Tectonostratigraphy of the Glen App area, Southern Uplands, Scotland: anatomy of an Ordovician accretionary complex

Abstract

The stratigraphy and structure of the Glen App coastal area, northwestern Southern Uplands, are characterized by structural repetition of both chaotic and coherent units of the Middle Ordovician (Caradocian) Tappins Group. The chaotic unit, the Currarie Formation, comprises blocks, including basalt, chert and terrigenous sedimentary rocks, set in folded and in places sheared red to grey siliceous mudstone. The coherent unit is turbiditic, and is divided into the Dalreoch Formation in the north and the Corsewall Formation in the south. The former is characterized by inclined, isoclinal folds and thrusts, whereas the latter shows a homoclinal dip, although both are largely overturned northwards. Small-scale duplexes are observed within the coherent turbidites of the Dalreoch Formation and chaotic units. Larger duplexes, identified on the basis of mapping, are oriented at an oblique angle to the general trend of regional structures. The chemistry of basalt blocks within the Currarie Formation shows a within-plate type, suggesting an origin as seamounts. The tectono-stratigraphy of the area is interpreted as the result of accretion of a substantial seamount at a subduction zone during Mid-Ordovician time. Thrust interleaving of chaotic and coherent units is inferred to have taken place in and above the décollement zone, while thick turbidite beds without strong folding were probably preserved below the décollement and emplaced below a large-scale nappe. Interpretation of the Glen App area as part of an accretionary complex confirms that an ocean basin remained open in part of the Iapetus system, at least until Caradocian times.