Characterization of Contractional Deformation Features on Mercury from Finite Element Modeling and Altimetric Profiles from MESSENGER’s Flybys

The MESSENGER spacecraft has executed three ~200-km altitude flybys of Mercury in the period between January 2008 and September 2009, prior to insertion into orbit about the planet in March 2011. From imaging and laser altimetry profiles obtained during these flybys and previous imaging from Mariner 10, contractional features, specifically high-relief ridges and lobate scarps, have been identified ,and are observed to have formed in a distributed manner over the parts of Mercury’s surface so far imaged. This is in contrast to analogous structures on the Moon that are concentrated in mare units and related to subsidence of surface loads, and some features on Mars that formed in regularly-spaced patterns reflecting the global stress field associated with Tharsis. Altimetry provides a quantitative description of the surface expressions and geometries of these features that provides information relevant to the structure of the lithosphere at the time of formation as well as the manner in which strain was accommodated during Mercury’s evolution. In order to relate the spatial distribution and properties of these structures to the geodynamics and shallow interior structure of Mercury, we employ a finite element approach. Using the two-dimensional FEM code LAYER, which solves for the velocity and stress fields of strongly non-Newtonian materials, we model lithospheric behavior under compression for plausible crust and mantle rheological profiles, surface temperatures, temperature gradients, fault weakening parameters, and model dimensions. In this formulation, faults arise spontaneously in response to included strain rate localization mechanisms rate and are not built into the finite element grid a priori. We track the evolution of strain accommodation for an imposed uniform strain rate, ranging over various model inputs to define the parameter space that results in faults with surface expressions comparable to those observed. We are able to define sets of parameters that allow for single, sparsely spaced faulting with significant vertical offset and inferred fault angle at depth consistent with the observed features. On the basis of likely strain rates from thermal history calculations, localized deformation on Mercury must have been predominantly brittle in nature with minimal weakening. To explain observed kilometer-scale vertical offsets requires the protracted accumulation of strain over geologic time scales. Allowable ranges of heat flow and thermal gradient are consistent with published thermal models.