The extensional strength of the continental lithosphere: its dependence on geothermal gradient, and crustal composition and thickness

Abstract

Summary The response of lithosphere to an applied tectonic tensile force and the resulting stress distribution with depth has been investigated using a mathematical model incorporating the elastic, plastic and brittle behaviour of lithospheric material. Lithospheric strength is shown to be primarily controlled by lithospheric rheology and as a consequence is critically dependent on geothermal gradient and lithospheric composition. The rheologies of the upper crust, lower crust and mantle are assumed to be controlled by dislocation creep in quartz, plagioclase and olivine respectively. The critical level of tensional tectonic force required to generate geologically significant strains has been calculated as a function of surface heat flow, and the predicted lithosphere strength compared with available levels of tensile tectonic force arising from subduction plate boundaries and isostatically compensated plateau uplift loads. The model predicts significant extensional deformation in regions with surface heat flow >65 mWm −2 subjected to a tensile tectonic force of 3 × 10 12 N m −1 and is in good agreement with observed examples of intraplate extension. Lithosphere strength is critically controlled by the crustal thickness since the quartzofeldspathic rheology of the crust is weaker than the olivine rheology of the mantle. A decrease in crustal thickness thus increases the strength of the lithosphere. However, lithospheric extension also increases the geothermal gradient serving to weaken the lithosphere. The rate of extension is critical in determining which of these processes predominates. Fast strain rates (> 5 × 10 −15 sec −1 ) produce a weakening of the lithosphere (i.e. strain softening) while slower strain rates lead to strengthening of the lithosphere (strain hardening). Extensional style is consequently controlled by the lithospheric extension rate; fast extension producing, through strain softening, intense localized lithospheric extension with high (potentially infinite) β values, and slow extension, through strain hardening, giving broader regions of lithosphere extension with finite β values of the order of 1.5. For intermediate geothermal gradients ( q = 55−70 mWm −2 ) the model predicts a low stress-low strength region at the base of the crust due to the contrast between plagioclase and olivine rheology at the Moho. Other low-strength regions are predicted within the crust at major compositional (and rheological) boundaries. These low-strength zones are expected to control the location of detachment horizons by which crustal extension occurs particularly at slower strain rates. High geothermal gradients favour the shallower detachment horizons at the expense of the deeper horizons. The reverse is true for low geothermal gradients.

The onset of the North Atlantic Igneous Province in a rifting perspective

“…Studies on lithospheric strength in a laterally homogeneous undeformed lithosphere have been dealt with in a number of studies (e.g. Kohlstedt, Evans & Mackwell, 1995;Hirth & Kohlstedt, 1996;Kusznir & Park, 2002;Van Wijk & Cloetingh, 2002;Jackson et al 2008). In brief, these authors concluded that increased heat flow and high rates of spreading generally resulted in a net weakening of the lithosphere and conversely, very slow spreading rates and low heat flows could result in a net strengthening.…”

Rifting and the Volcanic-Tectonic Evolution of the Izu-Bonin-Mariana Arc

“…Total crustal thickness varies between 10 and 15 km in the forearc (east to the outer arc high and north of 28° N), but may reach 20 km beneath the arc. Thicker crust (which is Theologically weak compared to the mantle) and higher heat flow have been shown to weaken the lithosphere (Vink et al, 1984;Steckler and ten Brink, 1986;Kuznir and Park, 1987). I therefore infer that the frontal arc volcanoes create a linear zone of lithospheric weakness that controls the location of rifting (B.…”

Kinematics and geometry of structures in the southern limb of the Paraíba do Sul divergent structural fan, SE Brazil: a true transtensional shear

“…Glazner and Bartley (1985) suggested that a balance between radioactive decay and denudation rates of a continental crust duplicated by thrusting may cause strong softening which enhances post-thrusting extension. Moreover, the lower strength of the continental crust in relation to mantle material suggests that thickened continental lithosphere is weaker than normal thickness continental and oceanic lithospheres (Brace andKohlstedt 1980, Kuznir andPark 1987), and hence it will be able to flow under its own weight (Molnar and Lyon-Caen 1988). So, if we consider the strong preexistent anisotropy due to structural discontinuities formed during convergence, and the rheological controls that lower the strength of duplicated continental crust (Teyssier and Vanderhaeghe 2001), the orogenic belts are suitable sites for extensional flow regimes (Norton 1986).…”