Tectonic Settings of Archean Sedimentary Basins with possible Implications for Plate Tectonics on the early Earth Ken Eriksson Subsidence in a sedimentary basin is a product of: 1) sediment loading, and 2) tectonics. Is tectonic component related to plate tectonics?

Tectonic subsidence mechanisms and resultant sedimentary basins

Archean sedimentary associations Bimodal volcanic + terrigenous conglomerate/sandstone: Rift? Quartz arenite +/- carbonate: Intracratonic sag / passive margin? Terrigenous conglomerate/sandstone: Compressional foreland? and Collisional graben? Calcalkaline volcanic + volcaniclastic conglom./sandstone: Forearc? Sandstone, mudstone, BIF, mafic/ultramafic: Shelf to plume?

Modified from McKenzie, 1978) (From McKenzie, 1978; Wernicke, 1985) Pure Shear Model Simple Shear Model Total Subsidence (S) = Initial Rift-related Subsidence (Si) + Thermotectonic Subsidence (Sth) (Pure Shear Model Modified from McKenzie, 1978)

Widespread Thermal Subsidence represented by the Chuniespoort Group (2.6-2.5 Ga) – preserved in a structural basin Initial Subsidence in isolated grabens and half grabens represented by the Ventersdorp Group (~2.7 Ga)

P – Pretoria C – Chuniespoort V – Ventersdorp Based on Two Way Travel Times of Tinker et al. (2002) 1)Thickness of Venterdorp is ~4 km 2) Thickness of Chuniespoort is ~1.8 km P – Pretoria C – Chuniespoort V – Ventersdorp (from Tinker et al., 2002)

Sth (Chuniespoort) = 1.8km For crustal thickness: Thicknesses 1 β = S (∫s/v - ∫m) tc (∫c - ∫m) Si (Ventersdorp) = 4.0km Sth (Chuniespoort) = 1.8km S = 5.8km For crustal thickness: 30km, β = 1.23 40 km, β = 1.17 (i.e. ~ 20% extension) β = Amount of crustal extension ∫m = Mantle density (3.3g/cm3) ∫c = Crustal density (2.8g/cm3) ∫s/v = Average density of sediment/ volcanic fill (2.8g/cm3) S = Total Subsidence tc = Thickness Continental Crust (from Bickle and Eriksson,1982)

The Witwatersrand and Pongola successions (3.0-2.8 Ga) make up the “Greater Witwatersrand Basin” with greater than 10 km of Total Subsidence (based on Tankard et al., 1982)

Greater Witwatersrand Basin is interpreted as a (From Nelson et al., 1995) Greater Witwatersrand Basin is interpreted as a Retroarc Foreland Basin passing southwards into a passive margin - comparable to modern South America Overall maximum grain size decreases from northwest to southeast The two structural basins are separated by a peripheral bulge related to syndepositional crustal loading

Auriferous conglomerates are anomalous Conglomerates overlie low-angle unconformities Pebbles and heavy minerals in auriferous conglomerates (placers) were concentrated by winnowing of footwall (Blenkinsop and Eriksson, 1995) Unconformities are a product of syndepositional warping/ folding in the basin Evidence for syndepositional warping? (From Tweedie, 1986)

Other evidence for syndepositional warping/folding (From Antrobus et al., 1986) (From Antrobus and Whiteside, 1964)

Barberton Greenstone Belt Fig Tree and Moodies Groups (3.3-3.2 Ga) are interpreted by Jackson, Eriksson and Harris (1986) as foreland basin deposits associated with south-to-north crustal shortening. Others have interpreted the Moodies Group as rift deposits (from Jackson, Eriksson and Harris, 1986)

Evidence for crustal shortening Northern Barberton Greenstone Belt Southern Barberton Greenstone Belt Evidence for crustal shortening Syndepositional folding and thrust faulting 2. Intraformational unconformities 3. Recycling

Moodies conglomerates are dominated by chert clasts and subordinate mafic and felsic volcanic, granitic, jaspilitic clasts + rare clasts of conglomerate and botryoidal vein quartz Clast types imply recycling of Onverwacht and progressive unroofing of granitic rocks + recycling of Fig Tree and Moodies

Fig Tree sandstones (graywackes) are lithic and Moodies sandstones are quartzose This trend indicates recycling of Onverwacht and progressive unroofing of granitoids Fig Tree

Moodies Moodies Moodies developed in shallow-water alluvial and tidal environments – “Molasse” Fig Tree developed in deeper water environments – “Flysch” Fig Tree

Pilbara Block, Western Australia Late-stage strike-slip faults (From Krapez and Barley, 1988) Pilbara Block, Western Australia From Krapez, 1984 Late-stage strike-slip faults transect the Pilbara Block Lalla Rookh Basin (~3.0 Ga) is interpreted by Krapez as a pull-apart basin

Evidence for marginal fault control on basin development Great thickness relative to basin size Elongate shape of basin Asymmetry of facies distribution Vertical stacking of facies Dominance of longitudinal infill

Superior Province is transected by late-stage, east-west trending (From Mueller and Corcoran, 1998) Superior Province is transected by late-stage, east-west trending strike-slip faults Duparquet Basin is one of a number of interpreted strike-slip basin deposits in the Superior Province with similar characteristics to the Lalla Rookh Basin in the Pilbara (From Mueller and Corcoran , 1998)

CONCLUSION On the modern Earth, sedimentary basins are a product of crustal thinning, thermal contraction, thrust loading and pull-apart related to mantle convection and plate tectonics. Recognition of these basin types in the Archean record may similarly imply plume activity and relative plate motions.