The structural development of the Lewisian rocks on the north shore of Loch Torridon, Ross-shire.

Abstract

SYNOPSISNine phases of deformation, D1-D9, are recognised in the Lewisian complex of Torridon. D1-D5 pre-date the Scourie dykes and are thus of Scourian age, whilst D6-D9 post-date the dykes and belong to the Laxfordian.The N.-S. to NE.-SW. S3 foliation which is dominant in the north of the area has been affected by NW.-SE. large-scale (F3A) and subsequent small-scale (F4) folds and related axial-planar foliation (S4). S4 was originally regarded as Laxfordian by Sutton (in Sutton and Watson 1951), but has been assigned here to the late Scourian or Inverian. In D5 both large and small scale F5 folds were produced with SE.-plunging axes, affecting sub-horizontal belts of S4. L5 is co-axial with L4.The earliest Laxfordian structure S6 is usually parallel or sub-parallel to dyke margins. In D7 were formed F7 folds, S7 axial-planar foliation and L7 lineation which increase in intensity towards the south. S7 is co-planar with S4 and S6, and L7 is co-axial with L4. The D8 structures are restricted to narrow NW.-SE. belts in which F8 small-scale folds and L8 lineations were formed. The last penetrative structure is the S9 mylonite banding affecting certain NW.-SE. belts and is co-planar with S4 and S7.Correlations are made with other areas in the southern Laxfordian belt. C O N T E N T S Introduction Lithological units Structural relationships of dykes and gneisses Structural history A. The pre-dyke structure (i) The D3 and pre-D3 structures (ii) The D4 structures (iii) The D5 structures B. The post-dyke structure (i) Hot shears in the dykes (ii) The D6 structures in the dykes (iii) The D7 structures in the dykes (iv) The D8 structures (v) The D9 structures C. Geometry of the large-scale structures Conclusions

Internal evolution of the major Precambrian shear belt at Torridon, NW Scotland

“…Thus they date the hereogeneous deformation and production of flaggy gneisses as Laxfordian. Cresswell (1972) used several criteria to show that in many shear zones (his 'monoclines') the flaggy fabric was in existence prior to dyke emplacement, and is therefore Inverian (Fig. 3).…”

“…To confirm or negate this it is necessary to determine movement directions and senses in each set of shears. If we take the foliation and lineation in the high-strain zones to approximate to the shear plane and direction, then Cresswell's (1972) stereonets of Inverian foliation and lineation can be used to deduce the relation between shear directions and shear planes (Fig. 6c).…”

Early Proterozoic structure and kinematic evolution of the southern mainland Lewisian

“…1. Simplified structural map of the southern mainland region based partly on the detailed structural maps of Park (1964Park ( , 1970a, Cresswell (1972), Crane (1978), Niamatullah (1984) and on published Geological Survey 6 in. sheets.…”

“…The distinction between Inverian and Laxfordian deformation is made primarily by reference to the relationships of the structures to the Scourie dykes (eg see Park & Cresswell 1972, 1973Cresswell 1972 andCrane 1978). However, the long time period (2400-c. 2000 Ma) during which dyke emplacement may have occurred, and the uncertainty over the precise age of individual dykes within the region, render it impossible to fix accurately either the end of Inverian deformation or the beginning of Laxfordian as originally defined.…”

“…Detailed structural mapping of parts of the region took place in the period 1958-1973 by a number of workers (Park 1964;1970a;Elliott 1964;Ghaly 1966;Keppie 1967;Bhattacharjee 1968;Cresswell 1972;and Crane 1978). The usefulness of much of the earlier work however is limited by a lack of general agreement over correlation and nomenclature.…”