Nonlocalized faulting in a thick lithosphere : application to lunar contraction

Abstract

We reexamine the longstanding hypothesis that lunar contraction is constrained by the lack of a visible global system of compressive faults. We model the lunar lithosphere as a layered elastic medium that fails according to a Mohr-Coulomb criterion. We use elastic constants inferred from lunar seismic profiles, and use a finite element code to model the response of this lithosphere to contraction. We find that fault localization and propagation are strongly affected by the thickness of the lithosphere. A thin lithosphere promotes fault localization by extending through the entire lithosphere and thus enabling large stress relief and large displacements. For a thick elastic lithosphere the mode of faulting is less localized and many faults form in the upper part of the lithosphere, each with small displacements. Furthermore, localization in a thin lithosphere enables fault propagation through a compliant layer, such as a 1-3 km megaregolith layer, while for a thick lithosphere faults cannot penetrate this layer. Thus, the lack of an observed global system of compressive faults, similar to the locate scarps observed on the surface of Mercury, may not be due to the absence of an episode of global contraction on the moon, but rather due to the thickness of the lithosphere at that time.