford et al., 2020; N. Daczko et al., 2001; Hollis et al., 2003; Klepeis et al., 2016; Schwartz et al., 2016; Stowell et al., 2010; Tulloch et al., 2011). Magmatism also migrated west, further weakening the continental interior and helping to create a lateral strength contrast between older, cooler, and stronger middle crust of the outboard arc and hot, weak lower crust of the Gondwana margin (Figure 11).…”

“…Sense of shear indicators in the Grebe‐IC and GSSZs are abundant and described elsewhere (Allibone & Tulloch, 2008; Blatchford et al., 2020; N. R. Daczko et al., 2001; Daczko et al., 2002c; Daczko et al., 2002b; Klepeis & King, 2009; Klepeis et al., 2004; Marcotte et al., 2005; McGinn et al., 2020; Scott et al., 2011, 2014). We used the results of kinematic analyses cited in these publications, which follow excepted methods of shear zone analysis (e.g., Díaz‐Azpiroz et al.…”

“…displacements on them nearly vertical (Blatchford et al, 2020;, resulting in little tilting in an E-W direction. The unfolded profile (Figure 4b), combined with layer thicknesses and the thermobarometric data, allowed us to reconstruct the relative depths of structures more precisely than with the geobarometric data alone (Figure 5; Table 1).…”

“…In northern Fiordland, it produced the Indecision Creek shear zone (Buriticá et al., 2019; Klepeis & King, 2009; Klepeis et al., 2004; Marcotte et al., 2005), the Mt Daniel shear zone (Bhattacharya et al., 2018; Klepeis et al., 2004), and the Pembroke Thrust (N. R. Daczko et al., 2001; Klepeis & King, 2009). In central and southern Fiordland, it produced the Grebe (Buriticá et al., 2019; McGinn et al., 2020; Negrini et al., 2018; Scott et al., 2011, 2014), George Sound (Allibone, Jongens, Turnbull, et al., 2009; Klepeis et al., 2004), Straight River (King et al., 2008), and Misty (Blatchford et al., 2020) shear zones. Mid‐crustal deformation also formed the South Adams Burn (SAB) (Blatchford et al., 2020) and Caswell (Daczko et al., 2002b) thrust belts, and, in the lower crust, the Breaksea and Malaspina Domes (Betka & Klepeis, 2013; Klepeis et al., 2016; Miranda and Klepeis, 2016).…”

“…In central and southern Fiordland, it produced the Grebe (Buriticá et al., 2019; McGinn et al., 2020; Negrini et al., 2018; Scott et al., 2011, 2014), George Sound (Allibone, Jongens, Turnbull, et al., 2009; Klepeis et al., 2004), Straight River (King et al., 2008), and Misty (Blatchford et al., 2020) shear zones. Mid‐crustal deformation also formed the South Adams Burn (SAB) (Blatchford et al., 2020) and Caswell (Daczko et al., 2002b) thrust belts, and, in the lower crust, the Breaksea and Malaspina Domes (Betka & Klepeis, 2013; Klepeis et al., 2016; Miranda and Klepeis, 2016). On Stewart Island, the Gutter shear zone, the Freshwater Fault System and the Escarpment fault (Allibone & Tulloch, 2008) form part of the Early Cretaceous system.…”