Page 292 - Reservoir Geomechanics

271 Stress fields

collision of two continental plates. As oceanic plates subduct into the viscous lower
mantle additional resistive forces add to the collision resistance forces acting at shallow
depth. Another force resisting plate motion is that due to transform faults, although, as
discussed below, the amount of transform resistance may be negligible.
While it is possible to specify the various stresses associated with plate movement,
their relative and absolute importance in plate movement are not understood. Many
researchers believe that either the ridge push or slab pull force is most important in
causing plate motion, but it is not clear that these forces are easily separable or that
plate motion can be ascribed to a single dominating force. This has been addressed by
a detailed series of finite element models of the stresses in the North American plate
(Richardson 1992).

Topography and buoyancy forces
Numerous workers have demonstrated that topography and its compensation at depth
can generate sizable stresses capable of influencing the tectonic stress state and style
(Artyushkov 1973). Density anomalies within or just beneath the lithosphere constitute
major sources of stress. The integral of anomalous density times depth (the density
moment of Fleitout and Froidevaux 1983) characterizes the ability of density anomalies
to influence the stress field and to induce deformation. In general, crustal thickening
and lithospheric thinning (negative density anomalies) produce extensional stresses,
while crustal thinning and lithospheric thickening (positive density anomalies) produce
compressional stresses. In more complex cases, the resultant state of stress in a region
depends on the density moment integrated over the entire lithosphere. In a collisional
orogeny, for example, where both the crust and mantle lid are thickened, the presence
of the cold lithospheric root can overcome the extensional forces related to crustal
thickening and maintain compression (Fleitout and Froidevaux 1983). Zoback and
Mooney (2003) showed that regional intraplate relative stress magnitudes are generally
predictable from buoyancy forces derived from lateral variations in the density and
structure of the lithosphere.

Lithospheric flexure

Loads on or within an elastic lithosphere cause deflection and induce flexural stresses
which can be quite large (several hundred MPa) and can perturb the regional stress
field with wavelengths as much as 1000 km (depending on the lateral extent of the
load (e.g. McNutt and Menard 1982). Some potential sources of flexural stress influ-
encing the regional stress field include sediment loading along continental margins and
the upwarping of oceanic lithosphere oceanward of the trench, the “outer arc bulge”
(Chapple and Forsythe 1979). Sediment loads as thick as 10 km represent a potentially
significant stress on continental lithosphere (e.g. Cloetingh and Wortel 1986;Turcotte