The Caledonides of Scotland and Ireland
The Scottish Caledonides
The Highland Border
Origin of the Tay Nappe, Scotland - Geoff Tanner Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK.
The Tay Nappe is a major isoclinal fold whose inverted lower
limb occupies most of the Highland Border region in Scotland, and whose hinge
zone can be traced for over 300 km. This originally recumbent fold affects the
Neoproterozoic to Ordovician Dalradian Supergroup, and forms the
foundation to the 460-470 Ma Grampian orogenic belt. The ‘Highland Border
Complex’, which occupies a narrow strip to the SE of the main
Dalradian outcrop has recently been divided into two parts: the Trossachs Group
and the Garron Point Group. The Trossachs Group is autochthonous and in
stratigraphical and structural continuity with the Southern Highland Group; it
includes the Leny Limestone, the Margie Limestone and the River North Esk and
North Glen Sannox pillow lavas. The Garron Point Group is allochthonous and
consists of the Highland Border Ophiolite and an overlying sequence of black
slates with chert, pillow lavas and umbers. The Dalradian rocks have been
affected by four phases of folding (D1- D4); the Tay Nappe formed during the
first two phases and was later folded by the D4 Cowal Antiform. Although the
geometry of the Tay Nappe is well understood, its origin and kinematics are
uncertain. The problem addressed here concerns the relationship between the Tay
Nappe and the Highland Border Ophiolite, and their role in the Grampian orogeny.
In 1958, Robert Shackleton introduced the concept of structural facing and used
it to demonstrate that the Aberfoyle Anticline, considered previously to be a
normal, upward-facing anticline closing to the southeast, was a downward-facing,
or synformal, anticline. He deduced that a large part of the South-west Grampian
Highlands consists of rocks that are upside down because they belong to the
lower limb of this originally recumbent, south-facing structure, which he named
the Tay Nappe. Since then many models have been proposed to explain the
evolution of this enigmatic structure, including: high-level gravitational flow;
expulsion of D1 nappe-like folds from a central ‘root-zone’ (fountain of nappes
model); ophiolite obduction; and back-folding related to NW-directed thrust
movement. None of these models has gained universal acceptance.
However, it is generally agreed, that having formed during D1, the ‘nappe’ was
emplaced by top-to-SE-directed simple shear during D2, as indicated
by the pattern of deformed early (S1) cleavage stripes within the S2
microlithons. The Highland Border Ophiolite forms a discontinuous outcrop <1
kmwide,
along the Highland Border from Bute to Stonehaven. It has a thick, locally
developed ‘sole’ of finely banded hornblende–epidote–albite–bearing amphibolite
at Scalpsie Bay on Bute, and also NE of Balmaha on Loch Lomond where it can be
followed for 4 km along strike. In each case the underlying rocks, regardless of
whether they belong to the Southern Highland Group or the Trossachs Group have
been made schistose and are affected by contact metamorphism within a zone a few
m thick. The main body consists of one or two thick units of serpentine
conglomerate, together with spinel-bearing harzburgite, massive ultramafic rock,
and units of jasper >2 m thick. The most consistent member of the assemblage is
a thick sheet of orange-brown-weathering, carbonated serpentinite, or
ophicarbonate. Serpentine conglomerate is seen in all stages of alteration to
ophicarbonate, which preserves ghost relics of serpentine clasts set in a
dolomite-magnesite-rich matrix, cut by numerous thin quartz veins. On the
islands in Loch Lomond (Inchmurrin and Inchcailloch), and at Lime Craig Quarry (Aberfoyle),
the ultramafic rocks are associated with Ni- and Cr- rich conglomerates, which
pass laterally into ophicarbonate. The standard ophiolite sequence is not seen
at any one place, and an unknown portion of the body is always hidden under the
(commonly faulted and folded) unconformable cover of Lower and Upper ORS strata
to the SE. An understanding of the role of the ophiolite in the Grampian Event
depends on the following (i) the base of the ophiolite can be seen in the field
to rest upon different members of the Southern Highland Group and of the
Trossachs Group, accompanied by contact metamorphism in each case. (ii) The
ophiolite lies on the upper, southward younging limb of the Tay Nappe, with the
basal contact of the ophiolite being at a small angle to the D1 cleavage in the
footwall. (iii) The mean orientation of the D1 stretching lineation in rocks
beneath the ophiolite is almost precisely orthogonal to the strike of the basal
plane of the ophiolite. (iv) The stretching direction (X) in the Dalradian
country rocks does not appear to change orientation as it is tracked from the
D1-dominated zone to the D2- dominated zone. (v) Pseudomorphs of an unknown
contact metamorphic mineral are euhedral in a narrow contact zone beneath the
ophiolite, and overprint the mylonitic emplacement fabric. Outwith the contact
zone they are elongated in the stretching direction. Based upon this evidence, a
new model for the Tay Nappe is proposed in which the pre-obduction formation of
the proto-Tay Nappe during D1 results from early initiation of NW-directed
subduction beneath the Dalradian sedimentary prism, which included the Trossachs
Group. The ophiolite is considered to have formed the floor of a small
extensional basin, floored by altered continental mantle, in a setting analogous
to the Ligurian ophiolites in the Alpine belt. Closure of this basin, and
obduction of the upper part of the ocean floor onto an already folded Dalradian
sequence is inferred to have taken place before obduction, during the late
Arenig, of the Clew Bay-Ballantrae-?Shetland Ophiolite onto the Midland Valley
arc farther to the SE. Contrary to previous interpretations the ophiolite
appears to be neither responsible for the formation, nor the modification, of
any of the structures in the Grampian orogenic belt. Subsequent D2 deformation
took place under the same shear regime as D1 and with the same stretching
direction, causing further translation of the Tay Nappe to the SE. This pattern
continued into D3, and was terminated by an island arc collision, resulting in
the D4 Highland Downbend that folded both the Tay Nappe and the ophiolites.
The Irish Caledonides
The Lake District and Wales