ONSET TIME AND DURATION OF CORONA ACTIVITY ON VENUS: STRATIGRAPHY AND HISTORY FROM PHOTOGEOLOGIC STUDY OF STEREO IMAGES

Abstract

The details of stratigraphic units and structures making up six coronae and their regional surroundings on Venus were examined using full resolution Magellan images and stereoscopic coverage. Altimetry and stereoscopic coverage were essential in establishing the local stratigraphic relationships and the timing of corona-related topography. The degree of preservation of signatures of earlier corona-related activities and the scale of later corona-related activities vary significantly from corona to corona. We compared the geologic sequence in each corona to regional and global stratigraphic units, placing the coronae in the broader context of the geologic history of Venus. The results of this study were compared with earlier analyses bringing the total number of corona considered to about 15% of the total corona population. We found that corona started forming soon after tessera formation and largely spanned a significant part of the subsequent geologic history of Venus, over about 200–400 million years. Topographic annulae were initiated in early post-tessera time but were largely completely formed by the time of emplacement of regional plains with wrinkle ridges. Some coronae ceased activity by this time, while others continued until closer to the present, although showing evidence of waning activity. Coronae-associated volcanism dominated many coronae during this later stage. Convincing evidence of pre-regional plains corona- related volcanism was not found in the population examined here. We conclude that coronae formed in a two stage process; the first stage (tectonic phase) involved the annular warping of early extensive stratigraphic units of volcanic origin and the second (volcanic phase) involved coronae-related lava flow activity and local fracturing. For the vast majority of coronae, the first tectonic phase was largely complete prior to the emplacement of the regional plains (Pwr, plains with wrinkle ridges). The vast majority of corona-related volcanic activity (emplacement of Pl, lobate flows) occurred subsequent to the emplacement of regional plains. We found no evidence of coronae initiation in substantially later periods of the observed history of Venus.