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Lithospheric Stress
Ch. 5, p. 79-‐85; 88-‐
1. Stress in the Lithosphere: Much of our understanding of deformation has involved considering the state of stress at the time of deformation. Despite its importance though, stress cannot be directly measured in the lithosphere. It can only be inferred based on observations of resultant strain (i.e., deformation).
[Fig. 5.9. Joints in Permian sandstone in the Canyonlands of Utah. The joints provide an indicator of the state of stress in which they formed]
2. Measuring/Inferring Stress: The techniques used to infer stress include:
(1) Borehole breakouts (2) Overcoring (3) Hydraulic fracturing (4) Earthquake focal mechanisms (5) Orientations of geologic structures
[Fig. 5.1. Techniques for inferring stress: (a) Borehole breakouts. (b) Overcoring. (c) Hydraulic fracturing. (d) Geologic structures] [Fig. 5.2. Breakouts along tunnel walls indicate stress orientations]
3/4. Measuring/Inferring Stress: The techniques used to infer stress include:
(1) Borehole breakouts (2) Overcoring (3) Hydraulic fracturing (4) Earthquake focal mechanisms (5) Orientations of geologic structures
[Fig. 5.5. Orientations of tension cracks in Iceland indicate stress orientations] [Fig. 5.12. Earthquake focal mechanisms indicate the principal strain axes, which relate to the stress orientations]
5. Stress Perturbations: Stresses can be locally perturbed by topography, openings (caves, tunnels, and boreholes), and geologic structures. These effects should be considered when choosing locations to infer stress orientations.
[Fig. 5.3/5.4. Local perturbations to the orientations of the principal stresses caused by the effects of topography (left) and existing geologic structures (right)]
6. Stress Maps: The stress measurement techniques can be used to generate maps of principal stress orientations around the world. Some techniques allow stress magnitudes to be inferred too.
[Fig. 5.13. World Stress Map, showing the orientation of the maximum horizontal stress in different tectonic environments]
7. Source of Stress: Strain is the end-‐result of the cumulative stress state; however, many factors contribute to creating stress. These include: (1) overburden (lithostatic stress, rgh); (2) tectonic stress (slab pull; ridge push; plate collision; lateral sliding; basal drag); (3) plate bending (vertical loads; subduction; curvature effects on a non-‐ spherical planet); (4) thermal effects; (5) fluid pressure (water; oil; gas).
[Fig. 5.14. Tectonic contributions to the overall stress state]
8. Limits to Stress Magnitudes: The Mohr-‐Coulomb criterion also indicates that there is a limit to how big the stress magnitudes can be, particularly the differential stress at any particular depth.
[Fig. 7.10. Mohr circles for different states of stress at the instant of frictional sliding, given by the Mohr-‐Coulomb failure line]
9. Limits to Stress Magnitudes: We generally assume that the vertical stress (σv ) is always a principal stress and is equal to the overburden or lithostatic stress, ρr gh (dry rocks) or (ρr – ρw)gh (wet rocks). In the absence of other stress components, the lithostatic reference state is isotropic: σ 1 = σ 2 = σ 3 = σv. A typical gradient is 25-‐30 MPa/km.
[Fig. 5.6. Measurements of vertical stress and fluid pressures as a function of depth in (a) Norway and (b) North Sea and other locations]
10. Limits to Stress Magnitudes: Fluid pressure in rocks will be hydrostatic from the water table down if the pores and fractures are fully connected up to the surface. Confined units may experience overpressure (p (^) f > hydrostatic), which is more prone to fracturing rock.
[Fig. 5.6. Measurements of vertical stress and fluid pressures as a function of depth in (a) Norway and (b) North Sea and other locations]
11. Limits to Stress Magnitudes: In general, horizontal stresses differ from σv , indicating other contributors to the stress state. We differentiate the maximum horizontal stress (σH or S (^) H ) from the minimum horizontal stress (σh or S (^) h ). Which principal stress these represent depends on the tectonic setting.
[Figure. Horizontal stresses S (^) h in a sedimentary basin in the U.S. (Twiss & Moores, 2007)]
12. Limits to Stress Magnitudes: If σv = σ 3 and S (^) H = σ 1 (contraction), stresses fall to the right of the lithostat (limited by green line).
If σv = σ 1 and S (^) h = σ 3 (extension), stresses fall to the left of the lithostat (limited by the purple line).
Rocks are stronger in compression than tension.
[Figure. limits to the value of horizontal stresses in extensional and contractional tectonic settings. Assumes C= MPa and μ = 0.6 (Twiss & Moores, 2007)] [Fig. 5.16. Rock strength versus depth for different lithologies]
22/23. Salt Mobility: Salt motion is not simply a buoyancy phenomenon. Because it behaves like a fluid, it also responds to pressure variations, flowing to regions of low pressure.
This may happen due to differential loading (changes in thickness of overburden), regional slopes, or erosion.
(e.g., Colorado River in the Grabens region of Canyonlands National Park –> Meander anticline).
[Fig. 19.12. Migration of salt in response to differential loading and regional slopes] [Figure. Meander anticline salt diapir along the Colorado River, Utah]
24. Salt Diapirism Mechanisms: The mechanisms behind salt migration include:
(1) Active diapirism: this is where the salt forces the adjacent rocks aside, causing folding and faulting. (2) Passive diapirism: after creation of a diapir, the salt structure grows at a rate that keeps pace with adjacent sedimentation. (3) Reactive diapirism: extension by normal faulting creates accommodation space for the salt. (4) Thrust related: in contractional settings, salt may migrate along thrust fault zones.
[Fig. 19.13. Different mechanisms by which salt migration may occur]
25. Salt Diapir Deformation: Salt domes may cause radial or concentric extension in the overlying rocks, which results in either radial faults or concentric faults, or both.
Deformation may also occur in the form of joints or deformation bands.
An example is Upheaval Dome in the Canyonlands of Utah. An instability caused by a Cretaceous meteorite impact caused prolonged salt diapirism that created radial and concentric structures superimposed on older, impact-‐ related dynamic deformation.
[Fig. 19.15. Different mechanisms by which salt migration may occur]
26. Salt Diapir Deformation: Upheaval Dome in Utah (top and bottom right images). This is the eroded root of an old meteorite impact site. Later diapirism created radial and concentric features. Bottom left: seismic image of the Silverpit crater in the North Sea, possible also of impact origin
[Box 19.2. Upheaval Dome]
27/28. Salt Diapir Deformation: [Figures. Field images of deformation around Upheaval Dome, Utah]
