IN SITU PARTIAL MELTING AND CRUSTAL DIFFERENTIATION ON VENUS: EVIDENCE OF GLOBAL SCALE METAMORPHISM?

Shield plains terrain may reflect unique evolutionary conditions on Venus. Tens of thousands of individual shields (<1-15 km diameter; <<1 km high) coalesce to form a variably developed ultra-thin mechanically strong ‘shield-paint’ layer across numerous 10⁶ km² and larger sized regions that appear blistered at 100’s of meters to km-scale. This globally extensive shield paint covers many areas delineated as ‘regional plains’ by other workers. Detailed mapping indicates that shield-paint formation and deformation occurred broadly synchronously, yet was time-transgressive. Shield-paint, which discontinuously coated deformed basal terrain, was itself subsequently deformed, and repainted locally. This layer records local reactivation and inversion structures. Locally fractures or wrinkle ridges deform shield-paint; rarely shield-paint appears to leak from fractures or ridge crests. Given shield distribution, individual paint sources must be local yet regionally extensive. Brittle strength of shield paint argues against a sedimentary origin (e.g. mud volcano). The layer could not result from flood-like lava that would necessarily be thicker and more continuous to extend across the topographically complex expanses the terrain covers; if a large magma body spawned individual shields tectonomagmatic patterns should reflect thermal stresses; however, delicate, regionally coherent, structures provide no such evidence. Likewise, magma originating at great depth would form large volcanic constructs; none are observed. Shield-paint appears therefore to emanate from regionally-distributed-point sources; the close spacing and small size of each ‘paint’ batch suggest that sources are located at shallow crustal levels. Global high T could trigger in situ near-surface dehydration-driven partial melting; melt could wick to the surface along fractures; individual ‘eruptions’ that emerged from numerous individual centers could coalesce into a extensive, yet discontinuous, ultra-thin shield paint layer; sustained low melt viscosity might result from inhibited latent heat release due to low convective heat loss at high atmospheric P and T. Shield paint, perhaps best envisioned as ultrametamorphic partial melt, could represent evidence of an ancient hot hydrous Venusian surface environment.