The Ochil Fault and the Kincardine Basin: Key Structures in the Tectonic Evolution of the Midland Valley of Scotland

Journal of the Geological Society, London, Vol. 153, 1996. pp. 573-587, 9 figs. Printed in Northern Ireland

The Ochil Fault and the Kincardine basin: key structures in the tectonic evolution of the Midland Valley of Scotland

J. RIPPON'.*, W. A. READ' & R. G. PARK' I Department of Geology, Keele University, Keele, Staffordshire ST5 SBG, UK 21nternational Mining Consultants Ltd, PO Box 18, Mill Lane, Sutton-in-Ashfield, Nottinghamshire, NGI 7 2NS, UK

Abstract: The OchilFault, one of the most prominenttectonic features in the Midland Valley of Scotland,juxtaposes Lower Devonian volcanic rocks against lateWestphalian strata, implying a possible vertical displacement of up to 4 km. The Kincardine Basin in its hanging-wall, although actively subsiding during the greater part of the Silesian, trends generally N-S, perpendicular to the Ochil Fault, and its Silesian sedimentary record shows little sign of tectonic control by that fault. It is proposed that the Ochil Fault was initiated, possibly as a sinistral strike-slip feature, in the Devonian, but acted as a sidewall fault during the early evolution of the Kincardine Basin, attributed to extension on a hypothetical pre-Brigantian fault along the Bo'ness Line on the east side of the basin until late Namurian times, when active extension ceased. Both faults were probablyburied during late Namurian and Westphalian times. Reactivation of theOchil Fault during end-Carboniferous N-S extension, dated by quartz-dolerite emplacement, was responsible for probably at least 2 km displacement. including the presently visible footwall uplift. It is suggested that the hanging wall may have contained a Permian basin, now removed.

Keywords: Scotland. Midland Valley, Carboniferous, faults, basins.

The Ochil Fault constitutes one of the most striking tectonic displacement,where strata of late Westphalian age to the features in the Midland Valley of Scotland,and its south are placed against Lower Old Red Sandstone volcanic relationship with the Clackmannan Syncline and Kincardine rocks to the north.This paper concentrates onthe West Basin on its downthrown side to the south is an intriguing Ochil Fault, with the Clackmannan Syncline and Kincardine tectonicproblem. East of Stirling, theline of the fault is Basin immediately to the south (Figs 1 & 2). The East Ochil marked bya prominent W-E-trending escarpment,some Fault is considered to have had a different geological history 640 m high. This sharply divides thelow ground of the Forth and is not considered here in any detail. Valley, underlain by theCarboniferous sediments of the The KincardineBasin, which subsidedrapidly during Clackmannan Syncline in the hanging wall, from the uplands most of the Upper Carboniferous (Silesian), lay between the of the Ochil Hills, formed by a N-dipping block of resistant West Ochil Fault and an area of reduced Silesian deposition Lower OldRed Sandstone (broadly equivalent to Lower to the south of a line between the east end of the Campsie Devonian) volcanic and intrusive rocks in the footwall (Figs Fault and the Forth estuary (Fig. 1). To the west, the basin 1 & 2). was bounded by the pileof Dinantian basalts forming the Onthe BGS 150000 Alloamap (sheet 39E) the eastern part of the Clyde Plateau massif, and to the east, by Clackmannan Syncline is seen to plunge gently northwards a zone of low subsidenceand persistent Carboniferous towardsthe Ochil Fault, against which the strata on both vulcanicity known as the Bo'ness Line (Read 1988), which flanks of the syncline abut almost at right angles. A similar run S in a NNE-SSW direction through Bo'ness [NTOO82]. geometry seems to have characterized the Kincardine Basin, A gap between the bounding highs on the west and south which was theprecursor of theClackmannan Syncline sides of the basin, known as the Denny Gap, connects with throughout much of the Carboniferous, since here too the the Kilsyth Trough to the SW (Fig. 1). The obvious spatial isopachs on both flanks generally approach the Ochil Fault connectionbetween themajor Ochil Fault andthe at a high angle (Read 1988). On the Bouguer anomaly map Kincardine Basin inits hanging wall suggests a genetic of the Midland Valley, the Ochil Fault stands out clearly as relationshipbetween the two structures, and the main a W-E line which separates a gravity high over the Ochil purpose of this paper is to examine the evidence for such a Hills from a gravity low over the syncline to the south. The relationship by reviewing thetectonic evolution of both line of the fault is also distinguishable, but is less obvious, on structures. the aeromagnetic anomaly map (Browne et al. 1987, figs, 32 & 33). All the above features emphasise an abrupt break in structuraland stratigraphical continuity across the Ochil Previous views Fault. B. N. Peach of theGeological Survey, whomapped the A particularly striking feature of the fault is the marked Stirling areaand the district to the westin 1865-1872, variation in southerly downthrow, which decreases rapidly, recognized the Ochil Faultas one of the most important in terms of stratigraphic displacement, both to the west and, fractures in the Midland Valley. Early seismological studies lessmarkedly, tothe eastfrom the point of maximum (Davison 1924) suggested thatthe Ochil Fault might be a 573

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reverse fault, but Haldane (1927) used outcrop evidence to demonstrate that the fault plane dips south in three places. Francis et al. (1970) describedevidence from detailed surfacemapping, shallow boreholesand geophysical techniques demonstrating that the Ochil Fault can be traced westwardsfrom the western end of theescarpment at Causewayhead [NS794 9541 through poorly exposed ground and dies out at [NS665 9521, 13 km west of Stirling. They suggested that the fault may have been initiated before the UpperOld Red Sandstone was deposited,and that it probably controlled subsidence and deposition during much of the Carboniferous. TheKincardine Basin hasbeen studied by Readand co-workers (see Read & Dean 1982 and references therein) whoanalysed the quantitative relationshipsbetween net subsidence of the basin and a series of bulk lithological variables, together with numbers of cycles, numbers of coals, etc., for various Silesian stratigraphic divisions. Several contrasting tectonic models have been proposed. Gibbs (1987, fig. 15) postulatedthat the Ochil Fault isa reverse fault, dipping north, with an implied combination of dip-slip and strike-slip displacement. He consideredthe Kincardine Basin in thecontext of a complex system of listric duplexes, inwhich theintermediate and upper fault

Fig. 2. Simplified geological map of the central/eastern part of the Midland Valley. centred on the Ochil Fault. See Fig. 5 for a detailed structural map.

