An analysis of noncylindrical and incongruous fold pattern from the eo-cambrian rocks of söröy, northern Norway

~ecf~~z~~~~s~cs, 18 (1973): 81-107 0 Elsevier Scientific ~blishing Company, Amsterdam - Printed in The Netherl~ds

AN ANALYSIS OF NONCYLINDRICAL AND INCONGRUOUS FOLD PATTERN FROM THE EO-CAMBRIAN ROCKS OF StjRtjY, NORTHERN NORWAY. I. NONCYLINDRICAL, INCONGRUOUS AND ABERRANT FOLDING DONALD M. RAMSAY and BRIAN A. STURT University of Lhmdee, Dundee, Scotland (Great 3r~t~~~~ University of Bergen, Bergen doorways (Accepted for publication August 2, 1972)

ABSTRACT Ramsay, D.M. and Sturt, B.A., 1973. An analysis of noncylindrical and incongruous fold pattern from the Eo-Cambrian rocks of Soroy, northern Norway. I. Noncylindrical, incongruous and aberrant folding. Tecfonophysics, 18: 81- 107. The earliest folds on Soroy are characteriz,ed by primary noncylindrism on all scales, and the range of noncylindrical fold styles described is widely developed. The pattern of quartz Cv-axes orientation in such folds remains symmetrically disposed to the fold axis, despite its variability. In such a fold regime the ‘pum~Uyan relationship’ between parasitic folds and the associated large:. structure frequently does not obtain and the parasitic folds are incongruous. Congruous and incongruous folds are genetically linked through an intermediate category, aberrant folds, which possess some features of both types. In the aberrant folds the parasitic structures are congruous while the layering is orientated at high angles to the axial surface, but are incongruous when this angular relationship falls below a critical threshold value. Two factors underly the apparent freedom for axial orientation in the incongruous parasitic folds: (1) primary noncylindrism of the fold regime; (2) the orientation of the layering relative to the axial surface and therefore the principal strain axes. The incongruous relationship underlies the widespread development of minor structures, with axes trending transverse to the major folds, and its regional implications are discussed in the light of the common occurrence in the Caledonian fold belt.

INTRODUCTION

The island of Siirb;y lies off the coast of West Finnmark (latitude 70”30’N) in northern Norway and is situated well within the main Caledonian fold belt. Two periods of major erogenic deformation, during Late Cambrian-Early Ordovician times, have imprinted their movement plans on the mixed assemblage of Eo-Cambrian metasediments and synerogenic igneous rocks which constitute the island (Ramsay and Strut, 1963; Sturt et al., 1967; Pringle and Sturt, 1969). The resultant fold patterns produced during each of these two periods of deformation are characterized by very different geometrical and symmetrological relationships.

82

D.M. RAMSAY AND B.A. STUR-I

Some characteristics

of the second fold structures (D2) have been described from cen-

tral-north

Soroy (Ramsay and Sturt, 1963; Ramsay, 1971) where the general orthorhombic

symmetry

of the movement

plan is reflected in the widespread development

folds on all scales. Within limited sub-areas the majority

of conjugate

of the minor folds are cyiindrical

in form and exhibit a relatively high degree of constancy

in axial orientation.

The general

regularity of the patterns of DZ-folding contrasts markedly with the triclinic symmetry the fabric produced during the first deformation. widespread development tures. A corollary

of noncylindrical

to the basic noncylindrism

acteristics of this folding is the frequent

This phase of deformation

of

caused the

folds on all scales from minor to major strucand one of the most striking geometrical development

of an incongruous

char-

relationship

be-

tween congenetic parasitic folds, relative to the associated larger fold and to each other. The ultimate expression of this incongruous relationship is the regional development of minor folds orientated

oblique to the major fold trend.

The present account is an examination

of the patterns and interrelationships

of non-

cylindrical and incongruous folding with some emphasis on their relevance in erogenic folding. In addition an attempt has been made to integrate the complex local patterns of minor and macroscopic long-standing

folding into the large-scale movement

problem of folds orientated

plan thereby elucidating

the

transverse to the main erogenic trend.

1:lRST PHASE I:OLDlNC

General setting The stratigraphy

of Sijrijy has been established

by Sturt and Ramsay (1965), Roberts

(1968) and Ramsay ( 197 1) and the general succession is: Hellefjord Schist Group Aafjord Group Falkenes Marble Group Storelv Schist Group Klubben

Psammite Group

In northwest Siirijy the recognizable large-scale structures of the early fold phase (the Breivik and Sorvaer folds) are clearly delimited by the distinctive lithologies of the Falkenes Marble and Aatjord Schist Groups, outcropping in belts trending north from Breivik and Sorvaer (Fig. 1). The Breivik structure is an overturned, asymmetrical, coupled fold with relatively simple Z-profile (Fleuty, 1964). Traced northwards from Breivik the complexity of this structure increases with the development of a recumbent syncline (the Aafjordnaeringen syncline) overlying the Breivik fold. The large Dl-fold lies in the core of a gentle open D2-synform ___

and has been little modified by the later deformation. -. _. _._ ___.._.._ ._.._._...-_--. --

Fig. 1. Structural map of northwest S%y *owing only Dl minor fold axes and representative attitudes of layering. Formations are delimited by dotted lines, but only three are ornamented: dots = Klubben Psammite; cross-lines = Falkenes Marble; random dashes = gabbro. inset shows whole island with outcrop of Falkenes marble and younger.

D.M RAMSAY AND U.A.STUR'I

84

The Sorvaer syncline is a deep, strongly compressed isocline, lying on the western limb of an upright, symmetrical

DZsyncline

(Fig. 1). The western margin of the D1-syncline

been replaced by a slide, causing the juxtaposition Klubben

Psammite. If the effects of D2 are removed by unrolling

the Dl-syncline

has

of members of the Aafjord Group with the symmetrical

synform,

can be seen to open westwards in contrast to the sense of facing of all the

other major hinges on the island. The large-scale D2-folds of this part of the island vary from upright to recumbent tures with amplitudes

of several hundreds-several

to 4 km. In the gently inclined,

non-inverted

(Dl) may be well preserved, tilted only slinky hinge areas and overturned

thousands

of metres and wavelengths up

limbs of the overturned

structures,

out of their original o~entations.

limbs, however, the D2 minor structures

Within the Aafjord Group where the crystallinity

struc-

dominate

early folds In the the fabric.

is low, many of the early minor folds

in the cores of the major structures are relatively simple in profile and develop a strong axial-plane flow cleavage. In the pelitic members of the Storelv schist, however, the crystallinity is much higher and the axial-plane schistosity so strong, that folds can only be detected where there are thin, interbanded quartzite ribs. This contrasts with the strongly asymmetrical folds of the Klubben Psammite where axial-plane schistosity is confined to the hinge and thick middle limbs, while in the strongly-thinned long limbs layer-parallel schistosity

is developed.

