Geodynamics - Tectonic Stress

Lecture Notes #4 Assertion: most of the stress tensor in the Earth is close to "lithostatic," τij ~ -ρgd δij, where ρ is the average density of the overburden, g is gravitational acceleration, and d is the depth of the point under consideration. Consider the following table of the lithostatic pressure at various points of interest: ocean ridge crest abyssal plain deep sea trench base of crust transition zone Core-mantle bndry Center of Earth depth (km) 2.5 4 12 30 600 2900 6400 Pa bars 25 MPa 250 40 MPa 400 120 MPa 1.2 kbar 1 GPa 10 kbar 20 GPa 200 kbar 140 GPa 1.4 Mbar 360 GPa 3.6 Mbar Assertion: Typical "tectonic" stresses have magnitudes in the range 0.3 - 300 MPa (33,000 bars). Conclusion: In considering tectonic problems, it is usually useful to subtract some isotropic reference state of stress. There are 2 common choices - the lithostatic stress, which leaves the nonlithostatic stress tensor, and the pressure field, which leaves the deviatoric stress field. But since we need to know the actual stress tensor in order to calculate the pressure, which requires knowing the solution to the problem in advance, it is most common to use the lithostatic stress field as the reference. In order to place these comments in context, let's look at the situation described in T&S problem 2-6, where a continent is in isostatic equilibrium with an ocean. To address the possible state of (nonlithostatic) stress in the region, we will follow an iterative approach. The "algorithm" can go as follows: 1) Assuming lithostatic stress, calculate the horizontal normal stress as a function of depth beneath both the continental and the oceanic structures. 2) Evaluate the "reasonableness" of this assumption by making a free body diagram by isolating a (thin) vertical "pill box" of material at the continent-ocean boundary and calculating the total forces acting on its sides, given this assumption. This can be done by integrating the normal traction as a function of depth to determine the total normal force (per unit length into the picture) acting on each of the two vertical faces of the pill box: Moho Fx length = ∫ σ dy n surface The normal traction exerted by the continent on the pill box is everywhere greater than the normal traction exerted by the oceanic structure on the pill box. The total force on the pill box is just the area shaded with horizontal hatchure in the figure below. Fig. 4_1 4) Since our assumptions have led to an unbalanced force on the pillbox that would lead to an infinite acceleration as the thickness of the pill box shrinks, something must be wrong with our assumption. So we need to modify our model to decrease the magnitude of the normal traction exerted by the continent, increase the normal traction exerted by the oceanic structure, or both. 5a) If we "fix" our problem in the continent, we require a nonlithostatic horizontal extension. 5b) If we "fix" our problem in the oceanic region, we require a nonlithostatic horizontal compression. 5c) Of course, another solution would be to have horizontal compression in the continent, with even more horizontal compression in the oceanic area. Note: Isostasy and lithostatic stress cannot exist simultaneously! Areas of thickened crust tend to have a state of stress that is more extensional than nearby regions of thin crust. Examples: Tibet, Southern California Transverse Ranges. Sources of tectonic stress are of great interest to geodynamics, particularly if variations in stress can be interpreted in terms of interesting processes. Recall, from equilibrium, τ ij , j + ρ f i = 0 If we can ignore body forces, then, for example, ∂τ xx ∂τ = − xz ∂x ∂z Fig. 4_2 Example: Stress transition near back-arc basin Fig. 4_3 Measurements: • “Flat jack” – null measurement Fig. 4_4 Given Tn across cut Fig. 4_5 Related techniques – overcoring Hydrofracture Problems – Near surface heterogeneity Difficult to do remotely Only gives present-day stress Other techniques – direction only Cheap or free! • Borehole breakout – relies on stress concentration around drill holes; spalling of borehole walls Fig. 4_6 Changes in stress direction Example – subduction zone Fig. 4_7 Fore-arc: Compression σ xxdev < 0 Back-arc: Extension σ xxdev > 0 ⇒ mantle flow “required” ∂σ xx ∂σ xz Equilibrium: + =0 ∂x ∂z {Outer rise – extension near surface due to bending (discussed later)} Other examples of change in stress orientation: US – Mid continent ∼ E-W Compression Basin and range ∼ E-W extension California – rotation of dev. Stress near faults Earthquake focal mechanisms Fig. 4_8 Types of faults Fig. 4_9 Fig. 4_10 Note: Angle of σ 1 to fault plane depends on fault properties.