Seismic reflection data from northern England are interpreted to show a northward dipping Caledonian thrust zone lying beneath the faulted southern margin of the Northumberland Trough. The thrust zone lies upon the updip projection of a northward dipping high conductivity layer in the middle and lower crust, identified by magnetotelluric sounding. Together, these features indicate the presence of a crustal-scale Caledonian shear zone, dipping north at an average angle of 20–25°. Extensional reactivation of the middle and upper crustal parts of the shear zone, in Carboniferous times, led to development of the Northumberland Trough. Reappraisal of the BIRPS NEC seismic profile suggests the presence of the same (or similar) crustal shear zone, reactivated to form the offshore extension of the Northumberland Trough. The shear zone probably formed as a major compressive or transpressive feature during late Caledonian (Acadian) continental collision. In the lower crust, the shear zone lies close to, or may actually represent, the Iapetus suture. At middle to upper crustal levels the shear zone lies wholly within Avalonian (English) crust, well to the south of the (faunally determined) Iapetus suture. Westward extrapolation of the shear zone indicates that it may crop out within the northern Lake District, perhaps as the Causey Pike and associated thrusts. The faunal suture is likely to subcrop north of the Lake District and Alston Block, beneath the central part of the Northumberland Trough.