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present in Sudbury breccia has important implications for the conditions under which the matrix was generated. Previous studies have utilised major and trace element geochemistry to argue for both in situ- and impact melt contributions to the breccia matrix (Thompson & Spray 1996; Randall 2004; Lafrance et al. 2008; Riller et al. 2010; Lieger et al. 2011). Preferential melting of mafic phases (Spray, 1992; Thompson and Spray, 1996; Plattner et al., 2003), mixing of parautochthonous melt within the footwall (Reimold 1991; Reimold et al., 2015) and injection and mixing of impact melt within the footwall (Lieger et al., 2011) have also been suggested as mechanisms to explain the differences in major element concentrations between the breccia matrix and adjacent country rock. Several previous studies have already established that rheological variations between different lithologies (such as mafic and felsic units) can create planes of weakness in the target rock that are activated during the excavation and modification stages of cratering, creating fault and fracture zones which may preferentially localize pseudotachylite development (Kenkmann et

al., 2013; Dressler 1984; Thompson and Spray 1996; Mungall and Hanley 2004) and possibly create pathways for the emplacement of impact melt into the footwall (Lieger er al., 2011). Localization of the formation of Sudbury breccia at lithological and rheological contacts would create a pseudotachylite consisting of a mixture of contributions from the host rocks. This is consistent with our models for Sudbury breccia at localities that require a mixture of local mafic and felsic footwall components, such as at McCreedy 153 East and Creighton Deep.

Although it is incorrect to assume that the results of the mixing model provide evidence of the definitive relative contributions from local footwall lithologies, the results do demonstrate that there is no need to invoke an impact melt component to construct the geochemical signature of the breccia, regardless of proximity to the SIC melt sheet (Fig. 2.8). Moreover, a wholly in-situ genetic model is complicated by the presence of transported material such as diabase clasts at West Murray Area breccia and paragneiss material at Old Creighton Town. In the latter case, gneissic material is locally present as a xenolith, <100 m from the sample site, implying a localised transport mechanism during breccia formation. Similar observations were made by Schwarzman et al. (1983) and Reimold (1991) at the Vredefort impact structure, who concluded that the 50–100 m distance between in-situ mafic wall rocks and mafic clasts hosted in a cross cutting pseudotachylite vein were the result of some transport in the breccia. This can be explained by thermal expansion or injection of melt and entrained clasts into local, low pressure, extensional zones (Melosh, 2005; Riller et al., 2010; Reimold et al., 2015). Riller et al. (2010) suggests that the latter process could have driven the injection of SIC-derived impact melt into the footwall at Sudbury, though it is also consistent with transportation of locally-derived frictional melt into tensile damage zones adjacent fault planes (Fedorowich et al., 1999; Kim et al., 2004). The presence of tensile features such as en-echelon and jig-saw fractures at Old Creighton Town, similar to those reported by several authors (Speers, 1957; Fedorowich et al., 1999; Riller et al., 2010), extending outwards from a 1–2 m wide zone of Sudbury breccia that may have created low- pressure zones that served to mobilize and mix the melt in the wider, breccia -forming zone. The injection of cataclastic and pseudotachylitic material in fault damage zones has been reported in several settings, including the West Musgrave Block, Australia (Glikson and Mernagh, 1990), and the Muddy Mountain thrust fault, Nevada (Engelder, 1974).

The pooling of melt into extensional zones may also explain the accumulation of thick zones of pseudotachylitic breccia at both the Vredefort and Sudbury impact structures. Previously, Spray and Thompson (1995) proposed that a more protracted period of fault slip, on the order of kilometers, taking place over the space of several minutes during crater excavation and modification, could produce thick accumulations of in-situ melt. Lubrication of the fault zone by the initial frictional melt (and thus termination of frictional melting) could be prevented by immediate draining of pseudotachylitic melt into extension zones within and adjacent to the fault planes (Melosh, 2005), as has been demonstrated to occur in the Okanagan Valley shear zone in Southern British Columbia (Brown et al., 2015). Furthermore, recent research by Lavallée et al. (2015) has demonstrated that fault-generated frictional melts do not display standard Newtonian viscous behaviour and that under extreme slip conditions, the low viscosity melt acts more like a viscoelastic semi-solid. If the strain rate exceeds the period of viscous response, the melt can experience lubrication failure whilst continuing to behave as a mobile, low viscosity liquid (Mungall pers. commum. 2016). This would enable frictional melting to continue, with melt being transported into fracture zones away from their origin. The presence of Sudbury breccia veins with banded, vitric margins (Fig. 2.4A, B, C) may represent the zones of frictional melting, whereas veins lacking such textures could signify cooler material that has been transported into non-melt generating fractures. This may also explain the lack of displacement observed in host rocks either side of some pseudotachylite veins at both Sudbury and Vredefort, which has been noted in several studies (Zubrigg 1957; Riller et al., 2010; Lieger et al., 2011; Reimold et al., 2015).

