Crustal Evolution and Deformation in a Non-Plate-Tectonic Archaean Earth: Comparisons with Venus

Abstract

Evidence for modern plate tectonics in the Archaean is equivocal to absent, and alternative environments for formation and deformation of greenstone sequences are summarized. We focus on proposals for an unstable stagnant lid basaltic plateau crust, with cratonization occurring initially above major mantle plumes. Archaean continental drift initiated as a result of mantle traction forces acting on newly-formed subcontinental mantle keels, with further cratonic growth occurring as a result of terrane accretion to the leading edges of the migrating cratonic nuclei.

Venus is presented as an analogue for a non-plate-tectonic Archaean Earth. Despite the absence of evidence for characteristic plate tectonic environments on Venus (i.e. subduction = trenches and volcanic arcs; seafloor-spreading = volcanic ridges and transforms), the form, scale, and geometry of folds, brittle-ductile shear zones, and faults interpreted on the surface of Venus from radar imagery are comparable to mid-upper crustal structures on Earth. Anastomosing rifts link coronae interpreted to form above upwelling mantle plumes. The Lakshmi Planum highland plateau in the western Ishtar Terra region of Venus lacks extensive, regional-scale internal deformation structures, but a fold-thrust belt produced mountains on its northern margin, folds and sinistral strike-slip faults occur on its NW margin, and both regional dextral and sinistral strike slip belts occur in a zone of lateral escape to its NE. Rift zones are present along the southern margin to Lakshmi Planum. The scale and kinematics of structures in western Ishtar Terra closely resemble those of the Indian-Asia collision zone, and we propose that lateral displacement of some coronae and ‘craton-like’ highlands or plana result from mantle tractions at their base in a stagnant lid convection regime, i.e. a similar regime as interpreted to have preceded development of plate tectonics on Earth.

In the Wawa-Abitibi Subprovince of the Superior Craton in Canada, the formation of granite greenstone sequences in a plume-related volcanic plateau and subsequent deformation can be generated through geodynamic processes similar to those on Venus without having to invoke modern-style plate tectonics. 3D S-wave seismic tomographic images of the Superior Province reveal a symmetrical rift in the sub-continental lithospheric mantle (SCLM) beneath the Wawa-Abitibi Subprovince, with no evidence for ‘fossil’ subduction zones. Major gold deposits and kimberlites are located above rift-bounding faults in the SCLM. Early rift structures localized subsequent deformation and hydrothermal fluid flow during N-S shortening and lateral escape ahead of a southwardly moving indenter (the Northern Superior Craton—Hudson’s Bay terrane) in the ca. 2696 Ma Shebandowanian orogeny. The geometry of reverse and strike-slip shear zones in the Abitibi Subprovince of the SE Superior Province is similar to that of shear zones developed in western Ishtar Terra, Venus, which also formed ahead of a rigid indenter whose displacement is attributed to mantle tractions. Similarly, shortening and rift inversion in the Abitibi is ascribed to cratonic mobilism where displacement of the N Superior Province ‘proto-craton’ resulted from mantle flow acting upon its deep lithospheric keel. Deformation in other Archaean cratons previously interpreted in terms of plate tectonics may also be the result of similar, mantle-driven processes.

Appendices

Appendix 1 Differences between Venus and Earth

Despite its similar size (0.94 times the Earth’s diameter), average density, and surface gravity (8.87 m/s² on Venus compared to 9.81 m/s² on Earth), Venus differs from Earth (McGill 1983; Schaber and Kozak 1990; Bonin 2012; Mocquet et al. 2011; NASA undated) in:

• The presence of a dense atmosphere (atmospheric pressure is 9.8 MPa compared to 0.1 MPa on Earth). Because of the dense clouds, visual means cannot be used to study Venus’ surface (hence radar is used for imaging surface features).

• Absence of surface water on Venus; note that liquid water was present on the surface of an Archaean Earth (Sagan and Mullen 1972).

• An atmosphere of mainly carbon dioxide and nitrogen.

• Differences in surface processes and chemical weathering.

• Absence of plate tectonics—mantle plumes dominate the tectonics of Venus.

• Likely absence of an asthenosphere (weak layer in mantle) .

• Its high surface temperature of 460 °C, with variations of ca. 100° depending on altitude. The difference in thermal structure results in a different rheological profile to Earth (Kohlstedt and Mackwell 2009).

• A liquid, instead of a solid, core.

• The absence of a magnetic field (either because a core dynamo does not exist or because temperatures are close to the Curie temperature).

  Head (1990a) and Hansen (2007b) add the additional differences that:

• Although Earth presents a bimodal hypsometric curve (i.e. cumulative elevation frequency curve), that for Venus is unimodal.

• Venus lacks obvious evidence of weathering, erosion, and sediment transport and deposition processes, or extensive sedimentary layers clearly deposited by wind or water.

• The thickness of crust on Venus is generally uniform and variations in topography are due to crustal thickening processes (e.g. in orogenic/fold belts) and due to local variations in the thermal structure.

• Differences in relative average crustal thickness between Earth (continental = 40 km/oceanic 5 km) and Venus (upland plateaus/ highlands = ‘continents’ 30 km compared to lowlands at ca. 15 km.

• The Earth has a larger percentage of ‘continents’. (Venus has two main highlands that are likened to continents on Earth: Ishtar Terra and Aphrodite Terra.)

• Lithospheric thickness estimates vary on Venus and may be equal or less than on Earth: 100–150 km is estimated by Smrekar and Parmentier (1996) and 100–200 km by Pauer (2004), however Kucinskasl and Turcotte (1994) estimate a lithospheric thickness of ca. 300 km beneath equatorial highlands of Venus. In comparison, lithospheric thicknesses of continental areas on Earth vary between 90 to 300 km (Conrad and Lithgow-Bertelloni 2006).

Appendix 2 Interpretation criteria for structural interpretation of Venus radar imagery

Interpretation criteria of SAR images for Venus in our study follow techniques used in previous SAR interpretations (e.g. Tuckwell and Ghail 2003; Romeo et al. 2005; Fernández et al. 2010; Romeo and Capote 2011):

• Marker horizons (i.e. with distinctive dark or light tones and/or textures) are used to trace regional fold closures and to interpret fold axial traces. Folds of tectonic origin are separated from volcanic flow features due to their similar orientation over 100s of kilometres, the regular wavelength of regional structures that are accompanied by superposed folds of shorter wavelength, their association with reverse, thrust, and strike-slip faults, and the formation of classical fold interference patterns; see discussion on folding by Romeo and Capote (2011).

• Faults with dominant strike-slip displacement are interpreted where marker layers or other structural features (e.g. fold closures or linear features in tessera terrains) are consistently offset in the same sense along the fault’s strike.

• Identical features as mapped on Earth (e.g. Wilcox et al. 1973; Sylvester 1988) are used to interpret brittle-ductile transcurrent shear zones. Folds or linear features progressively curve towards, and are offset across, linear structures. Faults with consistent lateral offset at a small angle (15° to 30°) to large, through-going structures are interpreted as Riedel shear arrays (c.f. Riedel 1929; Tchalenko 1968, 1970).

• Reverse or thrust faults or shear-zones are interpreted where linear features sub-parallel to axial traces of regional folds locally cross-cut folded lithological layering.

• Normal faults are linear features truncating lithological layering or other structural elements that bound down-thrown blocks (i.e. graben).

• Venus interpretations are aided by comparisons with radar interpretations of folds and shear zones on Earth that are constrained by field mapping (e.g. Rivard et al. 1999; Raharimahefa and Kusky 2006).