Page 37 - Global Tectonics
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24   CHAPTER 2



             Another indicator of lower crust composition is the   Table 2.2  Oceanic crustal structure (after Bott, 1982).
           elastic deformation parameter Poisson’s ratio, which
           can be expressed in terms of the ratio of P and S wave      P velocity  Average thickness
                                                                            −1
           velocities for a particular medium. This parameter           (km s )        (km)
           varies systematically with rock composition, from
           approximately 0.20 to 0.35. Lower values are character-  Water  1.5          4.5
           istic of rocks with high silica content, and high values   Layer 1  1.6–2.5  0.4

           with mafic rocks and relatively low silica content. For   Layer 2  3.4–6.2    1.4
           example, beneath the Main Ethiopian Rift in East Africa   Layer 3  6.4–7.0   5.0
           (Fig. 7.2) Poisson’s ratios vary from 0.27 to 0.35 (Dugda    Moho
           et al., 2005). By contrast, crust located outside the rift is
                                                          Upper mantle  7.4–8.6
           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 mafi c

           magma (Fig. 7.5).
             Undoubtedly, the lower crust is compositionally
                                                          The earliest refraction surveys produced time–dis-
           more complex than suggested by these simple geophys-
                                                        tance data of relatively low accuracy that, on simple
           ical models. Studies of deep crustal xenoliths and crustal
                                                        inversion using plane-layered models, indicated the
           contaminated magmas indicate that there are signifi -
                                                        presence of three principal layers. The velocities and
           cant regional variations in its composition, age, and
                                                        thicknesses of these layers are shown in Table 2.2.
           thermal history. Deep seismic refl ection investigations
                                                        More recent refraction studies, employing much more
           (Jackson, H.R., 2002; van der Velden et al., 2004) and
                                                        sophisticated equipment and interpretational proce-
           geologic studies of ancient exposures (Karlstrom & Wil-
                                                        dures (Kennett B.L.N., 1977), have shown that further
           liams, 1998; Miller & Paterson, 2001a; Klepeis  et al.,
                                                        subdivision of the main layers is possible (Harrison &
           2004) also have shown that this compositional complex-
                                                        Bonatti, 1981) and that, rather than a structure in
           ity is matched by a very heterogeneous structure. This
                                                        which velocities increase downwards in discrete jumps,

           heterogeneity reflects a wide range of processes that
                                                        there appears to be a progressive velocity increase with
           create and modify the lower crust. These processes
                                                        depth (Kennett & Orcutt, 1976; Spudich & Orcutt,
           include the emplacement and crystallization of magma
                                                        1980). Figure 2.17 compares the velocity structure of
           derived from the mantle, the generation and extraction
                                                        the oceanic crust as determined by early and more
           of crustal melts, metamorphism, erosion, tectonic
                                                        recent investigations.
           burial, and many other types of tectonic reworking
           (Sections 9.8, 9.9).
                                                        2.4.5  Oceanic layer 1
           2.4.4  The oceanic crust                     Layer 1 has been extensively sampled by coring and
                                                        drilling. Seabed surface materials comprise unconsoli-
           The oceanic crust (Francheteau, 1983) is in isostatic   dated deposits including terrigenous sediments carried
           equilibrium with the continental crust according to the   into the deep oceans by turbidity currents, and pelagic
           Airy mechanism (Section 2.11.2), and is consequently   deposits such as brown zeolite clays, calcareous and
           much thinner. Seismic refraction studies have con-  silicic oozes, and manganese nodules. These deep-sea
           firmed this and show that oceanic crust is typically 6–  sediments are frequently redistributed by bottom cur-

           7 km thick beneath an average water depth of 4.5 km.   rents or contour currents, which are largely controlled
           Thicker oceanic crust occurs where the magma supply   by thermal and haline anomalies within the oceans. The
           rate is anomalously high due to higher than normal   dense, cold saline water produced at the poles sinks and

           temperatures in the upper mantle. Conversely, thinner   underflows towards equatorial regions, and is defl ected
           than normal crust forms where upper mantle tempera-  by the Coriolis force. The resulting currents give rise to
           tures are anomalously low, typically because of a very   sedimentary deposits that are termed contourites (Stow
           low rate of formation (Section 6.10).        & Lovell, 1979).
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