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Workreport 2007-30

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Name:

Geological Mapping of the Region Surrounding the Olkiluoto site

Writer:

Paulamäki, S.

Language:

English

Page count:

90

Summary:

In 2002 and 2003, a geological mapping was carried out in the surroundings of Olkiluoto (referred here as the Olkiluoto region). The study was divided into two parts: (1) the structural geological mapping of the Olkiluoto region, and (2) the geology of the Eurajoki rapakivi stock with special emphasis on the brittle deformation of the rapakivi granite.



The aim of the structural geological mapping was to determine the history of the ductile deformation in the Olkiluoto region. The bedrock of the region shows structural patterns indicating a polyphase deformational history. On the basis of overprinting relationships, five successive ductile deformational phases have been defined.



The oldest structural feature that has been observed is a primary lithological layering, (S0) with the oldest deformational and metamorphic structure, the penetrative, slightly segregated foliation S1 (sub)parallel to the S0. No F1 folds have positively detected. In the next deformation phase D2, earlier structures were overprinted by tight to isoclinal, overturned F2A folds, with the penetrative S2A axial planar foliation. During the main stage of the D2 deformation phase, the earlier structures were overprinted by a foliation and/or segregation banding S2B, associated with abundant partial melting of the metasediments and the production of granite neosome veins. The migmatites thus produced were intensely flattened and sheared sub-conformably to the composite S0/S1/S2 foliation. The neosome veins were isoclinally folded at the same time, producing F2C folds.



In deformation phase D3, the migmatites were folded by F3 folding, producing tight folds with ductile shears. The formation of granitic melts continued during D3 deformation and granitic neosome veins and patches formed parallel to the axial planes, in the hinge zones of F3 folds and subparallel to the S2 foliation. The fold axes are usually gently plunging to the E or NE/SW. The axial plane is upright or overturned to the NW. Overturned fold structures suggest thrust-related deformation during D3. An axial plane foliation is only rarely developed. In the areas of intense D3 deformation, S2 foliation has more or less rotated into parallelism with the F3 axial plane (S3) so that the foliation therein can be described as S2/3 composite structure.



The earlier structures were re-folded and reoriented in the following deformation phase D4 with E-W compression, which produced F4 folds plunging ca. N-S and have vertical axial planes with associated parallel shearing. During D4 deformation, the S2/3 composite structures and F3 fold axes were zonally reoriented towards the trend of F4 axial plane (S4). The influence of D4 is strong in the central part of Olkiluoto, moderate in the areas east and southeast of Olkiluoto and weak in the area south of Olkiluoto. The youngest identifiable ductile structures are F5 folds, with the fold axes plunging mainly towards the SE. They are mostly very open outcrop-size folds or just small flexures.



The Eurajoki rapakivi stock is a satellite massif to the large Laitila rapakivi batholith and is composed of two main rapakivi granite types: hornblende-biotite granite (the Tarkki granite) and topaz-bearing microcline-albite granite (the Väkkärä granite). The Väkkärä granite consists of several types, which differ from each other in texture and/or mineral composition: contact type, even-grained type, porphyritic type, and coarse-grained type. Both sharp and gradual contacts exist between the different types.



The Tarkki granite is cut by partly topaz-bearing quartz-porphyry dykes, and fine- to medium-grained granite dykes with an approximate N-S strike. In the present mapping campaign, new types of dykes, not previously described within the Eurajoki stock, were found in the southern part of the porphyritic Väkkärä granite. The dykes are composite, built up during of several different phases of dyke formation.



The western contact of the Eurajoki stock (Tarkki granite) with the Palaeoproterozoic (Svecofennian) migmatitic country rock is probably gently dipping, the dip of the contact being about 20° to the west/northwest. The contact is very sharp and intact. A local, not previously described breccia zone was found near the western contact of the Tarkki granite with the migmatites. The breccia is composed of abundant fragments of migmatitic mica gneiss, 1–30 cm in diameter, lying in the rapakivi granite groundmass. The randomly oriented fragments are partly rounded, partly angular, most of the fragments being, however, more or less elongated.



Greisen veins and associated quartz (-beryl) veins occur both in the Tarkki and Väkkärä granites. In the Tarkki granite, the veins are mostly subvertical and strike E-W or ENE-WSW. In the Väkkärä granite the veins are more randomly oriented, and also lensoid, rounded and irregular greisen bodies occur.



A total of 933 fractures have been investigated throughout the area. Fracture frequencies (fractures/m) were measured across each outcrop or observation point in N–S and E–W traverses. The surface fractures form a very distinct system with two main fracture strikes, N-S and ENE-WSW. Both the Tarkki and Väkkärä rapakivi granites are “sparsely fractured”, the average fracture frequency being 0.7 fractures/m. Some outcrops may be “slightly fractured” (1 – 3 fractures/m) but “abundantly fractured” (>3 fractures/m) rock is rare and only occurs as narrow zones. The fracture frequency is not controlled by rock type, and the frequencies are the same in both the rapakivi granite types. The lengths of mapped fractures in the study area range from the lower cut-off of 1 m to 20 m, only one fracture being longer than 20 m. About 30% of the fractures are visible in their full length, their average length being 3.8 m.

Keywords:

Structural geological analysis, ductile deformation, evaluatory model, rapakivi granite, lithology, rapakivi contacts, composite dyke, brittle deformation, nuclear waste disposal, Olkiluoto, Eurajoki, SW Finland

File(s):

Geological Mapping of the Region Surrounding the Olkiluoto site (pdf) (2.9 MB)


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