ON THE THERMOMECHANICAL EVOLUTION OF CONTINENTAL LITHOSPHERE

Elastic thickness of continental lithosphere is modified by both mecha nical (thickening and thinning) events and thermal evolution. A simple thermo-mechanical model can account for the processes of tectonic alt eration and subsequent thermal reequilibration of lithospheric thermal structure. Mechanical thinning or thickening of the lithosphere direc tly affects iu thickness and flexural rigidity. After a tectonic event , thermal reequilibration proceeds at a rate dependent upon the amount of thinning or thickening and total lithospheric thickness, previous tectonic history, and the heat flux at the base of the lithosphere. He at flux at the base of the lithosphere is controlled by the efficiency of convective mantle heat delivery. Rapid heat delivery to the base o f the lithosphere retards lithospheric thickening, whereas slow conduc tive heat delivery allows lithospheric thickening at the maximum rate. The relative importance of thermal effects to mechanical alterations of lithospheric elastic thickness depends on the time scale of concern . Very thin lithosphere undergoes rapid thermal evolution such that co oling between tectonic events significantly thickens the lithosphere. For thick lithosphere, conductive heat transport is so slow that only minor changes in thermal structure and no significant changes in elast ic thickness take place between tectonic events. Model results with in finite stretching agree with observations of oceanic lithospheric thic kness evolution, showing a divergence from a cooling half-space at abo ut 70 m.y. Model results also agree with independent estimates of cont inental thermal evolution in the absence of tectonic events. On the lo ngest time scales approaching the age of the Earth thermal reequilibra tion is attained, and any tectonic effects are overprinted. The time s cale at which the transition between mechanically and thermally domina ted elastic thickness evolution occurs depends on the heat transport e fficiency of the convecting mantle. For high Nusselt numbers, thermal effects take longer to become important. Even for lower Nusselt number s, mechanical effects still dominate on time scales of 10(8) years. Cr ustal radiogenic heat production reduces the equilibrium lithospheric thermal and elastic thickness. It also introduces a dependence of equi librium thickness on tectonic crustal thinning or thickening. Model re sults indicate that on time scales of thrust sheet and sedimentary loa ding in foreland basins, mechanical effects dominate the evolution of elastic thickness except for very thin lithosphere. The model can be a pplied to the lithosphere beneath foreland basins so that observed bas in geometry can be used to constrain tectonic history prior to loading by thrust sheets and basin sediments.