ANALYSIS OF SPATIAL VARIATION OF FABRIC ORIENTATIONS AT THE KUKAS LAKE STRUCTURE (BALTIC SHIELD, NW RUSSIA)

Early Proterozoic Kukas lake structure is characterized by isoclinally folded volcanic and terrigenous sedimentary rocks highly metamorphosed at kyanite-sillimanite facies at about 1.9Ga. The lack of good stratigraphic marker beds and repetitive style of the deformation makes geological mapping and descriptions of the large scale structure unreliable.

Orientation data of planar and linear fabrics of ductile origin (supposedly related to the 1.9Ga metamorphic event) such as intersectional lineation of biotite, schistocity of biotite, mesoscopic fold axes and axes of rotation of porphyroblasts were collected in an area of about 15 km² inside this region.

Bulk distribution of all the fabrics shows a strong girdle of poles to the planar structures and normal to it a cluster of linear fabrics. This type of distribution together with absence of sudden spatial changes makes it impossible for traditional methods of structural geology to interpret observed continuous change from mostly schistose rocks with cluster type of biotite orientations distribution to ones with dominance of intersectional lineation fabric where biotite orientations are evenly distributed along a girdle.

However, such variations were modeled by Robin and Cruden^* for transpression zones. Their model predicts the spatial change of an orientation tensor (OT) in transpression zones. In this study, the OT was calculated at grid points spaced at about 150m in the 15 km² area.

For each node of the grid neighboring data points were selected and weighted based on the product of the distance between the node and a data point and the value of a bulk OT in the direction of the data point. Obtained sample was used to calculate the OT for the grid point. Then the tensors were reduced to scalars similar to Flinn's coefficient (K=[s1-s2]/[s2-s3]) and a coefficient of deformation (D=sqrt[2*s1^2+2*s1*s2+5*s2^2-4*s2-2*s1+1]). Trends of these scalar fields showed oblique orientation to the generating fabrics. This behavior agrees with the Robin and Cruden model^*, and indicates the studied area is in the NE side of a NW striking transpression zone.

Such interpretation agrees with tectonic settings of the zone.

^* Robin, P. F., and Cruden, A. R., 1994, Strain and vorticity patterns in ideally ductile transpression zones: J.Struct.Geol., v. 16, p. 447-466