The Outer Hebrides: a geological perspective

The Outer Hebrides: a geological perspective JANET WATSON WATSON, J. 1977. The Outer Hebr ides: a geological perspective. Proc. Geol. Ass., 88 (I ), 1-14. The Outer Hebrides form part of a horst block between the Atlantic margin of Europe and the marine troughs which occupy the Minch and Sea of the Hebrides. The geological history of the islands from late Palaeozoic times until the present day has been dominated by events connected with the break-up of the Laurasian supercontinent and the opening of the Atlantic Ocean-s-events which involved rifting and volcanic activity to the east and sea-floor spreading to the west. During an earlier period, from late Precambrian to mid-Palaeozoic times, the Outer Hebrides occupied a marginal position with respect to troughs flanking an older ocean which opened and subsequently closed on the site of the Caledonian orogenic belt in mainland Scotland. For much of this period also they functioned as a horst block. The behaviour of the Outer Hebrides as a geological unit has been controlled largely by the history and structure of its Lewisian basement which developed as a deep-seated metamorphic gneiss complex during the period 3000- I800 million years in early Precambrian times. The formation of a large region enriched in granite and the development of a grid of deep dislocations are among the events whose influence on the subsequent evolution of the Hebridean area are discussed. Department ofGeology, Imperial College, Prince Consort Road, London SW7 2BP.

CONTENTS I. INTRODUCTION .

2. THE BASEMENT . (a) Variations in bulk composition (b) Structural features . (c) The lower crust 3. THE CALEDONIAN CYCLE 4. THE DEVELOPMENT OF THE NORTH ATLANTIC REFERENCES

page 1 3 3 5 6 7 10

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1. INTRODUCTION The Outer Hebrides, standing at the extreme edge of the European continent, are dominated both scenically and climatically by the proximity of the Atlantic Ocean; and their geological relationships with the hinterland of Scotland are determined by structures formed during the initial stages of opening of this ocean. The Atlantic, however, is a young ocean; structures directly related to its formation date back only two or three hundred million years as against the two or three thousand million years recorded by the Lewisian basement of the Outer Hebrides themselves. Approaching the question from this point of view, one might infer that the salient feature of the present archipelago-its marginal position with respect to the Atlantic-was no more than a geological accident. In a longer perspective, the situation looks more complex, for the geological record shows that the Outer Hebrides have occupied marginal positions with respect to a number of older structures of different kinds. This coincidence , which can be matched at other continental margins, suggests that the siting of structures formed comparatively recently has been influenced in some way by the previous history of the crust. One of my objectives in attempting to examine the geological record in a long perspective is to enquire into some implications of this arrangement.

2

JANET WATSON 7 30 59

58

_

Tert iary igneous centre

oNo rt h Rona

4 30

SaintKilda ••

57

o,

,

50 KILO ME TRES

Fig. I. Sketch-map showing the Outer Hebrides in relation to north-west Scotland. Pre-Mesozoic rocks of the mainland region are not shown in detail

In terms of rock-units, the Outer Hebrides differ markedly from the regions immediately to the east (Table I, Fig. 1). The entire archipelago is underlain by Precambrian Lewisian gneisses whose structural and metamorphic development was completed some eighteen hundred million years ago. The long sequence of depositional, tectonic and metamorphic episodes (-1000-400 Ma) which culminated in the formation of the Caledonian orogenic belt in mainland Scotland is scarcely recorded in the Outer Isles, and the crucial events connected with the opening of the North Atlantic (-300-0 Ma) are directly recorded only by one patch of conglomerates and sandstones (the Stornoway Beds) and one swarm of early Tertiary dykes. The gaps in this record become significant when viewed in relation to the presence of thick late Precambrian and early Palaeozoic successions on the Scottish mainland and of Mesozoic and Tertiary deposits in the Minch and Inner Hebrides. The Lewisian basement of the Outer Hebrides is today almost devoid of overlying cover-rocks because these islands have functioned over hundreds of millions of years as parts of a positive block standing above sea-level or at any rate receiving only limited thicknesses of sediment. At the present day they form a fault-bounded horst between the continental margin on the one hand and the sediment -filled troughs which underlie the Minch and Sea of the Hebrides on the other. As much as a thousand million years ago they were part of a highland mass shedding detritus eastward into

THE OUTER HEBRIDES: A GEOLOGICAL PERSPECTIVE

3

troughs on the site of the future Caledonian orogenic belt. The persistent 'positive' character of the crustal block which supports the Outer Hebrides seems to have been built into it at an early stage and has controlled its behaviour through late Precambrian and Phanerozoic times.

