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Tectonic Significance of the Highland Boundary Fault, Scotland

September 19, 2008

By Tanner, Geoff

Abstract: None of the major roles assigned to the Highland Boundary Fault, such as a transcurrent fault with an orogen- parallel, sinistral displacement of several hundred kilometres, or terrane boundary, are confirmed. The Highland Boundary Fault can be traced NE-SW across Scotland for >240 km and defines the northern margin of the Midland Valley. Except for small outliers of Late Palaeozoic rocks on the NW side, it separates the Highland Border Ophiolite and Dalradian rocks of Neoproterozoic-Early Ordovician age (Grampian terrane) to the NW from Late Silurian-Early Carboniferous rocks (Midland Valley terrane) to the SE. The present Highland Boundary Fault is a steep, NW-dipping, post-Early Devonian, reverse fault that probably terminates SW of Bute. Stratigraphical and provenance links, and palaeostress data, limit post-Early Devonian lateral displacement to

The Highland Boundary Fault is marked by one of the most striking topographical features in the British Isles, a NE-SWtrending rampart forming the boundary between the Scottish Highlands and Lowlands. However, the nature of the fault is less clear: its inferred age varies from Ordovician to Middle Devonian (as a result of reactivation?), and it has been interpreted as a graben-bounding normal fault (George 1960), a major sinistral strike-slip fault (Ryan et al. 1995; Woodcock & Strachan 2000), a NW-dipping reverse fault (Allan 1928, 1940; Ramsay 1964) or a terrane boundary (Bluck 1984).

Most workers placed the Highland Boundary Fault between the Dalradian rocks and those rocks known since the 1980s, until recently, as the ‘Highland Border Complex’ (Barrow 1901; Jehu & Campbell 1917; Allan 1928, 1940; Bluck 1984, 2002). Others favoured the contact between either the Dalradian or the Highland Border Complex and the Early Devonian or younger rocks to the SE (Nicol 1862; Jehu & Campbell 1917; Anderson 1947; Friend et al. 1963; Henderson & Robertson 1982), or inferred that the main movement plane is no longer seen at the surface, being hidden beneath the hanging wall of a more recent fault (Bluck 1984). This diversity of interpretations reflects the fact that there is little evidence on the ground of a continuous, throughgoing fault plane, and a dearth of associated cataclastic or mylonitic fault rocks (Jones 1990).

Sporadic clusters of seismic activity mark the line of the fault; for example, in the Comrie area from 1608 to 1921, at Dunoon in 1985, and at Aberfoyle in 2003 (Fig. 2b) (Ottemoller & Thomas 2007). A detailed analysis by Ottemoller & Thomas of the Aberfoyle earthquake showed that it had an epicentre at 4 km depth, and resulted from oblique sinistral strike-slip with some normal movement, on a fault plane striking WSW-ENE and dipping at 65[degrees] NW.

The aims of this paper are to: (1) demonstrate, from a critical review of published work, augmented by the author’s unpublished mapping and field observations, the nature and timing of movements on the fault; (2) test recent models that require large sinistral strike-slip displacements on the Highland Boundary Fault in Devonian times (Soper & Woodcock 2003), of the order of several hundred kilometres (Dewey & Strachan 2003); (3) attempt to explain the intriguing parallelism of the Highland Boundary Fault to the fold traces of both the Highland Border Downbend Antiform (henceforth referred to as the ‘Highland Border Downbend’), a major late fold in the Grampian terrane, and the Strathmore Syncline, a primary fold in the Midland Valley terrane (Fig. 1b). The relationship is not a simple one, as the Lower Devonian sequence, which is folded by the Strathmore Syncline, cuts unconformably across the steeply dipping SE limb of the Highland Border Downbend.

