The long-term thermal consequences of rifting: implications for basin reactivation

Abstract

The attenuation of the continental crust during rifting and the subsequent filling of the rift-related accommodation alter the long-term thermal and mechanical state of the lithosphere. This is primarily because the Moho is shallowed due to density contrasts between the sediment fill and the crust, but also reflects the attenuation of the pre-existing crustal heat production and its burial beneath the basin, as well the thermal properties of the basin fill. Moho shallowing and attenuation of pre-existing heat production contribute to long-term cooling of the Moho and thus lithospheric strengthening, as has been pointed out in many previous studies. In contrast, basin filling normally contributes to significant Moho heating allowing the possibility of long-term lithospheric weakening, the magnitude of which is dependent on the thermal properties of the basin-fill and the distribution of heat sources in the crust. This paper focuses on the thermal property structure of the crust and basin-fill in effecting long-term changes in lithospheric thermal regime, with particular emphasis on the distribution of heat producing elements in the crust. The parameter space appropriate to typical continental crust is explored using a formalism for the heat production distributions that makes no priori assumptions about the specific form of the distribution. The plausible parameter space allows a wide range in potential long-term thermal responses. However, with the proviso that the accommodation created by the isostatic response to rifting is essentially filled, the long-term thermal response to rift basin formation will generally increase average crustal thermal gradients beneath basins but cool the Moho due to its reduction in depth. The increase in the average crustal thermal gradient induces lateral heat flow that necessarily heats the Moho along basin margins, especially in narrow rift basins. Using coupled thermo-mechanical models with temperature sensitive creep-parameters, we show that such heating may be sufficient to localise subsequent deformation in the vicinity of major basin bounding structures, potentially explaining the offset observed in some stacked rift basin successions.