Page 52 - Global Tectonics

THE INTERIOR OF THE EARTH 39

continental crust and upper mantle, respectively. The the thickness, composition, and pressure–temperature

results of these and other measurements (e.g. Ranalli & profile of continental crust, which makes ductile fl ow
Murphy, 1987; Mackwell et al., 1998) suggest that within in its lower parts more likely than it is in oceanic regions.
the oceanic lithosphere the upper brittle crust gives way The width and diffusivity of these zones make some of
to a region of high strength at a depth of 20–60 km, the concepts of plate tectonics, such as the rigid motion

depending on the temperature gradient (Fig. 2.26a). of plates along narrow boundaries, difficult to apply to
Below this depth the strength gradually decreases and the continents. Consequently, the analysis of continen-
grades into that of the asthenosphere. Continental tal deformation commonly requires a framework that
crust, however, is much thicker than oceanic crust, and is different to that used to study deformation in oceanic
at the temperatures of 400–700°C experienced in its lithosphere (e.g. Section 8.5).
lower layers the minerals are much weaker than the At the scale of large tectonic features such as wide
olivine found at these depths in the oceanic lithosphere. intracontinental rifts (Section 7.3), continental trans-
Whereas the oceanic lithosphere behaves as a single forms (Section 8.5), and orogenic belts (Section 10.4.3),
rigid plate because of its high strength, the continental deformation may be described by a regional horizontal

lithosphere does not (Sections 2.10.5, 8.5) and typically velocity field rather than by the relative motion of rigid
is characterized by one or more layers of weakness at blocks (e.g. Fig. 8.18b). Methods of estimating the
deep levels (Fig. 2.26b,c). regional velocity field of deforming regions usually

Figure 2.26c,d shows two other experimentally involves combining information from Global Position-
determined strength curves for continental lithosphere ing System (GPS) satellite measurements (Clarke et al.,
that illustrate the potential effects of water on the 1998), fault slip rates (England & Molnar, 1997), and
strength of various layers. These curves were calculated seismicity (Jackson et al., 1992). One of the challenges
using rheologies for diabase and other crustal and of this approach is the short, decade-scale time intervals
−15 −1
mantle rocks, a strain rate of (δε/δτ) = 10 s , a typical over which GPS data are collected. These short inter-
thermal gradient for continental crust with a surface vals typically include relatively few major earthquakes.
−2
heat flow of 60 mW m , and a crustal thickness of Consequently, the measured surface motions mostly

40 km (Mackwell et al., 1998). The upper crust (0–15 km reflect nonpermanent, elastic strains that accumulate
depth) is represented by wet quartz and Byerlee’s (1978) between major seismic events (i.e. interseismic) rather
frictional strength law (Section 2.10.2), and the middle than the permanent strains that occur during ruptures
crust (15–30 km depth) by wet quartz and power-law (Bos & Spakman, 2005; Meade & Hager, 2005). This
creep (Section 2.10.3). These and other postulated characteristic results in a regional velocity fi eld that

strength profiles commonly are used in thermomechan- rarely shows the discontinuities associated with slip on
ical models of continental deformation (Sections 7.6.6, major faults. Instead the displacements on faults are
8.6.2, 10.2.5). However, it is important to keep in mind described as continuous functions and the velocity fi eld

that the use of any one profile in a particular setting is taken to represent the average deformation over a
involves considerable uncertainty and is the subject of given region (Jackson, 2004). Nevertheless, regional

much debate (Jackson, J., 2002; Afonso & Ranalli, 2004; velocity fields have proven to be a remarkably useful
Handy & Brun, 2004). In settings where ambient condi- way of describing continental deformation. The
tions appear to change frequently, such as within methods commonly used to process and interpret them
orogens and magmatic arcs, several curves may be are discussed further in Sections 5.3 and 8.5.
necessary to describe variations in rock strength with Synthetic Aperture Radar (SAR) also is used to
depth for different time periods. measure ground displacements, including those associ-
ated with volcanic and earthquake activity (Massonnet
& Feigl, 1998). The technique involves using SAR data
2.10.5 Measuring to measure small changes in surface elevations from

satellites that fly over the same area at least twice, called
continental deformation repeat-pass Interferometric SAR, or InSAR. GPS data
and strain meters provide more accurate and frequent

Zones of continental deformation commonly are wider observations of deformation in specific areas, but InSAR
and more diffuse than zones of deformation affecting is especially good at revealing the spatial complexity of
oceanic lithosphere. This characteristic results from displacements that occur in tectonically active areas. In