THEMIS multispectral and thermophysical information combined with TES hyperspectral and albedo data is used as a powerful high spectral/spatial resolution tool to investigate the mineralogic heterogeneity of ancient Martian crust exposed in Ares Vallis bedrock. Three major spectral units are present in the upper Ares Vallis region: (1) a regional unit that is composed primarily of a high-silica (Si/O > ~0.35) component, with lesser amounts of plagioclase and pyroxene, and is associated with channel wall rock and surrounding plains; (2) a pyroxene- and olivine-rich rock unit exposed as a ~250 m thick contiguous layer in the wall rock of Ares Vallis, as well as in isolated exposures in the plains outside of the channel; and (3) a unit composed of plagioclase, pyroxene, and lesser high-silica component(s) (Si/O > ~0.35) that primarily occurs as low-albedo sand. The spatial and stratigraphic distribution of these units suggests that the pyroxene- and olivine-enriched unit may be extrusive and/or intrusive in origin, indicating that this region experienced either a single stage or repeated episodes of olivine-enriched magmatism during the first 1.5 Gyr of crust formation. This olivine enrichment may have been caused by a larger degree of mantle partial melting, facilitated by higher mantle temperature, melting from a more depleted mantle source, and/or less olivine fractionation relative to regional rock parent magmas. The olivine-rich unit is similar in thermophysical character to previously published olivine-bearing terrains on Mars, but the derived modal mineralogy consists of >~20% more pyroxene and less plagioclase than those terrains.