2. THE BASEMENT The highly metamorphosed plutonic, volcanic and sedimentary gneisses of the Lewisian complex which underlie almost the whole of the Outer Hebrides are portions of an early Precambrian province which extends eastward for more than 100 km into mainland Scotland (where it is largely hidden beneath younger deposits) and westward to the continental margin. Rocks of similar age and character in Rockall Bank and southern Greenland appear to be portions of the same ancient gneiss province which were united in a single continental mass during the period before the opening of the Atlantic. In general terms, the Lewisian complex varies remarkably little over the whole of its large outcrop-area. The principal rocks everywhere are coarse gneisses of granodioritic to tonalitic composition which are associated with subordinate gneisses derived from sedimentary and volcanic parents, and with differentiated basic intrusions, granites and pegmatites. The very high metamorphic grade of the gneisses indicates that the complex suffered metamorphism at great depths and high temperatures. The patterns of structures and the relationships of successive sets of intrusions record a history of deformation and metamorphism lasting for at least a thousand million years, from 2800-1800 Ma (Table 11). The early episodes in this history, notably the Badcallian gneiss-forming metamorphism which terminated at about 2700 Ma, were relatively uniform in their effects and were responsible for giving the gneisses their characteristic banded or streaky appearance. The later episodes were more irregular in their effects; strong deformation was concentrated mainly in steep narrow zones along which adjacent portions of the complex moved relative to one another; metamorphism was most intense in these shear-zones and in a number of other regions penetrated by abundant Laxfordian granite and pegmatite. In the present context, three aspects of the Lewisian basement are of special interest-its bulk composition relative to that of the corresponding rocks in the adjacent regions: the character and alignment of its principal structures; and the nature of the basement at depth. (a) Variations in bulk composition Although the Lewisian rocks of the Outer Hebrides have the same general characters as those of the Scottish mainland, the proportions of rocks which are relatively basic and relatively dense appear to be unusually low and the proportions of rocks approaching true granites in composition to be unusually high. Granulites, the driest and densest rock-types represented in the Lewisian complex, are almost restricted to the eastern fringe of the islands, whereas they crop out in or underlie considerable areas along the western seaboard of Scotland and appear to be of importance in the submarine outcrops of Rockall Bank (Roberts, Ardus & Dearnley, 1973) and the ShetlandHebrides shelf (Flinn, 1969; Bott & Watts, 1970). In an area representing some 15 per cent of the total land surface of the Outer Isles, the ordinary gneisses are intimately veined by leucocratic granite and pegmatite which make up to about half of the total bulk (cf. Myers, 1971). That this admixture of acid material extends to a considerable depth is shown by a pronounced bouguer gravity "low" centred on western Lewis and Harris where granitic material reaches its greatest concentration (McQuillin & Watson, 1973). The only other Lewisian region equally rich in granite at the present level of erosion-the type Laxfordian complex of northwest Sutherland-appears from the evidence of gravity and aeromagnetic surveys to be underlain by denser material, probably

-I>-

TABLE I Basement and cover in N.W. Scotland

WEST CRATONIC COVER LAID DOWN BEFORE AND DURING ATLANTIC OPENING

OUTER HEBRIDES Early Tertiary dykes (Stornoway Beds, one locality only) (major unconformity)

Tertiary sediments Early Tertiary plateau lavas, intrusive centres, dykes Mesozoic (? and U. Palaeozoic) sediments

tunconformitys Cambro-Ordovician transgressive marine series (unconformity) Torridonian formations

CRATONIC COVER LAID DOWN BEFORE ENDING OF CALEDONIAN CYCLE

CRYSTALLINE BASEMENT

THE MINCH AND INNER HEBRIDES

Lewisian gneiss complex -2800--1800 Ma

(major unconformityi Lewisian gneiss complex - 2800--1800 Ma

MAINLAND OF N.W. SCOTLAND EAST (FORELAND OF CALEDONIDES) Early Tertiary plateau lavas, intrusive centres, dykes (Mesozoic sediments locally) (unconformity) Cambro-Ordovician transgressive marine series iunconformity) Torridonian formations -1000--800 Ma with unconformity separating two divisions (major unconformity) Lewisian gneiss complex - 2800--1800 Ma

......

:>

z

~ ~

:> ..., CZl

o Z

THE OUTER HEBRIDES: A GEOLOGICAL PERSPECTIVE

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TABLE II History of the Lewisian basement LAXFORDIAN EVENTS

Deep-seated deformation and metamorphism affecting much of preexisting complex, further movement on deep dislocations, local injection of granite and pegmatite, end-stages dated at -1800 Ma.

