minor pegmatites and fine-grained uranium- and thorium-rich granitic dykes Ho¨gdahl et al., 1998.
2. Regional deformation zones: the Storsjo¨n – Edsbyn deformation zone and the
Hassela shear zone
On aeromagnetic anomaly maps, the SEDZ Bergman and Sjo¨stro¨m, 1994 appears as a 10 –
20-km wide and 300-km long zone between Eds- byn in the south and Lake Storsjo¨n in the north
Fig. 2. There is independent evidence that the SEDZ continues to the north-northwest beneath
the Caledonides Bergman and Sjo¨stro¨m, 1994. The SEDZ essentially separates \ 1.8 Ga rocks
mainly Ljusdal and Revsund granitoids and older Svecofennian supracrustal rocks to the east from
a younger, ca. 1.7 Ga TIB intrusion to the west.
Structural data and the recognition of various kinds of mylonites show that the SEDZ has been
active repeatedly Bergman and Sjo¨stro¨m, 1994. Ductile, retrograde and brittle – ductile mylonites
are the most common types along this deforma- tion zone. Dextral, transpressive shearing result-
ing in steep stretching lineations, has been suggested either to be connected to the emplace-
ment of the Ra¨tan intrusion, or a post-emplace- ment phenomenon. In the Revsund granitoid
affected by SEDZ-deformation, a coarse, dextral CS-fabric is the dominating structure. Sinistral
shear zones occur as conjugate sets, or occasion- ally as later overprinting structures.
The HSZ is localised along the boundary be- tween the Ljusdal Batholith and the older Sve-
cofennian metasedimentary
rocks greywacke-schist of the Bothnian Basin to the
north Bergman and Sjo¨stro¨m, 1994; Sjo¨stro¨m and Bergman, 1996; Fig. 2. Previously, this
boundary has been referred to as a primary fea- ture. It is a steep, WNW- to NW-striking domi-
nantly dextral shear zone formed under wrench conditions. Narrow sinistral zones, conjugate to
the dextral pattern, were probably formed during progressive bulk dextral deformation. Some of the
former display a retrograde character reflecting shear during a late stage of the deformation.
The dextral rotation of the HSZ into the SEDZ indicates either that the HSZ is older, or that they
formed simultaneously Bergman and Sjo¨stro¨m, 1994. The timing of the main ductile deformation
along HSZ is bracketed by its imprint on the 1.85 – 1.84 Ga Ljusdal Batholith Delin, 1993;
Welin et al., 1993, and its syn-metamorphic rela- tion to the regional low-pressure metamorphism
LPM
at \
ca. 1.82
Ga Claesson
and Lundqvist, 1995.
3. Local shear zone network