Noncylindrical fo1d.y The folds of the first phase are complex in their morphology and their mutual interrelationships. Through the several orders of magnitude recognised a high proportion of the structures possess a noncylindrical form (Turner and Weiss, 1963). Individual fold axes display plunge culminations and depressions which result in a sinusoidal wave-like.pattern in the axial direction (Fig.2). Depending on the fold attitude, individual axes may curve in azimuth or plunge through arcs of as much as 120”. The major Breivik fold for example, exhibits continuous tance of 8-10

variation in axial trend from north-south

to east-west,

over a dis-

km.

The most obvious axial curvature occurs in thin-bedded hthologies, especially that of psammitic ribs in mica schist. The wavelength of axial curvature seems related to profde wavelength and amplitude, which in turn are influenced by layer thickness (Ramberg, 1960). In general, the wavelength of axial curvature is at least two to four times the profile wavelength and frequently much larger. Where layer thickness is greater, as in the more massive psammites, the wavelength of axial curvature may be considerable and individual folds, over the distance of observation, may appear to be cylindrical. In a large outcrop therefore, it is possible to find fold axes of apparently constant attitude and others which are widely inconstant, depending on the lithologies. The curvature of an axis in individual folds may persist for only half a wavelength in the axial direction before dying out, while in others it may continue for several or many wave-

FOLD PATTERN FROM ROCKS OF SijRijY, NORWAY, I

85

b

C

e

f

Fig.2. Diagrammatic sketches of noncylindrical fold styles. a. Plane noncylindrical. b, d and e. Nonplane noncylindrical. c. Variable fold profile. f. Styles of axial curvature. See text for explanation.

86

D.M. RAMSAY AND B.A. STURI

lengths. In the plunge culminations

the curvature of the axes is usually smooth and gener-

ous, while in the plunge depressions it may be similar, much tighter, or very sharp (Fig.2f). The synclines between noncylindrical

anticlines

are frequently

formed, their form being dictated by the space requirements A significant

consequence

area, and even in individual in distribution.

of the dominating

anticlines.

of axial curvature is that the fold-axes measurements outcrops, display a widely varying though systematic

On the district scale of northwest

in any scatter

Soroy, axes of early folds are grouped

in a great circle-band

displaying no maximum

Despite this variation

the axial surfaces for equivalent

The noncylindrical

less regular and not as well

correlative with the major hinges (Fig.3). profiles are sensibly sub-parallel.

folds under discussion fall into several categories:

(la) Plane noncylindrical folds with planar axial planes (Turner and Weiss, 1963) (Fig.2a). Axes curve within the axial plane and the geometry of the profile remains sensibly constant, although the amplitude diminishes as the fold dies out. (1 b) Plane noncylindrical

folds similar to those of (1 a) but where the profile is incon-

stant. This change may be an opening or tightening inution

of amplitude

of the folds and commonly

as one traces the fold into an axial plunge depression.

Fig.3. Synoptic stereogram of 667 minor fold axes from northwest S&iiy.

it is a dim-

FOLDPATTERNFROM (2) Nonplane

8"

ROCKSOFS6RtiY,NORWAY,I

noncylindrical

folds with cylindrical,

curviplanar

axial surfaces (Fig.2b).

Axes curve within the axial surface, which is in turn curved about a rectilinear

axis which

can be oblique to the fold axis. The strike of the axial surface may be constant

or incon-

stant, i.e., the axis about which the axial surface is curved is horizontal

or plunging. The%

folds are asymmetrical

in some cases be-

coming disharmonic

to reclined and the profiles change significantly,

when traced up or down the axial surface (Fig.2c). This disharmonic

style suggests modification by later stage flattening (De Sitter, 1956; Ramsay, 1962) in which the maximum extension occurred parallel to the layering of the long limbs, causing, rotation of the mid-limb to complete unfolding in the other direction. (3) Nonplane

noncylindrical

in one direction

folds with noncylindrical

and to a reclined position

curviplanar

axial surfaces. In

these folds the axial surface is curved not only as in case (2), but is in addition curved about a transverse axis. The profiles of folds of this type are observed to vary markedly along the direction of the fold axis (Fig.2d). The abundance of fold axes curving within the trace of the axial surface together with the recurring similarity in style and range of axial orientation suggests that the noncylindrism is a primary phenomenon and not a consequence of refolding. Petrofabric evidence In order to ascertain whether the orientation pattern of the grain-fabric maintains a constant angular relationship with the fold axis in its different orientations a number of thin sections were cut normal to the fold axis, of a typical fold, from situations different

with ver)’

axial trend.

The fold examined was a plane noncylindrical fold from a thin-banded micaceous psammite in the Klubben Psammite Group. The limbs of the fold display marked attenuation and the axis curves from 270/30” to 3 16/32” within the trace of the axial plane. This fold was located in the steep limb of a macroscopic

monocline

in which the larger

minor folds of thicker psammite layers plunge gently and consistently as the folds in the less competent

lithologies have markedly inconstant

northwards,

where-

plunges.

The quartz grains measured in each thin section came from the same distinctive

thin

band and partial diagrams were prepared from several segments around the hinge. Postkinematic recrystallization of quartz produced a polygonal mosaic texture with fairly stable quartz-quartz junction where mica is not abundant. Mica diagrams from all three sections display partial peripheral girdles with monoclinic symmetry and maxima normal to the trace of the axial plane (Fig.4d). The synoptic quartz diagrams for each section have triclinic symmetry in patterns which although broadly conparable, differ in the detail of maxima positions (Fig.4a, b,‘c). A broken cleft girdle is symmetrically orientated with respect to the fold axis and there is a weak spread of poles on to the ac-plane. Rotation of the synoptical diagram for each section into the horizontal plane makes the broad cleft girdles and their respective westerly and northwesterly axis more obvious (Fig.4e, f, g). The fold axis therefore seems to have been a directional influ-

Fig.4. Petrofabric analysis of the noncylindrical fold shown in the inset. a-c. Quartz Cv-axes from locations A-C on the fold. d. Poles to 001 of muscovite. e-g. Quartz patterns of a-c reorientated into a horizontal plane. Figures to the left of the stereograms indicate the number of poles, those on the right the contour interval. The great-circle trace marks the axial plane and the dotted trace the UC-plane.

