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 displace- ment. 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 on the 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 pile of 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 towards the 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 that the Ochil Fault might be a 573 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/4/573/4888572/gsjgs.153.4.0573.pdf by guest on 25 September 2021 574 J. RIPPON ET AL. 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 the intermediate 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/4/573/4888572/gsjgs.153.4.0573.pdf by guest on 25 September 2021 O CH IL FAULT AND KINCARDINEANDFAULT OCHILSCOTLANDBASIN, 575 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 subsidence upon which a component of dextral strike-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 whether the 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.