Rothery, David A.; Thomas, Rebecca J. and Kerber, Laura
Prolonged eruptive history of a compound volcano on Mercury: volcanic and tectonic implications.
Earth and Planetary Science Letters, 385 pp. 59–67.
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A 27 × 13 km ‘rimless depression’ 100 km inside the southwest rim of the Caloris 19 basin is revealed by high resolution orbital imaging under a variety of illuminations to 20 consist of at least nine overlapping volcanic vents, each individually up to 8 km in 21 diameter. It is thus a ‘compound’ volcano, indicative of localised migration of the site 22 of the active vent. The vent floors are at a least 1 km below their brinks, but lack the 23 flat shape characteristically produced by piston-like subsidence of a caldera floor or 24 by flooding of a crater bottom by a lava lake. They bear a closer resemblance to 25 volcanic craters sculpted by explosive eruptions and/or modified by collapse into void 26 spaces created by magma withdrawal back down into a conduit. This complex of 27 overlapping vents is at the summit of a subtle edifice at least 100 km across, with 28 flank slopes of about only 0.2 degrees, after correction for the regional slope. This is 29 consistent with previous interpretation as a locus of pyroclastic eruptions. 30 Construction of the edifice could have been contributed to by effusion of very low 31 viscosity lava, but high resolution images show that the vent-facing rim of a nearby 32 impact crater is not heavily embayed as previously supposed on the basis of lower 33 resolution fly-by imaging. Contrasts in morphology (sharpness versus blurredness of 34 the texture) and different densities of superposed sub-km impact craters inside each 35 vent are consistent with (but do not prove) substantial differences in the age of the 36 most recent activity at each vent. This suggests a long duration of episodic 37 magmagenesis at a restricted locus. The age range cannot be quantified, but could be 38 of the order of a billion years. If each vent was fed from the same point source, 39 geometric considerations suggest a source depth of at least 50 km. However, the 40 migration of the active vent may be partly controlled by a deep-seated fault that is 41 radial to the Caloris basin. Other rimless depressions in this part of the Caloris basin 42 fall on or close to radial lines, suggesting that elements of the Pantheon Fossae radial 43 fracture system that dominates the surface of the central portion of the Caloris basin 44 may continue at depth almost as far as the basin rim.
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