low velocity zone (LVZ) occurs within most areas of the upper mantle at depths of ~100–250 km below the surface. The top of low velocity zone marks the contact between the strong lithosphere and the underlying, weak asthenosphere (Figure 1.3). The asthenosphere is more plastic than the lithosphere and flows slowly, rather than rupturing, when subjected to stress. The anomalously low rigidity of the LVZ has been explained by small amounts of partial melting (Anderson et al. 1971). This is supported by laboratory studies that suggest peridotite should be very near its melting temperature at these depths due to the high temperature. This is especially likely if it contains small amounts of water or water‐bearing minerals. Below the base of the low velocity zone (250–410 km), seismic wave velocities increase (Figure 1.2) indicating that the materials are more rigid solids. These materials are still part of the relatively weak asthenosphere which extends to the base of the transition zone at 660 km.
Seismic discontinuities marked by increases in seismic velocity occur within the upper mantle at depths of ~410 and ~660 km (Figure 1.2). This interval (~410–660 km) is called the transition zone between the upper and lower mantle. The sudden jumps in seismic velocity record sudden increases in rigidity and incompressibility. Laboratory studies suggest that the minerals in peridotite undergo transformations into new minerals at these depths. At approximately 410 km depth (pressures of ~14 GPa), olivine (Mg2SiO4) is transformed into more rigid, incompressible beta spinel (β‐spinel), also known as wadleysite (Mg2SiO4). Within the transition zone, wadleysite is transformed into the higher pressure mineral ringwoodite (Mg2SiO4). At approximately 660 km depth (~24 GPa), ringwoodite and garnet are converted to very rigid, incompressible perovskite [(Mg,Fe,Al)SiO3], also known as bridgmanite (Tschauner et al. 2014) and oxide phases such as periclase (MgO). The mineral phase changes from olivine to wadleysite and from ringwoodite to perovskite are inferred to be largely responsible for the seismic wave velocity changes that occur at 410 and 660 km respectively (Ringwood 1975; Condie 1982; Anderson 1989). Inversions of pyroxene to garnet and garnet to minerals with ilmenite and perovskite structures may also be involved. The base of the transition zone at 660 km marks the base of the asthenosphere in contact with the underlying mesosphere or lower mantle (Figure 1.2).
Figure 1.2 Major layers and seismic (p‐wave) velocity changes within Earth; showing details of upper mantle layers. Colors are as for Figure 1.1.
Figure 1.3 World map showing the distribution of major plates separated by boundary segments that end in triple junctions.
Source: From USGS.
The lower mantle (mesosphere)
The lower mantle, also called the mesosphere, extends from depths of 660 km to the core–mantle boundary at approximately 2900 km. Based on high pressure, high temperature laboratory studies, bridgmanite [(Mg,Fe,Al)SiO3], ferropericlase [(Mg,Fe)O], magnesiowustite [(Mg,Fe)O], stishovite (SiO2), and calcium‐rich ferrite (Ca,Na,Al)Fe2O4 are thought to be the major minerals in the lower mantle. Our knowledge of the deep mantle continues to expand, largely based on high temperature, high pressure laboratory studies and on anomalous seismic signals deep within the Earth. A deeper layer has been proposed at about 1600 km depth where the rigidity of the mantle may increase considerably (Miyagi and Marquardt 2015). Anomalous seismic velocities are particularly common in a complex zone, of variable thickness, near the core–mantle boundary called the D″ layer. The D″ discontinuity ranges from ~130 to 340 km above the core–mantle boundary. Williams and Garnero (1996) proposed an ultra‐low velocity zone (ULVZ) in the lowermost mantle on seismic evidence. These sporadic ultra‐low velocity zones may be related to the formation of deep mantle plumes within the lower mantle. Other areas near the core–mantle boundary are characterized by anomalously fast velocities. Hutko et al. (2006) detected subducted lithosphere which had sunk all the way to the D″ layer and may be responsible for the anomalously fast velocities. Deep subduction and deeply rooted mantle plumes support some type of whole mantle convection and may play a significant role in the evolution of a highly heterogeneous mantle, but these concepts are still controversial (Foulger et al. 2005).
1.4.3 Earth's core
Earth's core consists primarily of iron (~85%), with smaller, but significant amounts of nickel (~5%) and lighter elements (~8–10%) such as oxygen, sulfur and/or hydrogen. A dramatic decrease in P‐wave velocity and the termination of S‐wave propagation occurs at the 2900 km discontinuity which is Gutenberg discontinuity or core–mantle boundary (CMB). Because S‐waves are not transmitted by nonrigid substances such as fluids, the outer core is inferred to be a fluid. Geophysical studies suggest that the Earth's outer core is a highly compressed liquid with a density of ~10–12 g/cm3. Slowly circulating molten, iron‐rich, very viscous liquids in the outer core are believed to be responsible for the production of most of Earth's magnetic field.
The outer/inner core boundary, the Lehman discontinuity at 5150 km, is marked by a rapid increase in P‐wave velocity and the reemergence of low velocity S‐waves. This suggests that the inner core is rigid. The inner core is solid and has a density of ~13 g/cm3. Density and magnetic studies suggest that the Earth's inner core also consists of largely of iron, with nickel and less oxygen, sulfur, and/or hydrogen than the outer core. Seismic studies have shown that the inner core is seismically anisotropic; that is seismic velocity in the inner core is faster in one direction than in others. This has been interpreted to result from the parallel alignment of iron‐rich crystals or from a core consisting of a single crystal with a fast velocity direction. Recent discoveries suggest that the inner core is divided into two layers with the inner layer more rigid than the outer one and with a different orientation of its fast seismic wave direction (Ishii and Dziewonski 2002; Wang et al. 2015).
In this section, we have discussed the major layers of the geosphere, their composition, and their mechanical properties. This model of a layered geosphere provides us with a spatial context in which to visualize where the processes that generate earth materials occur. In the following sections we will examine the ways in which all parts of the geosphere interact to produce global tectonics. The ongoing story of global‐scale tectonics is one of the most fascinating tales of scientific discovery in the last century and new discoveries continue to be made in this one.
1.5 GLOBAL TECTONICS
1.5.1 Introduction
Plate tectonic theory has profoundly changed the way geoscientists view Earth and provides an important theoretical and conceptual framework for understanding the origin and global distribution of igneous, sedimentary, and metamorphic rocks (Chapters 7–18).
