The controls on the thermal evolution of continental mountain ranges

Abstract

This paper examines the controls on the thermal evolution of continental mountain ranges and investigates how pressure (P) and temperature (T) estimates from metamorphic rocks can be used to reconstruct the thermal histories of orogenic belts. We show that one-dimensional models can approximate the temperature structure of most continental mountain belts, and find that the metamorphic conditions typical of such tectonic settings can most easily be achieved by thickening of the mid- to upper-crust above a rigid lower crustal substrate. Homogeneous thickening of the entire lithosphere can only result in these conditions under unusual circumstances. This result implies that thickening above rigid lower crust has been the dominant mode of mountain-building preserved in the rock record. In this situation, modest rates of vertically distributed internal radiogenic heating (1±0.5 μW/m3) are able to produce typical metamorphic conditions and timescales, without the requirement for an external source of heat. In a given model, peak temperature conditions for rocks originally positioned closely in depth (e.g. 5 km apart) occur at times that can be separated by over 10 Myr, highlighting the spectrum of ages and conditions that can be produced by a single geometrically simple mountain-building event. The direction of approach in P–T space towards the highest-T conditions experienced by a rock (i.e. the curvature of the P–T loop) is the most diagnostic feature that allows the thermal and tectonic history of a mountain belt to be estimated, highlighting the importance of determining prograde P–T paths. Combining estimates of the peak-T conditions with multiple other sources of information (e.g. the timescales of metamorphism or the initial crustal thickness) can also allow the reconstruction of the mountain range evolution under most circumstances. Many configurations of continental mountain-building result in rocks transiently passing through regions with low average thermal gradients (e.g. 10–15°C/km), suggesting that such gradients should not be used to infer the presence of subduction. Analysis of existing P–T loops from the 3.2 Ga metamorphism in the Barberton terrane suggests that mountain-building processes at that time and place were similar to those in present-day mountain ranges, implying that large regions of rigid continental lithosphere were already in existence, and were underthrusting the margins of mountain ranges.