The Highland Border Complex, NE Scotland, revisited: its role in Laurentian rifting, Early Ordovician emplacement and later faulting at the Highland Boundary

The Highland Border Complex (HBC; e.g. River North Esk, Stonehaven) is here interpreted as part of a Cambrian–Early Ordovician hyperextended continent–ocean transition zone that was tectonically emplaced onto the S Laurentian margin related to Early Ordovician collision with a volcanic arc to the SE (Midland Valley). All HBC units include terrigenous sediment. Volcanics range from alkaline OIB, to variably enriched (North Esk), to MORB (Stonehaven). They include radiolarian chert and hydrothermal metalliferous deposits. A local conglomerate with locally stretched clasts (North Esk) includes chert and ultramafic detritus (with chromite). During and after emplacement, cross-cutting high-strain zones developed in response to progressive left-lateral/oblique ductile syn-metamorphic to brittle post-metamorphic stresses. Three main collision-related deformation events affected both the HBC and the adjacent Dalradian (North Esk), the first two under greenschist facies metamorphic conditions. Following peak metamorphism, the HBC and Dalradian in the Highland Border rotated to near vertical (Downbend). After exhumation via orogenic collapse and/or erosion, the Lower Old Red Sandstone (LORS) transgressed the Highland Border. In the North Esk, the basal LORS (Devonian Lintrathen ignimbrite) unconformably overlies the HBC, precluding this contact as the Highland Boundary Fault. The North Esk Fault and Highland Boundary Fault (Stonehaven) can be broadly correlated as a long-lived crustal discontinuity.