INTRUSION OF BASIC DYKE SWARM

-2400Ma

SCOURIAN EVENTS -2800-2400 Ma

(c) Late stages of deformation and metamorphism localised, deep dislocations developed (b) Early metamorphism leads to development of gneisses and granulites -2800-2700 Ma (Badcallian event) (a) Formation of early complex consisting of metamorphosed volcanic and sedimentary rocks, layered basic intrusions, tonalitic gneisses

of granulite facies (Institute of Geological Sciences, 1972; Bott, Holland, Storry & Watts, 1972). It seems probable, therefore, that the Outer Hebrides block incorporates in depth a larger proportion of buoyant, moderately granitic material than the Lewisian complex as a whole. (b) Structural features

The generalised boundary between a broad western region of amphibolite-facies gneisses incorporating a fair amount of granite and pegmatite and an eastern strip incorporating granulites runs roughly NNE through the Outer Hebrides. A similar trend shown by the isogals ofthe bouguer gravity map (McQuillin & Watson, 1973) indicates that this contrast persists in depth to define a broad lithological zoning parallel with the length of the archipelago. Over much of the islands, this large-scale structure is almost perpendicular to the NW-SE tectonic 'grain' imparted to the gneisses by foliations, fold-limbs and other minor structures measurable in the field. This discrepancy records the superposition of structures of differing ages and scales. The NNE lithological zoning appears to reflect an old and fundamental pattern on which smaller structures of varying trends were overprinted during the succession of late Scourian and Laxfordian episodes. The complex interference-patterns built up as a result of repeated deformation in metamorphic environments are traversed by a number of linear belts distinguished by indications of exceptionally intense deformation and in some instances by geophysical anomalies. These lineaments appear to mark steep zones of deep-seated dislocation along which adjacent gneiss blocks moved relative to each other. They are not faults in the sense of being clean breaks, but are ductile shear-zones formed at high temperatures and pressures during late Scourian and Laxfordian phases of deformation. They persist for minimum distances of 10 to 60 km and their length, with the facts that some are associated with geophysical anomalies and/or contain small intrusive bodies not seen elsewhere, suggest that at the time of formation they extended to the base of the crust. Two principal directions are followed by the lineaments (Fig. 1). NW-SE structures, running parallel to similar linear belts of high deformation in the Lewisian complex of the Scottish mainland, traverse the islands obliquely, the most conspicuous being those of southern South Harris and northern Lewis which are shown by geophysical evidence to continue for some 20 km across the Atlantic sea-floor. A single zone of SW-NE trend skirts the coast of western Lewis for 60 km (Davies, Lisle & Watson, 1975) and is truncated near the Butt of Lewis by the north-westerly lineament mentioned above. This SW-NE structure is seen, on a global reconstruction which allows for the opening of the Atlantic, to fall into place as one of a set of large transcurrent dislocations formed before 2000 Ma in North America and Greenland, possibly as accommodation structures related to bodily movements of an early Proterozoic supercontinent (Sutton & Watson, 1974). Only

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JANET WATSON

one lineament of this set is exposed in north-western Britain; but a number of large NE or NNE structures of later date traverse regions where the Lewisian basement lies below the present surface and it seems possible that some at least of these may be sited above similar dislocations. A spatial relationship between the ancient lineaments and the form of the Hebridean archipelago is readily apparent-the Atlantic coast of Lewis is defined by the edge of the north-easterly zone from the Uig Hills to the Butt, while the blunt northern tip of Lewis and the Sound of Harris are both defined by northwesterly structures (Fig. 1). Although the relative weakness of the strongly foliated rocks within the zones of deformation may have made them subject to preferential erosion, these coincidences appear to have a deeper significance in that younger faults tended to follow the basement structures. The tectonic and metamorphic patterns of the Lewisian complex as a whole did not attain their present complex form until the end of the Laxfordian event soon after 1800 Ma. Over much of the formerly adjacent regions of Greenland, tectonic and metamorphic activity had ended at an earlier date, roughly corresponding with the end of the Badcallian episode in Scotland. Over the period 2700-1800 Ma, the Lewisian complex formed part of a ring of early Proterozoic mobile belts enclosing the ancient Greenland stable massif. The eastern border of this cratonic massif does not lie within the British Isles. Such evidence as there is (Roberts & others, 1973) suggests that it lies between Rockall Bank and the coast of Greenland; it may perhaps coincide with the western edge of Rockall Bank at which two continental fragments parted in Tertiary times. (c) The lower crust The oldest metamorphic mineral assemblages which can be recognised in the Lewisian complex are those which date from the Badcallian phase of granulite or high amphibolite facies metamorphism at 2800-2700 Ma. The nature of these assemblages in several localities has been held to indicate that metamorphism took place at very high pressures. Estimates for granulites in southern South Harris and in the Scourie area of mainland Scotland by Wood (1975) and O'Hara (1975) respectively suggest pressures of 13 kb; and other estimates have been around 10 kb. If these figures are of the right order and if the inferred pressures were due entirely to the weight of overlying rocks, it would follow that the Badcallian granulites and gneisses originated at depths of 30-40 km, which would place them near the base of an average continental crust today. Progressive adjustments to lower pressures during the later Laxfordian stages of metamorphism (e.g, Dickinson & Watson, 1976) suggest that the enormous vertical movements required to bring rocks from such levels to the surface took place slowly from about 2200 Ma onward. Erosion had exposed Badcallian granulites along the western seaboard of Scotland by the time (-1000 Ma) when the oldest Torridonian formations began to accumulate. In the Outer Hebrides, erosion appears to have continued for at least part of the time over which Torridonian sediments were deposited and the rocks now exposed may have reached the surface at a later date. If the Archaean crust in north-west Britain was of roughly the same thickness as the average continental crust of today, uplift of 30-40 km should have had the effect of raising the base of that crust (that is, the Moho) to within a few kilometres of the earth's surface. The available geophysical data, however, indicate present crustal thicknesses of at least 20 km beneath northwest Scotland generally. This discrepancy can be accounted for either by supposing that the Archaean crust was originally some 50 km in thickness (cf. O'Hara, 1975) or that it was thickened during or after the period of elevation by the addition of material from below. The alternatives are of interest in the present context since the lineaments which provide major structural discontinuities in the Lewisian complex might be expected to stop short well above the level of the Moho if the lower crust were made of material plastered onto or thrust under the complex from below, but might extend the full distance to the base of a crust formed entirely of Lewisian material. The long history of activity on

