Mercury's Tectonic and Geodynamic History: 2. Contribution of Membrane–Flexural Strains to the Tectonic Record

Abstract

Previous analyses of Mercury's tectonic record have arrived at widely varying amounts of global contraction. Contraction also varies spatially, with some regions displaying a near-zero record of contraction. Here, we investigate the contribution of membrane–flexural strain from lithosphere loading to the tectonic record. Using an inversion of present-day gravity and topography for Mercury's interior structure and lithosphere deformation, we show that membrane–flexural strain can counteract contractional strain as well as add to it. Long-wavelength support from the mantle systematically predicts flexural uplift and extension in the northern and Budh-Sobkou rises as well as the Caloris basin and the Otaared and Du Fu regions, thereby explaining their relative contractional tectonic strain deficit. This correlation indicates ancient, yet long-standing, support of topography in these regions, which must have been growing concurrently with the major phases of planetary contraction. Persistent compositional mantle anomalies or crustal underplating best explain such regional and long-lasting support. Although our models predict membrane–flexural compression in the northern smooth plains, contributing to the overconcentration of wrinkle ridges, neglecting the compressional tectonics there would underestimate the contractional strain. We constructed a novel inversion of gravity and topography using tectonic strain as a proxy for membrane–flexural deformations. This model self-consistently constrains Mercury's crust and mantle structure, also predicts the existence of long-standing and deep density anomalies, and explains Mercury's heterogeneous tectonic record. Future work must consider membrane–flexural strain as an important contribution to Mercury's regional tectonic records, which can both increase and decrease the local strain, but with a negligible net effect on global contraction.