Structural control and Amazonian emplacement of mound-like edifices in Bernard Crater (Terra Sirenum, Mars)

The origin of mound-like edifices in the Martian southern highlands remains debated, particularly whether these features result from sedimentary extrusion or magmatic volcanic processes, and to what extent their emplacement is controlled by regional tectonics associated with the Tharsis stress field [1], [2]. Resolving this issue requires isolating the role of structural inheritance from local geomorphological effects within resurfaced basins.

Bernard Crater, located in Terra Sirenum (~20°–45°S; 140°–180°W), provides a well-constrained setting to address this problem. The crater (~128 km diameter) is superimposed on an ancient Noachian crust affected by sedimentary infilling and aqueous alteration [3], [4], and lies within a region structured by radial and circumferential fracture systems linked to the Tharsis tectono-magmatic province. Its floor was resurfaced during the Amazonian and hosts a dense population of mound-like edifices.

This study integrates CTX-based geomorphological mapping, structural analysis, THEMIS IR Night observations, morphometric measurements, and crater size–frequency distribution (CSFD) dating to determine whether mound emplacement is structurally controlled at regional scale, locally organized, or independent of the broader tectonic framework.

A total of 182 mound-like edifices were mapped within Bernard Crater, compared to 673 across the entire Terra Sirenum region. Their distribution is strongly heterogeneous, with a preferential concentration on the crater floor and absence on surrounding plateau surfaces. Their spatial organization is defined by clustered populations and linear alignments extending over several kilometers. Alignment analysis shows a dominant orientation between 150° and 170° (NNW–SSE), consistent with the principal orientation of mapped fractures within the crater.

At local scale, several mound chains are superimposed on concentric fractures and linear features, indicating that pre-existing discontinuities acted as pathways for ascent.

However, comparison with the regional structural framework reveals that mound organization is not systematically correlated with large-scale tectonic patterns across Terra Sirenum. Although regional fracture systems display multiple orientations related to Tharsis-induced stress fields [2], mound alignments are dominated by a single orientation and remain spatially restricted to specific sectors of the crater floor. The clustered populations further emphasize this localization, with high-density zones confined to limited areas rather than distributed along regional structural corridors. These observations indicate that mound emplacement is controlled primarily by local structural conditions within the crater rather than by direct expression of the regional tectonic framework.

Morphological analysis shows that the mound population is dominated by conical edifices with summit depressions, associated with less frequent collapsed structures and caldera-like features restricted to Bernard Crater. Morphometric data defines a coherent population: basal diameters range from ~80 to 1300 m, with most values between ~120 and 400 m; heights range from ~15 to 55 m, with a concentration between ~25 and 40 m; flank slopes are narrowly distributed between ~3° and 6°. These parameters indicate low-relief constructional landforms with consistent scaling relationships.

THEMIS IR Night observations (100 m/pixel) show that mound-bearing sectors correspond to distinct thermophysical domains within the crater, often associated with localized darker regions and linear features aligned with fractures. These patterns indicate that mound emplacement is embedded within structurally controlled surface units, although individual edifices are only weakly resolved at this scale.

CSFD analysis demonstrates that the crater floor was extensively resurfaced during the Amazonian (~250–500 Ma), defining the youngest geological surface within the study area. This timing postdates the main phase of Tharsis construction and indicates that mound emplacement occurred during a late stage of Martian geological evolution.

These results resolve the initial problem by showing that mound emplacement in Bernard Crater is structurally controlled but not directly governed by the regional tectonic framework of Terra Sirenum. Instead, emplacement reflects localized reactivation of inherited structures within the crater, which acted as preferential pathways for ascent during the Amazonian. The conical morphologies, vent-like summit depressions, restricted morphometric variability, and consistent alignment patterns are most compatible with small-scale monogenetic volcanic constructs [5], [6], while sedimentary extrusion remains a secondary and less consistent hypothesis [7].

Bernard Crater therefore records localized tectono-volcanic activity superimposed on an ancient crust, demonstrating that late-stage geological processes on Mars were controlled by local structural conditions rather than by direct expression of regional stress fields.

 

References

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