ence on the orientation of the quartz fabric. In the light of current interpretations of quartz C-axis girdles (Friedman, 1964) this pattern of grain orientation indicates rotation in a plane normal to the fold axis, irrespective of its orientation. The tightness of the fold together with the marked attenuation of the limbs indicates the influence of flattening and layer-parallel extension in the later stages of the fold’s history. The lack of sharpness in the girdle fabric is attributed to recrystallization during the flattening, superimposed on a syntectonic fabric. The one consistent tectonic element present in the fold-specimen is a faint mica fibre-lineation trending northeast-southwest on both long limbs of the fold. This is a stretching lineation representing X’, the major axis

~OLD~ATTERNFROM

x9

ROCKSOFS~R~Y,NORWAY,I

of the sectional deformation

ellipse in the plane of the layering (see more detailed discus-

sion in Part II). The path of this lineation the locus of X’-orientations.

in the various attitudes

of the layering represents

If the axial plane is equated with the XY-plane and the X-

axis is taken to coincide with the lineation

direction,

as seen on the layering when it par-

allels the axial plane, then it is obvious that neither the fold axis nor the related quartz fabric are in agreement with the orientation Genesis

of noncy~indrical

of the strain axes.

folds

Primary curvature of fold axes has been recorded by many authors and there have been several explanations given for the origin of such structures. In the following a brief review is given of work in this field with emphasis on the applicability to the examples on S6roy. In the Dalradian rocks of western Scotland, from zones of strong deformation, particularly

Voll(1960) describes noncylindrical folds within the overturned steep limbs of large

monoclines, which have constant axial disposition. Associated with these structures is a congenetic stretching lineation, which according to Voll has a regionally consistent trenc. and is oblique to the fold axes. These folds are similar in morphology and structural asscciation to the noncylindrical folds described from the thin-bedded lithologies on Soroy. However, it must be emphasized that on SijrGy it can be demonstrated that these noncy lindrical folds are not restricted to any one particular structural situation. Brynhi (1962) att~buted the arcuation of early linear structures in the ~rondheier ar ea of western Norway to inconstant orientation of the axial planes resulting from a lack of any constant sense of movement, together with some buckling of the layering before the penetrative

deformation.

In experimental compression of plastic materials Bhattacharji (1958) produced noncylindrical folds through non-uniform compression. The folds had a canoe-shaped form and axial curvature of a half wavelength. folds described by Ramsay (1962,

These are similar in form to the noncylindrical

1967). Some of Bhattacharji’s

folds displayed addition-

al curvature around steep transverse axes, so that the canoe-shaped outcrop in plan, i.e., they have become nonplane The simplest fold types of Bhattacharji %irijy, ~thou~

noncylindrical

folds have a curved

in form,

are similar to category (1) described from

the degree of axial curvature achieved in the experiments

of 3hattacha~~i

never reached the extremes observed on SZiy. The more complex types discussed by Bhattacharji have similarities with folds of category (3) on SiSriiy. Somewhat similar suggestions to the above are embodied in the four mechanisms pro. posed by Voll(l960)

to explain the origin of such structures:

(1) Unequal stretching along the strike of the layering (internal rotation) (see also Nabholz and Voll, 1963; Ramsay, 1967). (2) Thinning of limbs and subsequent detachment of hinges within the plane of the schistosity (see also Knill and Knill, 1958). (3) Buckling of layers prior to formation of the schistosity so that the layeringschistosity interaction produces a curvilinear trace (see also Ramsay, 1962; Brynhi, 1962).

D.M. RAMSAY

90

(4) Layering planes intersected

AND B.A. STUR-J

by shear planes of different attitudes (see also Brynhi,

1962). The range of noncylindrical and furthermore,

fold phenomena

these structures

i.e., there is no differentiation

described from Dl on Soroy is extensive,

occur repeatedly

within the same tectonic environment,

of the several types to particular

thors consider that any mechanism

situations.

Thus, the au-

proposed for the genesis of the noncylindrical

fold

structures on Soroy should have general application, and implicit in such a mechanism is non-uniform flow parallel to the axial surface, between adjacent folds (cf. mechanism 1 of Voll). In the simplest types (i.e., la and 1 b of the present study) such flow could result from non-uniform compres~on normal to the axial surface (cf. Bhattacharji, 1958; Ramsay, 1967). For the development of the more complex fold forms involving rotational strain, however, the non-uniform

flow in the axial surface can be the result of:

(1) Folds generated in a constrictional (2) Flattening

(k < X) superimposed

thus include a combination (3) Inhomogeneous

field (of general type k > 1; Flinn, on preexisting

of initial constriction

folds (Ramsay,

1962).

1962), which may

followed by flattening.

strain.

None of these mechanisms are mutually exclusive and indeed all may operate at some stage in the fold’s history, to produce the final form. The occurrence of a regular pattern in the minor fold axis and axial surface orientation, together with a consistent nite directional

restrictions

regional pattern of the stretch lineation on the inhomogeneous

pressional axes may well have been constant ation of the folds. In suggestion (2) the deformation

throughout

is initially

(see Part II), puts deti-

flow and suggests that the major comthe area during the initial cre-

constrictional,

and if the magnitude

of the

intermediate axis of strain has a quadratic extension just less than unity, the tendency to a symmetrical ‘egg-box’ pattern of folding will be restricted by the much greater shortening on the principal axis of compression.

With continued

intermediate strain axis diminishes and it ultimately strain becomes a flattening. INCONGRUOUS

deformation,

shortening

becomes an axis of extension

on the and the

FOLDING

Many folds of the first deformation demonstrate markedly anomalous relationships in the axial orientations of the ‘parasitic’ folds (De Sitter, 1956), on opposite limbs of the larger fold. In a typical example (F&S) the trend and plunge of the parasitic folds on one limb (A) may be coincident with the axis of the larger fold while those on the other limb (B) may be markedly divergent. As the axis of the larger fold is traced along its length, however, it may be found to curve into parallelism with the axial direction of the parasitic folds of limb B. Such ‘non-Pumpellyan’ divergence in parasitic fold interrelationships will be referred to as incongruous folding. Apart from their divergent axial orientations, the incongruous folds have ‘normal’ styles and sense of overturning towards the hinge of the larger fold. When the incongruously

FOLD PATTERN FROM ROCKS OF StiRijY, NORWAY, I

Fig.5. Diagrammatic sketch of the incongruous relationship. For explanation see text. trending parasitic folds impinge on the hinge of the larger structure

they either die out, or

curve into parallelism with it and the congruous folds of the other limb. Such relationships have been observed on several scales of magnitude, from single folds with amplitudes < 1 m to the large-scale Breivik fold, and as will be demonstrated, are a fundamental feature of the movement

plan rather than a unique and isolated occurrence.

