Brittle Deformation

Fracture A planar or curviplanar discontinuity

Forms as a result of brittle rock failure Under relatively low pressure and temperature

conditions in the earth crust

Rock fractures range in size from: Microcracks – Intragranular to intergranular

(fraction of a mm) Faults - Extend for hundreds of kilometers

Brittle Deformation Permanent change in rocks by fracture or sliding on fractures

Fracture: A discontinuity across which cohesion (Co) is lost

The term fracture includes three basic types of discontinuities: Extension fracture (type I)

Relative movement normal to fracture surface Shear fracture (type II & III)

Relative movement parallel to fracture surface Oblique extension (hybrid) fracture

Relative movement is oblique to the fracture surface

Vein: fracture filled by secondary minerals

Four categories of fracture observation

1. Distribution and geometry of fracture system

2. Surface features of fracture

3. Relative timing of fracture formation

4. Geometric relation of fracture to other structures

Fracture set and system Fracture set: a group of fractures with

similar orientation and arrangement Small extension fractures are referred to as

joint Systematic joints: have roughly planar

surfaces, parallel orientation and regular spacing (vs. non-systematic joints)

Fracture system: two or more sets of fracture affecting the same volume of rock

Sheet (exfoliation) joints

Are parallel to topography

Can form in any rock, but common in plutonic rocks that are exposed

Columnar Joints

Extension fractures characteristic of tabular extrusive igneous rocks i.e., form in lava flow, sill, dike

Other types of joint: Strike joint, dip joint, cross joint, oblique

joints

Extension fractures associated with shear fractures

Feather (pinnate) fractures – form en-echelon to a main brittle shear fracture

Gash fracture – are simiar to feather fractures, but filled with mineral, in ductile shear fractures Are sigmoidal (S- or Z-shaped)

What do we collect about fractures? Orientation (rose diagram, stereonet) Spacing Length Spatial pattern Relation to lithology Relation to layer (bed) thickness

Joints Are a type of fracture which form due to tension

They form parallel to the minimum tensile stress Perpendicular to maximum tensile stress Shearing is zero along joints when they form Also called cracks or tensile fractures

Joints form under: shallow depth, low confining pressure (Pc) elastic regime low temperature (T) high pore fluid pressures (Pf)

Joints Joints form perpendicular to the maximum

principal tensile stress i.e, along a principal plane of stress

Therefore joints dominantly show separation or opening of the walls of the fractures with no appreciable shear displacement parallel to the plane of the fracture Joints form through the Mode I crack surface

displacement Joints are commonly characterized by two

matching, rough, discontinuous, and curved surfaces, although they are approximated to be smooth, continuous, and planar

Tensile strength and jointing

Joint sets

Modes of crack surface displacement Individual cracks, when loaded, propagate, infinitesimally,

in three different modes: Mode I – Tensile (Opening) Mode

Tensile cracks form normal to the 3 (parallel to the 12 plane)

Crack opens infinitesimally perpendicular to the crack plane Crack grows in its own plane; no bending/changing orientation

Mode II – Sliding Mode One block moves parallel to the crack normal to the fracture front

Mode III – Tearing Mode One block moves parallel to the crack parallel to the fracture front

Modes of crack surface displacement

Modes of fracture

Mode II and III

Both are shear mode Do not grow in their own plane. As they start growing, they immediately either:

Curve Become mode I cracks Spawn new tensile, wing cracks

However, shear fractures and faults are not large mode II or mode III cracks

Joints The planar approximation is justified given that scale of

geometric irregularities (e.g. joint surface morphologies, curvature amplitude) is commonly very small compared to the size of the fracture surface

Termination of the two opposing surfaces at their distal edge or periphery (fracture front), i.e., a finite extent of the two walls Displacement is zero at the fracture fronts

Involve small relative displacement of the originally contiguous points compared to the in-plane dimensions of the fracture walls

En-echelon Tension Gashes

Vein

Antiaxial vs. syntaxial vein

Joint Spacing &Bed Thickness

Fracture Terminations

Shear Fractures

Fractures along which there has been shearing or displacement

Shear fractures are small, with small displacement

They occur in intact rock during brittle deformation

If the amount of displacement is significant and measurable, the shear fracture is called fault

Faults and joints

Shear Fractures

Shear fractures, form at an angle to the maximum compressive stress and show offset because of the shear traction along the fractures

