GEOLOGY OF LACHESIS TESSERA – A QUADRANGLE OF VENUS CONTAINING TESSERA TERRAINS, CORONAE, RIDGE BELTS, SHIELD FLOWS, PANCAKE DOMES, AND REGIONAL PLAINS

Here we present our progress towards completing a geologic map of Lachesis Tessera, a quadrangle in the northern hemisphere of Venus (25°-50°N, 300°-330°E). Regional plains Sedna and Guinevere Planitiae cover ~80% of the quadrangle. The region contains two prominent ridge belts and embayed fragments of 1-2 possible additional belts, three large central volcanoes, abundant small shield volcanoes and associated flow materials, pancake domes, three named coronae, many coronae-like features, and thirteen impact craters. A linear grouping of a prominent ridge belt, coronae, and coronae-like structures are oriented NW to SE in the southern half of the quadrangle. Structural features include radar-bright lineaments, graben, and wrinkle ridges, as well as broader ridges that may be local folds in regional plains. Several small circular features with dark interiors or rims but negligible relief have a mysterious origin—possibly a record of craters shallowly buried by regional plains.

Basement units are highly deformed, radar-bright, topographic highs embayed by regional plains and volcanic units. This includes tessera terrains, deformed by two or more orientations of ridges, grabens, or lineaments at high angles to each other, and are inferred as the oldest unit in the map region. Other basement units include less-deformed but still radar bright materials that are tessera-like or hummocky, but lack several orientations of cross-cutting deformation structures. Regional plains that embay basement units cover ~80% of the map area and include two distinct plains units distinguished by radar brightness. Distinct dark and mottled plains units are also delineated. Volcanic units superposed on top of regional plains include several flows from large shields, abundant clustered small shield flows, and many isolated flows. Impact craters in the map area range from 2.4-40 km diameter, including one doublet, and several crater flows and dark parabolas. This geologic map is updated from a draft completed by George McGill before his death.