Fracture 'Cleavage', Oslo, Norway - Interpreted

Three dike tips can be clearly seen, and at the time of formation the dike can be thought of as a fluid filled opening fracture that grew as the tip propagated into the host rock. There is an interesting apparent offset of the tip of the left most dike which could be taken as a small fault - yet, the adjacent bedding does not appear similarly offset, and instead this may represent some a complicated 3-D intrusion geometry. Most striking here is the relatively intense fracture set parallel to the dikes (traced in blue) that has a spacing of every cm or so. Many of the traces of this fracture set have small apparent offsets that are mainly right side away (sinistral in this perspective), although a few counter examples exist. It is important to realize that we do not know the direction of movement along these planes, only the resulting offset (more on this later). Some small thin calcite veins also occur along this surface. Such intense fracture surfaces with a more complicated character are often termed a fracture cleavage in the field. Likely these had a complicated history starting out as simple opening fractures and then being reactivated as a slip surface later. Solution along the fractures may have also occurred at some stage. These cut the bedding parallel veins in the lower, more massive limestone bed, giving a clear age relationship.

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