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

Abstract

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.

Geodynamic models of Archean continental collision and the formation of mantle lithosphere keels

“…[14] Recent papers [e.g., Rey and Houseman, 2006;Duclaux et al, 2007] demonstrate that two-dimensional (2D) experiments cannot adequately study Archean continental collision because three dimensional (3D) warm and buoyant lithosphere will undergo orogen-parallel ductile flow during orogenesis. In some of the experiments the lower crust accommodates convergence by undergoing pure-shear shear thickening and if given the opportunity will likely flow somewhat in the out-of-plane direction.…”

“…A third, though less popular, model envisages SCLM as a result of continental collision [e.g., Jordan, 1978]. However, previous studies on Archean continental collision have focused largely on increased continental geotherm induced lateral gravitational-driven flow of lithosphere [e.g., Rey and Houseman, 2006] and the dynamic interaction between the mantle and lithosphere [Cooper et al, 2006].…”

A matter of time: The importance of the duration of UHT metamorphism

“…More generally, such gravitational spreading is a natural progression in orogeny when collision ceases, providing a large mass of topographically elevated crust remains that is hot (>700°C) and hence able to flow at depth (Harley, 1989;Sandiford, 1989;Sandiford and Powell, 1990;Platt and England, 1993;Rey et al, 2001;Rey and Houseman, 2006;Jamieson and Beaumont, 2013). The numerical modelling indicates that heterogeneous domal flow of the hot middle and deep crust will occur contemporaneously with extension in the upper crust (Vanderhaeghe, 2009;Beaumont, 2011, 2013).…”

“…The total time that these granulites spend near their peak T conditions (>800°C) will be of the order of 40 Myr. Post-collisional gravitational spreading may remove the upper part of the thickened crust, the orogenic lid, or cause this lid to break apart, enabling the upward extrusion of deeperlevel rocks which will undergo high-T decompression and potentially form mantled migmatitic core complexes (Vanderhaeghe et al, 1999;Rey and Houseman, 2006;Vanderhaeghe, 2009). Alternatively, the structural architecture of the orogenic lid and ductile deeper crust may be reorganised in response to changes in tectonic boundary conditions, such as arrival of a strong crustal indentor or switch from orthogonal to oblique convergence (e.g., Schulmann et al, 2008).…”

“…Higher T p , Moho temperatures and heat productions in the crust-lithosphere system in the Archaean are considered to be favourable to the development of weak lithospheres (Marshak, 1999;Rey and Houseman, 2006) and hence facilitate the formation of UHO (Chardon et al, 2009). As a consequence, UHO are thought to be characterised by rapid accretionary crustal growth, juvenile magmatism and high geothermal gradients ).…”