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| | | | AH Hickman | | | | | Event type | tectonic: collisional orogeny | Parent event | | Child events | | Tectonic units affected | | Tectonic setting | basin: retroarc basin | Metamorphic facies | | Metamorphic/tectonic features | –– |
| | | Summary | The De Grey Event comprises tectonic and magmatic processes in the Pilbara Craton between the Prinsep Orogeny and the end of the North Pilbara Orogeny. All these processes, spanning a total of 150 Ma, are interpreted to be consequences of the subduction of an exotic plate under the northwestern margin of the craton. This subduction commenced at c. 3066 Ma following collision of the West Pilbara Superterrane with the East Pilbara Terrane. The amalgamation of these terranes closed the intervening Regal Basin, destroying a subduction zone that had developed the magmatic arc of the 3130–3093 Ma Sholl Terrane. Until the Prinsep Orogeny, compression of the Pilbara Craton due to interaction with the northwestern plate had been accommodated by the subduction zone within the Regal Basin. Following the orogeny, ongoing convergence resulted in a new subduction zone forming along the northwestern margin of the Pilbara Craton.
The northwestern subduction zone produced successive magmatic arcs in the northwestern Pilbara. Surviving remnants of these are preserved as the 3024–3007 Ma Orpheus Supersuite, the 3006–2982 Ma Maitland River Supersuite and the 2954–2919 Ma Sisters Supersuite. Detrital zircon ages in formations deposited after the Prinsep Orogeny suggest the existence of an earlier, c. 3066 to 3050 Ma supersuite. However, this has not yet been identified by mapping and geochronology, and it might be concealed off the northwestern Pilbara coast. Retro-arc and back-arc basins southeast of the magmatic arcs include the 3066–3015 Ma Gorge Creek Basin (sedimentary), the 3010-2991 Ma Whim Creek Basin (volcano-sedimentary) and the 3015–2931 Ma Mallina Basin (mainly sedimentary). Stratigraphic relations between the Whim Creek and Mallina Basins are obscured by a major strike-slip fault (Loudens Fault) along the contact. Likewise, the 2955–2941 Ma Bookingarra Group assigned to the northwestern shelf of the Mallina Basin is in tectonic contact with the central part of the basin. | | | Distribution | Dating from amalgamation of the East Pilbara Terrane and the West Pilbara Superterrane, the De Grey Event affected most parts of the northern Pilbara Craton. Only the Kurrana Terrane and the Mosquito Creek Basin, which were not connected to the East Pilbara Terrane until c. 2900 Ma, remained isolated from the event. Due to control of the event by the northwestern subduction, the geology of the northwestern Pilbara Craton was far more affected than that of the East Pilbara Craton. However, the Gorge Creek and Mallina Basins extend into the east Pilbara, as do granitic intrusions of the Sisters Supersuite. Mesoarchean reactivation of the Paleoarchean dome-and-keel crustal architecture of the East Pilbara Terrane is also interpreted to have been part of the event, possibly related to gravitational loading of the crust by deposition of the Gorge Creek and Croydon Groups, combined with crustal heating, resulting from an absence of volcanism in the east Pilbara between c. 3165 and 3000 Ma. | | | Description | Tectonic, magmatic and depositional processes between the c. 3070 Ma Prinsep Orogeny and the end of the North Pilbara Orogeny at c. 2919 Ma were directly related to ongoing collisional interactions of the northwestern Pilbara Craton with an exotic plate that had been converging from at least c. 3160 Ma (Krapež and Eisenlohr, 1998; Smith et al., 1998; Blewett, 2002; Van Kranendonk et al., 2002; Beintema, 2003; Smith, 2003; Hickman, 2004, 2012, 2016, 2021; Pike et al., 2006; Hickman et al., 2006, 2010). Amphibolite facies metamorphism during the c. 3160 Ma Karratha Event terminated and reversed previous plate separation of the East Pilbara and Karratha Terranes and is interpreted to have resulted from the initial collision (Hickman, 2016). Between 3160 and 3070 Ma, compression of the northwestern Pilbara Craton was accommodated by progressive closure of the Regal Basin. Once the basin closed between 3093 and 3070 Ma, the northwestern plate began to be subducted under the northwestern margin of the Pilbara Craton.
