A model for the crustal evolution of southern Scotland

Abstract

In the late Silurian, southern Scotland was a mature accretionary active margin of Sumatra‐type, on the southeastern edge of North America, while England was on a passive margin on the opposite side of the Iapetus Ocean. Southward subduction below England during the Middle Ordovician produced a magmatic arc which now lies close to the Iapetus suture, and thus argues for a missing forearc terrane. Tectonic erosion or strike‐slip removal may be responsible for some of this missing terrane to the north of the Lake District, but we now suggest that underthrusting below southern Scotland may also be an important cause. If the Ordovician forearc terrane of northern England is now below the Southern Uplands, we would have an explanation for the evidence for similar lower crustal foundations on either side of the surface trace of the Iapetus suture (velocity structure from the LISPB study, Sr/U‐Pb isotopic signatures and geophysical attributes of the c. 400 Ma granites). The under‐thrusting model still leaves room for considerable modification of both margins by large strike‐slip faults. Seismic refraction profiles of modern accretionary active margins (Sumatra, SW Japan) allow us to propose a likely velocity profile for late Silurian southern Scotland. We choose a Mesozoic forearc truncated by strike‐slip faulting, the Queen Charlotte Islands margin of the NE Pacific, as an analogue for the late Silurian southern Iapetus margin. Following previous suggestions of highly oblique and slow convergence during the final stages of closure of Iapetus, we combine the two profiles by under‐thrusting the southern margin to produce a model which involves minimal shortening in the collision zone. The crustal profile resulting from such a collision would be very similar to the modern southern Scotland profile, both in terms of the scale of the tectono‐stratigraphic terranes involved and of the resulting crustal velocity structure. The cessation of accretion after the mid‐Wenlock may be related to impingement of the English continental crust against the Scottish trench. Subsequent uplift of the Southern Uplands may be connected with the appearance of coarse detritus in the Lake District during the Silurian.