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 deforming them. Seven rock-stratigraphic units, three related time-stratigraphic units (Systems), and three geologic time subdivisions (Periods) are proposed to describe the vast majority of the areas under study. The first widespread geologic unit preserved on Venus is the highly deformed tessera terrain formed during the Fortunian Period, apparently the result of an event that destroyed the morphology of preexisting terrain and any superposed craters. Global image data reveal no evidence for extensive terrain dating from the pre-Fortunian time, comprising the first 80–90% of the history of Venus, although rocks dating from this period are almost certainly contained within the tessera. Immediately following the Fortunian an extensive period of plains volcanism began, the Guineverian Period, during 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 plains emplacement occurred and units of the Lavinia Group were deformed, some into extensive ridge belts, recording a change from distributed extensional deformation to often-focused compressional deformation. Plains of the Rusalka Group are the most widespread currently exposed, 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 plains volcanism then gave way to materials of the Atla Group, local volcanic edifices and flow units with sources associated with coronae and rifts that were emplaced in the late Guineverian 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 Venus and volcanism has continued at a reduced level relative to the earlier parts of the Guineverian 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 activity of endogenic origin which lasts until now. The lack of surface units representing the first 80–90% of the history of Venus is remarkable from the standpoint of its present low level of activity. This contrast, together with the emerging geologic history of the last 10% of the lifetime of Venus, suggest that catastrophic and/or episodic global processes may have characterized Venus in its earlier history. This factor may also provide an insight into earliest Earth history.