Geology of Venus: Mapping of a global geotraverse at 30° N latitude

Abstract

We mapped a swath of terrain on Venus that extends circumferentially completely around the globe at 30° N latitude representing about 11% of the surface of the planet in order to address the following questions: Do presently defined stratigraphic units prove useful for broad application to regional and global mapping? Do units described in one area occur in other regions of the planet and how laterally continuous are they? If similar sequences of units are found in different parts of the planet, do they correlate temporally (e.g., time-correlative), or do they represent similar sequences formed at different times (e. g., time-transgressive or independently locally repetitive)? We found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of Venus at this latitude, providing evidence that these units are not only global in extent, but also laterally contiguous. This gives confidence that the stratigraphic sequence can be extended to other parts of Venus for further testing and refinement. On the basis of this mapping, we find that several areas, including tessera massifs and Beta Regio, show a concentration of older units and appear to have been topographically high over much of the visible history of Venus. Tessera terrain is not uniformly distributed in some areas where these older units are concentrated. Ridge belts and fracture belts are not randomly distributed throughout the area, suggesting that they represent regional patterns of deformation. The altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest, with older units (tessera and earlier plains) at intermediate to high elevations, consistent with regional plains emplacement following the formation of the most significant regional topography. Sixty corona and corona-like features were found in the geotraverse, and the vast majority show evidence of being initiated in the earliest posttessera time (early Guineverian) in an extensional tectonic environment; few show evidence of post-regional plains volcanism, and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of Venus. Regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges (early part of the middle Guineverian), perhaps overlapping somewhat with its lower member. Evidence is found for changes in styles of volcanism with time, and tectonic activity changed in time in terms of pervasiveness, style, and areal distribution. On the basis of mapping of this globally continuous swath representing about 11% of the planet, we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time, rather than the "nondirectional" history proposed by Guest and Stofan [1999]. Copyright 2001 by the American Geophysical Union.