Structural analysis of a sampling of tesserae: Implications for Venus geodynamics

Understanding the nature and timing of tessera formation is fundamental to Venus tectonic and geodynamic models. Tesserae are commonly considered to exhibit complex deformation histories, to represent the oldest global stratigraphic unit, to have formed during a global phase of tesserization, and to require weak lithosphere for their formation. Although these characterizations of tesserae are gaining widespread mention in the literature, they are essentially unsupported by an assemblage of data, yet they figure prominently in the way we frame and address questions about Venus tectonics. Thus they are quickly becoming accepted paradigms despite a lack of scientific foundation. Open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of Venus. We examined tessera terrain in Ishtar Terra, crustal plateaus, and as inliers within the plains using high-resolution Magellan radar imagery. We describe several types of tessera terrain that record a wide range of structural histories. Fold and S-C terrains are found only in Ishtar Terra; ribbon, lava flow, and basin-and-dome terrains reside within the interior of crustal plateaus, whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus. Inliers are divisible into fracture-dominated and graben-dominated tesserae, although some inliers host early contractional fabrics. The range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit, Tessera deformation records local to regional surface strain patterns, and reflects near-surface rheology at the time of deformation. Progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time. Thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation. Tesserae likely formed in several tectonic environments, including (1) as a result of subsurface flow in Ishtar Terra, (2) as sequences of surface-layer extensidon and contraction in crustal plateaus, (3) as flooded crustal plateaus, and (4) as densely fractured surface layers-fractured as a result of corona and chasma formation. (C) 1996 Academic Press, Inc.