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. The conspicuous absence of wrinkle ridges on Ophir Planum may reflect burial by this later 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. The floor of this graben is only thinly mantled by intratrough deposits and is relatively intact; the trough shows no evidence for a chaotically broken and jumbled floor, as required by genesis models of collapse and subsurface drainage. 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.

Subtle asymmetries in topography and boundary fault development across Coprates Chasma may imply a gentle, down-to-the-north half-graben geometry for the trough. Normal faults that define the northern margin of western Coprates Chasma and those that deform the adjacent Ophir Planum plateau change trend systematically with position, implying a 30¿ rotation of the local horizontal principal stresses in this region during trough growth. 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. The revised crater age for western Ophir Planum indicates that the initiation of trough faulting and subsidence in the Valles Marineris system is early Late Hesperian and may have been synchronous across the system relative to the prevailing cratering rate. ¿American Geophysical Union 1991