Lessons from the first 100 minimum strain ellipsoids constrained in gneisses deformed at high metamorphic grade

Abstract

The Talbot 1970 method for constraining minimum strain ellipsoids has recently been updated by a computational approach and theory that can automatically constrain more complete strain ellipsoids with sufficient data. It is thus worth appreciating the lessons learned from the 100 minimum strain ellipsoids constrained mainly in gneisses deformed at high metamorphic grades published over the last four decades.In essence, the method measures the strikes and dips of either competent or incompetent active single layers with shortened or extended deformation structures (on scales of <1m) imparted by inhomogeneous strains to constrain ellipsoids on larger scales. The data reviewed here describe the homogeneous strains of ~100 localities (1–200m2) in 16 districts (10–~200km2) in 8 gneiss regions (1–200km2). Relating these ellipsoids to the natural reference frames provided by structures on intervening and larger scales allows study of the dynamics of deformation geology rather than merely structural geology.Several limitations expected on (outdated) theoretical grounds were overcome so that treating deformation fields as paths on a variety of deformation plots allowed recognition of multiple deformations, rotations, volume changes and even the distinction of contemporary pure and simple shears. By restricting measurements to single layers with length/thickness ratios >10, the competence contrast between the markers and their country rocks does not affect the ellipsoids — but can be seen to affect the structures that develop on intervening and larger scales.