THE OUTER HEBRIDES: A GEOLOGICAL PERSPECTIVE

7

some of these lineaments seems on the whole to favour the second alternative. So far as I know, no sub-horizontal discontinuity above the Moho which might represent the base of the Lewisian complex has been detected by geophysical means.

3. THE CALEDONIAN CYCLE (- 1000-400 Ma) In late Precambrian times, the zone of crustal mobility which was ultimately to develop into the Caledonian orogenic belt was differentiated from a great cratonic area comprising the western seaboard of Scotland, Greenland and northern Canada. For 600 million years down to the close of the Caledonian cycle the western craton, of which the Outer Hebrides formed part, was subject to tectonic regimes entirely different from those governing the development of the mobile belt; and the marginal position of the islands with respect to the Caledonian front determined their history. Among the earliest events related to the development of the Caledonides (Table I) was the definition of a system of north-easterly troughs filled by late Precambrian clastic and detrital sediments-the Torridonian on the craton, the Moinian in the mobile belt. These troughs appear to have been fault-bounded structures associated with horsts blocked out during phases of crustal extension that preceded the opening of an ocean (the proto-Atlantic or Iapetus) within the Caledonian tract. Stewart (1975) compares the tectonic setting in which the Torridonian sediments accumulated with that in which Triassic horsts and graben developed during the period preceding the opening of the Atlantic proper. It has long been known, from the inclination of current-bedding, that much of the detritus forming the youngest extensive Torridonian formation on the western craton-the Applecross Group-was derived from sourcelands to the west. By mapping out the directions of palaeocurrents Williams (1969) has concluded that the formation represents a number of overlapping alluvial fans deposited by rivers emerging from a mountain-front somewhere near the present position of the Outer Hebrides. This interpretation implies that while basins to the east of the Hebrides received Moinian or Torridonian sediments upward of four kilometres in thickness, an upland area to the west repeatedly rose to shed erosional debris into the basins. Such contrasts in behaviour suggest that block-uplift of the sourceland accompanied subsidence of the basins. The Outer Hebrides carry no Torridonian sediments, although such sediments occur in some of the Inner Isles and locally on the floor of the Minch. Both Williams (1969) and Stewart (1975) tentatively placed the original western edge of the main Torridonian basin near the eastern side of the Outer Hebrides. No large fault or fracture-system of the appropriate NNE trend traverses the islands themselves and therefore the boundary fault must either lie beneath the waters of the Minch or far to the west on the continental shelf. A major fault does indeed follow the east coast of the Outer Hebrides-the Minch fault, first identified by Dearnley (1962) and subsequently mapped out during geophysical and submarine studies (e.g. McQuillin & Binns, 1973). As seen today, it forms the western edge of the Mesozoic-Tertiary basins occupying the Minch and Sea of the Hebrides. There seems nothing improbable about the proposition that the present fault is the rejuvenated successor of a Precambrian dislocation that served as the boundary of a much older basin. The approximate parallelism between the southern portion of the fault as defined by its relationships with the cratonic cover and the deep basement lineament which skirts north-western Lewis suggests the possibility that the Minch fault originally developed above a similar basement structure, an idea which receives some support from the fact that an elongated slice of a meta-igneous body rather similar to bodies located in other lineaments (the Corodale gneiss of Coward, 1972) is carried on thrusts in South Uist which may root in the vicinity of the Minch fault. The first appearance of the Outer Hebrides as a geological entity was thus marked by their emergence as a rising block flanked on the east by subsiding troughs. The positive character of the unit has remained in evidence through most of the remaining billion years of geological history and

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JANET WATSON

may perhaps be attributed partly to the buoyancy resulting from the unusually acid composition of its basement. Although I assume (Fig. 2A & B) that cratonic cover-formations were deposited on the islands from time to time (notably during the widespread Cambro-Ordovician transgression) there is no reason to think that they ever attained great thickness. The tectonic and metamorphic activities which characterised the Caledonian belt over the period (~1OQ0-400 Ma) that saw the opening and closing of the Iapetus ocean and the associated compressive phases that ended with mountain-building and the development of the Moine thrustzone (Table I) were scarcely recorded in the Outer Hebrides. The basement (not only in the islands

A.