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leaves had developed as a 'carapace' to simpler strike-slip dextralstrike-slip component along the East Ochil Fault, shears in the basement, the upper leaf carrying the Silesian suggesting thata major basement fracture underlay and strata. Coward & Gibbs (1988) later postulated a series of linkedthe PaisleyRuck, theEast Campsie Fault and the majordextral strike-slip faults in thecentral part of the EastOchil Fault (Fig. l), activepossibly as early as the Midland Valley, linked by east-facing 'headwall' faults, one Brigantian,but concluded that most of the strike-slip of whichwas placed near Stirling. Dentith (1988)used movementoccurred at the end of the Silesian. A gently geophysical information to construct a model of the Ochil dipping detachment at depth beneath the Kincardine Basin Faultand Kincardine Basin in which the OchilFault dips wassuggested, following Dentith (1988), but placed at a steeply to the south, and a sub-horizontal detachmentlies at shallower level, possibly in subsurface evaporites within the depth below theKincardine Basin.This detachment was earlyDinantian Ballagan Formation. Both Dentith (1988) drawnat a refractor taken to be the base of theLower and Read (1989) suggested that the Ochil Fault may have Palaeozoic succession. been initiated as a late Caledonian sinistral strike-slip fault Haszeldine (1984) and Stedman (1988) proposed a W-E and later re-activated as a Carboniferous dip-slip fault. extensional stress system for the Midland Valley during the Inevaluating kinematic models of theDevonian- Namurian, whereas Dewey (1982) followed by Read (1988) Carboniferousperiod (e.g. see,most recently, Coward suggestedinstead thatDinantian WNW-ESE lithospheric 1993), the role of such an important structure as the Ochil extensionand rifting accompaniedby widespread alkali- Faultin the tectonic evolution of the MidlandValley is basalticvulcanicity was followed by Silesianthermal critical. This paper will therefore assess the evidence for the subsidenceupon which a component of dextralstrike-slip variousstages of movement of thefault, and for the motion was superimposed. movementdirection during each of thesestages, and Read (1989) discussed the evidence for a Carboniferous attempt to decide, for example, whether dextral strike-slip motionplayed a significant part in theCarboniferous movements. For this purpose the existing published data are evaluated,together with alarge amount of unpublished high-quality coalfield data from the hanging-wall area of the Ochil Fault.

The Kincardine Basin: sedimentological/stratigraphic n evidence The spatialrelationship between the Ochil Fault and the Kincardine Basin raises the problem of when the latter was initiated,and secondly of whetherthe basin is genetically related to the fault (in an early phase of its development), or whether the depositional system extended across the fault intothe present footwall.Much of the availablerelevant information relating to the hanging-wall stratigraphy (Fig. 3) can be summarised in the form of isopach and channel loci maps, which are presented in Fig. 4. These maps are based on a data set of deep, fully-cored boreholes. Most of these were drilled before 1975, and the relevant information has Plsan 1 Ltmoslono beenextracted from thearchive of non-confidential

C.hV Lm'lons boreholedata held by the BGS,Edinburgh. This upp., Him C0.l information has been supplemented by logs of more recent boreholes drilled by theNational Coal Board and British H CoalCorporation in connection with LongannetMine, enablingprevious knowledge to beextended northwards towards the West Ochil Fault, particularly for strata above the Upper Hirst Coal (Fig. 3). Throughoutthe period from thelate Devonian to the late Silesian, most sediments in the Kincardine Basin appear to have been derived from the north, principally from the metamorphicrocks of thesouthern Grampian Highlands (i.e. north of the Highland Boundary Fault, see Figs. 1 & 2), as is apparent from identifiable Dalradian clasts (Francis et al. 1970), backed by palaeocurrent vector evidence (Read & Johnson 1967).

Pre-Late Devonian Fig. 3. Simplified stratigraphic succession in the hanging wall of the Ochil Fault, showing the positions of horizons identified in the The section across the Kincardine Basin modelled from the paper. N-S MAVIS I1 seismicrefraction traverse (Dentith 1988)

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a.Location and scale b. Limestone Cod Formation above c. Llpper LimestoneFormation Black Metals Mari Band

d. Upper Linertaw, Formation aboM Cakny Limestom

Fig. 4. Isopach maps and channel locations illustrating the development of the Kincardine Basin during the Namurian. BL, Bo’ness Line: B, Blackfaulds borehole; WOF, West Ochil Fault; EOF, East Ochil Fault: isopachs in metres.

shows the ‘combined Lower Old Red Sandstone and Lower eastward-taperingwedge of uncompactableDinantian Palaeozoic’ succession thinning over a few kilometres from basalts, whichmay havesome bearing on thesubsequent more than 6 km to less than 2 km as it is traced southwards geometry of the basin. The marked eastward thickening of across the West Ochil Fault. This abrupt change is likely to the Lower Limestone Fm east of Stirling (Francis et al. 1970, indicatethat the thick pile of LowerOld RedSandstone fig. 17) indicates that the basin was already in existence by volcanics of the footwall is absent, or drastically reduced, in thelate Brigantian, but there is little information onthe the hanging wall, whichin turn suggests thatthe present central and eastern parts of the basin at this stage. Ochil Fault may coincide with a much older (pre-Upper Old RedSandstone) fault, possibly with astrike-slip displacement asproposed by Dentith (1988).Support for this Limestone Coal Formation (Pendleian, E,) hypothesiscomes from the suggestion of Conway et al. (1987) that the Strathmore Syncline, which lies NW of the Ochil Fault, is offset by some 20 km in the MAVIS I (N) Severalboreholes are core-proved over much of the refraction profile, implyingmid-Devoniana (Acadian) Limestone Coal Fm sequence throughout the Clackmannan sinistral strike-slip displacement. Syncline, while NationalCoal Board/British Coal seismic sectionsprovide supplementary data. Overall, the data density is sufficient to be able to define the broad thickness variations of the Formation. Blackfaulds borehole (Fig. 4b) Late Deuonian to Dinantian is particularly useful regarding the Ochil Fault relationships, lying within 1.5 km of the surface trace of the fault. The isopachmap of the Black MetalsMarine Band to Thesediments of theStratheden and Strathclyde Groups IndexLimestone interval (Fig. 3) showsan asymmetrical (formerlythe Upper Old Red Sandstone and Calciferous basin with gentlewestern and steepereastern flanks (Fig. Sandstone, respectively) and the overlying Dinantian basalts 4b).The basin is elongated N-S with theaxis running of the Clyde Plateau Volcanic Groupthin eastwards towards through Parklands borehole (E.2960 75; N. 692430) and the Stirling, atthe west end of the hanging wall of the West contours are cut off abruptly at the West Ochil Fault. It is Ochil Fault (Francis et al. 1970) (Fig. 2). Thus the western significant that the Black Metals to Index Limestone interval flank of the Kincardine Basin is probably underlain by an (c. 285 m) and gross lithology at Blackfauldsis little different

