Structural evolution of the Moine thrust belt between Loch More and Glendhu, Sutherland

Abstract

SYNOPSISReinvestigation and new mapping confirm that the area contains a number of large thrust sheets and imbricate slices of Cambrian cover and Lewisian basement but emphasise that they are bounded by an interconnecting network of WNW-directed faults. The network demonstrably propagated, piggy-back fashion, into the foreland since higher structures are cut or bulged up by lower imbricates. These new interpretations require a revision of structural units in the Glendhu area.The margins of culminations, which resulted from the continual stacking of imbricate slices, are the sites of extensional faulting which locally post-date thrust structures. However, some of the major thrusts in the area have primary extensional geometries in their footwalls and hanging walls since they cut down stratigraphic section towards the foreland. The proposed explanation is that bedding in the foreland Cambrian shelf was dipping gently to the SE, possibly in response to isostatic loading by the emplacement of the c. 10 km thick Moine thrust sheet. Datum-parallel thrust flats would then cut down stratigraphic section with an extensional sense. Early extensional strains which locally predate imbricate thrusts in those Cambrian quartzites in the immediate footwall to the Moine thrust may reflect a major gravity spreading component in the initial phases of the emplacement of the Moine sheet.The geometry of thrusts in the Glendhu culmination is illustrated on a balanced cross-section. The hanging wall of the Glencoul thrust, which includes a stack of interthrust Cambrian cover and Lewisian basement termed the Aisinnin imbricates, restores to a width of over 15 km. This can be added to the displacement on the Glencoul and lower thrusts as estimated from the offset of foreland structures, of 25-33 km, to give a total restoration for the footwall of the Moine thrust of at least 40 km.

Thrust sequences and the significance of low-angle, out-of-sequence faults in the northernmost Moine Nappe and Moine Thrust Zone, NW Scotland

“…Loch More, Glencoul and Knockan, Fig. 2; Coward 1982Coward , 1985aButler 1984). At the time of their discovery, three models were proposed that could be applied to account for the development of the Lochan Riabhach Thrust, as follows.…”

“…(1) Such thrusts have propagated through part of the thrust zone that had locally developed a more accentuated hinterland (eastward) dip as a result of flexural loading of the foreland by the developing nappes (Butler 1984). This model could explain the apparent extensional geometry across strike, but cannot account for the out-of-sequence character of the Lochan Riabhach Thrust.…”

Lateral variations and linkages in thrust geometry: the Traligill Transverse Zone, Assynt Culmination, Moine Thrust Belt, NW Scotland

“…As no Torridon Group rocks occur in the Foreland or in the thrust belt in this region, it cannot be determined whether post-Torridon movement occurred along the shear zone. However, c. 2 km south of Loch More is a major oblique ramp (Elliot & Johnson 1980) marking the termination of a large thrust sheet with a thick basement sliver, with the Bhuirich Dome also occurring here (Butler 1984, British Geological Survey 2007. It is likely that these major lateral changes in thrust architecture were at least in part rooted above the Laxford Shear Zone.…”

Inversion tectonics of the fold-and-thrust belt, western Taiwan

“…The misinterpretation of folds and thrusts as being related only to thin-skinned shortening without consideration of the possibility of the inversion of a sedimentary basin can have farreaching effects on the structural and tectonic interpretation of a region, leading to (1) use of the wrong method in section construction, (2) miscalculation of the amount of orogenic shortening, and (3) incorrect assumptions about the nature of structures at depth, both those directly beneath the fold-andthrust belt and those farther back within the hinterland of the mountain belt. Typical examples include the Sulaiman and Kohat Ranges, Pakistan (Coward, 1994(Coward, , 1996Coward and Butler, 1985) and the Moine thrust belt of the Scottish Caledonides (Elliott and Johnson, 1980;Stewart, 1982;Coward, 1988;Soper and England, 1995;Butler, 1984Butler, , 1997. It is, therefore, important to consider the concept of inversion tectonics in the interpretation of the geologic evolution of the Taiwan fold-andthrust belt.…”