ASYMMETRICAL FOLDING IN THE HAWICK ROCKS OF THE GALLOWAY AREA, SOUTHERN UPLANDS: COMMENT

Transcription

1 ASYMMETRICAL FOLDING IN THE HAWICK ROCKS OF THE GALLOWAY AREA, SOUTHERN UPLANDS: COMMENT SIRS Stringer and Treagus (1981) presented a largely admirable updating and synthesis of the structural evolution of the Hawick Rocks, ably backed up by their earlier analysis of non-axial planar cleavage within the area (Stringer and Treagus 1980). Two divergences from my own interpretations (Weir 1968, 1979) nevertheless call for comment. Firstly, Stringer and Treagus (1981, p. 131) asserted that '... the two-phase fold history that we propose conflicts with the more complex polyphase sequences of folding postulated for parts of the Galloway area by.... Weir (1968).' Admittedly; but I later went to some length (Weir 1979) to amend my original deformation chronology, which was in hindsight over-subdivided in respect of episodes within what we now mutually agree are Di and D2. In revising the chronology, cognizance was taken of the non-axial character of Si (Weir 1979, p. 172), well displayed in the Mossyard fold complex (Weir 1979, fig. 3); the T 3 ' folds of Skyreburn Bay (op. cit., fig. 1) were re-interpreted as F t buckle-folds rotated into steep plunges, possibly during a D 3 phase (op. cit., p. 173); and the 'F4' 'pseudo-ripples', correlated with a phase of Craig and Walton (1959), were recognized as large-scale F 2 crenulations, in which an exaggerated S2 strain-slip cleavage has developed as minute thrusts. In these respects my revised chronology essentially anticipates that of Stringer and Treagus. Secondly, the two authors challenge (Stringer and Treagus 1981, p. 134) the 'monoformal' folding concept of Craig and Walton (1959), proposing as an alternative that the Fi folds have a distribution which is regionally uniform, and '... commonly at hinges in one kilometre'. I still submit that this distribution is by no means uniformly valid for the coastal sections west of Gatehouse. These sections extend from the Ardwall shore, 2.5 km SW of Gatehouse of Fleet between National Grid references [NX ] and [NX ], westward of which coastal exposures fail. The distance is about 8-3 km, of which the northwestern 4 km to Kirkdale is across-strike and the northeastern 4-3 km is approximately strike-parallel but considerably wrench-faulted. These northeastern strike-parallel outcrops extend to Ringdoo Point (unnamed on the 1-inch O.S. map) at [NX ]. These outcrops, and a short buckle-fold tract centred on [NX ], conform best to Stringer and Treagus's proposals (cf. Figs. 1,2). Several fold limbs exceed 100 m and one approaches 400 m; in addition there is a notable concentration of large (150 m) buckle-folds in the southwestern half of the belt. Short (<10 m) limbs are largely concentrated into two 'rucks'. One comprises three fold-pairs, and is traceable for 0-5 km on the Scott. J. Geol. 19, (1), , 1983

2 14 LETTERS TO THE EDITORS Monoformal limbs Buckle-fold tracts -J Synclinal axes - Anticlinal axes Faults FIG. 1. Structural and locality map of the Silurian rocks of the coast west of Gatehouse. 1 Skyreburn folds, 2 Cardoness tract, 3a-c Newton buckle-fold tract, 4 Garvellan buckle-fold tract, together comprising Ringdoo-Ardwall belt; 5a-b Mossyard monoform; 6 Kirklaugh terrace; 7 Ravenshall monoform. Lowerhemisphere projections are of bedding poles; great-circle traces are of bisectors to bedding maxima in NW-facing limbs, which coincide with F2 axial surface maximum. Poles to latter also indicated. Projection a 5b 6 7 No. of readings Contour interval % 4, 8, 12 5, 20, 30 4, 7, 10 5, 10, 20 5, 10, 20 10, 20, 30 5, 10, 12 5, 10, 20 Cardoness shore around [NX ]; the second is exposed in Mossyard Bay at [NX ] and includes two fold-pairs. The belt as a whole is roughly divisible into 1) the domain of NE-steepening plunges, centred on Skyrcburn Bay and extending to the northeastern limit of exposure; 2) a domain which is incompletely exposed, but apparently dominated by NW-facing limbs, extending from Skyreburn to Cardoness (1-5 km); and 3) the 'large buckle-fold' domain between Newton and Mossyard Bay. Application of the t test of significance to the relative lengths of the NW- and SE-facing limbs within these domains, and to limb ratios between the domains, yielded significances of less than 95% in each case, suggesting a) that the differences are more apparent than real, and b) that the belt might be

