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Abstract

Subduction complexes preserving extensive coherent strata beneath lower slope sediments can be more useful than those dominated by melange for studying the sequence of deformation caused by accretion. In the southern Uplands, a 70 to 80-km-wide coherent accretionary complex comprising imbricated thin ocean-floor (basalt, metalliferous sediment, radiolarian chert, black graptolitic shale) and thick trench (volcaniclastic greywacke) deposits records 50 to 60 million years of accretion during northward subduction of Iapetus oceanic crust under Ordovician and Silurian southern Scotland, then part of the southern margin of ancient North America (Laurentia). Like many more recent coherent accretionary terranes, the Southern Uplands complex appears to have developed about a slow convergence-high sediment input subduction zone. Competency contrasts in the subducting sequence controlled the mesoscopic and macroscopic structural style and determined the type of strata accreted. Detailed mapping of three new map areas shows a generalized tectonic history in which compressive stress is taken up by: 1) initial decollement (offscraping) commonly at or above a black shale unit (the Moffat Shale Group) underlying the greywackes; 2) local fold development, during and after isolation of discrete linear packets of offscraped strata; 3) variable cleavage development, commonly transecting folds; 4) strain-hardening, tightening, and eventual locking of each packet so that deformation switches to a new packet at the base of the inner trench slope; 5) subsequent more-or-less bedding-parallel (commonly intense) imbrication within packets as they are uplifted and rotated.

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