Basin subsidence analysis, employing the backstripping method, indicates that fundamentally different basin-generating mechanisms controlled Silurian basin development in northern England and in Wales. Accelerating subsidence with time suggests that the dominant control on basin formation in northern England was flexure of the Eastern Avalonian lithosphere. Mid-Wales, however, displays dominantly decelerating subsidence, suggesting an extensional control. The errors involved with the backstripping method, particularly concerning Lower Palaeozoic application, have been addressed.

In conjunction with sedimentological and tectonic data, the subsidence patterns for the Windermere Supergroup indicate deposition in a flexurally-loaded foreland basin which migrated southwards with time. Similar data for the Powys Supergroup of Mid-Wales imply an extensional control on Silurian deposition, possibly in a strike-slip setting. However, later Pøídolí and early Devonian Welsh sedimentation may represent the onset of foreland basin deposition. The new data are consistent with a diachronous closure of the Iapetus ocean, related to rotational collision of Eastern Avalonia with Laurentia during the early Silurian. Early Silurian oblique collision produced transtension in Wales. Subsequent rotation of Eastern Avalonia produced more orthogonal collision. Thus, continental flexure was felt initially in northern England in late Wenlock times, followed by migration of the resultant foreland basin to the southeast into Wales in early Devonian times.

The versatility of the backstripping method is shown by this application to Lower Palaeozoic sequences.

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