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 Distribution and geometry of fracture system Surface features of fracture Relative timing of fracture formation 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 12 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