Abstract:We have identified several exposed dikes in Thaumasia Planum Mars using THEMIS, CTX, HiRISE and CRISM data. These dikes extend from tens to ∼100 kilometers in length with average widths of ∼50 m. They display classic ‘en echelon’ patterns while cross‐cutting preexisting geologic features, including extensive wrinkle ridges. Both the dikes and associated fissure eruption products have very blocky morphologies with ∼38% higher thermal inertia than the surrounding regions. The dikes are all enriched in Mg‐rich olivine relative to surrounding terrain, and a subset also contains elevated high‐calcium pyroxene, both of which indicate relatively primitive magma compositions. We propose that these dikes might have served as feeders for the olivine‐enriched flood basalts in this region, and may be derived from the Tharsis plume. These observations provide further evidence that the opening of Valles Marineris was facilitated by tectonic stresses following paths of preferential weakness along preexisting structures such as fractures and faults like those indicated by these dikes.