Unravelling the progressive strain-history of rocks in metamorphic areas, the example of the Somero area, SW Finland

Abstract

The progressive three-dimensional strain path of rocks during their metamorphic evolution and the relation between strain and structural elements were studied in the Somero area, SW Finland. All the methods used involve two basic steps: determination of two-dimensional strain ellipses on several planes and subsequent calculation of three-dimensional strain ellipsoids from the two-dimensional strain ellipses. Strain analyses were performed on: (1) deformed objects in agglomerates and xenoliths, providing finite strain values and (2) the spatial distribution pattern of cordierite and garnet porphyroblasts, reflecting F2 and F3-strain. The two types of strain markers have not been found together in one exposure. A test of the spatial distribution pattern method in terms of shape and orientation has been applied to cordierite porphoryblasts. These blasts show deformation substructures indicative of dislocation creep and have ellipsoidal shapes. Their shapes correlate closely with the results of the spatial distribution pattern method. The methods provide the possibility of obtaining strain data in metamorphic areas.