Rheological behaviour of mafic dykes deformed in a granite host, Wanna, Eyre Peninsula, South Australia
Introduction
Many Precambrian terrains contain large volumes of high-grade felsic gneisses that are commonly intruded by swarms of mafic dykes (e.g., Hoek, 1991; Dirks et al., 1994; Talbot and Sokoutis, 1995; Hanmer et al., 1997; Bunger et al., 2013; Dering et al., 2019). In such cases the mafic dykes are interpreted to have intruded during a period of crustal relaxation (crustal tension), prior to compressional orogenesis (Pollard et al., 1982; Hoek, 1991). Strain localized within mafic dykes and hosted in granites is known to produce inflation of dykes and boudinage (Hanmer et al., 1997). The elucidation of a structural history from dyke emplacement to their deformation commonly relies on the interpretation of structures within individual dykes and their host-rock (Hoek, 1991). These depend on several considerations such as viscosity contrast, strain, mechanical constraints and the deformation regime. Models for dyke propagation in brittle crust, predict shear failure on fault planes oriented ~30° to the dyke plane (Rubin and Gillard, 1998) and implies an apparently simple pressure-temperature paths and there may be a sequence of micro-earthquakes during progressive melt intrusion as a dyke moves upward in the mid-crust (White et al., 2019). Relatively little is known about the rheology of mafic rocks in the lower-crust, however mafic rocks are usually considered stronger than felsic ones (e.g. Wilks and Carter, 1990). However, as pointed out by Pearce et al. (2011), the production of weaker phases and grain size refinement may result in a reversal of this rheological behaviour. Similarly, Marti et al. (2017) suggest that during the high-temperature deformation of mafic rocks there is a strain-dependency on rheology, favouring viscous deformation with decreasing strain rate and increasing strain. This is also reflected in the deformation of mafic dykes already emplaced in lower-crustal rocks (Pearce et al., 2011). Where deformation occurs at high-metamorphic temperatures, mafic dykes may record boudinage structures associated with migmatites (Ghosh and Sengupta, 1999; Sims et al., 1994; Talbot and Sokoutis, 1992).
Boudinage of rigid bodies, such as mafic dykes, is generally considered to develop in mechanically layered rocks with contrasting viscosities (Goscombe and Passchier, 2003; Virgo et al., 2018) as a result of layer parallel extension (Goscombe et al., 2004). The difference in viscosity between a rigid object and its matrix has been cited as the major factor that determines the evolution of boudinage (Ghosh and Sengupta, 1999; Mandal et al., 2000; Bons et al., 2004; Goscombe et al., 2004; Rodrigues and Pamplona, 2018). In situations of high viscosity contrast, boudinage is controlled by the occurrence of fracturing with simultaneous shearing and extensional components, which affect the strained bodies in the ductile matrix (Mandal et al., 2000). A methodology for classifying end-members related to boudins was developed by Goscombe and Passchier (2003) and Goscombe et al. (2004). Nevertheless, this methodology does not clarify all boudin types, and does not account for the processes associated with the progressive development of boudinage. Establishing the sequence of events related to the boudinage process is complicated by a variety of intrinsic ambiguities and assumptions (Virgo et al., 2018). This is so when analysing a complexly deformed rock, particularly when its undeformed equivalent is not preserved (Potts and Reddy, 1999).
