geological evidence for neotectonic activity during deglaciation of the southern sperrin mountains,...

JOURNAL OF QUATERNARY SCIENCE (1999) 14 (1) 45–57 CCC 0267-8179/99/010045–13$17.50Copyright 1999 John Wiley & Sons, Ltd.

Geological evidence for neotectonic activityduring deglaciation of the southern SperrinMountains, Northern IrelandJASPER KNIGHT*School of Environmental Studies, University of Ulster, Coleraine, Co. Londonderry, BT52 1SA, Northern Ireland

Knight, J. 1999. Geological evidence for neotectonic activity during deglaciation of the southern Sperrin Mountains, Northern Ireland. J. Quaternary Sci., Vol.14, pp. 45–57. ISSN 0267-8179.

Received 23 November 1997, Revised 4 April 1998, Accepted 21 April 1998

ABSTRACT: In the southern Sperrin Mountains, Northern Ireland, stacked glacigenic sequencesthat accumulated during deglaciation (ca. 17 000–13 000 yr BP) overlie a basement of jointedand mascerated bedrock. The glacigenic sequences comprise interbedded glaciotectonic shears,diamictic breccias and rock rafts. At one site a normal fault with a metre-scale verticaldisplacement cuts through part of the sequence and is overlain by a glacial diamict. Sedimentsat an adjacent site show that faulting and associated hydrothermal activity was related toneotectonic reactivation of pre-existing Caledonian lineaments caused by ice unloading. Fromstratigraphical and directional evidence, fault reactivation occurred early in the deglaciationafter north central Ireland ice had retreated southwards into lowland areas, but before SperrinMountain ice readvanced from the north. This relationship provides evidence for the relativetiming of neotectonic activity in Northern Ireland, and demonstrates the effects of glacio-isostaticunloading near ice-sheet centres. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: ice unloading; glacio-isostatic rebound; fault reactivation; rock rafts; glaciotectonic shearing

Introduction

Neotectonic activity in previously glaciated terrains has beenrelated to differential rebound following ice unloading whenpre-existing structural lineaments and bedrock weaknessesare reactivated and other structures formed as the new stressregime is accommodated (i.e. Adams, 1989; Ringrose, 1989;Broster and Burke, 1990; Wallach and Chagnon, 1990;Morner, 1991; Lagerback, 1992; James and Bent, 1994;Thorson, 1996). Vertical and horizontal changes in stressrelated to land rebound (uplift) are manifested variously asstrike-slip and dip-slip (normal and reverse) faulting, and theformation of pop-ups where bedrock wedges are squeezedfrom rockhead in response to near-surface stress changes(Wallach and Chagnon, 1990; Rutty and Cruden, 1993;Wallach et al., 1993). Neotectonism may also changegroundwater hydraulic gradients, force fluid migration(Hickman et al., 1995; Massonnet et al., 1996), and causeseismic liquefaction (Ringrose, 1989; Lagerback, 1992;Mohindra and Bagati, 1996). Examining evidence for post-glacial neotectonic activity can throw light on the nature ofglacio-isostatic recovery and the differential bedrockresponse to ice unloading. This is especially important inthe north of Ireland, where sea-level recovery curves around

* Correspondence to: Dr. J. Knight, School of Environmental Studies, Univer-sity of Ulster, Coleraine, Co. Londonderry BT52 1SA, Northern Ireland

present coasts are fairly well known (Carter, 1982; Shawand Carter, 1994; Devoy, 1995; Lambeck, 1996), but wherethe magnitude of rebound over former inland ice dispersalcentres are unknown. This means that land recovery modelsare geologically unconstrained (McCabe, 1997), particularlywith respect to total ice thickness (loading) and deglaci-ation history.

The aim of this paper is to describe the structures andpatterns of bedrock and glacigenic features on the southernmargin of the Sperrin Mountains, Northern Ireland. Thestratigraphy of two exposures located near the Caledonian-age Omagh Fault records neotectonic activity followed bysouthward ice readvance from the Sperrin Mountains. Thischronology helps to constrain the relative timing and poss-ible causal mechanisms of neotectonic activity. A principleconclusion is that ice unloading of heterogeneous bedrockstructures led to metre-scale fault reactivation and associatedhydrothermal fluid flow along pre-existing structural lin-eaments.