Major features of the Highland Boundary Fault

It is recognized here that, apart from the small outliers on the Dalradian outcrop (Fig. 1b), the Highland Boundary Fault separates the Neoproterozoic-(?)Ordovician Dalradian Supergroup and the Highland Border Ophiolite, on its NW flank, from rocks of Late Silurian-Early Carboniferous age to the SE (Fig. 1b). The Dalradian succession includes the newly defined Trossachs Group (Tanner & Sutherland 2007), which overlies the Southern Highland Group. The Old Red Sandstone’ refers to a magnafacies of terrestrial red beds and lacustrine deposits, of late Silurian to earliest Carboniferous age. This informal lithostratigraphical term is used widely in the British literature referenced in this paper. The Lower, Middle, and Upper Old Red Sandstone are broadly equivalent to the Lower, Middle, and Upper Devonian. Two orogenic events have affected the area: the Grampian Orogeny (early Ordovician) affects the Dalradian rocks north of the Highland Boundary Fault, and the mid-Devonian (Emsian) Acadian Orogeny affects only rocks SE of the fault (Fig. 1c and d). No Scandian deformation has been recognized.

For most of its course, the Highland Boundary Fault has a remarkably straight trace, a feature normally associated with strike- slip faulting. On a regional scale it cuts across the SE limb of the Highland Border Downbend (D^sub 4^), excises the Tay Nappe (D^sub 1_2^), and is divisible into three segments (Fig. 1b). The NE segment is marked by an abrupt change in dip of the Lower Old Red Sandstone from

Pre-Devonian movement

The hypothesis that the former Highland Border Complex consists of an exotic terrane, emplaced against the Dalradian block by a major transcurrent movement along the Highland Boundary Fault following Grampian orogenesis, was based largely on the apparent contrast in age between the two units (Bluck 1985). This disparity has now been removed by the recognition that the Chitinozoa that reportedly gave Arenig to Ashgill ages for different parts of the Highland Border Complex (Curry et al. 1984) are of inorganic origin (Tanner & Sutherland 2007). Consequently, the Highland Border Complex is now seen to comprise an autochthonous Trossachs Group, in ‘stratigraphical continuity’ with the Southern Highland Group, in the sense that the two groups are structurally concordant and young in the same direction. If Batchelor (2004) is correct in deducing that metamorphosed airfall tuffs found in the Southern Highland Group close to the Highland Boundary Fault are of Tayvallich (600- 590 Ma) age, then there must, in places, be a non-sequence(s) or disconformity in the Southern Highland Group-Trossachs Group succession, albeit affected by local faulting.

The Dalradian metasediments are overlain by an obducted ophiolite (Tanner 2007a), the contact representing an ancient suture along which part of the floor of the sedimentary basin appears to have been emplaced and welded onto the upper limb of an already recumbent Tay Nappe (D^sub 1-2^) (Tanner 2007b). Thus, at the current level of exposure no continuous structural break identifiable as the Highland Boundary Fault has been observed in rocks older than the Early Devonian (for further discussion, see Tanner & Pringle 1999; Tanner 1997, 2007a). Several geophysical studies have been carried out in the Highland Border region to determine the 3D orientation of the Highland Boundary Fault. Qureshi (1970) carried out a gravity study of the Highland Boundary Fault between Balmaha and Aberfoyle (Fig. 2a), and concluded that it was a high-angle reverse fault. However, a more recent study (Dentith et al. 1992), which utilized a larger dataset and more density measurements, demonstrated that, along a NW- SE traverse (G6) through Callander (Fig. 2), the contact between the Dalradian rocks and the Highland Border Complex dips at 20[degrees] NW. This result has been quoted as evidence that the Highland Boundary Fault has a thrust-like geometry and dips gently to the NW (Dempster & Bluck 1991; Bluck 2002), whereas recent work has shown that it represents the attitude of the Dalradian-Highland Border Ophiolite contact rotated within the Steep Belt on the SE limb of the Highland Border Downbend. In the Keltie Water, close to the line of traverse G6, the Trossachs Group is inverted and dips at 40- 50[degrees] NW (Tanner & Pringle 1999) but the contact with the underlying ophiolite is not exposed. Throughout most of the Highland Border, the ophiolite lies on the upper limb of the Tay Nappe and, as a result of D^sub 4^ folding, changes orientation from gently southward dipping on Bute, through the vertical, to become gently NW- dipping in the vicinity of Callander. At Callander, the interface between the Highland Border Ophiolite and the Lower Old Red Sandstone, the true Highland Boundary Fault (although in part a modified unconformity), was modelled by Dentith et al. (1992) as a vertical plane.

