SHIELD VOLCANO CONCENTRATIONS IN INTRATESSERA BASINS ON VENUS: IMPLICATIONS FOR CRUSTAL PLATEAU EVOLUTION

Intratessera basins (ITBs) within crustal plateaus on Venus are irregularly shaped topographic lows flooded with low-viscosity lavas. Studying the volcanic history of ITBs relative to structure formation in the tessera may help resolve controversies over the formation mechanism of crustal plateaus. This study examined differences in concentrations of small shield volcanoes within intratessera basins and the timing of volcanic fill relative to formation of structural features.

Individual shield volcanoes in ITBs in four crustal plateaus were mapped and categorized on Magellan synthetic aperture radar (SAR), inverted SAR, and synthetic stereo images. The relative number of shield volcanoes in each basin shows that shield concentrations vary widely among the four plateaus analyzed, with no discernible pattern. Hundreds of small shield volcanoes occur in Tellus basins. Basins in Ovda Regio contain no recognizable shields, and basin fill generally appears featureless. Fortuna Tessera contains basins with and without shields, with no particular pattern in their distribution. In Alpha Regio, shield concentrations vary, with basins ranging from highly shielded to virtually featureless. All basins examined contain structures that crosscut basin flows as well as those that were flooded by basin flows, indicating that basin volcanism was contemporaneous with formation of at least some of the structures in the plateaus.

This work supports a model of crustal plateau evolution in which volcanism occurred during, rather than after, plateau formation on Venus. Variation in shield volcano concentrations suggests, however, that differences must have existed in processes involved in the formation of the ITBs in crustal plateaus. Any proposed model for plateau evolution should be flexible enough to account for this observed widespread variation in basin fill character among plateaus. One possibility consists of using large subsurface magma chambers and dike formations to explain the appearance of the flows in Ovda, and small, individually fed shield volcano constructs to explain the appearance of basins in Tellus, perhaps with combining the two models with a lithosphere thickening over time to explain observations at Fortuna and Alpha.