Tectonics of Syrtis Major Planum on Mars

Abstract

Mare ridges were caused by compressional tectonics and indicate the shortening of the planum surface foiled by lavas. At least two separate tectonic phases within Syrtis Major Planum can be found. The two central calderas are located on the southwestern continuation of the Nili Fossae graben zone at the junction of the N-S and NW-SE mare ridge sets. These central calderas were formed by surface collapses into relatively shallow magma chambers. Radial and concentric mare ridges around the two calderas represent a shortened surface environment within the large compressional megacaldera. Shortening was caused by sinking of the crust due to the lava load, plumbing of the magma chambers and cooling of the interiors. The main NW-SE ridge trend parallels highland faults of the major structural zone extending from Hesperia Planum to Vastitas Borealis. These NW-SE ridges indicate the large scale areal tectonic trend along the Scopulus Oenotria - Phison Rupes fault zone and support the idea of a main SW-NE compression. The N-S directed mare ridges of the northern planum area favour a change in compressional stress direction from SW-NE in the south to E-W in the northern planum, obviously due to the buried local topography. These linear mare ridges can also be interpreted as forming a large Isidis Planitia-concentric ridge circle connecting Nili Fossae to Libya Montes. Formation of the mare ridges was the youngest of the main tectonic phases involved within the area studied.