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									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 [1]. The diapiric        the southeast and northwest of Beruth B occurred at
            model [2] 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 [3], 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 [4], 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 [5]; 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 [6], 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. [2] Janes, D.M. et al., J. Geophys. Res., 97
            rift-spur may represent the earliest phase of intrusion,          (1992) 16055-16068. [3] Janes D.M. & S.W. Squyres,
            or perhaps deeper-seated intrusions within thicker crust          LPSC XXVI (1995) 673-674. [4] 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. [5]
            References: [1] 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. [6] 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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