The anatomy and evolution of a transpressional imbricate zone, Southern Uplands, Scotland

摘要

This paper describes in detail a spectacularly exposed transpressional imbricate zone from the Southern Uplands of SE Scotland. A highly heterogeneous assemblage of contemporaneous structures is preserved and is interpreted to have formed during bulk inclined triclinic sinistral transpression in an obliquely convergent thrust wedge. The resulting structures include a piggy-back imbricate system of closely related strike-slip detachments, highly curvilinear folds, oblique thrust faults and a clockwise-transecting cleavage. A series of sub-domains are recognised, which are interpreted to result from imperfect kinematic partitioning of the bulk transpression into either contraction- or wrench-dominated strains. In this case, the partitioning appears to be controlled by pre-existing lithological anisotropy and how this was subsequently modified by processes such as folding once deformation began. Our findings demonstrate that transpressional deformation zones can develop structural patterns geometrically very similar to those found in dip-slip fold and thrust belts. Such geometries are likely to occur in many other obliquely convergent wedges because deformation zone boundaries are very often inclined and lie at low-angles or sub-parallel to regional bedding. Imbricate systems inevitably form because the sedimentary anisotropy is a key mechanical control in their development. Our observations illustrate, therefore, the fundamental importance of measuring fault movement directions in the field using fault slickenline lineations in imbricate systems. It is possible that a number of 'imbricate zones' identified in other thrust wedges may turn out to be transpressional features.