Discussion

Measurements of the fracture frequency of both vertical and horizontal surfaces would be ideal to estimate the 3-dimensional fracture frequency. Most of the existing vertical surfaces in connection to the Lockne structure were blasted during road construction, and the artificial fractures from the blasting make them unsuitable for these kinds of studies (Priest 1993). There is, however, no visible difference in the fracture frequency of the vertical and horizontal surfaces of non-blasted rocks as in Fig. 2. It can, therefore, be assumed that an estimate of the fracture frequency of horizontal surfaces is representative for the comparable volume of rock.

In an authigenic breccia formed in-situ, the clast edges represent the fractures that formed in the target, hence the counting of the clast sides as fractures in this study. Earlier fractures in the target were erased by the formation of the authigenic breccia and cannot be identified. Fractures younger than the Tandsbyn Breccia crossing the outcrops are so few that they do not contribute to the fracture frequency. The fractures counted in the fracture frequency study are thus considered to be primarily impact-generated.

In section A (Fig. 3), the fracture frequency is high and shows no tendency to decline in either direction. A similar decline in the fracture frequency as observed in the southern part of section B would be expected in the north part of section A if this is a similar part of the structure. This indicates that the outer limit of the structure has not been reached in section A. Potential occurrences of crystalline rocks further to the north are covered by sedimentary rocks and cannot be investigated.

The systematic energy decay with distance from the impact center would create a gradient in the radial distribution of fracture frequency. The

Fig. 5. Frequency of electric resistivity readings from measurements at section A and B seen in figure 1. Tandsbyn Breccia has low resistivity values (<1000 to 7000 fim) and the crystalline basement has high values (15000 to 100 000 fim).

observations however, show a rather limited transition zone of 1100 m at the southern crater margin. The relatively narrow interval where the fracture frequency decreases towards the margin of the structure, and the step at the distance 1000 m from the start of the section contradict the systematic radial trend in fracture frequency assumed earlier.

The description of the Tandsbyn Breccia in Simon (1987) indicates an allochtonous nature of the rock type. On the other hand, the observed gradual change to less fractured basement and the predominance of locally derived clasts points to an authigenic breccia type. The homogenous fracture frequency and resistivity in the present investigations favors an allogenic formation process for the Tandsbyn breccia. The transition to authigenic breccia and undisturbed basement is a narrow zone of 1100 m that is only observed at the southern margin.

The distinct step in fracture frequency, also found in the resistivity distribution at the southern margin, shows that an allogenic, generally low resistive, part of the Tandsbyn Breccia is juxtaposed with an authigenic part with higher resistivity along a fault zone. An alternative interpretation of the step in fracture frequency would be non-systematic radial stress decay.

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