Two main types of incongruous fold-pattern have been observed: (a) The parasitic folds have a long wavelength, low amplitude of axial curvature

and

persist for considerable distances in the axial direction; in some cases comparable in extent to that of the larger fold (Fig.5). (b) The parasitic folds display axial curvature with a much shorter wavelength than that of the related larger fold axis. This is especially prominent in a mixed sequence of thick and thin-bedded lithologies. In case (a), several or all the axes of the incongruous

parasitic folds on one limb of the

larger structure may be parallel to each other, though curving out of phase with the mair fold axis and the parasitic folds on the other limb. In case (b), the axes of minor folds may all be at variance, so that in any cross-section there is a divergence of axial directions,

not only within the one limb, but also with the

matching structures in the complementary limb of the larger fold. On an exposed bedding plane these axial traces may curve and swirl in a manner reminiscent of ropy lava. The commonest fold-axis orientations, both in outcrop and in any synoptic plot of axial distribution are west-northwest-northwest and northeast-southwest. Where the axial curvature of individual structures can be observed for some distance the enveloping surface to the sinusoidal axes commonly trends west-northwest-north-west. This indicates that in the long limbs of the major folds the minor folds have a mean trend which is markedly oblique to the axes of the major folds. Data available on the Dl-fold structures in other areas of the island with different tectonic strike, confirm the incongruous relationship of many minor folds and underline the fundamental nature of the phenomenon in the movement plan; e.g., in north-central

92

D.M. RAMSAY AND B.A. STUR?’

Fig.6. Minor folds from a peninsula in north SSr8y, where the major fold axes and formations trend east-west. Circles ¬e macroscopic folds.

Sijriiy where the major folds trend east-west a large proportion of the minor folds maintain the oblique orientation to the major axial trend (Fig.@. RELATIONSHIP BETWEEN CONGRUOUS AND INCONGRUOUS FOLDS

On Soroy it is possible to recognize a spectrum of fold relationships and patterns which has at one end the orthodox Pumpellyan situation and at the other end the extreme variability and inconstancy of incongruous folds. Between these extremes is an intermediate type which the authors have termed aberrmtr folding. In this category some members of the parasitic assemblalgeare congruous and others diverge from the ‘normal’ congruous or Pumpellyan orientation. This type of folding has an important role in understanding the origins of noncylindrical and incongruous folds, revealing conclusively that they are a primary category and not the result of subsequent deformation. The aberrant folds, being a special expression of incongruous folds, are subject to the same controls and because the constituent members can also exhibit congruous relationships, they establish a link between the ‘normal’ Fumpellyan and incongruous folds. Some typical examples of aberrant folds wiIl reveal the nature of these folds and the parameters

which control

their development.

a small knoI12 km north of the inlet of Nordfjord (Fig_7), a thick development of the Klubben Psammite Group lies in the core of a recumbent Dl-anticline. These rocks have been refolded in open, symmetrical DZ-foIds of large amplitude and wavelength, On

FOLD PATTERN FROM ROCKS OF SijRGY, NORWAY, I

53

Fig.?. Locality map of S&Sy. 1 = Nordfjord; 2 = Klubben; 3 = Hoivik; 4 = Snakefold Point; 5 = Breivik. Ornamentation: dots = Klubben Psammite; lines = post-Klubben Psammite; broken lines = younger formations occurring within gabbro sheets.

plunging 5- 1.5” towards the northeast. On the limbs of these large folds are developed ~ymmetric~ step-folds of intermediate scale. The steep limbs of these folds are inclined at 50-90” while the gentle limbs dip between 10 and 25”. These macroscopic folds are characterized in turn, by abundant parasitic folds whose amplitude/wavelength ratio depends on the thickness of the psammite control horizons. The style of these folds is dependent

on the overall attitude

are symmetrical

of the layering, i.e., in the gently inclined portions

to weakly asymmetric~

they

and range from open to near isoclinal. In the

steeply inclined Layers the folds range from asymmetrical

to reclined and are sometimes

disharmonic, while the axial surfaces are inclined to the mean layering at acute angles and may even be parallel to it. Although these parasitic folds are quite regular with respect to style and sense of facing, it is in axial disposition that they become dis~ctive. As one traces the minor folds from the flat-lying layers into the steep zones, the minor structures display a characteristic divergence from the ‘normal’ orientation. Fig.8 is a typical example where the open folds of the flat limb are cylindrical and congruous with the larger fold for the limit of the exposure, i.e., 10 m, and for a length much greater than their wavelength. Immediately on entering the steep zone and the folds are asy~et~cal, they become noncylindrical with axes that may diverge from that of the modal fold by up to 90”. Individual folds display up to 120’ of plunge curvature. The axial curvature may be gradual or quite sharp, giving rise to geniculate bends. On re-entering the next flat belt the folds once again become cy-

D.M. RAMSAY AND B.A. STURT

b Fig.8. a. Macroscopic step fold with suite of parasitic folds I-24. b. Stereogram of

to folds 1 -24.

Wulff net, lower hemisphere.

lindrical and congruous. As stated the axial curvature in the steep belt is a marked feature and the axes of adjacent folds, even of the same layer, are seen to curve out of phase with each other, as in so many incongruous fold localities. This pattern is repeated throughout the locality (Fig.9) and in predominantly steep belts, where there are larger minor folds with gently disposed crestal areas, the plunge of minor crumples in these gentle regions is sub-horizontal and cylindrical. The various descriptive characteristics of the folds in both zones were examined to see if there was some striking morphological anomaly which would account for the incongm-

FOLD PATTERN

FROM ROCKS

OF SijRijY,

NORWAY,

I

b

../---? ?r

5 ._ $ g a

,’

.*

,’

,,I

. -. . ._:

z

y_ .:

.

’ :

.:

I

.

.;

.

-: . . ..

.

