The first kinematic controversy: Nappe Translations or not?

Törnebohm threw out his idea of large-scale nappe translations in 1888. This sparked a prolonged discussion that went on at least until the middle of the 20'th century.

We now agree that Caledonian nappes are transported up to several hundreds of kilometers from the west or northwest.

Reverse Nappe Translations?

After the advances in understanding kinematic indicators in mylonites during the late 1970's and 80's, I investigated (1986-92) the kinematics recorded by Caledonian mylonites in large parts of southern Norway. The result was surprising: in most cases the mylonitic structures indicated the wrong sense of vorticity. Thrusting was assumed to be to the (south)east, but the structures indicated northwesterly transport!!!

Todays model

Detailed investigations (Fossen 1992, 1993) showed that SE-directed structures did exist, but had consistently been overprinted by W and NW-directed structures (translation). The latter implied hinterland-directed nappe translations, which must have occurred along a NW-dipping décollement and thus was extensional. Although some indications of NW-directed movement had been reported, this was the first time that:

1) the consistency of the NW-verging structures on a regional scale was demonstrated (the structures were mapped from the coastal areas in the west to the foreland area SE of the Jotun Nappe);

2) that strong, mylonitic fabrics formed during the NW transport ;

3) that there was a consistent overprinting relationship between thrust-related E-verging fabrics and extension-related NW-verging fabrics, and

4) that the NW-verging structures, which are flatlying or presently indicate an apparent reverse sense of shear, must be related to stretching of the Caledonian crust.

The second kinematic controversy: orogenic or post-orogenic extension?

Whereas everybody agrees about the kinematic observations, there is some disagreement as to whether the Mode I (and II) extension or NW-directed nappe transport is a collapse-related, syn-convergent phenomenon or if it represent true extension across the Caledonian orogen. I have argued for the latter view, based on kinematic considerations; how can you translate the entire orogenic wedge about 30 km from the foreland to the hinterland in a convergent setting?

One of the arguments against the post-contractional or post-orogenic model concerns the timing of mild contractional deformation of uppermost Silurian deposits in the Oslo area (foreland). However, recallibration of the geologic time scale for the Silurian-Devonian time period) and the uncertainties still involved in calibrating stratigraphic and geochronologic data from this period) has completely eliminated this "problem". Also, recent Ar/Ar dating of Caledonian and extensional mylonites in the basal décollement zone (Fossen & Dunlap 1998) shows that the extensional and contractional fabrics are separated in time.

A discussion of this controversy is found in Journal of Structural Geology (1993, vol. 15, 1379-1383) and in Fossen 2000 (see publication list).

Several modes of extension

Following the extensional NW-directed nappe translation along a low-angle extensional detachment (reactivated basal thrust zone), the crust was broken into large pieces by somewhat steeper dipping ductile shear zones. Most of them are NW- or W-dipping, e.g. the Hardangerfjord Shear Zone, the Bergen Arcs Shear Zone and the Nordfjord-Sogn Detachment

From plastic to brittle deformation

As the crust cooled, the presently exposed section of the Caledonian orogen moved through the plastic-brittle transition zone. Hence ductile mylonite zones are overprinted by more brittle fault structures.