Torridonian NORTH TIP OF SKYE

B.

Outer Hebrides thrust .zone

LOCH TORRIDON

LOCH MONAR

Moine thrust zone

Carnbro., Ordovician

------basic lavas, sills and dykes

/

e---e = approximate erosion-level of present day

o I

20 ,

40 I

60 I

Kilometres

Fig. 2. Diagrammatic WNW-ESE sections illustrating stages in the tectonic development of the Outer Hebrides and adjacent parts of Scotland. (A) In late Precambrian times, 700 Ma. (B) -400 Ma, at the end of the Caledonian cycle. (C) - 50 Ma, after the main phase of tertiary igneous activity

THE OUTER HEBRIDES: A GEOLOGICAL PERSPECTIVE

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but even in the mainland portion of the craton which was undoubtedly overridden by metamorphic rocks of the Caledonides, Fig. 2B), remains astonishingly free from the effects of random shattering, shearing and retrogression so common near orogenic fronts. Apart from some rucks in the cover immediately below the Moine thrust-zone only one major Caledonian structure traverses the foreland-the Outer Hebrides thrust-zone which lies near the eastern side of the islands parallel to and some 80 km in advance of the Caledonian front itself. Throughout its length the Outer Hebrides zone keeps a roughly constant distance west of the Minch fault and it may be that this old basinmargin dislocation provided a built-in weakness which served to localise compressive stresses transmitted from the orogenic belt. The thrusts appear to have originated at roughly the same time as the Moine thrust (Sibson in discussion of Steel & Wilson, 1975) and achieved westward displacements of up to about 10 km (cf. Hopgood, 1971; Davies & others, 1975); at the present level of erosion, only basement rocks are involved but thrusting may have been expressed at higher levels by the development of westward-facing rucks in the cratonic cover (Fig. 28). The products of dislocation on the Outer Hebrides thrusts differ from those found along the Moine thrust-zone in ways which reflect the conditions under which displacement took place. The abundant product of frictional melting, pseudotachylyte, for which the islands are famous was formed by fast but intermittent jerky movements giving rise to repeated earthquakes (Sibson, 1975). The mylonites and other cataclasites which characterise the Moine thrust-zone on the other hand were produced by slower and more continuous creep without major earthquakes. The cold and dry basement of the Hebridean craton reacted by brittle fracture whereas the hotter and more mobile Caledonian metamorphic complexes which rode westward on the Moine thrust-zone behaved in a more ductile manner. The easternmost thrusts in the Outer Hebrides zone, which bring up rocks from a rather deeper level, are characterised by phyllonites (Coward, 1972) and Sibson (1977) concludes that at the level at which these structures were initiated crustal temperatures were high enough to allow slow yielding. Conspicuous differences in the range of crustal temperatures inside and outside the Caledonian orogenic belt appear to have been maintained for long periods. The absence of Caledonian granites and the scarcity of minor Caledonian intrusions not only in the Outer Hebrides but even in the mainland portion of the craton directly adjacent to the orogenic belt reflect an absence of hightemperature granite-forming conditions at depth. The isotopic dating technique most sensitive to rise in temperature-K-Ar dating of minerals-has failed to reveal any thermal modification of minerals in the basement of the foreland. The lack of a single apparent isotopic date in the timerange 1000-400 Ma among the scores of published determinations from Lewisian rocks and minerals strongly suggests that these rocks remained almost continuously below 20Q-300°C over this period. It would be interesting in the light of this influence to know the depth of the sections revealed at the present level of erosion on the craton and in adjacent parts of the Caledonides, and hence the geothermal gradients at successive stages of the orogenic cycle. Evidence on this point is hard to come by, the only estimate for the foreland known to me being Sibson's (1975) calculation that the Outer Hebrides thrust-zone, formed at about 400 Ma, developed at a depth between 1 and 10 km and most probably at 4-5 km. By the end of the Caledonian cycle, collision of European and American-Greenland plates had eliminated the Iapetus ocean and regional uplift had led to the emergence of the enormous Old Red Sandstone continent. The non-marine Old Red Sandstone formed from the erosion-products of the Caledonides accumulated in narrow basins in and to the south of the British sector of the orogenic belt. None is known on the western craton where the Devonian period was mainly one of uplift and erosion. By the opening of the next stage of geological history the level of erosion in the Outer Isles may have been no more than a kilometre or so above that of the present day. Tectonic disturbances in Scotland during Devonian to Carboniferous times were recorded

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JANET WATSON

principally by motions on major north-easterly transcurrent faults such as the Great Glen fault. The Minch fault was assigned to this set by Dearnley (1962) who inferred a sinistral displacement of well over 100 km on the basis of comparisons between the Lewisian structures of the Outer Isles and the Scottish mainland. The evidence on which Dearnley relied has not all been confirmed by more recent work and while the possibility that transcurrent movements took place undoubtedly remains, the suggested matching of southern South Harris with the Scourie-Laxford region of the mainland now seems of rather doubtful validity.