Another group of models call on a cataclastic process rather than a melt origin for the Sudbury breccia. Pseudotachylites are, unfortunately, highly susceptible to post emplacement recrystallization and alteration processes, owing to their fine grain size and meta-stable mineralogy (Kirkpatrick and Rowe, 2013). As a result, evidence of quenching from a melt can be obliterated in a relatively short span of geologic time. Sudbury breccia is widely documented to have acted as a preferential pathway for hydrothermal fluid systems, evidenced by the presence of post-impact hydrothermal-metamorphic titanites and amphiboles (Fleet et al., 1987; Bailey et al., 2004), feldspar alteration (Thompson et al., 1998) and the spatial association between Sudbury breccia and post-impact hydrous assemblages (Farrow and Watkinson, 1992; Hanley 2002; Hanley et al., 2004). At Halfway Lake, Sudbury breccia bears a striking resemblance to pseudotachylites found in tectonic fault zones (Rousell et al. 2003; Weirich et al. 2014; Macaudiere et al., 1985; Glikson and

Mernagh, 1990) and at the Vredefort impact structure (Lieger et al. 2011) (Fig. 2.4 A, B). On the other hand, Sudbury breccia in the South Range has a coarser matrix that could be the result of formation by cataclasis (Rousell et al. 2003). Following the impact event, the South Range was uplifted by 10 to 15 km along the South Range Shear Zone (Shanks and Schwerdtner 1991; Riller, 2005). Thus the coarser Sudbury breccia from locations such as Murray and Creighton could also be a result of a greater depth of formation compared with the North Range breccia. Although the lack of a definitive melt texture in South Range Sudbury breccia is problematic for a high temperature origin, if they are cataclastic in origin (as proposed by Rousell et al. 2003) then the breccia clasts should be more angular than for a melt breccia, due to partial melting or thermomechanical erosion in the latter scenario (Magloughlin, 1992; Lin, 1999). Image analysis of lithic fragments in Sudbury breccia from McCreedy 153 East, the West Murray Area and the Creighton Embayment show similar clast surface areas, with >75 % of clasts exceeding a roundness factor of 0.4 (eq. 5) (Fig. 2.5). This suggests thermomechanical erosion in a high temperature melt setting, rather than fragmentation or comminution under cataclastic conditions (Lin, 1999). The presence of discrete oxide inclusions, sub grain rotation and bulging recrystallization textures in South Range breccias are also suggestive of a matrix that crystallised at temperatures exceeding 700 °C (Stipp et al., 2002; Kirkpatrick and Rowe, 2013). Although this temperature exceeds lower amphibolite facies, which is considered the peak grade of post-impact metamorphic overprinting in the area (Fleet et al. 1987; Magyarosi 1998), it may reflect high temperature, contact metamorphism associated with the SIC aureole (Prevec and Cawthorn, 2002). Optically continuous patches of quartz (a.k.a ‘flood quartz’) and the sub-igneous textures observed in Sudbury breccia in the MCreedy East and Victor mines has previously been attributed to thermal overprinting by the SIC (Morrison 1994) and may also account for the lack of primary melt textures in the majority of samples from this study. Further out from the SIC and associated heat aureole, melt textures, such as vitric margins, have been better preserved, as noted in Sudbury breccia at Halfway Lake and in quartz diorite at the Trill offset dike by Smith et al. (2013) and Coulter (2016).