Northumberland
This north-south section across the Bewcastle area, near the boundary between Northumberland and Solway Basins, is a good representative model for their structure style. This is the setting for nationally important geothermal potential.
In Northumberland the Permian and Triassic is missing by erosion and the infill is mainly Carboniferous, which is about 4-5000 metres thick on the south flank, lying across Old Red Sandstone depocentres. The southern margin is defined by the syn-depositional Ninety Fathom and Stublick fault systems, separating it from the granitic Alston Block: these have very large throws, the rifting was mainly early Carboniferous. These major faults root onto the extensionally-reactivated Iapetus Suture, which was a plate boundary supporting Caledonian subduction of the Lake District northwards under the Southern Uplands.
The main structural block pattern is the same as in the Solway Firth, with huge ramp faults dipping southeast controlling sedimentation and subsequent deformation. The rifting and opening of northern North Sea in the early Permian was an east-west extension, so a simple model for the partial reversal of NE-SW faults rooting into the deep ramps is that it is transpressional and caused reactivation of the growth faults along with new faulting in cross-basin shortening. The compression shortened the Basin obliquely to create hangingwall rollovers on the steeper ramps.