Abstract
Alluvial fans are ubiquitous landforms in high-relief regions on Earth and Mars. They have a semi-conical shape and are located at the transition between highlands and adjacent basins. Alluvial fans can form by a range of processes including debris flows, which are water-laden masses of soil and rock with volumetric
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sediment concentrations exceeding 40%. In this thesis, I aim to (1) unravel the formative dynamics of debris-flow fans and, building on these insights, to (2) reconstruct hydrologic and climatic conditions in the last few millions of years on Mars from alluvial fan deposits (gullies). The dynamics of debris flows and debris-flow fans were studied in small-scale laboratory experiments. Debris-flow runout, and thus its hazardous effect, increases with channel and alluvial fan slope and with debris-flow volume and water fraction. There is an optimum relation between debris-flow runout and coarse-material fraction and clay fraction in the flow. Debris-flow fans were observed to form by autogenic sequences of backfilling, avulsion (i.e., channel shift) and channelization. These sequences are governed by large-scale topographic compensation. Gullies are catchment-alluvial fan systems found on crater walls on Mars. They are among the youngest landforms that may have formed by liquid water on Mars (they can be younger than 1 Ma), and therefore of critical importance in resolving the planet's recent hydrologic and climatic history. The formation of alluvial fans by cyclic sequences of backfilling, avulsion and channelization implies that fans comprise active and inactive sectors. Long inactive parts of alluvial fans are exposed to modification by secondary, post-depositional, processes, including weathering, wind erosion and fluvial erosion. Long-inactive fan surfaces are therefore likely to be masked by a surface morphology related to secondary processes. The effectiveness of post-depositional modification depends on the ratio between the characteristic time scales to build morphology by primary deposition and to modify morphology by secondary processes. On Mars the time scales of fan inactivity are large (~0.1-1 Myr), and therefore the surface morphology and texture of gully-fans is generally strongly modified, which hampers interpreting formative processes from surface morphology. Many Martian gullies dominantly formed by debris flows, as evident from sedimentological analysis of outcrops in gully-fans. The inferred debris-flow origin for many gullies implies limited and strongly ephemeral liquid water during gully-formation. For example in Istok crater, debris flows occurred at Earth-like frequencies (one flow every 1-100 yr) during short high-obliquity periods in the last million years on Mars. To form the debris flows, a decimeters thick layer of snow must have been present in Martian gullies.
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