3 LETTERS TO THE EDITORS 115 TABLE 1 Spacing of fold hinges in four contrasting tracts in the coast west of Gatehouse. tract l\l mean, m range, m hinges/km Ringdoc-Ardwall Mossyard monoform Kirklaugh terrace Ravcnshall mono.form considered as one unit for comparison. (The lengths of the limbs and their ratios give, within the belt as throughout the area, an approximation to a negative log-normal distribution, resulting in standard deviations close to, or even greater than, the means. Results have accordingly been calculated throughout on the logio of the readings, which give realistic relationships between means and standard deviations). A 14 km tract succeeds NW of Ringdoo Point, conforming entirely to the 'monoformal' style of Craig and Walton (1959). This tract, referred to herein as the Mossyard monoform, faces NW and is disturbed by 9 dextral fold-pairs spaced at a mean interval of 70 m (Table 2). A considerable strike fault, associated with secondary reddening, separates the monoform from the Ringdoo-Ardwall belt, and the sheet-dip is around 45, with extensive development of F2 flexures. The Mossyard monoform passes westwards, evidently without significant strike faulting (though probably with wrench-fault interference), into the Kirklaugh buckle-fold belt. This again displays 9 fold-pairs in its width of 500 m, and comprises a small first-order fold couple (Ramsay 1967, p. 355) with smaller second-order flexures on each limb. The folds are tight, approximately symmetrical and noticeably disharmonic (cf. Stringer and Treagus 1981, pi. 1). A strike fault through Corbie's Cove [NX ], which is also associated with significant reddening, bounds the Kirklaugh tract westwards, and introduces a second monoformal tract, the Ravenshall monoform. This monoform displays an even more impressive contrast to the Kirklaugh belt, being interrupted by only two small dextral fold-pairs in its T2 km of outcrop, and in having one continuous NW-facing limb 625 m wide. The Kirklaugh tract forms, in effect, a structural terrace separating the two monoforms, and the Ravenshall monoform has a sheet-dip of around 70. Table 2 illustrates the wide range of mean hinge-spacing among the four tracts

4 116 LETTERS TO THE EDITORS TABLE 2 and data of fold limbs in the fc mtrasting tracts SF_ -facinq limbs of logs logs means of logs s logs FUngdoo-Ardwail 1D Kirkiaugh terrace i Ravenshall monofc 333 jt-values, N-S limbs Signifi level Mean Mean limb s ratios s Between: ratios (logs)* _t_-values, limb signifi cance Rinqdcc-Ardwall $ Ringdoo- Ardwall Mossy; oncfc % Mossyard 8 Kirklaugh 99.9* Kirklauqh te 0.8 <= 9 5% Ringdoo & 2.1 Kirklaugh 95% * Values x 10 to eliminate negative logarithms. detailed. In order to assess the significance of this variation, outcrop lengths of NW-facing limbs in three of the tracts (the Ravenshall sample being too small to yield significant comparisons) were compared with those of associated SE-facing limbs. Ratios of limb lengths among the three tracts were also compared (Table 2). Application of the t test to means of the limb lengths yields significance levels of 95%, 99% and less than 95% for the Ringdoo-Ardwall, Mossyard and MOSSYARD MONOFORM RAVENSHALL MONOFORM 2W^ FIG. 2. Structural profiles of the Ravenshall, Kirklaugh and Mossyard tracts, and of the various domains within the Ringdoo-Ardwall belt. Numbering of fold domains corresponds to Figure 1.

5 LETTERS TO THE EDITORS 117 Kirklaugh belts respectively, denoting probabilities of 20:1, 100:1 and less than 20:1 against limb lengths in the respective belts displaying a chance distribution. These probabilities are taken to imply that there may be a significant contrast between limb lengths in the case of the Ringdoo-Ardwall belt; that there is a highly significant contrast in respect of the Mossyard monoform, which has a mean NW-/SE-facing limb ratio of 23-6:1; and no valid contrast within the Kirklaugh buckle-fold tract (Table 2). Comparison by the t test of limb ratios between the Ringdoo-Ardwall belt and the Mossyard monoform; the Mossyard monoform and the Kirklaugh terrace; and the Ringdoo-Ardwall belt and the Kirklaugh terrace show significance levels of 99-9%, 99-9% and 95% respectively, suggesting a highly significant contrast in the first two instances, and a possible contrast in the last. Thus it is concluded that in the case of the Kirklaugh terrace and the adjacent monoformal tracts a valid and accountable contrast does exist; and that there may also be a contrast between the monoforms and the Ringdoo-Ardwall belt, though this is rendered more difficult to substantiate because of incompleteness of outcrop and inhomogeneity of tectonic style in the Ringdoo-Ardwall belt. The author is indebted to Professor E. K. Walton for helpful and stimulating discussion. REFERENCES CRAIG, G. Y. and WALTON, E. K Sequence and structure in the Silurian rocks in Kirkcudbrightshire. Geol. Mag. 96, RAMSAY, J. G Folding and fracturing of rocks., McGraw Hill. STRINGER, P. and TREAGUS, J. E Non-axial planar Sj cleavage in the Hawick Rocks of the Galloway area, Southern Uplands, Scotland. J. Struct. Geol. 2, and Asymmetrical folding in the Hawick Rocks of the Galloway area, Southern Uplands. Scott. J. Geol. 17, WEIR, J. A Structural history of the Silurian rocks of the coast west of Gatehouse, Kirkcudbrightshire. Scott, f. Geol. 4, Tectonic contrasts in the Southern Uplands. Scott. J. Geol. 15, Department of Geology, University of St. Andrews, St. Andrews, Fife, KY16 9ST. J. A. WEIR MS. accepted for publication 12th November 1982

