Lunar and Planetary Science XXXII (2001) 1276.pdf The Complex Case of Beruth Corona. Richard Ghail, T.H. Huxley School, Imperial College, London, SW7 2BP, United Kingdom. R.Ghail@ic.ac.uk. Background: More than 350 coronae have been than its inner annulus. Shield and plains volcanism in identified on the surface of Venus . The diapiric the southeast and northwest of Beruth B occurred at model  most succesfully describes the morphology approximately the same time as the scarp faulting. and evolution of a generic corona located in regional After the formation of the annulus of Beruth B plains, isolated from other geological influences. volcanic deposits partially covered the region across However, many coronae occur in groups or chains, and Beruth C. These plains units were then folded in the are often associated with rift systems. Beruth Corona is east, forming a discontinuous, annulus-like, feature. one of many coronae located within Parga Chasma, at The most recent feature to have formed was the a junction between an east-west oriented arcuate rift northern dome in Beruth B, with its associated segment and a south-trending rift-spur. It is a complex volcanism, and the radial fractures that cross-cut all multiple corona consisting of three smaller coronae; other features in the region. The evolution of Beruth Beruth A is a 175-km diameter concentric double-ring Corona is, therefore, complex, with inter-related pulses corona; Beruth B is an asymmetric-multiple corona of activity occurring across the three components. The 200-km across; and Beruth C is a 150-km wide general succession is of eastward younging, with circular-concentric corona. It is here mapped in detail activity focussed on Beruth B. at the full resolution of the Magellan data (75 m pixel- The stratigraphy of the rift-spur is less certain, 1) to test the applicability of the diapiric model to a since volcanism appears to have been complex corona associated with a rift system. Because contemporaneous with the fracturing. The volcanic of the properties of the radar imaging system the platforms appear to be the youngest features within the stratigraphic relationships discussed here concentrate rift-spur and are younger than the volcanoes in the on the cross-cutting relationships between different faulted areas. The rift itself appears to have been structural features, such as faults and folds, and developing syntectonically with Beruth A, as fractures between these structures and other geological features, from both features intersect one another. such as volcanoes. Inferences: These complexities lead to a Geology: Two principal fracture groups are refinement of the diapiric model. This model requires identified: rift-parallel graben that also form annuli, an extremely thin lithosphere, as little as 2-km , to and radial graben. Interference patterns indicate some generate the annular moat and fractures. Given the degree of syn-tectonism but in general, the rift-spur likely thickness of the venusian crust , it is clear that and Beruth A are the oldest features and Beruth C the such diapirs are intra-crustal intrusions, or batholiths. youngest structures, with activity focussed at Beruth B. The East African Rift is thought to be underplated and In detail, the stratigraphy of Beruth Corona starts with intruded ; it is plausible that venusian rifts are also the rifting of plains material to form the main east-west intruded. These intra-crustal intrusions generate rift-arc. The earliest corona-forming event was the coronae by deforming the brittle upper crust, rather fracturing that produced the outer annulus of Beruth A. than the whole elastic lithosphere. They may be Rift-parallel graben continued to open throughout the preferentially elongated along the axis of the rift and growth of the outer annulus but ceased before the inner injected in several phases, inducing temporary annulus formed. During and following the last stages localised stresses into the overlying crust. These of corona growth, the corona was modified and cut by stresses interfere with the regional stress field, fracturing associated with the growth of the adjacent distorting the rift-parallel graben into local annular corona, Beruth B. The earliest structure within Beruth structures. Following ductile intrusion into the weak B itself is the southern radial dome. Fractures from the lower crust, the batholiths invade the stronger upper central graben cross-cut this dome, and hence formed crust by stoping and migmatisation, producing local some time later. These fractures continue west, to updoming and volcanism, and acting as source for merge with the outer annulus of Beruth A, and are radial dyke swarms. These radiating dykes are visible therefore contemporaneous with it. However, the at the surface as radial fractures , and in some cases central graben is cut by the east-west trending normal appear to feed fissure eruptions. So the observed fault scarps in the north, and hence is older than them. tectonic complexity results from a combination of The scarps merge with the inner annulus of Beruth A, brittle and ductile responses to batholith intrusions, confirming that the outer annulus of Beruth A is older together with interference patterns from the evolving Lunar and Planetary Science XXXII (2001) 1276.pdf THE COMPLEX CASE OF BERUTH CORONA. R. C. Ghail rift system. The corona-like arcuate fracturing in the 667-694.  Janes, D.M. et al., J. Geophys. Res., 97 rift-spur may represent the earliest phase of intrusion, (1992) 16055-16068.  Janes D.M. & S.W. Squyres, or perhaps deeper-seated intrusions within thicker crust LPSC XXVI (1995) 673-674.  Grimm R.E. & P.C. away from the main rift axis. Hess, in Venus II, S.W. Bougher, D.M. Hunten, R.J. Phillips, (1997) Univ. Ariz. Press, 1205-1244.  References:  Stofan, E.R., V.E. Hamilton, D.M. Karson J.A. & P.C. Curtis, J. African Earth Sci. 8 Janes & S.E. Smrekar, in Venus II, S.W. Bougher, (1989) 431-454.  Earnst R.E. et al., Earth Sci. Rev., D.M. Hunten, R.J. Phillips, (1997) Univ. Ariz. Press, 39 (1995) 1-58. Rift-Radial Graben Beruth B Darline Crater Beruth A Beruth C Rift-Parallel Graben R i f t - S p Marie u r Crater Figure 1 Beruth Corona. N Left-looking, Magellan cycle-1, 100 km synthetic-aperture radar mosaic, 1 : 5 , 0 0 0 , 0 0 0 scal e centred at 20oS, 233oE. S i n u so i d al P r o j ect i o n In this image, brighter surfaces result from rougher surfaces (at the 10 cm scale) or from radar-illuminated slopes. Black bars indicate data-loss.
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