An active thermoelastic rift mechanism

Abstract

Finite element solutions for stresses and displacements during rift development are obtained for a variable thickness, elastic lithosphere plate subjected to thermoelastic loading by conductive heating from an axial line source in the asthenosphere. The asthenosphere is modelled as a fluid foundation. The prerift lithosphere is modelled as an infinite continuous plate. A semi‐infinite plate is used to characterize the rifted lithosphere. Horizontal restraints are removed at the rift axis to model the transition from a continuous to separated lithosphere plate during rifting. Lithosphere strength envelope curves derived from olivine flow laws are used to determine the elastic lithosphere depth in the solutions. Conductive heating of a variable thickness, continuous lithosphere plate by a 2000 W/m line source at 60 km depth produces high stresses, oriented orthogonal to the forming rift axis, and small vertical displacements. The maximum uplift obtained in the continuous lithosphere plate models subjected to thermoelastic loading was 0.12 km at a distance of 120 km from the forming rift axis. This small uplift is accompanied by shallow tensile stresses of 80 MPa at a distance of 5 km from forming rift axis which decrease to 6 MPa at a distance of 210 km. Shallow horizontal thermoelastic tensile stresses are over 5 times greater than the horizontal stresses produced by thermal buoyancy loads near the forming rift axis. The transition from a continuous to rifted lithosphere is accompanied by an increase in lithosphere uplift at the rift axis from 0.11 to 0.42 km and a reduction in shallow horizontal stress magnitude by a factor of 30. Thermoelastic loading may be a source of shallow tensile stress in the lithosphere and a contributing factor for the initiation of rifting. Thermal loading may also produce postrifting topography changes. Thermoelastic loading of the lithosphere is proposed as a possible driving mechanism for rift formation which produces separation and the transition from a continuous to rifted lithosphere plate.