Geology of the Aberfoyle district: Faulting
|This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
Faulting has a very significant role in the gross geological structure in the Aberfoyle district, and the development of the sedimentary basins into which Devonian and Carboniferous rocks were deposited. Most postorogenic faulting activity occurred in the Palaeozoic, due to late Caledonian (Silurian) to Variscan (latest Carboniferous) tectonic events. Although the Highland Boundary Fault Zone represents a fundamental divide in the geology of the district, several aspects of the faulting are common to the domains north and south of this major structure.
Crustal loading by ice was significant within the district and adjacent areas during Devensian and Loch Lomond Stadial glaciations. Faults and joints within the district may have been reactivated to some extent during major isostatic adjustment, both during and following deglaciation, and there is some evidence for this in recent seismic activity located on the fault (Ottemoller and Thomas 2007). Fissuring occurs on the western summits of Craig More [NN 510 021], above Aberfoyle, and, very spectacularly, on the ridge north of Creag na h-Airigh [NN 536 109] to the east of Gleann nam Meann.
Highland Boundary Fault Zone (HBFZ)
This is the fundamental geological structure within the Aberfoyle district, separating the Palaeozoic rocks of the Midland Valley from the Neoproterozoic to middle Cambrian rocks of the Dalradian in the Highlands. It is a complex structure with a long history, reflecting its role in orogenic events at various times. Its significance as a major tectonic boundary is unquestioned, although its origins and significance as an early structure remain unclear (e.g. Dentith et al., 1992). Furthermore, the HBFZ has obscured and confused the nature of the original tectonic relationship between the Highland Boundary Complex and the Dalradian.
Although the HBFZ forms locally strong topographical features, the crags in the Arbuthnott Group conglomerates at Creag Dhubh [NN 540 024] being a notable example (P001225 and P219918), faults within the zone are generally very poorly exposed in the Aberfoyle district. A notable exception occurs in the Keltie Water, immediately south-west of Maol Ruadh [NS 4738 9646]. This fault is one of several in the vicinity which juxtapose Devonian, Highland Border Complex and Dalradian rocks in a highly complex zone. Throughout most of the district, the HBFZ appears largely unaffected by later transverse structures. However, immediately south of Loch Venachar, the HBFZ is displaced sinistrally by about 1 km to the south along the north-north-west-trending Bealach nan Cam and Bealch nam Beitheag faults. The influence of the HBFZ on Devonian and Carboniferous sedimentation is discussed further below.
The geometry of fault movement within the HFBZ is complex and has varied over time. In the Aberfoyle district and elsewhere, the dominant observed fault movements are in a reverse sense, across apparently high angle, north-west dipping fault planes. However, south-west of Loch Lomond, later normal (down-dip) fault movement has locally preserved Devonian and Carboniferous rocks on the north-west side of the fault zone. Fault movements thus continued at least into Late Palaeozoic times. Evidence for strike-slip movement is recorded locally by horizontal slickensides (Anderson 1951). This, coupled with the evidence for the faults being high-angle, north-westerly-dipping structures, has been interpreted as indicating major late Silurian–Early Devonian strike-slip movement that juxtaposed the different Highlands and Midland Valley terrains, and was probably related to Scandian orogenic events.
The nature of the HBFZ at depth is unclear (see also Section n on Concealed Geology). The interpretation of gravity anomaly data within the vicinity of the HBFZ by Dentith et al. (1992) suggested that the structure may be a low-angle thrust fault zone at depth, supporting the suggestion by Bluck (1984) that the present-day line of the fault is due to later thrusting in the Late Devonian and early Carboniferous. However, earthquake event data from recent minor earthquakes within the HBFZ are more consistent with a steep, north-westerly dipping structure (Ottemoller and Thomas, submitted).
Sinistral oblique/strike slip faulting within the Dalradian terrain
Significant sinistral oblique/strike slip faulting is a key element of the structure of the Highland terrain within the Aberfoyle district, and is dominated by the Loch Tay Fault Zone (Figure 9). This zone includes several major sinistral strike-slip faults and is about 5 km wide, trending approximately south-south-west from Glen Ample and Loch Lubnaig in the north-east of the district, to the Burn of Mar area in the south-west. Across the zone, rocks on the west side of the fault are displaced relatively to the south-west, bringing the Aberfoyle Slates, which lie close to the base of the Southern Highland Group, to within a kilometre or so of the Highland Boundary Fault.
