Abstract

We have developed a simple model to evaluate the gravity gliding of nappes in sedimentary terranes. Three factors play a critical role in the model: (1) the ratio of horizontal to vertical stress, (2) the rate of fluid flow to the detachment horizon, and (3) the permeability of lithologies immediately above that horizon. If the ratio of horizontal to vertical stress is less than one, this ratio limits the maximum attainable pore pressure through hydrofracture. Existing in situ stress measurements suggest that vertical hydrofracturing rather than gravity gliding might be the result of elevated pore pressures unless a low-strength “cap rock” exists. If the stress ratio exceeds one, a relatively simple equation relates the “cap rock” permeability and fluid flow necessary for gravity gliding. Effective cap-rock permeabilities less than 104 to 105 md are required for gravity gliding in a sedimentary basin. Based on available in situ permeability measurements, only shales and evaporites could have sufficiently low permeabilities on a regional scale.

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