Page 286 - Reservoir Geomechanics

267 Stress fields

of the processes that drive (or inhibit) lithospheric plate motions as well as the forces
responsible for the occurrence of crustal earthquakes – both along plate boundaries and
in intraplate regions. While such topics are clearly beyond the scope of this book, they
are briefly addressed here to provide a broad-scale context for the discussions of stress
at more regional, local, field and well scales that follow in Chapters 10–12.
Figure 9.1 is a global map of maximum horizontal compressive stress orientations
based on the 2005 World Stress Map data base. As with Figure 1.5, only data qualities
A and B are shown and the symbols are the same as those in Figure 1.5. While global
coverage is quite variable (for the reasons noted above in North America), the relative
uniformity of stress orientation and relative magnitudes in different parts of the world
is striking and permits mapping of regionally coherent stress fields. In addition to the
paucity of data in continental intraplate regions, there is also a near-complete absence
of data in ocean basins.
Figure 9.2 presents a generalized version of the global stress map that is quite similar
to the map presented by Zoback and others (1989) and showing mean stress directions
and stress regime based on averages of the data shown in Figure 9.1.Tectonic stress
regimes are indicated in Figure 9.2 by color and arrow type. Black inward pointing
arrows indicate S Hmax orientations in areas of compressional (reverse) stress regimes.
Red outward pointing arrows give S hmin orientations (extension direction) in areas of
normal faulting stress regimes. Regions dominated by strike-slip tectonics are distin-
guished with thick inward-pointing and orthogonal, thin outward-pointing arrows.
A number of first-order patterns can be observed in Figures 9.1 and 9.2:
1. In many regions a uniform stress field exists throughout the upper brittle crust
as indicated by consistent orientations from the different measurement techniques
sampling very different rock volumes and depth ranges.
2. Intraplate regions are dominated by compression (reverse and strike-slip stress
regimes) in which the maximum principal stress is horizontal. Such stress states are
observed in continental regions throughout the world and likely exist in regions
where data are absent. The intraplate compression seen in several ocean basins
(the northeast Indian Ocean and just west of the East Pacific rise, for instance) are
indicated by rare intraplate oceanic earthquakes.
3. Active extensional tectonism (normal faulting stress regimes) in which the maximum
principal stress is the vertical stress generally occurs in topographically elevated
areas of the continents. The areas of extensional stress near mid-ocean ridges in the
Indian Ocean are likely the result of cooling stresses in the crust near, but not along,
the spreading centers.
4. Regional consistency of both stress orientations and relative magnitudes permits
the definition of broad-scale regional stress provinces, many of which coincide with
physiographic provinces, particularly in tectonically active regions. These provinces
4
3
may have lateral dimensions on the order of 10 –10 km, many times the typi-
cal lithosphere thickness of 100–300 km. These broad regions of the earth’s crust