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One hypothesis might be that isolated plains coronae form through delamination (peeling away of the lower lithosphere as a result of density instabilities) and coronae along chasmata are upwellings. Conversely, if coronae all form by the same mechanism, differences in the morphologic characteristics of coronae between each geologic setting may provide constraints on regional lithospheric structure. For instance, the lithosphere along rifts or at hot spots might be thinner than in the plains.
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A series of 12 numerical cases were run in which plume and lithospheric parameters were varied. Delamination of the lower lithosphere developed in half of these cases. The initial temperature and width of the hot region at the base of the computational domain was varied from 1750°C to 2200°C and 50 to 100 km, respectively. The plume duration was varied from 50 to 430 My. The base of the high-viscosity thermal lithosphere is defined as the 1100°C contour. The lithospheric thickness range was 50 to 150 km; the range of residuum layer thickness was 0 to 200 km, where the thickness is counted as starting at the surface. The radial and vertical dimensions of the computational domain were varied from 250 to 800 km. In addition to these 12 cases, more than 50 cases were run with a radius of 2400 km. These cases were aimed at modeling large-scale hot spot-like features; 15 of these were reported on in (3). As noted in Table 1, a few of the corona topographic groups have only been predicted to date in the larger scale numerical models.
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The upwelling arises from a "hot patch" that consists of a high-temperature region at the base of the central axis of the computational domain in which temperature decreases linearly away from the axis. There was no attempt to match the depth of the box to a physical boundary such as the core-mantle boundary. Rather, the computational resources were devoted to modeling the top of the convecting boundary layer (that is, the lithosphere). Indeed, the origin of upwellings that create coronae is unknown, but it is believed to lie in the upper mantle on the basis of coronae size. There are no rigorous constraints on the duration of plumes on Venus. The maximum duration of plumes on Earth is also poorly constrained because of destruction of the lithosphere, but the oldest probable plume is 200 My [R. White and D. P. McKenzie, J. Geophys. Res. 94, 7685 (1989)]. Thus the range of plume duration was chosen to be comparable to that of terrestrial plumes while allowing for a large variation. In trying to model plumes that fit the observed range of topography, plume temperature (and thus buoyancy) trades off with plume duration. Again, the temperature of terrestrial plume heads when they reach the base of the lithosphere was used as a guide, with a typical peak temperature when the plume encounters the lithosphere of ∼1390°C or higher.
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The finite element grid was 90 by 90 elements; the finite difference grid had twice as many elements. The element spacing was nonuniform to give maximum resolution in the axial upwelling region and in the region where the plume interacts with the lithosphere. The vertical normal stress and shear stress were assumed to vanish at the bottom boundary and at the vertical outer boundary of the cylindrical domain. Boundary conditions were rigid (vanishing horizontal and vertical velocity) at the top of the cylindrical region. The surface and interior temperatures were 500° and 1300°C, respectively. Non-diffusing chemical variations were calculated with a particle-in-cell type method [K. Jha, E. M. Parmentier, J. Phipps Morgan, Earth Planet. Sci. Lett. 125, 221 (1994)]. More details of the numerical approach are given in (3) and references therein.
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We thank M. Parmentier for the use of his numerical code and J. Hall and E. DeJong for their assistance in creating the Magellan topography and image overlay. Supported by NASA Planetary Geology and Geophysics Program grants 151-01-70-71 to S.E.S. and 151-01-70-59 to E.R.S. The Jet Propulsion Laboratory-Caltech Cray Supercomputer used in this investigation was provided by funding from the NASA Offices of Mission to Planet Earth, Aeronautics, and Space Science. The research presented in this report was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.
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