The northwestern subduction zone produced successive magmatic arcs in the northwest Pilbara. Surviving remnants of these are preserved as the 3024–3007 Ma Orpheus Supersuite, the 3006-2982 Ma Maitland River Supersuite and the 2954–2919 Ma Sisters Supersuite. Detrital zircon ages in formations deposited after the Prinsep Orogeny suggest the existence of an earlier, c. 3066 to 3050 Ma supersuite. However, this has not yet been identified by mapping and geochronology, and it might be concealed off the northwestern Pilbara coast. Large retro-arc and back-arc basins southeast of the magmatic arcs include the 3066–3015 Ma Gorge Creek Basin (sedimentary), the 3010-2991 Ma Whim Creek Basin (volcano-sedimentary) and the 3015–2931 Ma Mallina Basin (mainly sedimentary) (Hickman, 2016). Original stratigraphic relations between the Whim Creek and Mallina Basins are uncertain due a major strike-slip fault (Loudens Fault) along the contact. This fault juxtaposed successions deposited over 100 km apart. Likewise, the 2955–2941 Ma Bookingarra Group, assigned to the northwestern shelf of the Mallina Basin, is in tectonic contact with the central part of the basin. The volcanic formations of this group have a total thickness of 2 km, but there are no volcanic units >100 m thick in the central part of the Mallina Basin.
Northwesterly to southeasterly convergence during the De Grey Event was the cause for most of the complex structural geology of the Central Pilbara Tectonic Zone, but deformation also took place in the east Pilbara. The earliest structures recognized are syndepositional extensional faults in the Farrel Quartzite (Van Kranendonk, 2004, 2010; Van Kranendonk et al., 2006) and Cleaverville Formation of the Gorge Creek Basin. Smithies (2004) reported fractures in the Cleaverville Formation on the northeastern Pilbara through which hydrothermal fluids deposited layers of dark-grey, fine-grained silica up to 0.5 m thick. In the northwestern Pilbara, numerous dolerite, dacite, and granophyre sills and dykes within the Cleaverville Formation are interpreted to have filled extensional fractures during intrusion of the Orpheus Supersuite. Hickman (2016) assigned these extensional structures to D3 in the northwestern Pilbara and subsequent deformation episodes are referred to using the same structural interpretation. Later structures in the northwestern Pilbara assigned to D4 (Hickman, 2016) are upright and tight to isoclinal folds in the Cleaverville Formation. At Mount Ada, 15 km south of Roebourne, east-southeasterly trending upright folds deform the Cleaverville Formation and a sill of the c. 3014 Ma Stone Yard Granophyre (Orpheus Supersuite). The minimum age of these structures is constrained by the fact that they are unconformably overlain by the c. 3010 Ma Warambie Basalt (Whim Creek Group). The proximity these folds to the Sholl Shear Zone suggests a 3015–3010 Ma phase of movement along the shear zone due to northerly to southerly compression. Locally thick sandstone units in the Warambie Basalt adjacent to the Sholl Shear Zone indicate deposition at the base of a fault scarp (Hickman, 2002).
Low-angle thrusts in the Warambie Basalt east of Mount Ada (Hickman, 2002) are likely to be local structures, produced by c. 3010 Ma movement along the Sholl Shear Zone, suggesting ongoing northerly to southerly compression of the northwestern Pilbara. No thrusting has been recognized in the 3009–2991 Ma Red Hill Volcanics overlying the Warambie Basalt. The deformation of the Warambie Basalt was assigned to D5 by Hickman (2016) and isoclinal folds in the lower part of the Croydon Group in the Mallina Basin might also be D5 structures. Smithies (1998) recorded early easterly to westerly trending folds in the Mallina Basin that deform the Constantine Sandstone and underlying units of the Gorge Creek Group, which are folded by later northerly trending folds, such as the D6 Powereena Anticline. Mapping on MOUNT WOHLER and SATIRIST reveals that the D5 folds are isoclinal and were possibly formed as recumbent folds. Northerly to north-northeasterly trending tight, upright and locally overturned folds (D2 of Smithies, 1998; D6 of Hickman, 2016) in the central part of the Mallina Basin include the Powereena Anticline, Croydon Anticline, and large folds northwest of the Croydon Anticline and south of the Satirist Monzogranite. A steep axial plane schistosity (S6) cuts across refolded D5 folds. The c. 2948 Ma Peawah Granodiorite cuts across the D6 folds and the structures are inferred to have formed at c. 2955 Ma. D6 and subsequent deformation and metamorphism, and igneous intrusion in the northwesterly Pilbara formed part of the 2955–2919 Ma North Pilbara Orogeny.
