Fractures

“cracks along which cohesion of the material has been lost”- planes of discontinuity…most common type of geologic structure

Two types of fractures:

(1) Faults: surface across which the rock has been displaced in a direction that is generally parallel to the fracture

(2) Joints: a fracture with NO displacement parallel to the fracture plane…displacement is perpendicular to the fracture…will look at in more detail in subsequent lecture

Very commonly filled with mineralization…

Fissure: special class of joints that have dilation >20 cm

Why study fractures?

EngineeringFluid flowEarthquake hazardsStress-strain significance

1. Engineering

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

2. Fractures can control subsurface fluid flow

paths

Courtesy of Zion National Park

3. Siesmic hazards

Freeway Damage from 1994Northridge Earthquake

4. Stress-strain signficance

Fractures record tectonism

Joints due to:1. Faulting2. Mechanical

Stratigraphy3. Fold Shape

Describing Faults

Faults: a fracture surface across which the rock has been displaced in a direction that is generally parallel to the surface itself.

Classification of faults:- each type of fault forms under different

geologic conditions- so, if we can characterize the nature of the

fault, we can conclude something about the geologic history...

Fault classifications are based on two basic observations:

(1) orientation of the fault surface

(2) nature of the displacement or offset across the fault

Also: to a lesser extent(3) Character of the fault surface

(This comes into recognizing the faults…we’ll look at this later)

Fault Classification

Orientation of fault surfaces:

Strike

Dip

strik

e

dip

Note: for mechanical reasons fault planes are seldom planar over long distances…thus, must take MANY strike and dip along a fault

Basic Fault Terminology (more to come later):Footwall – block sitting below the fault surface

Hanging wall – “ “ “ “ above the fault surface

NormalFault

Foot

wal

l

Han

ging

wal

l

Footwall

Hangin

g wal

l

Thrust/ReverseFault

Strike-SlipFault

Nature of slip along Faults

Nature of Fault Surfaces

Discrete fracture or fault zone

Nature of Fault Surfaces

SlickensidesStriationsGrooves

Reidel shears (photo by M.Miller)

Nature of Fault Surfaces

Joints and MicrofaultsFault brecciaVein FillingFault GougePseudotachylite

Nature of Fault Surfaces

Deformation BandsCataclastic rocks

and myloniteDuctile shear zones

Adirondacks Jay, NY

Nature of Fault Shapes

Faults are not infinite

Wytch Farm Oil Field, southern England (taken from Kttenhorn & Pollard, AAPG Bull, 2001)

Terminology of Slip

Slip: actual relative displacement on a fault. Orientation of the slip line Magnitude of displacement Relative displacement

Drag folds as slip indicators

Distortion of layers consistent with sense of shear

Reverse drag

Recognition of faults

1. Truncation and offset of rock units

2. Inconsistent stratigraphic relationships

Recognition of faults

3. Fault Scarps4. Fault Line Scarps

Photos by M. Miller

Recognition of faults

5. Triangular facets6. Erosional traces

Death Valley (photo by M. Miller)

Recognition of faults

7. Topographic irregularities Tectonic

geomorphology

http://www.intermargins.org/tsunami1.html#a2

Recognition of faults

8. Geophysics

From the University of Leeds, UK

From the University of Leeds, UK