The Southern Uplands as an accretionary prism: the importance of analogues in reconstructing palaeogeography

Abstract

Fore-arc regions, whether accretionary, erosional or composite, vary greatly both along strike and through time, in the geometries which they assume in response to subduction. There is no well-studied modern/Neogene margin which can be established as the 'type' accretionary prism. All are individual in some respect or other. Nor is there an example of a well-studied modern/Neogene accretionary margin which can be compared closely to the Southern Uplands, though there are several good analogues for particular stages of its history. Accretion took place along the Southern Uplands margin for c. 50 million years. The range of geometries on modern/Neogene accretionary active margins, and the extent to which the behaviour of those margins can change during periods as long as 50 million years, is reviewed, emphasizing the accretionary prisms of Mexico, SW Japan and the Makran. The interpretation of the Southern Uplands as an accretionary prism, as presented in the late 1970s and early 1980s, is reassessed in the light of advances made subsequently in understanding modern/Neogene prisms. The recent alternative model of Stone et al. for the Southern Uplands envisages that the terrane evolved by the opening and closure (by arc-subduction) of a marginal basin. The new model draws heavily on arc-type detritus derived from the oceanward side of the ancient Southern Uplands trough. There are areas along the SW Japan fore-arc where oceanward-derived tubidites are accumulating in a trench (Nankai Trough), and where active submarine arc-type volcanism is occurring outboard of a currently growing accretionary prism. Such features in an ancient terrane like the Southern Uplands do not necessarily mean that an alternative model must be sought involving whole new arc systems. Furthermore, analysis of the life-spans and sedimentary fills of modern/Neogene marginal basins begs the conclusion that the Southern Uplands' strata were not deposited in a marginal basin. It is most unlikely that the c. 50 Ma history of pelagic sedimentation preserved there can be manifesting anything other than the closure of a sizeable ocean basin, of a scale exceeding any known western Pacific-type marginal basin, and with a different sedimentation history from any such marginal basin.