Constraints on rift thermal processes from heat flow and uplift

Abstract

Heat is of fundamental importance in the genesis of continental rift zones. Heat flow data are available from the Baikal, Basin and Range (North America), East African, Rhine and Rio Grande rift systems, and most of the data indicate high heat flow, with means of 70–125 mW m −2, from the grabens in the rift systems. Data from the essentially non-volcanic sections of the East African rift system indicate normal to low heat flow, however. With the exception of the Basin and Range system, and parts of the Rio Grande system, the high heat flow appears to be restricted to the grabens, and is not measured on the broad domal or plateau uplifts associated with the rift systems. The uplifts associated with rifts typically are on the order of 1 to 2 km, with diameters of a few hundred kilometers or more, and the duration of the main phase of uplift is on the order of a few tens of million years, or less. These broad uplifts probably result from thermal changes in the lithosphere-asthenosphere system. Two thermal models of the lithosphere are developed for comparison with the heat flow and uplift data. The first model assumes very slow lithospheric thinning so that the lithosphere remains in a state of quasi-equilibrium during the thinning. The second model assumes that thinning occurs at a rate significantly faster than heat can be conducted into the base of the lithosphere, with thermal relaxation of the lithosphere occurring after the cessation of thinning. Comparison of uplift rates with the model results indicates that for uplift purposes in rift genesis, the lithosphere is in a state between the two models i.e. partial heating of the lithosphere, or close to the rapid thinning state. Some uplift is predicted to continue after thinning has ceased due to thermal relaxation in the lithosphere. With respect to surface heat flow, however, the rapid thinning model is always predicted to apply, and a surface heat flow anomaly is not predicted to develop until after thinning has stopped. Local (graben) heat flow anomalies are thought to be primarily due to convection of heat into the rift zones by ascending magmas. A regional conducted thermal anomaly is not predicted to develop until after uplift is essentially complete. These predictions are compatible with the available heat flow and uplift data.