6 ASYMMETRICAL FOLDING IN THE HAWICK ROCKS OF THE GALLOWAY AREA, SOUTHERN UPLANDS: REPLY SIRS We would like to reply fully to Weir's (1982) comments on our reinterpretation of the folding in the Hawick Rocks (Stringer and Treagus 1980, 1981) regarding the style and distribution of folds and the deformation chronology. The style of folding in the Ravenshall Point-Ringdoo Point coastal section near Gatehouse (Weir 1968, fig. 2; 1083, fig. 3) is considered by Weir (1983) to conform to the "monoformal" fold style of Craig and Walton (1959) who proposed that the fold structure of the Hawick Rocks is a major monoclinal fold stepping down to the north-west. The fold structure of the Ravenshall Point-Ringdoo Point section in our interpretation (Stringer and Treagus 1981, fig. 4a) comprises two asymmetrical, dextral (viewed north-eastward), first order Fj folds with south-eastward vergence; dextral and sinistral smaller scale Fi folds are developed respectively in the long limbs formed by belts of north-westward sheet-dip and in the short limbs formed by narrow belts of steep to gentle south-eastward sheet-dip (at Mossyard and Ringdoo Point, op. cit. fig. 4a). A dextral first order Fj fold east of Ravenshall Point (Weir 1983, p. 115 and fig. 3) constitutes a third such asymmetrical fold in the section. Alternating belts of steep to moderately steep north-westward sheet-dip and narrower belts of steep to gentle south-eastward sheet-dip, comparable in style and magnitude to those in the Ravenshall Point-Ringdoo Point section, have been mapped throughout the Hawick Rocks (Stringer and Treagus 1981, figs. 3a-c, 4a), and are consistent with our interpretation of a regional development of asymmetrical, south-eastward verging first order Fi folds of about m amplitude interspersed at c km intervals in the Hawick Rocks (op. cit., p. 134). Extensive inland mapping of the younging direction of beds in the Hawick Rocks of the Whithorn area west of Gatehouse (Rust 1963) and in the Glen Luce area north of Gatehouse (Gordon 1962) corroborates our interpretation; when the maps are divided into northward- and southward-younging strike belts, the respective ratio of the cumulative widths of the belts (about 1:0-45 in both areas) corresponds to asymmetrical first order F^ folds with south-eastward vergence. About 15 such folds may be inferred in the Whithorn area, at km intervals for 12 km across strike, and 10 within 8 km across strike in cross sections of the Glen Luce area (Gordon 1962). Narrow ( m) belts of approximately subhorizontal sheet-dip which occur locally between northward- and southward-younging belts in the Hawick Rocks, with symmetrical to asymmetrical Fj folds of varying intermediate scale (Stringer and Treagus 1981, figs. 3a-c, 4a), are interpreted as hinge zones of first order Fi folds. A few narrow ( m) "flat belts" of subhorizontal sheet-dip Scott. J. Geol. 19, (1), , 1983

