• Hellas;
  • Holden;
  • Mars;
  • alluvial fans;
  • impact crater;
  • valley networks

[1] The degradation of impact craters provides a powerful tool to analyze surface processes in the Martian past. Previous studies concluded that large impact craters (20–200 km in diameter) were strongly degraded by fluvial erosion during early Martian history. Our goal is to study the progression of crater degradation through time with a particular emphasis on the craters with alluvial fans and on the relative chronology of these craters. The geometric properties of 283 craters of >20 km in diameter were analyzed in two highlands of Mars, north of Hellas Planitia, and south of Margaritifer Terra, both known to contain craters with alluvial fans. Three classes were defined from morphology: strongly degraded craters with fluvial landforms and without ejecta (type I), gently degraded craters with fluvial landforms and preserved ejecta (type II), and fresh craters with ejecta and no fluvial landforms (type III). Our main result is that the type II craters that present alluvial fans have characteristics closer to fresh craters (type III) than degraded craters (type I). The distinctive degradation characteristics of these classes allowed us to determine a temporal distribution: Type I craters were formed and degraded between ∼4 Gyr and ∼3.7 Gyr and type II craters with alluvial fans were formed between Early Hesperian and Early Amazonian (∼3.7 to ∼3.3 Gyr). This chronology is corroborated by crosscutting relationships of individual type II craters, which postdate Late Noachian valley networks. The sharp transition at ∼3.7 Gyr suggests a quick change in climatic conditions that could correspond to the cessation of the dynamo.