2.2.6. Mantle Sills?

Although the mantle has proven far less heterogeneous than the crust, at least in terms of reflectivity, prominent mantle reflections have now been traced by a number of seismic reflection profiles. Perhaps the best known of these have been mapped by the BIRPS (British Institutions Reflection Profiling Syndicate) in northwestern Britain [79–81]. The mantle reflections which have received the most attention are dipping features, often interpreted as "fossil" subduction zones [82–85]. However, relative extensive subhorizontal reflections have also been observed in the mantle near Britain which resemble those in the upper crust that have been interpreted as sills (e.g., Figure 13). Some of these "flat" mantle reflections are spatially linked to the nearby dipping mantle reflections [86] and thus may be genetically related. In any case, they too may represent igneous sills, although other speculative explanations have been put forward (e.g., detachments [87]).

**Figure 13.** Strong, subhorizontal reflector in the mantle beneath northwest Great Britain from BIRPS marine data (After Snyder and Flack [86]). M indicates the position of the Moho at approximately 8 s (ca 25 km). Note that the top of this section corresponds to 5 s TWTT instead of the customary 0 s (surface).