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tothat at, say, Kincardine Bridge borehole (E.291655; the flanks. Howeverthere is no evidence of continued N.687150) some 8.3 km further south. Trend-surfaceanalysis vulcanicity along the Bo'ness Line (Read 1988). The isopach (Read & Dean 1967, fig. 4) shows that the total thickness of map of the formation (Fig. 4e) shows some similarities with coal increases fairly steadily eastwards across the Kincardine that of theUpper Limestone Fm in thatthe main Basin, but decreases abruptly close to the Bo'ness Line. A depocentre is still located near the Gartarry Toll borehole; similar abrupt decrease is seen in the overall proportion of however the contours now reveal a complete basin shape, no coal in the succession. According to Read and Dean (1967) longer truncated by the West Ochil Fault, and with no hint the loci of channels entering the basin from the NE tend to of asymmetry. The contour closures between the depocentre follow a NE-SW depression, the Tulliallan Trough, whereas and the West Ochil Fault lie up to 6 km from the fault (Fig. another set of chan nels entering from the NW swing round 4e). the west flank of the basin to join the first set in the SW of The beginning of this period was marked by uplift and thebasin nearthe Denny Gap (Fig. 4f). Duringthe erosion in the Grampian Highlands, resulting in a surge of deposition of the Limestone Coal Fm, active alkali-basaltic coarse-grained siliciclastic sediments from the north and the vulcanicity occurred along the Bo'ness Line, which is aligned appearance of a new and distinctive suite of heavy minerals with, and corresponds approximately to, the steeper eastern (Muir 1963). Muir identifies differentheavy mineral flank of the basin (Read 1988). contributions to thebasal Passage Fm sandstones in the MidlandValley from the NW (C-zircon)and the NE (K-zircon), and also identifies the same (Highland) sources Upper Limestone Formation (Pendleian-Arnsbergian, of supply near the top of the formation. This evidence was E1321 supported by the palaeochannel mapping of Read & Dean (1982, fig. 9). Uplift also occurred in the western part of the Over 30 modernand largelycored boreholes with Midland Valley where the main graben floor was tilted up supportinggeophysical logs andsurface reflection seismic anderoded (see Read 1989). Howeverthere is little (60km) acquired overthe last twentyyears in the wider indication of any local structural control in the Kincardine hanging-wall area, together with many earlier cored holes, Basinduring this period(other than theresidual provide an excellent data setdown to the level of the Upper development of the basin itself). HirstCoal (Fig. 3). As with theLimestone Coal Fm, contours of intervalswithin the Upper Limestone Fm are aligned N-S and meet the Ocil Fault at a high angle (Fig. 4c). The isopach map alsoshows a westward shift in the Coal Measures (Westphalian) position of the axis to about the Gartarry Toll borehole [NS The preserved Coal Measures of the Kincardine Basin are 9319131 some 2 km east of Clackmannan. The basinnow thought to belong almost entirely to the Scottish Lower and appears much more symmetricalbut continues to plunge Middle Coal Measures, and thus to be of largely Langsettian north. and Duckmantian age (i.e. Westphalian A and B, Francis et A wealth of data on major horizonintervals has been al. 1970, Read 1988). Post-Carboniferous erosion has largely examinedwithin the Clackmannan Syncline. None of the confined their present outcrop to the axis of the present-day mappedintervals shows any apparent relationship to the ClackmannanSyncline, where the youngest strata are West Ochil Fault in terms of either isopach pattern (e.g. Fig. preserved. Because the area of preserved Coal Measures is 4d) or seam quality and splitting. Indeed lithofacies mapping so small, itis onlypossible to drawisopachs of narrow usingcored borehole data in the nearfieldhanging wall stratigraphic intervals. However both the interval between indicates that one sand body, above and close to the Upper the Glenfuir and Mill Coals, and that between the Mill and Hirst Coal, trends at about 045" near the fault. In view of its Coalsnaughton Main Coal (all Langsettian) show an overall size and assumed derivation from the NE, this channel must tendency to thinN-wards towards the West Ochil Fault haveextended across the footwall for several kilometres. (Francis 1956, fig. 9a, b), repeating the pattern established in The thickmultistorey sandstones thatare typical of the the Passage Fm. formation are thought to be concentrated in the Denny Gap The sedimentology of theCoal Measures in the (Fig. l), with successive sandstones stacked vertically, sand Kincardine Basin is not well known and most information is having been transported westwards through this gap into the from old shallow boreholes and mine plans. However one adjacent Kilsyth Trough (Read 1988). significant palaeochannel is shown on the published 1:10560 Volcanic activity seemsto havepersisted along the sheet NS89NE eroding the Alloa Splint coal (Langsettian) Bo'nessLine, albeit at reduceda level, duringUpper over a width of around 0.3 km, and aligned NNW-SSE at a Limestone Fm deposition.Basin-centre vulcanism is also high angle to the West Ochil Fault trace, only 2 km to the present, assumed to be related to the SSW-NNE-trending north.Later Westphalian (Bolsovian; ?Westphalian D) Coalsnaughton Fault (see later), suggesting that it, too, may sedimentsare poorlyknown and are mainly redbeds, have been an active extensional (or transtensional) fault at particularly the Devon Red Sandstone (Francis et al. 1970). this time. Significantly, this fault runssub-parallel tothe Bo'ness Line. Summary Passage Formation (later Namurian, E2-G1,plus The KincardineBasin is defined andcharacterised by earliest Westphalian) several distinctive features. Because of post-Carboniferous erosion, complete borehole (1) Formation thicknesses increase significantly towards sections through the Passage Fm are confined to the central the basin centre, sometimes by over100% compared with part of the Clackmannan Syncline, and less is known about the basin margins.

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(2) The maindepositional basin occupiedbroadly the The Ochil Fault and associated structures same area, at least from Dinantian times to Duckmantian Structures directly associated with the Ochil Fault are here Coal Measures. considered to include all majorstructural elements lying (3) Rates of change of thickness are greatest along well generally within 10 km of the Ochil Fault, even though some defined ‘hinges’ just within the definable depositional basin; features may representolder or youngertectonic phases. this effectis particularly prominent in the Limestone Coal Theimmediate hanging wallof the Ochil Faulthas been Formation. investigated by NationalCoal Board/British Coal seismic (4) The isopachs generally,and the hinge zone in reflection and to a lesser extent by hydrocarbon exploration particular,bear no obviousgeometric relationship to the seismic. Some seismic lines extend up to, or just across, the OchilFault, but the hinge appearsto be aligned N-S or surfacetrace of thefault. The seismic lines are NNE-SSW nearto, and roughly parallel to,the Bo’ness supplemented by a number of modern boreholes, mainly to Line. the Upper Hirst Seam in the Upper Limestone Fm (Fig. 3). (5) Oneimportant lithologicalchange, at least in the Most of these holes are extensively cored and petrophysi- Limestone Coal Fm and Upper Limestone Fm, is the often cally logged; they provide immediate hanging-wall informa- rapid increase in coal thickness from the margin, across the tion, particularly in terms of bedding dips and incidence of hinge, and into the basin centre. This is far more prominent minor faults. thanvariations or percentagechanges in sandstone Themodern exploration data in thelater Namurian thicknessesand is assessed fromcomparisons between sequence are complemented by extensive structural proving mineworkings in thinnercoals along the west andeast by earlier mineworkings in the Westphalian seams, margins, and the significantly thicker coals proved in deep particularly in the Alva Syncline (Fig. 5). Although no bores towards the depocentre. longer accessible, these mine workings provide crucial data (6) Theroughly circular shape of the basin during on fault-adjacent strains, and the style and spacing of minor Passage Fm and Coal Measures deposition, along with the faults(up to 30 m displacement);further, some of the lack of evidencefor structural control, suggests thatthe workings lie within 100 m of theprojected subsurface basin development was controlled by a different mechanism position of theOchil Fault (as derived from surface and at this stage. seismic observationsdescribed below) andtherefore

Fig. 5. Main structural elements of the area adjacent to the Ochil Fault. ACCF, Abbey Craig Central Fault; LF. Langfauld Fault; SF. Sheardale Fault. Box indicates area of Fig. 6.

