Magma ascent at floor-fractured craters diagnoses the lithospheric stress state on the Moon

Abstract

On the Moon, floor-fractured craters (FFCs) present evidence of horizontal crater-centred magmatic intrusions. Crater floor uplift and moat formation indicate that these sill intrusions occur at shallow depths ( < 10 km). While a recent study has demonstrated that magma ascent below FFCs and mare-filled craters was triggered by crater unloading, the mechanism leading to the emplacement of shallow sills is still poorly understood. Here we show that the local stress field due to crater unloading is also responsible for the horizontalisation of the magma flow leading to sill-like intrusions. On Earth, caldera formation has been shown to similarly affect magma trajectories, inducing the formation of a sill-shaped storage zone. Magma ascent to shallow depths below FFCs was however made possible because of a regional tensional stress caused by mare loading on the lunar lithosphere. We show that the tensional stress generated by elastic lithosphere deformation caused by mare loading combined to the local crater stress field can explain the distribution of FFCs on the Moon, with the smallest FFCs being located over a larger distance range from the mare. In particular, FFCs distribution around Oceanus Procellarum is consistent with an average load thickness of ∼ 1 km. This study suggests that magma trajectory in the crust of terrestrial planets can provide a diagnostic of the lithospheric structure and state of stress.