Figure 6.1 (left) is a geological map of what I see when I look out of my window here in Grenoble, a view of the stacked thrust sheets of limestones and marls of the exterior zone of the French Alps. The satellite image on the right shows the distinctive “dome-and-basin” structure that characterises old (>3 Ga) granite- greenstone terrains. The photo is of the old yellowing cover of my copy of a book called Archaean Geology by Glover and Groves (1981). I checked on Google Earth for more recent images of the region but none illustrates the pattern as well. In the image, we see rounded, light-coloured, granitoid batholiths surrounded by cusp-shaped belts of dark-coloured metavolcanic and metasedimentary rocks. The pattern differs radically from that of the portion of the Alps in the left-hand image; there we see linear belts of mainly flat-lying volcanic and sedimentary rocks, intruded by sparse granitoids and cut by low-angle thrusts.
Le ft: Im age c ou rt es y o f Gé op or ta il ( w w w .ge op or ta il. gou v. fr) .
Figure 6.1 Two images of portions of the continental crust. On the left we see part of the geological map of France. Grenoble is in the middle of the Y-shaped valley in the top left of the image. On the right I show an old satellite image of the granitoid domes and cusp-shaped greenstone belts of the Pilbara region of Western Australia (from the cover of Archaean Geology by Glover and Groves, 1981).
The difference between the two structural styles is the focus of yet another debate in the general theme of formation of the continental crust. Once again there are two schools of thought. One of these holds that the style of deforma- tion in Archean granite-greenstone belts was not significantly different from those of the younger orogenic belts (Bickle et al., 1980; de Wit, 1982; Myers and Watkins, 1985; White et al., 2003; van der Velden et al., 2006). In this interpreta- tion, the domes and basins are interpreted as a fold-interference pattern. In this essentially uniformitarian view of the structure of the early crust, plate tectonics is assumed to operate, and the deformation is attributed to lateral plate move- ment. The compressive regime during deformation of the crust resulted in crustal thickening facilitated by numerous low-angle thrusts, and multiple episodes of deformation triggering several periods of folding.
The other school believes that the “dome-and-basin” structure is peculiar to the Archean, particularly the older parts, and reflects unusual conditions in the continental crust of that time (Bouhallier et al., 1983; Choukroune et al., 1995, 1997; Chardon et al., 2002; Van Kranendonk et al., 2004; Robin and Bailey, 2009). Vertical rather than horizontal movements are thought to dominate the deformation. The granitic domes are interpreted as rising diapirs of low-density felsic material and the surrounding supracrustal sequences as downward-moving portions of denser metavolcanic and metasedimentary rock. For such movements to occur, the lithosphere must have had a lower viscosity than that of the present crust.
Research on Archean structures and Tectonics by the Team from Geoscience Rennes – When I arrived at the University of Rennes in 1990 after my stint at the Max-Planck-Institut in Mainz, Pierre Choukroune was just becoming interested in the Archean. Pierre is a highly experienced structural geologist who had previously worked in younger mountain belts, particularly the Pyrenees and the Alps. A few years earlier, Raymond Capdevilla had led a group of petrologists and geochemists from Rennes – Sylvain Blais, Bernard Auvrey and Bor-ming Jahn – to study Archean and Proterozoic granitoids and greenstone belts in Finland, a project that led to the publication of a series of important early papers on Archean petrology, geochemistry and tectonics (e.g., Jahn and Shih, 1974; Blais et al., 1978; Martin et al., 1983). Soon after that, an exchange program was set up between Jean-Jacques Peucat and Hervé Martin in Rennes and Professors Mahabalashwar and Jayananda in the University of Bangalore in India with the aim of investigating the geochronology and petrology of the granitic rocks of the Indian Shield (Jayananda et al., 1995).
Pierre Choukroune led a number of trips to the Indian shield where he and his students investigated the structural style of the granitoids and their enclosing supracrustal sequences. In 1992, I went on one of these trips with the intention of following up the petrological-geochemical studies of granitoids that we had started in Mainz. To my disappointment, I saw very few rocks that I was happy to interpret as the products of crystallisation of granitic magmas. We spent most of the time working on rocks that had intruded at mid-crustal levels and had been through several stages of high-degree metamorphism and deformation. The rocks were foliated or lineated, and almost all
were invaded by migmatitic patches and veins that indicated partial melting. Pierre
used these observations to propose that the granitic domes had been emplaced not as granitic magmas, as had been suggested in some earlier studies, but as diapirs of only slightly molten granitoids and metamorphic rocks. With his PhD students, Hughes Bouhallier and Dominique Chardon, he published a series of papers (Bouhallier et al., 1983; Choukroune et al., 1995, 1997; Chardon et al., 2002) that developed the diapiric model for the dome-and-basin structure. The diagrams copied below (Figs. 6.3-6.5) are from sketches in Pierre’s notes.
At the same time, the debate between proponents of the two interpretations of the structure of Archean cratons was heating up. Those who favoured the fold-interfer- ence school were at that time developing structural models for greenstone belts that incorporated abundant low-angle thrusting and structural duplication of the volcano- sedimentary sections. This debate also continues to the present day, as reflected by the ongoing allochthonous vs. autochonous interpretations of greenstone belts in Canada and Zimbabwe (Kusky and Kidd, 1992; Bickle and Nisbet, 1993; Bickle et al., 1994). Maarten de Wit of the University of Cape Town had published an interpretation of the Barberton Belt in South Africa (de Wit, 1982) that incorporated many of these ideas, much to Pierre’s dismay. He was convinced that the structures he had studied in the Indian craton, and in other Archean regions in China and West Africa, were fundamentally different from those that he had mapped in the Pyrenees and the Alps. In 1986 we both went on another field trip, led by Steve McCourt of the University of KwaZulu-Natal who showed us the structures and geology of the Limpopo and Barberton Belts. What we saw only reinforced Pierre’s idea that the deformation and structure in the Barberton sequence was very different from that in the younger mountain belts and provided ideas in his subsequent papers on Archean structures and tectonics.
Pierre had previously worked in the northern and central parts of the continent, and was amazed by the excellent roads, public buildings and infrastructure of the Barberton region. Throughout the entire trip he repeated “ça, ce n’est pas l’Afrique!”