ANALYSIS OF FRACTURE ORIENTATION TRENDS ON EARTH AS A POTENTIAL ANALOGUE FOR TESSERA TERRAIN ON VENUS

The Magellan Mission (1989-1994) dataset continues to be the most detailed radar imagery of the Venusian surface, but the resolution is inadequate to test hypotheses regarding the formation of some surface features. We study analogous features on Earth where more data is available to gain insight into the possible formation of features on Venus. Tessera is a type of heavily fractured terrain on Venus, and its formation and composition are largely unknown. It is characterized by two or more sets of intersecting features; the exact patterns vary across the planet. Some of the patterns are interpreted as intersecting sets of joints. Visually similar features, on a smaller scale, occur in the Palms Granite of Joshua Tree National Park, prompting a digitization and statistical analysis of the fractures from satellite imagery. At the study site at Indian Cove, Cretaceous granitic plutons intrude into Proterozoic gneiss. The granite is heavily fractured, and 93 of the fractures were digitized using 1-m resolution NAIP Imagery, DEM imagery from Cal Atlas and the USGS Joshua Tree National Park Database in ArcMap. Fracture placement was verified using Google Earth. Fracture orientations were calculated using ArcMap and plotted on a rose diagram in Stereonet to identify and analyze sets of similar strikes. We see 3 distinct sets of fractures: Set 1, mean azimuth 072° ± 3°; Set 2, mean azimuth 333° ± 3°; Set 3, (only 7 fractures) mean azimuth 120° ± 6°. Uncertainties are given as 1σ. The fractures cut across the topography, indicating they are close to vertical; dips are assumed to be 90°. The average lengths are: Sets 1 and 2, 1.7 km; Set 3, 2 km. Sets 1 and 2 are oriented 99° from each other, suggesting they result from extensional jointing under two different paleostress orientations of σ₃. In the DEM data, the fractures of Set 2 have more dramatic topography, so that set likely formed first. The fracture sets appear mutually cross-cutting in the satellite imagery, but field data is needed to confirm this conclusion. The features mapped in this study are interpreted as extensional joints similar to, though on a smaller scale than, those found on tesserae. If tesserae are made of granitic or homogenous rock, their fractures may be formed in a similar way.