Nature of Crustal Reflectivity along the DEKORP Profiles in Germany in Comparison with Reflection Patterns from Different Tectonic Units Worldwide: A Review

Abstract

—Reflectivity of the continental crust displays many different patterns. The DEKORP lines are used as a basis for comparing and reviewing reflectivity in different tectonic units. The (brittle) upper crust generally exhibits only two types of reflectivity. It is either rather "transparent," preferably in some extensional provinces, or/and it shows traces of thrust and shear zones of former or present ruptures. As these zones have a low impedance interior (with few exceptions), their first reflection onsets have a negative polarity and evince strong, but short signals, which sometimes can be correlated over several kilometers. The (generally ductile) lower crust displays a completely different reflectivity. In warm, extensional and thin crusts the lower part is full of reflecting lamellae. It is suggested that this type of reflectivity has a thermo-rheological origin. The creation of lamellae must take place in a ductile material with contrasting impedance under extensional stresses. It can be associated with mineral alignment and corresponding seismic anisotropy. Destruction of lamellae may take place by a cooling process, transforming parts of the lower crust into a brittle regime. Small stresses might deform or break the lamellae and leave a certain dispersed reflectivity like that in some old (and cold) shields. There are no observations of reflecting lamellae in the upper crust or in the upper mantle. In all areas the Moho is the last reflecting band (reflection Moho), which most often is identical with the classical refraction Moho. There are isolated, mostly dipping, reflections in the uppermost mantle in zones where the last tectonic event, a delamination or subduction, was not succeeded by a heating process. The uppermost mantle is brittle again in most areas and may keep the memory of a (cold) collision over billions of years.