1

d

Fig.9. a. Synoptic plot of 176 parasitic folds from Nordfjord locality. b and c. Differentation of this suite into folds from flat and steep limbs respectively. d. Plot of axial deviation vs. angle subtended b/ ‘ayering and axial surface (SL A&‘). Dots are folds from steep limbs, crosses are folds from flat limb,;.

ous

behaviour of the steep zone folds. No consistent

differences in amplitude/wavelengtl-

ratio, wavelength-thickness or inter-limb angle emerged. The one significant relationship is between fold-axis variability and the angle subtended by the mean attitude of the layering and the axial surface (SL AAP, Fig.9d). The orientation of the cylindrical minor folds on the flat limb, being congruous with the larger fold axis, was taken as the modal attitude and the deviation of the folds on the steep limbs related to this. The resulting

96

D.M.RAMSAYANDB.A.S?‘URT

points fall within a field whose boundary of this boundary

surface has a hyperbolic

form. The inflexion

point

surface represents the critical angle between the two planar structures,

at

which fold axes become markedly inconstant. The symmetrical distribution of fold axes on the positive and negative side of the abscissa merely indicates respectively clockwise and anticlockwise

variation

of fold axes.

On the western coast of Donnesfjord,

on the small peninsula

of Klubben (Fig.7), there

is another display of D2 aberrant folding which differs significantly from the Nordfjord

examples. The layering in this thin-bedded

in certain respects

sequence of psammite

ribs

in mica schist varies in trend between 050 and 090”, and is part of the steep limb of a large northward

facing D2-antiform.

Minor folds (D2) are extremely

abundant,

ranging

from symmetrical with subhorizontal axial planes to asymmetrical and facing up or down dip of the layering. A number of these asymmetrical folds form conjugate pairs. Where the minor folds are symmetrical or slightly overturned they are sensibly homoaxial with mean plunge to west-southwest. A number of the minor folds are markedly overturned to reclined, but down the dip of the axial surface they merge into symmetrical folds. It is when the folds become overturned that their axes become incongruous and even noncylindrical. When fold-axis deviation from the modal attitude is plotted against SL A AP, a similar pattern to that at Nordfjord emerges, although the same range of variability was not developed (Fig.10). The distinctive feature of the Klubben occurrences is that the mean layering is sensibly constant, and it is the axial surface which displays variation from normal to parallel to the layering. Thus it appears to be the angular relationship between these two surfaces which is significant rather than any special orientation for one of them. In both these instances the inconstant folds were upright and variation was principally

in terms of plunge.

In an example of aberrant folding from Hoivik (Fig.7), the relationship

was observed

in the parasitic folds to an overturned asymmetrical Z-fold, with an amplitude of at least 10 m. The lithology is banded psammite with thin mica-schist partings and layers. Fig.1 1 summarizes the relationship with a systematic plot of fold-axis orientation. In the crestal region of the upper anticline and in the upper part of the steep mid-limb, the axial surface is sub-normal to the mean layering and minor folds are near-cylindrical for considerable distances in an axial direction. This orientation was taken as the modal attitude and all other minor fold orientations from the rest of the fold related to it. The pattern of axial deviation against SL A AP brings out the same pattern as already described, although in this case deviation involves azimuthal as well as plunge variation. This fold emphasizes the signifIca.nce of layering orientation relative to a fairly constant axial surface. When the layering of the mid-limb is traced from the upper anticlinal hinge it quite suddenly decreases in angle of inclination and, with this decrease in dip, the angle between it and the axial surface becomes acute and the minor folds abruptly become incongruous folds. The lower synclinal hinge, in contrast to the generous form of the anticline, is sharp and tight and there is no significant development of layering sub-normal to the axial surface, as in the crest of the anticline. The axis of this syncline diverges significantly from the comple-

.t

6 I=

1 I

P 60.. g

. .

< 3@ a

**

I, I .. I . . !I . :\ .

:

..+. ‘K . *.*>*._ *.. : . .* A.+ . .*.. . . .-:-*.*:.. . : *. * . :.. *es* . . I 30’ 60 90’SLAAP (I

b Fig. 10. Klubben, Donnesfjord. a. Axial deviation vs. SL A AP. b. 88 parasitic link different orientations of the same noncylindrical hinge.

mentary

fold axes. Dashed

lines

anticline and the parasitic folds of the crestal area are congruous with it. The

contrast in morphology between the anticline and the syncline explains why some folds display a more regular pattern of minor folds and others develop the distinctive aberrant fold pattern. This association between congruous and incongruous folds can also be shown to apply to the relationship between minor folds and the major structures and indicates how areas of folds with transverse orientation can be congenetic with the major folds. For approximately 1 km to the east and west of the village of Breivik there is a stretch of near-continuous road cutting through the major Dl-fold. In the cliff exposures behind the village sections of the major fold profile are clearly preserved. At the several locations where extensive measurements were recorded the relative position in the fold profile is hOWI-.

D.M. RAMSAY AND B.A. S’fUR I

DEV.

DEV.

30 bD S,AAP

90

Fig.1 1. Summary d&ram of Hoivik macroscopic fold. Stereogdms delimit fields containing parasitic folds from each limb. In the mid-limb stereogram one fteld (fine dots) indicates folds from the steeply inclined portion and the other (dashes) the folds from the gently inclined portion. The axial deviation vs. SL A AP plots for each limb are also given. In the most westerly station (Figl2a) there is a sudden development of variability in minor fold orientation as one traces from east to west. At the east end of this locality the asymmetrical folds face eastwards towards the anticlinal hinge with a constant axial orientation, congruous with the major fold. Eastwards from this locality to Breivik village, i.e., through the hinge of the major anticline the minor folds are congruous and regular (Fig. 12a). The overturned mid-limb of the major fold dips at 45’ or less to the west and here, in the incompetent lithologies of the pelitic Aafjord Group and pure marbles of the FaIkenes Marble Group, flattened minor folds are near isoclinal or reclined with axial surfaces subparallel to the overall layering. Axial orientation diverges markedly from that of the major fold (Fig.12b). In the core of the large syncline, minor folds are once again cylindrical and

FOLD

PATTERN

FROM ROCKS

OF SijRaY,

NORWAY.

I

b

a

C

Fig. 12. Differentiation of parasitic folds from the major Breivik fold. a. Circles = upper limb away from hinge; dots = vicinity of anticlinal hinge. b. Dots = steeply dipping portion of the overturned limb; circles = recumbent portions of mid limb. c. Folds from long lower limb. d. Axial deviation vs. SL iF\ AP.

congruous (Fig.12b). In the lower limb of the major syncline the more competent calcsilicate schist and talc-phyllites of the lower Falkenes Marble Group display easterly facing asymmetrical folds of simple open profile and the axial surfaces make a high angle with the layering. These folds have congruous gentle northerly plunges. In other lithologies where the folds are markedly overturned to recumbent they again become incongruous. To the east of Breivik, in the mica schist-thin psammite lithology of the upper Storelv

100

D.M.RAMSAY AND B.A.STUR'l

Group, the folds are all Z-style recumbent

and axes are invariably

oblique to the trend of

the major fold (Fig. 12~). Taking the trend of the major fold axis as the modal attitude, fold-axis orientation

the variability

of minor

can once again be correlated with the SL AAP and the critical value

marking the inconstancy

threshold is in the range of 40-50”

(Fig. 12d).

