Abstract

Detailed structural studies in the Wenlock age Southern Belt of the Southern Uplands have identified major controls on tectonic development. Original thickness and composition of sediment control the style of faulting: thin, poorly consolidated, interbedded sandstone/mudstone sequences form broad sheared zones, while thicker, well consolidated sandstone-dominated sequences fail along thin, bedding-parallel brittle fractures with vein infill. The structural evolution of the sheared zones provides an insight into melange development. Depth of burial within the accretionary pile governs the development of cleavage and intensity of post Fl deformational phases. The intensity of imbrication within accreted material probably relates to subduction rate with early Wenlock sequences being highly imbricated and mid Wenlock sediments preserved in internally coherent fault-bounded packets. Sinistral strike-slip movement controls the development of steeply plunging folds, sub-horizontal stretching directions and sideways facing bedding cleavage relationships, and strongly influences fabric development in sheared zones. Determination of the relative importance of these factors permits a fuller understanding of the deformational history.