Structures in tessera terrain, Venus: Issues and answers

Many workers assume that tessera terrain, marked by multiple tectonic lineaments and exposed in crustal plateaus, comprises a global onionskin on Venus. Because tesserae are exposed mostly within crustal plateaus, which exhibit thickened crust, issues of tessera distribution and the mechanism of crustal plateau formation (e.g., mantle downwelling or upwelling) are intimately related. A review of Magellan data indicates that tessera terrain does not form a global onionskin on Venus, although ribbon-bearing tesserae reflect an ancient time of a globally thin lithosphere. Individual tracts of ribbon-bearing tessera terrain formed diachronously, punctuating time and space as individual deep mantle plumes imparted a distinctive rheological and structural signature on ancient thin crust across spatially discrete 1600-2500 km diameter regions above hot mantle plumes. Plume-related magmatic accretion led to crustal thickening at these locations, resulting in crustal plateaus. Crustal plateau surfaces record widespread early extension (ribbon structures) and local, minor perpendicular contraction of a thin, competent layer above a ductile substrate. Within individual evolving crustal plateaus the thickness of the competent layer increased with time, and broad, gentle folds formed along plateau margins and short, variably oriented folds formed in the interior; late complex graben cut folds. Local lava flows accompanied all stages of surface deformation. In contrast to these conclusions, Gilmore et al. ( 1998) summarized post-Magellan arguments in favor of downwelling models for crustal plateau formation. In light of this discrepancy, we reexamine the regions investigated by these workers and evaluate their arguments against upwelling models. We show that Gilmore et al. ( 1998) did not differentiate ribbons from graben and therefore their proposed temporal relations are invalid; they disregarded shear fracture ribbons, thus invalidating their criticism of ribbon models; they misunderstood previous radargrammetric work that constrains ribbon geometry; and they relied solely on geometrical relations to constrain timing, violating kinematic analysis methodology. Their stratigraphic constraints on ribbon-fold temporal relations are invalid because they (1) misinterpreted implications of map relations; (2) did not isolate radar artifacts due to local radar slope effects from proposed material units; (3) chose a region for analysis that clearly shows the effects of younger tectonism and volcanism; and (4) presented map relations that cannot be reproduced. Their attempts to discount upwelling models of crustal plateau formation fail because they combine fundamentally different pre-Magellan and post-Magellan upwelling models. These misconceptions about the upwelling model and processes responsible for global warming (Phillips and Hansen, 1998), lead to serious errors in Gilmore et al.'s (1998) criticism. Furthermore, we show that the data of Gilmore et al. ( 1998) are actually more consistent with upwelling than downwelling models, consistent with arguments that tessera terrain is not global in spatial distribution.