Strain analysis in the Orijärvi triangle, southwestern Finland; in perspective of tectonic models

STEL, HARRY, VAN REES, DOUWE and SCHENKE, BART 1999. Strain analysis in the Orijärvi triangle, southwestern Finland; in perspective of tectonic models. Bulletin of the Geological Society of Finland, Number 71, Part 2, 329337. Finite strain analyses were performed in metavolcanic rocks (agglomerates and pillowed metabasalts) from the Orijärvi area, southwestern Finland. In this area, tight folding (F,) of layered metasedimentary rocks and development of a penetrative axial plane schistosity (S,) took place during an early stage in the Svecofennian orogeny. Within the domain, there is no evidence of older deformational structures or of a later deformation phase; except in narrow, late-stage shear zones that bound the Orijärvi triangle. During the orogenic evolution, a pervasive attenuation of the fabric in metavolcanic rocks has taken place. In order to quantify strain intensity and to investigate the relation of strain in metavolcanics and folding in metasediments, strain analyses were performed by the Rf/(p method on (sub)elliptical markers. Metavolcanic rocks demonstrate relatively homogeneous strain values on the outcrop scale, but strain parameters vary strongly on the scale of the study area. This variation in strain correlates with variation in rock composition; felsic metavolcanics demonstrate consistently lower strain intensities than mafic ones. Finite strain ellipsoids are prolate, indicating that the rocks underwent apparent constriction. X axes of strain are (sub)parallel to the regional fold axis. Constriction in the Orijärvi triangle is not in conflict with any of the current tectonic models. However, given the local structural setting, the most straightforward interpretation is that of progressive constriction by plutonic diapirism.