Early Devonian transpression to transtension in the South Mayo Trough, western Ireland: Small-scale structures related to relative plate motion

Abstract It is difficult to relate polyphase deformational sequences to relative plate motions in pre-Mesozoic plate boundary zones. We give an account of the structural evolution of the Ordovician–Silurian rocks of South Mayo, western Ireland, which form a well-preserved part of a narrow late early Devonian Erian collision zone between Laurentia and Avalonia. We review the structures that might be expected in an obliquely convergent plate boundary regime and demonstrate that the last, Erian, phases show an evolution from sinistral transpression through sinistral strike-slip to sinistral transtension. First, Silurian transpression caused zone narrowing and thickening with clockwise-transecting cleavage sequences and S/C fabrics. This was followed by strike-slip with banded shear fabrics and mylonite zones with steeply plunging isoclinal folds. Transtension followed to widen and thin the zone with the development of dextral kink bands and sinistral P-shears, some as giant open S-folds. Deformation was strongly partitioned between large areas with regional fabrics and four major shear zones. This sequence may be related to a change from sinistral oblique convergence to sinistral oblique divergence along the plate boundary zone between Laurentia and Avalonia. We relate this to the kinematic model developed in 2003 by J.F. Dewey and R.A. Strachan to explain the formation of late Silurian to mid-Devonian mainly “red-bed” basins during the progressive change in the motion of Baltica-Avalonia relative to Laurentia.