7 LETTERS TO THE EDITORS 119 which occur between belts of northward-young strata to the north and south, with asymmetrical to locally symmetrical F! folds of fairly constant intermediate scale (op. cit., figs. 3a, 3c, 4a), are tentatively interpreted as the short limbs of more open first order F^ folds. The flat belts are analogous in style to those in the "monoformal" folds of Weir (1983) and Craig and Walton (1959). Our interpretation of the style of intermediate scale Fj folds differs from that of Weir (1983) in some localities. Weir (1983, p. 115) records dextral fold pairs throughout the 14 km tract north-west of Ringdoo Point, whereas we have recorded sinistral Fi folds in the Mossyard and Ringdoo Point intervals of the coast section (folds No. 108 and respectively in Stringer and Treagus 1981, fig. 4a). Weir (op. cit., p. 115) records approximately symmetrical folds in his "structural terrace" east of Ravenshall Point between Corbie's Cove and Boatdraught; in the south-eastern half of this section, between National Grid Reference [NX ] and Boatdraught, we have recorded markedly asymmetrical (dextral) Fi folds (op. cit., fig. 4a and pi. 1), and Weir (1968, p. 35) described F^ folds in this belt with "north-facing limbs averaging 200 m from crest to trough and south-facing limbs only some 50 m" (i.e. dextral). The distribution of intermediate scale Fi folds in the Hawick Rocks is regionally uniform (Stringer and Treagus 1980, 1981) relative to the regionally unequal distribution proposed by Craig and Walton (1959, pp ) of about 140 folds in the flat-lying limb of their major monoclinal fold and only occasional folds in the steep limbs to the north and south. Locally, however, the distribution varies, mainly between 20 and 60 F a fold hinges/km (Stringer and Treagus 1980, p. 320); the frequency of about Fi fold hinges/km in sections across the northern margin of the Hawick Rocks (Stringer and Treagus 1981, p. 134) is consistent with the c. 40 Fi fold hinges in 2 km across the 060 strike between Ravenshall Point and Ringdoo Point (op. cit. 1981, fig. 4a). Weir (1983, p. 115 and tables 1, 2) demonstrates that within the Ravenshall Point-Ringdoo Point section, folds are sparse in the belts of steep north-westward sheet-dip relative to other parts of the section. This is evident in our cross section (Stringer and Treagus 1981, fig. 4a), where the contrast in frequency of intermediate scale Fi folds occurs between opposite limbs of the first order F\ folds, viz. 18 hinges/km in the belts of north-westward sheet-dip and 42 hinges/km in the intervening narrow belts of steep to gentle south-eastward sheet-dip at Mossyard and Ringdoo Point. A similar contrast is discernible in an equivalent section across the northern Hawick Rocks 5 km to the south-west along the west side of Wigtown Bay between [NX ] and Port McGean (Stringer and Treagus 1981, fig. 3c); the frequency of intermediate scale Fi folds is 4 11 hinges/km in the belts of steep north-westward sheet-dip, and 20 and 42 hinges/km respectively in the belts of steep south-eastward sheet-dip at Port McGean and between Innerwell Point andjultock Point. The contrast in frequency of intermediate scale Fi folds between opposite limbs

8 120 LETTERS TO THE EDITORS TABLE 1 Frequency and mean interlimb angle of Intermediate scale Fj folds 1n the long limbs, hinge zones and short limbs of first order F1 folds and in local flat belts, 1n the northern, central and southern Hawick Rocks, based on 800 Fj fold hinges along 24 km of coastal sections (Stringer and Treagus 1981, figs. 3a-c, 4a). Long limbs Hinge zones Short limbs Flat belts NORTHERN HAWICK ROCKS / a If I 15/km 46" m 60/km m 30/km m 47/km m CENTRAL HAWICK f a 38/km /km /km 45» 11 60/km 38* 1 ROCKS I 5195m 851m 1600m 161m SOUTHERN HAWICK f a N 35/km 45«12 57/km /km 50* 7 70/km 34* 3 ROCKS I 7234m 245m 1000m 372m f, frequency of Fj fold hinges per kilometre across strike; a, mean interlimb angle; N, number of long limbs, hinge zones, short limbs and flat belts; I, cumulative horizontal width. National Grid Reference (NX 100 km square) locations of the coastal sections: Northern Hawick Rocks [ ], [ ], [ ] Central Hawick Rocks [ , [ ] Southern Hawick Rocks [ ], [ ], [ ] of first order Fi folds occurs also in the southern Hawick Rocks (Table 1), but appears to be negligible in the central Hawick Rocks. The mean interlimb angle of Fj folds is essentially the same in both limbs throughout the Hawick Rocks (Table 1). In the belts of approximately subhorizontal sheet-dip interpreted as the hinge zones of first order Fj folds, the frequency and tightness of intermediate scale F] folds is on average persistently greater than in the limbs (Table 1); hence our observation (Stringer and Treagus 1981, p. 135) in the Hawick Rocks of "increases in the intensity of intermediate scale F] folding in the vicinity of larger scale F] fold hinges". The paucity of FT folds in the long limbs relative to the hinge zones and short limbs may be related to the proposed secondary origin of the asymmetrical first order F} folds, developed initially as recumbent folds concomitant with