Geological and tectonic setting

In Northern Ireland, regional geological provinces are separ-ated by major Caledonian-age northeast–southwest alignedlineaments associated with the strike-slip Highland Boundary

46 JOURNAL OF QUATERNARY SCIENCE

Fault and its related structures (Wilson, 1972; Max andRiddihough, 1975; Alsop and Hutton, 1993; Ryan et al.,1995). Geologically, the Sperrin Mountains are underlain byUpper Dalradian schists and metamorphics, and comprise anorthwest-dipping overturned anticline that has been meta-morphosed and dissected by movement along left-lateralstrike-slip faults during the Caledonian orogeny (Wilson,1972; Alsop and Hutton, 1993). These faults help delineateupland blocks such as Mullaghcarn and Slieve Gallion alongthe southern Sperrin margin (Fig. 1). Intrusive and metamor-phic rocks of the Ordovician-age Tyrone Igneous Complex(TIC) are present to the southeast of the Omagh Fault(Hartley, 1933). Tertiary volcanism deposited flood basaltsacross northeastern Ireland and emplaced dykes south of theSperrins (Wilson, 1972; Gibson and Lyle, 1992). This sum-mary confirms that tectonic events associated with changesin bedrock structures occurred throughout Northern Ireland’sgeological history (McCaffrey, 1997).

Glacial history

The Sperrin Mountains and adjacent areas were ice-coveredduring the Late Devensian glaciation (ca. 23 000–13 000 yrBP) (Colhoun, 1970). Ice radiated from upland dispersalcentres in County Donegal and the western Sperrin Moun-tains (Fig. 1). South of this area, ice flow was generallydirected south and southwestwards towards marine marginson the Irish west coast (McCabe, 1987; Knight and McCabe,1997). During deglaciation (ca. 17 000–13 000 yr BP) disper-sal centres in lowland areas of the Omagh Basin, LowerLough Erne Basin and Lough Neagh Basin were most domi-nant (Dardis, 1982; McCabe, 1987; Knight, 1997a). Thesplitting-up of Omagh Basin and Sperrin Mountain icemasses during glaciation probably occurred south of theSperrin Mountains near Mullaghcarn (Knight, 1997a). Sperrinice formed topographically controlled valley glaciers andretreated generally westwards through Sperrin Mountain rivervalleys (Colhoun, 1970). Omagh Basin ice formed a shield-like dome and retreated southwestwards from the Sperrinfoothills towards Lower Lough Erne (Knight, 1997a).

Exposures at Mountfield and Mountfield Quarry on thesoutheastern flank of Mullaghcarn are located below theupper drift limit (variably 240–270 m a.s.l.) and less than100 m distance from the Omagh Fault (Fig. 1). The exposuresare developed in thin, low-relief diamict sheets (few metresin thickness) that overlie heavily conjugate-fractured lavasand tuffs of the TIC. The exposures, extending 3–15 m tothe bedrock upper surface, show a range of glaciotectonicand neotectonic structures and are associated with OmaghBasin and Sperrin ice interaction during early deglacialstages.

Site descriptions

Mountfield

The exposure (up to 200 m long, 15 m high), comprises aplanated bedrock platform overlain by a diamictic brecciathat contains discontinuous rock rafts (Fig. 2). The bedrockupper surface has a well-defined dome-like morphology (1–2 m relief) (Figs 3 and 4). Topographic lows between dome

Copyright 1999 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 14(1) 45–57 (1999)

crests are infilled with angular bedrock fragments (,5 cmdiameter) which form a closely-packed, massive diamicticbreccia (0.6–2.5 m thick). Up-profile, the breccia faciesbecomes more texturally variable with flat-lying, subangular,bladed bedrock fragments (,25 cm diameter) supported bya dominant (.95%) coarse sand to angular granule matrix.Above dome crests, crushed bedrock granules form amassive, well-cemented breccia separated by glaciotectonicshears (0.5–1.2 m vertical spacing). Shears are both flat-lyingand inclined, laterally continuous across the exposure, anddemarcated by zones (0.5–2.0 cm thick) of crushed bedrockwith silty, mylonitised partings. Inclined shears (spaced 20–30 cm apart) are aligned en echelon, feed upwards fromthe flat-lying shears and generally dip northwards (,10°[range 8°–14°] towards 000° [range 350°–010°]; n = 10).Rock rafts (,4 m long, 0.8 m thick) are present in the lowerpart of the breccia facies, particularly over topographic lowson the bedrock surface (Fig. 4). Rock rafts are generallytabular to lensate and show disaggregated, broken margins.Rafts are supported wholly by the surrounding bedrock brec-cia and lie parallel to, and are defined by, flat-lying shears.Inclined shears sometimes demarcate the lateral margins ofindividual rafts (Fig. 3).

