GLOBAL STRATIGRAPHY OF VENUS - ANALYSIS OF A RANDOM SAMPLE OF 36 TESTAREAS

The age relations between 36 impact craters with dark paraboloids and other geologic units and structures at these localities have been stud ied through photogeologic analysis of Magellan SAR images of the surfa ce of Venus. Geologic settings in all 36 sites, about 1000 x 1000 km e ach, could be characterized using only 10 different terrain units and six types of structures. These units and structures form a major strat igraphic and geologic sequence(from oldest to youngest): 1) tessera te rrain; 2) densely fractured terrains associated with coronae and in th e form of remnants among plains; 3) fractured and ridged plains and ri dge belts; 4) plains with wrinkle ridges; 5) ridges associated with co ronae annulae and ridges of arachnoid annulae which are contemporary w ith wrinkle ridges of the ridged plains; 6) smooth and lobate plains; 7) fractures of coronae annulae, and fractures not related to coronae annulae, which disrupt ridged and smooth plains; 8) rift-associated fr actures; 9) craters with associated dark paraboloids, which represent the youngest 10% of the Venus impact crater population (Campbell et al ., 1992), and are on top of all volcanic and tectonic units except the youngest episodes of rift-associated fracturing and volcanism; surfic ial streaks and patches are approximately contemporary with dark-parab oloid craters. Mapping of such units and structures in 36 randomly dis tributed large regions (each similar to 10(6) km(2)) shows evidence fo r a distinctive regional and global stratigraphic and geologic sequenc e. On the basis of this sequence we have developed a model that illust rates several major themes in the history of Venus. Most of the histor y of Venus (that of its first 80% or so) is not preserved in the surfa ce geomorphological record. The major deformation associated with tess era formation in the period sometime between 0.5-1.0 b.y. ago (Ivanov and Basilevsky, 1993) is the earliest event detected. In the terminal stages of tessera formation, extensive parallel linear graben swarms r epresenting a change in the style of deformation from shortening to ex tension were formed on the tessera and on some volcanic plains that we re emplaced just after (and perhaps also during the latter stages of t he major compressional phase of tessera emplacement. Our stratigraphic analyses suggest that following tessera formation, extensive volcanic flooding resurfaced at least 85% of the planet in the form of the pre sently-ridged and fractured plains. Several lines of evidence favor a high flux in the post-tessera period but we have no independent eviden ce for the absolute duration of ridged plains emplacement. During this time, the net state of stress in the lithosphere apparently changed f rom extensional to compressional, first in the form of extensive ridge belt development, followed by the formation of extensive wrinkle ridg es on the flow units. Subsequently, there occurred local emplacement o f smooth and lobate plains units which are presently essentially undef ormed. The major events in the latest 10% of the presently preserved h istory of Venus (less than 50 m.y. ago) are continued rifting and some associated volcanism, and the redistribution of eolian material large ly derived from impact crater deposits. Detailed geologic mapping and stratigraphic synthesis are necessary to test this sequence and to add ress many of the outstanding problems raised by this analysis. For exa mple, we are uncertain whether this stratigraphic sequence corresponds to geologic events which were generally synchronous in all the sites and all around the planet, or whether the sequence is simply a typical sequence of events which occurred in different places at different ti mes. In addition, it is currently unknown whether the present state re presents a normal consequence of the general thermal evolution of Venu s (and is thus representative of the level of geological activity pred icted for the future), or if Venus, has been characterized by a sequen ce of periodic global changes in the composition and thermal state of its crust and upper mantle (in which case, Venus could in the future r eturn to levels of deformation and resurfacing typical of the period o f tessera formation).