A new view of the stratigraphic history of Venus

Studies of Venus using Magellan data have so far generated two views about the way Venus has evolved. On the one hand, Venus has been suggested to hav e had a history in which there was a series of epochs, each represented by a different volcanic or tectonic process on a global scale (Basilevsky and Head 1995; J, W, Head et al, 1996, Lunar Planet Sci, 27th, 525-526; P. T. B asilevsky et al. 1997, In Venus II, Univ, of Arizona Press, Tucson). This w e define as a directional history. On the other hand, there is evidence to suggest that coronae, rifts, wrinkle ridges, small and large edifices, and large flow fields have each formed throughout the portion of Venus' history revealed by presently exposed rack units. We propose that the plains have been built up by lavas erupted in a number of different styles, each occurr ing throughout the history represented by the exposed stratigraphy of the p lanet. Dates derived from crater counts accumulated from the combined area of specific types of feature such as coronae (e.g., M. H. Price et al, 1996 , J, Geophys, Res. 101, 4657-4672) must be interpreted with care as the met hod is based upon the assumption that features of like morphology have the same age. Detailed studies from full resolution Magellan data indicate that Venus has had a complex history in which most geologic processes have oper ated in a nondirectional fashion to a greater or lesser extent throughout t he planet's history. (C) 1999 Academic Press.