Long-wavelength Folding on Mercury: Lithospheric Boudinage in the Caloris Basin?

Both laser altimetric and stereo photogrammetric datasets returned by the MESSENGER spacecraft in orbit about Mercury reveal impact craters whose floors show systematic tilts away from topographically high regions. Such tilted craters indicate that Mercury's lithosphere has been affected by large-scale folding that, when mapped, is manifest as several long-wavelength and low-amplitude rises and troughs, interpreted as anticlines and synclines, that cross the planet. Topographic profiles across the syn- and anticlines show that folding can be described as more or less harmonic with wavelengths of 800 to 1300 km and amplitudes of 1.5 to 3 km. These dimensions show that folding accommodated shortening strains of only ~0.002%. Several syn- and anticlines are found in the region in and around the Caloris basin, the largest recognized impact basin on the planet. The topography within the basin is characterized by two anticlines, each trending approximately east-west and having crests that rise more than 2 km above a low-lying region near the basin center. Fault displacement analysis of several radial graben and circumferential ridges, together with crater excavation depths of spectrally distinct materials, yields stratigraphic information on the uppermost smooth volcanic plains in the basin's interior, revealing a pinch-and-swell structure to these units. Specifically, the plains are thicker in the vicinity of the topographic highs and thinner at the topographic low. We used numerical simulations with the two-dimensional module of the finite-element modeling code ADELI to explore how folding of the lithosphere on Mercury and the pinching and swelling might have been accommodated for a range of assumed boundary conditions and properties of the lithosphere and mantle, informed by recent geophysical data returned by MESSENGER. We find that continuing lithospheric folding with periodic emplacement of volcanic plains units can account for the observed topography and stratigraphy. In particular, we achieved best fits between our models and observations with a scenario involving lithospheric boudinage. Under this scenario, the emplacement of volcanic units on preexisting syn- and anticlines led to thickness variations across the plains, and further folding facilitated their pinching and swelling to ultimately produce an inversion of the original relative topography. This inversion is also evident in the order of mapped rises and troughs outside the basin, in that the proposed anticlines inside Caloris continue as synclines of greater amplitude along the same azimuthal trends outside the basin. These observations imply that lithospheric folding spanned an extended time interval that included the time of smooth plains emplacement. Loads induced by the smooth plains potentially influenced the folding, whereas strains of the folding are too low to have influenced the development of the complex faulting pattern in the basin. The concept of lithospheric boudinage in Caloris not only accounts for the observations of topography and stratigraphy, but also has implications for the rock mechanical behavior of Mercury's lithosphere and mantle in general.