On the aeromagnetic map of Great Britain (see inset on British Geological Survey (BGS) Sheet 56W (Glen Shee)) there is a pronounced magnetic anomaly at Blairgowrie, which is associated with surface exposures of the Highland Border Ophiolite. Modelling of gravity and magnetic data along a NW-SE line close to that of section B-B’ (Fig. 2a) (Crane et al. 2002) indicated the presence of an extensive near- horizontal sheet of ophiolite, within a few kilometres of the ground surface. The sheet is continuous for at least several kilometres on either side of the Highland Boundary Fault and is only slightly displaced vertically by the fault. As seen at Callander, the attitude of the ophiolite is controlled by the D^sub 4^ folding.

The D^sub 4^ structural model also explains the presence of possible xenoliths of the Highland Border Complex in a lamprophyre dyke located 8 km NW of the southern edge of the Dalradian outcrop on Loch Lomondside (Dempster & Bluck 1991). From the attitude of the southern limb of the Highland Border Downbend at this locality, it is predicted that the lamprophyre intersects the Highland Border Complex, including the ophiolite, at a depth of 3-4 km. Southward thrusting of the Dalradian rocks onto the Highland Boundary Fault is not required (see Bluck 1984).

The missing evidence

There is a period of

Early Devonian basin development

The Lower Old Red Sandstone presents a number of puzzling features, which arise from the preservation of small outliers of gently tilted or horizontal rocks of that age lying unconformably on the Dalradian-Highland Border Complex, NW of the line of the Highland Boundary Fault (Fig. 2a, outliers A and B).

First, no direct correlation is possible between the sequences seen in the outliers A and B and those across the fault in the immediately adjacent part of the Strathmore Basin (Allan 1940). second, there is a contrast in the provenance of the clasts: many of those in the outlier rocks are derived from the Dalradian basement, whereas those in the conglomerates SE of the fault are largely of recycled metaquartzite cobbles of unknown provenance, probably reworked from a conglomerate lying on the Dalradian surface (Haughton et al. 1990). Third, sediment dispersal directions for the outlier sequences are to the SE, with an apparently opposing direction of palaeoflow seen in the rocks immediately SE of the Highland Boundary Fault. To explain these and other features, Bluck (2000) proposed that the Lower Devonian rocks were deposited in a series of pull-apart basins formed in response to sinistral strike- slip movement on the Highland Boundary Fault. However, such a model is difficult to substantiate (Haughton & Bluck 1988, p. 291).

More problematical are the hypotheses proposed to explain the differences noted above between the Lower Old Red Sandstone sequences in the outliers and those in the adjacent basin; namely, (1) that there was an exotic basement block (C) between the Dalradian block and the Highland Boundary Fault that provided a source for metamorphic clasts in the basin (Bluck 1984), and (2) that an additional contemporary sedimentary basin(s), or part of a basin, had originally been present between the Dalradian block and the Strathmore Basin, and was ‘destroyed’ during a process that involved rotation of the Highland Boundary Fault from vertical to gently NW-dipping (Bluck 2000). These hypotheses are not supported by a recent geological resurvey of BGS Sheet 56W (Glen Shee) (British Geological Survey 1999), which established a single succession for the Lower Old Red Sandstone in the outliers and the basin, although displaced by the Highland Boundary Fault. In addition, on neither this sheet nor BGS Sheet 38E (Aberfoyle) (British Geological Survey 2005) is there any indication of a structural break(s) that could be related to the lateral expulsion of a suspect terrane. As noted above, geophysical data show that the Highland Border Ophiolite is continuous at depth beneath the line of the Highland Boundary Fault.

Evidence supporting minimal post-Early Devonian displacement on the Highland Boundary Fault

Outliers of horizontal to gently dipping Upper Old Red Sandstone or Lower Carboniferous strata occur SW of those discussed above (Fig. 1, Kintyre Peninsula; Fig. 2, outliers C-G). The rocks in these outliers become younger to the SW, indicating that as the Grampian block was progressively uplifted in the SW, it was onlapped by the Devonian-Carboniferous sequence (Anderson 1947). A mirror image of this pattern is seen in the Midland Valley SE of the Highland Boundary Fault (Fig. 2a), suggesting that although there was vertical displacement on the fault during this period, it was not accompanied by appreciable lateral movement. This conclusion is supported by a palaeomagnetic study of terrane relationships in the British Caledonides (Trench et al. 1989).

