A quantitative study of deformation mechanisms and finite strain in quartzites

Abstract

The Weverton quartzites in the Maryland Blue Ridge are deformed by one major period of greenschist-grade deformation. The components of finite strain due to different independent mechanisms have been measured for these rocks. The total strain is split up into two major components:

$$\varepsilon ^t = \varepsilon ^p + \varepsilon ^d .$$

The finite natural strain caused by dislocation creep (ɛ^d) is measured by a new technique using folded and stretched rutile needles which are good strain markers within the quartz crystals. Pressure solution strain (ɛ^p) is measured from the ratio of the area of new crystals and fibers to the whole rock area in principal sections. Grain boundary sliding is a dependent process which accompanies both mechanisms. Pressure solution obeys a linear Newtonian flow law, $\left| {\dot \gamma _0^p } \right| = A_p \left| {\tau _0 } \right|$, while dislocation creep obeys a power law of the form $\left| {\dot \gamma _0^d } \right| = A_d \left| {\tau _0 } \right|^n$ where $\dot \gamma _0^p ,\dot \gamma _0^d$ are octahedral shear strain rates, τ₀ is the octahedral shear stress and A _p , A _p and n are constants. A direct correlation between finite strain measurements and the operating flow laws can be made. Application of these methods and principles to a few field examples indicates that the rocks obey a flow law partly governed by each mechanism. Any set of physical conditions defines a unique flow law and there is a transition in creep behavior from dominantly Newtonian to a power law with increasing strain rate.