These examples, occurring as they do in folds of such differing scales, bring out the sig niticance of aberrant folding in the movement ous folds in a genetic perspective.

plan of Dl-deformation,

and put incongru-

Example from outside Stirtiv In the coastal strip 5-6 km south of the town of Girvan in southwest Scotland, Ordovician Ardwell flags are spectacularly with amplitudes thin sandstone

Upper

folded into a series of cascade or step folds,

upwards of 30 m (Williams, 1959). The lithology is a regular repetition

of

and siltstone or shale. At the south end of the section the larger folds plunge

gently northwards. On traversing northwards the plunge increases until at the north end of the section the folds, now much reduced in amplitude, plunge steeply northeastwards at between 50 and 75” in the steep layering. On the flat limbs of these step folds, minor folds vary from open symmetrical folds, with axial plunge down dip and congruous

to box-

with the larger fold plunge. On the steep

limbs the minor folds are markedly asymmetrical to reclined, with axes diverging from that of the associated large fold (Fig. 13). Fig.13a shows the relationship of this variability to the SL h AP angle and the pattern produced although less precise than the dxamples from Soroy, is still of the same type. Similar patterns have also been observed in the Ordovician rocks of Rhosneiger in Anglesey, North Wales, and in the middle Dalradian rocks of Islay, Scotland, tailed measurements of the structures have not been made.

although de-

Formation of incongruous folds Hills (1963) used the term ‘incongruous’ for minor folds, dependent or independent of a larger structure and possessing a different axial plunge. He observed that these exceptions to Pumpelly’s rule (Pumpelly et al., 1894) were frequent in ‘drag folds’ and were dependent on the relative movement of competent strata. Inconstant axial trends in adjacent minor folds have been described from the Fannich Forest area of the Scottish Highlands (Sutton and Watson, 1955), where axial variation occurs within a plane coinciding with the axial planes of the folds. These authors maintained that minor folds have considerable freedom in orientation and the situation where they are coaxial and congruous with the 1-r folds is a special case, albeit the one most commonly described and the one regarded as ‘normal’. Goguel(1962) questioned the universal validity of the Pumpeilyan relationship and cast doubt on the uncritical assumption that the same mechanical principles of folding on

FOLD PATTERN FROM ROCKS OF SijRijY, NORWAY, I

101

Fig.1 3. Ardwell shore, south of Girvan, Scotland. a. Axial deviation vs. SL /2 AP. b. Stereogram of 77 folds; dots = folds from flat limbs of macroscopic step folds; circles = folds from steep limbs.

the minor scale can be applied to the major folds, “. . . . . our efforts would be in vain if we were to seek a direct relation between the form of these minor folds and that of the major folds”. fn situations

where the major fold-axis orientation

can be found by means other than

employing minor fold orientation, it has frequently been reported that the Pumpellyan relationship is realistic and in certain deformation regimes, it is the prevalent pattern recognized. In more complex deformation, such as frequently obtained in erogenic belts, it may be difficult or impossible to dete~ine from minor structures the precise form of major folds and even less their axial orientation. In this situation the assumption of a Pumpellyan situation is unwarranted. Subsequent deformation phases aggravate and ob-

D.M. RAMSAY AND S.A. S’WRT

LO2

scure the problem, so divergence of minor structures be misidentified

from the ‘normal’ relationship

can

as minor folds of a different generation.

From the geometrical thors have concluded

styles and the patterns of orientation

that the problem of incongruous

vorced from that of noncylindrical The deformation

displayed, the present au-

folding on Soroy cannot be di-

folding and indeed is one aspect of this type of folding,

plan in which such associations

occur is obviously more complex than

the ‘normal’ situation of large folds with congruous parasitic folds. The ‘normal’ case can be, as Flinn (1962) suggests, the result of a simple and regular orientation of the original layering relative to the principal axes, in a bi-axial or a more complex strain pattern. Marked incongrui~ is consistency confined to specific situations, imply~g that the orientation of the layering is import~t, although this relationship to the principal strain axes must be different from that in the ‘normal’ case. In this event the restricted freedom of fold-axis orientation,

commonly

encountered

obtain and the folds are characterized

in simpler Pumpellyan

by noncylindrism,

situations,

although the variability

does not is con-

fined within specific planes. What emerges consistently from the examples of aberrant folding described in this account is the existence of a threshold of inconstancy, governed by the angular relationship of the mean layering to the axial surface. In a synoptic

diagram of the boundary

fields of

aberrant folding, from all the examples analysed (Fig.l4), there is a close correlation between them, with the critical angle of SL A AP ranging from 35 to 48”. The slight variation in the threshold value is probably situation.

a reflection

of offering

du~tihty contrasts in each

There must be in addition, however, something more fundamental as the angular relationships

in the strain pattern,

between layering and axial surface of the aberrant folding sit-

uations can be observed in many terrains of tight folding, without any corresponding constancy.

It is suggested that the distinctive

and significant

in-

feature of the tectonic pat-

tern in question is the basic noncylindrism of the folding, carrying with it the further implication of an asymmetrical relationship between fold and strain axes.

Fig. 14. Synoptic diagram of boundary folds to aberrant fold suites. S = Snakefoid Point; N = Nordfjord; H = Hoivik; B = Bnivik; K= Klubbcn; A = Ardwell.

103

FOLD PATTERN FROM ROCKS OF SijRijY, NORWAY, 1

Fig.15. Diagram of typical aberrant fold. See text for explanation.

