VOLCANISM AND TECTONISM IN THE LACHESIS TESSERA QUADRANGLE (V-18) REGION OF VENUS

We present the geologic map of the V-18 Lachesis Tessera quadrangle (25^o-50^oN, 300^o-330^oE) of Venus. Mapping was based on a 250 m/p Magellan cycle 1 synthetic aperture radar 1:5M scale controlled mosaic, but most of the analysis utilized 75 m/p FMAPS. Topographic information was derived from digital elevation models and from gridded elevation data; altimetry data were combined with SAR data to create synthetic stereoscopic images.

Stratigraphic units were defined primarily by their radar brightness and surface textures, although crosscutting relations, relative ages, and apparent association with topographic or structural features were also considered. The 29 units were grouped into six broad categories according to topographic setting or terrain type, including: 4 basement units (including “tessera materials”), 4 plains units (including “regional plains”), 11 volcanic units, 4 coronae units, 2 deformation belt units, and 4 impact crater units.

Volcanic materials superpose regional plains throughout V-18. Four central volcanoes have calderas surrounded by digitate flows of variable brightness. Large flow fields are commonly associated with clusters of small shield volcanoes. Isolated small shields, pancake domes, and digitate flows with no resolvable source are also identified.

A linear grouping of a prominent structural features runs NW to SE across the quadrangle. Breksta Linea, the largest fracture belt in V-18 (~500 km), ends at a raised magmatic complex with two calderas and three pancake domes. Zemire corona, Pasu-Ava corona and putative, partially imaged corona comprise the rest of the structural grouping.

Coronae in V-18 are widely separated and it is not possible to determine their relative ages. The structures defining the coronae cut materials of both regional plains units. The ages of the flows associated with the coronae relative to other materials are ambiguous, although most corona materials appear to be younger than adjacent regional plains.

V-18 craters range from 2.4 to 40 km diameter and are superposed on either regional plains or on flows that are superposed on regional plains. We also observe >10 dark spots between 10-50 km diameter that are without topographic relief; 6 have a distinct annulus. These are not uncommon features on Venus and are often interpreted to be airburst impact scars.