Modeling local crustal isostasy caused by ductile flow in the lower crust

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Abstract
Some continental regions display local isostatic uplift on a much shorter scale than that of classical isostasy, such as high ground associated with granites. This is best explained by compensatory channel flow in the lower crust. To investigate the mechanism, the uplift caused by an existing granitic body 30 km wide, extending to 10 km depth and of density deficiency −120 kg/m3 has been modeled by finite element analysis, as it rises toward isostatic equilibrium over 5 Myr by inward creep in the ductile lower crust. The upper crust is treated as elastoplastic with a specified strength envelope. The lower crust (15 to 36 km depth) is viscoelastic with either Newtonian or power law quartz rheology and the strong upper mantle down to 60 km is either elastoplastic or has olivine rheology. Models with channel flow produce much higher and sharper topography than those without it. Faulted models produce the highest and sharpest elevations; when preexisting faults are present, the uplift is strongly affected by the state of stress in the upper crust, with compression suppressing uplift on outward dipping faults and tension accentuating it (the opposite for inward dipping faults). The surface topography approaches isostatic equilibrium over 5 Myr at a decaying rate dependent on the type of rheology. The Moho is pulled upward as the surface rises during the earlier stages, but it eventually subsides toward its equilibrium level much more slowly than the surface approaches equilibrium. This linked deformation of surface and Moho allows approximate isostatic equilibrium of the upper crust to be established without inflow at the edges of the model, and it may produce long‐lasting bumps on the Moho.

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