REACTION WEAKENING AND EMPLACEMENT OF CRYSTALLINE THRUSTS: DIFFUSION CONTROL ON REACTION RATE AND STRAIN RATE

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: dg/dt = 2ⁿwkT/5hrx² for shear stress ³ 20 MPa, where n is a constant, w is a geometric factor, k is Boltzmann's constant, T is absolute temperature, h 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 ~10^-10 to 10^-12 m²/s over distances of 10-100 m results in strain rates of 10^-14 to 10^-13 s^-1 in the temperature range 200-400^oC. 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. This model can be applied to crystalline thrust in other orogenic belts.