Graben and extension in northern Tharsis, Mars

A morphometric analysis of north-south trending graben at 35 deg N in northern Tharsis is used to provide the first quantitative measurements of crustal extension in and strain for areas of the Martian crust that have undergone both extreme and mild deformation. These estimates also demonstrate the viability of the Viking data for making such quantitative assessment. Graben in the region accommodate about 8 km of post Early Amazonian east-west extension. This extension corresponds to a net regional strain of 0.45-0.61 percent; local strains vary from less than 1 percent to 5 percent. Extension is nonuniform and localized near 110 deg (south of Alba Patera and north of Ceraunius Fossae) because of reactivation of preexisting fractures in underlying basement and possibly due to superposition of stress fields from Tharsis and Alba Patera. A major mechanical discontinuity in the shallow crust, probably representing the base of or a strength contrast within the megaregolith, is observed. It occurs at a depth of 2 km on the profile flanks and is depressed to about 7.5 km near 110 deg due to loading of the crust by the volcano.