Thrust imbrication of Ordovician and Silurian submarine fan sequences overlapping pelagic deposits in the Southern Uplands has been interpreted in terms of an accretionary prism formed above a NW-directed subduction zone. Structural features invoked to support accretion are not definitive and could be explained in terms of a thin-skinned thrusting model. New palaeocurrent and compositional evidence from Llandeilo to mid-Llandovery age turbidites in the northern part of the Southern Uplands proves interdigitation of sediments with strongly contrasting petrography. Turbidites derived from the south contain significant quantities of fresh andesitic detritus whereas those from the north form more mature quartz-rich formations. This implies a back-arc situation; the turbidites being deposited in a basin with a relatively mature continental landmass to the north and a rifted continental fragment containing an active volcanic arc to the south. Oblique collision of the opposing continental margins of the Iapetus Ocean during the Llandovery caused the cessation of subduction. Underthrusting of the southern margin initiated a SE-propagating thrust stack which deformed the back-arc basin sequence and may eventually have ramped over the eroded and faulted remains of the volcanic arc. A southward-migrating foreland basin formed ahead of the rising thrust stack and is now represented by the late Llandovery Hawick Group and Wenlock sequences. Mid- to end-Silurian sinistral strike-slip resulted from oblique collision and produced a transpressional regime during which reactivation of deep-seated structures allowed the intrusion of lamprophyre dykes and granites.

Analytical procedures and results: U-Pb detrital zircon geochronologyDetrital zircons were analysed at the University of Alberta Radiogenic Isotope facility. Samples were crushed and zircons were separated and concentrated by standard techniques using a Wilfley table, heavy liquids and magnetic separator. The grains were ablated using a New Wave Research UP213 Nd:YAG with aperture imaging system. The wavelength was 213 nm, with a fluence of 3 Jcm -2 , a 4 Hz pulse rate and a spot size of 40 µm. Ablated ions were analysed with a Nuplasma multicollector ICP-MS with plasma power, gas flows, detector configuration and isotope measurements as described by Simonetti et al. (2005). Results from each sample site were recorded in 30 one-second integrations after a settling time of 3 s. Blanks and standards were recorded for the same duration as unknowns.For the Southern Uplands samples, analysed first, separated zircons were picked, avoiding cracked or altered grains, and ~200 grains were mounted in an epoxy mount and polished to a depth required to approximately expose grain centres. Ablation points were selected using a combination of electron backscatter images, reflected, and transmitted light, so as to avoid obvious inclusions, discontinuities and cracks. Grains were analysed in sequences of 10 or 12, preceded and followed by at least two analyses of standards. Results were normalized using the in-house standard LH94-15 with isotopic ratios determined by thermal ionization mass-spectrometry (TIMS) as quoted by Simonetti et al. (2005), yielding a concordant age of 1830 Ma.

A thick Ordovician marine flysch succession including greywackes, conglomer- ates, siltstones and shales occurs in West Nithsdale in the Northern belt of the Southern Uplands. Five new formations of Caradoc and ?Ashgill age are defined based mainly on petrographical and lithological evidence. The proposed correlation with areas up to 80 km distant to the SW, emphasises the persistent nature of the greywacke formations in the Southern Uplands when traced along strike. It is demonstrated that the main inliers of black shale in the Northern belt cannot be simple anticlines but are closely associated with large strike faults. The position of the investigated flysch succession in the accretionary prism model for the development of the Southern Uplands during the evolution of the Caledonides is indicated and its significance assessed.

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