The Wangkathaa Orogeny: an example of episodic regional ‘D2’ in the late Archaean Eastern Goldfields Province, Western Australia
Introduction
A common feature to all orogenic belts is the overprinting relationships of successive structural elements through time. Structural geologists describe these elements in terms of deformation events, and commonly give labels such as D1, D2, D3 etc. to place each deformation event in time and space. Regional correlation of local deformation histories is then tied to one or more significant sets of structural elements, so that a regional deformation framework can be established.
The Eastern Goldfields Province (EGP) is a late Archaean orogenic belt that is characterised by a prominent tectonic grain extending for ∼1000 km along the eastern edge of the Yilgarn Craton in Western Australia (Fig. 1). This tectonic grain has been considered to have developed by largely progressive trans-orogen ∼E-W shortening during the so-called regional ‘D2’ at ca. 2660 Ma (Swager, 1997), or at ca. 2650 Ma (Krapež et al., 2000). The ‘D2’ structural elements are used as ‘tie’ points for regional correlations, which is a valid approach if ‘D2’ is assumed to have been a single event. Correct identification of this ‘event’ is critical in establishing the deformation chronology and framework of the EGP, as ‘D2’ has been used by most workers as a basis of event correlation.
New evidence for a complex (polyphase) regional ‘D2’ event is beginning to emerge across the EGP. Detailed structural studies around Kalgoorlie (Bateman et al., 2001, Bateman et al., 2002), Laverton (Davis, 2001), and Mulgarrie (Davis, 2002), and Scotia-Kanowna Dome areas (Davis, 2002) show that there are multiple coaxial compressive events that developed the intense NNW-trending structural grain and associated fabrics. Coaxial ‘D2’ extension has now also been recognised. For example in the Laverton area, Davis (2002) and Newton et al. (2002) have recently described a late ‘D2’ orogenic collapse event that occurred in tandem with the last stages of ENE-WSW directed regional ‘D2’ shortening.
A key issue to resolve is whether the order of these multiple events reflects the episodic behaviour of the orogen or whether the events are simply part of a progressively evolving continuum. Identifying episodic as opposed to progressive deformation is particularly difficult where the inferred plate motions remain relatively constant (viz. ∼E-W) so that successive structural elements are generally developed parallel (coaxial and/or coplanar) to one another (viz. ∼N-S).
An unconformity is a chrono-surface, and it provides evidence for a temporal break between the rock packages above and those below. In the EGP, a chrono-surface is provided by the so-called late siliciclastic basins (Kurrawang, Pig Well-Yilgangi, Penny Dam, Jones Creek, etc.), which lie unconformably or disconformably on older (>ca. 2660 Ma) greenstone and granite (Fig. 1). These late basins have been deformed and metamorphosed, and have a set of pronounced NNW-trending structural elements that have been interpreted as ‘D2’ structures (see Swager, 1997, Krapez et al., 2000, Weinberg et al., 2003). These so-called ‘D2’ structural elements are (sub)parallel to similar structures below the unconformity. It is on this basis that most workers consider regional ‘D2’ to be younger than the late basins. However, structural relationships in the Welcome Well area (Kurnalpi Terrane) and the Ora Banda Domain of the Kalgoorlie Terrane (amongst others) show that ∼E-W shortening had occurred before the deposition of the late basins, as well as after them. Therefore, regional ‘D2’ was episodic, not progressive.
Weinberg et al. (2003) named the ‘D2’ event in the Kalgoorlie Terrane the Kalgoorlie Orogen. The applicability of this term is geographically restricted to this terrane and, as defined, considers a progression of ∼E-W compression from the latest stages of ‘D2’ (D2b) to younger events (D3 and D4 of previous workers). In contrast, we introduce the term Wangkathaa Orogeny (after the aboriginal tribal group of the region), which incorporates the complete sequence of ∼E-W compressional and extensional events involved in the development of the architecture of the entire EGP, and the latest events in the Southern Cross Province to the west. Unravelling these separate and complex stages of the Wangkathaa Orogeny is important for:
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establishing regional deformation correlations and the timing of events;
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establishing fold geometries through time and their impact on the rotation and/or translation of early structures and any mineral deposits from their original position/orientation (Bateman et al., 2002);
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establishing the kinematics and timing of fault and shear zone activity;
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understanding fluid flow and the changing axes of mean-stress to identify dilational and constrictional areas, and;
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an improved understanding of Archaean tectonic processes in relation to younger and generally better understood orogens.
Section snippets
Regional structural setting of the Eastern Goldfields Province
The pronounced NNW-oriented structural trend of the EGP is developed by the regional fault pattern and elongate granitoid bodies (Gee, 1979). The regional-scale faults form an anastomosing network of high-strain zones that bound a number of terranes or structural domains (Swager et al., 1992, Myers, 1997) that are elongate or lensoid in map pattern shape, and separate different greenstone successions. From west to east across the EGP, the terranes include the Kalgoorlie, Gindalbi, Kurnalpi,
Examples of episodic ‘D2’ in the Eastern Goldfields Province
The unifying deformation feature of the eastern Yilgarn Craton is the major ∼E-W (ENE-WSW) compressive event (generally regarded as regional ‘D2’) that was superimposed on pre-existing structural elements (Table 1). Correct identification of this ‘event’ is critical in establishing the deformation framework and chronology of the EGP, as it has been used by most workers as a basis of event correlation (e.g. Swager, 1997).
New results presented here show that at least two approximately coaxial
Timing of ‘D2’ deformation and metamorphism
The Pig Well-Yilgangi basin places lower and upper limits on the timing of D2a and D2b, respectively, in the Welcome Well area of the Kurnalpi Terrane (Fig. 2). Age data for the late basins are generally not well constrained, however, the intrusion of the 2662±5 Ma Yilgangi porphyry (Nelson, 1996) into the basin constrains D2a as older and D2b is younger than this age. Additional age constraints come from cross-cutting granitoids (Fig. 2), including: the Bulla Rocks Monzogranite (2660±5 Ma; Black
Implications of an episodic (diachronous?) Wangkathaa Orogeny
New results presented here show that the climax regional event (‘D2’) was not a function of progressive ∼E-W shortening after the last components of the greenstone sequence (i.e. the late siliciclastic basins). In our postulated model the regional ‘D2’ (Wangkathaa Orogeny) was more complex. Our model for the EGP has significant implications for regional correlation of deformation events, the regional tectonic architecture, low-pressure high-temperature metamorphism, Archaean tectonic processes,
Conclusions
The ‘D2’ Wangkathaa Orogeny of the Eastern Goldfields Province involved episodic switching of the tectonic mode (compression–extension–compression) between ∼2665 and ∼2655 Ma. The sequence of events was firstly an early stage of ∼E-W shortening (D2a); followed by extension/inversion with late siliciclastic basin formation and normal fault movement (D2E), and finally; a second stage of ∼E-W (ENE-WSW) compression (D2b). The final stage of shortening inverted the late basin-bounding faults as
Acknowledgements
We would like to thank Ed Chudyk, and Tania Fomin for their processing of the seismic reflection data. Terry Brennan and David Beard provided unfailing GIS and cartographic support. Joe Mifsud provided excellent cartographic service, and Angela Riganti for providing the base map for Fig. 1. Oliver Holm and Alan Whitaker provided insightful reviews of early drafts. Journal reviews by Brett Davis and an anonymous reviewer improved the clarity of the paper. Work reported here was conducted as part
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