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contribute to assessment of its fault plane dip in the upper that the NW or NE faults in the footwall are more likely to sequences. These mine workings extend laterally up to 2 km be responsible for the seismic activity. away from the Ochil Fault, over a vertical range of around 400 m. Structure of the hanging wall

The Ochil Fault Folds. The published 1:10560 mapsNS89NE, NS99NW, NS99NE (Geological Survey, editions of 1959), summarized on the published 150000 sheet 39E (Alloa) illustrate three Surface trace. The Ochil Fault has been traced eastwards by small E-W synclines preserving Westphalian Coal Measures boreholesand geophysics from near Boquan Home Farm in the immediate hanging-wall of the Ochil Fault, the largest [NS670 9521 east of Kippen,where its throwseems to be of which is the Alva Syncline (Fig. S). The cross sections of negligible, to apoint immediately north of Stirling, from Fig. 6, through the Alva Syncline, illustrate a steepening (up where it can easily be traced at the surface along the Ochil to 20" overa distance of about 2 km) of thenortherly escarpment (Fig. S). Here,the fault divides into two beddingdips which characterisethe coalfield overall for branches which re-unite after 3-4 km (Francis et al. 1970). some 10 km southfrom the probable area of maximum Thefault then runs east-northeastwards along the foot of throw of the OchilFault. The mineworkingsand seismic the prominent escarpment past Alva, Tillicoultry and Dollar data show that the incidence of minor faults is no greater to a point [NS 993 9941 about 3 km ENE of Dollar (Haldane thanthat typical of many UK coalfields, andthere is no 1927; Francis et al. 1970) where the Arndean Fault branches increase in numbers of minor faults on the approach to the southeastwards,appearing totake upa large part of the Ochil Fault. The northern, steep, south-dipping limb of the throw. The Arndean Fault may actually represent the con- AlvaSyncline is interpretedas normal drag againstthe tinuation of the mainstructure. East of this junction,the Ochil Fault and the northerly dip as reverse drag (Gibson et Ochilescarpment becomes lower and less steep, butcon- al. 1989). tinues east-northeastwards, whereas the named Ochil Fault itself swings southeastwards in two southward concave arcs E-W faults. A series of south-dipping major normal faults to a point [NT 149 9641 18 km east of Dollar, from where it with displacements of up to hundreds of metres characterize continueseast-northeastwards with amuch reduced but the coalfieldsouth of the OchilFault (Figs 5 & 6). These variable throw. Seen from around 20 km to the south, the faults are usually essentially planar in overall cross section present topographic form of the West Ochil Fault footwall where proved, although in plan the fault traces sometimes hills showsa smooth arc, the highestpoint of which is show some concavity towards the hanging-wall. They typi- roughly coincident with the areaof likely maximum throw at cally have moderate fault plane dips of around 45" S, and Alva. have little or no effect on mappable sedimentary features. Each of them has prominent reverse drag, particularlyin the hanging wall. Although Francis & Walker (1987) suggested Fault atdepth. Four crosssections, originally drawn on a that some of the E-W faults acted as growth faults, none in scale of 1:5000, illustrate the area of maximum throw, and this area can be shown to have significantly affected sedim- integrate the key information in the immediate hanging wall entation, at least in the upper Namurian and Westphalian, (Fig. 6). Althoughmasked by superficialdeposits on each with the possible exception of the Langfauld Fault (Fig. S) line of crosssection, theoutcrop of the OchilFault is wherecertain Namurian coals show significant thickness essentially well controlled. Outcrop and mining data indicate variations in bothhanging wall andfootwall, along zones a fault plane dip varying from 60" to 70" to the south in the coincident with the faulted extent. However, these authors upper 1 km,and the migrated positions of the available alsopresent evidence that Namurian alkali-dolerites were seismic reflectors at depth indicate a continuing steep fault emplacedbetween existing faults and used them as risers. plane dip (around 65" S) to at least 2.5 km depth, supporting Given the context of large scale extension during the main the original interpretation of a southerly dip (Haldane 1927) phase of Ochil Fault movement (see below), it is considercd based on outcrop descriptions. These observations, together likely that these W-E faults were rotated by some 10" to 15" with the descriptions of the footwall and hanging-wall geol- (i.e. from c. 60" to c.45") as the main fault developed. ogy which follow,indicate a large south-dipping fault on which the main displacement was essentially normal. NW-SE faults. Twolarge NW-SE faults meet the Ochil Fault east of Tillicoultry [NS920 9701 (Fig. S), viz. the Shear- Seismic activity. Thearea of the OchilFault has ex- dale and Arndean Faults (Francis et al. 1970, p. 254). It is perienced recent seismicity, and this has led to speculation considered likely thatthese are splays of the OchilFault, about its structureat depth. The seismicityhas typically contributing to the eastern breakup from its maximum dis- been in swarms of minor felt events, interspersed with quiet placement. The SheardaleFault has a maximum throw of periods of several decades (see Francis et al. 1970). Most felt around 200 m, the Arndean of probably over 1000 m (Fran- eventshave been located north of the OchilFault rather cis et al. 1970). thanalong it,despite the distribution of settlements/ observers being mainly in the valley to the south, along the NE-SW to NNE-SSW faults. Faults on this trend are gene- hanging wall, and Haldane (1927) noted other faults in the rally better developed in the Westphalian, rather than the area whichcould have been active, in particular,some Namurianpart of thesequence. Most are small, with less prominentNW-SE and SW-NE faults in the OchilFault than 5 m vertical throw at maximum. Two fault zones are footwall. In the light of theevidence that the Ochil Fault particularly prominent; the main zone constitutes the Coals- dipssouthwards, to at least 2.5 km depth, it is concluded naughton Fault (Francis et al. 1970, p.253) and the other

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Fig. 6. Simplified cross-sections of the immediate hanging-wall of the Ochil Fault. UH, Upper Hirst Coal; AS. Alloa Splint Coal ; LY, Lower Nine Feet Coal; U5, Upper Five Feet Coal; CNM. Coalsnaughton Main Coal: TF, Two Foot Coal: AC. Alloa Cherry Coal .