The hybrid shear fractures are discontinuities with a mixed mode of opening and shear and form oblique to the plane of the fracture as a result of both tensile normal stress and shear stress

Terminology

Fracture front: The line separating the fractured region from the un-fractured part of the rock

Fracture trace: Intersection of the fracture with any surface

Fracture tip: The termination of the fracture along the trace of the fracture

Joint Surface Morphology

Surface morphology of joints show evidence for initiation, propagation, and arrest

Theoretically, mode I loading in an isotropic, homogeneous material should lead to a smooth propagation with a mirror smooth surface

Joints however are not smooth, because rocks are commonly neither homogeneous nor isotropic

Out-of-plane Propagation

The orientation of the maximum tensile stress in front of a single crack tip may not be parallel to the normal to the parent crack

Cracks propagate so that “new’ portions of the crack remain normal to the local maximum tensile stress

This requires a crack to leave the plane of the parent crack in order to maintain its orientation relative to the local stress field

This out-of-plane propagation is so common in microscopic scale that leads to the formation of rough, non-smooth fractures surfaces

Crack Propagation Paths

Out of plane propagation is characterized by a combination of two end member crack propagation paths:

Twist: leads to segmentation of the crack into several smaller crack planes. Represents a rotation of the local maximum tensile stress

in the initial yz plane. Rotation is about an axis in the crack plane || to

propagation direction

Tilt: Causes the crack tip to rotate without segmentation. Rotation is about an axis in the crack plane _|_

propagation direction

Mesoscopic Joint Surface Although microscopic out-of-plane propagation is

common, joints appear smooth on the mesoscopic scale.

This is due to the homogeneous nature of the remote stress field

Even when the crack leaves its plane, the general propagation path remains normal to the remote maximum tensile stress

The microscopic out-of-plane propagation leads to the development of joint surface morphology

Plumose Surface Morphology Helps to interpret rupture nucleation, propagation, and

arrest Develops largely due to local twists and tilts during

propagation of a fracture which would otherwise be planar. Barbs: surface irregularities

In homogeneous rocks, barbs trace to the point of origin (an original crack)

In inhomogeneous rocks (sandstone, shale), barbs radiate from either a bedding plane or an inclusion in the bed (e.g., fossil, concretion, clast)

The point of origin may vary from bed to bed in shale or siltstone

Joint Initiation and Beds If joints initiate from bedding plane, they

often originate from irregularities such as ripples or sole marks

Joints in a bed often initiate from a common feature such as the upper bedding plane

Fossils and concretions often originate joints

in adjacent beds

Rupture Propagation The progress of rupture from the origin to the final arrest

leads to the formation of some patterns that are printed on the surface of joints

Mirror zone: Area immediately adjacent to the point of origin. Forms under small tip stress values, not big enough to

beak the material at oblique angles

Mist zone: Forms when larger stresses break bonds at oblique

angles to crack plane. On the fine scale, this oblique cracking forms a non-

smooth (misty) zone, separating the mirror from the hackle zone

Hackle zone

Forms when there are local components of twist during crack propagation.

Form when propagation occurs at a critical velocity when cracks branch or bifurcate

Hypothesis I: High velocities shift the maximum local tension away from the existing crack plane

Hypothesis II: High velocities form secondary cracks. The main crack then branches to follow the secondary cracks

Plumose Structures Also called feathers, hackle plumes, striations, or barbs.

These record rupture motion

Consists of an axis from which barbs mark the direction of the rupture front as portions diverge away from the plume axis

Barbs become more pronounced toward the edge of the beds away from the axis Barbs represent long, narrow planes oblique to the main

fracture plane. Barbs form similar to hackle marks due to twist

Plumose Structures …

Plumose structures have different shapes: Axes can be straight to curved Barbs vary from uniform to symmetrical

to asymmetrical about the axis

These variations reflect the degree to which rupture velocity was uniform during propagation

Arrest of Rupture Arrest lines show up as ridges or cusped waves

normal or subnormal to the direction of propagation of cracks

At arrest lines a large component of tilt is involved in the out-of-plane crack propagation

Arrest lines may be the boundaries between areas with perceptible barbs and areas with no barbs.

Rock joints may show several arrest lines in a row or a single arrest line at the end of a long fracture

Several arrest lines represent intermediate slowing or stopping points for a rupture as it moves through the rock to form a joint