Page 36 - Global Tectonics

THE INTERIOR OF THE EARTH 23

changes in chemical composition, while more grada- middle crust is some 11 km thick and ranges in depth
tional velocity boundaries are normally associated from 12 km, at the top, to 23 km at the bottom (Rudnick
with phase changes that occur over a discrete vertical & Fountain, 1995; Gao et al., 1998). The average lower
interval. crust thus begins at 23 km depth and is 17 km thick.
Models for the bulk chemical composition of the However, the depth and thickness of both middle and
continental crust vary widely because of the diffi culty lower crust vary considerably from setting to setting. In
of making such estimates. McLennan & Taylor (1996) tectonically active rifts and rifted margins, the middle and

pointed out that the flow of heat from the continental lower crust generally are thin. The lower crust in these
crust (Section 2.13) provides a constraint on the abun- settings can range from negligible to more than 10 km
dance of the heat producing elements, K, Th, and U, thick (Figs 7.5, 7.32a). In Mesozoic–Cenozoic orogenic
within it, and hence on the silica content of the crust. belts where the crust is much thicker, the lower crust may
On this basis they argue that on average the continental be up to 25 km thick (Rudnick & Fountain, 1995).
−1
crust has an andesitic or granodioritic composition with The velocity range of the lower crust (6.8–7.7 km s ,
K 2O no more than 1.5% by weight. This is less silicic Section 2.2) cannot be explained by a simple increase of
than most previous estimates. The abundance of the seismic velocity with depth. Consequently, either the

heat producing elements, and other “incompatible” ele- chemical composition must be more mafic, or denser,
ments, in the continental crust is of great importance high-pressure phases are present. Information derived
because the degree to which they are enriched in the from geologic studies supports this conclusion, indicat-
crust reflects the extent to which they are depleted in ing that continental crust becomes denser and more

the mantle. mafic with depth. In addition, the results from these
studies show that the concentration of heat-producing
elements decreases rapidly from the surface down-
wards. This decrease is due, in part, to an increase in
2.4.2 Upper continental crust metamorphic grade but is also due to increasing propor-
tions of mafi c lithologies.
Past theories of crustal construction suggested that the In areas of thin continental crust, such as in rifts and
upper continental crust was made up of rocks of granitic at rifted margins, the middle and lower crust may be
composition. That this is not the case is evident from composed of low- and moderate-grade metamorphic
the widespread occurrence of large negative gravity rocks. In regions of very thick crust, such as orogenic
anomalies over granite plutons. These anomalies belts, the middle and lower crust typically are com-
demonstrate that the density of the plutons (about posed of high-grade metamorphic mineral assem-
−3
−3
2.67 Mg m ) is some 0.10–0.15 Mg m lower than the blages. The middle crust in general may contain more
average value of the upper crust. The mean composition evolved and less mafic compositions compared to the

of the upper crust can be estimated, albeit with some lower crust. Metasedimentary rocks may be present in
uncertainty due to biasing, by determining the mean both layers. If the lower crust is dry, its composition
composition of a large number of samples collected could correspond to a high-pressure form of granulite
worldwide and from analyses of sedimentary rocks that ranging in composition from granodiorite to diorite
have sampled the crust naturally by the process of (Christensen & Fountain, 1975; Smithson & Brown,
erosion (Taylor & Scott, 1985; Gao et al., 1998). This 1977), and containing abundant plagioclase and pyrox-
composition corresponds to a rock type between grano- ene minerals. In the overthickened roots of orogens,
diorite and diorite, and is characterized by a relatively parts of the lower crust may record the transition to
high concentration of the heat-producing elements. the eclogite facies, where plagioclase is unstable and

mafic rocks transform into very dense, garnet-, pyrox-
ene-bearing assemblages (Section 9.9). If the lower
2.4.3 Middle and lower crust is wet, basaltic rocks would occur in the form of
amphibolite. If mixed with more silicic material, this
continental crust would have a seismic velocity in the correct range.
Studies of exposed sections of ancient lower crust
For a 40 km thick average global continental crust suggest that both dry and wet rock types typically are
(Christensen & Mooney, 1995; Mooney et al., 1998), the present (Oliver, 1982; Baldwin et al., 2003).