ANALYSIS OF LACHESIS AND ZIRKA TESSERAE IN THE V-18 QUADRANGLE OF VENUS

The Lachesis Tessera quadrangle (V-18) of Venus is bounded by 300^° and 0^° E, 25^° and 50^°N. The oldest materials in the V-18 quadrangle are tessera and tessera-like materials. They are primarily located in two areas along the western boundary of the quadrangle, where the northern exposures are named Lachesis Tessera, and the southern exposures are named Zirka Tessera. While approximately half of Lachesis Tessera occurs within the V-18 quadrangle, Zirka is more areally extensive to the west in the Beta Regio (V-17) quadrangle.

Zirka Tessera is the topographically highest feature in the quadrangle. It is very bright in SAR images, and is deformed by grabens, ridges, and penetrative lineations, with at least two dominant trends at high angles to each other. These structural features dominate texture down to the scale of resolution, and they very likely are responsible for the overall radar brightness of tessera materials. Tessera materials are sharply embayed by all other mapped units in the quadrangle. Adjacent plains materials truncate almost all structures superposed on tessera, and also commonly flood grabens that cut tessera material. Tessera material is older than all other materials based on truncation relations and density of superposed structures, and thus is inferred to be the oldest material in the quadrangle. There is no evidence in this quadrangle to determine the relative ages of materials within isolated patches of tessera.

Lachesis Tessera does not exhibit a pattern of two or more well defined deformation trends at high angles to each other, as is generally the case for tessera terrain: rather, it is characterized by multiple deformation trends that generally are spatially distinct from each other, hence the absence of cross-cutting trends that generally define tessera terrain. On the other hand, Lachesis Tessera is radar bright and highly deformed. It also is extensively flooded by plains materials, which likely is at least partially responsible for the lack of the clear-cut cross-cutting relations that normally characterize the trends of structures deforming tessera material. Although higher than the immediately surrounding plains, Lachesis is relatively low elevation compared to the rest of the quadrangle.