Structural development of Coprates Chasma and western Ophir Planum, Valles Marineris Rift, Mars

A portion of Valles Marineris was mapped in detail in order to clarify the dominant processes responsible for the formation of Coprates Chasma. New crater counts indicate that the caprock on western Ophir Planum plateau has a Late Hesperian crater age, whereas trough floor preserved in western Coprates Chasma has an Early to Late Hesperian crater age. Caprock on western Ophir Planum correlates in relative age with the Syria Planum Formation, and the caprock may overlie Lower Hesperian ridged plains material. Trough floor material in Coprates Chasma correlates in relative age with ridged plains material on the adjacent Lunae Planum and Coprates plateaus and represents a structurally coherent block displaced downward by normal faulting. The crater counts and detailed structural relationships demonstrate the commonly accepted view that Coprates Chasma occupies a graben. Formation of Coprates Chasma by keystone collapse of locally elevated topography is not supported by available topographic data, but a general association between the trough and volcanotectonic activity in the Tharsis region is considered to be a likely explanation for the trough forming stresses. Faulting on Ophir Planum appears related to the faulting that created the Coprates Chasma trough. The geology and structure of Coprates Chasma are comparable to those of other troughs such as Melas, Ius, and perhaps Candor chasmata, suggesting that these troughs may also have formed as grabens.