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

Abstract

In common with most thrust belts, the Moine Thrust Zone, Scotland, preserves a forelandpropagating sequence of thrusting. However, significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial. Remapping of the northernmost Moine Thrust Zone has defined a thick belt of mylonites in the centre of which lies the ductile WNW-directed Moine Thrust. The base of the mylonite belt, however, is marked by a regionally significant, low-angle, west-directed brittle fault, here termed the Lochan Riabhach Thrust, that truncates thrust-related structures in its footwall. Comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at Durness indicates that the Lochan Riabhach Thrust cuts down-section in its footwall and therefore appears to be extensional. Locally, it is unequivocally breached by underlying imbricate thrusts. We prefer to interpret the Lochan Riabhach 'Thrust' as an extensional fault that formed during a hiatus in thrust stacking. We relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common, although possibly unrecognized, features in many other thrust belts.

Progressive fold and fabric evolution associated with regional strain gradients: a case study from across a Scandian ductile thrust nappe, Scottish Caledonides

“…1), metamorphic grade is difficult to determine accurately. However, the presence of biotite, muscovite and garnet in pelites, coupled with hornblende, biotite and garnet in the amphibolite suggests upper greenschist to lower amphibolite facies similar to that observed along-strike on the north coast (Soper & Brown 1971;Winchester, 1974;Moorehouse & Moorehouse, 1977;Holdsworth 1989;Holdsworth et al 2001Holdsworth et al , 2006Holdsworth et al , 2007.…”

Earthquake deformation in the northwestern Sierras Pampeanas of Argentina based on seismic waveform modelling

“…The Ancasti Fault seems to be the youngest structure of the eastern ranges since it truncates the imbricated faults of the Sierra de Ambato, such as is interpreted for similar out-of-sequence roof-thrust geometries of other regions (e.g. Butler, 2004;Holdsworth et al, 2006). All these main faults in the Sierras de Ambato and Ancasti are west vergent in contrast with the faults in the Sierra de Velasco, which have opposite vergence.…”

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

“…These thrusts are rather continuous along strike and do not show a pronounced transverse zone across the projected trace of the shear zone (Holdsworth et al 2007;British Geological Survey 2002b). Farther SE (south and east of Eriboll) there are lateral ramps in the thrust belt (Holdsworth et al 2006); the nature of the Rispond Shear Zone here is unclear and a link between the two structures would require further study. 2) The Laxford Shear Zone is a wide shear zone (Beach et al 1974;Goodenough et al 2009;this volume), the southern part of which lines up with Loch More.…”