The seismic structure of the continental crust and upper mantle of North America

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Abstract
The seismic structure of the crust and upper mantle provides critical information regarding lithospheric composition and evolution. There are large variations in fundamental properties such as crustal thickness, crustal and upper-mantle velocity structure, and the depth to the lithosphere/asthenosphere boundary, which are interpretable in terms of processes that have formed and modified the lithosphere. Much of what we know about the seismic structure of the lithosphere has been accumulated over the past 30 years from seismic refraction profiles and surface-wave studies. Modern seismic refraction studies use denser arrays of seismic sources and recorders, improving the resolution and reliability of crustal models. Within the last 15 years, the deep-seismic reflection technique has been widely applied and has provided fundamental new insights into the structure and physical properties of the crust. Recently, seismic investigations have provided a fresh look at the properties of the Moho and the upper mantle where we may discover the “driving forces” of continental tectonics. In this discussion we define the lithosphere as the crust and the portion of the upper mantle above the seismic low-velocity layer (asthenosphere) that occurs at a depth of 60 to 200 km and is generally more evident in the shear-wave structure than in the compressional-wave structure. The low-velocity layer contrasts with the base of the crust (Moho), which is very pronounced in compressional-wave structure and is defined as the depth below which the seismic velocity (measured on a reversed seismic refraction profile) is greater than 7.6 km/s. Where the crust/mantle boundary has been examined in detail, it appears to consist of a laminated transition zone with a thickness of 2 to 5 km. The thickness of the Earth’s crust is highly variable; typical oceanic crust has a thickness of about 7 + 3 km (excluding the water column), and continental crust typically has a thickness of 25 to 50 km.