The Siluro-Devonian evolution of the southern Midland Valley of Scotland

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Abstract
Geological studies along the southern margin of the Midland Valley of Scotland, together with previously published work, suggest a strike-slip model for its evolution during Silurian and Devonian times. The data emphasize the ‘older Caledonian’ influence and persistent, albeit intermittent, transcurrent activity along the Southern Upland Fault which is the boundary between the Midland Valley and Southern Upland terranes. A comparison of the geological characteristics of the Silurian and Devonian rocks in the southern Midland Valley with characteristics from strike-slip basins elsewhere suggests that the Midland Valley rocks were deposited and locally deformed under sinistral strike-slip regimes with two main episodes of transtension — the first acting in Llandovery—Wenlock times and the second in Lower Old Red Sandstone times. A strike-slip model involving transpression may explain why there is a strong unconformity between the Llandovery—Wenlock and the Lower Old Red Sandstone strata in the Pentland Hills and Girvan inliers but only a marked change in sedimentary facies at the equivalent horizon in the inliers of the central Midland Valley. The Lower Old Red Sandstone was probably deposited in a transtensional regime, but the succession is complicated by the widespread Lower Old Red Sandstone calc-alkaline magmatic event. The mid- Devonian deformation is also variable in its effects and the more easterly trend of theen echelonfold axes relative to the Southern Upland Fault trend is indicative of sinistral transpression. Carboniferous and older rocks were affected by smaller dextral movements on the Southern Upland Fault and related faults which started during Namurian times when a change to an overall dextral regime took place.