DEPOSITIONAL AND GEOTECTONIC HISTORY OF THE GALA AREA, EASTERN SOUTHERN UPLANDS, SCOTLAND

The Ordovician and Silurian successions between Falahill and Galashiel s encompass six flysch-dominated formations: the Upper Ordovician Port patrick and Shinnel Formations representing the Leadhills Group, the L landovery Mindork, Garheugh, and Buckholm Formations together comprisi ng the Gala Group, and a formation indeterminate of age within the Haw ick Group. Southward ensialic andesitic volcanic arc and northward low - to medium-grade sialic sources contributed sediment, whilst ophiolit ic and subduction-related sources made minor contributions. Deposition took place firstly, in a SE-migrating back-arc basin bordering the no rtherly source, the Laurentian continent. Subsequent NW-directed under thrusting led to formation out of the back-arc basin of an imbricate t hrust stack which migrated southeastwards. Ultimately a foreland succe ssor basin formed ahead of the rising thrust stack. Flysch units are t ypically associated with linear outcrops of Moffat Shales which are th e loci of major steep SE-translating reverse faults, two of which part icipate in a late-stage sinistral strike-slip duplex with large-scale imbrication. The faults divide the succession into a sequence of tecto nostratigraphic blocks, successively younger to the SE. At least six o f the ten blocks customarily recognised in the Southern Uplands, Block s 3-8, are represented, some of which coincide with single or complete formations.