A fault gouge (0.5 m thick, .3 m long, aligned 040°–220°) bisects the bedrock surface between two domes andis demarcated by brecciated and mylonitized bedrock withsilty partings aligned parallel to the fault plane (Fig. 3).Displacement of bedrock blocks on either side of the faultshows it to be a normal fault with a relative vertical displace-ment of ca. 1 m and throw towards 130°. Lateral displace-ment is unknown. The upper part of the fault gouge issheared out by a continuous, flat-lying shear plane (Fig. 3).The area between the down-thrown block and hanging wallis occupied by a drag folded breccia (0.7 m high, 2.0 mlong). Upper and lower surfaces of the drag fold are definedby glaciotectonic shears (Fig. 5). The direction of shearingand dragging suggests north to south ice flow from theSperrin Mountains (Knight, 1997b).

Mountfield Quarry

This exposure (up to 200 m long, 4 m high) comprises adiamictic breccia, small rock rafts and glaciotectonic shears.These sediments (2.0–3.5 m thick) overlie an irregular bed-rock surface, which varies laterally from sharply erosionalto poorly defined with angular bedrock protrusions anddisplaced rock fragments (Fig. 6). The diamictic breccia isvariable in thickness (1–3 m thick) and texture and is com-posed of local bedrock granules and angular pebbles (,4 cmdiameter). The exposure is cut by laterally continuous (,8 mlong) glaciotectonic shears, which are marked by thin(several centimetres thick) silty partings (Fig. 7). The lowerpart of the profile (,1.5 m above bedrock) is characterisedby flat-lying planar to listric shears, which are usually stackedbut which may overlap and cut one another out. In theupper part of the profile, shears are inclined and show anortherly dip (,10° [range 5°–15°] towards 005° [range350°–015°]; n = 16).

Rock rafts are located mainly in the lower part of theprofile (Figs 6 and 7). Rafts (,3.5 m long, 0.5 m thick) aretabular to lensate, show silty coatings of crushed bedrock,and are defined and separated by flat-lying shears. Primarybedrock fracture patterns are preserved unchanged within therock rafts, and the interlocking bedrock blocks are cleanlydissected by the raft surfaces. Between-raft breccias are

47NEOTECTONIC ACTIVITY IN NORTHERN IRELAND

Figure 1 Location map showing generalised ice flow patterns (McCabe, 1987), topography and places named in the text: (a) generalisedgeological map showing major faults (Wilson, 1972); (b) location of described sites in the southern Sperrin Mountains.

generally absent. Both breccia and rock raft facies lie withinand overlie a bedrock ‘hollow’ (1.5–2.0 m deep, 2.5 macross) (Fig. 7). The sides of the hollow are steep and planarbut do not strike in the same direction (010°–190° and080°–260°), thereby suggesting they form a wedge shape in


three dimensions. The hollow contains vertically aligned andalternating beds (,20 cm thick) of massive brecciated bed-rock fragments and silty diamict, including a clay-rich, green-coloured band (Fig. 8). This structure (20–40 cm wide, .2 mhigh; strike 020°–200°), developed within the bedrock brec-


Figure 2 Generalised facies log and palaeoflow direction, Mountfield: (a) description and inferred processes of sediment emplacement, (b)reconstructed envelope of ice–bed interface processes and (c) phases of ice activity.

(a)

(b)

Figure 3 Sediments at Mountfield: (a) photograph showing the fault gouge (F), drag fold (D; also shown in Fig. 5) and rock rafts (R). Notethe sharply planated and domal bedrock upper surface. The section is 3 m high; (b) interpretive sketch.

cia, shows a number of vertically aligned zones. The outer-most zone comprises thin (several millimetres thick) andvertically continuous clay-rich layers. The innermost zonecomprises unaltered bedrock fragments that still contain ves-icles derived from the lavas and tuffs bedrock. X-ray diffrac-tion (XRD) analysis of the silt–clay (,63 mm) component ofthe outermost zone (Fig. 9) shows the presence of illite andmontmorillonite which is consistent with medium tempera-ture (300–400°C) alteration by circulating fluids (Lippman,1982; Velde, 1985). The vertically aligned beds on either


side of the green-coloured band are truncated by flat-lyingglaciotectonic shears that show sediment displacement fromnorth to south (Figs 7 and 8). Displacement decreasesupwards from 75 cm at the top of the hollow to 15 cm inthe upper part of the exposure profile and can be tracedwith reference to the vertical band, which thins and isprogressively displaced upwards (Figs 7 and 8).


