Lithospheric scale gravitational flow: the impact of body forces on orogenic processes from Archaean to Phanerozoic

In the Archaean, the combination of warmer continental geotherm with a lighter sub-continental lithospheric mantle suggests that gravitational forces played a more significant role in continental lithospheric deformation. To test this hypothesis, we compare the evolution of the deformation and the regional state of stress in ‘Archaean-like’ and ‘Phanerozoic-like’ lithospheres submitted to the same boundary conditions in a triaxial stress-field with imposed convergence in one direction. For plausible physical parameters, thickening of normal to cold Phanerozoic lithospheres produces relatively weak buoyancy forces, either extensional or compressional. In contrast, for Archaean continental lithospheres, or for anomalously warm Phanerozoic lithospheres, lateral gravitationally-driven flow prevents significant thickening. This conclusion is broadly consistent with: (1) the relative homogeneity of the erosional level now exposed at the surface of Archaean cratons, (2) the sub-aerial conditions that prevailed during the emplacement of up to 20 km of greenstone cover, (3) the relatively rare occurrence in the Archaean record of voluminous detrital sediments, (4) the near absence of significant tectonic, metamorphic and magmatic age gradients across Archaean cratons, (5) the relative homogeneity of strain across large areas, and (6) the ubiquitous presence of crustal-scale strike slip faults in many Late Archaean cratons. One of the most contentious issues in Archaean geology is the significance of the particular features seen in the surface geology of most Archaean cratons. These involve the characteristic strain pattern of domes and basins; the ubiquity (in particular in Late-Archaean cratons) of strike-slip faults; the relative homogeneity at the craton scale of the finite strain field, the metamorphic facies and the erosion level; together with the particular timing of tectonics, magmatism, and metamorphism that seem to develop craton-wide within a very narrow time window (Binns et al. 1976; Choukroune et al. 1997; Galer & Mezger 1998; Hamilton 1998; Qiu et al. 1999; Rey et al. 2003). Based on the same field evidence, these features are interpreted by some authors as evidence of platetectonic processes (e.g., Snowden & Bickle 1976; Myers & Watkins 1985; Treloar & Blenkinsop 1995), and by others as evidence of gravitational instabilities (McGregor 1951; Collins 1989; Ramsay 1989; Jelsma et al. 1993; Bouhallier et al. 1993; Chardon et al. 1998) possibly related to plume activity (Choukroune et al. 1995; Warren & Ellis 1996). In this threedecade-long ongoing debate, the differences in mechanical behaviour of Phanerozoic and Archaean continental lithospheres have not been properly considered, yet regional finite strain fields depend significantly on the thermomechanical properties of the continental lithosphere. Consequently, the particular features observed in Archaean cratons could be related, in part, to contrasting lithospheric mechanical properties rather than to contrasting tectonic processes. In this paper we aim to illustrate the relatively greater importance of gravitational forces in the tectonic evolution of ancient continental lithospheres. Based on triaxial numerical experiments that compare the evolution of Phanerozoic-like and Archaean-like lithospheres under horizontal convergence, we show that ancient continental lithospheres could have been under an increased stabilizing influence of gravitational forces that promoted homogeneous deformation and therefore impeded the development of strain localization along linear belts. If the buoyancyderived stress were greater in the Archaean, then lateral escape (strike-slip faults and crustal-scale lateral crustal flow) rather than large-scale thrust and thickening could have become the primary response to tectonic convergence. We also suggest that the secular cooling From: BUITER, S. J. H. & SCHREURS, G. (eds) 2006. Analogue and Numerical Modelling of Crustal-Scale Processes. Geological Society, London, Special Publications, 253, 153–167. 