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Caithness Field Club

Which Way Did The Great Glen Fault Move?
J Saxon

It has been assumed from various evidence that the Great Glen Fault was a dextral wrench fault with a throw of many tens of miles. Supporting evidence for this was the suggestion  that the Foyers and Strontian Granites were once part of the same pluton, the two masses having become separated by movement along the Great Glen Fault. These granites are of Caledonian age.

There is, however, contradictory evidence that the fault is sinistral; such a movement in post-Devonian times of some 15km, can be inferred from the mismatch of the Old Red Sandstone on both sides of the fault. The Brora-Helmlsdale Fault, which is clearly associated with the Great Glen Fault, can also be shown to have moved sinistrally by at least the same amount in Post-Jurassic times.

Further evidence shows that metamorphic events of Caledonian age do not match up well unless a sinistral direction is assumed. To further confound the evidence is the fact that all the other important wrench faults in the world can be shown to be sinistral.

One criticism of the theory that the Foyers and Strontian granites were once part of the same mass is comparisons with other granites on or near the Lairg Granite, mother of the Helmsdale Granite. Mr. Colin Frith, a graduate of Thames Polytechnic, who was employed for a period during his degree course at Dounreay, undertook to make analytical comparisons between the various granites as the project for his final year degree course. His findings are given below. Specimens were obtained of the Foyers, Strontian, Lairg and Helmsdale granites for the purpose of analysing them. The lead concentrations in parts per million were as follows:-

*This specimen had yielded 420+ - 150 ppm by and independent and different analytical technique.

Strontium isotopic compositions were determined for the four granites .which showed that the Foyers, Strontian, and Lairg granites emplaced by the same mechanism but only that the Helmsdale Granite may have been emplaced by a different mechanism i.e. the Helmsdale granite may have originally had the same strontium isotropic composition when emplaced and may have since undergone secondary changes.

It is noted that the Helmsdale granite has considerably altered by secondary events and was also extensively weathered.

Eight rare earth elements (La, Yb, Sm, Tb, Nd, En, Cc, Ln) ,were determined for one sample of the Strontian, Foyers, Lairg and Helmsdale granites. The results tend to show again that the Strontian, Foyers and Lairg granites are similar and the Helmsdale granite is dissimilar.

No positive evidence can be shown from these data regarding the movement of the Great Glen fault except that a match up is possible in either direction between the Foyers granite and the Strontian for a dcxtral wrench fault, and between the Foyers and Lairg for a sinistral wrench fault.

Published April 1976 Bulletin