4. THE DEVELOPMENT OF THE NORTH ATLANTIC

The formation of the North Atlantic proper did not begin until late Mesozoic and Tertiary times when basins floored by oceanic crust opened between Britain and Rockall Bank and between Rockall Bank and Greenland, while related magmatic activity took place at the continental margins. For over 300 million years before these events, the Outer Hebrides lay within the Laurasian supercontinent, remote from orogenic belts. That period was not uneventful, for the disruption of the supercontinent was preceded by a phase of crustal extension during which the development of rift-systems blocked out the shapes of the future continental fragments. One branch of these fracture-systems (initiated in late Palaeozoic or Triassic times) ran through the Hebridean region while others traversed the North Sea and passed east and west of Rockall Bank. On a global scale, it is generally accepted that the Atlantic fracture-systems, developing from Arctic to Antarctic, followed the recently stabilised Caledonian-Appalachian-Pan-African orogenic belts. Off Britain, where the Caledonides swing rather abruptly westward, the Atlantic margin cuts a corner through the edge of the western craton, leaving the Caledonian province north of Shetland and rejoining it west of Ireland. The region which I have been discussing therefore constitutes an exception to the general rule, a fragment apparently predestined for the Greenland-American plate which remained attached to the European plate. Some geologists (e.g. Roberts, 1974) see little relationship between the siting of the local Atlantic fracture-systems and the arrangement of old tectonic units in the continental crust. Others including myself, see indications of basement control acting on various scales. Although the Mesozoic to Tertiary fracture-systems bordering on or branching from the North Atlantic trespass into pre-Caledonian tectonic provinces west of Britain, they skirt round the Archaean massif in southern Greenland, as such systems have tended to do throughout the world. Perhaps more significantly, Tertiary igneous activity in Britain was concentrated at centres which lie inside, or within a few kilometres of, the Caledonian province. Most of the centres are located on or near old fractures or near the margins of upthrown blocks. But horsts such as the Outer Hebrides which lie wholly within the pre-Caledonian craton are penetrated by only two central complexes, those of St. Kilda and Rockall, whereas at least nine lie within or marginal to the Caledonides. With the possible exception of St. Kilda, no centre is located in regions underlain by Lewisian granulites. These features suggest that variations in the lithospheric conditions controlling magmatic activity were related to the age of the continental crust. On a regional scale, it is apparent that when the horsts and graben of the Atlantic system developed in northwest Scotland, the Outer Hebrides resumed their old role as a positive block. Fault-bounded basins on their eastern flank developed in the Minch and Sea of the Hebrides where up to three kilometres of Mesozoic and Tertiary sediments accumulated east of the old Minch fault (Figs. 1 and 3). To the west, the much deeper Rockall trough floored by oceanic crust opened up in late Mesozoic times. Deposits related to the basins in the Minch overlap onto the islands only in eastern Lewis where red breccias and sandstones of the ?Triassic Stornoway Beds rest directly on the basement. The character of these sediments, which do not appear to contain Torridonian or

