GLOBAL MAP OF RIBBON TESSERA TERRAIN, VENUS: RECORD OF A RICH ANCIENT HISTORY

Ribbon tessera terrain (rtt) represents the oldest locally exposed unit across much of Venus. Unit rtt—marked by distinctive contractional (folds), extensional (ribbons), and S-C-like shear structures—characterizes crustal plateaus (2-3 million km ² quasi-circular highlands) and is preserved in large and small kipukas across the lowlands. We are creating a global rtt outcrop and structural fabric map in order to understand rtt formation, and implications for Venus evolution. Data are gleaned from VMap quadrangles using linked images (C1 to full-resolution Magellan SAR and synthetic stereo normal and inverted images), and transferred to the ArcGlobe ^TM GIS environment. Unit rrt is exposed across most of Venus, with few areas rtt-free. Three outcrop patterns emerge: 1) large quasi-circular tracts of tessera similar to crustal plateaus, although not elevated; 2) long curvilinear arcs, 100s to 1000+ km long with varying widths up to 250-400 km; the arcs in general have long aspect ratios, in strong contrast with the more blob-like tracts; and 3) small scattered, yet grouped exposures as small as tens of km ², yet occurring across regional areas covering 1-3 million km ². Structural fabrics across large tracts reveal locally complex patterns similar to that preserved in crustal plateaus; fabrics generally parallel arcuate outcrops with arc-parallel folds parallel and orthogonal ribbons; locally S-C-like shear fabrics parallel arcs. Fabric trends describe coherent patterns between dispersed small outcrops across extensive regions, and provide evidence for the subsurface extent of rtt. Overlapping arc illustrate global scale cross cutting relations, with fabric trends provide further evidence of temporal relations between rtt domains; shear fabrics record local cross-cutting relations. Isopach surfaces (depth to rtt) provide a 3D view, constraining the thickness of surface layers above subsurface rtt layers. The global rtt map reveals a rich ancient history for Venus.