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Archaean Basin Margin Geology and Crustal Evolution: an East Pilbara Traverse

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Archaean Basin Margin Geology and Crustal Evolution: an East Pilbara Traverse Research article Journal of the Geological Society Published online June 6, 2017, updated August 30, 2017 https://doi.org/10.1144/jgs2016-127 | Vol. 174 | 2017 | pp. 1090–1112 Archaean basin margin geology and crustal evolution: an East Pilbara traverse Wouter Nijman1*, Armelle Kloppenburg2 & Sjoukje T. de Vries1 1 Department of Earth Sciences, Faculty of Geosciences, Utrecht University, PO Box 80115, 3508 TC Utrecht, the Netherlands 2 4DGeo Structural Geology, Daal en Bergselaan 80, 2565 AH The Hague, the Netherlands * Correspondence: [email protected] Abstract: A palinspastic reconstruction of a 100 km long traverse through Archaean rocks of the East Pilbara, Western Australia, includes new observations of the deformation preceding the now visible greenstone belt pattern. The restoration is time-calibrated with all available U–Pb datings. Between incompletely preserved basin sequences, two superposed Palaeoarchaean volcano-sedimentary basins (the Coongan and Salgash Basins) are separated by an eastwards time-transgressive interface tentatively interpreted as an onlap surface. For over 140 Ma, the basin margin architecture was structurally controlled by superposed extensional growth fault arrays (D1) with associated dyke swarms in a curved pattern spatially not related to that of the actual distribution of granite domes and greenstone belts. The basins are interpreted to have formed by collapse after arching above hotspots due to phase transitions by mini-subduction of slabs of cooled water-saturated basalt towards the base of an originally c. 45 km mafic crust. At c. 3.31 Ga, the extension was replaced by plate-driven regional NW–SE compression (D2) inferred from NW-over-SE shear and ramp-and-flat thrusts, partly reversing offsets of the D1 extension. The recognition of widespread D2 pre-doming compression is important because it triggered the c. 3.18 Ga start of formation of the dome-and-keel pattern (D3) visible today, which culminated at c. 2.9 Ga. Supplementary material: Eight figures, numbered ‘Supplementary material fig. a–h’, and a GPS list of observation sites are available at: https://doi.org/10.6084/m9.figshare.c.3808243 Received 28 September 2016; revised 19 April 2017; accepted 21 April 2017 Throughout the last three decades of research on the Archaean, the important facies changes and, if so, are these driven by the crustal old intriguing questions about the nature of the early Earth’s crust, structural development? An early example of such an approach its rigidity and thickness, the style of its structural development and related to our study area can be found in DiMarco & Lowe (1989). In the balance between vertical diapirism (Hamilton 1998) and Utrecht University’s project the emphasis was on the relationship tangential plate motion (De Wit 1998) played, and still play, an between sedimentation and structural control, both in the Pilbara important role (e.g. Blewett 2002; McCall 2003; Hickman 2004; and the South African Barberton Greenstone Belt, a relationship for Van Kranendonk et al. 2004, 2007; Van Kranendonk 2010a; De Wit the Palaeoarchaean (3.6 – 3.2 Ga) that turned out to be difficult to et al. 2011; Van Hunen & Moyen 2012; Gerya 2016). compare with Phanerozoic settings. For the Mesoarchaean Concurrent with the extensive 1:100 000 mapping project of the (3.2 – 2.8 Ga), on the contrary, the influx of the first large Geological Survey of Western Australia (GSWA) in the Archaean volumes of coarse-clastic arkosic sandstones, both in the Pilbara Pilbara Craton of NW Australia (for summaries see Hickman and in Barberton (Zegers et al. 1998), heralded the facies 2012a; Hickman & Van Kranendonk 2012), Utrecht University (the assemblage familiar to Phanerozoic orogens related to plate motion. Netherlands) was running a project with a twofold approach: one At present, opinions diverge on the timing of the successive group working from originally deeper crustal levels of metamorphic deformation phases with respect to the onset of greenstone belt and highly deformed rocks upwards, the other from uppermost formation, the latter since Collins et al. (1998) generally related to a crustal and less metamorphic rocks downwards, in order to meet in process of crustal convective overturn. Here, we distinguish three conclusions about the structural development of the craton. successive phases of deformation: D1 extension, D2 compression Mapping at a range of scales, including detailed (c. 1:10 000) and D3 related to steepening of the greenstone sequence and mapping of relatively small key areas (Figs 1 and 2), was combined geomorphological ‘belt’ development. The term ‘phase’ should be with geochronological efforts to determine the larger-scale absolute understood as an – in comparison to the Phanerozoic – long-lasting time relationships. The work related to the Pilbara was published in episode of uniform, though possibly pulsating, structural