None of the major roles assigned to the Highland Boundary Fault, such as a transcurrent fault with an orogen-parallel, sinistral displacement of several hundred kilometres, or terrane boundary, are confirmed. The Highland Boundary Fault can be traced NE–SW across Scotland for >240 km and defines the northern margin of the Midland Valley. Except for small outliers of Late Palaeozoic rocks on the NW side, it separates the Highland Border Ophiolite and Dalradian rocks of Neoproterozoic–Early Ordovician age (Grampian terrane) to the NW from Late Silurian–Early Carboniferous rocks (Midland Valley terrane) to the SE. The present Highland Boundary Fault is a steep, NW-dipping, post-Early Devonian, reverse fault that probably terminates SW of Bute. Stratigraphical and provenance links, and palaeostress data, limit post-Early Devonian lateral displacement to <30 km. The fault formed in conjunction with the Strathmore Syncline (to the SE) during the Acadian (Middle Devonian) Orogeny, in a transpressive regime that caused uplift of the Grampian block and a small sinistral movement on the Highland Boundary Fault. The location and orientation of the present-day Highland Boundary Fault may have been controlled by rejuvenation of an unseen, steeply inclined, strike-slip fault formed 460–420 Ma ago, a period for which no geological record is preserved in the Highland Border.

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