Structural consequences of sill formation below lunar craters inferred from scaled analogue experiments

Although mechanisms of impact cratering have been studied intensely by numerical modelling and field analyses, an outstanding problem concerns long-term crater modification. Localized deformation in form of radial and concentric crater floor fractures are prominent post-cratering structural vestiges of lunar impact craters. Two mechanisms were proposed to explain the formation of floor fractures: isostatic re-equilibration of crust underlying crater floors, and emplacement of horizontal igneous sheets below craters. Due to thick and cool lunar upper crust, the latter mechanism has been regarded as the more plausible one to account for the presence of floor fractures in lunar craters. However, the structural consequences of magmatic inflation on surface deformation in combination with crater floor morphology has not been analyzed systematically in 3D.

We use scaled analogue experiments to model the deformational behavior of upper crust following crater formation to explore the structural and kinematic consequences of sill formation below crater floors with different depths and diameters. Our experiments were scaled to the lunar physical conditions. The initial diameter-to-depth ratios of lunar craters were based on numerical modelling. Granular material simulating the Moon’s brittle upper crust was filled into a 60 cm by 60 cm size tank. Craters with specified morphologies, depths and radii were “drilled” into this material by a rotating blade. Sills were simulated by variably sized flat, circular balloons of plastic foil, emplaced into the granular material below model craters and inflated by a pumped-in fluid. For each experiment, the sill was first inflated and then deflated to model intrusion and evacuation of magma, respectively. Surface deformation within and around the crater was monitored with a 4-D digital image correlation system allowing us to quantify key parameters including surface uplift as well as the distribution and evolution of strain. The results of our scale models enabled us to quantify the geometry and distribution of brittle deformation of lunar upper crust.

Our experiments show that inflation of balloons caused radial and concentric dilation fractures in the overlying granular material. Fracture patterns were more controlled by the depth to the top surface of balloons rather than by crater floor morphology. For the duration of fluid inflation into shallow model sills, surface uplift was focused in the crater center and associated with rather prominent fractures. Upon deflation, concentric normal faults developed at the inner crater rim, and this corresponds to the terraced crater margins ubiquitously observed at lunar craters. Interestingly, model craters are characterized by more diverse fracture patterns, compared to lunar craters. This may be due to brittle deformation above sills during inflation, allowing for magma to erupt from natural sill reservoirs. It is, therefore, unlikely that natural sill systems attain the structural maturity of our modelled equivalents. Hence, evacuation during inflation in natural systems can account for the presence of less prominent fracture patterns compared to the ones in modelled, more “mature” sill systems.