Super-deep reflection profiling: exploring the continental mantle lid

Reflections from the continental mantle lithosphere have now been reported from common mid-point (CMP) profiles at numerous sites around the world. These reflections include: (1) distinct dipping events that appear to originate in the lower crust and continue some distance into the upper mantle; (2) kilometer scale, discontinuous, diffusely distributed, or isolated reflections at varying depths; (3) continuous, sub-horizontal reflections tens of kilometers in extent, often interpreted as the ‘base of the lithosphere’ in spite of a wide variation in depth and; (4) very deep reflections in the mantle lithosphere (> 125 km). The first category of mantle reflections is most commonly observed in proximity to crustal sutures, lending credence to their interpretation as fossil subduction zones. Isolated, discontinuous mantle reflections are by far the most common and, though observed in both ancient and modern collisional environments, are difficult to interpret due to both their limited extent and a corresponding lack of three-dimensional control. Sub-horizontal mantle reflections appear in disparate tectonic environments in at least nine locations worldwide. Of special note are the various reports of specific reflections at ∼80–100 km (22–24 s twt) in the North Sea, eastern Europe, and the Urals which suggest a more ubiquitous mantle horizon. The deepest events imaged thus far in a controlled source CMP survey are the ‘super-deep’ mantle reflections of the URSEIS project that have been identified at ∼135–165 km (35–43 s) and ∼225 km (56 s). Of the many reported mantle reflections, these are the most likely to be associated with the base of the lithosphere although such an interpretation averts the question of why an essentially thermo-mechanical transition should be reflective at high frequencies (10–20 Hz).