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| | | | | | | | | | | | | | | | | | | | | | | | Department of Mines, Industry Regulation and Safety |
| | | | Geological Survey of Western Australia |
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| | | | AH Hickman | | | | | Event type | tectonic: collisional orogeny | Parent event | | Child events | | Tectonic units affected | | Tectonic setting | orogen: collisional orogen | Metamorphic facies | | amphibolite: green hornblende |
| Metamorphic/tectonic features | cataclastic; folded; foliated; mylonitic; schistose |
| | | Summary | The Karratha Event was a 3165–3144 Ma metamorphic event in the northwestern Pilbara Craton. The event was first identified in the Karratha Terrane when a 3160 ± 158 Ma date (K−Ar, hornblende) was obtained from the 3270–3261 Ma Karratha Granodiorite. Additional evidence of a thermotectonic event at this time came from 3156 ± 4 Ma and 3152 ± 5 Ma dates on zircons in the same granodiorite. Subsequently, a 3144 ± 35 Ma ⁴⁰Ar/³⁹Ar age from the 3280–3270 Ma Ruth Well Formation adjacent to the Karratha Granodiorite was interpreted as the cooling age of amphibolite-facies metamorphism.
The timing of the Karratha Event coincides with a major change in the evolution of the Central Pilbara Tectonic Zone. Rifting and continental breakup of the East Pilbara Terrane (East Pilbara Terrane Rifting Event) between 3280 and 3165 Ma took place in three stages, the last of which involved plate separation of the Karratha and East Pilbara Terranes. This formed the Regal Basin, a rift basin composed of MORB-like basaltic crust. However, plate separation was abruptly terminated at about 3165 Ma when the Karratha and East Pilbara Terranes began to reconverge. This change is interpreted to have resulted from the collision between the Karratha Terrane and a large plate northwest of the Pilbara Craton. Continued convergence of this northwestern plate over the next 250 Ma was responsible for most features of the Mesoarchean evolution of the Pilbara Craton. | | | Distribution | Metamorphism between 3165 and 3144 Ma is recorded in the Karratha Granodiorite (Kiyokawa, 1993; Kiyokawa and Taira, 1998; Smith et al., 1998), Ruth Well Formation (Beintema, 2003) and granitic rocks within the Tabba Tabba Shear Zone (Beintema, 2003). Kloppenburg (2003) recorded evidence of a c. 3150 Ma metamorphic event in the Limestone Shear Zone of the Mount Edgar Dome (East Pilbara Terrane) and U–Pb data indicate a disturbance event in the Golden Eagle Orthogneiss at c. 3150 Ma (GSWA 178012, Nelson, 2005). However, separation of the East Pilbara and Karratha Terranes at this time does not favour a direct correlation with the Karratha Event. | | | Description | Metamorphism of the Karratha Event coincides with the earliest deformation in the northwestern Pilbara Craton (Smith et al., 1998; Hickman, 2001, 2004, 2016; Hickman et al., 2001; Van Kranendonk et al., 2002; Beintema, 2003). Tectonic structures include low-angle thrusts and recumbent folds extensively developed in the 3220–3160 Ma Nickol River Formation and in the c. 3200 Ma Regal Formation. The largest fault is the Regal Thrust at the base of the Regal Formation, which is interpreted to have obducted the Regal Formation across the Nickol River Formation and the Karratha Terrane across an area of at least 3000 km² (Hickman et al., 2010). Large-scale isoclinal folds in the Nickol River Formation southeast from Mount Regal are interpreted to have been formed as recumbent folds, but were later deformed during folding of the Prinsep Dome. Intrafolial isoclines are relatively common within the Nickol River Formation of the Mount Regal area (Hickman et al., 2000; Hickman, 2001), which are probably minor structures related to the recumbent folds.
A bedding-parallel tectonic foliation (S1) preserved in metasedimentary rocks of the Nickol River Formation (Hickman et al., 2000; Hickman, 2001) and in metabasalt of the Regal Formation is interpreted to have initially formed parallel to the thrusts, but was reactivated by parallel shearing during later tectonic events. The Regal Thrust is exposed 13 km southeast of Karratha on the southern limb of the Prinsep Dome. Here, a finely laminated silicic mylonite has been isoclinally folded and the isoclinal folds have been refolded by later tight to isoclinal folds (Hickman et al., 2010). Because deformation and metamorphism related to the Karratha Event are spatially associated with the Regal Thrust, it is likely that the event was closely related to the obduction (Hickman, 2004).
