An introduction to the Moine Thrust Belt
The NW margin of the ancient Caledonian mountain belt on the British mainland is a strip of faulting termed
the Moine Thrust Belt. Back in the 1880s, when the Geological Survey were mapping these features the geologists
called it the "zone of complication". And what complication! For geologists used to gently dipping strata
which simply forms a simple rock sequence the "zone of complication" is especially trying. To see how
Peach, Horne and colleagues sorted out the structure you'll need to visit the Highland
Controversy part of this web-site. Here we can briefly introduce some of the structures of the Moine Thrust
Belt. The belt runs from near Whiten Head on the north coast of Sutherland right down to Sleat on the Isle of
Skye. Offshore continuations have been proposed through the Sea of the Hebrides and on to the west of Ireland.
In
any tour of the thrust belt, a good place to start is the Moine Thrust itself. This forms the capping structure
to the "zone of complication". It carries a sheet of rock chiefly made up of metamorphosed sediments
(the "Moine") and their older, Lewisian-like basement. Most geologists reckon the Moine Thrust has moved
over 100 km - but it is hard to be sure how far. During these movements the rocks on either side of the thrust
were strongly sheared and streaked out. This makes a new type of rock, formed by deformation, called mylonite.
In places these strongly sheared rocks are over 100m thick. All the mineral grains in the mylonite are streaked
out and can become aligned, needle-like, in the direction of shearing. By measuring the alignment direction
of grains geologists can establish in which direction the Moine thrust moved. The answer is towards WNW - and
this direction is remarkably consistent all the way down the Moine Thrust Belt.
Below the Moine Thrust, the thrust belt itself varies in width from 0 to 12 km wide. It is made up of complex
stacks of rock sheets composed of rock types found on the Caledonian foreland - namely Lewisian basement together
with Torridonian and Cambrian sediments. In places the Lewisian basement forms sheets a few hundred metres thick,
for example at Glencoul. Here the Glencoul Thrust carries basement rocks which preserve, in essentially unmodified
form, geological structures that were produced much earlier in the history of the Lewisian. So the process of
thrust need not damage the sheets of rock that are being carried, excluding those caught up along the thrust itself.
In the case of the Glencoul Thrust this damage is limited to within a metre or two of the thrust plane. The neat
thing is that the old Lewisian structures in the thrust sheet can be matched with those on the foreland. The offset
of these markers allows geologists to deduce the amount of movement on the Glencoul Thrust - about 30 km.
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Click on the icon to reconstruct the movement on the Glencoul Thrust. |
Apart from sheets of Lewisian basement, the Moine Thrust Belt is known for the development of so-called "imbricate
structures". These are thin slices, chiefly of Cambrian sediments, that are repeated on thrusts time and
again. Sometimes these are found beneath the over-riding sheets of basement (as at Glencoul), at other times
directly beneath the Moine thrust (as at Foinaven). In some places the amount of stacking can become very pronounced
indeed. Near the head of Loch Assynt geologists have traced out dozens of repetitions of the distinctive Fucoid
Beds and Salterella Grit units. 
The
map is zebra-like stripy. By knowing the original thickness of the units involved in these imbricate structures
it is possible to work out just how much original length of strata has been stacked up. In places like Foinaven
(where it's chiefly Pipe Rock that's stacked) and southern Assynt (where it's Durness limestone) the remarkable
conclusion is that the original lengths of the strata involved were over 50 km. Now these rocks occupy a length
of just a few kilometres - testifying to the great displacements represented in the Moine Thrust Belt.
If you return to the thrust belt contents you can see examples of cross-sections through
the Moine Thrust Belt and general stuff about thrusts.
Extra information
The following notes are provided for those interested.
Fault-rock types and deformation conditions
There is a range in fault rock type found on east-west sections through the thrust belt. Structurally higher,
eastern units are generally marked by mylonitic textures and dominantly crystal plastic deformation microstructures.
These are accompanied by chlorite-grade metamorphic assemblages (c. 350oC). To the west, in structurally
lower thrust zones the fault rocks tend to be dominated by cataclastic fault rocks. This transition is a time
sequence from early, hotter and deeper shearing carried up on higher level, cataclastic faults. However, complete
evaluation of the fault rock patterns is difficult because of widespread lateral branching of thrusts making
north-south correlations very uncertain. The extent to which temperature played the dominant role rather than
strain rate is not clear.
You can see images of fault rocks by visiting the deformation part of the
site.
Dating thrusting
The fundamental constraint on the timing of deformation in the Moine Thrust Belt is provided by the unusual
igneous intrusions of Southern Assynt. The age of the Borrolan complex is about 430 Ma. The intrusions are cut
by the Moine Thrust and geophysical data suggest that they are cut at depth. All minor intrusions in Assynt, where
outcrops are clear, can be shown to pre-date thrusting. So thrusting post dates 430 Ma. However, there are indications
in the poorly exposed ground of southern Assynt that some of the later parts of the Borrolan complex cross-cut
the major Ben More thrust. So this suggests that thrusting was active at about 430 Ma.
In the 1990s there were various attempts to date directly the mylonites of the Moine thrust using the Rb-Sr
system applied to recrystallised micas. There are difficulties with these methods but the dates obtained range
from 430 to about 410 Ma for samples collected just south of Ullapool and for the Knockan area.
Deep structure of the Moine Thrust
During the 1980s there was considerable debate regarding the deep structure of the Moine Thrust Belt. The motivation
came from the planning and execution of deep seismic reflection profiling off the coast of Sutherland. The debate
was kicked off by Jack Soper and Tony Barber (1982) who simply projected the surface geology to depth beneath
northern Scotland. Their deep rooting model shows the Moine Thrust sweeping down to the base of the crust at
about 30o from its surface outcrop in Scotland. But in this model Soper and Barber did not take into
account the large displacements locked up in the Moine Thrust Belt. Rob Butler and Mike Coward (1984) pointed
out that these displacements, applied to the Soper and Barber model, implies that Cambrian sediments were deposited
directly on mantle rocks. This is not sensible. So the Butler and Coward model has the Moine Thrust with a gently
dipping profile under the northern Highlands.
It might be hoped that the geophysical data could shed light on the debate. In the 1980s various deep seismic
lines were shot offshore by the BIRPS group. Unfortunately these are not conclusive. A cursory glance suggests
that the offshore continuation of the Moine Thrust cuts down to the lower crust, rather as predicted by Soper
and Barber. But the problem is that the offshore area is a sedimentary basin with lots of east-dipping faults
that formed much later than the Moine Thrust. It is not at all clear how these later structures have controlled
the seismic images or even if the offshore geology matches well with the onshore. So the debate remains unresolved.
Unfortunately BIRPS funding stopped before onshore seismic data could be acquired.
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