9 LETTERS TO THE EDITORS 121 oceanward (south-eastward) thrusting at a subduction zone (Stringer and Treagus 1981, pp ). The origin of the high frequency and relatively small mean interlimb angle of intermediate scale Fi folds in the flat belts that occur between northward-younging steep belts (Table 1) is uncertain; the localized development of the six flat belts with a cumulative cross-strike length of only 835 m in 24 km of cross section, however, appears to preclude the regional significance attached to this style of structure by Craig and Walton (1959) and Weir (1979, 1983). Regarding the deformation chronology of folding in the Hawick Rocks, we acknowledged (Stringer and Treagus 1981, p. 139) that two principal phases of folding in the Ordovician and Silurian rocks of southwest Scotland had recently been proposed by Weir (1979). However, the more complex polyphase structural chronology postulated in the Gatehouse area (Weir 1968) had not been revised; we therefore reinterpreted the critical overprinting relationships in the Gatehouse area in terms of Fj and F 2 folding (Stringer and Treagus 1981, p. 140). Furthermore, our interpretation of the deformation chronology in relation to the style of F] and F 2 folding in the Hawick Rocks (Stringer and Trcagus 1980, 1981) differs from that proposed by Weir (1979) in several basic aspects, as follows. (i) FT folds and transecting Si cleavage developed synchronously (Stringer and Treagus 1980, pp ), v. major Fi fold generation preceded development of Si cleavage (Weir 1979, p. 172). (ii) Di origin of the locally steep plunge of Fi folds (op. cit. 1980, pp ), v. rotation of Fi folds into steep plunges during post-f 2 cross folding (Weir 1979, pp ; 1983, p. 113). (iii) South-eastward asymmetry of first order F] folds, and predominantly south-eastward vergence of smaller scale Fi folds (op. cit. 1980, 1981), v. major NNW-facing Fj monoforms, and smaller scale Fi folds upright or overturned generally to the NNW (Weir 1979, p. 171). (iv) F 2 folds lack regular vergence, varying from recumbent symmetrical folds to asymmetrical folds with axial surfaces inclined up to 50 SE and NW (op. cit. 1981, figs. 3a, 3c, 4a and p. 139), v. F 2 folds "comprise SE-verging monoforms" and axial surfaces dip c. 40 south-eastward (Weir 1979, p. 172). (v) Essentially uniform regional development of F t folds (Table 1) and development of F 2 folds mainly in the northern Hawick Rocks (op. cit. 1980, p. 318). v. "Evidence of deformation, particularly structures produced during the first two phases, increases southwards..." (Weir 1979, p. 171). These discrepancies have resulted in different concepts of the origin of the Fi and F 2 folds in the Hawick Rocks. Weir (1979) appears to favour the view that the Fi folding is related to decollement of the sedimentary cover on subducting oceanic basement, and the F 2 folding to later oceanward thrusting accompanying sequential accretion of sediments; we have ascribed the Fi folding to the latter event, and we would relegate the F 2 folding to vertically directed forces acting locally within the evolved accretionary prism upon sub vertical strata least affected by Fi folds.

10 122 LETTERS TO THE EDITORS REFERENCES CRAIG, G. Y. and WALTON, E. K Sequence and structure in the Silurian rocks of Kirkcudbrightshire. Geol. Mag. 96, GORDON, A. J The Lower Palaeozoic rocks around Glenluce, Wigtownshire. Univ. Edinburgh Ph.D. thesis (unpubl.). RUST, B. R The geology of the area around Whithorn, Wigtownshire. Univ. Edinburgh Ph.D. thesis (unpubl.). STRINGER, P. and TREAGUS, J. E Non-axial planar S t cleavage in the Hawick Rocks of the Galloway area, Southern Uplands, Scotland. J. Struct. Geol. 2, and Asymmetrical folding in the Hawick Rocks of the Galloway area, Southern Uplands. Scott. J. Geol. 17, WEIR, J. A Structural history of the Silurian rocks of the coast west of Gatehouse, Kirkcudbrightshire. Scott. J. Geol. 4, Tectonic contrasts in the Southern Uplands. Scott. J. Geol. 15, Asymmetrical folding in the Hawick Rocks of the Galloway area, Southern Uplands: comment Scott. J. Geol. 19, J Department of Geology, P. STRINGER 1 University of New Brunswick, J. E. TREAGUS 2 P.O. Box 4400, Fredericton, New Brunswick Canada E3B 5A3 2 Department of Geology, University of Manchester, Manchester, M13 9PL MS. accepted for publication 6th September 1982