The history of global strain and geodynamics on Mars

The tectonic record of Mars is dominated by compressional wrinkle ridges on volcanic surfaces, and these structures have been widely used as a record of tectonic and geodynamic evolution. This study analyzes the density of compressional tectonic structures and inferred strain as functions of time, using the lengths and heights of compressional ridges together with geologic estimates of surface age. Our analyses confirm an apparent peak in compressional strain in the late Noachian and early Hesperian, and comparatively lower values before and after. The lower tectonic strain in the early and middle Noachian relative to the early Hesperian reflects the incompleteness of the ancient compressional tectonic record of strain, as older surfaces should accumulate more strain than younger surfaces. This strain deficit necessitates other means of accommodating contractional strain in the ancient crust, including distributed strain and small-scale faulting. The abrupt decrease in the accumulated tectonic strain and strain rate after the early Hesperian reflects a rapid episode of global contraction followed by much lower rates, in conflict with models of steady-state mantle evolution. The decreasing strain rate may be explained by a changing mantle rheology due to volcanic outgassing. Alternatively, the strain history may be explained by the dominance of mantle plumes in the late Noachian and early Hesperian associated with the formation of Tharsis and the early Hesperian volcanic provinces, which could have led to a pulse of rapid contraction caused by disequilibrium cooling and volcanic outpourings. The tectonic record of strain on Mars - including the incomplete ancient record and evidence for strong departures from steady-state models - may have implications for the interpretation of the tectonic record and thermal evolution of other bodies as well.