The major faults in this system within the Aberfoyle district are the Loch Tay Fault itself, on the east side of the system, and the Loch Ard Fault on the western side. Although the Loch Tay Fault is traditionally seen as a major Highlands structure, analysis of the distribution of the faults within the district and in adjacent ground to the south-west in the Greenock district, shows that elements of this structure apparently continue across the Highland Boundary Fault, running into the Gartness Fault. Between Loch Tay and Loch Ard faults, numerous, more southerly trending faults occur in the ground between Creag Innich [NN 492 034] and the Duke’s Pass (Figure 9). Here, they commonly form steep, eastwards-dipping, planar surfaces in the arenites of the Creag Innich Sandstone, and are clearly seen on air photographs. The obliquity of these structures to the Loch Ard Fault, and their orientation relative to it, suggest that in a sinistral strike slip regime these faults may have acted as R1 Reidel Shears, which develop between two bounding shear structures (Ramsay, 1987, 420, page 529). Although their ready recognition is partly a function of the good outcrop, it is also significant that they are concentrated immediately north-west of the point where the Loch Tay Fault converges with the Highland Boundary Fault. This suggests that these faults developed as movement on the Loch Tay Fault was transferred over to the Loch Ard Fault.
The Loch Tay Fault is well exposed in cliffs immediately adjacent to a forestry track on the north side of Meall Ear [NN 5302 0273] (P643960). Faulted rock is exposed over some 100 m. Several sets of slickensides are present on surfaces in coarse clastic lithologies at the eastern end of the exposure. All plunge at shallow angles, indicating a predominantly strike-slip displacement. The fault is also exposed in Glen Ample [NN 588 144] and the Stank Glen [NN 577 105], where there is much broken, altered and weathered rock, and several metres of fault gouge.
Strike slip displacement on the Loch Tay Fault is estimated to be of the order of several kilometres in Central Perthshire (Treagus 1991), with dip-slip throws of the order of several hundred metres or more. The throw on the Loch Tay Fault in the Aberfoyle district is difficult to assess because of the lack of unambiguous lithostratigraphical control either side of the fault. However, when the regional fold structures are compared to those in the Stuc a’ Chroin–Ben Vorlich area, immediately north and north-east of the district, the lateral displacement appears to be about 6.5 km, with a relative downthrow to the north-west of about 500 m.
Strike-slip displacement on the Loch Tay Fault brings rocks low down in the Southern Highland Group (viz the Aberfoyle Slate Formation) to within 1 to 2 km of the HBFZ on the west side of the fault, near Aberfoyle. To the east of the fault, the Southern Highland Group rocks adjacent to the HBFZ are at much higher stratigraphical levels, and the Aberfoyle Slate is not exposed (see Neoproterozoic to Cambrian). The displacement also affects the disposition of structures, most particularly D2 to D4. West of the Loch Tay Fault, D2 structures can be found as far south as the Dukes Pass–Creag Innich area. To the east, D2 structures occur only at the northern limit of the district.
Late Caledonian and early variscan earth movements
From late Silurian to early Carboniferous, periodic movements of the Highland Boundary Fault Zone and related fracture systems influenced the tectonic and sedimentary development of the Midland Valley.
During the early Devonian, the central part of the Aberfoyle district was part of a major, north-east trending subsiding basin, centred on the Midland Valley and flanked to north and south by upland areas. It has been suggested, largely on hypothetical grounds (Bluck, 1978), that formation of this basin involved strike slip and reverse movements on the Highland Boundary Fault Zone. Comparison of the Apparent Polar Wander Paths for the Highlands and the Midland Valley suggest, however, that they have occupied similar positions in relation to each other since prior to late Silurian times (Trench et al., 1989). Similarly, provenance studies on various igneous clasts from the Lower Devonian sedimentary rocks also suggest that the Highlands and the Midland Valley were essentially in their present relative positions by that time (Haughton et al., 1990). However, these studies do not preclude significant strike slip movements prior to the late Silurian, as proposed by Bluck (1985). During the Early Devonian, intermittent uplift on the Highland Boundary Fault Zone would have rejuvenated Highland Source areas, and this is likely to have maintained supply of coarse detritus to the Midland Valley basin. However, many of the clasts in the sedimentary rocks are polycyclic and do not obviously reflect a first-cycle Highland source.
