Processes of long-term crater modification inferred from scaled analogue experiments

Abstract

Although the mechanisms of large-impact cratering have been extensively studied, the structural effects of long-term crater modification, operating for tens of thousands of years after impact, are poorly understood. Localised deformation in the form of crater floor fractures (CFFs) are known from large craters across celestial bodies. On Earth, CFFs in large craters are radial and concentric and filled with impact melt. Similarly, fractured crater floors are prominent post-cratering structural vestiges of lunar impact craters. Two mechanisms have been proposed to explain the formation of CFFs: emplacement of horizontal igneous intrusion, and long-term isostatic re-equilibration of target rocks beneath crater floors. Using scaled analogue experiments, I systematically investigated the structural consequences of both mechanisms on long-term crater floor deformation. Craters with specified morphologies, depths and radii were excavated into the material using a novel rotating blade. Analogue modelling observes the deformation of granular and viscous materials, acting as rock analogues, on a laboratory scale. In order to apply appropriate scaling, it is necessary to know the properties of the materials used. I present the characteristics of 22 granular and 4 viscous materials used at the UHH-Tec laboratory. These include the mechanical properties of a novel flour-sand mixture developed to model crater modification processes. In addition, I provide measurements of the relative permittivity values of common analogue materials. The experimental surfaces were monitored using 3D digital image correlation, which allowed me to quantify key structural parameters such as the distribution and evolution of surface fractures. This provides, for the first time, a quantitative relationship between diameter, depth and fracture geometry of crater floors. I used granular materials simulating the brittle upper crust of the Moon to investigate the structural consequences of sill formation below crater floors. Sills were simulated by flat, circular balloons emplaced in the granular material at varying depths below model craters. For each experiment, the model sill was inflated and then deflated to simulate intrusion and evacuation of magma. My experiments showed that fracture patterns were controlled more by the depth of the flat balloons below the surface than by the morphology of the crater floor. The modeliv craters are characterised by radial and polygonal fracture patterns. In nature, these fracture patterns may allow magma to erupt immediately from sill reservoirs. I conclude that natural lunar sill systems are unlikely to reach the structural maturity previously proposed. The deformation behaviour of the upper crust, scaled to Earth, following crater formation was modelled using a two-layer modelling setup to investigate the structural and kinematic consequences of crustal relaxation. The structural evolution was systematically analysed for different initial crater floor depths and diameters. The uplift of the crater floor is achieved by long wavelength subsidence of the crater periphery and can operate on time scales of thousands of years in nature. I conclude that the observed patterns of floor fractures, including impact melt dykes, may be caused by long-term crater floor uplift compensated by lateral crustal flow towards the crater centre.