Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
4734389 | Journal of Structural Geology | 2007 | 14 Pages |
Abstract
In the southern Appalachians, the Blue Ridge-Piedmont crystalline thrust sheet was emplaced onto low-grade Late Precambrian and Paleozoic sedimentary rocks in the footwall along a basal detachment consisting of phyllosilicate-rich mylonites (phyllonites). The phyllonites developed first by mechanical breakdown of feldspar followed by chemical breakdown to white mica in the presence of a pore fluid. Diffusion of solute in the pore fluid is the rate limiting step in controlling reaction rate and also the strain rate. Assuming solute diffusion follows the Stokes-Einstein equation, the shear strain rate is given by â
γ/â
t=2nÏkT/5ηrx2 for shear stress â¥20 MPa, where n is a constant, Ï is a geometric factor, k is Boltzmann's constant, T is absolute temperature, η is water viscosity, r is the atomic radius of the diffusing species, and x is the diffusion distance. A bulk diffusion coefficient in the range of â¼10â10 to 10â12 m2/s over distances of 10-100 m results in strain rates of 10â14 to 10â13 sâ1 in the temperature range 200-400 °C. It is concluded that greenschist grade crystalline thrust sheets develop on pre-existing basement faults that become weak during reaction softening and localize into high strain phyllonite zones in which pore fluid diffusion controls reaction rate and strain rate.
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Geology
Authors
Kieran O'Hara,