Identifying multiple detachment horizons and an evolving thrust history through cross-section restoration and appraisal in the Moine Thrust Belt, NW Scotland

Many thrust systems, including parts of the Moine Thrust Belt, are commonly interpreted as rather simple imbricate fans, splaying from a master detachment (floor thrust) at depth. We use field observations and geological map data to construct cross-sections through the Achnashellach Culmination, southern Moine Thrust Belt, Northwest Scotland, to test this interpretation. Initially cross-sections are constructed by assuming a single lower detachment; line length imbalances and thrust trajectory mismatches between deformed and restored-state sections indicate an invalid model. Significant differences in horizon lengths between two rock units are seen, indicating the position of a second detachment which, when incorporated into the deformed-state cross-section creates a valid structural model. The presence of this second detachment accounts for complex geometries seen at outcrop, and indicates that the Achnashellach Culmination is likely to have formed by the sequential activation of two detachment horizons. This new structural model has been derived using an iterative workflow involving cross-section construction, section balancing and integration of field observations from across the study area, ensuring model validity in three dimensions. This workflow is applicable to other systems in general.