Hectometric-scale mounds on Mars: insights from Bernard Crater and surrounding terrains in Terra Sirenum, Mars

Hectometric-scale mound-like features in Terra Sirenum, Mars, have been hypothesized to result from either sedimentary volcanism or small-scale igneous volcanic activity. Discriminating between these processes is essential for understanding the subsurface dynamics, tectonic evolution, and hydrological history of the region. This study provides a comprehensive structural, morphological, and preliminary mineralogical analysis of more than 700 mounds distributed across Terra Sirenum, focusing particularly on the Bernard Crater area, a Noachian-aged impact structure modified by subsequent tectono-magmatic processes. Our work integrates high-resolution orbital datasets (CTX, HiRISE, MOLA, CRISM) into a GIS-based analytical framework, allowing detailed mapping, morphometric analysis, crater size-frequency dating, and mineralogical assessments. The objectives are to characterize the emplacement context of the mounds, identify possible formation mechanisms, and explore the relationship between mound distribution and regional tectonics. The analysis reveals that mound features across Terra Sirenum exhibit significant morphological variability. Morphologies include pitted cones, flat-topped mounds, clustered forms, and aligned mounds along structural trends. In Bernard Crater, mounds frequently occur in association with concentric and radial fracture systems, and their spatial clustering suggests a strong structural control on emplacement. Notably, several features within Bernard Crater display morphologies consistent with collapsed volcanic conduits and dike-fed structures, offering crucial evidence supporting an igneous origin. Crater size-frequency distribution analysis dates the surfaces hosting the mounds to the Noachian epoch (~3.7–4.1 Ga), while some areas within the Terra Sirenum basin suggest resurfacing events during the Hesperian-Amazonian. These results indicate that mound emplacement spanned significant geological timescales, potentially linked to episodic tectonic and magmatic activity associated with the evolution of the Tharsis region. Structural mapping highlights a clear correlation between mound alignments and the regional fault and graben network, particularly those associated with the Sirenum Fossae extensional system. Mounds tend to align parallel to the major graben trends or cluster along secondary fractures, suggesting that tectonic structures acted as preferential pathways for subsurface material ascent. This spatial organization is consistent with mound emplacement mechanisms involving dike intrusions or fault-assisted fluid migration. Preliminary mineralogical analysis using CRISM targeted hydrated and mafic mineral phases indicative of fluid-related processes or igneous activity. Localized detections of alteration minerals, although not definitive, point toward the interaction between subsurface fluids and the surrounding rock matrix during or after mound formation. Comparative analysis with terrestrial analogues strengthens the interpretations. While certain morphological characteristics of the Martian mounds resemble mud volcanoes observed in tectonically active regions such as Azerbaijan and NE China, key differences are apparent and small features from igneous volcanism in Arizona and Iceland. The association of many mounds with fracture corridors, the presence of summit pits suggestive of vent structures, and the absence of widespread mudflows or brecciation argue against a purely sedimentary volcanic origin. Instead, similarities with small igneous cones and dike-induced structures in rift settings, such as those in Iceland, appear more compelling. A central scientific question addressed by this study is whether the observed mound features primarily result from sedimentary extrusion processes (e.g., mud volcanism) or from magmatic activity associated with shallow dike emplacement and small-scale volcanic eruptions. The structural control, morphometric characteristics, and comparative terrestrial analogues collectively favour an interpretation where igneous processes played a major role, particularly within the Bernard Crater area. Nonetheless, given the morphological equifinality between sedimentary and igneous features and the limitations of orbital datasets, a contribution from sedimentary processes cannot be entirely ruled out. Localized episodes of fluid-assisted extrusion, possibly involving groundwater or volatile-rich materials, may have contributed to mound formation in some areas, especially in topographic lows where clustering is observed. In conclusion, the integrated structural, morphological, and preliminary mineralogical evidence suggests that the small mound features in Terra Sirenum, and particularly within Bernard Crater, are more consistent with an igneous volcanic origin than with sedimentary processes. Mound formation appears to have been structurally controlled by extensional tectonics, with subsurface dike propagation likely facilitating localized surface expressions. These findings have significant implications for understanding the tectono-magmatic evolution of Terra Sirenum and the broader highland-lowland transitional region on Mars.