Three detailed studies provide further links across the Highland Boundary Fault, as follows.

(1) The Lintrathen ignimbrite, found at, or near, the base of the Lower Devonian sequence between Dunkeld and Glenbervie, 12 km WSW of Stonehaven, may be correlated across the fault (Paterson & Harris 1969) (Fig. 2, locations la-d, two of which are arrowed), thus limiting the lateral displacement to less than a few tens of kilometres (Trench & Haughton 1990).

(2) A comprehensive geochemical study, supported by some radiometric dating, of granitoid clasts in part of the Lower Devonian Crawton Group in the Crawton sub-basin showed that they could be related to specific source areas in the Dalradian block (Haughton et al. 1989) (Fig. 2, locations 2).

(3) A geochemical study of detrital garnets from sandstones in the Lower Old Red Sandstone on the northern limb of the Strathmore Basin showed that they could be linked to those in restricted areas of the Dalradian outcrop to the NW (Haughton & Farrow 1989) (Fig. 2, locations 3a and b).

When taken together, these results provide irrefutable evidence for restricting post-Early Devonian lateral movement on the Highland Boundary Fault to a few tens of kilometres.

Post-Early Devonian (Acadian) fault movements

In the Grampian terrane, structures in the Southern Highland Group (Dalradian) form a coherent pattern from Arran to Stonehaven, with no evidence of a major post-Grampian structural discontinuity (Fig. 2a and b). The Highland Border Ophiolite has been emplaced onto, and cuts across, the right-way-up limb of the Tay Nappe and both are cut out by the Highland Boundary Fault (Fig. 2b), and unconformably overlain by the Lower Old Red Sandstone.

In the Midland Valley terrane, serial cross-sections across the Lower Devonian rocks of the Strathmore Basin (Fig. 2c) show the constant profile of the syncline, and

The overlap zone in the Balmaha region contains a thick sequence of Upper Old Red Sandstone onlapping a very irregular topography. The geology of this zone provides a useful test of the displacement sense: reverse fault motion has resulted in a pull-apart basin, whereas sinistral strike-slip movement would have formed a restraining bend accompanied by thrusting and en echelon folds at

Thus, three independent investigations using different approaches have demonstrated that sigma^sub 1^ was approximately orthogonal to the Highland Boundary Fault trace over a distance of 200 km and that small amounts of local non-coaxial deformation (Fig. 2d) differ in shear sense and cancel out. These results also remove the possibility that a large sinistral displacement, in Early Devonian times, could have been disguised by a subsequent large dextral movement in Carboniferous times.

The currently exposed Highland Boundary Fault is a steeply inclined NW-dipping reverse fault of Acadian age. The axial trace of the Strathmore Syncline runs parallel to the Highland Boundary Fault for >200 km and its axial plane has a dip similar to that of the fault (Fig. 2b). These structures resulted either from orthogonal shortening or from transpression in which northsouth regional compression was partitioned into two components, normal and parallel to the Highland Boundary Fault (Jones & Tanner 1995). The latter component of sinistral fault-parallel simple shear would have resulted in a displacement on the Highland Boundary Fault of the order of a few kilometres. This would have generated sufficient strike-slip movement to explain the occurrence of near-horizontal slickenside lineations seen in places along the Highland Boundary Fault. The fault does not have a geometry compatible with transtension, and any geometric complications are due to linkage, as in the Balmaha area, and not to the development of a flower structure (see Dewey & Strachan 2003).

Post-Acadian movements

The Upper Old Red Sandstone rests unconformably on the folded Lower Old Red Sandstone, with strata of Middle Old Red Sandstone age being absent from the Highland Border Zone. The Upper Old Red Sandstone is locally tilted to near vertical close to the Highland Boundary Fault, in the same manner as the Lower Old Red Sandstone, but on a smaller scale. Normal faulting with a downthrow to the north occurs in places, especially in the overlap zone at Balmaha, in Glen Fruin, and in Old Red Sandstone near Helensburgh (Fig. 2b, section E-E’).