In the east Pilbara Craton, there is evidence of a disturbance event at c. 3150 Ma (Kloppenburg, 2003; GSWA 178012, Nelson, 2005). It is possible that prior to c. 3130 Ma subduction in the Regal Basin, convergence of the northwestern plate resulted in minor compression of the East Pilbara Terrane and Mosquito Creek Basin. | | | | | | Geochronology | | | Karratha Event | Maximum age | Minimum age | Age (Ma) | 3165 | 3144 | Age | Mesoarchean | Mesoarchean | Age data type | Inferred | | References | | |
| The maximum age of the Karratha Event is constrained by geochronological data from the 3223–3165 Ma Soanesville Group that was deposited during crustal extension of the East Pilbara Terrane Rifting Event (Hickman, 2016, 2021). The Karratha Event took place during plate convergence that commenced at about the time when the c. 3165 Ma Flat Rocks Tonalite intruded the Empress Formation at the top of the Soanesville Group. The composition of the tonalite and isotope data (Sm–Nd and Lu–Hf) suggest it was derived by partial melting of juvenile basaltic crust along the contact of the East Pilbara Terrane and the Regal Basin (Hickman, 2021). U–Pb zircon dates on the Flat Rocks Tonalite are 3166 ± 5 Ma (GSWA 142948, Nelson, 2000b) and 3164 ± 4 Ma (GSWA 142946, Nelson, 2000a). Accordingly, the maximum age of the Karratha Event is inferred to be c. 3165 Ma.
Metamorphism in the northwestern Pilbara was dated at 3160 ± 158 Ma (K−Ar, hornblende) by Kiyokawa (1993) and 3144 ± 35 Ma (⁴⁰Ar/³⁹Ar age) by Beintema (2003). Additional evidence of a metamorphic event was indicated by 3156 ± 4 Ma and 3152 ± 5 Ma dates on zircons from a c. 3265 Ma granodiorite (JS43, Smith et al., 1998). Smith et al. (1998) interpreted the c. 3155 Ma age of these zircons to reflect thermotectonic resetting during deformation and metamorphism.
The minimum age of the Karratha Event constrained by evidence that it pre-dated the 3130–3093 Ma Sholl Terrane. The Sholl Terrane is composed of mafic–felsic volcanic rocks of the 3130–3110 Ma Whundo Group and tonalite–trondhjemite–granodiorite (TTG) intrusions of the 3130–3093 Ma Railway Supersuite. The terrane was formed above a subduction zone resulting from compression of the juvenile basaltic crust of the Regal Basin (Hickman, 2016, 2021). Extensive U–Pb zircon dating of the Whundo Group and the Railway Supersuite by Smith et al. (1998), Smith (2003) and by GSWA (reviewed by Hickman, 2016) provided no evidence for igneous activity earlier than c. 3130 Ma. Additionally, the Whundo Group does not contain the style of deformation (thrusting and recumbent folding) present in the Nickol River and Regal Formations. These lines of evidence suggest that the Karratha Event took place before c. 3130 Ma. A ⁴⁰Ar/³⁹Ar age of 3144 ± 35 Ma (KB34, Beintema, 2003) was recorded in an amphibolite of the Ruth Well Formation just north of the Sholl Shear Zone. Beintema (2003) interpreted this to be a cooling age after amphibolite facies metamorphism. The minimum age of the Karratha Event is therefore inferred to be c. 3144 Ma. | | | Tectonic Setting | The timing of the Karratha Event coincides with metamorphism and deformation when separation of the East Pilbara and Karratha Terranes was replaced by convergence (Hickman, 2016, 2021). Evidence for this change is provided by different stratigraphic and structural features of the 3223–3165 Ma Soanesville Basin, compared to those of the entirely Mesoarchean Sholl and Regal Terranes. Features in the Soanesville Basin indicate deposition during crustal extension, whereas characteristics of the Sholl and Regal Terranes indicate crustal compression. Metamorphism and deformation during the Karratha Event is consistent with collision of the Karratha Terrane with another plate to the northwest. | | | | References | Beintema, KA 2003, Geodynamic evolution of the west and central Pilbara Craton: A mid-Archean active continental margin: Geologica Ultraiectina, Utrecht University, Utrecht, The Netherlands, PhD thesis (unpublished), 248p. | Hickman, AH 2001, Geology of the Dampier 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 39p. View Reference | Hickman, AH 2004, Two contrasting granite–greenstones terranes in the Pilbara Craton, Australia: Evidence for vertical and horizontal tectonic regimes prior to 2900 Ma: Precambrian Research, v. 131, p. 153–172. | Hickman, AH 2016, Interpreted bedrock geology of the east Pilbara Craton (1:250 000 scale), in East Pilbara Craton: a record of one billion years in the growth of Archean continental crust by AH Hickman: Geological Survey of Western