Shear zones and structural analysis of the Loimaa area, SW Finland
Pitkälä, Iiro (2019-06-18)
Julkaisu on tekijänoikeussäännösten alainen. Teosta voi lukea ja tulostaa henkilökohtaista käyttöä varten. Käyttö kaupallisiin tarkoituksiin on kielletty.
Julkaisun pysyvä osoite on:
The study area is located in the western part of the Häme belt, where deformation has partitioned into folded- and sheared domains. The N-S and the ENE-WSW striking shear zones form the most prominent structures. In areas with lesser shear zone deformation, the regional scale synforms and antiforms can be distinguished. The main goals of this study are: (i) to map the structural features, determine their evolution and draw a structural map within the study area, (ii) determine the kinematics of the two major shear zones and their correlation with other nearby shear zones, (iii) determine the relationship between the shear zones and folds, (iv) determine the age and temperature of the shearing. Three regional-scale folded areas can be distinguished in addition to the two shear zones. These folds and shear zones are inferred to characterize separate deformation phases of the same orogenic event. The folds are parts of the continuous fold chain which has formed due to NW directed compression prior to the shear zones. In the central area the shear zone deformation merges with the fold structures, which has caused a very complex crustal structure. Both of the shear zones characterize mainly oblique dip-slip shear sense under ductile conditions. The west-side-up shear sense seems dominant within the N-S striking shear zone, while in ENE-WSW shear zone it is the south-side-up. The N-S striking shear zone could have merged with the Kynsikangas and Kolinummi shear zones due to the reactivation of shearing forming one major shear zone. The ENE-WSW striking shear zone is interpreted to be a part of the Hämeenlinna shear zone in its western part. Based on the dynamic quartz recrystallization the deformation temperature in the N-S striking shear zone is estimated between 480°C and 530°C, while in the ENE-WSW striking shear zone it is 420-480°C in the east and 300-380°C in the west. The attempted age determination of the N-S shearing was performed with U-Pb zircon geochronology from a syntectonic granite, from which at least three zircon populations were found. The youngest of these yielded weighted average and upper intercept ages of c. 1.87 Ga. However, all the populations are regarded inherited from the source.