The mantle reflector pattern beneath the Scottish and Irish Caledonides is perhaps the most widely studied individual structure in the uppermost continental mantle. Previous investigations interpreted the reflector as a thrust related to the Caledonian orogeny (Silurian-Early Devonian) or as a normal fault or shear zone active during post-Caledonian (e.g., Devonian or Permo-Triassic) extension. This study uses all available deep seismic reflection data to map mantle reflector structure along the Caledonian orogen and to constrain better its geometrical and tectonic relation to regional geology. The distribution of the mantle reflector shows that it is not spatially related to either Caledonian compressional or subsequent basin extensional structures in a simple way. A structural contour map shows two distinct discontinuous surfaces consisting of a steeply plunging antiform-synform pair beneath the West Orkney basin (north of the Scottish mainland) and the F¿roe-Shetland basin west of Shetland, respectively, separated by an inferred zone of structural disruption. A working hypothesis for this study is that these two surfaces may have once been part of a continuous, more or less planar surface that has been offset and deformed by either (or perhaps a combination) of two processes: (1) differential extension of crust that was at least partially coupled to the uppermost mantle beneath the West Orkney and F¿roe-Shetland basins; (2) a broad, approximately orthogonal (i.e., east-west trending) zone of simple shear corresponding to the disrupted zone that was largely confined to the upper mantle but reactivated to deform the crust. Our results also suggest that the mantle reflector structure may be inherited from Neoproterozoic events associated with rifting of the Laurentia-Baltica supercontinent. ¿ American Geophysical Union 1995