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just east of it which dips into the Coalsnaughton Fault in a (3) There is a variety of faultorientations. The most probableantithetic relationship. The CoalsnaughtonFault prominent faults trend E-W, subparallel to the West Ochil achieves a maximum throw of around50m in the West Fault, with fault plane dips generally to the south, in some OchilFault hanging-wall but it is laterallyextensive, and cases less than 45". Other orientations recall the prominent traceable for many km as a zone, although compartmental- NE-SW and NW-SE faults in the footwall of the WOF to ised into segments by the W-E faults. the north. Although the Coalsnaughton Fault itself has no surface (4) The faultintensity, in terms of maximum throws, exposures. and proving mineworkings are long-abandoned, faulttrace lengths and spacing, is low, except within recent mineworkings in the Upper Hirst Seam intersect the particularfault zones; this observation is qualitative,and parallelantithetic fault zone, the main element of which based on visual assessment, as the area is too small for a strikesat 020" with a steep (75" W)fault plane dip, and rigorousstatistical study. However it is assumed thatthe exhibitssubhorizontal slickensides. The overall pattern of fault recording by mining surveyors was comprehensive at fractures proved in the Upper Hirst shows a fault zone with their level of detection, which would have been to the high overlappingfault segments, the arrangement of which standard required by UK mining legislation; where coal was indicates sinistral strike slip. worked, it is unlikely thatany throw greater than 45 cm Other largefaults on this NE/NNEtrend are would have been unrecorded in these relatively thin seams. represented by the northeastern end of the Campsie Fault There appears to be no straightforward relationshipbetween and the Carnock Fault, both of which lie along-strike of the the numbers of larger and smaller faults as found in fault Coalsnaughton Fault to the SW (Fig. 5). Faults on this trend populationstudies in the English EastPennine coalfield are also present in the Lower Devonian igneous complex in (Watterson et al. 1966). It is concludedthat the principal theOchil Fault footwall, one of which alsolies faults (i.e. the West Ochil Fault and the faults in its wider approximately along-strike of the Coalsnaughton Fault zone hanging wallwith throws of >loom) have generally been (see below). Differential subsidence to the SE seems to have re-used,rather than further extension promoting new taken place across the line of the Campsie Fault during de fractures.More specifically, it would appearthat the position of the upper part of the Limestone Coal Fm. It is mechanics of themain-phase faulting required movement therefore possible that a major large fault on this trend is mainly on existingfractures lying at ahigh angle tothe presentat depth below the coalfield inthe hanging wall, contemporary minimumhorizontal stress, which maythus with the known lesser faults inheriting the trend. be inferred to have been oriented NNW-SSE to N-S. The possibility that the Coalsnaughton Fault continues to depth is suggestedpartly by thelateral persistence of the Quartz-doleriteintrusions. Boththe Ochil Fault and the zone,but also by thelikelihood that it was a locus for ArndeanFault are invaded by quartzdolerite intrusions volcanic activity in Upper Limestone Fm times. A series of (Fig. 7) known to beof late or post-Carboniferous age, since agglomerates, together with basalts, is known at Tillicoultry adjacent Westphalian coals at Dollar show enhanced rank- borehole (E.292 764 N.696 528), mainly between the Calmy ing . Other,small, quartz dolerite sills are knownin the and Plean 1 Limestones (Fig. 3) indicating a vent probably Arndean hanging-wall (published 1:lO 560 Geological within 0.5 km of theborehole, which intersectsthis fault Survey sheet NS99NE) and in the Castlebeg and Arndean aroundthe basal Westphalian horizon. Other nearby boreholes, indicating invasion of the hanging-wall to at least boreholes along the line of theCoalsnaughton Fault show 500m away fromthe faultplane, at horizons around the tuffs within the Upper Limestone Fm, probably thickening Castlecary Limestone (Fig. 3), which gives some indication towardsthe fault and suggesting a line of small vents. of the extent of the contemporary hanging-wall strain. Volcanism of thisage onthe eastern margin of the Clackmannan Syncline towards the Bo'ness Line has already Structure of the footwall been noted. The Coalsnaughton Fault may therefore represent a long The footwall geology derivedfrom the published 1:10560 established system, perhapsextending (in theLower maps in the area under discussion consists of a thick pile of Devonianbasement) through both the hanging wall and footwall of the Ochil Fault, andwhich may have a significant I I outcrop quartz - dole .-.- strike-slip element, at least in part of its development. emplacements along

Fault patterns in the Alva Syncline. Mineworkings, now in- accessible, in the Westphalian seams of the Alva Syncline provide records of the fault patterns there, from which the

following generalizations may be made. (?quartz - doleme) quartz . dolerlte (1) All the faults are recorded by miningsurveyors as sdls around + ~ast~ecaryLimestone + normal. The possibility of an unrecorded strike-slip element horlzon, Ihkely limtt may beaddressed indirectly by studyingthe fault trace pattern, and by consideration of the throw distribution after I! themethod of Rip pon(1985). Using bothcriteria, it is concluded that the style is essentially normal, although there i' Mldlands Valley S111 (quartz dolente) 3 remainsthe possibility of some strike-slip element, at depth. details uncertam: locally .=I lncreasmg devolstollsatlon of LCF especially on NE-trending faults. coals wlth depth 300 9; (2) Few of the faults in the Alva Syncline have surveyed I verticaldisplacements greater than 10m in thesequence Fig. 7. Late Carboniferous/Permian igneous intrusions in the Ochil down to Upper Hirst Coal (Fig. 3) and most are less than Fault/Kincardine Basin area. A, Arndean borehole: C, Castlebeg 5 m. borehole.

