Dikes: key link between Thaumasia Planum, Tharsis, and Valles Marineris?

The area called Syria-Thaumasia is a big triangular block of Mars just east of three giant Tharsis volcanos: Arsia, Pavonis, and Ascraeus Montes. Valles Marineris bounds the block on the north, while its southwest and southeast sides are defined by two large highlands that converge toward the south.

PUSH, THEN CRACK. Volcanic dikes (red lines) slice across compression-made wrinkle ridges in Thaumasia Planum. The dikes show a texture and composition different from the lavas that form the surface of Thaumasia Planum, and scientists suggest they offer clues to the evolution of the area. (Image taken from Figure 1 in the paper.)

As one of the most unusual pieces of terrain on Mars, the Syria-Thaumasia region and its origin have long presented a geological puzzle. After studying its features in detail, a team of researchers led by Jun Huang (China University of Geosciences in Wuhan) reports that a network of volcanic dikes offers clues to the origins of Thaumasia Planum, at the eastern end of the big triangle. Their report was published in Geophysical Research Letters (September 6, 2012).

“We have identified several exposed dikes in Thaumasia Planum,” the team writes. “These dikes extend from tens of kilometers to about 100 kilometers [about 6 to 60 miles] in length with average widths of about 50 meters [160 feet].” Volcanic dikes form when molten magma pushes upward into cracks and fissures in the crust. The dikes in Thaumasia Planum extend generally east-west and cut across pre-existing geologic features. These include many large wrinkle ridges produced by horizontal compression. When seen in high-resolution images, both the dikes and the erupted material next to them appear very blocky. Their surfaces also appear substantially harder and more solid than the surface materials elsewhere in the surrounding area.

“We propose that these dikes might have served as feeders for the olivine-enriched flood basalts found in the region,” the scientists write. They add that the basalts may have come from the same magma source that fed the whole Tharsis area, a hypothesized “plume” of molten rock rising from deep in the Martian mantle.

They explain that a geologic history for Thaumasia Planum begins with the rise of volcanism in Tharsis. This compressed Thaumasia Planum. Then sheets of olivine-rich basalt from the mantle flooded over the landscape. This was followed by the formation of wrinkle ridges from compression and dikes from extension, all driven by the forces from still-growing Tharsis. The dikes, they note, could have helped to feed the lava flows. Then dust and volcanic ash covered the region, while impacts and wind erosion exposed the olivine-rich basalts here and there.

The dikes run generally parallel to Valles Marineris, the scientists remark, so “it is plausible that the fractures associated with Valles Marineris provided the path for dike emplacement.” Thus the dikes “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.”