Structural geometry, tectonic history and deformation mechanisms in the Moine thrust zone near Ullapool, N.W. Scotland

The Moine thrust is an example of a foreland thrust belt. The thrusts propagated from the hinterland towards the foreland so that higher) older thrusts were deformed by movement on the younger, lower thrusts. The Moine thrust was later reactivated as a low angle extension fault and cut through the structures which had originally deformed it. Horses are an important structural component of the thrust zone, and were accreted to the thrust sheets by collapse of footwall ramps and asperities. The bedding geometry within the horses is classified, and allows an insight into the original geometry of the ramps and asperities. The extension faults, tectonic veins, hydraulic fractures and stylolites in the lowest thrust sheet (Sheet IV) are considered to have formed as a result of gravitational loading by the overlying thrust sheets. Using a semi-quantitative model it is shown that high pore fluid pressures were needed to form the extensional structures. Differential stress was estimated from Sheet IV by using twin analysis in carbonates, and by a new method based on observations of modes of rock failure. The palaeostress estimates support a two stage model of thrust movement: firstly, a compressional stage, during the initiation and propagation of the thrusts, when differential strýss was ca. 1300 bars; and secondly a loading event during translation of the thrust sheets, when differential stress was ca. 500 bars. It is suggested that the driving mechanism during this second stage was gravity spreading. Fault rocks show that the dominant deformation mechanism was cataclasis along the thrust planes. Illite crystallinity studies show a widespread heating event in the foreland which is interpreted as a thermal response to thrust sheet emplacement. Temperatures reached 2750C approximately 20 Ma after the main thrusting event. Three stages of movement are proposed at 440-430 Ma, ca 425 Ma, and 425-414 Ma. This is based on the relationships between syn-thrusting intrusions and thrust planes combined with radiometric dates from the Moine sediments.