Implications of Venus Evolution Based on Ribbon Tessera Terrain Relations within Five Large Regional

Introduction. Venus' ribbon tessera terrain [1,2] (rtt) records ancient environmental conditions significantly different than contemporary Venus [3,4], and geologic relations are consistent with rtt representing the oldest locally exposed unit across Venus [5], although individual tracts of rrt may have formed at differnet times [6]. We have constructed a global geologic map of rtt exposures and structural fabric. In this contribution we examine global rtt outcrop and structural patterns , and rtt outcrop and structural relations within five large regional areas: (A) lowland environment , (B) lowland-volcanic rise transition, (C) volcanic rise environment, (D) mesoland environment , and (E) a single planitia basin. These areas individually and collectively provide a diverse record of long histories and evolutionary processes preserved on Venus' surface. Ribbon tessera terrain. Ribbon tessera terrain (rtt), characterized by generally orthogonal ribbons and folds [1-2] constitutes Venus' locally oldest surface unit [3], although diverse tesserae could have different ages [2,6]. Long to short-wavelength folds record layer shortening; ribbons and graben record layer extension [1-3,6-7]. Locally , volcanic deposits flood structural lows within tessera fabric. Although debate exists as to whether folds dominantly pre-or postdate ribbons , most workers agree that: a) fold and ribbon formation broadly overlapped in time, b) graben formed late, and c) volcanic activity accompanied tessera fabric formation [2-3,6-10]. Ribbons represent brittle extensional structures in which spacing reflects the depth to a brittle-ductile transition [1-2]. Thermal modeling indicates that ribbon fabric formation requires an extremely high thermal gradient [2-3]. The regular spacing of ribbons across millions of km 2 [11-12] implies a thermal control for the base of the upper strong layer corresponding to a regional brittle–ductile transition depth [2,4,13]. Calculated heat flow values for ribbon formation requires values similar to terrestrial mid-ocean ridges; additionally, the estimated depth to the crustal solidus requires either ex