In this paper we are able to demonstrate the temporal relationship between multiple deformation features, developed synchronously in a set of mafic dykes. We use crosscutting relationships to establish a relative temporal order of the structural elements and representative geometrical data from a combination of observations across multiple mesoscopic outcrops. The study area is in the Kimban mobile belt, Eyre Peninsula, South Australia (Fig. 1), where there are particularly good examples of mafic dykes, emplaced in a granite host, where the dykes have been extensively boudinaged. We are not focusing on polyphase boudinage as described by Virgo et al. (2018), but processes that are occurring close to the peak of a granulite facies metamorphic event related to the Kimban Orogeny. We will first describe the nature of undeformed mafic dykes recognized in low-strain areas (Section 2.3). This will then be followed by a detailed description of structural relationships in a region of more intense Kimban strain (Section 3). Furthermore, this paper will provide insights into the tectonic evolution occurring in the eastern margin of the Gawler Craton, particularly in zones of transpressional tectonics recognized by several authors (Steward et al., 2009; Hand et al., 2007; Vassallo and Wilson, 2002; Parker et al., 1993). Emphasis will be placed on: (1) structural features that can be produced by deforming a swarm of mafic dykes; (2) an indication of the effects of stress refraction across rheological boundaries; and (3) provide a qualitative understanding of the progressive changes in the mechanical parameters of layered rocks during deformation.
Section snippets
Regional setting
The study area is located 20 km south of Port Lincoln, on the south-eastern margin of the Gawler Craton (Fig. 1), where the tectonic history is dominated by the ca. 1730–1690 Ma Kimban Orogeny (Hand et al., 2007; Parker et al., 1993). Structures related to this tectonic episode are mainly confined to a 100 km wide Kimban mobile belt and rock units associated with the Donington Suite magmatism which lies east of the Kalinjala Shear Zone (Fig. 1). The Kalinjala Shear Zone, a subvertical
Structural and temporal relationships in deformed mafic dykes and granite host
The host-rocks to the Tournefort Dykes at Wanna, are exposed on a sub-horizontal wave-polished platform, which is dominated by a megacrystic granite and a felsic orthogneiss, and is referred to as the ‘Wanna Megacrystic Granite Gneiss’ by Cowley et al. (2017). The main foliation (KS1) strikes north-northwest (Fig. 5) parallel to the trend of the Kalinjala Shear Zone. Although overprinting relationships between structures in a single lithology are present, relative timing relationships between
Formation of pegmatite and metamorphic conditions
The pegmatite veins that cross-cut the mafic dykes, or accumulated in boudin necks and within the orthogneisses suggest a local generation of partial melts during the growth of K-bearing minerals, e.g. biotite and subhedral alkali feldspar. The addition of K2O to the mafic composition suggests that melt infiltration played an important part in the rheological evolution of the mafic dykes, which may be linked to melt infiltration during prograde metamorphism. Sawyer (1991) and Tait and Harley
Discussion
The structural style of the Kimban Orogeny (1730–1690 Ma) at Wanna is subtly different from the regional deformation pattern. On a regional scale KD1 is related to the intracontinental dextral strike-slip motions and constrictional strains related to the evolving Kalinjala Shear Zone (Vassallo and Wilson, 2002). While KD2 was characterized by an east-west flattening and east-verging localized thrusts and upright cylindrical folds particularly in areas away from the Kalinjala Shear Zone (
Conclusions
The Tournefort Metadolerite dykes were emplaced after protracted extension and thermal perturbations associated with the Donington Suite magmatism. This was followed ~80 Ma later by a chronology of events that show two kinematically related episodes of deformation (KD1–KD2) under granulite facies metamorphic conditions. KD1 reflecting slightly more constrictional strains and KD2 reflects more flattening strains. Both resulted in a strong component of east-west shortening and a north-south
Author statement
Field work for this paper was undertaken by the three authors with support of Australian Research Council Grant A39533031 with further work undertaken by Wilson in 2019. Hoek contributed enlightening field discussions while supported by ARC post-doctoral funding and contributed to an initial draft manuscript; but was not involved with the current versions of the paper. The research reported here was part of Vassallo's PhD thesis. A final manuscript was put together by Wilson with revisions
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Detailed mapping of the south eastern portion of the Eyre Peninsula was initially undertaken with support of ARC Grant A39533031 with further work undertaken after the 2019 SGTSG conference at Port Lincoln. JDH contributed enlightening field discussions while supported by ARC post-doctoral funding. The research reported here was part of JJV's PhD thesis which was supported by an Australian Postgraduate Award. Thomasin Bales contributed to this work as part of her Honours project in the Wanna
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