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M.T. Bland, A.P. Showman (University of Arizona)
Ganymede�s pervasive 5-10 km-wavelength grooves have been suggested to result from a necking instability during an epoch of lithospheric extension, but to date few quantitative studies of groove formation have been performed. Using a linearized analytical model, Dombard and McKinnon (2001) calculated growth rates of a necking instability as a function of wavelength and demonstrated that, under conditions of high heat flow, the fastest-growing modes have wavelengths and growth rates consistent with Ganymede�s grooves. However, questions remain as to whether nonlinearities influence groove formation; furthermore, it is important to elucidate how such instabilities respond to finite surface topography. To address these issues, we present a numerical investigation of necking instabilities and the resulting topography. We use the two-dimensional finite-element code Tekton to simulate the extension of a stiff surface layer overlying a ductile substrate. Both Newtonian and power law flow regimes have been explored. The latter case employs recent rheological laboratory data for both dislocation creep and grain-boundary-sliding flow mechanisms. Preliminary simulations indicate that extensional necking instabilities can occur under a range of conditions, many of which may be relevant to Ganymede. The form of the surface topography produced by these instabilities varies as a function of the strain rate, amount of extension, initial topographic perturbation, and rheological parameters. We will summarize in detail how these parameters affect the instabilities and compare our results to those of Dombard and McKinnnon (2001). This project is supported by NASA PG&G and NSF Planetary Astronomy.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.