Geology of Sinai Tholus, Thaumasia Plateau, Mars

A new geologic map of the Sinai Tholus region on the Thaumasia plateau, Mars reveals the structural, magmatic, and stratigraphic complexity that can be found in the Tharsis region on Mars, and may help answer key questions about the origin of the subvolcanic basement, evolution of the regional stress field, development of Valles Marineris, and magmatic history of the region. Sinai Tholus is a topographic high in a relatively flat, lava flow covered part of Thaumasia Plateau, just south of the central part of Valles Marineris. Here, five stratigraphic units and one generalized impact unit have been mapped at a 1:200,000 scale. The Late Noachian Sinai Tholus Formation (lNst) is the oldest (at least 3.65 Ga based on crater retention age), and the most rugged. This unit is likely megaregolith consisting of altered impact breccia, sedimentary units, and lava flows. Numerous grabens strike NW (159°), NE (42°), and EW (86°), with EW-striking faults being unique to lNst and the oldest of the three sets. Similarly unique to lNst are linear ridges; these likely exhumed igneous dikes have higher olivine abundances according to the spectral analyses of CRISM and are erosionally resistant topographic highs. Their average strikes (18° and 74°) are different enough from the grabens to indicate an earlier, magmatic-related extension resulting in their formation. lNst is only exposed along an eroded normal fault face; much of it is covered by the Late Hesperian Highlands of the Sinai Dorsa Formation (lHsd) (3.6 Ga). A patera in this region may have been an eruptive source for this unit, indicating an igneous origin. However, CRISM data indicate higher abundances of low temperature alteration minerals than in the underlying Sinai Tholus Formation. This, combined with its highly erodible nature, makes it appear that lHsd was deposited or altered during a wet, oxidizing period. It was then cut by the NE-striking faults and uplifted about 1 to 1.5 km along an EW-trending normal fault. These two units were subsequently eroded in the south, exposing lNst and its igneous dikes and fault sets. This is further supported by the presence of erosional, seemingly fluvial, escarpments defining the boundary between lNst and lHsd. The Late Hesperian Nia Fossae Formation (lHnf) was deposited as a series of fissure-fed basaltic lavas around 3.6 Ga, and its vents likely covered. Further extension caused NW-trending faults to cut all units. The Late Hesperian Ridged Plains of the Sinai Planum Formation (lHsp) also formed around 3.6 Ga as a younger series of fissure-fed basaltic flood lavas. lHsp covered much of lHsd, lHnf, and the patera. The region then underwent tectonic contraction, possibly due to planetary cooling, as evidenced by wrinkle ridges with a consistent strike (146°) similar to those found globally. The uniquely stratified Early Amazonian Louros Valles Formation (eAlv) formed about 1.9 Ga and buries the northern exposures of lHsd and lHsp. CRISM and CTX imagery indicate that this unit consists of thin alternating light and dark layers containing clays and sulfates. Similar beds have been noted on the north side of Valles Marineris and could have been part of a large, ephemeral lake before being cut by the canyon.