Geology 229Engineering Geology

Lecture 28

Structural Geology(Reference West, Ch. 10)

Structural Geology1. Folds2. Faults3. State of stress and Faulting4. Stress perturbation caused by tectonic features

Research Methodologies:Traditionally, geological knowledge has come to us by a process of induction. This is the reasoning from some particular geologic observations or individual cases of geologic phenomena to a general conclusion.

In contrast, In Physics, it is more often to use deduction(as suggested by Albert Einstein):

Reasoning from a known principle that applies to a general construct in order to explain a particular geologic phenomenon observation.

There are Four ingredients in this approach:1, general boundary conditions;2, geometry of the structure;3, constitutive behavior of the materials;4, specific boundary conditions and initial condition.

Atypical example of the deduction approach is the classic mathematic physics.

Physics + constitutive relations + boundary conditions + initial conditions.

boundary value problems.

The structural Problems:Structural Geology studies rocks that have been deformed by earth stresses. These studies include descriptions of

1, Position;

2, Attitude;

3, Sequence.

From observations and infer the cause and process of the deformation.

Examples:Horizontal and vertical movement in sedimentary rocks;Intrusion of magma bodies in forming igneous rocks;

The induction approach applied to structural geology is the observations of rock deformation:

folding:

Ductile, or plastic deformation, a slow process, with relative hot and soft materials;

faulting:

brittle, a rapid to instantaneous process, with relatively cold material, usually caused by compressive force.

Material Property Geological example

Rubber elastic response of rocks to the passage of seismic waves

Clay plastic deformation in a ductile shear zone

Honey viscous flow of lavaGlass brittle fractured rocks

Ductile deformation: folding

Folding:

ductile, or plastic deformation, a slow process, with relative hot and soft materials.

(West, Figure 10.2)

(West, Figure 10.3)

(West, Figure 10.6)

(West, Figure 10.7)

t=Wsinθ

(West, Figure 10.8)

(West, Figure 10.9)

Brittle deformation: types of rock fractures:

Fractures:

narrow openings along which the rock mass has lost grain to grain contact.

Joints:

rock fractures along which no movement has occurred parallel to the joint surface, perpendicular movement may occur –joints are simple Mode I openings.

Types of rock fractures (cont.):

Shear zones:

rock fractures along which some movement has occurred but not a great amount, at the level of a few centimeters. Usually, shear zone occurs in weak materials with rich of water, and in great depth.

Faults:

fractures along which significant movement has occurs, much more than that associated with shear zones. The level of displacement is on the order of meters to kilometers, even to hundred kilometers.

Global plate tectonics, seismicity, and stress regimes

World Stress Map

State of stress in the crust:

Lithostatic:

all 3 principle stresses are equal to σv=ρgz, rarely occurs;

Reverse faulting regime:

the 2 horizontal stresses are all greater than the vertical stress σv=ρgz;

Strike slip faulting regime:

the maximum horizontal stress is greater than, and the minimum horizontal stress is less than the vertical stress σv=ρgz;

Normal faulting regime:

the 2 horizontal stresses are all smaller than the vertical stress σv=ρgz;

Stress-Depth relationships for the three states of stress

Both engineering community and Geological science community recognize the magnitude of the vertical stress has a depth dependence of ρgz.However, in engineering community, it is a common practice to assume the horizontal stress is about 1/3 of the vertical stress, if there is no (and usually very hard to measure) horizontal stress data. Their rationale is:

If the engineers keep this idea to a greater depth it is definitely untrue. If it is true, there will never be reverse faulting in the earth crust.‘Standard state’: σh=σv;‘Perfect lateral constraint’: σh=σv/3;The stress in the crust is closer to the standard state.

0.251 0.75 3

vh v v

σνσ σ σν

= = =−

Fault geometry:

Hanging wall;

Foot wall;

Strike;

Dip;

Rake.d

a

bv

u

x

North

zy

East

Faulting Geometry

a: Strike; b: Rake; d: Dip; u: Slip Vector; v: Fault Normal

Types of faulting:

1, normal faulting:

hanging wall goes downward;

2, reverse faulting:

hanging wall goes upward;

3, strike slip faulting:

The 2 walls go horizontally in the opposite directions against each other;

left-lateral strike slip;

right-lateral strike slip.

GPS observations of crustal deformation in the San Francisco Bay area crossing the San Andreas fault

Field recognition of faults:1, Landform features:

Offset/truncation of geologic features such as mountain chains, valleys, and flow channels;

2, Abnormal stratigraphic sequences:

repetition or omission;

3, Fault plane features:

polished rock surface;

fault gauge;

drag of bed.

The surface expression of the San Andreas Fault at the Carrizo Plains, CA.

The Kunlun Fault, Tibet Plateau

The clay formed by fault gauge

Offset of a flow channel caused by the fault generated by the 1993 Cocosili Earthquake, Tibet plateau.

Stress perturbation caused by some geologic features

Stress perturbation caused by a dense rift pillow

The horizontal displacement field superimposed with the contour of the vertical displacement on surface in the New Madrid seismic zone. The unit of the color bar is in meters. The maximum horizontal displacement is 14 m.

The trajectory of the horizontal principal stress superimposed with the contour of the horizontal shear strain expressed by the engineering shear strain on surface.