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Lower Old Red Sandstone volcanics with minor intercalated partly due to the difficulty of distinguishing such a structure sediments. These strata have an essentially NE-SW regional from the pre-existing northwestwards dips of the Lower Old strike,and dip at 10-20" NW.This regional structure is Red Sandstone. clearly pre-UpperOld Red Sandstone in age,since it is The possibility was considered that one or both of the truncated by theunconformable, gentlysouth-dipping, refractor horizons identified on the MAVIS I1 south profile Upper Old Red Sandstone at the western termination of the (Fig. 8)acted as detachment horizons for the extension, West Ochil Fault. Only immediately north of the fault itself following Dentith (1988).Refractor 1 is identified by does the dip appear to relate to movement on the fault, in a Dentith (1988) as the base of the Upper Old Red Sandstone, zone up to 500 m wide, where the strike is parallel to the while refractor 2 is identified with the base of the Palaeozoic faulttrace, and the dip appears to become shallower to (or top of the crystalline basement). However the fact that near-horizontal,suggesting thepresence of poorly a the fault plane has a uniform dip of about 65" to at least developed footwall anticline. This structure corresponds to 2.5 km,coupled with the relatively shallow depth of the thezone of maximumthrow and the highestpart of the reflectors (the lower reflector lies at a depth of only about escarpment. 4 km)suggests that the listric model is probably The faults in the West Ochil Fault footwall consist of two inappropriate in thepresent case. Amore appropriate sets: a prominent set trending NW-SE, locally bending into modelfor the WestOchil Fault is considered to be the a N-S orientation,and a numericallyminor NE-SW set flexural cantilever model (Kusznir & Egan 1989) from which (Fig. 5). In addition, north of Dollar, an ENE-WSW fault footwall uplift andbasin geometries for variouscombina- parallels the main fault, and further east, three E-W faults tions of flexural rigidity andamount of extensioncan be occur,also parallel tothe mainfault. The NW- to calculated (see below). north-trending set, effectively cross-faults, are concentrated atthe western end of the WestOchil Fault. Thesefaults contribute to the rapidwestwards decrease in throw referred Discussion toearlier, and may perhaps reflect footwalldeformation during uplift, accommodatingincreased footwall uplift The initiation of the Ochil Fault towards the east. Twolarge NE-SWfaults run northeastwards from The abrupt change in thickness of layer 2 (Lower Old Red Tillicoultry andDollar respectively. One of thesefaults Sandstone/Lower Palaeozoic) on the MAVIS I (N) seismic (probably the eastern one, since it has the larger throw) may refraction profile (Conway et al. 1987) from 6 km to 2 km representthe northward continuation of theprominent acrossthe Ochil Fault is stronglysuggestive of anearly NE-SWCoalsnaughton fault zone described in the phase of Devonian activity . The western continuation of the hanging-wall(see above)and interpreted aspossiblea present fault is overstepped by theUpper Old Red basement lineament. Sandstone, and it is probable therefore, that the precursor of Jointdirections in the LowerDevonian volcanic rocks the OchilFault was initiated in lower to mid-Devonian cluster around 045" and 135", with a third set parallel to the times,possibly as part of the sinistral strike-slip tectonic WestOchil Fault only near the fault trace.This contrasts regime then operating in the Midland Valley (e.g. see Bluck with a pervasive E-W coal cleat trend in the hanging wall. 1984;Soper & Hutton 1984; Hutton 1987) (Fig. 9a). The The contrast in bothjoint and fault directionsnorth and southwardcontinuation of thethick Lower Devonian south of the WestOchil Fault strongly suggests thatthe volcanic pile presently exposed in the footwall may thus now footwall structurepredominantly reflects mucha older lie buried beneath the eastern flank of the Kincardine Basin. deformation(essentially pre-UpperOld Red Sandstone), However field evidencefrom the volcanic sequence itself and that the effects of the end-Carboniferous movement are (pyroclasticbeds become thicker, coarser and more much more obvious in the hanging wall than in the footwall. numerous towards the West Ochil Fault) suggests possible venting along the line of the fault, in which case the abrupt change in thicknessacross the fault may be(wholly or Net throw and displacement on the West Ochil Fault partly) original. The totalnet throw on the present West Ochil Fault is difficult to quantifyprecisely, because therelevant Relationship between the Kincardine Basin and the stratigraphy in the footwall ismissing. However,assuming Ochil Fault thatat least an attenuated(halved?) Namurian to Westphaliansequence extended northwards onto the At first sight, it may be tempting to visualize the Kincardine footwall(see later),and adding the present topographic Basin as a simple case of a hanging-wall basin produced by relief of 640m,a minimumthrow of about 2100m is Silesian movements on the West Ochil Fault. However there indicatedatthe point of maximumpresent uplift. In are several problems with this interpretation. addition,an unknown thickness of UpperDevonian and (1) The basin-form contourpatterns reflect thickness Dinantian beds (possibly up to a further 2000m) may also variations in the coals, shales and limestones, which are the have been removed from the footwall, although, as indicated lithofacies most sensitive to subsidence and other bathymet- later, the footwall may have been emergent for at least part ric factors,and these illustrate a gradually evolving basin of this period. formthroughout theNamurian. However themain The reverse drag or rollover structure in the hanging wall movement onthe WestOchil Fault as revealed by the is indicated by thenorthward plunging region of the hanging-wallstructure is lateCarboniferous/early Permian Clackmannansyncline, which extends for about 9.5 km in age(see below), and therefore there can be no simple southwardstowards the River Forth (see Fig. 8)and relationship between the hanging-wall strain relating to this steepensat its northern end into the AlvaSyncline. The eventand the formation and evolution of the(earlier) lack of anobvious northwards tilt in the footwallmay be Kincardine Basin.

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N S

I------\ I

Fig. 8. (a) Simplified cross-section from the Ochil Fault to the Firth of Forth, based on the MAVIS I1 seismic restored length 14.5 km refraction profile (Dentith 1988), with additional interpretation by the present authors, indicating the inferred geometry of the hanging wall. L ORS. Lower Old Red Sandstone (Lower Devonian): LCF, Limestone Coal Formation: ULF, Upper Limestone Formation; PF, Passage formation: the Alva Syncline contains Westphalian Coal Measures. (b) Restored section to base Passage Formation.

(2) The isopach patterns of Fig. 4 which describethe parallel to it as would be expected were the two structures shape of the basin during the Silesian show a rather narrow simply related. basinstrongly elongated ona N-S trend,approximately (3) The isopach patterns are not centred adjacent to the perpendicular to that of the West Ochil Fault, rather than sector of present maximum throw on the West Ochil Fault,

Fig. 9. Kinematic interpretation of the tectonic setting of the Ochil Fault and Bo’ness Line from Devonian to late Carboniferous/early Permian times. Dark half-arrows show strike slip movement: light arrows show extension/compression directions. (a) Lower Devonian; (b) Dinantian/lowet Namurian: (c) late Carboniferous: (d) end-Carboniferous/early Permian. See Fig. 1 for explanation of ornament and abbreviations (CL, Glasgow: ST, Stirl- ing: ED, Edinburgh: HBF, Highland Boundary Fault: WOF, West Ochil Fault: EOF. East Ochil Fault; PR, Paisley Ruck: CF, Campsie Fault: IGF, Inchgotrick Fault: KLF, Kerse Loch Fault: SUF, Southern Upland Fault: BL, Bo’ness Line; CS, Clackmannan Syn- cline; FSS, Falkirk-Stane Syncline: BA, Burntisland Anticline; LS, Leven Syn- cline; MB, Mauchline Basin).

