Abstract

Comparisons of deep seismic reflection profiles from a variety of tectonic regions that span a large fraction of the Earth's history lead to a threefold categorization of reflection patterns near the crust-mantle transition: type I patterns have no distinct reflections (including no reflection Moho at all); type II patterns have one or more subhorizontal reflections, and type III patterns have reflections that project from the crust into the mantle. Subdivisions within each of these are based upon the geometric relationships between the reflection Moho and reflections immediately above and, in some cases, below. Hence, type I examples may have essentially no reflectivity near the crust-mantle transition, or they may be characterized by crustal reflectivity that fades downward. Type II examples may have a single prominent reflection, or they may have several subhorizontal reflections. Type II patterns may also have crustal reflections that are either listric downward or that appear to be truncated at or near the base of reflectivity. Type III examples may have a single prominent reflection that projects below the Moho (as delineated by seismic refraction data), or they may have a series of subparallel dipping reflections. More than one type of reflection Moho may be visible along a single profile, sometimes changing within distances of only a few tens of kilometers. This diversity of both geometry and character leads to the conclusion that the variable appearance of the reflection Moho is a consequence of variations in the thermal and structural evolution of the crust-mantle transition at many scales from a few tens to many hundreds of kilometers.

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