For a 40 km thick average global continental crust (Christensen & Mooney, 1995; Mooney et al., 1998), the middle crust is some 11 km thick and ranges in depth from 12 km, at the top, to 23 km at the bottom (Rudnick & Fountain, 1995; Gao et al., 1998). The average lower crust thus begins at 23 km depth and is 17 km thick. However, the depth and thickness of both middle and lower crust vary considerably from setting to setting. In tectonically active rifts and rifted margins, the middle and lower crust generally are thin. The lower crust in these settings can range from negligible to more than 10 km thick (Figs 7.5, 7.32a). In Mesozoic-Cenozoic orogenic belts where the crust is much thicker, the lower crust may be up to 25 km thick (Rudnick & Fountain, 1995).
The velocity range of the lower crust (6.8-7.7 km s-1, Section 2.2) cannot be explained by a simple increase of seismic velocity with depth. Consequently, either the chemical composition must be more mafic, or denser, high-pressure phases are present. Information derived from geologic studies supports this conclusion, indicating that continental crust becomes denser and more mafic with depth. In addition, the results from these studies show that the concentration of heat-producing elements decreases rapidly from the surface downwards. This decrease is due, in part, to an increase in metamorphic grade but is also due to increasing proportions of mafic lithologies.
In areas of thin continental crust, such as in rifts and at rifted margins, the middle and lower crust may be composed of low- and moderate-grade metamorphic rocks. In regions of very thick crust, such as orogenic belts, the middle and lower crust typically are composed of high-grade metamorphic mineral assemblages. The middle crust in general may contain more evolved and less mafic compositions compared to the lower crust. Metasedimentary rocks may be present in both layers. If the lower crust is dry, its composition could correspond to a high-pressure form of granulite ranging in composition from granodiorite to diorite (Christensen & Fountain, 1975; Smithson & Brown, 1977), and containing abundant plagioclase and pyroxene minerals. In the overthickened roots of orogens, parts of the lower crust may record the transition to the eclogite facies, where plagioclase is unstable and mafic rocks transform into very dense, garnet-, pyroxene-bearing assemblages (Section 9.9). If the lower crust is wet, basaltic rocks would occur in the form of amphibolite. If mixed with more silicic material, this would have a seismic velocity in the correct range. Studies of exposed sections of ancient lower crust suggest that both dry and wet rock types typically are present (Oliver, 1982; Baldwin et al., 2003).
Another indicator of lower crust composition is the elastic deformation parameter Poisson's ratio, which can be expressed in terms of the ratio of P and S wave velocities for a particular medium. This parameter varies systematically with rock composition, from approximately 0.20 to 0.35. Lower values are characteristic of rocks with high silica content, and high values with mafic rocks and relatively low silica content. For example, beneath the Main Ethiopian Rift in East Africa (Fig. 7.2) Poisson's ratios vary from 0.27 to 0.35 (Dugda et al., 2005). By contrast, crust located outside the rift is characterized by varying from 0.23 to 0.28. The higher ratios beneath the rift are attributed to the intrusion and extensive modification of the lower crust by mafic magma (Fig. 7.5).
Undoubtedly, the lower crust is compositionally more complex than suggested by these simple geophysical models. Studies of deep crustal xenoliths and crustal contaminated magmas indicate that there are significant regional variations in its composition, age, and thermal history. Deep seismic reflection investigations (Jackson, H.R., 2002; van der Velden et al., 2004) and geologic studies of ancient exposures (Karlstrom & Williams, 1998; Miller & Paterson, 2001a; Klepeis et al., 2004) also have shown that this compositional complexity is matched by a very heterogeneous structure. This heterogeneity reflects a wide range of processes that create and modify the lower crust. These processes include the emplacement and crystallization of magma derived from the mantle, the generation and extraction of crustal melts, metamorphism, erosion, tectonic burial, and many other types of tectonic reworking (Sections 9.8, 9.9).
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