The main deformation event between c. 3015 and 2955 Ma in the eastern Pilbara was a major re-activation of the dome-and-keel structures that had formed between 450 and 200 Ma earlier in the East Pilbara Terrane east of the Lalla Rookh – Western Shaw Structural Corridor. The Mesoarchean doming took place after deposition of the 3022–3015 Ma Cleaverville Formation, because this is strongly deformed around the margins of many domes. Evidence that deformation of the Cleaverville Formation commenced shortly after its deposition is provided by an erosional unconformity separating it from the c. 3015 Ma Cundaline Formation in the Marble Bar greenstone belt (Williams, 1999). The minimum age of the doming is poorly constrained and may have differed between domes. In some greenstone belts, the Cleaverville Formation is inclined moderately to steeply away from the centres of domes, but in others it is subvertical within narrow, deep graben-like structures of the type described by Hickman (2001). An excellent example of this is at Coppin Gap (Zone 51, MGA 200190E 7688040N), where subvertical and tight to isoclinally folded beds of the Cleaverville Formation occupy the faulted boundary between the Mount Edgar and Muccan Domes. Vertical deformation of the east Pilbara crust is not readily related to the horizontal deformation in the northwestern Pilbara during the De Grey Event. Instead, it is likely that the Mesoarchean doming was a response to the same crustal conditions that produced the Paleoarchean doming, namely, gravitational pressures on the crust (in this instance, resulting from deposition of the Gorge Creek Group) combined with crustal heating. Rising temperatures in the Mesoarchean east Pilbara crust are likely through an absence of volcanic heat loss between c. 3165 and 3000 Ma and for reasons given in the 'conductive incubation' model of Sandiford et al. (2004). Conductive incubation involved the burial of radiogenic heat-producing elements beneath an accumulating supracrustal succession.
Deformation in the east Pilbara that was related to the northwesterly to southeasterly compression of the craton during the De Grey Event includes tight to isoclinal folding in the Wodgina, Pilbara Well, Cheearra, East Strelley, Pincunah and Goldsworthy greenstone belts. It is notable that all this deformation, assigned to east Pilbara deformation D13 by Hickman (2021), was confined to those parts of the East Pilbara Terrane west of the Lalla Rookh – Western Shaw Structural Corridor, where extension during the 3280–3165 Ma East Pilbara Terrane Rifting Event is interpreted to have substantially reduced the thickness of the crust. This distribution means that there is no obvious interference between these horizontal D13 structures and the Mesoarchean domes, although they appear to have formed at about the same time. The original orientation of the tight to isoclinal folds indicates northerly to southerly or north-northeasterly to south-southwesterly compression. Folds and faults that deform D13 structures in greenstone belts west of the Lalla Rookh – Western Shaw Structural Corridor are assigned to D14 (Hickman, 2021). The folds trend north-northeasterly, are upright, open to tight and have subvertical axial planes (Van Kranendonk et al., 2010). A crenulation cleavage is locally developed where D14 deforms S13. Based on a structural correlation with D6 folds in the Mallina Basin, the age of D14 structures is interpreted to be about 2955 Ma and part of the North Pilbara Orogeny. The difference between the trend of D13 and D14 indicates west-northwesterly to east-southeasterly compression. | | | | | | Geochronology | | | De Grey Event | Maximum age | Minimum age | Age (Ma) | 3066 | 2919 | Age | Mesoarchean | Mesoarchean | Age data type | Inferred | | References | | |
| The maximum age of the De Grey Event is constrained by a date of 3066 ± 4 Ma (GSWA 169016, Nelson, 2002) on the Elizabeth Hill Supersuite intruded during the Prinsep Orogeny. The minimum age of the event is indicated by a date of 2919 ± 3 Ma (GSWA 142883, Nelson, 1998) on the youngest intrusion of the Sisters Supersuite, which was intruded at the end of the North Pilbara Orogeny. | | | Tectonic Setting | The De Grey Event spanned a c. 150 Ma period during which subduction on the northwestern margin of the Pilbara Craton produced a series of magmatic arcs and related retro-arc and back-arc basins. The earliest arcs formed along the northwestern side of the craton, probably about 150 to 200 km southeast of the subduction zone, but migrated southeast during the event (Hickman, 