(a)

(b)

Figure 4 Sediments at Mountfield: (a) photograph showing a rock raft (upper arrow) immediately above the lowest part of the domalbedrock upper surface (lower arrow), separated by vaguely bedded diamictic breccia. Section is 3 m high; (b) interpretive sketch.

Site interpretations

Both sites show similar features and are interpreted together.The sites record cycles of glaciotectonic activity, evidencedby the presence of flat-lying shears throughout the exposureprofiles, which are interbedded with unsheared, massivediamictic breccias and rock rafts (cf. Banham, 1977; Bennand Evans, 1996) (Figs 4 and 7). Displacement of brecciaand rock raft facies during each shearing event is consistentlyfrom north to south (Knight, 1997b). The closely interlockingand angular nature of the breccia, and absence of matrix,suggests a low capacity for elastic deformation (Banham,1977). Sheared zones marked by silty, mylonitised partingssuggest that shearing occurred by frictional failure of individ-ual breccia fragments that were crushed during shearing.Meltwater or porewater lubrication along the shear zone wasnot a dominant process, as evidenced by the preservation ofsilts along the shears. Distinct, planar and laterally continu-ous shears separating individual beds, lack of sedimenthomogenization, and preserved internal bedding and otherstructures suggest that rock rafts and breccias were emplacedwithout internal deformation (Croot, 1987).

At Mountfield, shearing and brecciation was followed byfaulting, which changed the basal topography and allowed


the formation of a discrete pod of breccia that was drag-folded during shearing, and also truncated the upper part ofthe fault gorge (Fig. 5c). Bedrock pulverisation on the up-ice nose of the drag-folded pod reflects high-pressure glaci-otectonic impacting (Menzies, 1982; McCabe and Dardis,1994) (Fig. 5). This caused ‘shock’-style deformation (clastfracturing and brecciation) of adjacent bedrock fragments.Lower pressure deformation styles in the down-ice directioninclude drag folding and homogenisation (mixing offragments). At Mountfield Quarry the position between domecrests of both rock rafts and the drag-folded pod (Figs 3 and4) may reflect preferential raft migration to low-pressure sitesin areas with a thick sediment pile (cf. Hart and Boulton,1991). This contrasts with sediments over dome crests, whichare more heavily impacted (sheared) and less able to deformelastically (Fig. 4). This relationship also suggests that shearedsurfaces were successively abandoned by later ice stages ofice activity, thereby allowing sediments to be stacked verti-cally in aggradational sequences.

Evidence for glacially induced neotectonism

Three lines of geological evidence suggest that neotectonicand related activity was present in the southern Sperrin


Figure 5 Interpretive sketch of the Mountfield exposure (see Fig. 3): (a) structural relationship between the main bedrock blocks, faultgouge, drag fold and sheared surfaces; (b) detailed sketch of proximal to distal changes in drag fold internal structures; (c) proposed modelfor fault gouge and drag fold formation (stages i–iii).

Figure 6 Generalised facies log and palaeoflow direction, Mountfield Quarry: (a) description and inferred processes of sedimentemplacement, (b) reconstructed envelope of ice–bed interface processes and (c) phases of ice activity. Active (shearing) and inactive(meltout) phases characterised ice behaviour.



Mountains during the last deglaciation. This emphasises theclose relationship between neotectonic and glaciotectonicprocesses, which can be differentiated on a directional,stratigraphical and geomorphological basis (Fig. 10).

1. The normal fault at Mountfield was reactivated by shallowcrustal extension and bedrock slippage (gouge formation)during Sperrin ice flow, which was aligned at an angleto fault strike (Figs 10 and 11). A neotectonic origin isattributed to this feature because, stratigraphically, north-to-south glaciotectonic shears pre-date formation of thefault gouge because they sheared the bedrock surfacebelow the drag fold (Fig. 5c). This infers that faulting musthave occurred between two distinct phases of subglacialshearing. Scale of the observed vertical displacement atMountfield can be compared favourably with glaciallyinduced faulting described from the Puget lowlands,Washington (5–9 m, Thorson, 1996), and northern Scandi-navia (,30 m, Lagerback, 1992; ,15 m, Arvidsson,1996).

