REGIONAL AND GLOBAL STRATIGRAPHY OF VENUS - A PRELIMINARY ASSESSMENT AND IMPLICATIONS FOR THE GEOLOGICAL HISTORY OF VENUS

Photogeologic analysis of Magellan images for 36 widespread sites and several larger areas permits the definition and characterization of a sequence of mappable stratigraphic units and tectonic structures defor ming them. Seven rock-stratigraphic units, three related time-stratigr aphic units (Systems), and three geologic time subdivisions (Periods) are proposed to describe the vast majority of the areas under study. T he first widespread geologic unit preserved on Venus is the highly def ormed tessera terrain formed during the Fortunian Period, apparently t he result of an event that destroyed the morphology of preexisting ter rain and any superposed craters. Global image data reveal no evidence for extensive terrain dating from this period are almost certainly con tained within the tessera. Immediately following the Fortunian an exte nsive period of plains volcanism began, the Guinerverian Period, durin g which the majority of Venus was volcanically resurfaced. During the Guineverian early widespread plains of the Sigrun Group were deformed by extensive and closely-spaced graben systems. Continued widespread p lains emplacement occurred and units of the Lavinia Group were deforme d, some into extensive ridges belts, recording a change form distribut ed extensional deformation to often-focused compressional deformation. Plains of the Russalka Group are the most widespread currently expose d, and are characterized by extensive development of wrinkle ridges of compressional origin. These Guineverian Period plains must have been emplaced and deformed over a relatively short period of time (probably less than about a hundred million years) because the vast majority of impact craters are superposed on the plains, and the crater retention age of this surface is of the order of 300-500 Ma. This extensive pla ins volcanism then gave way to materials of the Atla Group, local volc anic edifices and flow units with sources associated with coronae and rifts that were emplaced in the late Guinerverian And Aurelian Periods . The Aurelian Period, defined by impact craters with dark parabolas, is interpreted to extend from the present back to about 30-50 Ma ago. During this period extensive rifting occurred in several areas of Venu s an volcanism had continued at a reduced level relative to the earlie r parts of the Guinerverian period. Our observations favor a model in which the observed part of the geologic history of Venus (which is the last 20% or less) started with catastrophic tectonic deformation and volcanic resurfacing, followed by the period of declining surface acti vity of endogenic origin which lasts until now. The lack of surface un its representing the first 80-90% of the history of Venus is remarkabl e form the standpoint of its present low level of activity. This contr ast, together with the emerging geologic history of the last 10% of th e lifetime of Venus, suggest that catastrophic and/or episodic global processes may have characterized Venus in its earlier history. This fa ctor may also provide an insight into earlier Earth history.