The thermal structure and thermal evolution of the continental lithosphere

Abstract

For thermal considerations, the lithosphere is defined as the outer layer of the Earth in which heat transfer is dominated by conduction. Low density continental crust caps the continental lithosphere preventing its subduction, and thus it has, in general, a long and complex thermal history. The thermal regime in the near surface zone of the continental lithosphere is commonly complex, and is responsible for much of the scatter in shallow heat flow data, the main data set used to examine the thermal structure and evolution of the continental lithosphere. A review of continental heat flow data shows that while heat flow is commonly high in areas of recent tectonic activity, there is no simple relationship between heat flow and age of tectogenesis. Low heat flow values are found in some areas of recent tectonic activity, and high heat flow values are found in some areas of older tectonism. An analysis of the parameters controlling the thermal structure of stable continental lithosphere shows that for the same heat flux into the base of the lithosphere, the main factors controlling temperatures within the lithosphere and surface heat flow are the quantity and distribution of heat producing elements within the lithosphere. In regions of recent tectonic and/or magmatic activity, perturbations in the thermal structure of the lithosphere are a function of the style and intensity of the tectonothermal disturbance. In hot spot and extensional settings, the lithosphere is thinned and its thermal gradient increased. The thermal effects at strike-slip boundaries are generally only of local importance, but can be significant when lithospheres with different thermal structures are juxtaposed. A variety of thermal phenomena are observed and predicted for zones of convergence, including low heat flow associated with subduction, high heat flow associated with magmatic activity, and thermal inversions associated with underthrusting. In seventeen continental areas, sufficient data are available to estimate the contribution of upper crustal heat production to the surface heat flow. This contribution is highly variable, and does not appear to correlate with crustal age for Proterozoic and Phanerozoic sites, but appears to be generally lower and less variable in Archean sites. In contrast, the component of heat flow from below the upper heat producing layer, the reduced heat flow, is relatively uniform, around 27 mW m −2 for sites which last experienced tectonism or magmatic activity in pre-Mesozoic times, but is highly variable for younger sites. From the uniform reduced heat flow for pre-Mesozoic sites, the stable thickness of the continental lithosphere is estimated to range from 90 to 220 km, thinner lithosphere being associated with higher heat production in the lithosphere. From this thickness, thermal perturbations in the lithosphere associated with tectonothermal activity are predicted to last no more than a few 100 m.y., a prediction consistent with available reduced heat flow data. Some prolongation of thermal relaxation is expected due to post-tectonic sedimentation and erosion, but these effects appear to be negligible at sites of Early Paleozoic age and older. The scatter in surface heat flow values at these sites is primarily the result of crustal heat production variations and near surface effects. Relatively low and uniform heat flow at Archean sites perhaps reflects relatively low and uniform crustal heat production at these sites.