Rheology of the lithosphere in space and time

Abstract

The rheology of the lithosphere is a factor of primary importance in the kinematics and dynamics of mountain belts. This paper attempts to clarify the role of rheology in orogenesis, by applying simple physical principles to the analysis of tectonic processes. The emphasis is on broad generalizations leading to order-of-magnitude estimates. Since the rheology of lithospheric materials is strongly dependent on temperature, the discussion opens with a review of continental and oceanic geotherms and an assessment of their reliability. Then the brittle (frictional) and ductile (high-temperature creep) properties of the lithosphere are considered. In the brittle field, particular attention is paid to the problem of fault reactivation, which is shown to be more likely in extensional than in compressional regimes. In the ductile field, a summary of creep parameters for the most common lithospheric materials is presented. The central concept of rheological profiles (strength envelopes), essential to the estimation of the depth variations of lithospheric rheology, is discussed with reference not only to its applicability but also to its limitations (a two-dimensional example from the Canadian Cordillera is given). Processes related to the rheological properties and layering of the lithosphere — gravitational collapse in thickened and softened crust, tectonic inversion following the detachment of a lithospheric root, lower crustal ductile flow with consequent relaxation of Moho topography — are analysed semi-quantitatively, mainly to show that they are indeed likely to be important players in geodynamics. Finally, in a brief Archaean detour, some possible important differences between present-day and Archaean oceanic lithosphere are examined, and the conclusion is reached that in all likelihood plate tectonics, although at different rates, has been the main agent of orogenesis during most of the history of the planet.

Depth of faulting and ancient heat flows in the Kuiper region of Mercury from lobate scarp topography

“…where p is the density, g is the acceleration due to the gravity (3.7 m 5-2 for Mercury), z is the depth and ex is a coefficient depending on the stress regime (which is 3 for pure compression, appropriate for thrust faulting; e.g., Ranalli, 1997 (Peplowski et al, 2011), and we estimated U abundance by assuming the chondritic ThjU ratio of 3.6 (Morgan and Anders, 1980;Taylor and Scott, 2005). Therefore, the used crustal heat production rates range between 1.7 x 10-4 and 8.7 x 10-4 mW m-3 at the time of scarp formation.…”

Mechanisms for folding of high-grade rocks in extensional tectonic settings

“…2). Ductile flow in the middle to lower crust has been recognised as an important deformation mechanism during rifting (Wernicke, 1990;Ranalli, 1997;Westaway, 1998;Gartrell, 2000;see Fig. 2 for examples).…”

Thermal and mechanical structure of the central Iberian Peninsula lithosphere

“…Experimental investigations centred on the deformation properties of several rocks have yielded rheological laws related to the first-order mechanical behaviour of the continental crust (e.g., Brace and Kohlstedt, 1980;Kuznir and Park, 1984;Carter and Tsenn, 1987;Kirby and Kronenberg, 1987;Tsenn and Carter, 1987;Ranalli, 1997). Different lithologies at varying depths, each with characteristic mechanical properties and deformational responses to a given regime of temperature, pressure, strain rate and fluid pressure, lend supports to the phenomenon of rheological stratification of the lithosphere.…”