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

The term tessera has been used to describe regions of deformed venusian crust exhibiting two or more intersecting sets of structural elements; however, tessera includes terrains formed by a variety of spatially and temporally discrete tectonic processes. Tessera fabric characterizes highland plateau structure, and thus understanding the nature of this deformation is critical to understanding the mode of highland plateau formation. Many tessera fabrics include ribbons, folds, and late graben. In this paper, we refine the geometry of ribbons through geologic mapping and radargrammetric analysis of type ribbon structures at southwestern Fortuna Tessera; we extend our findings to ribbon fabrics at Thetis Regio. Any model of ribbon formation must account for the following constraints on ribbon geometry. (a) Ribbon-forming lineaments exhibit sharp contrasts relative to adjacent materials. (b) Ribbon-bounding lineaments form a distinct pattern alternating between radar-dark and radar-bright, which represent trough walls oriented away from and toward the satellite, respectively. (c) Ribbons form long, narrow troughs that alternate with parallel, narrow ridges; ridges and troughs display extreme length:width aspect ratios. (d) Trough walls are near vertical. (e) Troughs are shallow with consistent shallow depth along individual troughs and in adjacent troughs. (f) In some cases (e.g., southwest Fortuna Tessera) trough walls are parallel and matched and would exhibit a close fit if the trough was closed; in these cases trough walls merge laterally forming V-shaped terminations. Trough floors are smooth and flat, lacking small-scale interior lineaments. (g) In other cases (e.g., Thetis Regio) ribbons display (a)-(d), but trough walls are defined by a series of subparallel lineaments including local interior lineaments, and trough floors ramp up to join trough walls displaying parallel rather than V-shaped terminations. Trough walls would not display a close fit if closed. We propose two member types of ribbons, tensile-fracture ribbons and shear-fracture ribbons. Tensile-fracture ribbons display features (a)-(f) and formed by the opening of tensile fractures of a thin brittle layer above a ductile substrate. They require a near fracture-free shallow crust and very shallow depth to the brittle-ductile transition (BDT) (<1 km). Shear-fracture ribbons display features (a)-(d) and (g) formed under near-surface transitional-tensile failure conditions or due to reactivation of steeply oriented preexisting fractures resulting in steep-sided graben. Formation of tensile-fracture ribbons would be favored with a sharp BDT at very shallow depth, whereas a broader and somewhat deeper BDT would favor formation of shear-fracture ribbons. In both cases, the thickness of the strong upper layer is quite thin (<1-2 km), and ribbon structures likely formed prior to long-wavelength folds, which require a greater effective elastic thickness and a deeper depth of support (~6 km). The presence of ribbon structures within highland plateaus favors an upwelling model for highland plateau formation, in which crustal thickening results from magmatic underplating related to a mantle upwelling or mantle plume. In order for the plume to be able to anneal mechanically the crust as required by ribbon formation, the lithosphere would likely have to be quite thin. These implications are consistent with highland plateaus as an ancient signature of mantle plumes on thin lithosphere, whereas volcanic rises, which are presently thermally supported, reflect thick lithosphere. Phoebe Regio represents a transitional lithospheric thickness.