0305-8719/06/$15.00 # The Geological Society of London 2006. of the continental geotherm, combined with the decrease in the buoyancy of the sub-continental lithospheric mantle (SCLM), have changed the thermo-mechanical properties of the continental lithosphere in such a way that the impact of gravitational forces on lithospheric deformation has decreased over geological time. Argand Ratio: assessing the impact of gravitational forces on modern and ancient continental lithospheres Scaled models of orogenic systems depend on a few dimensionless parameters that describe the relative importance of different processes in the stress and thermal balance. Amongst these parameters the Argand Ratio characterizes the ability of gravitational forces to intervene in lithospheric deformation (Fig. 1). The Argand Ratio (AR) can be defined as the local ratio of the gravitational stress (arising from lateral variation of gravitational potential energy and thus of density) to the averaged lithospheric strength at a nominal strain rate (i.e., the stress-driving deformation). The ratio AR may be compared with the Argand Number (Ar) of England & McKenzie (1982, 1983), a global parameter by which one can quantify the overall regional balance between gravitational stress and viscous stress in an indentation problem (see also Houseman & England 1986; Buck & Soukoutis 1994; Schmalholz et al. 2002, 2005). AR is obtained from Ar by replacing the nominal buoyant stress factor in Ar with the locally variable buoyant stress (relative to a reference column), and similarly replacing the nominal viscous stress scale factor in Ar with the locally variable strength of the lithosphere. The resulting Argand Ratio can be used to describe the evolution during deformation of the balance between buoyant stress and the strength of the lithosphere. The Argand Ratio measures the tendency of the lithosphere to deform in response to the variation of gravitational potential energy (DGPE), and therefore is a measure of the ability of gravitational forces to intervene in lithospheric deformation. In modern continental lithospheres, the Argand Ratio may reach values !1 when large DGPE exists and/or when the orogenic system involves a weak lithosphere (i.e., a warmer geotherm). In Tibet as well as in the Basin and Range Province of the western USA, it has been argued that present-day crustal flow is strongly influenced by forces induced by large gradients in gravitational potential energy (England & Molnar 1997; Jones et al. 1996; Flesch et al. 2000). A warmer geotherm reduces the strength of the continental lithosphere and, as a rule of thumb, it is inferred that a thickened continental crust spreads under its own weight (i.e., Argand Ratio !1 for a given effective strain rate) when the temperature at the Moho is above c. 7008C (e.g., Sonder et al. 1987). In modern continental lithospheres, such a thermal condition is likely to be transient and associated with either crustal thickening or the rise of the lithosphere/asthenosphere interface under continental areas. In the Archaean however, the larger radiogenic heat production, possibly coupled with a stronger basal heat flow, would lead to warmer continental geotherms. For radiogenic-element contents compatible with the present-day average composition of Archaean cratons (Taylor & McLennan 1986) and plausible mantle heat flow, the steady state temperature at the Moho in the Late Archaean could have been close to 7008C (Rey et al. 2003). In fact, one can argue that this temperature could have buffered the thickness of the continental crust, since a thicker crust would have spread (Bailey 1999). Hence, the Archaean continental lithosphere would have been less likely to develop large gradients of crustal thickness. In addition, there is solid evidence that the buoyancy of Archaean continental lithosphere was enhanced by a sub-continental lithospheric mantle, more depleted, and therefore less dense, than that of Phanerozoic lithosphere (3310 kg m vs 3370 kg m, Griffin et al. 1998). Upon lithospheric thickening, a more buoyant sub-continental lithospheric mantle would produce relatively greater extensional forces, and therefore a larger Argand Ratio. Figure 1a illustrates the consequences of a warmer geotherm on the integrated strength of the lithosphere (i.e., the denominator of AR). Figure 1b illustrates the effect of a more buoyant lithospheric mantle on the local gravitational stress (i.e., the numerator of AR). Both effects imply that local gravitational forces might have been more important relative to in-plane stresses arising from distant plate boundaries in the Archaean than in the Phanerozoic. Models: assumptions and simplifications Keeping in mind that our objective is to illustrate the difference in lithospheric deformation due to contrasting buoyancy and thermal structure, we assume here that Archaean-like and Phanerozoic-like lithospheres are similar in all but two aspects: the density profile and the temperature profile. There are a number of problems when P. F. REY & G. HOUSEMAN 154