THE OUTER HEBRIDES: A GEOLOGICAL PERSPECTIVE

11

Palaeozoic pebbles, and the alignment of palaeocurrent indicators within them (Steel & Wilson, 1975) indicate that they were derived from a land-area near the site of the island. During the early stages of basin-formation, therefore, Lewisian rocks were at least locally exposed and subject to erosion (any pre-existing cover-rocks having already been cleared away) and, although later Mesozoic formations may have overlapped onto the islands, these seem unlikely to have been thick (Fig. 2e). To the north of the Hebrides, horst blocks of crystalline basement alternating with fault-bounded basins containing a few kilometres of Mesozoic and Tertiary sediments have been located geophysically on the continental shelf west of Shetland (Flinn, 1969; Bott & Watts, 1970). The main horsts, though roughly parallel to the Outer Hebrides, do not line up with the archipelago and are separated from it by a zone of irregular structure and submarine topography (Flinn, 1973) in which stand the Lewisian islets of North Rona, Sule Sgeir and Stack Skerry. The southern border of this zone of disharmony is marked by a bouguer gravity feature which coincides with the northwesterly basement shear-zone passing through the Butt of Lewis. The north-westerly sub-Mesozoic ridge separating the basins of the Minch and Sea of the Hebrides is also associated with a bouguer gravity gradient (McQuillin & Binns, 1973; 1975) and deep lineaments in the basement may have influenced the development of the late Palaeozoic to Mesozoic fracture-patterns in both regions. Seen in perspective, the Outer Hebrides appear to have played a similar structural role during two cycles of sea-floor spreading, that giving the Iapetus ocean which opened to the east of the islands and that giving the Atlantic to the west. One remains, however, in doubt as to the significance of this coincidence. Along much of the Atlantic margin outside Britain a 'sill' in which the continental basement stands relatively high separates the ocean proper from a system of ensialic fault-troughs; and the Hebridean structure, in conforming to this general pattern, could be regarded as a natural product of the process of sea-floor spreading. The presence of older block-structures, themselves perhaps related to the architecture of the basement, may however have had some effect in diverting the Atlantic fracture-system from the Caledonian province into the edge of the adjacent craton in north-west Britain. The Tertiary magmatic activity at the Atlantic margin had no known parallel during the Iapetus cycle. The main igneous centres, as already noted, lie in or alongside the Caledonian province, avoiding the Outer Hebrides where the basement is entirely pre-Caledonian. The thick sills which underlie the lava-plateau of northern Skye extend to within a few kilometres of the coast of Harris (Walker, 1931) and it seems not unlikely that the feather-edge of the lava pile itself spread onto the Outer Isles. The vesicularity of many Tertiary dykes along this coastline suggests, however (G. P. L. Walker, personal communication), that the thickness of such lavas (plus that of any Mesozoic and basement rocks subsequently removed by erosion) amounted to no more than about a kilometre. By the end of the igneous events, the peaks of the volcanic centres in the Inner Hebrides probably stood high above the land surface of the Outer Isles. Although the volume of basic magma emplaced as dyke-swarms in the Outer Hebrides was small, the importance of the fracture-systems associated with them suggests that considerable disturbances of the basement were involved. The nature of the structures developed is indicated not only by the alignment of the dykes themselves, but also by that of linear negative aeromagnetic anomalies (Institute of Geological Sciences, 1972) and of very numerous fractures and joints etched by erosion and clearly revealed on aerial photographs (Fig. 3). The great majority of all these types of features run N.W. or N.N.W. in conformity with the general alignment of Tertiary dykes in north-west Britain. From North Harris to Barra, many dykes line up with the swarms centred on Skye and Mull, though the frequent absence of magnetic anomalies linking these centres with the Outer Isles across the basin of the Sea of the Hebrides suggests that magma may have risen higher in the basement block than in the Mesozoic strata. In Lewis, a remarkable set of close straight

12

JANET WATSON

o I

20 I

Scale

'l."';'

e;,':.0....': -,"\

~\..

.....

.

'v'-.... ~

~ .;

.'

'

~ ....

0 .:

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Fig. 3. Fracture-patterns in the basement of the Outer Hebrides drawn from aerial photographs; inset. major post -Caledonian faults and centres of depo sition

THE OUTER HEBRIDES: A GEOLOGICAL PERSPECTIVE

13

fractures extends N.N.W. from the Park district to Loch Roag. Most of these fractures contain no dykes and probably mark the roof zone above a dyke-swarm. Pronounced magnetic anomalies associated with them cross the Minch fault without apparent displacement and pass through the Shiant Islands which are made of a thick differentiated basic sheet. An apparent southward deflection of the anomalies from the Shiants links the swarm with the northern tip of Skye completing a pattern reminiscent of that which links the Skye and Mull swarms. The distribution of dykes in the Outer Isles thus conforms to the general pattern which suggests emplacement during sinistral movements roughly parallel with the continental margin (cf. Wilson, Ade-Hall, Skelhorn, Speight & Dagley, 1974). The termination of igneous activity in north-western Britain some fifty million years ago brought the history of local crustal movements almost to a close. The combined effects of subaerial and glacial erosion since that date have no doubt lowered the land-surface somewhat. If we accept the figure of a kilometre as the maximum probable thickness of cover on vesicular Tertiary dykes near sea-level on the east coast, the maximum rate of erosion could average out at no more than a millimetre in fifty years, and it seems quite probable that the summits of the Harris mountains still rise almost to the level of the early Tertiary land-surface. In a long perspective, it appears that the crustal block which includes the Outer Hebrides has maintained itself at much the same level relative to the sea for a very long period, perhaps since the late stages of Torridonian sedimentation. Any renewal of tectonic activity in the future seems likely to depend on the closing of the Atlantic, with the consequent approach of the dispersed continental fragments of Greenland and Rockall Bank. In such circumstances, the Outer Hebrides, with the other basement 'highs' in the collision zone, might renew their role as positive blocks between nappe-complexes developed from the basinfillings on either side. REFERENCES BOTT, M. H. P. & A. B. WATTS, 1970. Deep sedimentary basins proved in the Shetland-Hebridean continental shelf and margin. Nature, Lond., 225, 265-8. BOTT, M. H. P., I. G. HOLLAND, P. G. STORRY & A. B. WATTS, 1972.Geophysical evidence concerning the structure of the Lewisian of Sutherland, N.W. Scotland, Jl. geol. Soc. Lond., 128,589-612. COWARD, M. P., 1972. The eastern gneisses of South Uist. Scott.J. Geol., 8, 1-12. DAVIES, F. B., R.I. LISLE&I. WATSON, 1975. The tectonic evolution of the Lewisian complex in northern Lewis, Outer Hebrides. Proc. Geol. Ass., 86,45-61. DEARNLEY, R., 1962. An outline of the Lewisian complex of the Outer Hebrides in relation to that of the Scottish mainland. Q. JI geol. Soc. Lond., 118, 143-76. DICKINSON, B. & J. WATSON, 1976. Variations in crustal level and geothermal gradient during the evolution of the Lewisian complex of N.W. Scotland. Precamb. Res., 3, 363-74. FLINN, D., 1969. A geological interpretation of the aeromagnetic maps of the continental shelf around Orkney and Shetland. Geol. Jl, 6,279-92. FLINN, D., 1973. The topography of the sea-floor around Orkney and Shetland and in the northern North Sea. Jl geol. Soc. Lond., 129,1-38. HOPGOOD, A. M., 1971. Structure and tectonic history of Lewisian gneiss, Isle of Barra, Scotland. Krystalinikum, 7,27-60.