During the Middle Devonian, compressive earth movements due to the Acadian Orogeny briefly brought about marked changes in the palaeogeography of central Scotland. The Lower Devonian rocks were folded into north-east trending structures, most notably the Strathmore Syncline, the steep northern limb of which lies adjacent to the Highland Boundary Fault. Major reverse movements on elements of the Highland Boundary Fault Zone resulted in the erosion of any Lower Devonian rocks deposited to the north of the fault system, whilst within the Midland Valley basin, Lower Devonian rocks were deeply eroded, particularly along anticlinal axes. Continued uplift prevented deposition of Middle Devonian sediments. The direction of the palaeoslope within the Midland Valley was reversed during this period. Rivers that drained to the south-west in the Early Devonian were replaced by an axial drainage pattern directed towards the north-east in the Late Devonian.
Upper Devonian strata in the Aberfoyle district appear to represent, in part, the distal equivalents of coarse detritus seen to the north-west of the Midland Valley basin. This detritus was probably derived from a Highland source area, maintained as an area of positive relief by continued uplift along the Highland Boundary Fault Zone. The proximal nature of the clasts in the Rosneath Conglomerate exposed in the Ben Lomond district to the west, suggests that the Rosneath Fault was an active component within the Highland Boundary Fault Zone during this period. To the south of the district, particularly in areas along the axis of the main Midland Valley basin, the Rosneath Conglomerate may be overlain by, and partly equivalent to, the Stockiemuir Sandstone Formation, a generally upward fining sequence of alluvial sandstones that suggests declining tectonic activity. The deposition of the Inverclyde Group was preceded by uplift, erosion and renewed subsidence, again focused along the Highland Boundary Fault Zone.
Variscan earth movements
Further reactivation of the Highland Boundary Fault Zone probably occurred during the early Carboniferous (Paterson et al., 1990). Most faults affecting lower Carboniferous and older strata in this area have the principal north-east and NORTH-WEST trends developed widely in the Midland Valley; NORTH-WEST-trending faults are particularly common in the vicinity of the Highland Boundary Fault (Figure 9).
In the Burn of Mar area [NS 445 925], faulting is more complex than elsewhere immediately to the south-east of the HBFZ. Here, Upper Devonian and lowermost Carboniferous rocks are preserved in a small graben. Notably, faults with orientations parallel to the Loch Tay–Loch Ard fault systems are present in this area, south of the Highland Boundary Fault Zone. This suggests that interaction of the sinistral strike-slip ‘Highland’ structures with the Highland Boundary Fault Zone may have resulted in the development of complementary structures in the northern margin of the Midland Valley, as discussed above.
Approximately north-west trending later fractures are also abundant in the Aberfoyle district and are probably of Variscan age or younger. Most of these structures are major joints with little displacement across them, but some are faults with minor oblique slip displacements. In addition, normal, dip-slip faults in this set are particularly common between Gualann and Loch Venachar, where they displace the Devonian Arbuthnott and Garvock Group rocks. This later fracture set is seen prominently on aerial photographs and has a significant effect on the orientation of some major topographical features, apparently controlling, in particular, the orientation of the south-east end of Loch Katrine, Glenfinglas and Milton Glen. These lines of weakness have subsequently been exploited and enhanced by erosive processes, particularly during Quaternary glaciation. Several features also appear to have affected the Highland Boundary Fault. The largest of these, the Bealach nan Carn and Bealach nam Beitheag faults in the Menteith Hills, have inferred horizontal displacements of 700 and 300 m, respectively, near Invertrossachs [NN 561 051], on the south side of Loch Venachar (Figure 9). Many of the faults shown between Balleich and Gualann have been inferred from lineaments on aerial photographs.
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<ref>tag; name "Dentith 1992" defined multiple times with different content
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