2. At Mountfield Quarry, the bedrock hollow is interpretedas a glaciotectonic pop-up (Wallach and Chagnon, 1990;Rutty and Cruden, 1993; Wallach et al., 1993) resultingfrom tectonic compression of near-surface bedrock duringSperrin ice readvance. This structure, aligned approxi-mately parallel to the strike of the normal fault at Mount-field, may be a by-product of more general fault reacti-vation following changes in stress regime (Adams, 1989;Einstein and Dershowitz, 1990; Wallach and Chagnon,1990; Rutty and Cruden, 1993; James and Bent, 1994)(Fig. 11c). The geometry of the hollow is defined bysharp, planar surfaces which are interpreted as faults andwhich are aligned at an angle to Sperrin ice flow (Fig.10). Infilling of the ‘hollow’ with bedrock-derived brecciassuggests that further tectonic compression following pop-up detachment did not occur and that the ‘hollow’ didnot close up.

3. The XRD analysis (Fig. 9) shows the green-coloured bandat Mountfield Quarry (Figs 7 and 8) to be related tomedium-temperature hydrothermal fluid escape througha fault or rock fracture (Velde, 1985). Lateral zoning ofmineral alteration is consistent with this idea (Velde,1985, p. 116), and the preservation of vesicular bedrockfragments in the core of the structure does not favourother interpretations such as dykes. It is also consistentwith evidence that solute and hydrothermal fluid move-ment is related to and accompanies faulting (Hickmanet al., 1995; Mozley and Goodwin, 1995; Peltzer et al.,1996). Fluid flow may be driven by the geothermal heatflux acting along bedrock lineaments (Hickman et al.,1995), and in Ireland enhanced heat fluxes have beenassociated with igneous bodies, such as the TIC, andother structural features (Brock, 1989). Hydrothermal min-eralisation is also known to have been focused along theOmagh Fault (McCaffrey and Johnston, 1996; Wilkinsonand Johnson, 1996).

Discussion

Tectonic and glaciotectonic setting

The stratigraphical successions described were formed bysubglacial shearing, bedrock brecciation and transport ofbedrock rafts, coupled to changes in the nature of the


substrate caused by neotectonism. Directional indicators (Fig.11) show that shearing occurred during north-to-south flowof Sperrin Mountain ice towards the Omagh Basin (Knight,1997b). The sediment successions also demonstrate that neo-tectonic and glaciotectonic activity may have very similarstratigraphical signatures, and may be closely related pro-cesses in deglaciating environments characterised by changesin subglacial stress regimes (cf. Owen, 1989) (Figs 10 and11). Neotectonic and glaciotectonic processes can be dis-tinguished with reference to inferred strain regimes, thedirection of the principle stress axis imparted by ice flowdirection (Fig. 11c), and the relative timing of movementalong faults and shears (Richard and Krantz, 1991). Theoccurrence and alignment of the normal fault (Mountfield)and pop-up structure (Mountfield Quarry) is consistent withproglacial or subglacial compression, followed by tectonicrelaxation along pre-existing lineaments behind the ice mar-gin. This means that neotectonism in the southern Sperrinsmay have involved strain accommodation under both com-pressional and extensional regimes (Broster and Burke, 1990;Palmer et al., 1995), possibly as a result of strain jumpingbetween different fault segments (Fig. 11).

Bedrock structures at Mountfield and Mountfield Quarrysuggest a combination of both neotectonic and glaciotectonicprocesses because they are aligned at an angle to ice-flowdirection and Omagh Fault strike (Fig. 10). Major fault planesmay act to focus the direction of stress propagation byenhancing contrasts between the stress tensors (s1, s3) (Evanset al., 1995). This may have conditioned the alignment ofthe pop-up wedge, which is aligned approximately parallelto the Omagh Fault and at an angle to ice-flow direction. Thepop-up ‘hollow’ is infilled with and overlain by brecciatedbedrock, rock rafts and shears that share a similar directionalcomponent and depositional setting. In the southern SperrinMountains, the size and longevity of the Omagh Fault mayhave historically downplayed the significance of movementalong smaller associated strike-slip and dip-slip faults. Bed-rock blocks, delimited by faults and other structures, arealso likely to respond differentially to ice unloading (i.e.Eyles and McCabe, 1989).

