DARK SPOTS SUPERPOSED ON THE REGIONAL PLAINS OF THE LACHESIS TESSERA QUADRANGLE (V-18), VENUS
Some of these maculae are consistent with the appearance of fine debris accumulations , while others show expression of annular features that cannot be explained by debris alone. Some of the annular features also exhibit diffuse bright rims, similar to those interpreted as pyroclastic deposits around volcanic features . However, none of these features have any notable topographic relief, as would be expected in association with volcanic features. These features are found exclusively on the regional plains that cover about 80% of the V18 quadrangle and are not geographically clustered.
Interpretation of these observations is constrained by the ~5 km resolution of topography which cannot distinguish smaller-scale features. The recently funded VERITAS mission to Venus will improve the resolution of radar imaging and topographic models, which may shed further light on the origins of these dark features.
References:  Ford, J. P. (1993). Guide to Magellan image interpretation. National Aeronautics and Space Administration, Jet Propulsion Laboratory, California Institute of Technology.  Campbell, B. A., Morgan, G. A., Whitten, J. L., Carter, L. M., Glaze, L. S., & Campbell, D. B. (2017). Pyroclastic flow deposits on Venus as indicators of renewed magmatic activity. Journal of Geophysical Research: Planets, 122(7), 1580-1596.