regime. PhD theses1 (Zegers 1996; Beintema 2003; Kloppenburg 2003; De The evident and high demand for geochronological data in these Vries 2004; Strik 2004; Van den Boorn 2008) and in articles studies of Archaean rock resulted in a wealth of SHRIMP and Argon (see further references in this paper). datings. Notwithstanding the density of the sampling and the With respect to the Palaeoarchaean, important questions to be relative accuracy and precision of the age determination, the tackled were: what could be learned about the crustal structural methods are still insufficient to assess, for instance, continuity or development and the driving force behind it from sedimentological discontinuity within the stratigraphic column. Therefore, it is an characteristics such as basin shape and fill? From a sedimentolo- obvious pitfall to oversimplify stratigraphic correlations and gical perspective specifically: are we, or are we not, dealing with overlook lateral variations in stratigraphy. We present a synthesis of our bottom-up and top-down approaches, aiming to identify relationships between structural development and 1Publications of the Utrecht Pilbara project are indicated with * in the list of references. sedimentary response. To that end we revisited East Pilbara and © 2017 The Author(s). Published by The Geological Society of London. All rights reserved. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Archaean basins, East Pilbara traverse 1091 compiled a c. 100 km long east–west traverse through the craton continuation of the Doolena Gap Greenstone Belt over the (Fig. 2;locationmapinFig. 1). It extends from Bamboo Creek Talga Talga River along the south side of the Muccan through the Coppin Gap and Marble Bar belts westwards over the Dome. North Pole Dome to just beyond the Mulgandinnah Fault zone. The (3) the Marble Bar Sector, the north-striking, western part of traverse integrates data from previous publications with new the Marble Bar Greenstone Belt; observations from key locations along the traverse (Fig. 2, sites 1 – (4) the Glen Herring Synclinorium (new name after Glen 22; see GPS list of the locations in the Supplementary material). Herring Creek); and The traverse combines well-preserved Palaeo- and Mesoarchaean (5) the North Pole Dome (further abbreviated as NP Dome). structural, sedimentological and volcanological features with a To avoid unnecessary complexity in the description, the relatively good time control from U–Pb datings (Figs 2 and 3b). A name ‘Panorama Greenstone Belt’ for the greenstone cover stepwise palinspastic reconstruction of the traverse (Fig. 3a→b→c; of the NP Dome will not be used. legend in Fig. 1) allows for an interpretation of the basin configuration and concurrent lateral facies changes. In combination with interpretations of structures that formed at originally deeper Remarks on the stratigraphical correlation crustal levels, we arrive at a model of structural evolution of the The legend (Fig. 1) of the corridor map presents the dated Archaean crust of the Pilbara, which in several aspects differs from stratigraphical sequence used in this paper. The following the protocontinental model proposed by Van Kranendonk et al. correlations differ from those made in the GSWA maps: (2015a). North Pole, McPhee and Razorback Cherts: the Razorback Chert of the Coppin Gap Sector ( just south of site 11 in Fig. 2)is The East Pilbara traverse observed correlated here with the McPhee Chert in the Marble Bar Sector (sites 12 – 15) and the North Pole Chert (Dresser Fm) in the NP Corridor map and construction of the traverse Dome (site 20). As a consequence, the underlying sequence of the Razorback Chert is designated to the Talga Talga Subgroup Corridor map and traverse (De Vries et al. 2006; cf. section ‘Razorback Chert and Shear Zone’ Primary sources for the corridor map (Fig. 2) and the cross-section below). (Fig. 3a)are: Mount Ada Basalt, Apex Fm and the Antarctic Creek Member: in (1) the new 1:100 000 GWSA geological map sheets: North the NP Dome the top of the Mt Ada Basalt is placed at the contact Shaw (2nd edn, Hickman 2012b; explanatory note by Van with the, in places discontinuous, felsic volcanic/chert unit known Kranendonk 2000), Marble Bar (Hickman & Van as the Antarctic Creek Member (stratigraphic level of sites 10c and Kranendonk 2008), Coongan (Van Kranendonk 2004; 22) of the Mt Ada Basalt (Australian Stratigraphic Units Data Base). 207 206 explanatory note by Van Kranendonk 2010b), Muccan The age of the Antarctic Creek Member ( Pb/ Pb age of 3470.1 (Williams 1998; explanatory note by Williams 1999) and ± 1.9 Ma, Byerly et al. 2002) corresponds with that of the lower part Mt Edgar (map and explanatory note by Williams & Bagas of the Duffer Fm further to the east in the Bamboo Creek Sector. We 2007a, b); notice that these map sheets will be further consider the felsic volcanic component of the member as a pinch- mentioned in the text without references; out of the Duffer Fm between the Mt Ada Fm and the next younger – – 207 206 (2) new field observations, air photo and satellite image basalt unit, the Apex Fm.
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