Discussion

Recent work has demonstrated that there is an obducted ophiolite, but no Dalradian ‘Highland Border Complex’ terrane boundary of pre- Early Devonian age, in the Highland Border region of Scotland (Tanner & Sutherland 2007). Likewise, postEarly Devonian lateral movement across the Highland Boundary Fault is restricted by stratigraphical and provenance links to a few tens of kilometres. Thus, if a major strike-slip displacement has occurred within the Highland Border Fault Zone, as required by current plate tectonic models for this period to explain the presence of Scandian deformation in the Northern Highlands terrane (Dewey & Strachan 2003), it must have taken place along the present fault line between 460 and 420 Ma.

This hypothetical fault, the proto-Highland Boundary Fault, must have cut the Highland Border Ophiolite and brought it into contact with some far-travelled terrane that would now occupy the Midland Valley. Unfortunately, the Midland Valley terrane can be identified as ‘exotic’ only if its present-day geology cannot be incorporated into the plate tectonic model proposed for the Grampian Orogeny. As most models for this orogeny are based upon what is inferred to be present beneath the Palaeozoic cover of the Midland Valley, this leads to a circular argument. In the absence of field or geophysical evidence, the problem will be resolved only as our knowledge of the Midland Valley basement is increased (i.e. Bluck et al. 2006; Downes et al. 2007), new models for the Grampian Orogeny are proposed (Tanner 2007b), and plate tectonic reconstructions for NW Europe in Devonian times are refined further (i.e. Woodcock et al. 2007).

The same uncertainty over the early history of the Highland Boundary Fault bedevils attempts to explain the near-parallelism of the axial surfaces of the Strathmore Syncline and Highland Border Downbend to the Highland Boundary Fault plane along the NE segment of the Highland Boundary Fault. Harte et al. (1984) postulated that the Downbend formed as a result of the presence of a basement ‘step’; that is, it is a ‘forced’ fold formed in response to uplift of the interior part of the fold belt on a steep NW-dipping, blind thrust. The same interpretation could be applied to the Highland Boundary Fault, with the Strathmore Syncline developed as a hanging- wall syncline. Although Allan (1940) inferred that a precursor Highland Boundary Fault was active during Early Devonian time, there is still an interval of some 30-40 Ma between the formation of the Strathmore Syncline and the Highland Border Downbend, and any link between them is broken if the Highland Boundary Fault had a previous history as a major strike-slip fault.

The significance of the marked topographical feature, which separates the Highlands from the Lowlands of Scotland, is now clear. It results from the presence of a narrow zone of mechanically weak and easily eroded rocks (Trossachs Group and serpentine-rich ophiolite) between the mountainous terrain formed by the Dalradian schists and greywackes, and the lesser feature formed by the steeply dipping sandstones and conglomerates of the northern Midland Valley to the SE. Movement on the Highland Boundary Fault has accentuated this contrast, but it did not cause it.

In conclusion, the tectonic significance of the Highland Boundary Fault has been much overrated, especially in the last decade. The currently exposed fault is not a fundamental, terrane-bounding dislocation, its lesser role as a large transcurrent fault is seriously questioned, and it did not give rise to the major topographic feature with which it is associated.

I thank the Leverhulme Trust for support during the writing-up of this work, and M. Shand (Glasgow) for the digital cartography. The comments and suggestions from referees A. L. Harris and N. H. Woodcock led to material improvements in the interpretation proposed here, and are much appreciated.

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Received 7 February 2008; revised typescript accepted 4 April 2008. Scientific editing by Rob Strachan

GEOFF TANNER

Department of Geographical and Earth Sciences, University of Glasgow, Glasgow Gl2 8QQ, UK

(e-mail: geoff.tanner@virgin.net)

Copyright Geological Society Publishing House Sep 2008

(c) 2008 Journal of the Geological Society. Provided by ProQuest LLC. All rights Reserved.




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