Engebøfjellet
Structural geology
RutilNett
By Alvar Braathen (chapter 3 in Korneliussen et al. 1998)
Deformation of the Engebøfjellet eclogite and the surrounding rocks may be separated into six stages, designated D1 to D6. Prior to deformation magmatic layering (?) may have existed in the rocks. Metamorphic conditions and mineralogy for this priamary phase can not be established due to later, overprinting metamorphic events. The first observable stage, D1, is shown as isoclinal folds (F1) of the mineralogical banding in the protolith rocks. A dominant transposition foliation (S1), often parallel to this banding, was developed. Metamorphic conditions during the first stage of deformation probably reached eclogite facies, as indicated by formation of garnet and omphacite (Carswell, 1990; Yardley, 1989). Orientation of the F1 axes, which have a subhorizontal, E-W orientation, may be explained by N-S shortening. However, later non-coaxial deformation, with potential rotation, make any conclusion speculative.
After formation of the S1 foliation, an elongated omphacite formed in the eclogite rocks (post-D1). Since this mineral has grown superimposed on to the foliation, and with a random orientation, it probably relates to a period of static P conditions.
During the following D2 stage, the S1 foliation is folded into tight to isoclinal F2-folds. In these structures, a spaced cleavage generated near the fold-hinges. In the F2 fold-limbs, which suffered the most D2 strain, a new penetrative foliation (S2) was formed. This foliation, which is very fine grained and mylonitic(?), represents the dominant fabric in the Engebøfjellet Eclogite. One good example of a F2-fold is illustrated in cross-section D-D' (Fig. A4.2 in Korneliussen et al. 1998), whereas the Fureåsen area (see Appendix 2 in Korneliussen et al. 1998), near the top of Engebøfjellet, shows well exposed outcrop-scale structures. Metamorphic conditions during the D2 stage reached eclogite facies, as suggested by the stable minerals garnet+omphacite along the S2 foliation. Field observations of kinematic indicators in the D2 high strain zones reveal both dextral and sinistral shear-sense for separate zones, respectively, i.e. parallel to the WNW-ESE lineation. One way to interpret this pattern is that the eclogite body suffered regional, coaxial N-S shortening during the D2 phase.
The following D3 stage folds the S2 foliation into tight F3-folds. A spaced cleavage formed in the hinges of these folds, whereas distinct meter-wide shear-zones (S3) generate in the fold-limbs. At other places, the eclogitic S2 foliation is reactivated as shear-zones. In both cases the eclogite is retrograded into (garnet-) amphibolite. Growth of amphibole+garnet+plagioclase in these late high-strain zones indicate amphibolite facies, garnet zone metamorphic conditions (Yardley, 1989) during the D3 stage.
The D4 stage is characterized by the formation of meter-wide shear zones (S4), which have been observed at a few locations. One of these structures cuts through the F3 fold in the quarry (described above); thus it post-dates the F3-folding. Along the margin of the S4 high-strain zones, the eclogite is retrograded into amphibolite, whereas the shear-zones contain amphibole+plagioclase(?)+biotite+epidote+quartz. The lack of garnet, and growth of biotite, suggest that these shear-zones formed under epidote-amphibolite to upper greenschist facies, and biotite zone conditions (Yardley, 1989), maybe from a progressive continuation of the D3 stage.
Shear-sense indicators (e.g., Hamner and Passchier, 1991) for both the D3 and D4 stages, such as composite shear fabrics, shear-folds and wings around mechanical strong bodies, support non-coaxial sinistral shear. This movement was subhorizontal, parallel to the stretching lineation, and resulted in internal shortening by stacking and folding of the first order lens. Thus, the deformation mechanism probably was sinistral transpression.
The S4 shear-zones and all older structures are truncated by sub-vertical N-S striking joints of the D5 stage. These joints are filled with epidote + quartz (+ carbonate + chlorite), whereas the wall-rocks often show cm-wide zones where omphacite+ blue amphibole+garnet is replaced by green amphibole. These mineral associations indicate epidote-amphibolite to greenschist facies conditions during the D5 stage. The N-S, vertical orientation of the fractures, and their tensile nature, suggest that this deformation relates to N-S, horizontal shortening.
It is worth to mention that similar fractures are found in eclogites several km south of Engebøfjellet. There, steep tensile fractures strikes NW-SE. This change in orientation from N to S may relate to late, open folding in the area; folds that possibly are associated with W-directed movement in the detachment zone. These folds are suggested to have a late Devonian - Early Carboniferous age (e.g. Torsvik et al. 1987). Thus, these late, tensile fractures constitute a possible marker that may help to establish late-/post-Caledonian folding of the Precambrian basement.
The last deformation observed in the rocks, the D6 stage, is present as two populations of brittle faults: (i) N-S striking, moderately to steeply east and west dipping normal faults, and (ii) sub-vertical, NNW-SSE and NE-SW striking strike-slip faults. Slip on the faults is indicated by separation of markers in the rock, as well as lineations on the slip-surfaces (slickensides). These kinematic indicators suggest formation of the faults during N-S shortening, which triggered E-W extrusion/extension. Greenschist facies or lower P-T conditions during the D6 stage is suggested from the brittle style of deformation. This type of structures are suggested by Braathen (1998) to reflect Carboniferous - Permian deformation in the region.
The suggested deformation sequence and its tectonic implications is important in a regional perspective. The steep E-W orientation of foliations (S2 and S3) seen in the Engebøfjell area are not continuous to the E. There, the main foliation successively changes to moderate N dips. However, the high-strain zone of Engebøfjellet appears to be continuous into the eastern area. Thus high-strain zone bounds an open box-like antiform further S, which has an other high-strain zone as its southern margin. Internal foliation in the antiform probably predates the shearing in the fold-limbs. In the Fureviknipa area W of Førde, a distinct fold-pair is evident. The southern antiform is open and upright, and is probably truncated by the detachment zone, whereas the synform further N is tight and S-verging. The upper limb of the synform reveals a high-strain zone that may correlate with the Engebøfjell shear-zone.
Thus, at least two old foliations can be distinguished in the regional map-pattern, in addition to the mylonitic foliation in the detachment zone. The oldest foliation, which could be Sveconorwegian in age (1000-1200 Ma), occurs in hinge-areas of regional folds. There, isoclinal sheet-like folds within the foliation indicate that significant strain affected the rock at this stage, and that primary, possibly older structures were transposed. A younger foliation is superposed on the older fabric in high-strain zones that are found in the fold-limbs. This foliation, which can be correlated with the S3 and/or S4 fabric of Engebøfjellet, reveals subhorizontal stretching lineations, whereas shear-sense indicators suggest sinistral shear. Eclogites and eclogite facies fabrics, probably Caledonian in age (Andersen and Jamtveit 1990), are retrograded into amphibolite-facies rocks in the D3/D4 high.