Insight from the Noachian-aged fractured crust to the volcanic evolution of Mars: A case study from the Thaumasia graben and Claritas Fossae

Although most of the large volcanic landforms on Mars have been extensively studied, this is not the case for kilometer-sized landforms whose origin remains uncertain and which might provide important insights into the evolution of Martian volcanism. Previously, different populations of small-scale putative scoria cone volcanoes have been described in Tharsis, specifically in areas where the heavily fractured crust of the Noachian- and Hesperian-age escaped Amazonian resurfacing by younger lava flows. Therefore, we decided to explore a region of old fractured crust in Tharsis, Claritas Fossae, to search for signs of local-scale volcanic activity. Here, we present the results of a mapping campaign focused on the morphological and spectral properties of small-scale constructional mounds in the Noachian-age Claritas Fossae region that was affected by Hesperian- to early Amazonian-aged fracturing. By using data from Context Camera (CTX) and High Resolution Imaging Science Experiment (HiRISE), we mapped 39 mounds superimposed on the ancient crust and determined their morphological and morphometrical properties. The majority of the mounds are elongated (WNW- trending) with steep flanks and without associated flow units. The general appearance of studied edifices is consistent with a volcanic origin and their shapes suggest that they have been emplaced by effusion of viscous, volatile poor lavas incapable of significant flow. In addition, a spectral investigation utilizing the data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) reveal that some of the mounds showed evidence for high concentrations of low-calcium pyroxenes (LCP). Based on the relative stratigraphy, we demonstrate that volcanic activity is responsible for their formation and postdates the main N-S-trending fracturing of the Claritas Fossae, suggesting the formation of a volcanic field in the period spanning through the Late Hesperian and the Early Amazonian. However, morphological evidence such as well-preserved volcanic fissures and fresh-appearing slopes of some of the edifices might suggest even younger age. Among the mapped edifices, we found a wellpreserved circular-shaped cone with a short-distance flow-like unit spreading from a caldera-like structure that challenges a scenario in which all the edifices are old. Such relatively young ages of volcanic activity combined with LCP-rich compositions might have critical implications for understanding the temporal evolution of magma compositions on Mars. Moreover, the observed shapes and spectral characteristics of the studied edifices appear to be quite different from other volcanic fields of similar geological settings observed so far on Mars. Therefore, we underline the importance of studying old and heavily fractured terrains that escaped later resurfacing by widespread younger lava flows, in order to search for evidence of small-scale volcanism and better understand local Martian magmatic systems.