ABSTRACTVarious order duplex structures are described from oceanic sequences of basaltic and associated pelagic–hemipelagic sedimentary rocks in the Ordovician (northern) part of the Southern Uplands accretionary complex. The general structure of the terrane as a whole strikes ENE, but each component lithological tract strikes NE or more northerly, oblique to the regional trend, making an en echelon outcrop pattern. Further oblique relationships between structures and lithologies can be mapped at larger scales, up to 1 km scale or more. These duplex structures are thought to be originally SE-verging, now partly overturned to the NW. Differences in the en echelon geometry, either sinistral or dextral, are explained by variable plunge of the original structures.Peach & Home's first regional map of the Southern Uplands suggests an en echelon pattern of lithologies, implying large-scale duplex structures across the whole terrane. Here, the duplex structures are regarded as ubiquitous at both regional and smaller scales, suggesting considerable horizontal shortening. This was accommodated by such structures during underplating and out-of-sequence thrusting, in all parts of the accretionary prism, but particularly in the deeper tectonostratigraphic levels. The duplex structures are characteristic of ancient décollement zones.
K-Ar ages of pelagic sedimentary rocks from Jurassic accretionary complex in eastern Yamaguchi Prefecture, Southwest Japan and their geologic significance.