Coincident seismic reflection/refraction studies of the continental lithosphere: A global review

Nearly 50 coincident seismic reflection/refraction studies to depths of at least the Moho provide an improved understanding of the continental lithosphere. Some conclusions include the following: (1) A transparent upper crust, a common observation on vertical reflection profiles, cannot generally be correlated with velocity gradients or low‐velocity zones. Rather, a commonly transparent upper crust may be explained by short‐wavelength, steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source‐generated noise. (2) The reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity (6.6–7.3 km/s) and to consist of laminated high‐ and low‐velocity layers with typical thicknesses of 100–200 m. (3) Landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high‐ and low‐velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin. (4) The crust‐mantle boundary may differ sufficiently when imaged with vertical incidence and wide‐angle data to justify the retention, for the present, of the concept of separate reflection and refraction Mohos. While there is good evidence that these features are coincident within measurement uncertainties in most regions, recently recorded data from the Basin and Range admit the possibility for noncoincidence in that area. (5) Upper mantle reflections which cannot be migrated into the lower crust remain rare, despite isolated unequivocal examples. Thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust. The wide‐angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come. (6) There appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments; these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes. (7) No truly three‐dimensional coincident experiment (i.e., including three‐dimensional migration) has been conducted, but some three‐dimensional data have been collected using both methods. Measurements of attenuation, Poisson's ratio, and anisotropy within the crust using coincident data sets remain frontiers.