Strain partitioning in transpression zones: examples from the lapetus suture zone in Britain and Ireland

Abstract

Kinematic partitioning in transpression zones is well documented and leads to the development of discrete domains of contraction- and wrench-dominated deformation. Although widely reported there are few detailed descriptions of such partitioning in ancient transpression zones. The lapetus Suture Zone comprises a broad band of Lower Palaeozoic rocks that were deformed within a sinistral transpression zone immediately prior to or during the Acadian phase of the Caledonian Orogeny. Excellent coastal exposure of these rocks in SE Ireland, the Isle of Man and SE Scotland allow detailed comparative studies of the deformational patterns and processes within an ancient transpression zone to be made. All three areas preserve a highly heterogeneous assemblage of contemporaneous structures, including folds, interlinked strike-slip detachment faults and a regional cleavage that locally transects in a clockwise sense. Geometrically and kinematically different assemblages of these structures define a series of fault bounded, structural domains that are interpreted to result from the kinematic partitioning of a regional triclinic transpressional deformation. This interpretation lends support to assertions that many transpression zones are triclinic in nature. However, the geometric and kinematic patterns in the different domains suggest a strain model where the triclinic transpressional strain is partitioned into monoclinic end-members (dip-slip non-coaxial contraction and strike-slip simple shear) rather than into domains of pure shear and oblique simple shear. Clockwise transacted cleavage, sinistral strike-slip faulting and zones of sideways and downwards facing sinistrally verging folds have been used to infer sinistral transpressive strain. This project suggests that such features are particularly obvious in regions where significant amounts of partitioning have occurred. The nature and distribution of strain partitioning appears to be controlled by the presence of mechanical heterogeneities on regional and local scales, e.g. the proximity to weak tract-bounding faults and the presence of lithologically controlled regions of high pore fluid pressures, respectively.