Structural geology of Easter Ross, Scotland: implications for movement on the Great Glen fault zone

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Abstract
The integration of a recently-drilled exploration well, sited near the town of Tain, Ross-shire, with a seismic database and onshore exposures allows a new interpretation of the structure in the Easter Ross peninsula, NE Scotland. It is now evident that previous interpretations, which highlighted the importance of NE–SW-striking lineaments, need some modification. Instead, numerous N–S to NNE–SSW-trending folds and faults affect Devonian sequences and show evidence of compressional reactivation of structures which had previously-experienced extension. These inversion structures only appear to be dissected by NE–SW- and E–W-striking faults and extensional fractures in areas adjacent to the Great Glen fault zone. Integration of the onshore data with results from adjacent offshore areas suggests that the peninsula has undergone a complex structural history since the Early Devonian. It appears to have been characterized by Early Devonian (probably rift-related) extensional faulting and subsequent Mid-Late Devonian regional (thermal) subsidence. The restriction of similar structural relations to Devonian sequences on offshore seismic lines from adjacent areas suggests that the contractional and inversion structures, which similarly affect the Late Palaeozoic sequences in onshore areas, were probably related to Permo-Carboniferous structural events rather than younger deformation. Further integration of offshore data with the occurrence of Jurassic sequences on the coast suggests that the peninsula may be considered an integral part of the Inner Moray Firth basin and experienced a similar Mesozoic subsidence history to offshore areas before undergoing regional uplift and dissection due to reactivation of the Great Glen fault zone during the Cenozoic. There is no evidence to suggest that this fault zone had any significant control on basin development, sedimentary sequences or structural geometries during Palaeozoic or Mesozoic periods of extension, and it probably only had minor effect during Permo-Carboniferous inversion. Similarily, its role in tectonic events after the Early Cretaceous appears to have been limited to local deformation with little evidence for any large-scale strike-slip offset along its length. The unusual occurrence of Jurassic coastal exposures in Easter Ross appears to represent fault-bounded slices of sediment downfaulted to accommodate the formation of a ‘negative flower structure’ in response to limited, dextral strike-slip movement. The results suggest that the Great Glen fault zone was a strike-slip fault system effectively locked during phases of extension in the Inner Moray Firth, but was reactivated during phases of inversion when extensional slip vectors sub-paralleled the lineament.