Evolution of a previously thickened thermochemical lithosphere: Application to Venus

Abstract

Magellan images have revealed areas with high topography on Venus like Maxwell Montes, interpreted as the result of a lithospheric thickening, associated to a global compressional stress regime. In addition to compressional structures, Maxwell Montes displays younger extensional structures and magmatism at its summit. We present a numerical model to explore the main factors that may explain the chronology of compressional and magmatic events observed on Maxwell Montes. In particular, this study aims to quantify the process of destabilization of a thickened lithosphere by taking into account thermal and chemical instabilities. Indeed, the contribution of basaltic/eclogitic rocks and depleted mantle rocks is not negligible in mantle dynamical models. The initial situation in the upper mantle is represented in a two-dimensional cartesian domain by a stationary state of thermal convection. The thermal lithosphere is then instantaneously thickened and buoyant tracers of depleted mantle and basalt are placed in the lithospheric mantle and crust, respectively. This material distribution controls both the thermal evolution of the lithosphere and its recycling after thickening. Assuming a continuous and slow basalt/eclogite transformation, the lithosphere acts as an insulating layer during 100 Myr before an effective recycling can occur. For an instantaneous basalt/eclogite transformation, combination of basaltic crust and depleted upper mantle prevents recycling of the lithosphere during the first few millions years. The recycling is all the more enhanced as the basalt transforms to eclogite because it is an ongoing process. It is twice as fast (20 Myr) as the recycling of a purely thermal lithosphere (40 Myr) but the recycled rocks of a thermochemical lithosphere represent less than the recycled rocks of a thermal one because of the great stability of the depleted layer. This results in a thermochemical lithosphere thicker by several tens of kilometers. At an upper mantle scale, this chemical diversity favors the mixing of chemical heterogeneities and the perturbation of internal dynamics. This involves the impingement of a hot depleted diapir at the base of the lithospheric root. For a sufficiently hot mantle, a partial melting zone is trapped inside the lithospheric root and induces magmatism about 80-100 Myr after the lithospheric recycling. This could explain the late volcanic flows observed in Maxwell Montes. Detailed modeling of a thermochemical lithosphere also could explain the formation and evolution of Maxwell Montes as evidenced by volcanic and tectonic features. Copyright 1999 by the American Geophysical Union.