INSTITUTE OF GEOLOGICAL SCIENCES, 1972. Aeromagnetic map of Great Britain, Sheet I, 1:625000. McQUILLIN, R. & P. E. BINNS, 1973. Geological structure in the Sea of the Hebrides. Nature, Phys. Sci., 241,2-4. McQUILLIN, R. & I. WATSON, 1973. Large-scale basement structures of the Outer Hebrides in the light of geophysical evidence. Nature, Phys. Sci., 245, 1-3. McQUILLIN, R. & P. E. BINNS, 1975. Geological structure of the Minches, the Sea of the Hebrides and the adjacent north-west British Shelf. Canada's continental margins and off-shore petroleum exploration (ed. C. J. Yorath, E. R. Parker and D. I. Glass) Can. Soc. Petrol. Geol. Memoir, 14,283-94. MYERS, I. S., 1971. The late Laxfordian granitemigmatite complex of western Harris, Outer Hebrides. Scott. J. Geol. 7, 254-84. O'HARA, M. J., 1975. Great thickness and high geothermal gradient of Archaean crust: the Lewisian of Scotland (Extended Abstract). Int. Conference on Geothermometry and Geobarometry, Penn. State Univ. Oct. 1975. ROBERTS, D. G., 1974. Structural development of the British Isles, the continental margin and the Rockall Plateau. The Geology of continental margins (ed. C. A. Burk and C. L. Drake) Springer-Verlag, 343-59. ROBERTS, D. G., D. A. ARDUS & R. DEARNLEY, 1973. Pre-Cambrian rocks drilled from the Rockall Bank. Nature, Phys. Sci., 244,21-3.

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SIBSON, R. H., 1975. Generation of pseudotachylyte by ancient seismic faulting. Geophys . J. Roy . Astr. Soc. 43,775-94. SIBSON, R. H., 1977. Fault rocks and fault rnechanisms. Jlgeol. Soc. Lond., 133,191-213. STEEL , R. J., & A. C. WILSON, 1975. Sedimentation and tectonism (? Permo -Triassic) on the margin of the North Minch basin, Lewis, Jl geol. Soc. Lond. , 131, 183-202 . STEW ART, A. D., 1975. 'Torridonian' rocks of Western Scotland. In Harris, A. L., & others (eds) A correlation of Precambrian rocks in the British Isles. Geol. Soc . Land. Sp. Pub. 6,43-51. SUTTON, J. & J. WATSON, 1974. Tectonic evolution of continents in early Proterozoic times. Nature, Lond., 274,433-5.

WALKER, F., 1931. The dolerite isles of the North Minch. Trans. Roy . Soc . Edinb., 56,753-66. WILLIAMS, G. E., 1969. Characteristics and origin of a Precambrian pediment. Jl Geol., 77, 183-207. WILSON, R. L., J. M. ADE -HALL , R. R. SKELHORN, J. M. SPEIGHT & P. DAGLEY, 1974. The British Tertiary igneous province: palaeomagnetism of the Vaternish Dyke -swarm, north Skye, Scotland . Geophys. J. Roy. Astr. Soc ., 37,23-30. WOOD, B. J., 1975. The influence of pressure, temperature and bulk composition on the appearance of garnet in orthogneisses-an example from South Harris, Scotland . Earth Planet . Sci. Leu.• 26, 299311. Received 5 July 1976