STRAIN ELLIPSOIDS FROM INCOMPETENT DYKES - APPLICATION TO VOLUME LOSSDURING MYLONITIZATION IN THE SINGO 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 st rain ellipsoids (X greater than or equal to Y greater than or equal to Z) in the Palaeoproterozoic Singo deformation zone of Sweden. Ductile flows are found empirically to be homogeneous within two scale-ranges : 2-10 m localities in a 3 x 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 a xes in locality ellipsoids rotate from subhorizontal in Singo gneisses with natural strains of <(epsilon)over bar>(s) approximate to 1.2, to subvertical in mylonites (2 < <(epsilon)over bar>(s) < 3) and ultramy lonites (<(epsilon)over bar>(s) > 3) where X parallels the ubiquitous mineral lineation. Several of the theoretical limitations of working w ith homogeneous strains can be bypassed in practice. Tie-lines between ellipsoids constructed by applying different methods to different mar kers in the same rocks demonstrate that our strain field is also the s train path. Individual ellipsoids are constructed assuming no volume c hange-but their strain path reveals a uniaxial volume loss of approxim ate to 3% per 10% total shortening of Z in an otherwise pure shear. Re plotting locality ellipsoids with respect to the orientation of the di strict ellipsoid reveals that gneisses in the Singo zone are the resul t of cryptic (approximate to 1.86-1.83 Ga) transpression that led to ( approximate to 1.83-1.6 Ga) extrusion along dip-how mylonites of gneis s lenses that may be the roots of former nappes.