Arcuate graben of Venusian volcano-tectonic structures: The last phase of tectonic activity?

Astronomy Division, Department of Physical Sciences, PO Box 3000, 90014 University of Oulu, Finland e-mail: petri.kostama@oulu.fi

Received: 25 June 2003
Accepted: 5 July 2004

Abstract

Coronae and novae are two examples of Venusian volcano-tectonic structure types. Both these structures are considered either as surface expressions of diapirs or as representations of mantle upwellings. Half of the novae are located within coronae. The recent studies show that, in contrast to the earlier assumptions, at least the last activity of the nova usually postdates the coronae formation. This is possibly due to the reactivation of the nova location or increase in the activity of the formation process of the feature (Aittola & Kostama [CITE], JGR, 107, 5112). In these cases, the most recent phases of activity are the nova related features, the radial structures and the lava flows produced by the nova. However, in some examples, suites of very young arcuate graben are seen adjoining the sloping flanks of the structures. The studies of the volcano flanks of Earth have shown that similar horseshoe-shaped scarps are formed after an eruption which indicate the occurrence of failure of the scoria cone. It is also possible to have a lateral spreading of the cone resulting in fracturing of coherent material due to plastic flow of subsequent material. We have found evidence of these graben in at least four of the studied corona-nova paired structures. In this study, we concentrate on measuring and analysing these suites of tectonic modification. As the earlier studies have shown, these corona-novae structures are usually very young in respect to their surrounding geology and thus the deformation of the lava flows, i.e. the graben sets could actually represent the last stage of development of the corona-novae structures of Venus.