ESTIMATION OF MECHANICAL AND TRANSPORT PARAMETERS IN FOLDING OF A LEUCOSOME LAYER MODIFIED BY DIFFUSIONAL MASS TRANSPORT

Syntectonic mass transport modifies folding and necking in layered rocks. For interfacial transport by diffusion for a stiff layer embedded in a soft medium, both treated as power-law fluids, coupling of transport and deformation is governed by the parameter W=2h₁MV₀²/(Ön₁H³). Here,1-component transport down a chemical potential gradient is ~ M=(c₀/RT)(Dd), h₁=medium effective viscosity, n₁=medium stress exponent, H=layer thickness, V₀=specific volume, c₀=mean concentration, D=bulk diffusivity, d=fluid film thickness, R=gas constant, & T=temperature.

Results from an analysis of folding are applied to a set of data for a fold train in a 3mm thick coarse quartz-K-spar leucosome layer embedded in quartz-biotite-plagioclase gneiss. Quartz lenses in the inner hinge regions of the folds indicate interface separation and diffusive transport. A simple fit of (fold arclength)/(layer thickness) to the dominant wavelength/thickness ratio L_d/H=2p/k_d » 4 - 5, the ratio of quartz lens thickness to fold amplitude to Wk_d³/(1 + Wk_d³) » 0.1 - 0.15, and a requirement of adequate instability, q_d > 20, yields n ³ 9, R=h₁/h £ 1/16, and a strongly constrained transport number W » 0.1. A value of n₁=3 is used for the soft medium.

A refined fit, taking into account bulk layer thickening, will be used to further constrain n and R. Modification of the analysis to treat an example of mass transport in the medium by melt percolation down a pressure gradient rather than diffusion down a chemical potential gradient requires replacement of M by M’=K/h_f, where K is permeability and h_f is melt viscosity.