2016). Two of the three basins (Gorge Creek and Mallina) extended across most of the northern Pilbara Craton. The third basin (Whim Creek) comprised volcanic and intrusive rocks and was constricted to a belt immediately southeast of the arc, represented by granitic intrusions of the Maitland River Supersuite. Compression and closure of the Mallina Basin resulted in the North Pilbara Orogeny between 2955 and 2919 Ma. This orogeny completed the evolution of the crust in the northwestern and central parts of the northern Pilbara Craton, although post-orogenic fractionated granitic intrusions, possibly related to a migrating hot spot (Hickman, 2016), were emplaced at about 2850 Ma. | | | | 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. | Blewett, RS 2002, Archaean tectonic processes: a case for horizontal shortening in the North Pilbara granite–greenstone terrane, Western Australia: Precambrian Research, v. 113, no. 1–2, p. 87–120. | Hickman, AH 2001, East Pilbara diapirism: New evidence from mapping, in GSWA 2001 extended abstracts: new geological data for WA explorers edited by Geological Survey of Western Australia: Geological Survey of Western Australia, Record 2001/5, p. 23–25. View Reference | Hickman, AH 2002, Geology of the Roebourne 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 35p. 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 2012, Review of the Pilbara Craton and Fortescue Basin, Western Australia: Crustal evolution providing environments for early life: Island Arc, v. 21, p. 1–31. | 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, Interpreted bedrock geology of the east Pilbara Craton (version 2) (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, Huston, DL, Van Kranendonk, MJ and Smithies, RH 2006, Geology and mineralization of the west Pilbara — a field guide: Geological Survey of Western Australia, Record 2006/17, 50p. 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 | Krapež, B and Eisenlohr, B 1998, Tectonic settings of Archaean (3325–2775 Ma) crustal–supracrustal belts in the West Pilbara Block: Precambrian Research, v. 88, p. 173–205. | Nelson, DR 1998, 142883.1: foliated porphyritic syenogranite dyke, South of Mulgandinna Hill; Geochronology Record 352: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Nelson, DR 2002, 169016.1: foliated leucocratic biotite quartz diorite, Mount Gratwick; Geochronology Record 140: Geological Survey of Western Australia, <www.dmpe.wa.gov.au/geochron>. View Reference | Pike, G, Cas, RAF and Hickman, AH 2006, Archean volcanic and sedimentary rocks of the Whim Creek greenstone belt, Pilbara Craton, Western Australia: Geological Survey of Western Australia, Report 101, 104p. View Reference | Sandiford, M, Van Kranendonk, MJ and Bodorkos, S 2004, Conductive incubation and the origin of dome-and-keel structure in Archean granite–greenstone terrains: A model based on the eastern Pilbara Craton, Western Australia: Tectonics, v. 23, no. 1, doi:10.1029/2002TC001452. | Smithies, RH 1998, Geology of the Mount Wohler 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 19p. View Reference | Smithies, RH 2004, Geology of the De Grey and Pardoo 1:100 000 sheets: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 23p. 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 2004, Geology of the Carlindie 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 45p. View Reference | Van Kranendonk, MJ 2010, Geology of the Coongan 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 67p. View Reference | 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. | Van Kranendonk, MJ, Hickman, AH, Smithies, RH, Williams, IR, Bagas, L and Farrell, TR 2006, Revised lithostratigraphy of Archean supracrustal and intrusive rocks in the northern Pilbara Craton, Western Australia: Geological Survey of Western Australia, Record 2006/15, 57p. View Reference | Van Kranendonk, MJ, Smithies, RH, Hickman, AH, Wingate, MTD and Bodorkos, S 2010, Evidence for Mesoarchean (~3.2 Ga) rifting of the Pilbara Craton: The missing link in an early Precambrian Wilson cycle: Precambrian Research, v. 177, no. 1–2, p. 145–161, doi:10.1016/j.precamres.2009.11.007. | Williams, IR 1999, Geology of the Muccan 1:100 000 sheet: Geological Survey of Western Australia, 1:100 000 Geological Series Explanatory Notes, 39p. View Reference |
| | | | Recommended reference for this publication | Hickman, AH 2024, De Grey Event (PCDG): 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 28 May 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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