The importance

of layer orientation

can be appreciated

from Fig.15. Where the atti-

tude of layering is sub-normal to the axial surface symmetrical folding is accompanied by a bulk extension normal to the layering and parallel to the axial surface (Xv). The competent control horizons exert a strong influence

on the pattern of strain parallel to XY

and especially the possible range of orientation of the fold axes in this zone. The strain in the XY-direction which would be required to allow significant axial variation, is obviously much greater than the competent control horizon would allow. Even if such folds become noncylindrical, with wavelength much shorter than the larger fold, the extent of curvature is restricted and the folds are little more than shallow periclines. In the steep limbs of the larger fold, the layering rotates towards the axial surface as deformation proceeds and the principal compression comes to act across the layering, causing an increase in layer-paral. lel strain. The restraint of the control horizon on axial freedom of the developing parasitic folds is now missing, and the fold axes are free to adopt any orientation within the axial plane, which now makes an acute angle to the layering or even lies within it. If the axial surface is equated with the XY-plane of strain, then the onset of inconstancy would appear to reflect the passage from a layer-thickening and shortening, to a layer. thinning and extension of the boundary

regime. This threshold, marked in Fig.14 by the inflexion

points

curves, is held to be analogous to the plane of no finite longitudinal

strain. The consistent distribution of both the congruous and incongruous folds in a single great circle, suggests an underlying simplicity and regularity in the strain pattern. The narrowness of the girdle reflects the consistent

similarity in the style of fold profiles and lin b

attitudes, especially the recurring inclination of 10-30” of the long limbs. The common plane containing the fold axes would appear therefore, to be a significant one in the movement plan, probably analogous to the XY-plane of the strain ellipsoid. The pattern of minor folds, with its simplicity and apparent independence of the orientation of the major-fold axes, is interpreted by the authors as a reflection of the regional strain, rather than the product of local strain-fields related to the geometry of the major

104

D.M.RAMSAY AND B.A.STURT

folds. The major folds therefore, enjoy no special orientation strain axes by virtue of their Iarge ~mensions,

relative to the principal

indeed the pattern of major folds over the

scale of Sijriiy and adjacent islands to the northeast

is reminiscent

of a noncylind~cal

fold

outcrop. In assessing the chronological there are several significant

interrelationship

considerations.

of the incongruous

If the incongruous,

and ‘normal’ folds

oblique folds were to be

considered as a separate phase of deformation, post-dating the formation of the major fold and the minor folds which are congruous with it, it becomes difficult to reconcile their distribution and in particular their absence from the weak and eminently susceptible lithologies of the Aafjord Group and Falkenes Marble, in the hinge areas of the Breivik folds. Conversely, the large recumbent folds are traceable for many kilometres, but have associated congruous minor folds confined to the very narrow and specific belts of the hinge zones. A further point of significance in the argument is the fact that the axes of the smaller incongruous folds maintain their markedly oblique relationship to the major folds, as the latter swing from north---south into an east-west orientation, i.e., in the easwest zone many of the minor folds trend approximately north-south. This has been confirmed by the authors in several areas along the outcrop of the Breivik core in north Sijray. The similarity of the relationship between small and major folds to that observed directly in individual macroscopic folds, together with the evidence of aberrant folds, supports the conclusion that the two apparently differing fold suites are congenetic expressions of the same complex strain-field and that the incongruous folds represent a particular expression of the fundamental noncylindrism of the fold pattern. IMPLICATIONSOFNONCYLlNDRICALANDlNCONGRUOUSFOLDlNG The integration

of locally deduced movement

work. especially when these are not mutually protracted

speculation

within the literature.

plans into a regional kinematic

congruous,

has led to a considerable

The most prominent

approach in tectonic analysis correlates the symmetry

frameand

and widely supported

of the tectonic fabric with that of

the causal deforming movements (Sander, 1934; Turner, 1957). In this approach, one central axiom is that the symmetry of the movement plan can be determined from a statistical analysis of minor and major fold axes and lineations, together with other features of the megafabrics and microfabrics. Where linear elements, including fold axes, are established geometrically as b-lineations but trend transverse to the major fold axes, nappes or thrust fronts, a double phase of folding has been invoked, i.e., B 1 B’ or B A B’ tectonites. A recurring feature of the tectonic fabrics in the marginal zones of several erogenic belts is the presence of transversely orientated folds and mineral lineation (Phillips, 1937; Cloos, 1946; Anderson, 1948; Kvale, 1953; LindstrSm, 1958; Dalziel and Bailey, 1968). This alignment of linear elements is approximately coincidental with the translation direction deduced from thrust or nappes attitudes and, although having the morphology of bstructures, they have been regarded as part of the thrusting fabric and to be lineations in a. In such an interpretation the movement plan deduced from minor structures, whilst re-

FOLD PATTERN FROM ROCKS OF SOROY, NORWAY, I

fleeting the strain of individual

105

local domains, cannot be extrapolated

to the large-scale

regional picture. More recent studies (Johnson, 1956; Lindstrom, 1958; Dalziel and Bailey, 1968; Hooper, 1968) indicate that the small isoclinal folds of the thrust zones belong to the prethrust deformation.

In addition,

these folds can be traced back into the erogenic hinter-

land, into areas well clear of any zone affected by the thrusts. The appreciation

that the

folds and lineations predate the thrusting, however, merely relegates the problem of transverse folding back in time. On Soroy, which is situated well within the Caledonian

erogenic belt, thrusting

can be

removed from the argument as the transversely orientated minor folds are directly related to the large-scale folds. Here the oblique to transverse relationship is evidently a primaryc phenomenon

reflecting the complex pattern of strain.

Crampton

(1913) in considering

the large transversely

orientated

southeasterly

the Beinn Dronaig area, some distance to the east of the Moine Thrust in northern concluded

that they were synchronous

with orogen-parallel

caledonoid

folds in Scotland,

folds and caused

by uneven advance of the major fold hinges. In terms of the present paper they are large noncylindrical folds. McIntyre (1952) and Rutledge (1952) on the other hand, concentrsting on small-scale structures, took the counter-view that the southeasterly trend which dominates the minor fold fabric in this area was the only significant northeast-southwest compression.

one and resulted from

Lindstrom (1958) proposed that the transverse folds and lineations in the thrust zone of northern Sweden originated by rotation of folds, from initial parallelism with the ore. genie belt, into the transverse orientation. This rotation was considered to have been accompanied translation.

by intense flattening

and stretching

into parallelism with the direction

of thrust

Voll(l960) described the penetrative transverse lineation throughout the Caledonide:. and Norvegides as a stretching lineation, created when shortening normal to the schistosity reached a certain critical level. This, he suggests, originates from the interaction

of two

sets of slip planes intersecting in ‘a’ and which, if unequally developed, can produce a rotational axis normal to the main fold axis, B, and the lineation therefore parallels the trans. verse fold axis. The persistent stretch lineation on Soroy, however, is not exclusively par. allel to the fold axis and can be orientated anywhere between parallel and normal to it (see Part II). In the southwest Highlands Voll(l960) have a predominantly southeast-southwest bent structures,

whose northeast-southwest

demonstrated that the early minor structures trend, in marked contrast to the large recum,, axes have been convincingly

demonstrated

from the stratigraphy (Clough, 1897; Bailey, 1922; Shackleton, 1958). Voll concluded that, as the transversely orientated structures are the first tectonic imprint and in addition do not change their orientation when they are traced round the hinge of the large recumbent Cowal anticline, the nappes, if they exist, must predate the regional metamorphism. The implication of the present study, however, is that the transverse structures described

