Pre-tectonic reconstruction of the Islay anticline: implications for the depositional history of Dalradian rocks in the SW Highlands

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Abstract
Summary: The shape and orientation of the finite strain ellipsoid has been determined at more than 50 locations around the Islay anticline. Restoration of the ellipsoids to their original spherical form with the appropriate adjustments in orientations and positions of parts of the Anticline allow it to be ‘unfolded’. Original stratigraphical thicknesses were generally twice present day values and constant across the area with the exception of those of the Jura Quartzite which had much more marked thickness variations than are now observed. These probably resulted from syndepositional normal faulting with NE-SW crustal tension (a similar stress system controlled the intrusion of the Jura epidiorite dykes subsequently). This unstable phase in the depositional environment interrupted the otherwise uniform subsidence during Argyll Group deposition. A NE-SW striking (Caledonide) palaeostope did not develop until the close of the Argyll Group with the very energetic deposition of the Crinan Grit turbidites. Palaeocurrent data, especially for the less competent lithologies, are unreliable unless the suggested corrections are made for the effects of penetrative tectonic strain.