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

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 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. [Display omitted] •The implications of 100strain ellipsoids constrained in 8gneiss regions are reviewed in terms of deformation geology•Ellipsoids describe homogeneous strains on scales 1-100m2 and 10-200km2 with structures resulting from inhomogeneous strains on other scales•Most (98%) ellipsoids appear oblate because of uniaxial volume loss along Z of ~30vol%.•Upper orogen shortened horizontally when early heat flow was low but vertically when later thermal gradient was high•Same single planar anisotropy controlled all 3 main orogenic deformations: buckling, flow and anisotropic loss of silica