Martian Chaos terrain fracture geometry indicates drainage and compaction of laterally heterogeneous confined aquifers

Abstract

The interlocking plateaus of martian chaotic terrain have long been inferred to relate to Hesperian outflow-channel megafloods. Numerous hypotheses have been invoked to explain the formation of the hundreds-of-kilometer-scale depressions that chaoses are found in, and the mechanisms by which the fractures formed. Hypotheses range from mechanisms involving water, e.g., ice melt, overturn of sediment-covered paleolakes, or submarine landslides, to purely magmatic processes, such as caldera formation, to exotic endmembers including clathrate decomposition. These interpretations of martian chaos are largely based on photogeological mapping of individual chaoses, and have mostly neglected analysis of the chaos fracture network and its relationships with the chaos basin. Here, we show, based on analysis of 35,964 fracture blocks across 18 different chaoses and 6 terrestrial analogs, and supported by novel volumetric measurements of chaos terrain deposits and intervening void spaces, that the geometry of martian chaoses is best explained by depressurization and compaction of an underlying confined aquifer. Block size distributions are incompatible with magma chamber collapse analog experiments. We show that sedimentary fill in chaos basins is inhomogeneously distributed, with layers thickening towards the chaos center, as in terrestrial sedimentary basins. The relationship between fracture block thickness and area is explained by the same power law that describes fracture spacing and layer thickness in weak terrestrial sandstones. The presence of some chaoses with blocks that are higher than surrounding plains implies repressurization of some sub-chaos aquifers. Hesperian-aged water or ice may remain within ∼1–3 km of the surface beneath these landforms.