Nature of the Moho beneath the Scottish Highlands from a receiver function perspective

The geology of Scotland documents a protracted geological history from Precambrian basement formation through the Caledonian Orogeny and to the Tertiary opening of the Atlantic. A temporary deployment of 22 closely spaced broadband seismic stations in Scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure. With teleseismic P-wave receiver functions, we use the H−κ method to determine variations in bulk crustal parameters: crustal thickness (H) and VP/VS ratio. Mean crustal thickness is ~28km, varying from ~23km in the NE highlands and increasing to 30km near the Highland Boundary Faultin the southern part of the study area. Mean VP/VS values of ~1.76 show no significant variation across the study area. An abrupt increase in crustal thickness of ~4.5km NW across the Moine Thrust is not easily linked to Tertiary–Recent tectonic activity. Instead it appears that Scotland's crust has retained features for hundreds of millions of years since it was first formed, despite abundant volcanic activity in Tertiary times. Our work favours the view that the Moho is a compositional boundary rather than a mineralogical one defined by a reaction in P–T space.