Fig. 1: Evidence for erosion in the southern portion of Salus tessera.

a Left-looking Cycle 1 Magellan SAR image (central coordinates 4.4°S, 48.2°E); north is up, showing tessera (light grey and lineated) with a re-entrant area that has been flooded by younger plains lavas (featureless dark grey). White band marks absence of Magellan SAR data, but topographic data are available in this region. The topographic relief between the valley (covered by lava flows) and bordering tesserae hills is about 100 m (along profile line A–A’). Dotted portion of brown line extrapolates across a topographic data artefact (appearing as a topographic ‘hole’) in the dataset. Red portion of topographic profile corresponds to the portion flooded and embayed by plains lavas. Edge of main flooding (red line) on left side extrapolated from the SAR image on both sides of the data gap. b–d End-member generalized alternative geological interpretations for cross-section A–A’ of which only the third (d) fits the observations: b a purely tectonic interpretation whereby these landforms are a central syncline and bordering anticlines; c a purely fluvial erosional interpretation, or d initial tectonic activity (synclinal folding) followed by fluvial erosion. T = tesserae and L = younger mafic lava flows. e Elevated oblique view (looking north) generated by draping the SAR image onto Magellan altimetry topography using the software ArcScene. Vertical exaggeration is 20× and the viewing elevation angle is about 20°. The tessera is orange and the younger mafic lava flows are light yellow, and the smooth yellowish-white band is the missing data. Noteworthy are the white dashed lines in e that correspond to prominent lineament sets that may reflect inward dipping strata on both flanks of the inferred syncline.