Crustal Plateau Collapse on Venus: Evidence from the Pasom-mana

Introduction: Over the past several years, a number of workers have offered compelling evidence that Venusian crustal plateaus were created by mantle upwelling [1], [2], [3], [4]. Recent geophysical modeling has further suggested that, on Venus, an initially uncompensated crustal plateau created by upwelling should decay into a torroidal shape in three dimensions and a ring shape in map view [5]. Such a decayed plateau should appear on the surface of Venus as a ring of complexly-deformed terrain (tessera), perhaps flooded or partially flooded by younger lavas. A number of workers have noted the resemblance between arcuate patches of tessera in the plains and the rims of crustal plateaus [6], [7]. No one, however, has previously systematically studied isolated rings and patches of tessera to determine whether they might, in fact, be the remnants of collapsed plateaus. The existence of such areas would provide crucial evidence for resolving controversies over the origin and evolution of crustal plateaus on Venus. The Pasom-mana Tessera region on Venus (centered on 30S, 37E) consists of a topographically high ring approximately 1000 km across rising 1-3 km above both a low center and the surrounding plains (figure 1). This study aims to determine whether this ring defines the rim of collapsed and previously unrecognized crustal plateau and whether the detailed geologic history of the area is consistent with early plateau formation and subsequent collapse and burial. Pasom-mana Region: Full resolution SAR images and synthetic stereo images of the Pasom-mana Tessera region (figures 1 and 2) were examined. The evidence strongly indicates that the Pasom-mana ring is a small, partially buried crustal plateau. First, the ring is about 1000 km by 1200 km, within the size range of crustal plateaus. Second, the presence of a high ring consisting of tessera (A, B, and C in figure 1) and deformed flows with kipukas of tessera (D in figure 1) is also consistent with this interpretation. Third, around the south and southeast portions of the rim, folds in the tessera consistently define an arcuate ring parallel to the margin of the plateau, with ribbons at a high angle to the margin (A in figure 1). Such consistently-oriented fold and ribbon terrain is typical in the rims of crustal plateaus. In the southwest, northwest, and east central portions (B, C, and D in figure 1), folds and ribbons are randomly oriented, which is typical of fold and ribbon terrain in the interiors of crustal plateaus. Fourth, tessera is only shallowly buried on the inside of the ring, consistent with the interpretation that tessera lies beneath the younger flows in the center of the ring. All of these features are consistent with the formation of a single original plateau of tessera rather than isolated and unrelated patches of tessera. Tessera throughout the region is flooded and embayed by a series of younger flows. Synthetic stereo images provide clear evidence that differential motion between exposed tessera and flows occurred throughout the time period of plains formation in the area. Evidence includes the following: flooded grabens that are clearly warped up and over the southern part of the ring; flows that have been warped up against high areas of tessera (figure 2); and flows that are warped completely over ridges of tessera.