Structural characterization of brittle deformation zones in crystalline rocks: contribution to SKB site investigation study for the disposal of highly radioactive nuclear waste

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Giulio, Viola; Nordgulen, طystein; Saintot, Aline; Venvik-Ganerّd, Guri
Holding Date 04 October 2008

The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigation at two different locations in south-eastern Sweden, Forsmark and Laxemar-Simpevarp, with the aim of identifying the most suitable area for the construction of a deep repository for the disposal of highly radioactive nuclear waste. As a part of this programme, the Geological Survey of Norway (NGU) has undertaken a study of predominantly brittle structures (i.e. brittle deformation zones, faults, fractures and associated fault rocks) with the aim to document their character and kinematics as an input to an evaluation of the brittle deformational history of the regions under investigation. The results are used in the development of the conceptual model for especially brittle deformation zones in the target rock volumes.
All cored boreholes drilled were logged using the Boremap methodology and oriented BIPS images were acquired for the whole length of the holes. Single-hole interpretation studies identified possible deformation zones within the cores based on fracture frequency, rock alteration, geophysical logs, radar reflectors, visual inspection etc. In order to better characterize these possible deformation zones in the boreholes, NGU initially carried out field structural mapping on selected outcrops and trenches. The results of the field study were then integrated with the constraints derived from the logging of chosen deformation zone intervals in a variety of drill cores. Oriented structural features were systematically documented, described and characterized for a number of structurally relevant zones. In addition to the mapping of planar structural features, particular attention was paid to shear striations along broken fault planes. These linear features were oriented and measured with the help of fracture orientation data from Boremap and a drill core holder, which allowed the core to be correctly positioned in 3D space. Statistically significant fracture and striation orientations and fault-slip data allowed the determination of the kinematics of many of the deformation zones investigated as well as the identification of multiple reactivation events. The data were also used to compute paleostress tensors and therefore to frame individual deformation events into a consistent sub-regional brittle deformation scheme. When present, fault rocks were studied in order to better constrain the deformation mechanisms that controlled the brittle structural evolution at the two sites. Petrographic studies and SEM work on thin sections prepared from samples of representative fault rocks provided a valuable tool to unravel the details of the local brittle history. Information gathered during the study provided in summary a robust basis for the fine-tuning of the single-hole interpretation work and for the 3D modelling of the target rock volumes. Examples will be presented of the characterization of deformation zones and of the paleostress inversion study.