D.M. RAMSAY AND B.A. S-WR’I

106

by Voll could well be interpreted anomalous position

as being congenetic

The present account stresses the important macroscopic

structures,

possibility

ping. On Soroy it is possible to demonstrate

that the pattern of small and

this pattern is the regional expression

tell the

directly or through stratigraphic

map-

a close genetic link between the ‘normal’ and

transverse suites of folds of the early deformation

is a fundamental

and the

reflecting the strain in local domains, does not necessarily

overall story of the large-scale structures identified

ual structures,

with the nappe formation,

relative to the nappe hinge, which he emphasizes, loses its impact.

phase. The widespread occurrence

of the incongruous

relationship

of

observed in individ-

or between groups of closely associated folds, and imphes that this pattern feature of the regional movement

plan.

ACKNOWLEDGEMENTS

The authors wish to record their thanks to the Natural Environmental Research Council for a research grant; travel support from the University of Dundee and Central Research Fund of London University; field support from the Geological Survey of Norway. In addition the authors would like to thank their many colleagues for helpful discussion and criticism.

REFERENCES

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Bhattacharji, S., 1958. Theoretical and experimental investigations on cross-folding. J. Geol., 66: 625-667. Brynhi, I., 1962. Structural analysis of the Gr$neheia area, Eikefjord, western Norway. &or. Geol. Tidsskr., 42: 331-369. Cloos, E., 1946. Lineation, a critical review. Geol. Sot. Am. hfem., 18: 122 pp. Clough, CT., 1897. The geology of Cowal. Geol. Surv. G.B., Mem. Geol. Surv. Scotl., 333 pp. Crampton, C.B., 1913. In explanation of sheet 82. Geol. Surv. G.B., Mem. Geol. Suw. Scotl., 114 pp.

Dalziel, I.W.D. and Bailey, SW., 1968. Deformed garnets in a mylonitic rock from the Grenvilie Front and their tectonic significance. Am. J. Sci., 266: 542-562. De Sitter, L.U., 1956. Structural Geology. McGraw-Hill, London, 552 pp. Fteuty, M.J., 1964. The description of folds. Geol. Assoc. Proc.. 75: 461-492. Flinn, I)., 1962. On folding during three-dimensional progressive deformation. Q. J. Geol. Sot. Lond.. 118: 385-433. Fourmarier, P., 1936. Essai sur la distribution, I’allure et la genbse du clivage schisteaux dans les Appalaches. Ann. Sot. Geol. Be/g., 60: 69- 131. Friedman, M., 1964. Petrofabric Techniques for the Determination of Principal Stress Directions in Rocks. State of Stressin the Earth’s Crust - Proc. Int. Conf.# Santa Monica. Elsevier, New York, N.Y., pp. 451-553. Goguel, J., 1962. Tectonics. Freeman, New York, N.Y.. 384 pp. Hills, E.S., 1963. Elements of Structural Geology. Wiley, New York, N.Y., 483 pp. Hooper, P.R., 1968. The ‘a’ lineation and the trend of Caledonides of northern Norway. Nor. Geol. Tidsskr.. 48: 261-268.

FOLD

PATTERN

FROM ROCKS

OF SijRijY,

NORWAY,

107

I

Johnson, M.R.W., 1956. Conjugate fold systems in the Moine thrust zone in the Lochcarron and Coulin forest areas of Wester Ross. Geol. Mag., 93: 345-350. Knill, J.L. and Knill, D.C., 1958. Some discordant fold structures from the Dalradian of Craignish, Argyll and Rosguile, Co. Donegal. Geol. MQg., 95: 497-510. Kvale, A., 1953. Linear structures and their relation to movement in the Caledonides of Scandinavia. Q. J. Geol. Sot. Lond., 109: 51-74. Lindstrijm, M., 1958. Tectonic transports in three small areas in the Caledonides of Swedish Lapland. Lunds Univ. Arsskr. N.F. Avd. 2, 54: 3. McIntyre, D.B., 1952. The tectonics of the Beinn Dronaig area, Attadale. Trans. Edinb. Geol. Sot., 15: 258-264. Nabholz, W.K. and Voll, G., 1963. Bau und Bewegung im Gotthardmassivischen Mesozoikum bei Ilanz (Graubiinden). E&g@? Geol. Helv., 56: 755-808. Phillips, F.C., 1937. A fabric study of some Moine schists and associated rocks. Q. J. Geol. Sot. Land., 43: 581-620. Pringle, I. and Sturt, B.A., 1969. The age of the peak of the Caledonian orogeny in West Finnmark, North Norway. Nor. Geol. Tidsskr., 49: 435-436. Pumpelly, R., Wolff, J.E. and Dale, T.N., 1894. Geology of the Green Mountains in Massachusetts. U.S. Geol. Surv. Mem., 23: l-206. Ramberg, H., 1960. Relationships between length of arc and thickness of ptygmatically folded veins. Am. J. Sci., 258: 36-46. Ramsay, D.M., 1971. The structure of northwest SGriiy. Nor. Geol. Undersiik., 269: 15-20. Ramsay, D.M. and Sturt, B.A., 1963. A study of fold styles, their association and symmetry relationships from Soroy, North Norway. Nor. Geol. Tidsskr., 43: 41 l-430. Ramsay, J.G., 1962. The geometry and mechanics of formation of ‘similar’ type folds. J. Geol., 70: 309-327. Ramsay, J.G., 1967. Folding and Fracturing ofRocks. McGraw-Hill, London, 568 pp. Roberts, D., 1968. The structural and metamorphic history of the Langstrand-Finfjord area, S%y, northern Norway. Norg. Geol. Unders., 253: 160. Rutledge, H., 1952. The structure of the Fannich forest area. Trans. Edinb. Geol. Sot., 15: 317-321 Sander, B., 1934. Petrofabrics and orogenesis. Am. J. Sci., Ser. 5, 28: 37-50. Shackleton, R.M., 1958. Downward-facing structures of the Highland border. Q. J. Geol. Sot. Lond., 113: 361-392. Sturt, B.A., 1971. Contact phenomena Nor. Geol. Unders., 269: 180-181.

of the syn-erogenic

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Hasvik gabbro,

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of the Breivikbotn

northern

area, Soroy,

Norway. northern

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