Constraints on the relative timing of neotectonicactivity

The event sequence at Mountfield Quarry suggests that bed-rock brecciation and glaciotectonic shunting took place inseveral episodes. These are: (i) pop-up formation and devel-opment of the bedrock hollow. Infilling with verticallyaligned breccia beds probably occurred at the same time.(ii) Generation of bedrock breccia overlying the infilledhollow. This breccia was at least 1 m thick. (iii) Hydrother-mal activity and formation of the vertically aligned, clay-rich band which extends and thins from the bedrock hollowupwards through the breccia facies. The fluids therefore hada deeper source and escaped to areas of lower confiningpressure. (iv) Glaciotectonic shearing to at least 1 m depthwithin the breccia facies, and en masse southward displace-ment of these sediments. An upward decrease in the dis-placement of successive shearing events suggests that glaci-otectonic penetration into the sediment pile decrease overtime. (v) A second major period of bedrock brecciation andshearing, which extends across the site. Faulting at Mount-field is also constrained between two glaciotectonic shearingevents (Figs 2 and 5).

From stratigraphical and sedimentary evidence, therefore,

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Figure 9 Plot of XRD results for a clay sample removed from the outermost edge of the green-coloured vertical band at MountfieldQuarry. (a) Raw sample showing illite peaks; (b) glycolated sample showing montmorillonite peaks.

fault reactivation and related hydrothermal activity at Mount-field and Mountfield Quarry occurred after the north centralIreland ice-cover split up into distinct Omagh Basin andSperrin ice masses, but before or concurrent with Sperrinice readvance. These glacial episodes therefore act to con-strain the relative timing of neotectonic activity, and itspossible causal mechanisms. Microfaulting described byAdams (1989) from eastern Canada occurred within a fewthousand years of deglaciation. This is too long after initialice unloading to be compared with the Mountfield examples.Liquefaction caused by post-glacial seismic activity occurrednear the margin of the Loch Lomond Stadial ice mass atGlen Roy, Scotland, at around 10 000 yr BP (Ringrose, 1989).Calculated earthquake moment magnitude (Mw) was 5.9according to deformation style and distance from inferredearthquake epicentre (Ringrose, 1989). This scale of seismicactivity along the Great Glen Fault, similar in age, extent,orientation and structural characteristics to the Omagh Fault,


Figure 10 Lower hemisphere stereoplot showing the orientationof various neotectonic and glaciotectonic structures in theMountfield region. The Omagh Fault is shown as a verticalstructure (dip unknown).


Figure 11 Schematic sketches of suggested tectonic regime in the southern Sperrin Mountains: (a) regional-scale sketch showing extent ofdip-slip and strike-slip faulting; (b) local-scale sketch of changes in basement structures, marked with approximate locations of examplesdescribed. No absolute spatial scale or orientation is implied; (c) block diagrams showing the orientation of principle stresses undertectonic and glaciotectonic regimes.

suggests that similar events may have occurred across thenorth and west of Ireland. Tectonic or glacioteconic activitymay have driven both faulting and associated hydrothermalfluid migration along pre-existing bedrock weaknesses(Peltzer et al., 1996).

Conclusions

Geological evidence for glaciotectonic and neotectonic pro-cesses and events in the southern Sperrin Mountains showsthe close temporal relationship between these elements dur-ing early deglacial stages (ca. 15 000 yr BP). Neotectonicactivity (ca. 1 m vertical magnitude) occurred after the ice


mass covering this area split up into individual lobes, butbefore ice remaining in the Sperrins actively readvanced.Neotectonic reactivation of structural lineaments upondeglaciation is important because the geological evidencefor such activity has not been commonly described outsidethe Great Lakes region and Scandinavia (Adams, 1989; Bros-ter and Burke, 1990; Lagerback, 1992; Thorson, 1996). Thisevidence is also important because it may prove the effectsof glacio-isostatic uplift in core areas of glaciation. This isof particular relevance in the north of Ireland where upliftcurves of coastal zones have been well-studied (i.e. Carter,1982), but where there is no inland evidence whatsoeverfor the magnitude of glacio-isostatic rebound. This also con-firms the possibility that regional-scale differential tectonicand glacio-isostatic effects were important along the marginsof the last British ice mass (Eyles and McCabe, 1989;McCabe, 1997).


Acknowledgements I thank Marshall McCabe for commentinghelpfully on early drafts of the paper; Tony Hamilton for discussion;Nigel McDowell for processing the photographs and Mark Millarand Kilian McDaid for drafting. I also thank Yoma Megarry (Queen’sUniversity, Belfast) for organising the XRD analysis; Peter Wilsonfor discussion of results; and JQS referees Peter Coxon and LewisOwen for their useful comments. This work was supported by aDENI CAST award with the Department of the Environment (NI).

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geological evidence for neotectonic activity during deglaciation of the southern sperrin mountains,...

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