Shear zones and structural analysis of the Loimaa area, SW Finland

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

LITHOSPHERE 2018: TENTH SYMPOSIUM ON STRUCTURE, COMPOSITION AND EVOLUTION OF THE LITHOSPHERE: PROGRAMME AND EXTENDED ABSTRACTS

Shear zones of various ages and orientations are common in Southern Finland. In the study area, E-W and N-S trending shear zones are the dominant structural feature. Mylonitic foliations were identified from the most intensely sheared rocks. Ductile shearing has mainly been of dip-slip type. Structural mapping revealed several larger map-scale folds, which appear to be relatively continuous across the study area from SE to NW. In the central area, folding interfered with the shear zones causing a complex crustal structure such as associated with the Uunimäki mineralization. Aeromagnetic and lithological maps, field observations, stereographic projections and oriented thin sections were used to determine the structural features of the study area.</p

LITHOSPHERE 2018: TENTH SYMPOSIUM ON STRUCTURE, COMPOSITION AND EVOLUTION OF THE LITHOSPHERE: PROGRAMME AND EXTENDED ABSTRACTS

Field work was conducted in the Kullaa area in SW Finland to study the tectonic evolution and structural setting of the gold mineralisations in the area. Our structural data and interpretation recognises the structural complexity associated with the mineralised zone. We suggest that the NE-SW trending faults in association with the second-order structures related to the NW-SE trending Kynsikangas shear zone have controlled the precipitation of the gold-bearing fluids.</p

LITHOSPHERE 2018: TENTH SYMPOSIUM ON STRUCTURE, COMPOSITION AND EVOLUTION OF THE LITHOSPHERE: PROGRAMME AND EXTENDED ABSTRACTS

The Uunimäki gabbro was studied by zircon U-Pb geochronology which yielded an age of ~1.89 Ga, making it one of the oldest plutonic rocks in the Häme Belt. Geochemical analysis of the gabbro reveals that it lacks several characteristics for typical subduction zone rocks: (i) it does not have a negative Ta-Nb anomaly compared to average NMORB-composition, (ii) it shows a rather unfractionated REE pattern, (iii) it lacks clear enrichment of fluid-mobile elements (e.g. Ba, Rb, Th, Pb). Structurally, the Uunimäki gabbro is located at the intersection of several regional features: (i) steep NE-plunging folds, (ii) a ENE-WSW-trending deformation zone immediately to the north and (iii) a large N-S-trending deformation zone to the west. The gabbro itself has been deformed under both brittle and ductile conditions by primarily NW-SE-trending faults and shears.</p