Ribbon Terrain Formation, Southwestern Fortuna Tessera, Venus: Implications for Lithosphere Evolution

near-surface transitional-tensile failure conditions or due to The term tessera has been used to describe regions of dereactivation of steeply oriented preexisting fractures resulting formed venusian crust exhibiting two or more intersecting sets in steep-sided graben. Formation of tensile-fracture ribbons of structural elements; however, tessera includes terrains would be favored with a sharp BDT at very shallow depth, formed by a variety of spatially and temporally discrete tectonic whereas a broader and somewhat deeper BDT would favor processes. Tessera fabric characterizes highland plateau strucformation of shear-fracture ribbons. In both cases, the thickness ture, and thus understanding the nature of this deformation is of the strong upper layer is quite thin (,1–2 km), and ribbon critical to understanding the mode of highland plateau formastructures likely formed prior to long-wavelength folds, which tion. Many tessera fabrics include ribbons, folds, and late grarequire a greater effective elastic thickness and a deeper depth ben. In this paper, we refine the geometry of ribbons through of support (p6 km). geologic mapping and radargrammetric analysis of type ribbon The presence of ribbon structures within highland plateaus structures at southwestern Fortuna Tessera; we extend our favors an upwelling model for highland plateau formation, in findings to ribbon fabrics at Thetis Regio. Any model of ribbon which crustal thickening results from magmatic underplating formation must account for the following constraints on ribbon related to a mantle upwelling or mantle plume. In order for geometry. (a) Ribbon-forming lineaments exhibit sharp conthe plume to be able to anneal mechanically the crust as required trasts relative to adjacent materials. (b) Ribbon-bounding lineaby ribbon formation, the lithosphere would likely have to be ments form a distinct pattern alternating between radar-dark quite thin. These implications are consistent with highland and radar-bright, which represent trough walls oriented away plateaus as an ancient signature of mantle plumes on thin from and toward the satellite, respectively. (c) Ribbons form lithosphere, whereas volcanic rises, which are presently therlong, narrow troughs that alternate with parallel, narrow ridges; mally supported, reflect thick lithosphere. Phoebe Regio repreridges and troughs display extreme length : width aspect ratios. sents a transitional lithospheric thickness.  1998 Academic Press (d) Trough walls are near vertical. (e) Troughs are shallow