Strain ellipsoids from incompetent dykes: Application to volume loss during mylonitization in the Singِ gneiss zone, central Sweden

We describe how to constrain bulk strain ellipsoids by distinguishing the attitudes of mullioned from planar contacts in incompetent dykes. Our method is demonstrated by using metabasaltic dykes to construct strain ellipsoids (X ≥ Y ≥ Z) in the Palaeoproterozoic Singö deformation zone of Sweden. Ductile flows are found empirically to be homogeneous within two scale-ranges: 2–10 m localities in a 3 × 3 km district. These alternate with three scale-ranges in which inhomogeneous strains generated decimetre-scale mullions and 30 m wide mylonites in a gneiss zone over 3 km wide. X axes in locality ellipsoids rotate from subhorizontal in Singö gneisses with natural strains of 3s≈ 1.2, to subvertical in mylonites (2 < 3s < 3) and ultramylonites (3s > 3) where X parallels the ubiquitous mineral lineation. l of the theoretical limitations of working with homogeneous strains can be bypassed in practice. Tielines between ellipsoids constructed by applying different methods to different markers in the same rocks demonstrate that our strain field is also the strain path. Individual ellipsoids are constructed assuming no volume change—but their strain path reveals a uniaxial volume loss of ≈3% per 10% total shortening of Z in an otherwise pure shear. Replotting locality ellipsoids with respect to the orientation of the district ellipsoid reveals that gneisses in the Singö zone are the result of cryptic (≈1.86-1.83 Ga) transpression that led to (≈1.83-1.6 Ga) extrusion along dip-flow mylonites of gneiss lenses that may be the roots of former nappes.