Australia, Report 143, Plate 1A. View Reference | Hickman, AH 2021, East Pilbara Craton: a record of one billion years in the growth of Archean continental crust: Geological Survey of Western Australia, Report 143, 187p. View Reference | Hickman, AH, Smithies, RH and Huston, DL 2000, Archaean geology of the west Pilbara granite–greenstone terrane and the Mallina Basin, Western Australia — a field guide: Geological Survey of Western Australia, Record 2000/9, 61p. View Reference | Hickman, AH, Smithies, RH, Pike, G, Farrell, TR and Beintema, KA 2001, Evolution of the West Pilbara granite–greenstone terrane and Mallina Basin, Western Australia — a field guide: Geological Survey of Western Australia, Record 2001/16, 65p. View Reference | Hickman, AH, Smithies, RH and Tyler, IM 2010, Evolution of active plate margins: West Pilbara Superterrane, De Grey Superbasin, and the Fortescue and Hamersley Basins — a field guide: Geological Survey of Western Australia, Record 2010/3, 74p. View Reference | Kiyokawa, S 1993, Stratigraphy and structural evolution of a Middle Archean greenstone belt, northwestern Pilbara Craton, Australia: University of Tokyo, PhD thesis (unpublished). | Kiyokawa, S and Taira, A 1998, The Cleaverville Group in the west Pilbara coastal granite–greenstone terrane of Western Australia: an example of a mid-Archaean immature oceanic island-arc succession: Precambrian Research, v. 88, p. 102–142. | Kloppenburg, A 2003, Structural evolution of the Marble Bar Domain, Pilbara granite-greenstone terrain, Australia: the role of Archaean mid-crustal detachments: Utrecht University, Utrecht, the Netherlands, PhD thesis (unpublished), 256p. | Nelson, DR 2000a, 142946.1: foliated biotite tonalite, Flat Rocks; Geochronology Record 297: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Nelson, DR 2000b, 142948.1: tonalite, Flat Rocks; Geochronology Record 298: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Nelson, DR 2004, 178012.1: biotite tonalite gneiss, Quartz Hill; Geochronology Record 96: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Smith, JB 2003, The episodic development of intermediate to silicic volcano-plutonic suites in the Archaean West Pilbara, Australia: Chemical Geology, v. 194, p. 275–295. | Smith, JB, Barley, ME, Groves, DI, Krapež, B, McNaughton, NJ, Bickle, MJ and Chapman, HJ 1998, The Sholl Shear Zone, West Pilbara; evidence for a domain boundary structure from integrated tectonostratigraphic analysis, SHRIMP U–Pb dating and isotopic and geochemical data of granitoids: Precambrian Research, v. 88, p. 143–172. | Van Kranendonk, MJ, Hickman, AH, Smithies, RH, Nelson, DN and Pike, G 2002, Geology and tectonic evolution of the Archaean North Pilbara terrain, Pilbara Craton, Western Australia: Economic Geology, v. 97, p. 695–732, doi:10.2113/gsecongeo.97.4.695. |
| | | | Recommended reference for this publication | Hickman, AH 2024, Karratha Event (PCK): Geological Survey of Western Australia, WA Geology Online, Explanatory Notes extract, viewed 05 August 2025. <www.dmp.wa.gov.au/ens> |
| | | This page was last modified on 05 March 2024. | | | | | Grid references in this publication refer to the Geocentric Datum of Australia 1994 (GDA94). Locations mentioned in the text are referenced using Map Grid Australia (MGA) coordinates, Zones 49 to 52. All locations are quoted to at least the nearest 100 m. Capitalized names in text refer to standard 1:100 000 map sheets, unless otherwise indicated. WAROX is GSWA’s field observation and sample database. WAROX site IDs have the format ‘ABCXXXnnnnnnSS’, where ABC = geologist username, XXX = project or map code, nnnnnn = 6 digit site number, and SS = optional alphabetic suffix (maximum 2 characters). All isotopic dates are based on U–Pb analysis of zircon and quoted with 95% uncertainties, unless stated otherwise. U–Pb measurements of GSWA samples were conducted using a sensitive high-resolution ion microprobe (SHRIMP) in the John de Laeter Centre at Curtin University, Perth, Western Australia. Digital data related to WA Geology Online, including geochronology and digital geology, are available online at the Department’s Data and Software Centre and may be viewed in map context at GeoVIEW.WA. | | | Further details of geological publications and maps produced by the Geological Survey of Western Australia are available from: Information Centre Department of Mines, Industry Regulation and Safety 100 Plain Street EAST PERTH, WA 6004 Telephone: +61 8 9222 3459 Facsimile: +61 8 9222 3444 |
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