Impact crater and basin control of igneous processes on Mars

Numerous martian impact craters have been heavily modified by processes restricted to the crater interior. A common expression of this modification is the presence of extensive fractures arranged in a concentric plan and typically forming moats that engulf the old crater wall. Although similar styles of modification occur on the moon, martian floor‐fractured craters display a greater diversity in morphology. Such craters are closely associated with major regional features and provinces, such as the Vallis Marineris system, the fretted terrains, and the martian plains. The well‐preserved record of this style of crater modification and its proximity to similarly preserved regional features imply that floor‐fractured craters represent crater‐controlled sites of a late‐stage and widespread pulse of igneous activity. Heat from such activity may locally thaw ground ice, resulting in the observed diversity in style of crater modification. Theoretical calculations show that heat released by a mafic sill beneath the brecciated zone of an impact crater may thaw trapped water‐ice at depth over periods on the order of 104–105 years. Thawed materials may gradually escape through peripheral fractures surrounding the crater floor. Alternatively, a metastable state of potential liquification can occur if the material is confined and the rate of thawing exceeds the rate of escape. This establishes conditions for catastrophic release of a warmed slurry which may produce the chaotic terrain and outflow channels as suggested by other investigators. Identified multiring basins associated with Margaritifer Sinus and the fretted terrains are proposed to represent broader scale control of igneous processes by old impact structures, in direct analogy with floor‐fractured craters. Moreover, the arcuate pattern of Noctis Labyrinthus and a concentric arrangement of massifs identify a proposed Tharsis impact basin centered on Syria Planum.