Leslie F. Bleamaster

Evaluation of high‐resolution images allows a systematic characterization of the walls of Dao and Harmakhis Valles, Mars. Variations in wall morphology are classified into three archetypes (exposed, mantled, and incised). Gullies are exposed by removal of semi‐competent mantling deposits, suggesting progressive development of feature‐types in an idealized sequence from i) mantled walls, to ii) incised mantle, and eventually iii) walls that display exposed gullies. Analysis of latitude, elevation, and wall orientation of 420 representative sample sites reveals spatial patterns that display preferential clustering of the two end member morphologies: exposed gullies are favored at lower latitude, higher elevation, and on east facing walls; mantled walls are more prevalent at higher latitude, lower elevation, and on south and southwest facing walls. These morphologies and their patterns are consistent with the hypothesis that gullies emerge from beneath an ice‐rich mantle that is degrading in response to local variations in total solar insolation.

The Ix‐Chel, Kuanja, and Vir‐ava Chasmata (IKVC) are a 7000‐km‐long collection of discontinuous topographic troughs and deformation suites. Collectively, they account for approximately 12.5% of the ∼55,000 linear km of chasmata on Venus; however, morphologically the IKVC differs significantly from the majority (75%) of Venusian chasmata because it lacks corona features. Coronae represent notable volumes of material both erupted onto the surface and intruded into the crust. Together with chasmata, coronae may also be important indicators of Venus's interior convective processes as well as significant contributors to Venus's heat loss. Although IKVC lacks coronae, detailed geologic and structural mapping along its length highlights previously unrecognized structural and volcanic relationships indicative of local uplift, collapse, and volcanism we call pseudocoronae. In light of this mapping, previous corona formation models (theoretical and numerical) are evaluated, and an amended hypothesis (diapir stagnation) is presented for pseudocorona formation. The diapir stagnation model proposes that variations of the crustal thickness perturb corona formation and thus affect their resultant surface morphology. Chasmata are accompanied by corona development when forming in regions of relatively thin, or normal (plains‐type), crust (<20 km) but lack corona features when influenced by crustal heterogeneities (mechanical and chemical) at depth associated with thick crustal plateau roots (35–80 km (e.g., Ovda, Thetis, and Phoebe Regios)). Beneath crustal plateaus, diapiric rise is inhibited by (1) the presence of a low‐density crustal keel that creates a deep‐seated density trap and (2) a composition‐dependent mechanically weak crust‐mantle boundary layer. Diapir stagnation can be characterized or accompanied by minimal broad doming of the surface, formation of radial and concentric extension structures, lateral dike propagation, and collapse, all of which may be accompanied by limited volume surface eruptions of the lowest density melt phases. Through time, the crust‐mantle boundary layer may repeatedly lift and subside through conductive heating and viscous flow, which may allow more voluminous melt migration and eruption; however, true coronae fail to develop.