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which lies between 3 km and 10 km further west. Moreover scenario is componenta of intra-CarboniferousW-E the basin centre migrates westwards with time, rather than extensionactivating the Bo’ness Line (see Fig. 9b).W-E southwards,wouldas expected be withnormal a extension is compatible with therecent Coward (1993) hanging-wallbasin, andthe basin centre in thelater model of the tectonic evolution of the Midland Valley, in Namurian, as shown by the closed isopach patterns, lies at which sinistral strike-slip is visualizedcontinuingas some distance from the fault. throughoutthe Carboniferous to bereplaced by dextral (4) The lack of marginal facies and the presence of some strike-slip onlyduring the end-Carboniferous movements, channel axes aligned at very high angles to the West Ochil which resulted in inversion of many of thepreviously Fault suggest that the footwall was not emergent or active formedstructures. The possibility of dextral strike-slip for much of the Namurian. On the other hand the channel movements in the MidlandValley during Dinantian/ patterns in Fig. 4f couldbe interpreted asresulting from Namuriantimes is notprecluded, but the evidence cited footwall uplift over the eastern sector of the fault; ie. the above offers no positive support for it. According to Coward channelsmay have been diverted around the ends of an (1993) theearly Carboniferous tectonics of Europe uplifted (though not necessarily emergent) fault segment in continuedto be dominated by thepattern established a similar manner to the major fans of the Sperchios Basin duringthe Devonian, with a sinistral strike-slip regime in describedby Eliet & Gawthorpe (1995). If the uplifted northernBritain alongthe major NE-SW Caledonian segmentwere submerged, it wouldexplain the lack of lineaments.A sinistral strike-slip regime in theMidland footwall-derived marginal facies. Valley,as implied by Coward (1993, fig. 8) wouldcreate The critical factor is thegeometry of the basin, which W-Eextension across the NNE-SSW Bo’ness Line, as indicatesa relationship with the NNE-SSW Bo’ness Line required by the above model. rather than with the West Ochil Fault. It is suggested that the basinwas initiated by extensionon a pre-Brigantian normalfault along the Bo’ness Line, with its known Namurian 1Westphalian evolution of the basin concentration of alkali-basaltic vulcanicity(Fig. 9b). There is The stratigraphic/sedimentological evidence from the early no trace of the continuation of the Bo’ness lineament into NamurianLimestone Coal and Upper Limestone Forma- the Ochil Fault footwall (where it should be clearly visible tionssuggests that the basin was rapidlysubsiding at this because of the lack of Carboniferouscover) and it is time. As explained above, it is believed that this subsidence therefore concluded that the basin may have been bounded was dueto a continuation of activeextension along the along its northern margin, roughly along the present outcrop Bo’ness line onits eastern flank, whichwas acontinuing of theWest Ochil Fault, during the period of active locus of volcanic activity. The geometry of the channels extension (i.e. Dinantian to earlyNamurian times). It is suggests that the drainage pattern may have been influenced further suggested that the precursor Ochil Fault during this by a topographic high in the region of the present maximum periodacted as a sidewall fault confiningactive extension uplift area of thefootwall of theWest Ochil Fault for at anddepression to itssouth side. Thepresence of large least part of this period. Since there is no surface fault along channels during at least the Limestone Coal and Passage Fm the Bo’ness line cuttingthe Namurian rocks of the times indicate that the area of deposition, if not the basin as LimestoneCoal and Upper Limestone Formations, the such, certainly extended northwards well beyond the West active faulting postulated for the Dinantian may have been Ochil Fault. replaced by a monoclinal structure. Althoughdetailed sedimentological information is lackingfrom the CoalMeasures, the general pattern of Dinantian extension in the Midland Valley thicknessvariation in the Passage Fm andCoal Measures indicates a continuation of the basin to at least Westphalian The prevalence of extensionalbasins within the Midland B times. The indications from the depocentre and channel Valleygenerally during the Dinantian, together with the locations provide no evidence of tectonic activity either on adjacent alkaline vulcanicity of the Clyde Plateau lavas to theOchil Fault or onthe Bo’ness Line. Nor is the basin the west, suggest that the Kincardine Basin was initiated by shape, in the Passage Fm (Fig. 4e)and theWestphalian, an active extensional fault at this time. It is one of a number characteristic of thetype of basincontrolled by active of UpperCarboniferous depositional basins trending faulting. Thethermal subsidence produce froma roughly N-S within the MidlandValley and including in particular, the Leven-Midlothian Basin to the east (see Fig. Dinantian/Namurian extensionalphase would generatea 1). The initiation and early development of these basins is much larger-scale basin, more comparable with the Midland Valleyas whole,a and although this mechanismmust attributed by Stedman(1988) to W-Eextension and by Leeder (1988) to N-S extension.Several authors point to contribute to the continuation of regional subsidence during the later Silesian, the continuation of the local Kincardine the importance of dextral strike-slip movements (see Read 1989, Dentith & Hall 1990, Burn 1990 and Dailly 1990). The basin shape during this period should probably be attributed to theto effects sedimentloading, accentuated by WSW-ESEtrend of aseries of linearvent systems and of differential compaction of the greater thickness of coal and multipledykes which fed the Campsie volcanics might shale lithologies in the basin. suggesta component of NNW-SSEextension during the early Dinantian. Howeverfrom our evaluation of theKincardine Basin Lale Carboniferous inversion itself, there is no evidence of contemporary N-S extension across the OchilFault (e.g. all thecontemporary igneous The obvious N-S foldstructures in this part of the Midland activity is relatedtheto NNE-SSWBo’ness and Valley (Fig. 2), includingClackmannanthe Syncline Coalsnaughtonlineaments) and that amore likely tectonictogether with theneighbouring Burntisland Anticline and

