THERMAL CONTROLS ON FLEXURE OF UNDERTHRUST CONTINENTAL LITHOSPHERE

Abstract

We model the thermal and mechanical structure of Precambrian foreland lithosphere subject to a range of initial thermal states and evolving thermal perturbations beneath an orogen. Temperatures in the lower crust exert a critical control over the flexural response by regulating crust–mantle decoupling. By careful consideration of crustal heat production and temperature-dependent thermal conductivity, we verify that at least half the surface heat flow in Archean terranes is supplied by the crust and find that Moho temperatures in Proterozoic lithosphere may be as high as 800 K (525 °C). Such ‘hot’ Precambrian forelands may have an effective elastic thickness of less than 10 km. The ability of crustal heating beneath the orogen to enhance weakening of continental lithosphere is strongly dependent on the convergence rate. A rapidly underthrust foreland is insufficiently heated by thermal diffusion into the zone of maximum lower crustal deformation to significantly modify the flexural signature. This process has a minor effect at the 5 mm year−1 or greater convergence rates characteristic of most active orogens, but it may cause a marked reduction in lithospheric strength following the cessation of convergence. We propose that the large, ∼10–100 km range of observed mountain belt effective elastic thicknesses can be entirely explained by the known variability of Precambrian surface heat flow, crustal heat generation, rock thermal conductivity, and lithospheric rheology without invoking any other mechanisms.