Lithosphere response to externally and internally derived stresses: a viscoelastic stress guide with amplification

Lithosphere subjected to an externally derived horizontal stress undergoes creep in the lower lithosphere resulting in the decay of lower lithosphere stress and the associated amplification of stress within the upper lithosphere. This stress response of lithosphere has been investigated for a lithosphere model with power-law stress and temperature dependent viscoelastic properties. The rate and extent of stress decay and associated stress amplification is greatly dependent on the lithosphere geotherm. Oceanic lithosphere subject to an applied stress of ± O.1 kb undergoes upper lithosphere stress amplification of × 1.5, × 1.8 and × 2.0 at 104, 106 and 108 yr respectively. At 106 yr the effective lithosphere thickness is reduced to approximately 40 km. The stress decay and amplification proceeds more rapidly for an applied stress of ± 1.0 kb and in the case of a tensile applied stress results in some upper lithosphere fracture. For continental and Basin and Range type lithosphere the comparable stress amplification at 106 yr, for a stress of ± 0.1 kb, is × 2.0 and × 6.5 with effective lithosphere thicknesses of 60 and 20 km respectively. The large values of stress amplification for the Basin and Range lithosphere result in complete upper lithosphere fracture which gives rise to a cyclic process of upper lithosphere faulting and lower lithosphere creep in which extensive lithosphere deformation can occur. The stress amplification process also occurs for stresses generated by lateral density contrasts. For an isostatically compensated plateau uplift structure, deviatoric stresses of the order of 1 kb can be generated in the upper lithosphere by this process and are sufficient to cause tensile fracture of the upper lithosphere. The response of viscoelastic lithosphere to constant geometry bending stresses has also been examined and results in substantial but not complete reduction of the bending stresses.