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

The details of stratigraphic units and structures making up six corona e and their regional surroundings on Venus were examined using full re solution Magellan images and stereoscopic coverage. Altimetry and ster eoscopic coverage were essential in establishing the local stratigraph ic relationships and the timing of corona-related topography. The degr ee of preservation of signatures of earlier corona-related activities and the scale of later corona-related activities vary significantly fr om 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 o f corona considered to about 15% of the total corona population. We fo und that corona started forming soon after tessera formation and large ly spanned a significant part of the subsequent geologic history of Ve nus, over about 200-400 million years. Topographic annulae were initia ted in early post-tessera time but were largely completely formed by t he rime of emplacement of regional plains with wrinkle ridges. Some co ronae ceased activity by this time, while others continued until close r to the present, although showing evidence of waning activity. Corona e-associated volcanism dominated many coronae during this later stage. Convincing evidence of pre-regional plains corona-related volcanism w as not found in the population examined here. We conclude that coronae formed in a two stage process; the first stage (tectonic phase) invol ved the annular warping of early extensive stratigraphic units of volc anic origin and the second (volcanic phase) involved coronae-related l ava flow activity and local fracturing. For the vast majority of coron ae, the first tectonic phase was largely complete prior to the emplace ment of the regional plains (Pwr, plains with wrinkle ridges). The vas t majority of corona-related volcanic activity (emplacement of Pl, lob ate flows) occurred subsequent to the emplacement of regional plains. We found no evidence of coronae initiation in substantially later peri ods of the observed history of Venus.