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Leven Syncline, were formed by E-W compression during a therefore that 640 m of footwall uplift is of the right order of phase of lateCarboniferous inversion (e.g. seeCoward magnitude for the estimated amount of extension. 1993)during which many of thenormal faults produced The marked difference in structural pattern between the during the Dinantian extension may have been re-activated LowerDevonian footwall and the Carboniferous hanging in reversemode (Fig. 9c). It is significant thatthe many wallmay beexplained by thedevelopment of theformer small NNE-SSW folds in the hanging wall of the Ochil Fault during the Devonian or Dinantian tectonic phases and the are concentrated on the east flank of the Kincardine Basin, latterduring the end-Carboniferous phase. Thus the east of, and parallel to, the Bo’ness Line (see Fig. S), where prominentNW-SE to NNW-SSEfault/joint system may theCarboniferous cover was probablymuch reduced in reflect a transverse Devonian ‘Caledonoid’ extensional joint thickness. pattern, whereas the dominant E-W faults and coal cleat in the hanging wall reflect theend-Carboniferous N-S extension.A N-S extensiondirection (i.e. approximately pure dip-slip) duringthe end-Carboniferous phase is End-Carboniferous movement on the Ochil Fault consistent with the dextral strike-slip model for the Midland Valley as a whole, as indicated, e.g. by Coward (1993) (see The presentOchil Fault, together with the suite of E-W Fig. 9d). faults in its hanging-wall and the related strain effects (e.g. coalcleat) is the result of animportant phase of N-S extension (Fig. 9d).This event seems certain to have Was the Clackmannan syncline a Permian (or Permo- occurred during late Carboniferous to early Permian times, Triassic) basin? coincidingwith theemplacement of thewidespread quartz-doleriteintrusive suite, which is locallyrepresented An obvious question arising outof the above analysis,which by sheet-like bodies along the line of the fault itself and of datesthe main visible fault displacement toaround the some of its splays, and which yield a late Carboniferous age Carboniferous-Permianboundary, is whetherthe hanging- of 303 f 5 Ma (Foster & Warrington 198.5). It is assumed wall basin wasfilled by Permian (or evenPermo-Triassic) thatthe emplacement of theintrusions coincideswith the sediments, now removed by erosion. A local analogy would extensionalmovements and that the magma would have be with the MauchlineBasin in Ayrshire (Fig. l), where facilitated movement on the West Ochil and Arndean Fault about 610 m of Permian volcanic rocks and continental red planes, thuspartly explaining the lack of additional beds overlie the Westphalian sequence (Eyles et al. 1949). fracturing in the hanging wall proximal to the WestOchil This structure is bounded by major lineaments. TheNE-SW Fault.The coalcleat parallel tothe W-Efaults in the to ENE-WSWInchgotrick Fault to thenorth and the hanging wall are carbonate mineralized, probably reflecting NE-SWKerse Loch Fault to thesouth which mayhave fluid migration during this extensional phase. acted as extensional faults controlling the basin formation. The hanging-wall structureincludes the AlvaSyncline It should be noted that Mykura (1967) believed that the andthe larger-scale 9.5 km regional tilt formed by the Mauchline Basinwas controlled by aNW-SE lineament northward plunge of the Clackmannan Syncline. The extent (seealso McLean 1978; Francis 1978; Russell & Smythe and size of this combinedstructure is consistentwith a 1978). Howeverdykes with petrological affinities tothe vertical displacement of the hanging-wall of about 1460 m. Permian Mauchline lavas have an E-W trend, as have the Assumingthat at least anattenuated Namurian to distinctive late-Carboniferous tholeiitic basalts(Eyles et a1 Westphaliansequence extended northwards onto the 1949, Monro, in press), suggesting that N-S extension may footwall, and adding the footwall uplift, estimated from the well have been responsible for the Mauchline Basin also. presenttopographic relief onthe fault scarp to be about It is unlikely that the West Ochil Fault was re-activated 640 m, a minimum throw of about 2100 m is estimated at the duringmore recent movements (e.g. duringTertiary point of maximumpresent uplift. Therestored section extension)since there is no evidence of fracturing of the acrossthe hanging wall (Fig. 8) indicatesan extension of Permo-Carboniferous intrusions. 1.75 km (12%). Overall,the post-Carboniferous burial history of the Footwall uplift and basin geometry can be calculated for OchilFault/Kincardine Basin area is unlikely to have variouscombinations of amount of extensionand flexural involved burial depths greater than thosetypical of most UK rigidity using the flexuralcantilever model of Kusznir & coalfields, as assessed from coal ranks where unmodified by Egan (1989). Thisfault model takes into account lower igneous effects; theseare comparable, for example, with lithosphere extension and its resulting thermal effects, and those of thesouthern margin of thePennine Basin of also the effects of sedimentary loading of the hanging wall. northern England. An effective lithosphere elastic thickness of between 3 and 10 km is believed to be a realistic value in the case of natural basins studied by Kusznir & Egan (1989) and Marsden et al. Regional context (1990).Applying themodel to theOchil Fault, using an effective elastic thickness of S km and extensions of 2 and It is notpossible to give acomplete explanation of the 3 km,footwall uplift of about 30% of thetotal throw is structures considered in this paper without reference to the predicted(Egan, pers. comm.). Doubling the effective broader tectonic context of the Midland Valley as a whole, a elastic thickness to 10 km produces only a slight change in full treatment of which is outside the scope of this paper. this ratio.This figure is identical tothe estimate (30%) However it should be noted that, in comparison with some obtained by assuming that the present topographic relief on of the well documentedfault/basin structures in the the fault scarp (c. 640m) represents the exhumed ‘basement’ literature, the Kincardine Basin is quite small, and may be expression of the footwall uplift. It may beconcluded regarded as only a sub-basin within a wider basin occupying

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much of thearea of the MidlandValley. Inthe latter References context,the Kincardine and Levenbasins correspond to BLUCK,B.J. 1984. Pre-Carboniferous history of the Midland Valley of sub-basinsseparated by anintrabasinal high overthe Scotland. Transactions of the Royal Sociery of Edinburgh, 75, 275-279. BurntislandAnticline, and the Kilsyth Trough to the SW, BROWNE,M.A.E., ROBINS,N.S., EVANS,R.B., MONRO,S.K. & ROBSON, P.C. althoughdefined as a separate structure by the isopachs, 1987. The Upper Devonian and Carboniferous sandstonesof the Midland may be regarded as linking with the Kincardine Basin (see Valley of Scotland. In: Inuestigafion of the Geothermal Potential of the [/K. British Geological Survey, Keyworth. Fig. 1). In a similar way, the Ochil Fault hanging-wall basin BURN,M.J. 1990. Controls on the geometry of Carboniferous deltas: Easr Fife resultingfrom the end-Carboniferous phase of movement U.K. and East Kentucky, U.S.A. PhD Thesis, Oxford Polytechnic. musthave been considerably wider than the present CONWAY,A., DENTITW, M.C., DOODY,J.J. & HALL,J. 1987. Preliminary Clackmannan Syncline. investigation of uppercrustal structure across the Midland Valley of Scotland. Journal of the Geological Society London, 144, 965-985. COWARD,M.P. 1993. The effect of Late Caledonian and Variscan continental escape tectonics on basement structure. Paleozoic basin kinematics and Conclusions subsequent Mesozoic basin development inNW Europe. In: PARKER, (1) Aprecursor fault tothe present WestOchil Fault is J.R. (ed.) Petroleum Geology of Northwest Europe: Proceedings of the thought to have been active either during Lower Old Red 4th Conference. Geological Society, London, 1095-1 108. - & GIBBS.A.D. 1988. Structural interpretation wirh emphasis on Sandstonetimes, or in themid-Devonian, prior to extensional tectonics. Joint Association forPetroleum Exploration deposition of the Upper Old Red Sandstone, possibly with a Courses ( UK.) Course Notes, No. 75. sinistral strike-slip displacement (Fig. sa). DAILLY,P.A. 1990. The Late Palaeozoic and Early Mesozoic strucrure and (2) The initiation and evolution of the Kincardine Basin evolution of the Solway and Vale of Eden Basin Complex. PhD Thesis, University of Oxford. duringDinantian to early Namurian times is attributed to DAVISON,C. 1924. A history of British earthquakes. Cambridge. E-Wextension on ahypothetical normal fault along the DENTITH,MC. 1988. Geophysical constraints on upper crustal structure in the Bo’ness line; the precursor West Ochil Fault is considered to Midland Valley of Scotland. PhD Thesis, University of Glasgow. haveacted as a sidewall fault boundingthe basin to the -& HALL,J. 1990. MAVIS: Geophysical constraints on the Carboniferous basin of WestLothian, Scotland. Transactions of the Royal Society of north (Fig. 9b). The Bo’ness fault may have been replaced Edinburgh, Earth Sciences, 81, 117-126. by a monocline by the early Namurian. DEWEY,J.F. 1982. Platetectonics and the evolution of the British Isles. (3) Thelater evolution of theKincardine Basin(late Journal of the Geological Society, London, 13, 371-412. NamuriantoWestphalian) is attributedtothe post- ELIET,P.P. & GAWHORPE,R.L. 1995. Drainage development and sediment supply within rifts. examplesfrom the Spercios basin, centralGreece. extensional stage where the continuation of the basin shape Journal of the Geological Society, London, 152, 883-893. is attributed to the effects of sediment loading, particularly EVLES.V.A.. SIMPSON.J.B. & MACGREGOR,A.G. 1949. The Geologyof on basin-fill clay rocks and coals. Central Ayrshire. Memoirs of the Geological Survey of Gt Britain. (4) TheClackmannan Syncline, together withneigh- FORSTER,F.C. & WARRINGTON.G. 1985. 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Received 9 October 1995, revised typescript accepted 12 February 1996. Scientific editing by Alan Roberts.

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