Evolution of the Northern Belt, Southern Uplands: implications for the Southern Uplands controversy

Abstract

Abstrart: Conodont and graptolite dating provide a high resolution biostratigraphical framework for correlation across the Southern Upland Fault. The establishment of an early Caradoc age for the red chert and volcanic sequences within the Northern Belt of the Southern Uplands confounds the accretionary prism model. and our data support the view that following the obduction of the Ballantrae ophiolite, fan complexes prograded southeastwards across a narrow, fault-bounded shelf into the deep basin of the Northern Belt. The fan complexes were initiated by uplift in the hinterland, caused by the intrusion of plutons into the Midland Valley Terrane. Sedimentation was controlled by regional extension, subsidence and eustasy through the Llanvirn and Caradoc. Basic lavas, with attenuated within-plate geochemical signatures, were erupted during the early Caradoc peak of extension and may have just post-dated the initiation of southwards subduction beneath the Lake District.

The Southern Uplands Terrane in Scotland – a notional controversy revisited

“…Anderson might well have added the third possibility for the origin of the Northern Belt that had been raised by Armstrong et al (1996). This extensional fore-arc interpretation was then developed further by Armstrong & Owen (2001) Thereafter, following collision of the outboard arc and the southward thrusting of the Ordovician tracts, the Silurian successions of the Southern Uplands built up as in a foreland basin thrust belt; there are similarities at this stage with the earlier, back-arc to foreland basin model of Stone et al (1987).…”

“…An alternative proposal by Armstrong et al (1996) for the origin of the Ordovician Northern Belt was based on detailed biostratigraphy and correlation with the coeval Girvan succession of the Midland Valley Terrane. Armstrong et al (1996) regarded the Northern Belt strata as having been deposited as prograding submarine fans at a subsiding and extending continental margin, effectively as the more distal equivalent of the Girvan succession although requiring a more varied provenance. Basic lava and chert intimately associated with the some of the Northern Belt sedimentary units were thought to be indicative of an attenuated rift environment.…”

“…The entire terrane had been proposed as an accretionary prism formed from the sedimentary cover of Iapetus oceanic crust as it was subducted beneath the margin of Laurentia (McKerrow et al 1977, Leggett et al 1979; subduction operated continuously from the mid-Ordovician until the late Silurian, when the Iapetus Ocean closed. Alternatively, the northern (Ordovician and perhaps oldest Silurian) strata had been interpreted as the deposits of either a back arc basin (Morris 1987;Stone et al 1987) or an extending and subsiding fore-arc (Armstrong et al 1996). The southern (Silurian) strata had been interpreted as originating either in a foreland basin that migrated onto the Avalonian continental margin following mid-Silurian closure of the Iapetus Ocean (Stone et al, or in a successor basin lying above the Laurentia-Avalonia continental suture following endOrdovician closure of the Iapetus Ocean (Murphy & Hutton 1986).…”

“…Barnes et al 1989) Conodonts were used by Armstrong et al (1996) to supplement the graptolite biostratigraphy of the Northern Belt and to strengthen correlation with the coeval Midland Valley successions, an important aspect of their fore-arc model. Their interpretation, and the broader regional synthesis by Armstrong & Owen (2001), was also aided by correlations of the shelly fauna, notably brachiopods.…”

“…The model for the Southern Uplands Northern Belt proposed by Armstrong et al (1996), which envisaged its deposition within an extensional basin formed on the Laurentian continental margin, also fails the 'thermal history test'. Although the proposed basin developed above thinned continental crust, the necessary eruption within it of volcanic units such as the Downan Point Lava Formation (the northernmost tract of the Tappins Group adjacent to the Ballantrae Complex) and the Bail Hill Volcanic Group (Figure 1a) would argue strongly for an elevated heat flow in an extensional setting.…”